Guidelines for the welded fabrication of nickel alloys

Guidelines for the welded fabrication of nickel alloys
NiDI
Nickel
Development
Institute
Guidelines for
the welded fabrication of
nickel alloys for
corrosion-resistant service
A Nickel Development Institute
Reference Book
Series NO 11 012, 1994
The material presented in
this publication has been
prepared for the general
information of the reader
and should not be used or
relied on for specific
applications without first
securing competent advice.
The Nickel Development
Institute, its members, staff
and consultants do not
represent or warrant its
suitability for any general or
specific use and assume no
liability or responsibility of
any kind in connection with
the information herein.
This report was prepared by
Richard E. Avery and
Arthur H. Tuthill
Consultants in the United States
to the Nickel Development Institute
Guidelines for
the welded fabrication of
nickel alloys for
corrosion-resistant service
Contents
Introduction........................................................................................................................................ i
PART I – For the welder .................................................................................................................... 1
Physical properties of nickel alloys ............................................................................................... 1
Corrosion resistance of nickel alloy welds..................................................................................... 1
Avoid crevices.......................................................................................................................... 2
Embedded iron ........................................................................................................................ 2
Effect of surface oxides from welding ....................................................................................... 2
Other welding-related defects................................................................................................... 2
Welding qualifications................................................................................................................... 3
Welder training ............................................................................................................................. 3
Weld joint penetration................................................................................................................... 3
Weld joint design.......................................................................................................................... 4
Preparation for welding................................................................................................................. 4
Cutting and joint preparation .................................................................................................... 5
Oxides and other surface layers ............................................................................................... 5
Contaminating elements........................................................................................................... 5
Chlorinated solvents................................................................................................................. 6
Health hazards......................................................................................................................... 7
Fixtures and positioners ........................................................................................................... 7
Backing materials..................................................................................................................... 7
Tack welding............................................................................................................................ 7
Welding processes ....................................................................................................................... 8
Shielded metal arc welding....................................................................................................... 8
Electrode types...................................................................................................................... 8
Electrode storage................................................................................................................... 9
Welding current ................................................................................................................... 10
Electrode handling ............................................................................................................... 10
Arc Starting and Stopping ................................................................................................ 10
Weld Puddle Control........................................................................................................ 10
Out-of-Position Welding................................................................................................... 10
Weld Spatter ................................................................................................................... 10
Gas tungsten arc welding....................................................................................................... 10
GTAW equipment ................................................................................................................ 11
Power.............................................................................................................................. 11
Current controls............................................................................................................... 11
Cooling............................................................................................................................ 11
Electrodes ....................................................................................................................... 11
Nozzles ........................................................................................................................... 11
System leaks................................................................................................................... 11
Shielding gases ............................................................................................................... 11
Filler metals..................................................................................................................... 11
Operator guidelines................................................................................................................ 12
Arc initiation .................................................................................................................... 12
Arc stopping .................................................................................................................... 12
Arc shielding ................................................................................................................... 12
Nickel alloy filler metals ................................................................................................... 12
Gas metal arc welding............................................................................................................ 12
GMAW arc types.................................................................................................................. 13
GMAW equipment ............................................................................................................... 13
Consumables....................................................................................................................... 13
Other welding processes........................................................................................................ 14
Welding nickel alloy pipe ............................................................................................................ 14
Types of pipe welding ............................................................................................................ 14
Instrument piping ................................................................................................................... 14
Automatic welding.................................................................................................................. 14
Manual welding...................................................................................................................... 14
Purging during pipe root welding ............................................................................................ 16
Post-fabrication cleaning ........................................................................................................ 16
Surface contaminants .......................................................................................................... 16
Embedded iron .................................................................................................................... 16
Mechanical damage ............................................................................................................... 17
Safety and welding fumes ...................................................................................................... 17
PART II – For the materials engineer ............................................................................................. 19
General guidelines for nickel alloys............................................................................................. 20
Preheat and interpass temperature ........................................................................................ 20
Post-weld heat treatment ....................................................................................................... 20
Filler metal selection for corrosive environments .................................................................... 20
Group A- Nickel and nickel-copper alloys ............................................................................... 20
Alloys 200 and 201 ............................................................................................................. 20
Alloy 400 and R405 ............................................................................................................. 20
Salt and brine environments ............................................................................................ 21
Hydrofluoric acid service.................................................................................................. 21
Group B - Chromium-bearing alloys ....................................................................................... 21
Group C - Nickel-molybdenum alloys...................................................................................... 21
Group D - Precipitation-hardening nickel alloys ...................................................................... 21
Dissimilar-metal welds .......................................................................................................... 22
Procurement guidelines .............................................................................................................. 22
Surface finish ........................................................................................................................ 22
Nickel alloy castings ................................................................................................................... 23
Source Inspections ................................................................................................................ 24
Radiographic inspection....................................................................................................... 24
Liquid penetrant inspection .................................................................................................. 25
Weldability test .................................................................................................................... 26
Pressure test ....................................................................................................................... 26
Certification ......................................................................................................................... 26
Heat Treatment.................................................................................................................... 26
NiCrMo alloys.................................................................................................................. 26
Nickel, Nickel-copper, Nickel Molybdenum alloys ............................................................. 26
Chemistry .............................................................................................................................. 26
Casting repair by welding ....................................................................................................... 27
Ni, NiCu and NiMo alloys, Group A and C alloys .................................................................. 27
Filler metals ......................................................................................................................... 27
Post-weld repair heat treatment ........................................................................................... 27
Welding nickel alloy castings.................................................................................................. 27
Procurement checklist for nickel alloy castings ....................................................................... 27
PART III – For the design engineer ................................................................................................ 29
Design for corrosion service ....................................................................................................... 29
Tank bottoms ......................................................................................................................... 29
Tank bottom outlets ............................................................................................................... 29
Bottom corner welds .............................................................................................................. 30
Attachments and structurals ................................................................................................... 30
Heaters and Inlets.................................................................................................................. 31
Pipe Welds ............................................................................................................................ 31
Weld overlay, sheet lining, and clad plate ................................................................................... 32
Weld overlay .......................................................................................................................... 32
Submerged arc welding ....................................................................................................... 32
Gas metal arc welding ......................................................................................................... 33
Shielded metal arc welding .................................................................................................. 33
Weld overlay guidelines ....................................................................................................... 33
Base metal dilution .......................................................................................................... 33
Base metal interface........................................................................................................ 33
Sheet lining............................................................................................................................ 33
Clad steel .............................................................................................................................. 34
Bibliography.................................................................................................................................... 35
Acknowledgement .......................................................................................................................... 35
Tables
Table 1 – Wrought nickel alloys by group .................................................................................... 1
Table 2 – Influence of physical properties on welding nickel alloys .............................................. 1
Table 3 – Nickel alloy cutting methods ........................................................................................ 5
Table 4 – Embrittling elements.................................................................................................... 6
Table 5 – Matching composition filler metals for nickel alloys ...................................................... 9
Table 6 – Comparison of GMAW arc modes for nickel alloys..................................................... 13
Table 7 – Nominal composition of groups A – D wrought nickel and nickel alloys ...................... 19
Table 8 – Nominal composition of cast corrosion resistant nickel alloys..................................... 19
Table 9 – Matching filler metals of the comparable solid solution alloys ..................................... 22
Table 10-A – Filler metal alloy identification for bare and covered electrodes .............................. 22
Table 10-B – Suggested filler metals for dissimilar metal welds................................................... 23
Table 10-C – Suggested filler metals for dissimilar metal welds................................................... 24
Table 11 – Specifications for procurement of groups A – D wrought nickel and nickel alloys........ 25
Table 12 – Surface finishes for nickel base alloy sheet, strip, and plate....................................... 25
Table 13 – Comparisons of weld overlay, sheet lining, and clad plate.......................................... 33
Figures
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
1 – Incomplete fusion in pipe root pass weld .................................................................... 2
2-1 to 2-5 – Typical joint designs for sheet and plate........................................................ 4
3 – Sample showing sulphur embrittlement of a Nickel 200 sheet..................................... 6
4 – Typical backing bar designs for use with and without a backing gas ........................... 7
5 – Tack weld sequence to provide uniform weld gap.......................................................8
6 – The arc zone in the SMAW process ........................................................................... 8
7 – The Gas Tungsten Arc Weld (GTAW) process ......................................................... 10
8 – The basic components of the Gas Metal Arc Weld (GMAW) process ........................ 12
9 – Typical joint design for pipe with consumable insert ................................................. 15
10 – Typical joint design for pipe welded with open root joint and hand-fed filler metal ..... 15
11 – Standard consumable insert shapes, ANSI/AWS D10.11 ......................................... 15
12 – Typical pipe purging fixtures .................................................................................... 16
13 – Flat bottom, square corners – worst ......................................................................... 29
14 – Flat bottom, rounded corners – good corners – poor outside .................................... 29
15 – Flat bottom, rounded corners, grouted –good inside, poor outside............................ 29
16 – Flat bottom, rounded corners, drip skirt – good inside, good outside......................... 29
17 – Concave bottom rounded corners–good inside, good outside, fatigue resistant ....... 29
18 – Dished head – best inside, best outside, fatigue resistant......................................... 29
19 – Side outlet above bottom – poor .............................................................................. 29
20 – Centre outlet, above bottom – poor .......................................................................... 29
21 – Side outlet, flush – good .......................................................................................... 29
22 – Centre outlet recessed – good ................................................................................. 29
23 – Side outlet, flush, sloped – best ............................................................................... 30
24 – Centre outlet, recessed, sloped – best ..................................................................... 30
25 – Corner weld from inside – poor inside, worst outside................................................ 30
26 – Corner weld from both sides – poor inside, good outside.......................................... 30
27 – Side wall in lieu of corner weld – best inside, good outside, fatigue resistant ............ 30
28 – Tray support, staggered strength weld – severe crevice ........................................... 30
29 – Tray support, full seal weld top – good crevice resistance ........................................ 30
30 – Tray support, full seal weld top & bottom – best crevice resistance........................... 30
31 – Reinforcing pad, staggered welds – adequate strength ............................................ 30
32 – Reinforced pad, seal weld – best crevice resistance................................................. 30
33 – Position of angles .................................................................................................... 31
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
34 – Position of angles .................................................................................................... 31
35 – Position of channels ................................................................................................ 31
36 – Stiffeners and baffles............................................................................................... 31
37 – Corner baffle cut-out – good .................................................................................... 31
38 – Heat exchanger, baffle cut-out–good ....................................................................... 31
39 – Poor and good designs for the location of heaters in a vessel .................................. 31
40 – Poor and good designs for mixing concentrated and dilute solutions ........................ 31
41 – Pipe weld with incomplete penetration – severe crevice ........................................... 32
42 – Pipe recessed, flange and pipe, same alloy – good.................................................. 32
43 – Pipe flush, pipe and flange same alloy – better ........................................................ 32
44 – Stub–end, flange carbon steel or ductile iron – very good ........................................ 32
45 – (A) Horizontal (standard) – poor............................................................................... 32
(B) Sloped – very good ............................................................................................ 32
Figure 46 – Weld joints for liners ............................................................................................... 34
Figure 47 – Joint designs for clad steel ..................................................................................... 34
Introduction
Part II, FOR THE MATERIALS ENGINEER,
describes the types of nickel alloys; it reviews how
their metallurgical and corrosion characteristics are
affected by welding and covers some of the more
specialized aspects of fabrication such as heat
treating. A number of useful references are included to assist in the selection of electrodes, rods,
and filler metals for solid solution alloys. Additional
tables cover the selection of electrodes and rods for
dissimilar metal welds. Guidelines are included for
material procurement of castings along with suggestions for supplementing the specifications with
additional requirements and tests to assure the
quality of the finished castings.
Part III, FOR THE DESIGN ENGINEER, provides
a number of design examples showing how the
corrosion performance of nickel alloys used in
process tanks can be enhanced through thoughtful
design. A generous number of figures illustrate the
configurations which improve the prospects for
successful performance in corrosive environments.
The discussion also treats weld overlay, sheet
lining, and clad plate as alternative means of providing corrosion protection using nickel alloys. A
number of welding processes are briefly evaluated
as tools for achieving the desired results with each
of these alternates.
This publication is presented in three parts with
each, in turn, focused toward the primary interests of the welder, the materials engineer, and
the design engineer.
Part I, FOR THE WELDER, assumes that the
welders and others involved in welded fabrication
are familiar with the basic techniques used in
carbon steel fabrication and have had limited
experience with nickel alloys. The discussion
treats many areas of concern to the welder and
gives practical suggestions concerning the effects
of shop practices in maintaining the corrosion
resisting properties of the nickel alloys. The
importance of proper storage and protection of
the surfaces, proper cleaning combined with the
proper cleaning materials is stressed both before
and after welding. Welding and welding training
and qualification are discussed as well as arc
management during the welding process. A
number of commonly used welding processes are
covered to furnish a perspective of the particular
features that ensure improved results. Finally a
brief discussion introduces the particular considerations involved in welding pipe. The discussion
takes a “how to” approach useful to the nonengineer but the material covered is also a good
reference for the materials and design engineer.
i
For the welder
Part I
For the welder
ments similar to those for the precipitation-aging
stainless steels. Some physical properties and
their influence on welding are shown in Table 2.
Part I focuses on the fabrication and welding of
nickel alloys as they relate to the welders and
production personnel engaged in fabrication of
nickel alloys for corrosion service. Table 1 shows
the wrought and cast nickel alloys by group.
Corrosion resistance of nickel
alloy welds
Physical properties of nickel
alloys
The performance of nickel alloy equipment in
corrosive service is subject to the care taken by
welders and others on the shop floor. Sound,
high-quality welds are the single most important
objective; however, to achieve this objective,
there are a number of factors that require attention during fabrication. These factors will be
addressed in detail.
The physical properties of solid solution nickel
alloys, Groups A, B, and C, are quite similar to
the 300 Series austenitic stainless steels. The
solid solution nickel alloys cannot be strengthened by heat treatment, only by cold working.
Group D alloys, the precipitation hardening nickel
alloys, are strengthened by special heat treatTable 1
Wrought nickel alloys by group
Wrought alloys
Wrought alloys
Alloy No.
UNS No.
Alloy No.
UNS No.
Group A – Nickel and nickel-copper solid solutions alloys
200
N02200
400
N04400
201
N02201
R-405
N04405
Group B – Chromium-bearing solid solution alloys
825
N08825
59
N06059
G-3
N06985
686
N06686
G-30
N06030
622
N06022
600
N06600
C-22
N06022
690
N06690
C-276
N10276
625
N06625
Group C – Nickel-molybdenum alloys
B-2
N10665
B-4
N10629
B-3
N10675
Group D – Precipitation hardening alloys (used for corrosive service)
K-500
N05500
725
N07725
625 PLUS®
N07716
718
N07718
Cast alloys ASTM A494A 494M
Alloy No.
UNS No.
CZ-100
M-35-1
N02100
N24135
CW-6MC
CY-40
CW-2M
CX2MW
N26625
N06040
N26455
N26022
N-7M
N30007
Table 2
Influence of physical properties on welding nickel alloys
Property
Alloy(s)
Remarks
Melting Point
All
Melting point is 55°-165°C (100-300°F) lower than SAE 1020 steel, tending to allow faster
welding for the same heat or less heat for the same speed.
Magnetic Response
Nickel 200, 201
Magnetic up to 360°C (680°F) subject to arc blow similar to carbon steels.
Alloys 400, R-405
May be magnetic or non-magnetic at room temperature, depending on composition variations.
All others
Similar to austenitic stainless steels .
Electrical Resistance
Nickel 200, 201
Low, similar to SAE 1020 steel.
Others
Varies with composition. Compared to Type 304, alloy 400 is 25% lower while the chromiumbearing alloys are up to 200% higher. High electrical resistance may cause overheating in some
covered electrodes.
Thermal Expansion
All
All nickel alloys are closer to carbon steels than are the austenitic stainless steels. This results
in less warpage and distortion than comparable stainless steel fabrications and lower residual
stresses in welding to low alloy steels.
Note:
While wrought austenitic stainless steels are non-magnetic, some stainless welds and castings may be slightly magnetic as a result of the presence
of a delta ferrite. Nickel-chromium and nickel-chromium-iron wrought and cast alloys do not contain ferrite and do not exhibit a magnetic response.
1
For the welder
Avoid crevices
The detection and treatment of embedded iron
is discussed under the topic entitled Postfabrication cleaning, later in this section.
It is well-recognized that butt welds should be
full-penetration welds to provide optimum
strength. In corrosion service, there is another
reason for full penetration welds. Crevices resulting from inadequate penetration, when exposed
to certain corrosive environments, are potential
sites for crevice corrosion. Avoiding crevices is
mainly a design responsibility, but it is helpful for
those actually making the equipment to assist in
eliminating crevices wherever possible. A typical
example of an undesirable crevice resulting from
incomplete fusion of a pipe root pass weld is
shown in Figure 1.
Effect of surface oxides from welding
It is a well-established fact that heat tint may
reduce the pitting and crevice corrosion resistance of austenitic stainless steels in some environments. Surface oxides in the form of heat tint
result from welding on the reverse side of a plate
or sheet or from the oxide formed in the heataffected zone (HAZ) next to the weld surface.
For example, oxides that form on the inside of a
vessel from welding lifting lugs, stiffeners, or
similar items, on the outside can be particularly
damaging. Such oxides should be removed
down to clean metal.
Studies by Silence and Flasche have shown
that there is less need for heat tint removal from
chromium-containing nickel alloys than from
austenitic stainless steels. The tests were made
on three nickel alloys and three iron-base alloys
using a range of FGD-representative oxidizing,
reducing, and oxidizing acid-chloride environments. These findings are in general agreement
with field experience. The authors have, however,
encountered cases in non-FGD environments
where heat tint removal was essential to corrosion-resistance in nickel-chromium-molybdenum
alloys. Such cases are infrequent but do suggest
that the best practice is to remove heat tint on the
wetted side of the fabrication, whether it occurs
on the welded side or on the side opposite the
welded surface, for example, where lugs or
stiffeners may be welded to the outer surface of
a tank.
Data on the effect of surface films on the nonchromium-bearing nickel alloys is even more
sparse. In the absence of such data for the wide
range of potentially corrosive environments
where nickel alloys are applied, the conservative
approach is to provide the cleanest and most
oxide-free surfaces that are economically practical.
Figure 1 Incomplete fusion in pipe root pass weld
Embedded iron
When new nickel alloy equipment develops rust
spots, it is nearly always the result of embedded
free iron. Surface rusting is objectionable from an
appearance point of view and might also be the
cause of pitting corrosion as it is with austenitic
stainless steels in certain environments. Furthermore, the iron or rust may act as a contaminant
in the process that it services, thus affecting the
product purity.
Following are a few common sense precautions
that can greatly reduce the chances of contamination.
• Protect iron and steel surfaces that might
come into intimate contact with nickel alloys
by using wood, plastic, or cardboard to prevent iron contamination.
• Use clean stainless steel wire brushes or
abrasive disks or wheels that have never
been used on iron or steel. Iron-contaminated
wire brushes or abrasive disks may introduce
embedded iron to the nickel alloy surface.
• Do not leave nickel alloy sheets or plates on
the floor where they are exposed to traffic.
Store sheet and plate stock in a vertical
position.
• Separate nickel alloy fabrication from other
types of metal fabrication. Steel grinding,
cutting, and blasting operations can introduce
embedded iron to the nickel alloy surfaces.
Other welding-related defects
A number of additional welding-related defects
and their suggested treatments follow.
• Arc strikes on the parent material damage the
alloy’s protective film and create crevice-like
imperfections. Weld stop points may create
pinpoint defects in the weld metal. Both imperfections can be avoided by using proper techniques and, if they occur, should be removed by
light grinding with clean, fine-grit abrasive tools.
• Weld spatter creates a tiny weld where the
2
For the welder
molten slug of metal touches and adheres to
the surface. The protective film is penetrated
and tiny crevices are created where the film is
weakened.
• Weld spatter can easily be eliminated by
applying a commercial spatter-prevention
paste to either side of the joint to be welded.
The paste and spatter are washed off during
cleanup.
• Some nickel alloy electrode coatings contain
fluorides which can leach out and cause
corrosion if the slag is not completely removed. Slag particles can also create crevices, additional places for corrosion to begin.
Slag is difficult to remove, particularly small
particles, when there is a slight undercut or
other irregularity. The usual removal is done
with wire brushing, light grinding, or abrasive
blasting with iron-free abrasives.
rate WPS is needed. A change in a base metal
from one P-Number to another P-Number requires requalification. Joints made between two
base metals of different P-Numbers require a
separate WPS, even though qualification tests
have been made for each of the two base metals
welded to themselves. The P-Numbers of common nickel alloys follow.
P-number
Base metal
41
42
43
44
Nickel 200, 201
Alloy 400
Alloys 600, 625, 690
Alloys B-2, C-22, C-276
Not all alloys have been assigned a P-Number.
For example, cast alloys and Group D, the precipitation hardening alloys, do not have P-Numbers.
Alloys without a P-Number require individual qualification even though similar in composition to an
alloy already qualified. If an alloy is not listed in
the P-Number tables, the alloy manufacturer can
advise if a P-Number has been recently assigned.
Welding qualifications
It is standard practice for fabricators of process
equipment to develop and maintain Welding
Procedure Specifications (WPS) for the various
types of welding. The individual welders and
welding operators are tested and certified by
satisfactorily making acceptable qualification
weldments. There are a number of society or
industry codes that govern welding
qualifications, but the two most widely used in
the U.S. for corrosion resistance equipment are:
• American Society of Mechanical Engineers,
ASME Boiler and Pressure Vessel Code —
Section IX, Welding and Brazing Qualification;
• American Welding Society, Standard for
Welding Procedure and Performance Qualification — AWS B2.1.
Each country typically has its own individual
codes or standards. Fortunately, there is a trend
toward the acceptance and interchange of
specifications in the interest of eliminating
unmerited requalification.
The identification of essential variables that
establish the need for a new procedure qualification test is the common element of these codes.
These essential variables differ for each welding
process, however, they also share some common
factors. Changes in any of the following items
are considered to be essential changes.
- base metal (P-Number);
- filler metal (F-Number);
- metal thickness;
- shielding gas;
- welding process.
ASME Section IX classification of P-Numbers is
often the first determinant as to whether a sepa-
Welder training
In complying with welding specification codes
such as ASME and AWS, welders must pass a
performance test. A welding training program is
essential to prepare welders for the performance
test and training is equally essential to assure
quality production over the long term.
Ample training and practice time should be
provided for welders who have not had experience with the particular nickel alloys. For example, the welding characteristics of chromiumbearing solid solution nickel alloys, Group B, are
similar to those of austenitic stainless steels.
Skilled stainless steel welders can usually
adjust quickly to welding Group B alloys. The
same welders, however, may find Nickel 200
and Alloy 400 welding fillers rather different
because of the sluggish nature of the molten weld
metal. Once briefed on the specific elements
that change for any alloy or filler metal, they can
proceed confidently and productively to make
high quality welds.
In addition to the particular base metal and
welding process, training should include information and practice on unusual welding positions
as well as on the shapes to be welded such as
pipe and thin sheets.
Weld joint penetration
Butt welds should be full-penetration welds to
produce full strength and optimum performance
in corrosion service. Fillet welds need not be full3
For the welder
Preparation for welding
penetration welds as long as the sides and ends
are welded to seal off voids that might collect
product. Pipe welds that lack full penetration
invite crevice corrosion which creates a high
stress point at the root. For this reason, pipe
welds should be full-penetration welds for best
performance.
The care taken in preparation for welding is time
that yields improved weld quality and a finished
product that gives optimum service. Important
preparation steps follow.
Weld joint design
Molten nickel alloy weld metal is considerably
less fluid than carbon steel and somewhat less
fluid than stainless steel. The depth of weld
penetration is also not as great. Within the nickel
alloy group, there is a difference in fluidity and
weld penetration depending upon the amount of
nickel that is present. For example, commercially
pure Nickel 200/201 is most viscous and yields
a shallow weld bead. To compensate for these
features, nickel alloy joints have a wider bevel,
narrower root face, and wider root opening. The
welding process also influences weld joint dimensions. For example, a spray arc gas metal arc
weld (GMAW) has a deeper penetration weld
bead than other arc welding processes so thicker
root faces may be used.
Typical joint designs for sheet and plate are
shown in Figure 2-1 through 2-5.
Figure 2-3 Typical double “V” joint for plate.
Figure 2-4 Typical single “U” joint for plate.
Figure 2-1 Typical square butt joint for sheet.
Figure 2-2
Typical single “V” joint for sheet and plate.
Figure 2-5 Typical double “U” joint for plate.
4
For the welder
more of an influence on the depth of penetration
and bead shape of gas tungsten arc welds
(GTAW) than on arc welds made with higher heat
input arc welding processes. Pre-weld cleaning of
thin gauge sheet or strip, e.g., 0.5mm (0.020 in.)
and thinner, is a critical requirement to prevent
weld defects. Vapor blasting is a common cleaning method for this gauge of material.
When repair welding is required on equipment
used in chemical service, it is especially important
to ensure the removal of surface contamination with
careful pre-weld cleaning. The cleaning objective is
to remove the embedded contamination by grinding, abrasive blasting, or neutralizing the surface
prior to repair welding. Acid-contaminated surfaces,
are neutralized with a mild basic solution and
alkaline-contaminated surfaces are neutralized with
a mild acid solution. A thorough hot water rinse
should always follow the neutralizing treatment.
For example, if caustic has been in contact with
the nickel alloys for an extended period of time it
may be embedded. If not removed prior to welding, the weld and heat-affected zone often develops cracks. Removal requires grinding, abrasive
blasting, or neutralizing with an acid solution such
as 10% (by volume) hydrochloric acid (followed by
a thorough hot water rinse).
Cutting and joint preparation
Nickel alloys are cut as shown in Table 3. They
share the same cutting methods as those used
for stainless steels. Oxyacetylene cutting of nickel
alloys (without iron-rich powder additions) results
in the formation of refractory oxides, preventing
accurate, smooth cuts. As you will observe in
Table 3, the thickness and shape of the parts
being cut largely dictates which of the cutting
methods is most appropriate.
Oxides and other surface layers
All oxides and dross from thermal cutting should be
removed by grinding, machining, or abrasive blasting. Oxides of elements in nickel alloys such as
chromium, nickel and particularly titanium and
aluminum in the precipitation hardening alloys, are
high melting point oxides and are not fused by the
weld metal. An oxide film can become trapped in
the solidifying weld, resulting in a defect that is
difficult or impossible to detect by radiography.
Surface oxides may be present as a result of
heat treating or they may exist on equipment that
has been exposed to high temperatures. In some
high temperature service environments, a carburized or sulphurised surface layer can develop.
All such layers should be removed by grinding or
machining the area to be welded. Wire brushing
does not remove the tightly adhering oxides or
other surface layers. While wrought product forms
or castings in the as-received condition are
normally free of oxides, it is good practice to
condition a one-inch wide band on both the top
and bottom surface of the weld zone to bright
metal with a medium grit flapper wheel or disk.
This is particularly important when nickel-molybdenum, Group C, alloys are welded by shielded
metal arc welding (SMAW). The slag can interact
with any mill scale and cause cracking at the toe
of the weld.
Surface oxides or a rough surface can have
Contaminating elements
There are a number of elements and compounds that must be removed from the surface
prior to welding or heat treating. If not removed,
the heat from welding can cause cracking, weld
defects, or reduced corrosion resistance of the
weld or HAZ. The elements penetrate at the grain
boundaries and the metal is said to be
“embrittled”. The elements to be avoided, the
type of defect generated, and the common
sources of the elements are shown in Table 4.
Weld defects, reduced corrosion resistance, or
embrittlement are caused by a combination of
temperature along with the presence of one of the
Table 3
Nickel alloy cutting methods
Method
Material thickness
Comments
Shearing
Sawing & Abrasive Cutting
Machining
Sheet/Strip, Thin Plate
Wide range of thicknesses
Wide range of shapes
Prepare edge exposed to environment to remove tear crevices
Remove lubricant or cutting fluid before welding or heat treating
Remove lubricant or cutting fluid before welding or heat treating
Plasma Arc Cutting (PAC)
Wide range of thicknesses
May be used for gouging
backside of weld
Wide range of thicknesses
Grind cut surfaces to clean metal
Powder metal cutting with
iron-rich powder
Carbon Arc Cutting
Cut less accurate than PAC, must remove all dross
Used for gouging backside of
weld and cutting irregular shapes
Grind cut surfaces to clean metal
5
For the welder
wiping with solvent-saturated cloths. Other acceptable methods include immersion in, swabbing with, or spraying with alkaline emulsion,
solvent, or detergent cleaners, or a combination
of these. Vapor degreasing, steam, with or without a cleaner; or high-pressure water jetting can
also be utilized. American Society of Testing and
Materials, ASTM A380, Standard Recommended
Practice for Cleaning and Descaling Stainless
Steel Parts, Equipment, and Systems, is an
excellent guide for fabricators and users.
A typical procedure to remove oil or grease
includes the following steps:
• Remove excess contaminant by wiping with
clean cloth;
• Swab the weld area (at least 5cm (2 in.) each
side of the weld) with an organic solvent such
as an aliphatic petroleum, a chlorinated
hydrocarbon, or blends of the two. (See
cautionary remarks which follow.) Use only
clean solvent (uncontaminated with acid,
alkali, oil, or other foreign material) and clean
cloths;
• Remove all solvent by wiping with clean, dry
cloth;
• Check to assure complete cleaning. A residue
on the drying cloth can indicate incomplete
cleaning. Where size allows, either the waterbreak or atomized test are effective checks.
If alkaline cleaners containing sodium carbonate
are used, the cleaners themselves must be
removed prior to welding by spraying or scrubbing with hot water. Selecting the solvent cleaner
involves considerations beyond just the ability to
remove oil and grease. Two precautions follow.
Figure 3 Nickel 200 showing sulphur embrittlement
of sheet on the right side caused by
inadequate cleaning. Photo courtesy Inco
Alloys International Inc.
listed elements. The depth of attack varies with the
embrittling element, its concentration, and the
heating time and temperature. Group A nickel
alloys are most susceptible. Figure 3 shows a
typical example of sulphur embrittlement on a
Nickel 200 sheet. The area of an alloy that becomes embrittled cannot be restored and must
be discarded. Carbon or carbonaceous materials left
on the surface during welding may be taken into
solution. The resulting high carbon layer lowers
the corrosion resistance in many environments.
A number of methods and materials exist for
removing the kinds of contaminants mentioned
earlier. Metallic contaminants and materials which
are not oil or grease-based can be removed by
mechanical means such as abrasive blasting or
light grinding. It is essential that the blasting
material or abrasive disk be free of contaminants
such as free iron. A nitric acid treatment, followed
by neutralization can effectively remove some low
melting point metals without damage to Group B,
chromium-bearing alloys, but this treatment may
attack other nickel alloys.
Oil or grease based (hydrocarbon-based) contaminants must be removed by solvent cleaning;
they are not removed by water or acid rinses.
Large weldments are usually hand-cleaned by
Table 4
Embrittling elements
Elements
Sulphur, Carbon
Effect/Defect
Reduced corrosion resistance
Sulphur, Phosphorous
Cracking in welds and HAZ
Chlorinated solvents
Many commercial solvents contain chlorides and
are effective in cleaning machined parts and crevice-free components. While chlorinated solvents are
acceptable for use on nickel alloy, they can present
a corrosion problem to stainless steel alloys. Fabricators often use a non-chlorinated solvent for both
stainless and nickel alloys to avoid the risk of using
a chlorinated solvent on stainless steel.
Common Sources of Elements
Hydrocarbons such as cutting fluids, grease, oil
waxes, and primers
Marking crayons, paints, and temperature-indicating
markers
Tools such as lead hammers, copper hold-down or
backing bars, zinc-rich paint, galvanized steel
Any of the above
Cracking in welds and HAZ
Lead, Zinc, Copper
(low melting point metals)
Any of the above
Shop dirt
6
For the welder
Health hazards
The term health hazard has been defined as
including carcinogens, toxic agents, irritants,
corrosives, sensitizers, and any agent that damages the lungs, skin, eyes, or mucous membranes. Each organization should assure that the
solvents used are not harmful to personnel or
equipment. In addition to the toxic effect, consideration must be given to the venting of explosive
fumes, safe disposal of spent solutions, and other
related handling practices. Knowledge of, and
compliance with federal, state, and local regulations is a necessity.
Solvents used for pre-weld cleaning include, but
are not limited to the following:
• Non-chlorinated: toluene, methyl-ethyl-ketone, and acetone;
• Chlorinated solvent: 1-1-1 trichloroethane.
All of the aforementioned solvents must be
handled in compliance with the regulator requirements and the manufacturer’s instructions.
Figure 4 Typical backing bar designs for use with
and without a backing gas
welding, the copper bar chills the weld to solid
metal without melting the copper. The arc should
not be misdirected to the extent that copper is
melted and incorporated into the alloy weld or
weld cracking can result. It is good practice to
pickle after welding to remove traces of copper
from the surface, particularly if solution annealing
is to follow welding.
Argon backing gas provides excellent protection
to the back side of gas tungsten arc welds
(GTAW). It helps control penetration and maintain
a bright, clean surface.
While nitrogen has been used as a backing gas
for stainless steels and chromium-bearing nickel,
Group B alloys, it should never be used for Group
A or C, non-chromium-containing alloys. Nitrogen
might also cause weld metal porosity in welds
made with the GTAW process which have inadequate filler metal.
When copper backing bar or an inert gas backing purge is impractical, there are commercially
available tapes, pastes, and ceramic backing
products that can be utilized. These offer some
protection from burn-through but little protection
from oxidation, so final cleaning by abrasive
means or acid pickling is needed after welding
when these backing materials are used.
Fixtures and positioners
Fixtures are usually designed for each particular
assembly and hold the parts together throughout
the welding operation. When fixtures are attached
to positioners, there is a further advantage in that
welding can be done in the most convenient
position. Some advantages of using fixtures are
summarized as follows:
- Better joint match-up;
- Less tacking and welding time;
- Minimized distortion;
- Accurate assembly.
It is essential that the mating pieces be carefully
aligned and matched for good quality welding.
When one member is considerably thicker than the
other, for example, a tank head that is thicker than
the shell, the head-side should be machined to a
taper of 3:1 or more to reduce stress concentrations. Joints with varying root gap require special
adjustment by the welder to avoid burn-through or
lack of penetration. When the volume of identical
parts is large, use of fixtures is more easily justified.
Backing materials
A backing material should be used in welding
sheet or plate, unless both sides of the joint can
be welded. Without a backing, the back side may
have erratic penetration with crevices, voids and
excessive oxidation. Such defects reduce weld
strength and can initiate accelerated corrosion.
Copper, with its high thermal conductivity, is the
material most-often used for backing bars. Typical backing bar designs for use with and without
a backing gas are shown in Figure 4. During
Tack welding
Joints not held in fixtures must be tack-welded to
maintain a uniform gap and alignment along the
entire length. The tacks should be placed in a
sequence to minimize the effect of shrinkage. In
fitting two sheets, tack welds should be placed at
each end and then the middle section as a shown
7
For the welder
in Figure 5 (A). Figure 5 (B) shows how the
sheets close up when the tack welding
progresses from one end.
Nickel alloys have a thermal expansion close to
that of ordinary steel so distortion from welding is
less than is experienced with stainless steel.
Tack welds in nickel alloy fabrications are about
the same number and size as those required for
carbon steel.
The length of tack welds may be as short as
3mm (0. 125 in.), or a small spot of weld metal for
thin material to over 25mm (1 in.) long for heavy
plate sections. More importantly, the shape of the
tack should not cause a defect in the final weld.
Heavy or high tacks or abrupt starts and stops
should be contour-ground. Bead shape is easier
to control with the GTAW process, making it a
good choice for tack welding. Before tack welds
are incorporated into the final weld, they must be
wire-brushed or ground to clean metal. They
should be inspected for crater cracks and any
cracks should be ground out.
Welding processes
This section provides information to assist in formulating nickel alloy welding procedures for the
shielded metal arc welding (SMAW), GTAW, and
gas shielded metal arc welding (GMAW) processes. The areas covered in earlier sections of
this publication such as base metal properties,
joint designs and preparation for welding are
common to all welding processes and are not
repeated.
Shielded metal arc welding
SMAW is a versatile process, widely used for
welding nickel alloys when the shapes or quantities do not justify automatic welding. The welding
is performed manually with the welder maintaining control over the arc length while directing the
arc into the weld joint.
SMAW is frequently referred to as covered
electrode or stick welding. The electrode is a
solid wire covered by an extruded flux coating,
although some manufacturers use a cored wire in
lieu of the solid core wire.
The electrode coating supports the following
functions:
• Initially at the arc start, the electrode core
burns back faster to form a cup which in turn
projects droplets and increases the pinch
affect. This action supports the capability to
weld out-of-position;
• It may provide alloy addition to the weld
deposit. Usually the core wire is of similar
composition to the deposited weld metal, but
some electrode manufacturers make very
large alloy additions through the coating and
rely on complete mixing or alloying in the weld
puddle. Because of this practice, it is not
advisable to remove the flux in order to use
the uncoated core wire for GTAW or any
other process;
• The gaseous envelope from the flux decomposition excludes oxygen and nitrogen from
the molten weld metal;
• The molten slag formed on top of the weld
protects the weld metal from contamination by
the atmosphere and helps to shape the bead.
Electrodes for SMAW are available for all solid
solution nickel alloys, Groups A, B, and C but not
for the precipitation hardening alloys, Group D.
The arc zone in the SMAW process is shown in
Figure 6.
Figure 5 Tack weld sequence to provide uniform
weld gap
Electrode types — Nickel alloy covered electrodes are classified according to chemical composition of undiluted weld metal. The matching
composition filler metals for nickel and nickel
Figure 6 The arc zone in the SMAW process
8
For the welder
Table 5
Matching composition filler metals for nickel alloys
Base metal
Common
UNS No.
Name
N02200
Nickel 200
N02201
Nickel 201
N04400
Alloy 400
1
Note
Alloy R-405
Alloy 825
N08825
Alloy G-3
Alloy G-30
Alloy 600
Alloy 690
Alloy 59
Alloy 625
Alloy C-22
and 622
Alloy C-276
Alloy 686
N06985
N06030
N06600
N06690
N06059
N06625
N06022
Alloy B-2
Alloy B-3
Alloy B-4
CW-2M
CW-6MC
CY-40
CX2MW
CZ-100
M-35-1
N-7M
N10665
N10675
N10629
N26455
N26625
N06040
N26022
N02100
N24135
N30007
N10276
None
Bare electrode and rod
ANSI/AWS A5.11-90
UNS No.
Coverage electrode and rod
ANSI/AWS 5.14-89
UNS No.
ERNi-1
N02061
ENi-1
W82141
ERNi Cu-7
N04060
ERNiCrMo-3 or
ERNiFeCr-1
ERNiCrMo-9
ERNiCrMo-11
ERNiCr-3
TM
Inconel F.M.52
TM
Nicrofer S5923
ERNiCrMo-3
ERNiCrMo-10
N06625
ENiCu-7
ENiCu-7
ENi CrMo-3 or
TM
Incoloy W.E.135
ENiCrMo-9
ENiCrMo-11
ENiCrFe-3
TM
Inconel W.E.152
VDM 2.4609
ENiCrMo-3
ENiCrMo-10
W84190
W84190
W86112
ERNiCrMo-4
TM
Inco-Weld F.M.
686 CPT
ERNiMo-7
ERNiMo-7
ERNiMo-7
ERNiCrMo-4
ERNiCrMo-3
ERNiCr-3
ERNiCrMo-10
ERNi-1
ERNiCu-7
ERNiMo-7
N10276
None
ENiCrMo-4
TM
Inco-Weld W.E.
686 CPT
ENiMo-7
ENiMo-7
ENiMo-7
ENiCrMo-4
ENiCrMo-3
ENiCrFe-3
ENiCrMo-10
ENO
ENiCu-7
ENiMo-7
W80276
None
N06985
N06030
N06082
N06059
N06625
N06022
N01665
N10665
N10665
N10276
N06625
N06082
N06022
N02061
N04060
N10665
W86985
W86030
W86182
None
W86112
W86022
W80665
W80665
W80665
W80276
W86112
W86182
W86022
W82141
W84190
W80665
1
Note : SMAW is the preferred welding process.
2
Note : Group D – Precipitation hardening alloys. Contact the base metal producers for filler recommendations
3
Note : Inconel, Incoloy, and Inco-weld are registered trademarks of the Inco family of companies. Nicrofer is a registered trademark of VDM
Nickel Technologies AG.
recommendations, since the temperature often
varies with the particular coating, but lacking
this information, commonly used temperatures
are as follows:
- Storage of opened electrodes: 110°C (225°F);
- Recondition bake: 260-315°C (500-600°F).
The nickel-molybdenum Group C coating formulation is a low hydrogen type and moisture pickup must be closely controlled. If the electrodes
are exposed to moisture pick-up, they can be
reconditioned by heating to 315-370°C (600700°F) for two to three hours.
Moisture in the coating can cause hydrogen gas
generation in the weld, leading to weld porosity.
The porosity may be within the weld metal or
may reach the surface just as the metal solidifies,
forming visible surface pores. The porosity can
occur in butt welds when the moisture content of
the coating is high, but more often, it occurs in
fillet welds.
Moisture in the weld is not the only cause of
weld metal porosity. Welding on painted, greasy,
or oily surfaces may lead to porosity of the wormhole type.
alloys are shown in Table 5. Most nickel alloy
electrodes are designed to operate on direct
current, electrode positive, although some can
operate on alternating current.
The type of coating is not identified in nickel
alloy electrodes as it is for carbon and stainless
steel electrodes. As with covered electrodes of
other alloys, the flux formula is the proprietary
secret of its manufacturer. Nickel alloy electrode
coatings are best described as lime-titania type
coatings; they cannot be classified as either lime
or titania types because both compounds are
used.
Electrode storage — Nickel alloy electrodes are
normally furnished in packages suitable for long
storage. After the package is opened, the electrodes should be stored in heated cabinets at the
temperature recommended by the manufacturer.
If the electrodes have been over-exposed to
moisture, they should be reconditioned by a
higher temperature bake using the manufacturer’s suggested time and temperature. It is preferable to obtain the manufacturer’s specific
9
For the welder
soon learn the amount of weave needed to obtain
the correct bead contour for various joint conditions. The acceptable width of weave is limited to
a dimension no wider than three times the electrode core diameter.
Welding current — The recommended current
ranges for each package of electrodes is usually
printed on the package. The current ranges may
vary significantly from one alloy family to another.
The electrical resistance of Group B core wires is
much higher than Group A core wires so the
recommended currents for Group B are substantially lower. Excessive current overheats the
electrode coating which, in turn, causes a loss of
arc force and difficulty in directing the arc near
the end of the electrode.
Electrode handling — Arc Starting and Stopping — The same techniques for arc starting and
stopping used for low hydrogen carbon steel
electrodes, such as type E7018, are applicable to
nickel alloy welding.
Some guidelines follow:
• Strike the arc at some point in the joint so
that the metal is remelted. An arc strike away
from the weld may have cracks and unless
removed, may result in lower corrosion resistance in that area;
• Do not abruptly extinguish the arc, leaving a
large weld crater. A depression will form as
the metal solidifies, often with a slag-filled
pipe or cracks in the center of the crater
depression. One acceptable technique is to
hold the arc over the weld pool for a few
moments and then move quickly back, lifting
the arc from the completed weld. Another
technique is to extinguish the arc against one
of the joint side walls after filling the crater.
Weld spatter — Under correct welding conditions, there should not be excessive spatter.
When high spatter does occur, it may be caused
by one of the following factors: excessive arc
length, excessive amperage, incorrect polarity, or
excessive moisture in the electrode coating. All of
these factors are under the control of welding
personnel and can be readily corrected. Magnetic
arc blow can also cause excessive spatter. The
corrective measures are the same as those used
for other metals such as ordinary steel.
Gas tungsten arc welding
The gas tungsten arc weld (GTAW) process or
tungsten inert gas process (TIG), as it is frequently called, is widely used and is well-suited
for welding nickel alloys. It frequently is the only
process used for welding precipitation hardening,
Group D alloys. An inert gas (usually argon) is
used to protect the molten weld metal and the
tungsten electrode from the air. Filler metal in the
form of bare wire is added as needed, either by
manual or automatic feeding into the arc. The
process is illustrated in Figure 7. GTAW can be
used to weld material as thin as a few mils or as
thick as the heavy gauges. Usually, faster weld-
Weld puddle control — It is necessary to maintain a short arc length for control of the weld
puddle. In downhand welding, the electrode is
positioned ahead of the puddle and at an incline
of 20 or more degrees from the vertical (a drag
angle). This is also described as backhand
welding. The angle improves control of the molter
flux and eliminates slag entrapment.
Out-of-position welding — Out of position welding should be done with a 3.2mm (0.125 in.) or
smaller diameter electrode using a shorter arc
and lower current than for downhand welding. In
vertical welding, a range of electrode angles is
often used varying from 20 degrees drag (backhand) to 20 degrees push (forehand) depending
on welder preference.
Nickel alloy weld metal does not flow or spread
like most other metals and requires placing it to
the desired spot in the joint. For proper bead
placement, some weave or manipulation is
needed. The amount of weave depends on such
factors as joint design, welding position, and the
type of electrode. With a little practice, welders
Figure 7 The Gas Tungsten Arc Weld (GTAW)
process
10
For the welder
excellent emissive qualities, although other
tungsten electrode types are acceptable. Opinions differ regarding electrode size for various
amperages. Some favour using different diameters for a number of specific current ranges
while others use a size such as 2.4mm (0.09 in.)
for a much wider current range. Also, the electrode end preparation preferences vary. One
commonly used configuration is a 20° to 25°
taper with the tip blunted to a 2.5mm (0.10 in.)
diameter.
ing processes are used for material thicknesses
over 3.2mm (0.125 in.).
Some of the advantages of the GTAW process
for welding nickel alloys follow.
- No slag to remove — minimizes post-weld
cleanup;
- All position welding capability — particularly
useful for pipe welding;
- No weld spatter;
- No alloy loss during welding;
- Good as-welded surface — minimal finishing
required.
Nozzles — Nozzle or gas cups come in a wide
variety of shapes and sizes and it is often best to
match the nozzle to the weld joint or application.
Larger cup diameters provide better shielding
gas protection to the weld while smaller nozzles
help maintain a more stable arc and allow better
visibility. An alternate is the gas lens which creates a laminar flow through use of special
screens inside the nozzle. The flow of inert gas is
projected a considerable distance beyond the
end of the nozzle, giving both better gas protection and good visibility.
GTAW equipment — Power — Direct current,
electrode negative, (DCEN), (straight polarity)
current is standard. Pulsed-current power is
another option. In this type there is a pulsating
high rate of current rise and decay. This current
mode is well-suited to welding thin materials and
joints which have poor fit-up. Pulsed-current is
also useful in making the root pass of pipe joints.
A high-frequency or capacitor discharge starting
feature is often included in the power source.
This starting feature is also available in a separate “arc starter” control for use with conventional
constant current power sources. This allows an
arc to be initiated without a scratch start, a procedure that risks contamination of the tungsten
electrode.
Newer power sources provide a “lift start”
feature that allows the electrode to be positioned
on the work before applying power. The arc is
established when the torch is lifted from the work.
The power source controls the current during the
lift start and practically eliminates the risk of electrode contamination. The advantage of this
method over high frequency starting is that it
eliminates possible interference to nearby electronic equipment such as telephones, radios, and
computers.
System leaks — With any welding process using
inert gas, it is important that all gas lines and
connections be checked to ensure freedom from
leaks in the system. If a leak is present, for example, in a gas line, air will aspirate into the inert
gas stream rather than the internal gas escaping,
as might be expected.
Cooling — Torches are either air or water
cooled. The air-cooled variety is limited to lower
currents than the water cooled units.
Shielding gases — Pure argon, helium, or
mixtures of the two are used for shielding gas
in welding nickel alloys. The oxygen-bearing
argon mixtures used in GMAW welding should
not be used in GTAW welding because of rapid
deterioration of the tungsten electrode. Nitrogen additions are not recommended for the
same reason and also because they introduce
the possibility of weld metal porosity in the nonchromium-bearing nickel alloys. In manual
welding, argon is the preferred shielding gas. It
provides good penetration at lower flow rates
than helium and less chance of melt-through.
Helium produces a higher heat input and
deeper penetrating arc which may be an advantage in some automatic welding applications. Argon-helium mixtures may improve the
bead contour and wettability. Hydrogen additions (up to 5%) maybe added to argon for a
hotter arc and more uniform bead surface in
single pass automated welds.
Electrodes — The 2% thoriated tungsten electrodes are in most popular use because of their
Filler metals — The correct filler metals for
GTAW welding of nickel alloys are shown in
Current controls — In addition to the current
controls at the power source, it is often useful to
have a foot pedal or torch-mounted hand current
control. This control allows the welder to increase
or decrease current during welding to adjust to
conditions such as poor fit-up. A further advantage to this type of control is that the welder can
slowly reduce the current (and the weld pool) at
arc stops to eliminate crater cracks.
11
For the welder
Table 5. Straight lengths are normally used for
manual welding and spool or coil wire is used for
automatic welding. Conventional quality control
practices to assure clean wire and avoidance of
material mix-up are essential. Bare wire for
GTAW should be wiped clean before using and
stored in a covered area.
tion are possible only when ample filler metal
additions are made. It is difficult to define just
how much is ample and to measure it. Experience suggests that at least 50% of the weld
metal should be from filler metal addition. With
adequate amounts of filler metal in the joint, it
then becomes important that filler metal mixing
takes place before the weld solidifies, otherwise
segregated spots of melted base metal and
melted filler metal may exist. Uneven melting of
filler metal along with fast solidification rates can
cause this type of segregation.
Operator guidelines
Arc initiation — Arc initiation is made easier by
devices such as high frequency, capacitor, or lift
start features (described earlier), or pilot arcs. In
the absence of these devices, a scratch start is
used which risks contaminating the electrode and
the metal being welded. Where practical, starting
tabs adjacent to the weld joint are useful in eliminating damage to the base metal.
Gas metal arc welding
In the GMAW process, an arc is established
between a consumable, bare wire electrode and
the work piece. The arc and the deposited weld
metal are protected from the atmosphere by a
gas shield, comprised mainly of the inert gases,
argon and/or helium. Small amounts of carbon
dioxide may be used for better wetting, arc action, and bead control. The process is referred
to as MIG when an inert shielded gas is used
and MAG when an active gas is used.
The advantages of GMAW over GTAW and
SMAW are summarized as follows:
- Faster welding speeds;
- No slag, minimizing post-weld cleanup;
- Ease of automation;
- Good transfer of elements across the arc.
Arc stopping — Take care when extinguishing
the arc to decrease the size of the weld pool,
otherwise crater cracking is likely as the weld
solidifies. In the absence of a foot pedal or hand
current control described earlier, or a power
source current decay system, decrease the arc
pool by increasing the travel speed before lifting
the electrode from the joint. Good arc-stopping
practice is particularly important in the root pass
of welds that are welded from only one side. If
cracks occur in this situation, they may extend
completely through the root, presenting a difficult
repair. After the arc is broken, hold the torch over
the crater for several seconds to allow the weld
to cool under protection of the argon atmosphere.
Arc shielding — Nickel alloys are easy to weld
with the GTAW process. The alloys are relatively
insensitive to marginal shielding compared to
reactive metals such as titanium or zirconium. It is
good practice, however, to provide ample shielding protection to both the weld puddle and backside. It is also a good idea to keep the filler metal
within the inert gas envelope during welding.
If the process has a potential short-coming, it is
that the weld may look good but may have inadequate filler metal. In some weld joints, inadequate filler can result in a concave bead that
has a tendency for centerline cracking. Adequate
filler metal addition produces a slightly convex
weld bead. Another result of inadequate filler
metal may be porosity in the weld, particularly in
the non-chromium bearing nickel alloys.
Nickel alloy filler metals — Nickel alloy filler
metals often contain elements that are not in the
base metal to control porosity or improve resistance to cracking. Welds of the desired composi-
Figure 8 The basic components of the Gas Metal Arc
Weld (GMAW) process
12
For the welder
slope wire feed, pulse rate, and transfer mode;
consequently, the process is more complex and
expensive. The new synergic pulsed arc power
source has made operation simpler by providing
only one or two control dials for the operator. The
remaining parameters are adjusted automatically
or are programmed into the power source. The
synergic mode power source has many advantages over the original fixed frequency pulse
mode and is largely replacing fixed frequency
units.
The welding current used in most power
sources is the direct current electrode positive
(DCEP) – reversed polarity. This current gives
deeper penetration and a more stable arc than
direct current electrode negative (DCEN) –
straight polarity. The DCEN approach finds its
best use in applications requiring shallow
penetration such as overlay welding.
Consumables — Some of the most popular
shielding gases used in GMAW are shown in
Table 6. Argon is the shielding gas that is usually
used for spray arc GMAW. Short circuiting and
pulsed arc GMAW use a variety of shielding
gases. A mixture of 90% helium, 7.5% argon, and
2.5% CO2 is a popular mixture in North America.
In Europe, helium is quite expensive so a mixture
of 90% argon, 7.5% helium, and 2.5% CO2 is
often the mixture of choice. Whatever the combination, the shielding gas should contain at least
97.5% inert gases (argon, helium, or a mixture of
the two).
In MAG welding, the use of active gas such as
carbon dioxide often presents extra work in
multiple-pass welds. Many nickel alloy filler
metals contain elements such as aluminum and
titanium that form very refractory and tightly-
The basic components of the GMAW process
are shown in Figure 8.
GMAW arc types — The type of metal transfer
in GMAW has a profound influence on the process characteristics to the extent that it is often
misleading to make general statements about
GMAW without indicating the arc transfer mode.
The three modes in most frequent use in welding
nickel alloys are spray arc, short circuiting arc,
and the pulsed arc.
The spray arc process is characterized by high
deposit rates and high heat input. The arc is quite
stable, but welding is generally limited to the flat
position.
The short circuiting arc provides a low heat input
transfer mode and therefore minimizes distortion
in welding thin gauge material. It is most useful
for single-pass welding, but has limitations when
used on multiple pass, thick joints. The process
is somewhat prone to lack of fusion defects. In
addition, the weld beads tend to be rather convex; this may necessitate grinding each bead to
assure full penetration to the side wall.
The pulsed arc mode of gas metal arc welding is
an excellent compromise between spray arc and
short circuiting arc modes in the general fabrication of nickel alloys as shown in Table 6.
GMAW equipment — The same power
sources, wire feed mechanisms, and torches
used for welding ordinary steels are used for
nickel alloys. Plastic liners in the wire feed conduit are helpful in reducing drag. The GMAW
process has more weld parameter controls than
the GTAW and SMAW processes. The GMAW
process controls amperage, voltage, current
Table 6
Comparison of GMAW arc modes for nickel alloys
Material/weld variables
Spray arc mode
Short circuiting mode
Pulsed arc mode
Typical thickness welded
3mm (0.125 in.) min., 6mm (0.25 in.)
and thicker is normal
flat
highest
1.6mm (0.062 in.)
1.2mm (0.045 in.)
250-350 amps
Argon
1.6mm (0.062 in.) and up
1.6mm (0.062 in.) and up
all
lowest
0.8 or 0.9mm
(0.030 or 0.035 in.)
70-130 amps
90% Helium
7.5% Argon
2.5% CO2
or
69% Argon
30% Helium
1% CO2
or
75% Argon
25% Helium
all
intermediate
0.9 or 1.2mm
(0.035 or 0.045 in.)
60 to 150 amps
75% Argon
25% Helium
or
75% Helium
25% Argon
or
90% Argon
7.5% Helium
2.5% CO2
Welding positions
Relative deposition rate
Typical wire diameter
Typical welding current
(1)
Shielding gas
(1) Other gas mixtures are used, however, the shielding gas normally contains at least 97.5% inert gas, i.e., argon, helium, or a mixture of the two.
13
For the welder
adhering oxides that must be removed prior to
the next weld pass. Frequent interpass or
interlayer grinding is required to remove the
oxides from the surface. For this reason shielding
gases with carbon dioxide additions are often
limited to single-pass welds of Group B, chromium-bearing alloys where the better wetting and
more stable arc is important in short circuiting
and pulsed arc welding.
The preferred filler metals in GMAW nickel
alloys are shown in Table 5. The most widely
used diameters are 0.9mm,1.2mm, and 1.6mm
(0.035 in., 0.045 in., and 0.062 in.) but other
diameters are available.
procedure and technique is such an extensive
topic, it is only highlighted in this publication.
Types of pipe welding
The pipe size, equipment available, and welder
skill or experience determine to a large extent the
type of pipe weld and joint design that is best for
a particular application. The particular nickel alloy
is usually of secondary significance. In fact, there
is a great deal of commonality in welding a range
of alloys from carbon steel to stainless steel to
the nickel alloys. A discussion on some of the
most common piping joints follows.
Instrument piping
Other welding processes
Instrument piping, usually about 13mm (0.5 in.)
and less in diameter, is often joined by socket
welds or it may be mechanically joined. Elements
of a good socket welding procedure include
GTAW root and cap pass and a gap of 1.6mm
(0.062 in.) between the end of the pipe and the
socket face. Use of an internal purge prevents
oxide formation on the pipe ID which may be of
significance in some piping systems. SMAW for
the root pass is difficult because of the small pipe
diameter, but more importantly, this process
presents the possibility of slag entrapment with
its attendant risk of contributing to corrosion.
The most frequently used solid solution alloys in
Groups A and B can be welded by most of the
other commercial welding processes. The use of
another process may offer advantages over
those available from the SMAW, GTAW, and
GMAW processes and should be evaluated for
high production or special fabrications.
Submerged arc welding, SAW, has been used
for welding thicknesses starting around 6.4mm
(0.25 in.) and thicker and for overlay welding.
Commercial fluxes are available for use with
standard filler metals for welding Nickel 200
(N02200), and alloys 400 (N04400), 600
(N06600), and 625 (N06625). Contact the manufacturers of fluxes and filler metals for information
on SAW or other solid solution alloys.
Plasma arc, electron beam, and laser welding
are used with increasing frequency and the
resistance welding processes; spot, seam, projection, and flush welding are readily adaptable to
most nickel alloys. The development of nickel
alloy flux-cored arc products has been slow and
few alloy fillers are available
Oxyfuel welding, OFW, is seldom used today.
Many nickel alloys can readily pick up carbon from
the flame which reduces their corrosion resistance.
Brazing can be used to join nickel alloys to themselves or to a number of other alloys. Brazing is
not usually used for severe corrosion environments
such as the applications discussed herein.
Automatic welding
There are a number of commercially available
GTAW orbital pipe welding units that can be used
for welding nickel alloys. Orbital welds are preferred on all 75 mm (3 in.) and smaller diameter
piping as compared to threaded joints and socket
welds which have built-in failure-prone crevices.
In welding other metals such as thin-wall stainless steel, orbital welds are often made using a
tight butt with no filler metal addition. In welding
nickel alloys, however, it is most desirable to add
filler metal for Group A alloys and is often preferable for Groups B and C alloys.
In welding wall thicknesses over about 1.6mm
(0.062 in.), a bevel is used and a consumable
insert or automatically fed filler is used for the
root pass. After the root pass, the joint is
completed with the automatic GTAW or a higher
deposition process depending upon the wall
thickness and piping configuration.
Welding nickel alloy pipe
Piping systems are a very vital part of many industrial process plants. The fabrication and welding
techniques for pipe are somewhat different from
those used for tanks, pressure vessels, and similar
equipment. One major difference is that in piping
systems, the internal root is seldom accessible for
backside welding so the root pass must be made
correctly from the outside. Since pipe welding
Manual welding
A large quantity of nickel alloy piping is manually
welded. Manual welding may be the choice when
automatic welding equipment is not available, the
project does not merit the expenditure, or the
14
For the welder
popular use are shown in Figures 9 and 10.
The standard consumable insert shapes are
shown in Figure 11 and are available in a number
of nickel filler alloys to ANSI/AWS A5.30 — 79.
Class 1; 3, and 5 are often used for nickel alloys.
Classes 3 and 5 are often easier to fuse than the
larger volume of Class 1. The consumable insert is
placed into the joint and tacks are made between
the insert and the pipe. The interior of the pipe must
be purged to prevent oxidizing the tacks. The rise of
the molten pool indicates that the insert is completely fused. With experience, the welder observes
this change and adjusts travel speed accordingly.
When the pipe can be rotated, the root pass is
completed without stopping. When the pipe is in a
fixed position, welding is usually done in sectors,
alternating from side to side.
In tacking joints without consumable inserts, or
open root welds, as they are called, there is a
strong tendency for the shrinking forces to pull
the joint closed. To maintain the desired gap, it
maybe necessary to use spacers and to increase
the size and number of tack welds. Spacers are
usually short lengths of suitable diameter, clean
filler wire. Any cracked or defective tack welds
should be ground out. Both ends of the tacks on
open root welds should be tapered to aid in
fusing into the root weld.
The need to maintain a proper gap during root
pass welding is two-fold. First, a consistent and
uniform gap aids the welder in producing the
optimum ID root contour. The other reason for a
uniform root gap is the need to maintain the
optimum root pass chemical composition.
pipe configuration and accessibility are bettersuited to manual welding.
In manual welding nickel alloys, as with steel
piping over about 13mm (0.5 in.), the three likely
procedures are:
- The use of backing rings;
- Consumable inserts;
- Open root joints with hand-fed filler metal.
Backing rings are a very poor choice for nickel
alloy process piping. If the SMAW process is used,
slag may be trapped between the pipe ID and the
backing ring creating a potential corrosion site. In
addition, backing rings can reduce flow in the pipe
and they become a site for crevice corrosion.
The remaining two procedures, the use of
consumable inserts and the open root joint with
hand-fed filler metal, are equally good selections
for the root pass manual welding of nickel alloys
with the GTAW process. Both procedures produce high quality root welds in the hands of
capable welders. The two types of joint designs in
Figure 9 Typical joint design for pipe with
consumable insert
Figure 10 Typical joint design for pipe welded with
open root joint and hand-fed filler metal
Figure 11 Standard consumable insert shapes,
ANSI/AWS D10.11
15
For the welder
Purging during pipe root welding
versed, that is, about 19L/min. (40 ft.3/hr.) purge
and 5L/min. (10 ft.3/hr.) torch.
In either practice an internal pressure build-up
must be avoided or a concave root will result. In
extreme cases, a hole may occur completely
through the root. The purge gas exit hole should
be sufficiently large that it does not contribute to
pressure build-up in the pipe. The tape on the
outside of the joint is peeled back in advance of
the weld arc. Near the end of the root pass, the
purge flow rate should be reduced to a very low
level to prevent a blow-back.
After the root pass, the internal purge should be
maintained during the next two fill passes in order
to minimize heat tint (oxidation) on the inside
weld surface. This is especially important when it
is impractical to pickle after welding.
For those needing more information on GTAW
root pass pipe welding, there are a number of
technical articles and specifications available. Two
excellent sources are the American Welding Society publications listed in the general references to
this publication. While they are written mainly for
steel, most information is applicable to nickel alloys.
The pipe interior must be purged with an inert
gas prior to and during the GTAW root pass.
Failure to use a purge can result in heavily oxidized ID root surface with substantially lower
corrosion resistance. Purging is usually done
with pure argon, but helium may also be used.
Purging is a two-step operation, the first being
done prior to welding to displace air inside the
pipe. To save time and purging gas, baffles on
either side of the weld joint are often used to
reduce the purge area.
Open root weld joints should be taped and dead
air spaces vented prior to purging. The internal
purge atmosphere should be essentially free of
oxygen and moisture in order to obtain a root
surface with little or no surface oxide. In practice,
it is difficult to specify a single oxygen limit that
can be consistently obtained with all piping configurations, joint fitup conditions and other variations. The maximum amount of oxygen should
be in the order of 0.5% but every effort should
be made to obtain a lower level. At about 0.5%
oxygen, the root is oxidized, but not to the degree
that a “sugary” weld bead is obtained. Typical
purging fixtures are shown in Figure 12.
Post-fabrication cleaning
All too often, it is assumed that the fabrication,
be it a tank, pressure vessel, or pipe assembly, is
ready for service after the final weld is made and
inspected. Post-fabrication cleaning may be as
important as any of the fabrication steps discussed. The surface condition of the nickel alloy
is equally important where the product must not
be contaminated, as in a pharmaceutical, food, or
nuclear plant; as well as where the alloy must
resist an aggressive environment, as in a chemical or other process industry plant.
Some guidelines on post-fabrication clean-up
follow.
Surface contaminants — Examples of typical
contaminants include grease, oil, crayon
markers, paint, adhesive tape and other sticky
deposits. Such contaminants can adversely
affect product purity and in some environments
may foster crevice corrosion. Typically, these
contaminants can be removed by spraying or
scrubbing with a detergent or solvent.
Figure 12 Typical pipe purging fixtures
After the proper purity level has been reached,
the purge flow rate is adjusted. When welding
carbon steel and stainless steel, the common
practice is to use a purge flow rate of about
5L/min. (10 ft3/hr.) and a torch flow in the order of
16L/min. (35 ft3/hr.). In welding nickel alloys with
an open butt, Haynes International has found that
there is less root oxidation with the rates re-
Embedded iron — During fabrication operations, iron particles can become embedded, then
they corrode in moist air or when wetted, leaving
tell-tale rust streaks. In addition to creating an
unsightly condition, the iron particles might initiate
local attack or, when used in process equipment,
they might affect product purity.
16
For the welder
defects is often specified by the fabrication specification and may vary from 10% to 25% of the
total thickness. When weld repair is needed, it
can be made by SMAW, GMAW, or GTAW
processes.
GTAW is usually used because of greater ease in
making small repair welds. Filler metal should
always be added and wash passes or cosmetic
welds should never be allowed because of the
risk of weld cracking and reduced corrosion
resistance.
Some tests to detect embedded iron follow:
• Spray the surface with clean water and inspect for rust streaks after 24 hours;
• Immerse the surface in a 1% sodium
chloride solution for 12 to 24 hours or use as
a spray in enclosed spaces such as tanks;
• For small areas, use a ferroxyl test. Apply as
a warm solution and allow at least two hours
before checking. The solution jells as it cools.
The formation of blue spots indicates the
presence of free iron.
The composition of one ferroxyl test solution is
as follows:
Agar-agar
10g
Potassium ferricyanide
1g
Sodium chloride
1g
Water
1000 ml
Free iron can be removed by an acid pickle
treatment. First, the surface must be cleaned of
any oil or grease, otherwise the acid is ineffective. An effective pickling solution for Group A
and C alloys follows:
Hydrochloric acid
30ml
Ferric chloride
11 g
Water
1000ml
For Group B chromium-bearing alloys, a nitrichydrofluoric acid solution of 10 to 20% nitric acid
and 2% hydrofluoric acid is quite effective.
When pickling is not practical, abrasive
blasting, fine grit flapper wheels, or disks can be
used. Glass-bead blasting or walnut shells
produce good results. With any blasting, care
must be taken to assure that the abrasive is free
from iron or other foreign material that could
contaminate the surface.
Safety and welding fumes
Safety rules for welding nickel alloys are essentially the same as for all metals as they pertain to
areas such as electrical equipment, gas equipment, eye and face protection, fire protection,
labelling hazardous materials, and similar items.
The American Welding Society publishes a good
reference guide on welding safety, entitled Safety
in Welding and Cutting (American National
Standard Institute/Accredited Standards Committee, ANSI/ASD, Z49 1-88).
Proper ventilation to minimize the welder’s
exposure to fumes is important in welding and
cutting all metals, including nickel alloys. In
addition to good ventilation, the welders and
cutters should avoid breathing the fume plume
directly, by positioning the work so that their
heads are away from the plume. The composition
of welding fumes varies with the welding filler
metal and welding process.
Arc processes also produce gaseous products
such as ozone and oxides of nitrogen. Concern
has been expressed in welding with nickel alloy
consumables because of the chromium and, to a
lesser extent, the nickel, usually present in the
welding fume. Good ventilation minimizes the
potential health risk. The International Institute of
Welding has developed a series entitled Fume
Information Sheets for Welders which offer
internationally accepted guidelines for fume
control.
Mechanical damage
When reconditioning is needed to repair surface
damage, the repair is usually made by grinding or
by welding and grinding. Shallow defects are first
removed by grinding, with a clean, fine-grit abrasive disk, a flapper wheel , or a pencil-type
grinder. The maximum grinding depth to remove
17
18
For the materials engineer
Part II
For the materials engineer
dure Specifications or Quality Control documents.
Topics covered include the effect of welding on
corrosion resistance, post-fabrication heat treatment, and guides for material procurement.
Table 7 shows the nominal composition of
wrought nickel and nickel alloys. Table 8 shows
the nominal composition of cast nickel alloys.
This section is for the engineer who needs further
information about the metallurgy and fabrication
practices that are appropriate for wrought and cast
nickel alloys. Refer to Part I for suggestions concerning good storage practices. Additional information in this discussion, which was not included in
Part I, may be useful for formulating Welding ProceTable 7
Nominal composition of groups A through D and nickel alloys
(1)
Alloy
UNS No. Ni
C
Cr
Mo
Fe
Co
Cu
Group A Nickel and nickel-copper solid solution alloys – Composition percent
200
N02200
99.5 0.08
0.2
0.1
201
N02201
99.5 0.01
0.2
0.1
400
N04400
66.5 0.2
1.2
31.5
R-405
N04405
66.5 0.2
1.2
31.5
Group B Chromium-bearing solid solution alloys – Composition percent
825
N08825
42
0.03
21.5
3
30
2.25
G-3
N06985
46
0.01
22.2
7
19.5
2.5
2
G-30
N06030
42
0.01
29.7
5
15
2.5
1.8
600
N06600
76
0.08
15.5
8
0.2
690
N06690
61.5 0.02
29
9
59
N06059
60
0.01
23
15.7
0.7
625
N06625
61
0.05
21.5
9
2.5
686
N06686
56
0.01
21
16
2.5
622
N06022
56
0.01
21.2
13.5
4
1
C-22
N06022
56
0.01
21.2
13.5
4
1
C-276
N10276
58
0.01
15.5
16
5.5
1
Group C Nickel-molybdenum alloys – Composition percent
B-2
N10665
70.5 0.01
28
B-3
N10675
63
0.005
2
30
2
B-4
N10629
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Group D Precipitation-hardening alloys – Composition percent
K-500
N05500
64
0.2
1.0
30
725
N07725
57
0.1
20.7
8
9
60
0.1
21.5
8.2
5
625Plus® N07716
718
N07718
52.5 0.04
19
3
18
Al
Ti
Cb
0.1
0.9
0.2
0.2
0.2
0.1
(2)
3.6
Mn
Si
W
0.2
0.2
1
1
0.2
0.2
0.2
0.02
0.5
0.5
0.7
0.5
0.2
0.25
0.50
0.40
0.2
0.2
0.2
0.5
0.2
0.2
0.5
0.2
0.04
N/A
N/A
N/A
0.5
1.5
N/A
2.7
0.25
0.2
0.5
0.6
1.3
1.3
0.9
3.3
3.3
5.2
0.8
0.2
0.1
0.2
N/A
B
Other
S.04
3.7
3
3
3.7
1.5
N/A
N/A
N/A
0.2
0.2
(1) Includes small amount of cobalt if cobalt content is not specified.
(2) Includes tantalum also.
625 Plus® is a registered trademark of Carpenter Technology Corp.
Table 8
Nominal composition of cast corrosion resistant nickel alloys ASTM A494
Ni, NiCu, NiMo Groups
Composition percent
Alloy
UNS No.
Ni
Cr
Mo
Other
Designation
0.75 Cu, 1 Si
97
N02100
CZ-100
29.5 Cu, 2 Fe,0.3 Cb
65
N24135
M-35-1
1.5 Fe
31.5
0.5
66
N30007
N-7M
NiCrFe & NiCrMo Groups
Composition percent
CW-6MC
N26625
60
21.5
9.0
Fe, 3.8 Cb
CY-40
N06040
75
15
7 Fe
CW-2M(1)
N26455
65
16
16
1 Fe, 0.5 W
CX2MW
N26022
56
21
13.5
4 Fe, 3 W
Wrought
Counterpart
Nickel 200
Alloy 400
Alloy B2
Alloy 625
Alloy 600
Alloy C (1)
Alloy C-22
(1) CW- 6M and CW-12MW, two earlier cast counterparts of alloy C have been superseded by CW-2M and are therefore omitted from this
compilation. CW-2M is a cast counterpart of the older wrought alloy C. There is no true cast counterpart of alloy C-276.
19
For the materials engineer
composition welds corrode preferentially to the
base metal, non-matching composition filler metals
should be used that are both compatible
metallurgically with the base metal and are cathodic
to the base metal in the particular environment.
Selection should be made by knowledgeable
material specialist or by on-site evaluation tests.
It is important to remember that most welding
codes specify that the non-matching filler metal
welds be treated as a dissimilar metal welds and
indicate the need for a separate welding procedure specification and welding procedure test.
General guidelines for nickel
alloys
Guidelines that apply to all nickel alloys are
discussed first. Since the general guidelines are
not appropriate to all groups, specific headings
indicate the appropriate considerations for
Groups A, B, C, and D alloys.
Preheat and interpass temperature
Preheat of nickel alloys is not required except to
bang the metal in the area to be welded to room
temperature or to a typical shop temperature to
prevent moisture condensation. A maximum
interpass temperature of 175°C (350°F) is widely
used although one base metal producer is more
conservative and recommends a maximum of
950C (200°F).
Group A — Nickel and nickel-copper
alloys
Welders soon discover that the welding characteristics of nickel and, to a lesser degree, nickelcopper alloys are somewhat different from the
chromium-bearing nickel alloys or the austenitic
stainless steels. Primary among the differences
is the low viscosity or inability of the molten weld
metal to spread or flow in the joint; however,
competent welders soon become accustomed to
this and are able to produce quality welds. The
materials engineer who is aware of this viscosity
difference in advance is better prepared to cope
with the false “materials problem” reports from
uninitiated shop personnel.
Post-weld heat treatment
In most all instances, solid solution nickel alloys
do not require a post-weld heat treatment for
corrosion resisting service. Precipitation hardening alloys require heat treatment after welding to
develop full strength. When heat treatment or
stress relief is required for specific applications;
for example, to anneal following cold forming, or
for dimensional stability, the user should consult
the nickel alloy producer’s literature or its staff for
specific recommendations.
Prior to any heat treatment, it is essential that all
alloy surfaces be thoroughly cleaned of oil, grease,
paint, or markings, and similar contaminants to
avoid catastrophic corrosion during heat treatment.
The method of heating and cooling and the amount
of sulphur in the furnace atmosphere must be
controlled or the alloys can be damaged.
Alloys 200 and 201 — Nickel 200 and 201
differ in the amount of carbon, 0.15% maximum
in Nickel 200 and 0.02% maximum in 201. Prolonged exposure of Nickel 200 in the temperature
range of 425-650°C (800-1200°F) precipitates
graphite. For this reason Nickel 201 is recommended for service in the 315-650°C (6001200°F) temperature range. Nickel filler metal
welds (ENi-1 and ERNi-1) are not subject to
graphite precipitation and are used for welding
both Nickel 200 and 201.
Equipment intended for caustic service provides
an exception to the general rule that post-fabrication heat treatment is not normally required. A
stress relief treatment of 700°C (1300°F) for 1/2
hour followed by a cooling rate of 90°C (200°F)
per hour is a standard procedure to relieve
stresses as a safeguard against corrosion cracking in caustic service for these alloys.
Filler metal selection for corrosive
environments
Nickel alloys are normally welded with matching
composition filler metals as shown in Table 5,
Part I. In sea water and some environments,
nickel-copper (Group A) alloy welds made with
matching composition filler metals may be anodic
to the base metal and corrode preferentially by
galvanic corrosion. This condition may be attributed largely to the fact that many “matching
composition” filler metals are not of identical
composition; some elements have been added or
amounts adjusted for better weldability. Another
factor to consider is that weld metal may also
become anodic to the base metal as a result of
segregation as it solidifies.
When experience demonstrates that matching
Alloy 400 and R405 — Alloy 400 is readily
welded by all the common welding processes
discussed in Part I. Alloy R-405 is a free-machining
grade of alloy 400, containing 0.025-0.060%
sulphur and is available as rods or bars. Parts
made of alloy R-405 usually involve little or no
welding, but when welding is required, it is good
20
For the materials engineer
metals for all three alloys and should detect no
difference between the three alloys. For this
reason the subject was not discussed in Part l,
For the welder.
Alloy B-2 has been the standard nickel-molybdenum alloy for a number of years, having replaced
the older alloy B. Alloy B had a shortcoming in
that it required a solution anneal at a temperature
of 1175°C (2150°F) after welding to eliminate
carbide precipitates in the weld heat-affected
zone and to restore corrosion resistance. A
modification of the alloy composition resulted in
the formulation of alloy B-2 which demonstrates
acceptable corrosion resistance in the as-welded
condition. This development made possible the
construction of fabrications too large to be solution annealed.
Work with alloy B-2, however, revealed a problem: it experiences a phase transformation during
brief exposure to temperatures in the range of
595-815°C (1100-1500°F). Such exposure can
result in cracking during base metal manufacturing operations or annealing by fabricators after
cold working. Recent alloy modifications by two
different metal producers have overcome the
595-815°C (1100-1500°F) low ductility problem
and associated cracking of alloy B-2. The result
of their work is the introduction of the two alloys:
B-3 and B-4.
This is a very brief treatment of the nickelmolybdenum alloys. Fabricators new to these
alloys should contact the producers before undertaking complex fabrications.
practice to make generous filler metal additions and
to minimize the amount of base metal melted, thus
reducing the amount of sulphur in the weld. Alloy
R-405 welds made with the SMAW process are
often less affected by sulphur from the base metal
than welds made by GTAW or GMAW.
Salt and brine environments — The standard
matching composition filler metals for welding
alloy 400 are shown in Table 5, in Part I. It is
important to note, however, that in salt or brine
environments, alloy 400 matching composition
welds may become anodic to the base metal and
suffer galvanic corrosion attack. To solve this
problem in brine environments, nickel-chromium
type electrodes are used such as ENiCrFe-2 and
ENiCrMo-3. Welds made with these electrodes
are cathodic to the base metal and thus, resist
galvanic corrosion.
Hydrofluoric acid service — Welded alloy 400
equipment used in hydrofluoric acid service
should receive a post-weld stress relief to avoid
stress corrosion cracking. The stress relief
treatment is performed at 540-650°C (10001200°F) for one hour followed by slow cooling.
Group B — Chromium-bearing alloys
The nickel-chromium, nickel-iron-chromium, and
nickel-chromium-molybdenum alloys may exhibit
carbide precipitation in the weld heat-affected
zone, a condition similar to that encountered in
austenitic stainless steels. In most environments, however, the sensitization of these nickel
alloys is not sufficient to affect the corrosion
resistance; as a result, solution annealing is
seldom required. Two factors function to reduce
sensitization: very low carbon levels (as a result
of recent improved melting practices), and the
use of stabilizing additions of titanium and
columbium in many alloys.
A post-weld heat treatment to prevent stress
corrosion cracking is recommended when alloy
600 is used in high-temperature, high-strengthcaustic-alkali service. The stress relief treatment
is performed at a temperature of 900°C (1650°F)
for one hour or at 790°C (1450°F) for four hours
with a slow cool.
Group D — Precipitation-hardening
nickel alloys
The precipitation-hardening nickel alloys have
limited use in corrosion resisting service so this
publication covers only the most important guidelines. Consult with the alloy manufacturers for
more detailed information.
The precipitation-hardening nickel alloys are
used in applications requiring corrosion resistance and a need for greater mechanical strength
or higher hardness than is obtainable with the
corresponding solid solution alloys. The precipitation-hardening or age hardening, as it is often
called, is accomplished by the addition of increased amounts of titanium and aluminum along
with special heat treatments. The heat treating
temperatures vary from 600-760°C (11001400°F) depending upon the alloy and specific
properties desired. The hardenable alloys in the
soft or solution-annealed condition have about
the same strength as the comparable solid solution alloy.
Group C — Nickel-molybdenum alloys
The materials engineer involved in fabrication of
nickel-molybdenum alloy equipment should be
aware of the background behind the three
grades; alloys B-2, B-3, and B-4, along with the
precautions required in post-fabrication heattreating. The welder will use matching filler
21
For the materials engineer
Of the hardenable alloys covered in this publication, i.e., alloys K-500, 725, 625 PLUS® and 718,
only alloy 718 has a matching composition filler
metal (ANSI/AWS A5.14 ERNiFeCr-2) that can be
strengthened by heat treatment. In practice this
has seldom presented a problem since most
applications do not involve welds that must develop the same strength as the base metals. An
example is the fillet weld used for attachments.
When full weld metal strength is not required, the
practice is to use a filler metal of the comparable
solid solution alloy as shown in Table 9 that
follows.
gested filler metals by identification (shown in
Table 10-A). These DMWs represent the nickel
alloys covered in this document welded to each
other and to some of the common steels.
The fillers indicated are those that are capable
of making metallurgically sound welds using
proper welding procedures with the SMAW,
GMAW, and GTAW processes. In making
DMWs, it is desirable to keep the base metal
dilution to a minimum and to keep the amount of
base metal melted into the weld uniform along
the length of the weld.
Base metal dilution is more easily controlled with
the SMAW process and to almost the same
degree with the GMAW process. In the manual
GTAW process, the amount of filler metal added
(and conversely the amount of base metal
melted) may vary considerably depending on
welder technique. For this reason, welder training
and qualification is particularly important for
DMWs made with the GTAW process.
Table 9 Matching Filler metals of the comparable
solid solution alloys
Alloy
Bare Electrodes
ANSI/AWS A5.14
Coated Electrodes
ANSI/AWS A5.11
K-500
725 & 625 PLUS®
718
ERNiCu-7
ERNiCrMo-3
ERNiCrMo-3
ENiCu-7
ENiCrMo-3
ENiCrMo-3
Procurement guidelines
Dissimilar-metal welds
Table 11 shows the principal wrought nickel and
nickel alloys, UNS numbers, and specifications for
the product forms. The seamless and welded tube
and pipe material specifications reference a
general requirement specification that governs
areas common to all nickel and nickel alloy pipe
and tubing such as dimensional tolerance, check
analysis, and inspection methods. The specifications are: B751, General Requirements for Nickel
and Nickel Alloy Seamless and Welded Tubing
and B775, General Requirements for Nickel and
Nickel Alloy Seamless and Welded Pipe. It is
good practice to order material to the specifications shown in the table rather than by trade
name.
In dissimilar-metal welding, the properties of three
metals must be considered; the two metals being
joined and the filler metal used to join them. For
example, if one of the metals being joined is welded
using preheat when welding to itself, preheat
should be used in making a dissimilar-metal weld
(DMW). Another variable might be the need for a
post-weld heat treatment. On occasion there may
be a conflict in that the optimum control for one
metal is undesirable for the other. In this case, a
compromise is needed. This is one reason the
development of a DMW procedure often requires
more study than for a conventional similar-metal
welding procedure. (ref. NiDl 14018)
Table 10-A, which follows, presents the filler
metal alloy identifications which are referenced in
Tables 10-B and 10-C. Tables 10-B and 10-C list
the dissimilar metal weld (DMWs) and the sug-
Surface finish
Surface finish, an important factor, is not
covered in the alloy-product form specifications
Table 10-A
Filler metal alloy identification for bare and covered electrodes
Class No.
1
2
3
4
5
6
7
8
9
10
Base alloy
Alloy 200
Alloy 400
Alloy G-3
Alloy G-30
Alloy 600
Alloy 600
Alloy 625
Alloy C-22
Alloy C-276
Alloy B -2
Bare electrodes and rods
Covered electrodes
ANSI/AWS A5.14-89
ANSI/AWS 5.11-90
ERNi-1
ERNiCu-7
ERNiCrMo-9
ERNiCrMo-11
ERNiCr-3
—
ERNiCrMo-3
ERNiCrMo-10
ERNiCrMo-4
ERNiMo-7
22
ENi-1
ENiCu-7
ENiCrMo-9
ENiCrMo-11
ENiCrFe-3
ENiCrFe-2
ENiCrMo-3
ENiCrMo-10
ENiCrMo-4
ENiMo-7
For the materials engineer
Table 10-B
Suggested filler metals for dissimilar metal welds
Alloy (UNS)
200 (N02200)
201 (N02201 )
CZ-100 (N02100)
400 (N04400)
K-500 (N05500)
M-35-1(N24135)
825 (N08825)
G-3 (N06985)
G-30 (N06030)
600 (N06600)
CY-40 (N06040)
690 (N06690)
625 (N06625)
725 (N07725)
625 Plus (N07716)
718 (N07718)
CW-6MC (N26625)
59 (N06059)
686 (N06686)
C-22 (N06022)
622 (N06022)
CX2MW (N26022)
C-276 (N10276)
CW-2M (N26455)
B-2 (N10665)
B-3 (N10675)
B-4 (None)
4 & 6% Mo
Stainless steels
300 Series
Stainless Steels
Carbon and
Low alloy steels
200
201
CZ-100
400
K-500
M-35-1
825
20 Mo-4
20 Mo-6
1, 5, 6, 7
1, 8, 3
1, 3, 4
5, 6, 7
5, 6
5, 6
3, 7, 8
4, 7, 8
4, 7, 8
1, 5, 6
1, 5, 6
5, 6
5, 6
5, 6, 7, 8
5, 6, 7, 8
3, 7, 8
3, 7, 8
4, 7, 8
4, 7, 8
5, 6
1, 5, 6, 7
5, 6, 7
5, 6, 7, 8
3, 7, 8
4, 7, 8
5, 6
5, 6
1, 5, 6, 7
5, 6, 7
5, 6, 7, 8
3, 7, 8
4, 7, 8
5, 6
5, 6
1, 8
5, 6
7, 8
3, 7, 8
4, 7, 8
5, 6
5, 6
1, 5, 6, 9
5, 6
7, 8, 9
3, 7, 8, 9
4, 7, 8, 9
5, 6
5, 6
1, 10
10
7, 10
3, 8, 10
4, 8,10
7, 8, 10
7, 8, 10
1, 7, 8
5, 6
7, 8
3, 7, 8
4, 7, 8
5, 6, 7
5, 6, 7
1, 5, 6
5, 6
7, 8
3, 7, 8
4, 7, 8
5, 6
5, 6
1, 5, 6
2
5, 6
3, 7, 8
4, 7, 8
5, 6
5, 6
G-3
G-30
600
CY-40
690
1,2
alloy castings to this ASTM specification, however, does not assure quality castings. The best
assurance of obtaining quality castings lies with
the capability, experience, and integrity of the
producing foundry. Unfortunately this point is
too often overlooked and factors such as price
prevail. Large users of castings can profit by
visiting potential foundry suppliers to assess
their technical and production capabilities. The
purchasers should consider the supplemental
ordering requirements which follow.
Direct procurement of nickel alloy castings by
end users is unusual. The end user normally
buys castings in the form of pumps, valves and
components already assembled into OEM-furnished equipment. The following considerations
apply to the entity actually purchasing the cast-
presented previously. Surface finish for nickel
alloy sheet, strip, and plate is not standardized
as it is for stainless steel. Table 12 presents
information on surface finishes available from
two nickel base alloy producers. Where surface
finish is important, the purchaser must review
and negotiate the requirements with individual
nickel alloy producers.
Nickel alloy castings
The principal cast nickel alloy designations, UNS
numbers, compositions, and wrought counterparts are shown in Table 8. The suggested filler
metals are shown in Table 5, Part I. Procurement
of nickel alloy castings would seem to be straightforward because all castings are covered by one
specification, ASTM A494. Specifying nickel
23
For the materials engineer
Table 10-C
Suggested filler metals for dissimilar metal welds
Alloy (UNS)
625,725
625 Plus,
718, CW-6MC
C-22
622
CX2MW
59
686
CW-2M
CW-2M
B-2
B-3
B-4
200 (N02200)
201(N02201)
CZ-100 (N02100)
400 (N04400)
K-500 (N05500)
M-35-1 (N24135)
825 (N08825)
G-3 (N06985)
G-30 (N06030)
600 (N06600)
CY-40 (N06040)
690 (N06690)
625 (N06625)
725 (N07725)
625 Plus (N07716)
718 (N07718)
CW-6MC (N26455)
59 (N06059)
686 (N06686)
C-22 (N06022)
622 (N06022)
CX2MW (N26022)
C-276 (N10276)
CW-2M (N26455)
B-2 (N10665)
B-3 (N10675)
B-4 (None)
4 & 6% Mo
Stainless steels
300 Series
Stainless Steels
Carbon and
Low alloy steels
7, 8
7, 8
7, 8
7, 8, 9
7, 8, 9
7, 8, 9
7, 10
7, 8, 10
8, 10
9, 10
7, 8
7, 8
7, 8
8, 9
8, 10
7, 8
7, 8
7, 8
8, 9
8, 10
7, 8
7, 8
7, 8
8, 9
8, 10
radiographic examination, liquid penetrant examination, weldability tests, and pressure tests are
examples of some measures that are available to
further control the quality of nickel alloy castings.
These additional quality assurance provisions
may be specified by the purchaser and should be
substantiated by certification that the foundry
complied with the specifications. A few observations concerning the effective use of these supplemental measures follow.
ings. The supplying foundry should thoroughly
review and understand the specifications which
must be carefully written by the purchaser.
A very important property of nickel alloys is their
corrosion resistance ASTM A494, to which nickel
alloy castings are normally procured for corrosion
resisting service, covers composition, mechanical
properties, and heat treatment. Corrosion tests
are not a part of ASTM A494 and, therefore,
require a special arrangement between the
purchaser and supplier. In practice, however,
corrosion testing of each lot or heat of material is
seldom justified except for unusual services.
There are supplemental requirements that can
assist users in obtaining nickel alloy castings
which embody the inherent corrosion resistance
of these alloys. Source inspections utilizing
Source Inspections
Radiographic inspection — Radiographic
inspection should be considered when the castings are subject to high and/or cyclical stresses
and when mechanical strength, as well as corrosion resistance, is important. In addition, if
24
For the materials engineer
Table 11
Specifications for procurement of groups A through D wrought nickel and nickel alloys
ASTM Specifications unless otherwise noted
(1)
(1)
Plate sheet
Rod bar
Seamless
Welded
strip
forgings
tube pipe
tube & pipe
Group A Nickel & nickel-copper solid solution alloys
B725
B161
B160
B162
N02200
200
B730
B725
B161
B160
B162
N02201
201
B730
B725
B165
B164
B127
N04400
400
N/A
N/A
B164
N/A
N04405
R-405
Group B Chromium-bearing solid solution alloys
B423
B425
B424
N08825
825
B705
B619
B622
B581
B582
N06985
G-3
B626
B619
B622
B581
B582
N06030
G-30
B516
B167
B166
B168
N06600
600
B517
N/A
B167
B166
B168
N06690
690
B619
B622
B574
B575
N06059
59
B626
B704
B444
B446
B443
N06625
625
B705
N/A
N/A
N/A
N/A
N06686
686
B619
B622
B574
B575
N06022
622
B626
B619
B622
B574
B575
N06022
C-22
B626
B619
B622
B574
B575
N10276
C-276
B626
Group C Nickel-molybdenum alloys
B619
B622
B335
B333
N10665
B-2
B626
B619
B622
B335
B333
N10675
B-3
B626
N/A
N/A
N/A
N/A
N10629
B-4
Group D Precipitation-hardening alloys
K-500
QQN286
QQN286
QQN286
QQN286
N05500
Mil-N-17506
Mil-N-17506
Mil-N-17506
725
N/A
N/A
B805
N/A
N07725
625-Plus®
N/A
N/A
B805
N/A
N07716
N/A
AMS 5589/90
B637
B670
N07718
718
Alloy
UNS No.
Fittings
Condenser
tubing
B366
B163
B366
B163
B366
N/A
B163
N/A
B704
B366 B163
B366
N/A
B366
B366
B163
N/A
N/A
B163
N/A
B366
N/A
N/A
B366
N/A
N/A
B366
N/A
B366
N/A
B366
N/A
B366
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
(1) B751 “Standard Specification for General Requirements for Nickel and Nickel Alloy Seamless and Welded Tube” also applies in addition to
the individual alloy pipe or tube specification.
N/A Alloy is not available in this product form to an ASTM specification.
surface conditions allow corrosion to proceed
beyond the casting surface, subsurface defects may
allow the degradation of the longer term corrosion
resistance of the casting. For such applications,
radiographic inspection may be justified.
The added expense of radiographic inspection is
usually not justified for castings where sound metal
on the wetted surfaces and flange faces is the
primary service requirement. In such cases, a liquid
penetrant examination is a less expensive choice.
Table 12
Surface finishes for nickel base alloy sheet, strip,
and plate
Producer A
Thickness
Sheet and strip
up to 0.64cm (0.25 in.)
Sheet and strip
0.48cm (0.19 in.) and up
Producer B
Thickness
Sheet and strip
up to 0.32cm (0.125 in.)
Plate over 0.32cm
(0.125 in.)
Finish
CR Cold Rolled or
Pickled Plate
Descaled or
as Rolled (Hot Rolled)
Finish
CR (Cold Rolled) (2B)
Liquid penetrant inspection — Liquid penetrant
inspection after rough machining can identify
surface defects that may become sites for corrosion and cracking. Wetted surfaces and flange
HRAP (Hot Rolled,
Annealed, and Pickled)
25
For the materials engineer
faces inside the bolt circle are examples of surfaces where such defects can be removed with
light grinding or minimal weld repair.
Liquid penetrant inspection of non-wetted surfaces can lead to unnecessary cosmetic repairs
and should be avoided unless justified by
unusual circumstances. The fillet areas on the
outer surfaces of castings between the bodies
and flanges are good examples of areas that are
particularly prone to persistent minor penetrant
indications. These indications are usually inconsequential and difficult to eliminate completely.
1220°C (2225°F) is better for alloys CW-2M and
CX2MW. The annealing temperature as well as
the water quench can present problems to some
suppliers. The higher furnace temperature is
needed to put potentially harmful precipitates into
solution. The water quench is needed to ensure
that these potentially harmful precipitates remain
in solution and do not precipitate out during
cooling.
Many foundries do not have furnaces capable
of reaching the required solution annealing temperatures. They may offer to heat treat at somewhat lower temperatures for longer times. Both of
these exceptions are detrimental to the finished
product quality and should be refused. Lower
solution annealing temperatures allow
precipitates to form which are harmful to corrosion resistance and longer times allow more of
the harmful precipitates to form. To verify that
these conditions do not occur, procurement
documents should require that furnace charts be
supplied showing that the specified solution
annealing temperature was actually reached.
Some foundries may request exception to a
water quench fearing that the casting may crack.
While the water quench is an excellent check on
general quality in addition to preventing precipitation of undesirable second phases, there may be
configurations that are prone to cracks. These
need to be reviewed on a case by case basis by
qualified and experienced metallurgical specialists. No exception to the water quench test
should be allowed without this qualified evaluation.
Weldability test —The weldability test as
specified in ASTM A494 is optional, but can be
one of the best assurances that the alloy does
not have harmful levels of trace elements and
that the heat treatment, if required, has been
properly performed. The test, Figure 1(b) of
ASTM A494, is the preferred test and includes a
bend test and macro examination.
Tramp or unwanted elements usually result
from poor material and scrap circuit control.
Foundries with good quality control programs
seldom encounter weldability problems.
The purchaser must decide how extensively the
weldability test should be applied, i.e., to every
heat of material or to a selected or random
number of heats. Generally, the test should be
applied to all heats supplied by a new foundry
source. It is prudent to follow the same policy for
alloys new to a particular foundry until a confidence level has been established.
Pressure test — Hydrostatic or air testing is
performed by many foundries on pressure type
castings. Although these tests are not a requirement of ASTM A494, they should be specified as
an additional requirement. Pressure tests should
be performed after rough machining and before
weld repair.
Nickel, Nickel-copper, Nickel-Molybdenum
alloys — Ni, NiCu and NiMo alloys are less
sensitive to heat treatment and weld repair than
NiCrMo alloys. Nickel and NiCu do not require a
solution annealing treatment and are used in the
as-cast condition. NiMo alloys are annealed at a
temperature of 1093°C (2000°F) followed by a
water quench. A temperature of 1093°C
(2000°F) is easily reached in most alloy foundry
furnaces.
Certification —To assure that the requirements have been met, the purchaser can, and
should, request the manufacturer’s certification
stating that the material was manufactured,
sampled, tested, and inspected in accordance
with the full material specification including all
supplemental tests requested.
Chemistry
It is essential that the alloy composition of all
nickel alloys be within the ranges specified by
ASTM A494, however, not all of the tramp elements which can be deleterious to the welding of
nickel alloys or to their corrosion resistance are
identified in ASTM A494.
The composition of the as-cast surface of the
casting may differ from the specified composition
due to carbon pickup from some molding materi-
Heat Treatment — NiCrMo alloys — ASTM
A494 requires a solution annealing temperature
of 1175°C (2150°F) followed by a water quench
for the NiCrMo alloys: CW-6MC, CW-2M, and
CX2MW shown in Table 8. Experience has
shown that a solution annealing temperature of
26
For the materials engineer
als, chromium depletion during annealing, or
other casting-related surface changes. When
the casting is machined, the surface layer is
removed and the composition of the machined
surface closely approaches the bulk chemistry.
The following three suggestions can help to
ensure the chemical integrity of the castings you
receive.
1. Require a weldability test, described earlier,
to assure that tramp elements are not
present in an amount to cause defects from
welding.
2. Assure that the foundry has the capability to
make reliable chemical analyses. This can
be confirmed by an audit performed by a
qualified analytical chemist and/or by a
check analysis performed by an outside
laboratory skilled in analysing nickel alloys.
3. When the as-cast surface is the surface that
must resist corrosion, it is prudent to require
that the composition at the surface be within
the ranges applicable to the particular alloy.
Post-weld repair heat treatment — ASTM
A494 leaves post weld heat treatment (PWHT)
after foundry weld repairs as an optional operation. Procurement documents should require
PWHT and water quench after all weld repairs
as an exception to ASTM A494 for the NiCrMo
alloys. The quench is as important as actually
reaching the required solution annealing temperature. The quench keeps the deleterious
phases in solution and is an excellent check on
the general quality of the casting.
Welding nickel alloy castings
The guidelines for welding wrought nickel alloys
covered in Part I should also be used in welding
nickel alloy castings, whether making casting
repairs or incorporating the castings into fabrications. All oil, grease, machining lubricants, and
similar substances should be removed with a
suitable solvent. All surface oxides and “casting
skin” should be removed next to the weld joint by
machining or abrasive grinding. Failure to remove
this layer usually contributes to weld defects. Abrasive grinding wheels should be dedicated to grinding only nickel alloy castings and should not be
previously used on carbon or low alloy steel.
In fabrication welding, nickel alloy castings are
joined with the same welding processes used for
the wrought forms. In the repair of castings, the
welding process employed often depends upon
the size of the casting and size of the defect.
GTAW is the process of choice in repairing small
castings and in repairing small, shallow defects
on any size of casting. Similar composition filler
metal should be added in making GTAW repairs.
The SMAW process is most-often used by foundries for the repair of larger defects and has the
advantage of lower heat input than GTAW for
similar repairs. Some foundries have developed
GMAW procedures for welding repair and are
able to realize greater welding efficiencies.
Casting repair by welding
It is good practice to rough-machine and pressure-test all nickel alloy castings before weld
repairs are made. This procedure allows weld
repairs to be made before final heat treatment
and avoids most of the problems that arise
when defects are uncovered in machining.
Major weld repairs are defined in ASTM A494.
Welding heats the area adjacent to the weld into
a range where phases may precipitate that can
be deleterious to corrosion resistance of the
NiCrMo alloys. Post weld heat treatment
(PWHT) and rapid quenching after weld repairs
restores full corrosion resistance — if and only if
— the high temperatures required for solution
annealing are actually reached.
Ni, NiCu and NiMo alloys, Group A and C
alloys — Although welding heats the area
adjacent to the weld of Ni, NiCu and NiMo alloys
to high temperatures, there are no deleterious
phases that precipitate in these solid solution
alloys. Since PWHT is not required, procurement is simplified.
Procurement checklist for nickel
alloy castings
In procurement, recognize that nickel alloys
have been selected for their extreme corrosion
resistance. Using ASTM Specifications is an
essential first step, but does not protect the user
from receiving nickel alloy castings with degraded
corrosion resistance. Assurance of quality is up to
the purchaser and the supplemental requirements he places on the foundry, above and
beyond the basic ASTM requirements. Following
Filler metals — The filler metals shown in
Table 5, Part I should be used for all weld repairs
unless experience has shown over-matching
composition filler metals are required for the
specific environment of concern. An example of
a need for over-matching filler metal is discussed
earlier in Part II, in the topic entitled Salt & brine
environment.
27
For the materials engineer
by supplier or by a qualified laboratory;
d. Weldability tests;
e. Rough machining and pressure tests;
f. Certification.
2. Process control requirements and verification:
a. Higher solution annealing temperature;
b. Furnace charts;
c. Water quench;
d. Weld repair and post weld repair heat
treatment.
is a checklist of requirements supplementary to
those in ASTM A494 that are frequently necessary in order to obtain the quality and corrosion
resistance inherent in the basic composition:
1. Source inspections, including one or more of
the following:
a. Radiographic inspection;
b. Liquid penetrant inspection;
c. Analysis of surface and/or bulk chemistry
28
For the design engineer
PART III
For the design engineer
Design for corrosion service
actually worse from the outside as condensation
is funnelled directly into the bottom-to-pad crevice. The grout used to divert such condensation,
Figure 15, helps initially, but soon shrinks and
becomes a source of maintenance.
The drip skirt shown in Figure 16 is the best
arrangement for flat-bottom tanks. The concave
bottom and the dished-head bottom on supports,
Figures 17 and 18, are very good and are superior to all flat-bottom tanks not only in corrosion
resistance but also in fatigue resistance.
Fatigue stresses from filling and emptying are
seldom considered in design, but they can be
significant and have led to failures in flat-bottom
tanks. The concave and dished-head designs
can withstand much greater fatigue loadings than
can flat bottoms.
Thoughtful design can improve corrosion resistance and obtain better service from less expensive grades of nickel alloys. There are two
cardinal rules to keep in mind.
1. Design for complete and free drainage.
2. Eliminate or seal weld crevices.
Tank bottoms
Figures 13 through 18 show six common tank
bottom designs. The square-corner-flat-bottom
design shown in Figure 13 invites early failure
from the inside at the corner weld where sediment collects, increasing the probability of undersediment crevice attack. In addition, the flat
bottom-to-pad support invites rapid crevice corrosion when moisture penetrates the underside.
The rounded-bottom design shown in Figure 14
is much more resistant from the inside, but is
Figure 13 Flat bottom,
square corners — worst
Figure 14 Flat bottom,
rounded corners — good
corners — poor outside
Figure 15 Flat bottom,
rounded corners, grouted
— good inside, poor
outside
Figure 16 Flat bottom,
rounded corners, drip
skirt — good inside, good
outside
Figure 17 Concave
bottom rounded corners
— good inside, good
outside, fatigue resistant
Figure 18 Dished head
— best inside, best
outside, fatigue resistant
Tank bottom outlets
Residual water in the bottom of nickel alloy
tanks is a potential source of tank bottom failures.
Side outlets and centre outlets shown in Figures
19 and 20 provide a convenient construction
configuration but invite early failure of tank bottoms. Not only is a layer of stagnant liquid held
on the tank bottom, but sediment and accumulated contamination cannot easily be flushed out.
The flush side outlet and the recessed bottom
outlet as shown in Figures 21 and 22 allow the
bottom to be completely drained of sediment,
29
Figure 19 Side outlet
above bottom — poor
Figure 20 Centre outlet,
above bottom — poor
Figure 21 Side outlet,
flush — good
Figure 22 Centre outlet
recessed — good
For the design engineer
severe crevice between
the angle and the inside
wall of the vessel. Over
time, this crevice fills
with sediment and other
contaminants, inviting
premature failure from
crevice corrosion.
Figure 28 Tray support,
Figure 29 shows the
same tray support with staggered strength weld
— severe crevice
a continuout seal weld
at the top. This change prevents contaminants
i
l
ld
leaving it clean and dry. The sloped designs
shown in Figures 23 and 24 improve further on
these designs, facilitating the drainage of
residual liquid and sediment.
Figure 23 Side outlet,
flush, sloped — best
Figure 24 Centre outlet,
recessed, sloped — best
Bottom corner welds
When the sidewall forms a right angle with the
bottom, the fillet weld is seldom as smooth as
shown in the cross section of Figure 25. It is
usually rough and it frequently varies in width to
compensate for variations in fit-up. Sediment
tends to collect along the weld. It is difficult to
remove, and leads to under-sediment crevice
attack. Welding along the outside as shown in
Figure 26 improves the resistance of the joint to
crevice attack from the outside; however, rounding the corner and moving the weld to the
sidewall as shown in Figure 27, improves it
further. The corrosion resistance from both
sides as well as the fatigue resistance are
improved by this last refinement.
Figure 25 Corner weld
from inside — poor
inside, worst outside
Figure 29 Tray support,
full seal weld top — good
crevice resistance
Figure 30 Tray support,
full seal weld top &
bottom — best crevice
resistance
Figure 31 Reinforcing
pad, staggered welds —
adequate strength
Figure 32 Reinforced
pad, seal weld — best
crevice resistance
from migrating into the crevice. The angle-tosidewall crevice is still open from the bottom, but
this is a much less severe crevice. While this
crevice is still subject to vapor penetration, it is
not vulnerable to the lodging of solids. Figure 30
shows a full seal weld at the top and bottom of
the tray support angle. With this addition, the
crevice is fully sealed and represents the best
design for crevice corrosion resistance.
Figure 31 shows a reinforcing pad frequently
used to weld other attachments. The intermittent
weld creates a severe pad-to-sidewall crevice
inviting premature failure. Completing the seal
weld as shown in Figure 32 requires very little
additional time but greatly improves the corrosion
resistance of the pad.
Figure 33 shows structural angles positioned to
allow drainage, an important factor in preventing
crevice corrosion. Angles should never be positioned as shown in the top view of Figure 34.
Figure 26 Corner weld
from both sides — poor
inside, good outside
Figure 27 Side wall in lieu of corner weld — best
inside, good outside, fatigue resistant
Attachments and structurals
Attachments create potential crevice corrosion
sites. Figure 28 shows a tray support angle with
intermittent welds made with the intention of
providing adequate strength, however, there is a
30
For the design engineer
Figure 33 Position of
angles
Figure 37 Corner baffle
cut-out — good
Figure 34 Position of
angles
Figure 38 Heat
exchanger, baffle cut-out
— good
The best position for complete drainage is shown
in the lower view.
When structural shapes are used, they should
be positioned with open side down so that liquids
will drain freely. When this preferred positioning
is not possible, drain holes should be drilled
about every 305mm (12 in.) in the centre as
shown in the middle view of Figure 35.
Figure 39 Poor and good designs for the location of
heaters in a vessel
poor location of heaters creates hot spots which,
in turn, may result in higher corrosion in the area
between the heater and the vessel wall. The
good design avoids hot spots by centrally locating
the heater.
When a concentrated solution is added to a
vessel, it should not be introduced at the side as
shown in the poor design of Figure 40. Side introduction causes concentration and uneven mixing
at the sidewall. With the good design, mixing takes
Figure 35 Position of channels
Continuous fillet welds on stiffeners and baffles,
as shown on the right side of Figure 36, seal the
severe stiffener/baffle-to-horizontal-plate crevice.
The staggered fillet welds shown on the left side
of Figure 36 leave the joint vulnerable to crevice
corrosion.
Figure 36 Stiffeners and baffles
Baffles in tanks and heat exchangers create
dead spaces where contaminants and sediment
can collect and where full cleaning is difficult.
Figure 37 shows a cut-out at the lower corner of
a tank baffle and Figure 38 shows a cut-out in the
lower portion of a heat exchanger tube support
plate. Both arrangements reduce the likelihood
of the accumulation of contaminants and facilitate
cleaning.
Figure 40 Poor and good designs for mixing
concentrated and dilute solutions
place away from the sidewall. It is also good
design practice to introduce feed below the liquid
level to avoid splashing and drying above the
liquid line.
Pipe Welds
Small diameter nickel base alloy piping, 51 mm
(2 in.) and less in diameter is more frequently
socket welded than butt welded. The emergence
of automatic orbital welding promises to greatly
Heaters and Inlets
Heaters should be located so they do not cause
hot spots on the vessel wall. In Figure 39, the
31
For the design engineer
reduce the use of socket welds. The crevice of a
socket weld joint is less damaging to nickel alloys
than to stainless steel due to the considerably
greater corrosion resistance of the nickel alloys.
Nevertheless, the crevice provides a site of lower
corrosion resistance in some aggressive environments. To circumvent this weakness, specify
orbital butt welds wherever practical.
Figure 41 shows a circumferential pipe weld with
incomplete penetration. Pipe welds should be full
penetration welds for best corrosion performance
and for full weld joint efficiency. Welding codes
such as those of the American Society of Mechanical Engineers (ASME) and the American
Petroleum Institute (API) require full penetration
butt welds. When such codes are not imposed,
used on nickel alloy pipe. In this case a stub-end
arrangement as shown in Figure 44 is preferred.
In order for piping and heat exchanger tubing to
drain completely, it is necessary to slope the
piping or heat exchanger tubing just enough so
that water is not trapped in the slight sag between
supports. Figure 45 shows how a water film tends
to remain in horizontal runs of pipe or tubing, and
how water drains when sloped.
Figure 45 (A) Horizontal (standard) — poor
(B) Sloped — very good
Weld overlay, sheet lining, and
clad plate
Figure 41 Pipe weld with incomplete penetration —
severe crevice
To offset the relatively high cost of solid nickel
alloy construction and still provide a highly corrosion-resistant layer of alloy, weld overlaying,
sheet lining, or clad plate are often viable design
options. Construction using these techniques or
products is well-suited to those applications
where the full metal thickness is not required for
mechanical purposes or corrosion resistance. For
economic reasons, the backing material is
usually carbon steel, but other steels are feasible.
Some features of the three designs are summarized in Table 13.
A discussion on welding and fabrication techniques of each of the processes follows.
the purchaser should specify that pipe welds be
full penetration welds. In addition, limits should
be placed on the weld concavity and convexity.
Common limits are maximums of 0.8mm (0.03
in.) concavity and 1.6mm (0.06 in.) convexity.
Three good pipe-to-flange welding designs are
shown in Figures 42, 43, and 44. The recessed
arrangement shown in Figure 42 avoids the need
Weld overlay
Figure 42 Pipe recessed,
flange and pipe, same
alloy — good
Weld overlay surfacing is well-suited to covering
thick sections of items such as tube sheets, large
diameter shafts, and the walls of thick-section
pressure vessels. The substrate is usually carbon steel or, on occasion, a low alloy steel. The
weld overlay may be made by a number of different welding processes; the choice is usually
based on the process that gives the highest
deposition rate and acceptable quality overlay for
the particular application.
Comments on the welding processes for overlay
welding follow.
Figure 43 Pipe flush,
pipe and flange same
alloy — better
for machining or grinding smooth the surface
of the weld on the flange
face in Figure 43. Both
of these are suitable
when the flange is of the
same material as the
pipe. Neither is suitable Figure 44 Stub-end,
when carbon steel or
flange carbon steel or
ductile iron flanges are ductile iron — very good
Submerged arc welding — Deposition rates
using SAW are high, a 35 to 50% increase over
32
For the design engineer
Weld overlay guidelines — The effect of base
metal dilution and the profile of the overlay/base
metal interface are two areas of concern in weld
overlay work. These concerns are common to
overlays made with any welding process.
GMAW overlay capability. SAW fluxes are commercially available for use with most of the common nickel alloy bare wire and strip filler metals.
With wire electrodes, a diameter of 1.6mm (0.062
in.) yields better results than the larger diameters
characteristic of steel or stainless steel SAW.
Base metal dilution is normally controlled so that
only two weld layers are needed unless the
surface is to be machined, in which case three
weld layers may be required. The as-welded
surfaces are smooth enough to be dye-penetrant
inspected with no special surface preparation
other than wire brushing. All welding must be
done in the flat position unless the equipment is
specially adapted for horizontal welding.
Base metal dilution — Usually the objective is
to provide a weld overlay in which the top weld
surface has a composition equivalent to the base
metal to achieve a similar corrosion resistance.
This means a minimum of two weld layers and
often three along with careful control of welding
parameters to minimize penetration into the base
metal. Each additional layer adds significantly to
the total cost, so there is a strong incentive to
minimize the number of weld layers. Two ways to
do this follow:
• Where the overlay composition is a chromiumbearing alloy containing iron, use a low iron,
high alloy filler metal. This suggestion might
be implemented, for example, by substituting
filler metals such as ERNiCrMo-3, ERNiCrMo-4
or ERNiCrMo-10 in an overlay weld which
calls for alloys such as 825 or G-3.
• For nickel and nickel-copper overlays, do not
specify lower iron limits than are needed for
satisfactory corrosion performance.
Gas metal arc welding — GMAW overlays are
usually made using the spray arc or pulsed arc
mode. The spray arc mode has the advantage of
higher deposition rates, but all welding must be
performed in the flat position. Base metal dilution
tends to be higher with GMAW welding than with
other processes. The favoured method employs
automatic welding with an oscillating torch movement.
Pulsed GMAW overlays are usually done with a
filler wire diameter of 1.2mm (0.045 in.), compared to a wire diameter of 1.6mm (0.062 in.)
used with the spray arc mode. Deposition rates
are lower, but all-position welding is possible.
Pulsed arc mode GMAW may be done with either
manual or automatic set-up.
Base metal interface — The second concern is
the interface profile. Ideally the interface profile
perpendicular to the direction of welding should
be a nearly straight line, free of “spikes” of base
metal between weld beads. An uneven profile is
more prone to weld cracking and may fail to pass
a side bend test which is often required by codes.
Shielded metal arc welding — Deposition rates
are relatively low, but the process is useful in
overlaying small areas and irregular, out-ofposition surfaces where automation is not justified.
Facings on vessel outlets and trim on valves are
good examples of suitable applications.
Sheet lining
Sheet lining has been used for over 60 years to
cover metal substrates with a more corrosion
Table 13
Comparisons of weld overlay, sheet lining, and clad plate
Design
Weld overlay
Sheet lining
Clad plates
Applications
Covering substrate sections of unlimited
thickness and varied shapes, such as tube
sheets.
Multi-layers (2 or 3 minimum) needed to
compensate for dilution
Covering broad areas of existing or new
construction substrate with thin alloy sheet
Extensive sheet forming is required in applying to
complex shapes
Constructions where large size plates are an
advantage
Roll-bonded plates limited to 64-76mm (2.5-3 in.)
thickness
33
Remarks
Solid bond of alloy to substrate providing good heat
transfer and mechanical strength
Generally not competitive where sheet lining or clad
plate is acceptable
Local repair of alloys in process equipment
Applying liner to selective high corrosion/erosion
areas
Not suited to vacuum or heat transfer applications
Need for plug or arc spot welds for mid-sheet
attachment
Where an integral bond between alloy and backing is
needed for vacuum or heat transfer applications or for
construction of storage and pressure vessels
Tight welding controls are required for minimum iron
dilution on the alloy side
For the design engineer
Clad steel
resistant alloy. Stainless steels, nickel, and copper
alloys are common lining alloys. Essentially the
same application technique is used for all alloys.
The sheet characteristics and the welding practices
used in the sheet lining process are summarized
below; further details are available in NiDI technical
series publications Nos. 10,027 and 10,039 which
are included in the Bibliography.
• All pre-weld cleaning and preparation practices covered in Part I should be followed in
welding the alloy sheet to the substrate.
• Sheet thickness/size — A sheet thickness of
1.6mm (0.062 in.) is most widely used; thinner
sheet is more difficult to weld. To minimize the
amount of welding, the sheet sizes used are
as large as are practical to form and handle.
• Liner weld joints are usually either the overlap
joint or three-bead method shown in Figure
46. The overlap joint is preferred with 1.6mm
(0.062 in.) sheets when minimum weld dilution
from the substrate is essential. The threebead method is more often used for alloy
sheets that are 3.2mm (0.125 in.) and thicker.
• Mid-sheet attachments, when needed, are
made with plug welds through pre-punched
holes or, alternately, with a GMAW spot weld
through the sheet.
• Seal welds can be made by SMAW or GTAW,
but the pulsed GMAW process is most widely
used.
Nickel alloy clad steel is available as either a
roll-bonded or explosion-bonded product. Rollbonded clad is produced by hot-rolling a thick
section sandwich of steel and alloy starter plates.
In the course of hot-working, a metallurgical bond
is formed between the two metals. The normal
thickness of roll-bonded clad plates is 5mm
(0.187 in.) up to 64-76mm (2.5 or 3 in.) with the
alloy representing 10 to 20 % of the total thickness. In explosion-bonding, there is usually no
reduction after bonding, so the starting and finish
alloy and steel thicknesses are the same. This
allows relatively thin alloy sheet to be applied to
backing steel several inches thick.
Recommended joint designs for butt welding
clad steel are shown in Figure 47. Both designs
include a small root face of steel, i.e., the edge of
unbeveled portion of the joint, above the cladding
to protect the cladding during welding of the steel.
The steel side should be welded first with a steel
filler metal, usually by SMAW with a low hydrogen
electrode. It is important to avoid penetration into
the cladding when welding the first pass. Dilution
of the steel weld with the nickel alloy cladding
can cause weld cracking. Upon completion of the
steel side, the clad side is prepared by grinding
or chipping. Welding is done with the filler metal
recommended for welding solid alloy sections in
Table 5, Part I. To compensate for dilution by
steel, at least two and preferably three alloy
layers should be applied.
If the clad plate is 8mm (0.312 in.) or less, it is
generally more economical to use the nickel filler
metal for both sides of the joint. Refer to the
Lukens Steel Publication, included in the Bibliography, for further details on welding roll-bonded
clad metals.
Figure 46 Weld joints for liners
Figure 47 Joint designs for clad steel. (A) Material
4.8 to 16mm (3/16 to 5/8 in.) thick.
(B) Material 16 to 25mm (5/8 to 1 in.) thick
(courtesy of Inco Alloys International —
Joining Technical Bulletin)
34
Bibliography
American Welding Society- AWS B2.1, Standard
for Welding Procedure and Performance
Qualification.
Guidelines for the Welded Fabrication of NickelContaining Stainless Steels for Corrosion Resistant
Services, NiDI Technical Series No.11 007.
American National Standard Institute/Accredited
Standards Committee.
Guidelines for Welding Dissimilar Metals, by
Richard E. Avery, NiDI Technical Series No. 14 018.
ANSI/ASD, Z49 1 — 88, Safety in Welding and
Cutting.
Joining Technical Bulletin, Inco Alloys
International, Inc., Huntington, WV.
American Society of Testing and Materials,
ASTM A380, Standard Recommended Practice
for Cleaning and Descaling Stainless Steel Parts
Equipment, and Systems.
Specifying Stainless Steel Surface Treatment, by
Arthur H. Tuthill and Richard E. Avery, NiDI
Technical Series No. 10 068.
Stainless Steel Sheet Lining of Steel Tanks and
Pressure Vessels, by Richard E. Avery, Jonathan
D. Harrington, and William L. Mathay, NiDl
Technical Series No. 10 039.
ANSI/AWS D10.11 — 87, Recommended
Practices for Root Pass Welding of Pipe Without
Backing.
The Affect of Heat-Tint on Corrosion Resistance
of Alloys Used in FGD Systems, by W.L. Silence
and L.H. Flasche, Paper 358 NACE
CORROSION ’86.
ASM International Metals Handbook, Volume 6,
Welding, Brazing, and Soldering.
ASME Boiler and Pressure Vessel Code, Section
Nine, Welding and Brazing Qualifications.
The Back Purging and Welding Requirements for
Fabrication of HASTELLOY ® Alloy Pipe
Systems, Haynes International, Inc., Huntington,
WV.
Fabrication of HASTELLOY® CorrosionResistant Alloys, Haynes International Inc.,
Kokomo, IN.
Fabrication of Lukens Clad Steels, Lukens Steel
Company, Coatesville, PA
Welding and Fabrication of Nickel Alloys in FGD
Systems, by Richard E. Avery and
W. H. D. Plant, NiDI Technical Series No. 10 027.
Acknowledgement
The authors are indebted to Lee H. Flasche, Haynes International, Inc.; Leo E. Wildenthaller, Hobart Welding
Products; S. D. Kiser, Inco Alloys International, and Nickel Development lnstitute, D. J. Heath, D. E. Jordan
and Warren M. Spear for their valuable technical comments. We also want to recognize Raymond V.
Gendron, Tech. Text, for his technical writing assistance.
35
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement