Heat treatment technology
1.
Heat treatment of steels:
1.1
Define Heat Treatment.
1.2
State the objectives & purpose of Heat Treatment.
1.3
List the Heat treatment processes.
1.4
Know about the phase transformations on heating of steels.
1.5
Know about the decomposition of austenite on cooling.
1.6
Define isothermal transformation.
1.7
Explain the construction of Time Temperature Transformation Diagrams.
1.8
List out the factors affecting position of TTT curve.
1.9
Define Critical Cooling rate.
1.10 Explain the superimposition of continuous cooling curves.
1.11 Define a) Pearlite
b) Bainite & c) Martensite
1.12 Explain the mechanism of austenite to
a.
Pearlitie
b.
Bainitie
c.
Martensitic transformation.
1.13
State the effect of carbon content on Maximum hardness.
1.14 Define and state the purpose of
a) Annealing
b) Normalizing
c) Hardening
1.15 Mention the Annealing, Normalizing and Hardening temperatures of
eutectiod, hypo eutectiod, hyper eutectiod steels.
1.16 Explain
a) Annealing
b) Normalizing
c) Hardening processes.
1.17
List out the types of annealing.
1.18
Draw the microstructures at various temperatures of annealing
1.19
Define & state the purpose of spheroidizing annealing
1.20
Explain the spheroidizing process.
1.21 Draw the spheroidized structure.
1.22 Draw the normalized microstructure.
1.23
Compare annealing & normalizing.
1.24
Mention the stages of quenching.
1.25
Define & state the purpose of Tempering
1.26
Explain the Tempering process.
1.27
Describe the theory of Tempering.
.
1.28
State the effect of Tempering on microstructure and mechanical properties.
1.29 Define retained austenite.
1.30 Define Temper Embrittlement and state the causes and remedies..
2. Hardenability:
2.1 Define hardenability
2.2 Differentiate Hardness and Hardenability.
2.3 State different Quenching Media.
2.4 Define the cooling power of Quenching Media.
2.5 List out merits & demerits of each quenching medium.
2.6
Define critical diameter & ideal critical diameter.
1
2.7
2.8
Draw the relationship between critical diameter & ideal critical diameter.
Determine the relationship between ideal critical diameter, chemical
composition & hardenability.
2.9
Explain the Jominey end – quench test .
2.10
State the use of Jominey end quench test
3.
3.1
3.2
3.3
3.4
Grain size
Define grain size.
Determine the grain size by carburising method.
Determine the grain size by ASTM comparative Method..
State the measurement of Austenite grain size by Heyn’s intercept method
and Jefferies planimetric method.
4
Heat Treatment of Alloy steels:
4.1
State the purpose of alloying.
4.2
Mention the effect of alloying elements on the Fe-Carbide diagram &
hardenability.
4.3 Classify the alloying elements which tend to
a.
form carbides.
b.
graphitise the carbides
c.
stabilize the austenite
d.
stabilise the Ferrite
4.4 Give the composition, properties & applications of
a.
Nickel steels.
b.
Chromium steels
c.
Manganese steels
4.5
Define stainless steels.
4.6
Classify the stainless steels.
4.7 State the composition, properties and applications of
a.
Ferritic
b.
Austenitic
c.
Martensitic
stainless steels.
4.8 State the heat treatment process for ferritic, austenitic and matensitic stainless
steels.
4.9
Know about sensitisation & stabilisation.
4.10
List the causes and remedies of sensitisation.
4.11 Know about composition, properties and applications of Maraging
steels.
4.12
Define tool steels.
4.13 Give the composition of
a) Water hardening steels b) High-speed tool steels.
4.14 List the properties of high-speed tool steels.
4.15 Give the heat treatment procedure for tool steels.
4.16 Explain the heat treatment cycle for high-speed steels.
4.17 Define secondary hardening.
4.18
Know the Special cutting materials.
4.19
Know the chemical composition & properties of stellite.
4.20
Know the chemical composition, microstructure & properties of cemented
carbides.
2
5.0
5.1
5.2
5.3
5.11
Special heat treatment techniques:
Define case hardening.
Know the case hardening techniques.
Define
a) carburising
b) Nitriding
c) Cyaniding
d) Flame Hardening
e) Induction Hardening
5.4 Sate the types of carburising Techniques.
5.5 Explain
a) Pack carburising,
b) Liquid carburising
c) Gas carburising.
5.6
Enumerate the advantages & disadvantages of carburising.
5.7
List the applications of carburised steels
5.8
Draw the microstructure of carburised steel.
5.9
Explain the post carburising heat treatment.
5.10 Explain the nitriding process.
Know the effect of nitriding on properties of steels.
5.12
State the applications of nitriding.
5.13
Know about white layer.
5.14 Explain the cyaniding process.
5.15 Understand the composition of cyanide bath.
5.16 Enumerate advantages of cyaniding.
5.17 Explain the flame hardening process.
5.18 Explain the induction hardening process.
5.19 Define Austempering & Martempering
5.20 Explain the austempering process.
5.21 Mention the effect of austempering on properties of steels.
5.22 Explain the martempering process.
5.23 Mention the effect of martempering on properties of steel.
5.24 Know about defects in heat treated products.
5.25 Mention the causes and remedies of low hardness, soft spots, oxidation,
decarburisation, overheating, burning, quench cracks, distortion and warping.
5.26 Know about Widmanstatten structure.
6.0
Heat Treatment furnaces
6.1
Know about Heat treatment furnaces:
6.2 Classify the heat-treating furnaces based on design and their use
6.3
Explain the working principle of the following furnaces with a neat
sketch – Muffle, Salt bath, Bogie type, pit & rotary hearth.
7.0
7.1
7.2
7.3
7.4
3
Furnace atmospheres
Define the controlled atmosphere.
Know about the necessity of maintaining controlled atmosphere.
Know about the Exothermic and Endothermic atmosphere.
State the methods of controlling Furnace atmosphere.
8.0
8.1
8.2
8.3
8.4
8.5
8.6
Heat treatment of Non-ferrous Metals:
Know about the Heat treatment of Al-Cu alloy.
Explain solution treatment with the help of Al – Cu phase diagram
Explain precipitation hardening.
Explain the theory of Age-hardening.
List the Al – alloys respond to precipitation hardening
State the Heat treatment process for Brass and Bronzes
COURSE CONTENTS:
1.0
Heat Treatment of Plain Carbon Steels :
Introduction, Transformation of Austenite to pearlite, bainite, martensite,
construction of TTT curves and their applications, types of heat treatment
processes- Annealing, Normalising, Hardening and Tempering, Temper
embrittlement.
2.0
Hardenability :
Definition, difference between hardness and harden ability, critical & ideal
critical diameter, calculation of ideal critical diameter, jominey end quench test
process and its uses,.
3.0
Grain Size:
Effect of grain size on properties of steel, Grain size determination by.
Carburising method, ASTM Comparative method heyn’s intercept method,
Jefferies planimetric method,.
4.0
5.0
6.0
7.0
8.0
Alloy Steels & their Heat treatment:
Definition, Purpose of alloying, Effect of alloying elements on Fe – C diagram.
Classification of alloying elements, Nickel steels, Manganese steels, Chromium
steels, Stainless steels, Tool steels heat treatment, properties and uses.
Maraging steels.
Special heat – treatment techniques and defects in heat treated
products : Introduction, Chemical heat treatments -Carburising, Nitriding,
Cyaniding,. Surface hardening treatments-Flame and Induction hardening.
Austempering, Martempering and sub-zero treatment. Defects in the Heat
treatment.
Heat treatment Furnaces:
Classification according to design and use, Continuous and Batch furnaces.
Furnace Atmospheres:
Introduction, definition, need of controlled atmosphere, Types of furnace
atmospheres – Exothermic and Endothermic atmosphere, Method of controlling
of controlled atmospheres.
Heat treatment of Non-ferrous Metals:
Introduction, Precipitation hardening of Al-Cu alloys, Heat treatment of Brasses
and Bronzes.
REFERENCE BOOKS:
4
1. Heat Treatment Principles and Techniques - by T.V. Rajan Sarma, Ashok Sharma.
2. Engineeeering metallurgy Part I - by Raymond Higgins
3. Metallurgy for Engineers
- by E.C. Rollason ELBS.
4. Physical metallurgy and Heat Treatment
- by Lakhtin
5. Introduction to physical metallurgy
- Sydney H. Avener
6. Physical metallurgy for engineers
- Clark & Warney
7. Materials science and Metallurgy
- kodigere
Heat treatment of Plain Carbon Steels
1.1
Define Heat Treatment.
i) Definition of Heat Treatment: It is defined as a sequence of heating and cooling
operations timely applied to metals and alloys to get required mechanical
properties. The heat treatment involves modification of the microstructure of
metals and alloys.
ii) Steps involved in the heat treatment process
a) Heating the material to a specific temperature.
b) Holding the material at this temperature for a definite period, called soaking
c) Cooling the material to room temperature to obtain the required mechanical
properties (as shown in the above figure)
1.2
2.
5
State the objectives & purpose of Heat Treatment
Objectives and purpose of heat treatment
a) To increase the mechanical properties like ductility, hardness, fatigue, wear
resistance, abrasion resistance, etc.
b) To increase corrosion resistance, electrical and magnetic properties
c) To reduce or eliminate internal residual stresses
d) To refine the grain size of steels
e) To eliminate gases like hydrogen, nitrogen and oxygen
1.3
List the Heat treatment processes.
3. List of heat treatment processes: Depending up on the rate of cooling,
the heat treatment processes are classified as
1. Annealing (slow cooling):
i) Full annealing ii) Process annealing iii) Spheroidising annealing
2. Normalizing (air cooling) 3. Hardening/Quenching (fast cooling)
4. Tempering
1.4
4.
6
Know about the phase transformations on heating of steels.
Transformation into austenite on heating of steels
a) In hypoeutectiod steels, the microstructure at room temperature consists of
ferrite and pearlite. On heating the above steel sample in a furnace, Ferrite and
Pearlite starts to transform into Austenite phase, on crossing A1 (723ºC)
temperature. The Austenite phase formation is completed on reaching A3
temperature. This is clearly shown in the above Fe-C diagram.
b) In Eutectoid Steel, the microstructure at room temperature is Pearlite. On
heating this steel sample in a furnace, Pearlite transforms fully into Austenite
phase, on crossing A1 (723ºC) temperature.
c) In hypereutectiod steels, the microstructure at room temperature consists of
Cementite and Pearlite. On heating the above steel sample in a furnace,
Cementite and Pearlite starts to transform into Austenite phase, on crossing A1
(723ºC) temperature. The Austenite phase formation is completed on reaching
ACm temperature. This is clearly shown in the above Fe-C diagram.
1.5
7
Know about the decomposition of austenite on cooling.
5.
Decomposition of Austenite on cooling
Depending up on the cooling rate of steel, Austenite on decomposition produces
different transformation products like Pearlite, Bainite and Martensite besides
some pro-eutectoid phases.
i) Slow cooling produces equilibrium phases like Pearlite as found in the Fe-Fe3C
diagram
ii) Faster cooling rate (air-cooling) results in reduction of pro-eutectoid phases
formation but produces more pearlite content.
iii) Fast cooling such as quenching in water or brine solutions produces martensite
phase
Martensite and Bainite phases being non equilibrium phases are not found in
Fe-Fe3C diagram.
1.6
6.
Define isothermal transformation.
Iso-Thermal Transformation
Iso-thermal transformation is defined as the transformation which takes place at
constant temperature, on cooling.
Example: In case of eutectoid steel (0.8% C), the formation of Pearlite at a
constant temperature of 700ºC on cooling from a temperature above 723ºC is
called iso-thermal transformation.
r → α + Fe3C , at 700oC
1.7
7.
8
Explain the construction of Time Temperature Transformation Diagrams.
Construction of TTT diagrams
TTT diagrams represent time, temperature and transformation diagrams
TTT diagrams give an idea about effect of time and temperature on the type of
transformation product formed
Let us consider a eutectoid (0.8%C) steel sample.
The steps involved are:
1. Take a large number of small steel samples of 0.8%C and heat to the
austentizing temperature and soak for some time to get homogenized
austenite.
2. Transfer all theses samples quickly into a salt bath which is kept at constant
temperature just below723ºC
3. Transfer all these samples one by one at fixed intervals of time and quench in
brine or cold water. This converts the untransformed austenite into martensite
4. Metallographic study is done for all the samples to find out the start and finish
of the transformations into Pearlite, Bainite and Martensite
5. A graph is plotted between % transformations versus time as shown in the
figure
6. Similar studies are conducted at different temperatures below 7230C and times
of start and finish of iso-thermal transformation are noted
7. The times of start and finish of iso-thermal transformation and the
corresponding temperatures are plotted as shown in the below figure to obtain a
9
C-curve
1.8
List out the factors affecting position of TTT curve.
8. Factors affecting the position of TTT curve are
1. Composition of steel namely,
a) Carbon content: Depending on carbon content, the steels can be classified as
hypoeutectiod, eutectoid and hypereutectoid steels.
In hypoeutectiod and hypereutectoid steels, the TTT curve shifts to the extreme
left when compared to eutectoid steels. So, there is no possibility of martensitic
phase formation
b) Alloying elements: Austenite stabilizers like Ni and Mn shift the TTT curve to
the right. Cobalt will not shift the TTT curve to the right.
2. Grain size: Increase in grain size will also shift the TTT curve to the right
1.9
Define Critical Cooling rate.
9. Critical cooling rate: Critical cooling rate (CCR) is defined as the minimum
cooling rate beyond which martensite forms. If the cooling rate is less than the
critical cooling rate then pearlite or bainite forms.
It is a tangent to the TTT curve
1.10 Explain the superimposition of continuous cooling curves.
1.11. Define
a) Pearlite
b) Bainite
&
c) Martensite
11. Definition of
a) Pearlite: It is an equilibrium transformation product obtained by
eutectoid
reaction. It is a lamellar structure( alternate layers) of ferrite and
cementite. The appearance of pearlite resembles the surface of a mother of pearl
sheen. Hence named as pearlite. It is of two types namely upper pearlite and
lower pearlite.
b) Bainite: It is a non equilibrium transformation product of cementite and
ferrite obtained by super cooling the austenite below 500oC but above Ms
Temperature. It is of two types namely upper bainite and lower bainite.
c) Martensite: It is a supersaturated interstitial solid solution of carbon
atoms in ferrite. It is a non equilibrium transformation product. This is produced
through diffusion-less shear transformation of austenite. Martensite
transformation is designated by Ms and Mf temperatures.
1.12 Explain the mechanism of austenite to
10
a. Pearlite b. Bainite
c. Martensitic transformation
12. a) Mechanism of Austenite to Pearlite transformation
Fig. Mechanism of pearlite formation
The steps involved (as shown in the above figure) are
a) Initially cementite forms from unstable austenite
b) Diffusion of carbon from unstable austenite results in growth of cementite.
c) Diffusion of carbon from austenite results in lowering of carbon in austenite.
d) The carbon depleted Austenite is called Ferrite
e) Carbon rejection into austenite takes place while ferrite transformation
occurs
f) Hence, austenite adjacent to ferrite will enrich with Carbon
g) The enrichment continues till a new platelet of cementite nucleates.
h) This sequence will be repeated to produce pearlite.
1.13
13.
11
State the effect of carbon content on Maximum hardness.
Effect of carbon content on maximum hardness
In plain carbon-steels, hardness property is essentially due to
the martensite phase formation by quench- hardening. The hardness of the
martensite increases with the carbon content in it up to certain limit. If carbon
content is more than 0.8%C, further hardness increase is not possible. This is due
to more and more retained austenite formation. This is clearly shown in the above
figure.
1.14 Define and state the purpose of a) Annealing b) Normalizing c) Hardening
14.
Definitions of
a) Annealing: It is a heat treatment process in which the material is heated to a
pre determined temperature, holding at that temperature for certain time and
finally cooling at a slow rate.
b) Normalizing: It is a process of heating the material to a pre determined
temp, holding at that temperature for certain time followed by cooling in still air
c) Hardening: It is a process of heating the material to hardening temperature,
holding at that temp and then rapidly cooling (quenching) in water, or brine or oil.
i) Purpose of Annealing:
a) Relieve internal residual stresses developed during solidification, machining,
forging, rolling or welding
b) Improve or restore ductility and toughness
c) Enhance machinablity and eliminate chemical in-homogeneity
d) Refine grain size
ii)
Purpose of normalizing
a) To get enhanced mechanical properties
b) Refinement of grain size particularly for rolled and forged components
c) To eliminate pro eutectoid cementite in hyper eutectoid steels which is very
brittle
iii)
Purpose of hardening
a) To develop high hardness in the tool steels and die steels
b) To increase the wear resistance of steels
1.15 Mention the Annealing, Normalizing and Hardening temperatures of
eutectiod, hypo eutectiod, hyper eutectiod steels.
15.
12
i)
Annealing temperatures for plain carbon steels
a) In case of hypo eutectoid steels, annealing temperature is 30-50oC above the A3
line
b) In case of hyper eutectoid steels, annealing temperature is 30-50oC above the A1
line
ii) Normalizing temperatures for plain carbon steels
Hypo eutectoid steels and hyper eutectoid steels are heated to about 40-50oC
above upper critical temperature (A3 or Acm)
iii) Hardening temperatures for plain carbon steels
a) For hypo eutectoid steels, hardening temperature is 30-50oC above upper
critical line (A3)
b) For eutectoid and hyper eutectoid steels, hardening temperature is 30-50oC
above lower critical line (A1).
All the temperatures are shown in the above figure
1.16 Explain
13
a) Annealing b) Normalizing c) Hardening
processes.
1.17 List out the types of annealing
1.17
Types of annealing processes
17. Various types of annealing processes as shown in the above figure are
a) Full annealing b)
Iso-thermal annealing c) Diffusion annealing
d) Recrystallization annealing e)
annealing
Process annealing f)
Stress
1.18
Draw the microstructures at various temperatures of annealing
18. Microstructures at various temperatures of annealing
14
relief
The above figure clearly shows that during full annealing of hypo-eutectoid steels,
at
i) temperatures below A1, the microstructure consists of ferrite and pearlite
ii) Temperatures between A1 and A3, the microstructure consists of pro-eutectoid
ferrite and austenite
iii) Temperatures above A3, the microstructure consists of only austenite grains
iv) on furnace cooling to room temperature, the final microstructure consists of
ferrite and pearlite.
In case of full annealing of eutectoid steels, at
j) temperatures below A1, the microstructure consists of 100% pearlite
ii) temperatures above A1, the microstructure consists of only austenite grains
iv) on furnace cooling to room temperature, the final microstructure consists of
100% pearlite.
In case of full annealing of hyper-eutectoid steels, at
i) temperatures below A1, the microstructure consists of cemenite and pearlite
ii) temperatures between A1 and Acm, the microstructure consists of pro-eutectoid
cemenite and austenite
iii) on furnace cooling to room temperature, the final microstructure consists of
cemenite and pearlite.
1.19
Define & state the purpose of spheroidizing annealing
19. i)
Definition of spheroidizing annealing process
It is a heat treatment process in which cementite platelets in pearlite are converted
into spheroids. There is no micro structural change but shape of cementite changes
15
ii) Purpose of spheroidizing annealing process
The purpose of spheroidizing annealing process is to improve the machinability and
ductility of hyper eutectoid steels
1.20
1.20
Explain the Spheroidising process.
Explain Spheroidising annealing process
The above figure shows the temperature at which the spheroidizing annealing
process is carried out.
16
Various methods available to obtain spheroidized structure in steels are
a) Heating high carbon steels to above A1 and holding for prolonged time period
followed by slow cooling to a temperature just below A1 line.
b) Heating and cooling the steel alternatively above and below A1 line.
c) Iso-thermal annealing just above A1 followed by slow cooling to a temperature
below A1 and holding at this temperature till all the cementite changes in to
spheroids.
1.21 Draw the spheroidized structure.
iv) The microstructure of spheroidized high carbon steel.
Spheroidized high carbon steel, 800X
1.22 Draw the normalized microstructure.
.
17
1.23
Compare annealing & normalizing
19.
Compare annealing and normalizing
Annealing
For hypo-eutectoid steels heating is
done at 30-500C above A3 and for
hyper eutectiod steels at 30-500C
above A1
Furnace cooling is done
Coarse pearlite is produced due to
slow cooling
Normalizing
For hypo-eutectoid steels heating is
done at 40-50oC above A3 and for
hyper eutectoid steels at 40-50oC
above Acm
Air cooling is done
fine pearlite is produced due to air
cooling
Time required for heat treatment is
more
Internal stresses are less
Grain size distribution is more uniform
Time required for heat treatment is
less
Internal stresses are relatively more
Grain size distribution is less uniform
The annealed steel has less hardness,
tensile strength, and toughness and
yield strength.
The normalized steel has better
hardness, tensile strength, toughness
and yield strength
1.24
Mention the stages of quenching.
1.25
Define & state the purpose of Tempering
1.25 i) Definition of Tempering: Tempering is a heat treatment process which
consists of heating the quenched steel to some temperature below lower critical
temperature followed by air cooling or at any desired cooling rate.
ii) Purpose of tempering: To relieve internal residual stresses induced during
quenching
a) To improve the ductility and toughness of the steel at the expense of
hardness of martensite
b) To improve the elastic properties , c) To transform the retained austenite
1.26
Explain the Tempering process.
1.26. Tempering Process
Depending up on the tempering temperature, tempering process may be divided in
to 3 stages.
a) I stage tempering or low temperature tempering
b) II stage tempering or medium temperature tempering
18
c) III stage tempering or high temperature tempering
I stage tempering (low temperature tempering)
The hardened steel is heated to a maximum temperature of 250oC. At about 100oC
of tempering temperature, the following phase transformation occurs,
High carbon martensite →Low carbon martensite + Fe2.4C ( ɛ- carbide)
II stage tempering (medium temperature tempering)
This stage consists of heating the hardened steel in the temperature range of
350-500oC. In this stage of tempering, retained austenite transforms in to bainite.
The resultant microstructure consists of small sized Fe3C in a matrix of ferrite called
troostite.
III staged tempering (high temperature tempering)
This stage consists of heating the hardened steel in the temperature range of
500-680oC. Resolvable Fe3C is produced due to presence of fine Fe3C in a matrix of
ferrite. This structure is called as sorbite.
1.27
Describe the theory of Tempering
22.
Theory of Tempering
Tempering involves the diffusion of carbon out of supersaturated BCC structure.
This results in lattice strain relief to the structure. Tempering is also called as
softening of the steel.
1.28
State the effect of Tempering on microstructure and mechanical
properties.
1.29 Define retained austenite.
1.29 i) Define retained austenite: During hardening of steels complete
transformation of austenite into martensite does not take place. The amount of
austenite that remains untransformed is known as retained austenite
ii) Effect of composition on retained austenite: For plain carbon steels, the
amount of retained austenite increases with increase in carbon content. The
presence of alloying elements further increases the amount of retained austenite.
iii) The presence of retained austenite greatly reduces the mechanical properties of
steel
iv)The conversion of retained austenite into martensite results in increased
hardness, wear resistance and dimensional stability of steel
1.30 Define Temper Embrittlement and state the causes and remedies
1.30. i)Temper embrittlement
During tempering, either heating of steels in the temperature range of 350-550 oC
19
for prolonged time period or slow cooling in the same temperature range causes
increase in brittleness. This type of phenomenon is called temper embrittlement.
ii) Causes of temper embrittlement
a) The embrittlement is caused strongly by certain elements such as antimony (Sb),
phosphorous (P), Tin (Sn), Arsenic (As)
b) The embrittlement is increased greatly by Chromium (Cr) and Manganese (Mn)
iii)
Remedial measures
a) Steels that are susceptible should be quenched from 550oC to below 300oC
in order to prevent the temper embrittlement
b) Reduction of embrittlement causing elements during steel making process.
c) Addition of 0.5% molybdenum(Mo) to steels eliminates the temper
embrittlement
2. Hardenability
2.1 Define hardenability
i) Define Hardenability: The depth and distribution of hardness across a
cross section induced by quenching.
2.2
20
Differentiate Hardness
and Hardenability.
ii) Differentiate hardness and hardenability
Hardness
Hardness is resistance to indentation
The maximum hardness depends on
a)carbon content
b)critical cooling rate attained
c)100% martensite formation
Hardenability
The depth and distribution of hardness
across a cross section induced by
quenching
The maximum hardenability depends on
a)amount of alloying elements
b) Austenite grain size.
c)size of sample
d) Nature of coolant.
2.3 State different Quenching Media.
iii) Different quenching media: Quenching is a process of rapid cooling of steel
from austenite temperature. A medium that is used for quenching is known as
quenchant. Most of the quenchants commonly used are liquids, air and gases. The
various quenching media used are
a) Water b) Oil c) Air d) Brine e) Molten salts f) Polymer quenchants
2.4 Define the cooling power of Quenching Media.
2.5 List out merits & demerits of each quenching medium.
2.6
Define critical diameter & ideal critical diameter.
i)
Define critical diameter (Dc) and Ideal critical diameter (Di)
Critical diameter: It is defined as the diameter of the steel bar having 50%
martensite and 50% Pearlite at its center. Bars having diameter more than the
critical diameter will not harden throughout, whereas smaller diameters will.
Ideal critical diameter: The diameter of the steel bar which when quenched in an
ideal quenching medium would have 50% martensite at its centre.
18
2.7
21
Draw the relationship between critical diameter & ideal critical diameter.
ii) Relationship between Dc and Di
A number of curves are plotted in this graph, each curve belongs to different rates
of cooling. The rate of cooling is denoted by the H- value.
For different values of H, we can write from the figure as
H = ∞ then
Dc = Di.
H ‹ ∞ then
Dc ‹ Di
Fig: The ideal critical diameter as a function of carbon content and grain size
22
2.8
Determine the relationship between ideal critical diameter, chemical
composition & hardenability.
iii) Relationship between ideal critical diameter (Di), chemical composition
and grain size In Steels,
a) Di decreases with decreasing carbon content and decreasing grain size
b) Di decreases when carbon content increases beyond 0.8% C
c) Di increases with increase in the amount of alloying elements like
Manganese, Molybdenum, chromium and Boron
2.9 Explain the Jominey end – quench test.
2.9
Jominy end quench test
The Jominy end quench test is the standard method for measuring the
hardenability of steels. This describes the ability of the steel to be hardened in
depth by quenching
i)
Standard sample as per ASTM
The test sample is a cylinder with a length of 102 mm (4 inches) and a diameter of
25.4 mm (1 inch) as shown below
Test procedure:
a) The steel sample is normalized to eliminate differences in microstructure due to
previous forging, and then austentised
b) Preheat a furnace to 850°C and heat the sample to austentizing the temperature
c) Allow the specimens to soak at this temperature for one hour
d) The test sample is quickly transferred to the Jominey end quench apparatus
23
e) The sample is held vertically and water jet is sprayed up to 20 min, onto the
bottom of the sample until it cools to room temperature
f) Two flat surfaces are ground opposite to each other along the length of the
sample
h) The hardness is measured at intervals of 1/16 inch (1.6mm) from the quenched
end of the sample as shown below
i) The critical distance from the quenched end where the hardness changes most
rapidly is called the Jominy distance
2.10
2.10
State the use of Jominey end quench test
Uses of Jominy end quench test
Data from the Jominy end quench test can be used to determine
a) whether a particular steel can be sufficiently hardened in different quenching
media and with respect to different cross sections
b) Jominy test can be used to estimate Dc and Di for a given steel
c) The effect of microstructure on the hardenability of steels
d) The effect of alloying elements and grain size on the hardenbility of steels
3. Grain size
3.1
29.
Define grain size.
Grain Size
i)
Definition of grain size
It is the average ` diameter ` of a grain revealed in a two dimensional section.
24
ii) The properties of steel that are dependent on actual grain size are,
a)Tensile strength b)Yield strength c)Toughness d)Hardness e) Machinability
f) Hardenability
Fig. Inherently fine & coarse grained steels
iii) Classification of steels based on grain size
Depending on the grain size, steels are classified in to two types namely,
a) Fine grained steels b) Coarse grained steels
Fine grained steels:
The formation of fine grained steels (above figure) involves fast rate of cooling and
more number of nuclei .The grain size of these steels is small. Steels remains fine
grained even at temperatures as high as 10500C. Grain coarsening occurs beyond
10500C
Coarse grained steels:
The formation of coarse grained steels (above figure) involves slow rate of cooling
25
and less number of nuclei. The grain size of these steels is relatively larger. The
grain size of these steels increases slowly with increase in temperature.
iv) Comparison between fine grained steels and coarse grained steels
a) Fine grained steels have higher fatigue strength, yield strength and tensile
strength as compared to coarse grained steels
b) Coarse grained steels have better hardenbility than fine grained steels
c) Coarse grained steels have better machinability than fine grained steels
3.2
Determine the grain size by carburising method.
30.
Measurement of Grain size
i) Revealing Austenite grain size
Before measuring the grain size of steels, it is significant to reveal the austenite
grains. Some methods that are followed are,
a) In hypereutectoid steels, proeutectoid cementite network reveals the austenite
grain boundaries. In hypoeutectoid steels, proeutectoid ferrite network reveals the
austenite grain boundaries.
b) McQuaid-Ehn test: In this test, low carbon steels are carburized at 9250c for 8
hours followed by slow cooling in the furnace itself. A hypereutectoid case is
formed. The cementite network reveals the austenite grain boundaries
ii) Once the austenite grain boundaries are revealed, the grain size is measured by
the following methods,
3.3
a)
Determine the grain size by ASTM comparative Method..
ASTM comparative method
The method is most convenient and accurate for specimens consisting of equiaxed
grains. In this method the specimen is prepared and etched according to the
metallographic procedure. The microstructure of the specimen is projected at a
magnification of 100X and compared with graded ASTM standard grain size charts
(A.S.T.M No1 to No 8). The index number of the matching chart corresponds to the
grain size of the metal. It is called ASTM grain size number, designated by ‘N’
The no. of grains per square inch “n” is calculated using the relation
n = 2N-1
n = the number of grains observed per square inch at 100 X
N= ASTM grain size number
26
Fig. ASTM standard grain size charts
Steels with ASTM grains size numbers 1 to 2 correspond to coarse grained steels.
Steels with ASTM grain size number 6 to 8 correspond to fine grained steels
b)
Heyn`s intercept method
Fig. Heyn`s intercept method
This method is commonly used for grain size determination of metals and alloys
when the grains are not equiaxed.
Initially, metallographic preparation of the specimen is done. The method consists
of counting the number of grains intercepted by a line of known length at a
magnification, on the ground glass screen of a microscope or a photo micrograph.
A line of 0.005 inch in length at a magnification of 1000X is used. The grains
touched by the end of the test line are counted as half grains. The number obtained
27
is called mean intercept value, I as measurements are made at least three different
areas and then an average is taken.
Let the number of grains intercepted by a linear length of 0.005 inch at 1000X= ‘I’
then, the number of grains per inch at 1000X = I/0.005
then, the number of grains per square inch at 100X = ( 2* I)2, which is equal to the ,
n of equation
n = 2N-1
(2*I)2 = 2N-1
N
3.4
c)
= 2logI / log2 + 3
State the measurement of Austenite grain size by Heyn’s intercept method
and Jefferies planimetric method.
Jefferies planimetric method
Fig. Jefferies planimetric method
This method is suited to equiaxed grained steels.
The metallographically prepared specimen is viewed at a certain magnification on
the ground glass screen of the microscope or on a photo micrograph. The
magnification should be selected such that at least 50 grains can be counted in the
field. A circle or a rectangle of known area (usually 5000 mm2) is inscribed on a
photomicrograph or on the ground glass screen of the microscope. The grains
intercepted by the circumference of drawn circle are counted as half. The total
28
number of grains is finally counted.
Jefferies multiplier table
Magnification
used
1
10
25
50
75
100
150
200
250
300
500
750
1000
1500
2000
‘f‘
0.002
0.02
0.125
0.5
1.125
2.0
4.5
8.0
12.5
18.0
50.0
112.5
200
450
800
The number of grains per mm2
= total number of grains in area of 5000 mm2 (*) f
(f called the magnification factor or Jefferies multiplier is taken from the Jefferies
multiplier table)
29
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6.0 Heat Treatment furnaces
•
6.1 Know about Heat treatment furnaces:
•
The success of heat treatment depends on proper choice of heat treating furnace and
type of atmosphere maintained in this furnace.
•
All the heat treatment furnaces basically consist of a heating chamber lined with inside
with refractory lining and out walls are insulators to prevent heat losses. Heating is
done by firing of oil, gas. Laboratory furnaces use electricity for heating. Provisions for
controlling temp also incorporated.
•
6.2
Classify the heat-treating furnaces based on design and their use
•
6.2
Classification of Furnaces :
Based on Design:
1. Batch furnaces .
Based on use:
2. Continuous Furnaces
1.Annealing furnace.
2. Hardening furnace.
3. Tempering furnace.
4. Carburizing furnace.
6.3 Explain the working principle of the following furnaces with a neat
sketch
– Muffle, Salt bath, Bogie type, pit & rotary
hearth.
6.3 a. Muffle Furnace:
• Muffle Furnace: is a furnace in which
the heat source does not directly make
contact with the material being heat
treated is described as muffle furnaces.
• A muffle is a hollow cuboid or
cylindrical retort made of special
refractory material or non-sealing steel.
• used for Bright annealing, Nitriding,
Carburizing and Bright hardening.
1
b. Salt-bath Furnace
•
Salt bath furnace consists of an oval or rectangular container made of steel, cast iron
or a refractory pot which holds the molten salts.
•
The pot is heated and maintained at the required temp either by the fuel or by
electrical resistors.
•
Externally heated furnaces
•
Immersion heating element type furnace
•
The heating element is immersed in the salt and remain therefore for heating,
2
maintain the temp and even after shutdown of the furnaces
Immersed electrode type salt bath furnace
•
It has molten salt bath with immersed electrodes to supply the power.
•
The electrode used normally are mild steels
•
Electrodes flat with square or rectangular cross-section as the opposing surfaces of
such electrodes cause better concentrations of magnetic flux than a round surface.
Immersed electrode type salt bath furnace
•
Uses : Cyaniding ,Liquid carburizing, Liquid nit riding, Austempering , Martempering
Hardening and tempering.
3
c. Bogie hearth furnace (Fig.)
•
The bogie hearth with the loaded charge is put inside the furnace and then sealed with
granular and sealing troughs or solid seals.
•
Normally bogie hearth furnaces are used in temperature range 5400c to 11000c.
•
Bogie hearth furnaces are used for Stress-relieving, annealing and hardening of
components.
d. Pit furnace:
•
It is called pit furnace because most of its portion lies below the ground level.
•
Pit furnace consists of cylindrical refractory chamber.
•
A pit furnace consists of the furnace placed in a pit.
•
It has a cover or lid on top of the furnace.
•
The long and slender parts such as tubes, spindles, shafts, rods etc are suspended from
suitable fixtures from top.
•
Heating in such a manner reduces distortion and warpage.
•
Furnace is used for gas carburizing and other case hardening treatments.
•
The non scaling steel retort can help high degree of control of atmosphere.
4
e. Rotary-hearth furnace :
•
The furnace consists of a rotating hearth.
•
The hearth rotated along its vertical axis.
•
The components to be heat treated are charged through an opening.
•
After completion of heat treatment cycle heat treated components are taken out from
the same opening.
•
The furnaces are then useful where only one operation is used as the charging and
discharging.
•
The speed of rotation is so adjusted that the heat treatment cycle is completed by the
time the hearth completes one complete rotation.
5
7.0 Furnace atmospheres
•
7.1 Define the controlled atmosphere.
Controlled atmosphere: An atmosphere used to protect the metal from oxidation and
maintains the desired properties at the surface of the metal during heat treatment.
•
•
7.2 Know about the necessity of maintaining controlled atmosphere.
7.2 Necessity Maintaining of controlled atmosphere
•
To produce specific surface characteristics.
•
To produce chemical stability.
•
Prevention of oxidation and surface decarburization of steel parts which otherwise
causes loss of dimensions and lowered surface hardness and strength.
•
7.3 Know about the Exothermic and Endothermic atmosphere.
•
Exothermic atmosphere:
•
It is produced by partially burning natural gas, town gas, propane etc in the presence
of air and nickel oxide catalyst.
•
Exothermic Reaction :
CH4+2O2+7.6N2→CO2+2H2+7.6N2 this is an exothermic reaction
2CH4+O2+3.8N2→2CO+2H2+3.8N2 this is an exothermic reaction
These are two types:
Rich Exothermic (in Vol %)
•
N2 – 71.5
•
Co – 10.5
6
•
CO2 – 5
•
H2 – 12.5
•
CH4 – 0.5.
Rich Exothermic atmosphere.
•
These are used commonly for clean annealing and tempering of steel.
•
Brazing of Copper and Silver sintering of powdered metals.
Lean exothermic ( in Vol %)
•
N2 – 86.8
•
Co – 1.5
•
Co2 – 10.5
•
H2 – 1.2
Exothermic atmosphere use:
•
These are used for where intentional surface oxidation is needed.
•
Annealing of Copper.
•
Specialized low temp work.
Endothermic Atmosphere:
•
A pressurized air-gas mixture in the ratio of approximately 3:1 is passed through a
catalyst bed of nickel oxide contained in a retort,
•
Reaction temp is 1040-12000C. After the reaction is completed, the products are cooled
to below 3150C to prevent the formation of soot.
•
It is capable of producing a consistent atmosphere particularly, a carbon potential.
•
Being heated within a furnace hence preventing unintentional carburization or
decarburization
•
Endothermic reaction
•
2C3H8 + 3O2 + 12N2 → 6CO+ 8H2 + 12N2 ……….this is a endothermic reaction
•
Accurate control of the process is possible by variation of the air-gas ratio to produce
•
An atmosphere free from oxygen, Carbon monoxide and with a negligible amount of
methane.
•
Endothermic reaction : 2CO↔C+CO2
Uses:
Endothermic atmosphere used extensively for scale free annealing ,normalizing and hardening
of low ,medium, or high carbon steel.
Used in the carbon restoration of decarburization bar stock or forgings and
7
as carrier gas for gas carburization and carbon nitriding.
Rich Endothermic (in Vol %)
•
N2 – 39.8
•
Co – 20.7
•
H2 – 38.7
•
Ch4 – 0.8
Lean Endothermic (in Vol %)
•
N2- 45
•
Co – 19.6
•
Co2 – 0.5
•
H2 – 14.6
•
CH4 – 0.5
•
Endothermic Atmosphere use
•
Endothermic generators are widely used for heat treatment, brazing of carbon steels
without decarburization and used in sintering.
•
Used as a base gas for gas carburizing and carbo-nitriding of steel.
•
Used in bright hardening.
7.4 State the methods of controlling Furnace atmosphere.
Methods of controlling furnace atmosphere
Three main devices are used for controlling the furnace atmosphere
1. Infrared controlled.
2. Dew point measuring.
3. Oxygen probe.
Infrared method:
This method consists of taking sampling and measure CO and / or CO2 levels in it.
It is used commonly for exothermic and endothermic generator operations.
Dew point Method:
This method consists of taking sampling analyzing for N2 – H2.
Oxygen probe method:
It directly analyses the sample of gas inside the furnace without collecting a sample to give
carbon potential. Accuracy is ± 0.05%.
8
9
8. Heat treatment of Non-ferrous Metals
8.1
Know about the Heat treatment of Al-Cu alloy.
8.1 Heat treatment of Al-Cu alloy: LM 11(4.5%Cu) is a precipitation hardenable
alloy and produces good strength after precipitation hardening. It is mainly used
for aircraft castings and for highly stressed parts due to its good mechanical
properties and shock resistance. Eg. Duralumin
8.2 Explain solution treatment with the help of Al – Cu phase diagram
8.2 solution treatment with the help of Al – Cu phase diagram :
Solutionizing is the process of heating above the solvus to produce a single-phase
solid solution.
If the alloy is reheated to point 1(Solutionizing temperature) and soak it there,
all the excess β will be dissolved and the structure will be a homogeneous α solid
solution. The alloy is then cooled rapidly (quenched) to room temperature.
A supersaturated solution results, with excess β trapped in solution.
The quench is usually carried out in a cold-water bath or by water spray and
boiling water may be used as a quench medium to minimize distortion.
8.3 Explain precipitation hardening.
8.3 Precipitation Hardening: After quenching, precipitation of second phase
particles occurs.
Purpose: Precipitation hardening is used in Aluminum alloys to improve the
mechanical properties, such as strength and hardness.
Types of ageing: 1. Natural ageing and 2. Artificial ageing
•
Precipitation with time at room temperature is called natural ageing,
whereas precipitation at higher temperature is referred as artificial ageing
1
(120-200oC).
•
Ageing time may vary from 4 hours to 24 hours.
•
Spontaneous decomposition of supersaturated solid solution takes place
during ageing treatment.
Stages in Precipitation Hardening
•
In the first stage formation of clusters and GP zones occur.
•
During this process micro strains developed due to the formation of GP
zones (e.g.GP-II zones) thus give rise to maximum strengthening in the alloy
•
Higher ageing temperatures which cause considerable decrease in strength
and increase in ductility of the alloy (>250 oC) is known as overageing.
Precipitation hardening of AI-4.5% Cu Alloy: The precipitation process can
2
be explained by considering the aluminum rich portion of Al-Cu system.
•
The equilibrium solid solubility of copper in aluminum increases with
temperature
•
At 2500 C, the solid solubility of copper in aluminum is about 0.20%.
•
The maximum solid solubility of copper in aluminum is 5.65% at 5480 C.
8.4 Explain the theory of Age-hardening.
8.4 Controlled precipitation hardens the alloy. The hardness increases with
time of ageing
Steps in ageing process
1. Solutionizing is the process of heating above the solvus to produce a single
phase solid solution. Solutionizing requires heating to within a few degrees of
the solidus and does not heat above.
2. Quenching or rapid cooling to retain the single phase solid solution at room
temperature.
3. Ageing: natural ageing refers to holding at room temperature, whereas
artificial ageing is slightly elevated temperatures (100-200oC).
3
8.5 List the Al – alloys responds to precipitation hardening
8.5 Aluminum - silicon, Aluminum-copper and Aluminum-magnesium alloys.
8.5 State the Heat treatment process for Brass and Bronzes
• In general annealing heat treatment is given
• To improve toughness and ductility
• Cu+Zn = Brass, Recrystallization annealing, temp 650-700 oC.
• Cu+Sn = Bronze, Annealing temp is 500 oC
4