109MM0439

109MM0439
EFFECT OF HEAT TREATMENT ON
WEAR PROPERTIES OF
PLAIN CARBON STEEL
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
In
Metallurgical & Materials engineering
By
Tusharkanta Daspattanayak (109MM0439)
Kaibalya Mohanty (109MM0118)
Under the Guidance of Prof. A. BASU
Department of Metallurgical & Materials Engineering
National Institute of Technology, Rourkela
2013
I
ACKNOWLEDGEMENT
We wish to express our deep sense of gratitude to Prof. A. Basu, our guide, for
his consistent encouragement, guidance and support to carry out and complete
this project, and for giving us an opportunity to work on this project.
We are sincerely grateful to Prof. B.C Ray, Head of the Department of
Metallurgical and Materials Engineering, NIT-Rourkela, for providing all the
necessary facilities in the department.
We would like to express our deep sense of gratitude to Prof S.K Pratihar, Head
of the Department of Ceramic Engineering, for his timely help and support.
We would be highly obliged to extend our thanks to Mr. Uday Kumar Sahu and
Mr. Rajesh Patnaik, and Mr. Hembram for their immense support and help
rendered while carrying out our experiments, without which the completion of
this project would have been at stake.
We would also like to thank all the staff members of MME Dept., NITR and
everyone who in some way or the other has provided us valuable guidance,
suggestion and help for this project.
Date:
Tusharkanta Daspattanayak (109MM0439)
Kaibalya Mohanty (109MM0118)
Department of Metallurgical & Materials Engineering
National Institute of Technology, Rourkela- 769008
III
Abstract
A study was made of the effect of heat treatment upon the wear resistance of low and high
carbon steels as determined by the ball on a plate wear-testing machine (diamond indenter) under
combined action of rolling and sliding friction under pressures. Total ten samples of the steel (5
samples each from low carbon and high carbon range) were subjected to five different heat
treatments i.e. annealing, normalizing, oil quenching, water delay quenching and water
quenching heated to a temperature of 960˚C. One from each heat treated types was prepared for
microstructural and hardness studies. The hardness of the five different heat treated samples was
measured by Vickers hardness testing machine. Optical microscopies to study the microstructure
and Scanning electron microscopic analysis of wear surface have been done.
Annealed steels gave inferior wear-resisting qualities. Normalized steel showed less wear
resistance in the low-carbon range than the annealed steel, but in the high-carbon range
approached the relatively high wear resistance of the delay hardened steels. Hardened steels
(water quenched and oil quenched) showed a very rapid increase in wear resistance with
increased carbon content. The water delay quenched steel showed lesser wear resistance as
compared to oil quenched samples though the hardness of former is more than the latter.
The scanning electron microscopy images of wear surfaces of steels showed that this particular
type of wear (combined rolling and sliding friction under pressures) was generally accompanied
by severe distortion of the structure and the images demonstrated exactly what was interpreted
from wear-testing results.
A martensitic micro structure was the most favorable for resisting wear of the type studied.
Properly normalized high-carbon steels, which showed a structure of lamellar pearlite, had
almost as good wear resistance as the delay hardened steels. Annealed high-carbon steels,
showing a structure predominately spheroidized cementite, had relatively poor resistance to
wear. Apparently free ferrite or free cementite (as spheroids) is detrimental to the resistance to
wear.
Keywords: plain carbon steel, wear depth, sliding distance, penetration rate, delay quenching
IV
CONTENTS
Certificate
II
Acknowledgement
III
Abstract
IV
List of Figures
VII- VIII
List of Tables
IX
1. INTRODUCTION
1-2
2. LITERATURE SURVEY
3-14
2.1 The Fe-C Phase Diagram
3-4
2.2 Principle of heat treatment
5
2.2.1 Annealing
5-6
2.2.2 Normalizing
6-7
2.2.3 Quenching
7-8
2.2.4 Delay quenching (Special heat treatment process)
8-9
2.3 Wear
9-13
2.3.1 Different types of wear
9
2.3.1.1 Abrasive wear
9-10
2.3.1.2 Fretting wear
11
2.3.1.3 Corrosive wear
11-12
2.3.1.4 Fatigue wear or contact fatigue
2.3.1.5 Adhesive wear
12
12-13
3. EXPERIMENTAL PROCEDURE
15-21
~V~
4. RESULT AND DISCUSSION
22-34
4.1 Hardness measurement
22-23
4.2 Microstructure
24-26
4.5 Wear Behavior
26-31
4.3 Scanning Electron Microscopy
32-34
5. CONCLUSION
35
6. REFERENCES
36
~VI~
LIST OF FIGURES
Figure 2.1
Figure 2.2
The Fe-C equilibrium diagram up to 6.67 wt. % C. Solid lines
indicate Fe-Fe3C diagram
Schematic illustration of process of normalizing and annealing
Figure 2.3 (a)
Low stress abrasive wear
Figure 2.3 (b)
High stress abrasive wear
Figure 2.3 (c)
Gouging
Figure 2.4
Schematic representation of fretting wear
Figure 2.5
Schematic representation of contact fatigue
Figure 2.6 (a)
Schematic representation of adhesive wear
Figure 2.6 (b)
Oxidative wear
Figure 3.1
Abrasive cutting machines
Figure 3.2
Heat treatment furnaces
Figure 3.3
Micro Hardness Testing Machine
Figure 3.4
High resolution optical microscope
Figure 3.5
Wear testing machine
Figure 3.6
JEOL JSM-6480LV scanning electron microscope
Figure 4.1
Variation of hardness with carbon content and heat treatment
Figure 4.2
High carbon annealed sample at 200X
Figure 4.3
Low carbon annealed sample at 200X
Figure 4.4
High carbon normalized sample at 500X
Figure 4.5
High carbon water quenched sample at 500X
Figure 4.6
Low carbon water quenched sample at 500X
~VII~
Figure 4.7 (a)
Wear depth vs. sliding distance of annealed sample
Figure 4.7 (b)
Wear depths vs. sliding distance of normalized sample
Figure 4.7 (c)
Wear depth vs. sliding distance of oil quenched sample
Figure 4.7 (d)
Wear depths vs. sliding distance of water delay quenched sample
Figure 4.7 (e)
Wear depths vs. sliding distance of water quenched sample
Figure 4.8 (f)
Wear depths vs. sliding distance of low carbon steel samples at various
heat treatment operations
Figure 4.8 (g)
Wear depths vs. sliding distance of high carbon steel samples at various
heat treatment operations
Figure 4.9
Graph showing maximum penetration rate vs. heat treatment
Figure 4.10
SEM image of Low carbon annealed steel at 30X
Figure 4.11
SEM image of Low carbon annealed steel at 150X
Figure.4.12
SEM image of High carbon annealed steel at 30X
Figure 4.13
SEM image of High carbon annealed steel at 150X
Figure 4.14
SEM image of Low carbon normalized steel at 30X
Figure 4.15
SEM image of Low carbon normalized steel at 150X
Figure 4.16
SEM image of High carbon normalized steel at 30X
Figure 4.17
SEM image of High carbon normalized steel at 150X
Figure 4.18
SEM image of Low carbon water quenched steel at 30X
Figure 4.19
SEM image of Low carbon water quenched steel at 150X
Figure 4.20
SEM image of High carbon water quenched steel at 30X
Figure 4.21
SEM image of High carbon water quenched steel at 150X
~VIII~
LIST OF TABLES
Table 2.1
Various types of wear, their symptoms & appearance
Table 4.1
Hardness values of samples after different heat treatment
_
~IX~
1. Introduction
Steel, in modern age, has found a vast application in almost every field. India’s 33 per
cent growth in steel production in the last five years was second only to China among the topfive producing nations. Steels represent the most important group of engineering materials as
they have the widest diversity of applications of any of the engineering materials. Generally,
carbon is the most important element profoundly affecting the mechanical properties of the
steels. Increasing the carbon content of steels increases the hardness and strength. More-over,
plain carbon steels have moderate strengths and can resist satisfactorily ordinary temperatures
and atmospheres and also are available in large quantities, in quite large variations of shapes and
sizes with a much lower cost.
In their service periods, steels parts undergo heavy working stresses. As a result wearing
and tearing take place in the body parts. The quality of the steel products depends on the
condition of their surfaces and on surface deterioration due to use. Surface deterioration is also
important in engineering practice; it is often the major factor limiting the life and performance of
machine components. Wear may be defined as the unintentional deterioration resulting from use
or environment. It may be considered essentially a surface phenomenon. Wear is one of the most
destructive influences to which metal are exposed, and the importance of wear resistance needs
no amplification. The displacement and detachment of metallic particles from a metallic surface
may be caused by contact with (1) another metal (adhesive or metallic wear), (2) a metallic or a
nonmetallic abrasive (abrasion), or (3) moving liquids or gases (erosion). The above three types
of wears may be subdivided into wear under rolling friction or sliding friction and, further,
according to whether lubrication can or can’t be used [1].
~1~
Different heat treatment processes are adopted for achieving improved mechanical
properties like hardness, ductility; wear resistance etc. studies reveal that steels having higher
hardness values show a better resistance to wear. This is in accordance with Archard’s equation
i.e.
Q= KWL/ H
Where, Q= total volume of wear debris produced,
and H= hardness of softest contacting surface.
~2~
W= total normal load, L= sliding distance
2. Literature survey
A STEEL is usually defined as an alloy of iron and carbon with the carbon content between a
few hundreds of a percent up to about 2 wt.%. Other alloying elements can amount in total to
about 5 wt. % in low-alloy steels and higher in more highly alloyed steels such as tool steels and
stainless steels. Steels can exhibit a wide variety of properties depending on composition as well
as the phases and micro constituents present, which in turn depend on the heat treatment [3].
2.1 The Fe-C Phase Diagram
The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig. 1).
Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the
metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop,
especially in the low-temperature and low-carbon range, and therefore the metastable diagram is
of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or
metastable equilibrium) for different combinations of carbon concentration and temperature. We
distinguish at the low-carbon end ferrite (α-iron), which can at most dissolve 0.028 wt.% C at
727°C and austenite (γ-iron), which can dissolve 2.11 wt.% C at 1148 °C . At the carbon-rich
side we find cementite (Fe3C) of less interest, except for highly alloyed steels, is the δ-ferrite
existing at the highest temperatures [2].
Between the single-phase fields are found regions with mixtures of two phases, such as ferrite +
cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid
phase field can be found and below this are the two phase fields liquid + austenite, liquid +
cementite, and liquid + δ-ferrite.Some important boundaries at single-phase fields include: A1,
the so-called eutectoid temperature, which is the minimum temperature for austeniteA3, the
lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α
boundary· Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C
boundary. If alloying elements are added to the iron-carbon alloy (steel), the position of the A 1,
A3, and Acm boundaries and the eutectoid composition are changed [2].
~3~
Fig. 2.1 The Fe-C equilibrium diagram up to 6.67 wt. % C. Solid lines indicate Fe-Fe3C diagram;
dashed lines indicate iron-graphite diagram [2].
~4~
2.2 Principle of heat treatment
A combination of heating, soaking and cooling operationstimed and applied to a metal, or alloy
in the solid state in a way that will produce desired properties. These properties are dependent on
microstructure of the alloy, i.e. the nature, shape, size, distribution and amount of phases, which
are controlled by changes in the alloy constituents, and mainly by heat treatment. Different heat
treatments bring about a very wide range and distribution of phases [3].
A heat treatment process consists of:
(1)
(2)
(3)
Heating the steel to a high predetermined temperature, called
austenitization temperature.
Soaking for some period of time to obtain a homogeneous austenite
throughout the cross section of the steel part.
Cooling to room temperature at a particular cooling rate which depends
on the properties to be developed in the steel.
There are different kinds of heat treatment processes. The most common ones are:
(1) Annealing
(2) Normalizing
(3) Quenching
2.2.1 Annealing
ANNEALING is a generic term denoting a treatment that consists of heating to and holding at a
suitable temperature followed by cooling at an appropriate rate(normally very slow cooling),
primarily for the softening of metallic materials [2]. Generally, in plain carbon steels, annealing
produces a ferrite-pearlite microstructure. Annealing of a metal is performed generally for the
following purposes [9]:





Removal of internal stresses
Homogenization of structure
facilitate cold working or machining
improve mechanical or electrical properties
promote dimensional stability
In this project, we have adopted full annealing operation i.e. heating the steel to a temperature
above the upper critical temperature, soaking there for sufficient time to obtain homogeneous
austenite and then cooled in the furnace by switching it off. The austenitizing temperature for full
annealing is a function of carbon content of the steel. The relation is as follows:
For hypo eutectoid/eutectoid steels:
~5~
A.T= A3 + (20-40̊ C)
For hypereutectoid steel:
A.T= A1 + (20-40̊ C)
Where, A.T= austenitizing temperature
A1= lower critical temperature
A3= upper critical temperature
Three important factors that should be taken into prior consideration during full annealing are:



proper austenitizing temperature
soaking time
very slow cooling through A1
2.2.2 Normalizing
Normalizing is the process ofheating the steel above the upper critical temperature to attain
homogeneous single phase austenitic region, soaking at that temperature for a particular period
of time and allowing to cool freely in air. Typically, the work is heated to a temperature above
the upper critical line of the iron-iron carbide phase diagram, that is, above A 3 for hypo eutectoid
steels and above Acm for hypereutectoid steels. The austenitic temperature ranges for steels
depending on the carbon content are as follows:
Tnormalizing
====== ==
A3 + (40-60̊ C) for hypo eutectoid/eutectoid steels
=
Acm + (30-50̊ C) for hypereutectoid steels
Normalizing operation of steel is performed to achieve the following objectives:






to refine the coarse grains of the steel castings
to improve mechanical properties of plain carbon steel
to eliminate or reduce microstructural irregularities
to increase machinability of low carbon steels
to eliminate or break coarse cementite network in hypereutectoid steels
general refinement of structure prior to quenching operation
It differs from annealing in two important process variables:

Higher austenitizing temperature than annealing to attain more homogenization, so that it
could be super cooled much below A1 temperature before transformation takes place [3].
~6~

Air cooling being faster than furnace cooling helps austenite supercool much below A 1
temperature before transformation occurs.
Fig.2.2 Schematic illustration of process of
normalizing and annealing[2]
The resulting microstructure of normalizing operation consists of:



Finer proeutectoid ferrite grains
Much finer perlite
Low amount of proeutectoid ferrite than that in annealed structure.
2.2.3 Quenching
It is the process of heating the steel to proper austenitizing temperature, soaking at this
temperature to get a fine grained and homogeneous austenite, and then cooling the steel at a rate
faster than its critical cooling rate. Parent austenite thus transforms into martensite and the steel
becomes very hard.Successful quenching/hardening usually means achieving the required
~7~
microstructure, hardness, strength, or toughness while minimizing residual stress, distortion, and
the possibility of cracking.
The selection of a quenchant medium depends on the hardenability of the particular alloy, the
section thickness and shape involved, and the cooling rates needed to achieve the desired
microstructure. The most common quenchant media are either liquids or gases. The liquid
quenchants commonly used include:





Oil that may contain a variety of additives
Water
Aqueous polymer solutions
Water that may contain salt or caustic additives
Gaseous quenchants like helium, argon, and nitrogen.(These quenchants
are sometimes used after austenitizing in a vacuum.)
Quenching is performed for achieving following objectives:





Main aim of hardening tools is to induce high hardness and subsequently
improve their cutting ability.
Most of the machine parts and all the tools are hardened to improve their wear
resistance.
The main objective of hardening machine components made of structural steels is
to develop high yield strength with good toughness and ductility to bear higher
working stresses.
Stainless and high-alloy steels may be
Quenched to minimize the presence of grain boundary carbides or to improve the
ferrite distribution [3].
The austenitizing temperature of plain carbon steels depends on the carbon content of the
steel and is generalized as:
Tquenching= A3 + (20-40̊ C) for hypoeutectoid steels
A1 + (20-40̊ C) for hypereutectoid/eutectoid steels
2.2.4 Delay quenching (Special heat treatment process)
Delay quenching though is not a standard and required heat treatment process in industry but can
be useful to study the TTT diagram and effect of any accidental possible delay in actual practice.
~8~
Austenitized steel is taken out of the hardening salt bath held for some time so as to make the
temperature fall, and then quenched in suitable medium. This operation results in minimum
distortion as the temperature of quenching is low. High speed steels and case hardening steels are
often given this treatment.
2.3 Wear
Wear is erosion or sideways displacement of material from its "derivative" and original position
on a solid surfaceperformed by the action of another surface.Wear is related to interactions
between surfaces and more specifically the removal and deformation of material on a surface as
a result of mechanical action of the opposite surface. The need for relative motion between two
surfaces and initial mechanical contact between asperities is an important distinction between
mechanical wear compared to other processes with similar outcomes.The definition of wear may
include loss of dimension from plastic deformation if it is originated at the interface between two
slidingsurfaces.
2.3.1
Different types of wear
2.3.1.1 Abrasive wear
It involves ploughing of localized surface contacts through a softer mated material [5]. Nonmetallic particles usually cause abrasion but metallic particles can also cause abrasion. A
material is seriously abraded or scratched if the particle is harder than itself. Abrasive wear can
be divided into three different types: low stress (Fig. 2.3(a)), high stress (Fig. 2.3(b)) and
gouging (Fig. 2.3(c)). Low-stress abrasion or scratching can be defined as wear that involves a
relatively light rubbing contact of abrasive particles with the metal. The subsurface deformation
is minimal and shows scratches, hence the surface does not work harden.
High stress abrasion involves wear under stress, high enough to crush the abrasive and
hence more strain hardening occurs. Examples are abrasion by rolling-contact bearings, gears,
cams and pivots. Gouging abrasion is the high-stress abrasion that results in sizable grooves or
gouges on the worn surface. In this type of wear, strain hardening and deformation are the
~9~
dominant factors. This form of wear occurs on parts of crusher liners, impact hammers in
pulverizers, etc.
Factors affecting abrasion resistance are: microstructure, hardness, and carbon content
(for steel), e.g. for low stress abrasion, hard martensitic structure is preferable. Martensitic
stainless steel is used for its high hardness where high-stress abrasion occurs.
(a)
(b)
(c)
Fig. 2.3 (a) Low stress abrasive wear, (b) high stress abrasive wear, (c) gouging
~10~
2.3.1.2 Fretting wear
The process involves wear caused by the small amplitude vibrations (Fig. 2.4) at
mechanical connections e.g. at riveted joints [5].This is caused by the combined effects of
oxidation and abrasive wear. Two oscillating metallic surfaces produce tiny metallic
fragments that oxidize and become abrasive. Subsequently, wear progresses by mild
adhesive wear in combination with abrasive wear. Factors that influence fretting wear are
contact conditions, environmental conditions and material properties. Fretting is
influenced by parameters such as load frequency, amplitude of fretting motion, numbers
of fretting cycles, relative humidity and temperature [4].
Fig. 2.4 Schematic representation of fretting wear
2.3.1.3 Corrosive wear
It involves wear under the influence of corrosive reagent [5]. Corrosion is the process of
electrochemical removal of material form surface by dissolution whereas wear involves the
removal of material due to physical interaction between the two surfaces. These two processes
affect synergistically to accelerate the materials removal from its surface and termed as corrosive
wear. This type of wear involves the disruption and removal of the oxide film, leading to
exposure of active metal surface to the environment, dissolution or re-passivation of the exposed
metal surface, elastic field interaction at asperities in contact with the environment and the
interaction between the plastically deformed areas and its environment. In mining industries,
abrasive wear is aggravated by a wet corrosive environment. Carbon steel readily forms iron
~11~
oxide, which is removed by sliding and bumping of moving coal/ore causing continuous loss of
metal thickness. Stainless steel for its resistance to removal of oxide layer on its surface is
desirable for the application in corrosive wear environment.
2.3.1.4 Fatigue wear or contact fatigue
It occurs when a surface is stressed in a cyclic manner (Fig. 2.5). This type of wear can be found
in parts subjected to rolling contact, such as ball bearings and gears. Surface conditions, such as
finish, residual stress, hardness and microstructure play a role in determining the extent of
fatigue wear. Surface treatments e.g. nitriding, carburizing and shot peening which increase
surface hardness and improve residual stress distribution in turn could enhance resistance to
fatigue wear [5].
Fig. 2.5 Schematic representation of contact fatigue
2.3.1.5 Adhesive wear
This type of wear occurs due to sliding of two metallic components against each other under an
applied load where no abrasives are present (Fig. 2.6(a)). A strong metallic bond forms between
surface asperities of the contacting materials and hence the name ‘adhesive’ [10]. In case of
stainless steel, there is a thin oxide layer on the surface, which prevents the formation of metallic
bonding in between the asperities on the sliding surfaces. Hence, wear rates are low at low load.
~12~
This type of adhesive wear is known as mild wear or oxidative wear (Fig. 2.6(b)). When the load
applied on the surface is high, metallic bond forms between the surface asperities and the
resulting wear rates are high. This kind of wear is more prevalent where a lubricant cannot be
used e.g. chain-link conveyer belts, fasteners and sliding components in a valve. For stainless
steel, hardness affects adhesive wear resistance. For martensitic alloys, a minimum hardness of
53 HRC is required for excellent wear resistance. In austenitic stainless steel, the work hardened
hardness is critical due to the presence of the alloying additions that increase the stability of
oxide film. These factors increase the transition load required for severe corrosion to occur.
(a)
(b)
Fig. 2.6 Schematic representation of (a) adhesive wear, (b) oxidative wear
~13~
The various types of wear, there symptoms and appearance of the worn out surfaces are given
below.
Table 2.1 various types of wear, their symptoms & appearance [6].
~14~
3. Experimental procedure
As received samples of high and low carbon steel bars were cut into 10 mm thickness samples.
Around 10 samples (5 samples each from both the samples) were cut with the electric cutting
machine. Water was provided frequently while cutting in order to prevent it from over burning as
over burnt steel may make the heat treatment ineffective as grain refinement & other mechanical
property might not improve albeit of performing any kind of heat treatment for any number of
times. Electric cutting machine has a disc shaped sic plate which rotates at high speed to cut
through the body of the bar. Precaution should be taken in order to get accurate measurement as
high speed may displace the bar quite a bit; hence the samples may not be flat as well as might
not be usable. Safety while performing cutting is vital as burnt metal particles may come into
your eyes. Figure below shows the abrasive cutting machine.
Fig. 3.1 abrasive cutting machine
~15~
Then the samples were taken for different kinds of heat treatment operation. It was decided that 5
different kinds of heat treatment operation were to be adopted namely: - (1) Annealing
(2) Normalizing
(3) Oil quenching
(4) Water quenching
(5) Water delay
quenching.
The samples were tied with steel wires having their top portion a bit curvilinear (umbrella stick
shaped) in order to easy removal of samples from furnace after heat treatment as after heating to
high temperature you never expect your hand to do the removal operation. Then the tied samples
were placed inside a rectangular steel box fraught with charcoal. Charcoal protects the samples
from oxidation as steel samples are vulnerable to oxidation at such high temperature. Carburizing
is neglected at this temperature as holding time is less for diffusion to occur effectively on the
surface. But in reality a bit of carburization might occur over the surface.
Then the samples with their designation (as after removal there won’t be any confusion) were
place inside a ceramic lined rectangular heating furnace of maximum heating capacity 1500 0 C.
The temperature was set to 9600C. Actually it was decided to adopt a less temperature than this
but the furnace had certain fluctuations and to avoid any possible inter-critical range
austenitization, it was set at 9600 C. Even though full annealing range for high carbon steel is
way less than what we have adopted, it was decided to study the effect of same austenitizing
temperature affect also. A rate of heating of 150 C/min was set.
After the furnace reached that temperature 1 hour holding was done at that temperature. Then the
furnace was opened after making it switched off. To be annealed samples were put as it is inside
it so as to have furnace cooling i.e. slow cooling. To be normalized samples were put in air. For
water quenching, samples were put in water. For oil quenching, samples were put in oil. For
~16~
delay quenching, the sample was kept 30 seconds in the air before quenching in the water. Figure
below shows the rectangular ceramic lined furnace in which heat treatment was done.
Fig. 3.2 Heat treatment furnace
After that the samples were taken for grinding and polishing. First the samples were grinded in a
grinder in order to make the surface flat as well as to make it eligible for polishing. Then the
samples were polished by 1/0, 2/0. 3/0, 4/0 emery papers before these were being taken for
hardness measurement.
Vickers hardness as well as micro hardness was taken but accuracy of micro hardness was quite
good to be considered. The model of the instrument used for finding out the micro hardness was
LECO-LV700. Hardness of all the samples were measured in Vickers scale under a load of 50gf
and dwell time of 15sec & noted down. The Vickers hardness of the samples was compared by
plotting a graph between the hardness values and respective heat treatment performed.
~17~
Fig.3.3 Micro Hardness Testing Machine
Then the samples were taken for cloth and diamond polishing. After fine polishing the etching of
samples were done using Nital (98% ethyl alcohol& 2% HNO 3). Then the samples were taken
for microstructural observation through high resolution optical microscopy. Microstructures were
taken at 500X.
~18~
Fig. 3.4 High resolution optical microscope
Then the wear testing of all the samples was carried out. Wear testing in terms of sliding wear
resistance of all the samples was evaluated using a ball on disc type wear testing instrument
(DUCOM TR-208-M1) having a diamond indenter. Tests were carried out with an applied load
of 2kg, 2 mm wear track diameter, 20 rpm rotating speed and 15 min time for the wear test. The
results for the wear depth were obtained and the sliding distances were calculated by using the
values of the wear depth and the process parameters. Graphs were plotted between the wear
depth and sliding distance for different heat treatment operations as well as for different carbon
content samples having same heat treated. These graphs were analyzed.
~19~
Fig.3.5 Wear testing machine
~20~
Finally the wear surfaces were taken for SEM analysis.The instrument model used for the SEM
analysis was JEOL JSM-6480LV.A potential of 20KV was used for the scanning electron
microscope and the magnification used for the samples were 30X & 150X respectively.The SEM
images obtained for the wear surfaces of different samples were analyzed.
Fig.3.6 JEOL JSM-6480LV scanning electron microscope
~21~
4. Results and discussions
4.1 Hardness measurement
During micro hardness testing average of five individual reading on a particular sample was
taken and reported in Table 1.
Table 4.1 Hardness values of samples after different heat treatment
Heat treatment processes
Hardness values(VHN)
Low carbon steel samples
High carbon steel samples
Annealed
207.03
Normalized
270.06
678.50
735.66
796.46
Oil quenched
723.76
786.16
Water quenched
763.03
812.96
Water delay quenched
283.20
To get a graphical easy understanding of the same bar chart of hardness values are shown in Fig.
4.1
Hardness (VHN)
Hardness vs. Heat treatment
900
800
700
600
500
400
300
200
100
0
Fig 4.1 variation of hardness with carbon content and heat treatment
~22~
The graph shows that high carbon steel samples have higher hardness values as compared
to the low carbon steel samples. Annealing and normalized high carbon steel samples have
higher hardness compared to its low carbon counterpart. This is because high carbon steel
samples have higher cementite phase as compared to its low carbon counterpart. High carbon
quenched samples have higher hardness than its low carbon counterpart because super saturation
increases with increase in carbon content which has a vital role in significant difference in
hardness.
The order of hardness values of the different heat treated samples is as follows:
Annealed < normalized < oil quenched < water delay quenched < water quenched
Normalized sample has higher hardness than that of annealed sample because normalizing
involves higher under cooling resulting in finer structure than that of annealed structure. Thus
finer structure imparts higher hardness. Secondly, normalizing allows lesser time for
proeutectoid phase (ferrite or cementite) formation; consequently more amount of pearlite is
formed which imparts higher hardness.
Quenched samples have higher hardness than normalized samples due to formation of hard
martensite. Water being more severe quenching medium than oil, hence, water quenched
samples have higher hardness than oil quenched samples as faster cooling rate (water) generates
finer martensite than slow cooling rate (oil). Again, faster the cooling rate there will be less
chance of formation of pearlite or bainite which has less hardness as compared to martensite.
Water delay quenched samples have higher hardness than oil quenched samples as per data
obtained. The reason might be the formation of less amount of martainsite in oil quenched
samples as compared to delay quenched samples.
~23~
4.2 Microstructure
Optical microscopies of some samples are given below.
Fig 4.2 High carbon annealed sample at 200X
As indicated in the figure black phases are cementite which appears as networks throughout the
surface because of high temperature used instead of full annealing temperature range for high
carbon steels. Other phases are not visible but the colored marks might be due to burning.
Fig 4.3 low carbon annealed sample at 200X
~24~
Fig 4.4 High carbon normalized sample at 500X
Fig 4.5 High carbon water quenched sample at 500X
~25~
Fig 4.6 Low carbon water quenched sample at 500X
In general all the low and high carbon quenched (water and oil) samples show presence of
martensite and for obvious reason the amount is negligible in low carbon steel.
4.3 Wear Behavior
The data collected from wear test of all the samples were converted to graphs between the wear
depth and the sliding distance. The plots related to wear are shown below:
(a)
~26~
(b)
(c)
~27~
(d)
(e)
Fig. 4.7 (a) – (e): Variation of wear depth with sliding distance for low as well as high carbon
steels at different heat treatment operations
The above graphs show that high carbon steels are more wear resistant than low carbon steels in
all heat treatment operations. As hardness and wear resistance are synonymous in general, so,
steel samples having higher hardness (i.e. high carbon steel samples) have high wear resistant as
compared to steel samples having lower hardness.
~28~
Figures below show the variation of wear depth vs. sliding distance of same samples but
differently heat treated.
(f)
(g)
~29~
Fig 4.8 (f) & (g) graph showing variation of wear depth vs. sliding distance at different heat
treatment operations.
The carbon content apparently had little effect upon the wear of annealed steels. Rather
surprisingly, the wear of the low-carbon normalized steel was greater than that of the annealed
steel ofsimilar carbon content, but with increasing carbon content the wear rate of the normalized
steel decreased rapidly, approaching, in thehigh-carbon range, the relatively high wear resistance
of the delay hardened steel. Generally speaking, it is true that the harder the material, the better is
its wear resistance with the exception of the low-carbon normalized steel sample which showed a
higher wear resistance than the annealed steels. Low carbon annealed sample shows the softest
condition in the entire study as per the hardness value but as per the wear test is different due to
excessive ductility of the sample. High ductility shows pronounced adhesive wear mechanism,
i.e. the wear debris may re-weld on the track again leading to low wear depth result. Thus it
shows better wear resistant than the normalized sample in the graph.
From the study (data and the microstructures), it is evident that a martensitic structure is the most
favorable for resisting wear. Although both the annealed and the normalized steel in the highcarbon range are relatively soft as compared with the hardened steels, their resistance to wear
varies widely. The normalized high-carbon steel shows a resistance to wear almost as great as
that of the delay hardened high-carbon steels, while the annealed high carbon steel shows a
relatively poor resistance to wear.
The micro structure of these normalized and annealed high-carbon steel vary so much. They
might be divided into two classes. The micro structure of the normalized steel is of lamellar
pearlite; whereas that of the annealed steel is mostly spheroidized cementite and ferrite. This
spheroidization was the result of the relatively slow cooling undergone by the steels during the
annealingtreatment. According to Lehman [7], lamellar pearlite is quite resistant to wear as he
found in making wear tests on cast ironthat the main factor which governed the resistance to
wear of cast iron was the percentage of pearlite.
Evidently the presence of either free ferrite or free cementite (as spheroids) is detrimental to the
wear resistance of normalized carbon steel. That free ferrite is distinctly harmful in low-carbon
steels, all of which exhibited low resistance to wear [8].
As previously mentioned, the normalized low-carbon steel showed slightly less resistance to
wear as the same steel when annealed. The pearlite in the annealed steels was surrounded by
~30~
cementite networks. The normalized steel did not show this feature in itsmicrostructure. These
networks, being rather continuous and much stronger than the surrounding ferrite, are
undoubtedly responsible for the wear resistance of the low-carbon annealed steelbeing greater
than that of the same steel when normalized.
0.8
penetration rate(µm/sec)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Fig.4.9 graph showing maximum penetration rate vs. heat treatment
During the wear study of maximum depth of penetration, penetration rate was calculated
(dept/time) and is shown in Fig. 4.9. This also gives a comprehensive idea of the wear study and
shows similar trend as discussed from earlier figures. As explained above high carbon water
quenched sample shows least penetration rate & low carbon normalized steel shows highest
penetration rate.
~31~
4.4 Scanning Electron Microscopy
The worn surfaces obtained after wear tests were studied under SEM and shown below.
Fig.4.10 Low carbon annealed steel at 30X Fig.4.11 Low carbon annealed steel at 150X
Fig.4.12 High carbon annealed steel at 30X Fig.4.13 High carbon annealed steel at 150X
Above SEM images demonstrate clearly that high carbon annealed steel samples have
higher wear resistance than low carbon steel sample as evident from the width of track. In case of
low carbon it’s more irregular and distorted and the width of track is more. Hence more material
has lost in case of low carbon steel samples.
~32~
Fig.4.14 Low carbon normalized steel at 30X Fig.4.15 Low carbon normalized steel at 150X
As explained in the wear graphs low carbon normalized steel samples have least resistant to wear
as evident from the width and distortion of wear track.
Fig.4.16 High carbon normalized steel at 30X Fig.4.17 High carbon normalized steel at 150X
~33~
Fig.4.18 Low carbon water quenched steel
Fig.4.19 Low carbon water quenched steel
at 30X
Fig.4.20 High carbon water quenched steel
at 150X
Fig. 4.21 High carbon water quenched steel
at 30X
at 150X
From the wear images above it can be seen that there is more rubbing activity and depth of
penetration in case of low carbon quenched steel. Hence abrasive wear is prominent here as
compared to high carbon quenched steel.
High carbon normalized and in general all the quenched samples show abrasive wear mechanism
as in the wear track we can find more scratching and rubbing instead of re-welding.
~34~
5. Conclusion
The resistance to wear of carbon steels was studied by means of the diamond indenter on a ballon-plate wear testing machine in which the material is subjected to combined rolling and sliding
friction underpressures. Annealed carbon steels gave relatively poor wear-resisting qualities for
all carbon contents. Normalized steel showed low wear resistance in the low-carbon range, but
had greatly improved wear resistance in the high-carbon range. Low carbon annealed sample
shows the softest condition in the entire study as per the hardness value but as per the wear test is
different due to excessive ductility of the sample. High ductility shows pronounced adhesive
wear mechanism, i.e. the wear debris may re-weld on the track again leading to low wear depth
result. Thus it shows better wear resistant than the normalized sample. The hardened steel
showed low wear-resistance in the lowest carbon ranges, but as the carbon content increased the
wear resistance increased. The water delay quenched steel showed lesser wear resistance as
compared to oil quenched samples though the hardness of former is more than the latter. A
martensitic micro structurewas the most favorable for resisting wear of the type studied. Properly
normalized high-carbon steels, which showed a structure of lamellar pearlite, had almost as good
wear resistance as the delay hardened steels. Annealed high-carbon steels, if shows a micro
structure predominately spheroidized cementite, has relatively poor resistance to wear.
Apparently free ferrite or free cementites (as spheroids) are detrimental to the resistance to wear.
~35~
6. References
1] Avner Sidney H., Introduction to Physical metallurgy, New York, Tata Mcgraw-Hill, 1997
2] ASM Handbook, Heat treating, volume 4, U.S.A. (1991), pp. 85-87, pp. 102-104
3] Singh V., Physical Metallurgy. Delhi, standard publisher, Oct.1999
4] Standard Terminology Relating to Wear and Erosion, Annual Book of Standards, Volume
03.02, ASTM, 1987, pp. 243-250
5] ASM Handbook, U.S.A. (1992).volume 18, Friction, Lubrication, and Wear Technology,
pp.337-340, 462-468, 436-442
6] Behera Balaram B.K & Prasad Neeraj V, Erosion wear of materials, National Institute of
Technology, Rourkela, 2007: pp. 14
7] H. Lehman, January 13, 1927, Wear Tests on Cast Iron, Foundry Trade, pp. 36,; pp. 35.
8] Rosenberg Samuel J. Bureau of Standards Journal of Research, vol.7, April 29, 1931, The
Resistance to wear of carbon steels, pp. 428
9] B.R. Banerjee, Annealing Heat Treatments, Met. Prog., Nov 1980, p 59
10] A.W.J. DeGee and J.H. Zaat, Wear, Vol 5, 1962, p 257
~36~
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