Welding Lightweight Steel Framing Members

Welding Lightweight Steel Framing Members
FROM THE FOUNDATION
Welding Lightweight
Steel Framing Members
This article from the June 1988 Foundation Updates has been updated to
reflect current developments in the field of welding lightweight steel framing.
Updates is published by the Foundation of the Wall and Ceiling Industry.
By Kathy B. Sedgwick, Executive Director
One of the most common construction
applications of welding is panelization
and prefabrication with lightweight steel
framing, that is, steel ranging from 12
to 22 gage. When steel studs and joists
are prefabricated into curtain wall
assemblies and load bearing exterior
wall systems, the components are frequently welded together for strength and
rigidity.
Several types of welding are used in
lightweight steel framing applications.
The most popular is Shielded Metal
Arc Welding, commonly called stick
welding.
The popularity of Shielded Metal Arc
Welding (hereinafter referred to as
SMAW) is due to: (1) its versatility;
(2) portability of equipment; and (3)
the joint strength and quality of the
resulting weld.
SMAW is a versatile method which
can effectively weld metals as light as
20 gage. It can be used both in the shop
or at the job site. Because the power
supply leads can be extended for
relatively long distances, SMAW can be
performed at some distance from the
power source.
In addition, SMAW is adaptable to
multi-position welding in difficult locations. SMAW can weld joints in any
position that can be reached with an
electrode (even overhead and vertical
joints). Using bent electrodes, a welder
can create joints in blind areas normally inaccessible for most other welding
processes.
Specially designed assembly fixtures
(jigs) are used to hold components at a
convenient working level and assure
accurate dimensioning. These fixtures
in combination with other assemblyline techniques make SMAW a fast and
efficient method for accurate shop
fabrication.
Since SMAW electrode materials are
available to match the properties of most
ferrous base metals, the welding rode
and the framing member can be a
metallurgical match, resulting in
stronger welds.
The Shielded Metal Arc
Welding (SMAW) Process
In SMAW, an electric welding
machine produces an electrical current
which is conducted through a cable to an
electrode. The electrode is the welding
alloy or “stick,” which conducts electricity. When the electrode is brought
close to the base metal, an arc is produced. This arc is the electrical current
jumping across the gap between the
electrode and the base metal.
The electrical current meets considerable resistance at the gap between
the electrode tip and the base metal.
This build up of resistance results in an
arc of high temperature, ranging between 10,000 and 12,000° F.
This heat melts the base metal and
causes it to form a molten pool. The tip
of the electrode also begins to melt,
forming droplets, which mix or fuse
with the growing molten pool. As the
REQUIRED EQUIPMENT
Figure 1. Both the electrode and the
base metal are connected to different sides
of the power supply.
welder moves the electrode along the
base metal, the electrical current causes
the melting pool to flow away from the
electrode, creating a weld bead on the
base metal. Eventually the bead cools
and solidifies into a homogeneous alloy
of the base metals and the welding
electrode.
Theory of Electrical Current
An understanding of the basic theory
of electrical current is necessary to fully
comprehend the welding process. Electrical current is the rate of flow of electric charges. There are two types: Alternating Current and Direct Current.
Direct Current (DC) flows constantly in one direction, from negative to
positive, or from positive to negative.
This is referred to as polarity. Alternating Current (AC) travels in one direction for a period of time and then
reverses its direction in the circuit for
Construction Dimensions/August 1989
33
an equal period of time. This entire
cycle may be repeated many times per
second. The number of cycles occurring
per second is measured in units called
Hertz (Hz). For example, at 60 Hz (60
cycles per second) the current travels in
one direction for 1/120th of a second,
and then reverses its direction.
Voltage is the pressure that supports
the electric arc across the gap between
the electrode and the work surface.
Power is the rate of transforming,
transferring, or consuming energy. In
welding, power is rate of transferring
electrical energy from the power source
to the base metal.
Current is measured in amperes,
voltage in volts, and power in watts or
in kilowatts (1,000 watts).
Power is computed as the product of
current and voltage.
Current X Voltage = Power
34 August 1989/Construction Dimensions
Therefore, the power of a 300 ampere,
30 volt arc would be computed as
follows:
300 amp. X 30 V = 9000 watts
9000 watts = 9 kilowatts
The arc has a power rating of 9000
watts or 9 kilowatts.
Equipment
A welding machine (also called a
buzz box or hot box) provides the
correct voltage needed to maintain the
arc. As amperage (flow or current) increases, the faster the electrode is consumed. Generally, higher settings are
required for welding heavier gage steel
members.
Historically, stick welding of lightweight galvanized steel framing has
been accomplished with transformer/
rectifier welding units requiring either
single or 3-phase, 230-volt, 50 ampere,
D.C. electrical service. These welding
units can typically furnish a current output range of 30 to 250 amperes at 30
arc volts, with positive or negative
polarity. The welding of 12 to 20 gage
galvanized steel framing is normally
accomplished within the range of 30 to
75 amps or current.
While most early welding machines
used DC, some manufacturers have
developed AC equipment which offers
several advantages over DC. AC equipment usually costs less because it does
not require a rectifier to convert alternating to direct current. And, AC equipment can be used at some distance from
the power source without the resistance
buildup and overheating of cables common with DC equipment.
The primary advantage attributed to
DC welders is a uniform, continuous
flow of current to help maintain a
smooth, stable welding arc.
More recently, manufacturers have
introduced small, portable, automatic
wire feed units which utilize coils of
flux-core wire electrode with welding
machines in field applications; presently, these wire feed units are recommended with DC power sources. The
advantage of automatic wire feed units
is that the entire electrode is consumed, and there is less interruption to the
welder since the “stick” (electrode)
does not have to be replaced frequently
as it is consumed in the welding process.
Electrodes
Electrodes are made of various kinds
of metal wire, and their selection is
dependent on the composition of the
metal to be welded.
The American Welding Society
(AWS) has simplified the electrode
selection process by establishing a
classification system. Numerical codes
identify electrodes made by different
manufacturers which have the same
general welding characteristics.
With DC welding units, the electrode
selected will determine if the welding
unit is set for positive or negative
polarity.
The electrodes used in SMAW range
in size from 1/16 to 5/16 in. diameter,
and from 9 to 18 inches long. As an
electrode is consumed, the welder
replaces the electrode, strikes a new arc,
and continues the welding process.
Electrodes are also available in coils
of flux-core wire ranging in weight from
14 lbs, used in automatic portable wire
feeder units, to coils weighing several
hundred pounds for stationary, mounted
wire feed units used in the shop.
Electrodes are coated with a material
called a flux, which dissolves during
welding. The dissolving flux becomes
either a neutral or reducing gas (such
as carbon dioxide or carbon monoxide)
which surrounds the arc.
The electrode’s flux melts more slowly than the core wire so that the flux
projects slightly beyond the tip of the
electrode. This extension of flux concentrates and directs the arc stream, and
it protects the melting tip and the molten
puddle from the oxygen and nitrogen in
the surrounding air. In addition, the
chemicals in the flux keep the arc stable,
control metal fluidity, and prevent
porosity and the formation of hard spots
in the molten puddle.
As the weld progresses down the sur-
face of the base metal, a coating called
slag forms over the completed weld
bead. Although the slag must be wire
brushed away later, its formation serves
several useful purposes; it aids in fusion, floats impurities to the surface of
the weld beads, and insulates the welded point against the cooling (annealing)
effects of the atmosphere.
Types of Welds
The types of welds typically used for
joining light gage steel framing components to each other are lap joints, fillet
welds, and groove welds.
Lap Joints. Probably the joint most
frequently used in welding is the lap
joint (Figure 2), a joint between two
overlapping components. This joint
doubles the thickness of the metal.
Fillet Welds. Fillet welds are used to
fill in the corners created by the edges
of two steel framing members positioned at right angles to each other.
Figure 3 shows a lap joint with a fillet
weld. The cross section of the weld is
essentially triangular.
Groove Welds. Panelization with
lightweight steel framing employs a
variety of groove welds, including flare
bevel and flare vee groove welds.
is most often used to create a butt joint.
(For more information on selection of
welds, see AISI Section 4.2.1.)
Figure 2. Lap joint.
Figure 3. Lap joint with fillet weld.
Gas Metal Arc
Welding (GMAW)
Gas Metal Arc Welding (GMAW) is
an arc welding process in which a
stream of chemically inert gas — such
as Argon or CO2 — is fed through the
welding gun. The gas (or mixture of
gases) creates a shield which effectively controls the atmosphere. In this way
the gas shield produces much the same
results as the flux in SMAW. The gas
Figure 4. Flare bevel groove weld.
Figure 5. Flare vee-groove weld.
Perhaps the most prevalent weld is the
flare bevel groove weld (Figure 4). It is
similar to a simple fillet weld, except the
ends of the framing members are
beveled.
The flare vee-groove weld (Figure 5)
joins the beveled ends of two studs. It
Figure 6. Shielding gas protects the
molten metal from the atmosphere.
Construction Dimensions/August 1989
35
shield protects the electrode, the weld
pool, the arc and adjacent areas of the
framing member from atmospheric
contamination.
Because the gases used to shield the
electrode are chemically inert, the
GMAW process is often called metal
inert-gas welding (MIG). GMAW is also
referred to as Tungsten Inert Gas
Welding (TIG) when using a tungsten
metal electrode.
GMAW processes are controlled
through the use of three basic pieces of
equipment: (1) the gun; (2) the wire feed
unit; and (3) the power source.
The gun guides the electrode and conducts the electrical current and shielding
gas to the weld. In this way it provides
two important elements: (1) the energy
necessary to create and maintain the arc
and melt the electrode and (2) protection from the ambient atmosphere.
The wire feed unit and power sources
are normally coupled to make selfregulation of the arc length automatic.
This regulation is possible due to a constant voltage power source (flat voltampere curve) and a constant speed wire
feed unit.
Heat input is low, and so weld bead
penetration is very shallow, allowing for
welding out-of-position and on thin
gauge sections.
Figure 7. Basic GMAW equipment.
Automatic maintenance of the arc
length and current level means that
essentially the only manual controls required by the welder for semiautomatic
operation are gun positioning, current
level, guidance, and travel speed.
GMAW-S. Short circuiting mode of
metal transfer, called GMAW-S, is a
lower heat energy variation of the process. In this method, the electrode is put
in contact with the molten puddle on the
work, and the arc is established intermittently. The relationship between the
arc and the short circuiting is controlled
by the power source characteristics.
Construction Dimensions/August 1989
37
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