Heat treatment of materials in prosthetics

1. Heat Treatment
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2. Introduction to Heat Treatment
Heat Treatments
• Controlled process of heating and cooling of metals
• Alteration of physical and mechanical properties without changing the product
shape
• Sometimes takes place inadvertently due to manufacturing processes that either
heat or cool the metal such as welding or forming.
DEFINATION:
• A combination of heating & cooling operation timed & applied to a metal or alloy
in the solid state in a way that will produce desired properties.- Metal Hand Book
(ASM)
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3. Introduction to Heat Treatment
• The amount of carbon present in plain carbon steel has a pronounced
effect on the properties of a steel and on the selection of suitable heat
treatments to attain certain desired properties.
• Below are some major types of heat treatment processes:
• Annealing
• Normalizing
• Hardening
• Carburizing
• Tempering
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4. Introduction to Heat Treatement
Needs of Heat Treatments
• Often associated with increasing the strength of material
• Can also be used to obtain certain manufacturing objectives like
– To improve machining & formability,
– To restore ductility
– To recover grain size etc.
– Known as Process Heat Treatment
• Heat treatment done for one of the following objective:
– Hardening.
– Softening.
– Property modification.
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5. Introduction to Heat Treatment
• Hardening heat treatments particularly suitable for Steels
–Many phase transformation involved even in plain carbon steel and low-alloy steel.
• Other type of heat treatments equally applicable to ferrous & non-ferrous
Hardening Heat Treatment
• Hardening of steels is done to increase the strength and wear properties.
• Hardening (Quenching followed by Tempering) is intended for improving the
mechanical properties of steel.
• Generally increases hardness at the cost of toughness.
• Pre-requisites for hardening is sufficient carbon and/or alloy content.
– Sufficient Carbon - Direct hardening/Case hardening.
– Otherwise- Case hardening
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6. Hardening Heat Treatment
Hardening Heat Treatments:
• Direct Hardening
– Heating Quenching Tempering
• Austempering
• Martempering
• Case Hardening
– Case carburizing
– Case Nitriding
– Case Carbo-nitriding or Cyaniding
– Flame hardening
– Induction hardening etc
• Precipitation Hardening
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7. Softening Heat Treatment
• Heat treatment is necessary when a
large amount of cold working, such
as cold-rolling or wire drawing been
performed.
• Softening Heat Treatment done to:
–Reduce hardness
–Remove residual stresses
– Restore ductility
– Improve toughness
– Refine grain size
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8. Softening Heat Treatment
Purpose of Annealing:
• To relieve the internal stresses induced
due to cold working, welding, etc.
• To reduce hardness (i.e. to make the
steel soft) and to increase ductility
• To increase the uniformity of phase
distribution and to make the material
isotropic in respect of mechanical
properties
• To refine the grain size
• To make the material homogeneous in
respect of chemical composition
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Types of
Annealing
• Stress Relief: Reduce
stress caused by:
-plastic deformation
-non uniform cooling
-phase transform.
• Full Anneal (steels):
Make soft steels for
good forming by heating
, then cool in
furnace to get coarse P.
• Spheroidize (steels):
Make very soft steels for
good machining. Heat just
below TE & hold for
15-25h.
Process Anneal: Negative
effect of cold working by
(recovery/recrystallization)

9. Stress Relieving
• To reduce residual stresses in large castings, welded and cold-formed parts.
• Such parts tend to have stresses due to thermal cycling or work hardening.
• Parts are
– heated to 600 - 650ºC (1112 - 1202ºF)
– held for about 1 hour or more
– then slowly cooled in still air.
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10. Process Annealing
Process Annealing
• Used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon).
• Allows the parts to be soft enough to undergo further cold working without fracturing.
• Heated upto lower critical temperature line A1 i.e. 650ºC to 700ºC and holding for
sufficient time, followed by still air cooling.
• Initially, the strained lattices reorient to reduce internal stresses (recovery)
• When holding time is long enough, new crystals grow (recrystallisation)
• Material stays in the same phase through out the process
– Only change in size, shape and distribution of the grain structure
• This process is cheaper than either full annealing or normalizing
– As material is not heated to a very high temperature or cooled in a furnace.
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11. Spheroidization
Spheroidization
• Used for high carbon steels (Carbon > 0.6%) that will be machined or cold
formed subsequently.
• Heat just below the line A1 (727 ºC) and hold for a prolonged time followed by
fairly slow cooling.
• For tool and alloy steels heating range is upto 750 to 800ºC
• Formation of small globular cementite (spheroids) dispersed throughout the
ferrite matrix.
• Improved machinability and resistance to abrasion.
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12. Full Annealing
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Process of Full Annealing:
• The process consists of heating the steel to above
A3 temperature for hypo-eutectoid steels and
above A1 for hypereutectoid steels by 30-50oC,
holding at this temperature for a definite period
and slow cooling to below A1 or room
temperature usually in a furnace.
• Cooling rate varies from 30ºC/hr to 200ºC/hr
depending on composition.
• Due to slow cooling (furnace cooling), eutectoid
reaction occurs very nearly in accordance with
conditions represented in the Fe-C phase diagram.
• Normal slow cooling enables austenite to
decompose fully.

13. Full Annealing
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• Higher the austenite stability, slower the
cooling to ensure full decomposition.
• Alloy steels, in which austenite is very stable
should be cooled much slower than carbon
steel.
• Microstructure is coarse Pearlite with ferrite
or Cementite.
• Full annealing for hyper eutectoid steel is
required only for restoring grain size from
coarser grain size formed during hot working.
• Hypoeutectoid hot worked steel (rolled stock,
sheet, forgings, etc), castings of carbon &
alloy steels, may undergo full annealing.
Annealing

14. Full Annealing
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Annealing
> 0.5 Tm
Cold worked
structure
Equiaxed
structure
Effect of Annealing on Cold Working
• Cold working is the working or deformation of a material at room temperature or below its
recrystallization temperature.
Equiaxed
structure
Cold worked
structure
•Annealing leads to a equiaxed, stress free structure
which is softer and ductile
• Effect of strain hardening is lost and the material can be
further cold worked

15. Normalizing
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Process of Normalizing:
• The process consists of heating the steel to above
A3 temperature for hypoeutectoid steels and above
Acm (to break the network of pro eutectoid cementite)
for hypereutectoid steels by 30-50oC, holding at this
temperature for a definite period and slow cooling to
below A1 or room temperature usually in air.
• Due to air cooling which is slightly fast as
compared to furnace cooling employed in
annealing, normalised components show slightly
different structure and properties than annealed
components.
• Results a grain structure of fine Pearlite with pro-
eutectoid Ferrite or Cementite

16. Normalizing
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• Steel is normalized to refine grain size, make
its structure more uniform, or to improve
machinability.
• When steel is heated to a high temperature,
the carbon can readily diffuse throughout,
and the result is a reasonably uniform
composition from one area to the next.
• Steel is then more homogeneous and will
respond to the heat treatment in a more
uniform way.
• Normalizing treatment is more frequently
applied to ingots prior to working, and to
steel castings and forgings prior to hardening
for homogenizing or grain-refining
treatment.
Normalizing

17. Heat Treatment Temperatures
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Annealed Normalised
1. Less hardness, strength and toughness
2. For plain carbon steels, microstructure
shows pearlite almost in accordance
with the Fe-C diagram.
3. Pearlite is coarse and usually gets
resolved by the optical microscope.
4. Grain size distribution is more uniform
5. Internal stresses are least.
1. Slightly more hardness, strength &
toughness
2. For plain carbon steels, microstructure
shows finer pearlite than observed in
annealed components.
3. Pearlite is fine and usually appears
unresolved in the optical microscope
4. Grain size distribution is slightly less
uniform
5. Internal stresses are slightly more.
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18. Quenching
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Purpose of Hardening(Quenching):
• To harden the steel to the maximum level by
austenite to martensite transformation.
• To increase the wear resistance and cutting
ability of steel
Process of Quenching
• The process consists of heating to above the upper
critical temperature (A3) for hypoeutectoid steels
and above A1 for hypereutectoid steels by 50oC,
holding at this temperature for a definite period
for complete austenitizing for a sufficient time
and cooling with a rate just exceeding the critical
cooling rate of that steel to room temperature or
below room temperature.

19. Quenching
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Mechanism of Quenching
• Austenite to Ferrite transformation takes place by a
time dependent process of Nucleation & Growth
• Under slow or moderate cooling rates, the carbon
atoms diffuse out of the austenite structure (FCC)
forming ferrite (BCC) & cementite
(Orthorhombic)
• With increase in cooling rate, time allowed is
insufficient
Although some movement of carbon atoms take place
• The structure can not be BCC
• The carbon is trapped in solution
• The resultant structure, Martensite is a
supersaturated solution of carbon trapped in a
body centered tetragonal structure (BCT).
Hardening

20. Quenching Contd..
• Quenched steel (Martensite)
• Highly stressed condition
• Too brittle for any practical purpose.
• Quenching is always followed by
tempering to
– Reduce the brittleness.
–Relieve the internal stresses
caused by hardening.
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Martensite

21. Quenching Media
• Quenching media with increased degree of severity of quenching
– Normal Cooling
– Forced Air or draft cooling
– Oil
– Polymer
–Water and
– Brine
• quenching medium depends on
– Material composition
– Weight of job
• Aim is to have a cooling rate just bye-passing the nose of TTT curve for
– minimum stress
– minimum warping/crack during quenching.
• Cooling rate varies from surface to core: slower cooling towards centre.
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22. Tempering
• Tempering (formerly called drawing), consists of reheating a quenched steel to a suitable
temperature below the transformation temperature for an appropriate time and cooling
back to room temperature.
• Freshly quenched martensite is hard but not ductile.
• Tempering is needed to impart ductility to martensite usually at a small sacrifice in
strength.
The effect of tempering may be illustrated as follows.
• If the head of a hammer were quenched to a fully martensitic structure, it probably
would crack after the first few blows. Tempering during manufacture of the hammer im-
parts shock resistance with only a slight decrease in hardness.
• Tempering is accomplished by heating a quenched part to some point below the
transformation temperature, and holding it at this temperature for an hour or more,
depending on its size.
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23. Tempering
Process of Tempering:
• The process consists of heating the hardened
components to temperature between 100oC and
700oC (below A1) holding at the temperature for
specific period (1-2 hrs) and cooling to room
temperature usually in air.
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Purpose of Tempering:
• To relieve the internal stresses developed due to rapid cooling during hardening process (i.e.
austenite to martensite transformation).
• To reduce hardness and to increase ductility and toughness i.e. to reduce brittleness.

24. Tempering
• Tempering means subsequent heating
– to a specific intermediate temperature
– and holding for specific time
• Tempering leads to the decomposition of martensite into ferrite-cementite mixture
– Strongly affects all properties of steel.
• At low tempering temperature (up to 200ºC or 250ºC),
– Hardness changes only to a small extent
– True tensile strength increases
– Bending strength increases
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25. Tempering
• Higher tempering temperature reduces
– Hardness
– True tensile strength
– Yield point
– While relative elongation and reduction area increases.
• This is due to formation of ferrite and cementite mixture.
• At still higher temperature or holding time
– Spherodisation of cementite
– Coarsening of ferrite grains
• Leads to fall in hardness as well as toughness
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26. Martempering
The process has the following advantages:
• It results in less distortion and warping, since
the martensite formation occurs at the almost
the same time throughout the cross section of
the component.
• There is less possibility of quenching cracks
appearing in the component.
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Martempering is process the austenitized steel is cooled rapidly avoiding the nose of the
TTT curve to a temperature between the nose and Ms, soaked at this temperature for a
sufficient time for the equalization of temperature but not for long enough to permit the
formation of bainite and then cooled to room temperature in air or oil.

27. Martempering
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• Specially designed quenching technique.
• Quenched around 315ºC (above Ms).
• Held at this temperature for sufficient time to
homogenize surface & core temperature.
• Further quenched to MS through MF.
• The structure is martensite tempered to get
desired combination of Hardness and
Toughness.
Advantage over rapid quenching
– More dimensional stability
– Less Warping
– Less chance of quench crack
– Less residual stress

28. Austempering
• Here austenite transforms to lower bainite.
• Properties of bainite are Intermediate to
those of marteniste and pearlite and are very
much similar to that of tempered martensite.
• Most important advantage of austempering is
that it produces structures and properties very
much similar to tempered martensite without
involving the martensitic transformation as
in the hardening heat treatment:
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Austempering consists of cooling the austenitised steel with a rate exceeding the critical
cooling rate and then held at some constant temperature between the nose of TTT curve and
Ms temperature i.e. in the bainitic region, holding at this temperature for a sufficient period
for the completion of bainitic transformation and cooling to room temperature at any desired
rate.

29. Austempering
• Quenched around 315ºC (above Ms).
• Held at this temperature for sufficient time to
homogenize surface and core temperature.
• Undergo isothermal transformation from
Austenite to Bainite.
• Bainite has same composition as Pearlite
with much finely spaced structure (inter
lamellar spacing) is tough as well as hard.
• Suitable for direct use in many application
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30. Hardening
• Hardening is carried out by quenching a steel, that is cooling it rapidly from
a temperature above the transformation temperature.
• Steel is quenched in water or brine for the most rapid cooling, in oil for some
alloy steels, and in air for certain higher alloy steels.
• With this fast cooling rate, the transformation from austenite to pearlite
cannot occur and the new phase obtained by quenching is called martensite.
• Martensite is a supersaturated metastable phase and have body centered
tetragonal lettice (bct) instead of bcc.
• After steel is quenched, it is usually very hard and strong but brittle.
• Martensite looks needle-like under microscope due to its fine lamellar
structure.
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31. Surface Hardening
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Surface Hardening
• In many applications such as gears, crankshafts, etc. a hard and wear resistant
surface (case) is required with a tough centre (core) to withstand impact loads.
• Such a requirement is difficult to achieve by using a steel of uniform composition.
• Low C steels are ductile and tough but will wear due to less hardness and wear
resistant while high C steels are hard and wear resistant but have less toughness.
Medium C steels are Intermediate in properties to those of low and high C steels but
do not satisfy the requirements to the optimum level
Case (hard and
wear resistant)
Core (ductile
and tough)

32. Surface Hardening
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Such a problem can be solved by:
• Increasing the carbon in the surface of a low C steel and subsequently heat
treating the component in a specific manner to produce hard and wear resistant
surface (case) and a tough centre (core)
• Introducing nitrogen in the surface of a tough steel so as to produce hard nitrided
case with no subsequent heat treatment.
• By selectively hardening the surface only by the hardening heat treatment.
Surface hardening treatments like those mentioned below are carried out to
selectively harden the surface
1. Carburizing
2. Nitriding
3. Flame hardening
4. Induction hardening

33. Carburizing
• Method of increasing the carbon on the surface of a steel is known as carburizing.
• It consists of heating the steel to the austenitic region in contact with a carburizing
medium, holding at this temperature for a sufficient period and cooling to room
temperature.
• The carburized steel may be directly quenched / hardened from the carburizing
temperature or may be given a quenching / hardening heat treatment later i.e. after
cooling from the carburizing temperature.
Depending on the medium employed carburizing
can be classified into:
1. Pack carburizing
2. Liquid carburizing
3. Gas carburizing
Surface Hardening
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34. 1. Pack Carburizing
• The components to be carburized are packed with a carbonaceous material in steel or cast
iron boxes and sealed with clay.
• The usual carbonaceous material consists of charcoal and coke.
• The sealed boxes are heated to the austenitic region and kept at the temperature until the
desired degree of penetration is obtained.
Surface Hardening

35. 2. Liquid Carburizing
• In this method, carburizing is done by immersing the steel component in a carbonaceous
fused salt bath medium at a temperature in the austenitic region.
• The bath is composed of sodium cynide, sodium carbonate and sodium chloride.
3. Gas Carburizing
• Here the components are heated to the austenitic region in the presence of carbonaceous
gas such as methane, ethane or butane diluted with a carrier gas.
CH4 C + 2H2
Surface Hardening

36. Nitriding is accomplished by heating the steel in contact with a source of atomic nitrogen at a
temperature of about 550oC, i.e., ferritic region compared austenitic phase in carburising
process
Surface Hardening
• The atomic nitrogen diffuses into the steel and
combines with iron and certain alloying elements
present (1%Al, 1.5%Cr, 0.2%Mo) in the steel
and forms respective nitrides.
• The atomic nitrogen source can be a molten salt
bath for liquid nitriding containing NaCN
similar to that used in liquid carburizing or
dissociated ammonia
2NH3 2N + 3H2
Nitriding

37. Flame Hardening
• Flame hardening is a process heating the surface layer of a hardenable steel to its
austenitic range by means of a oxyacetylene flames followed by water spray quenching
to transform austenite to martensite.
• Heating to austenizing range, 30–50ºC above Ac3 (Hypoeutectoid) or Ac1
(Hypereutectoid).
• Quenching in suitable quenching media an tempering to refine grain size and reduce
stresses if required.
Surface Hardening

38. Induction Hardening
• In induction hardening the surface layer of a material is heated by induction heating by
passing a high frequency current through a induction coil.
• The high frequency alternating currents flowing through the inductor generates alternating
magnetic field which in turn induces eddy currents in the surface layer which rapidly
heats the surface.
• Heating to austenizing range, 30–50ºC above Ac3 (Hypoeutectoid) or Ac1 (Hypereutectoid).
• Quenching in suitable quenching media an tempering to refine grain size and reduce stresses
if required.
Surface Hardening

39. Case Hardening
• Objective is to harden the surface & subsurface selectively to obtain:
– Hard and wear-resistant surface
– Tough impact resistant core
– The best of both worlds
• Case hardening can be done to all types of plain carbon steels and alloy steels
• Selectivity is achieved
a) For low carbon steels
– By infusing carbon, boron or nitrogen in the steel by heating in appropriate
medium
– Being Diffusion controlled process, Infusion is selective to surface and subsurface
b) For medium & High carbon or Alloy steel
– By heating the surface selectively followed by Quenching
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40. Case Hardening
Case Carburizing
• Heating of low carbon steel in carburizing medium like charcoal
• Carbon atoms diffuse in job surface
• Typical depth of carburisation; 0.5 to 5mm
• Typical Temperature is about 950ºC
• Quenching to achieve martensite on surface and sub-surface
• If needed, tempering to refine grain size and reduce stresses
Case Nitriding
• Heating of steel containing Al in nitrogen medium like Nitride salt, Ammonia etc.
• Typical temperature is about 530ºC
• Nitrogen atoms diffuse in job surface
• Forms AlN, a very hard & wear resistant compound on surface & sub-surface
• Typical use is to harden tubes with small wall thickness like rifle barrel etc.
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41. Case Carbo-nitriding
Case Cyniding
• Heating of low carbon steel containing Al in cynide medium like cynide salt
followed by Quenching
• Typical temperature is about 850ºC
• Nitrogen & Carbon atoms diffuse in job
• Typical case depth 0.07mm to 0.5mm
• Forms very hard & wear resistant complex compounds, on surface & sub-surface
• If needed, tempering to refine grain size and reduce stresses
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42. Precipitation Hardening
• Also known as Dispersion or Age hardening
• Applicable to common non-ferrous metals and alloys and some spl. Steels
• Technique used for strengthening
– Al (Mg, Cu), Mg, Ti (Al, V) alloys
– Some variety of SS, Maraging Steel etc.
• Hardening in steel is mainly due to martensite formation during quenching
• common non-ferrous metals normally don’t respond to quenching
• A method where finely dispersed second phase precipitates in the primary matrix
• These precipitations lock the movement of dislocation causing increase in hardness
• Exploits phenomenon of super- saturation.
• Nucleation at a relatively high temperature (often just below the solubility limit)
–Maximise number of precipitate particles.
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43. Precipitation Hardening
• Lower the temperature an hold
– These particles grow in size
– The process called aging.
• Typical dislocation size is 5-30 nm
• Diffusion's exponential dependence upon
temperature makes precipitation
strengthening a fairly delicate process.
• Too little diffusion (under aging)
– The particles will be too small to
impede dislocations effectively
• Too much diffusion(over aging)
– Particle will be too large and
dispersed to interact with the majority of
dislocations
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Precipitation Hardening

44. Hardenability
• Ability of a metal to respond to hardening treatment
• For steel, the treatment is Quenching to form Martensite
• Two factors which decides hardenability
– TTT Diagram specific to the composition
– Heat extraction or cooling rate
TTT Diagram
• For low carbon steel, the nose is quite close to temperature axis
• Hence very fast cooling rate is required to form Martensite
– Causes much warp, distortion and stress
– Often impossible for thick sections
• Carbon and Alloy addition shifts the nose to right and often changes the shape
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45. Hardenability
Factors affecting cooling rate
• Heating Temperature
• Quenching bath temperature
• Specific heat of quenching medium
• Job thickness
• Stirring of bath to effect heat convection
• Continuous or batch process
• Hardenability is quantified as the depth up to which full hardness can be achieved
• Amount of carbon affects both hardness of martensite and hardenability
• Type and amount of alloying elements affect mostly hardenability
• The significance of alloying element is in lowering cooling rate for lesser
distortion and thick section
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46. Applications
The main applications are:
• The procedure for different heat treatment processes to get better properties of
material for different applications.
• Phase transformation at different temperature conditions and compositions through
iron carbon diagram to study material behaviour in different conditions.
• Application of TTT curve during heat treatment of materials.
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47. Course Outcomes
Course Outcomes:
• Understanding of different types of materials and their properties.
• Student will learn the structure and properties of ferrous and non-ferrous
materials.
• Understand the presently used or newly developed modern engineering
materials and their characterizations.
• Principles of heat treatment and types of heat treatment for steels.
• Understand the Principles of Iron- Carbon Equilibrium diagram and behaviour
at different composition and temperature.
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48. Recommended Books
• Smith, Engineering Foundation of Material Science and Engineering (McGaw Hill,
5th Edition)
• S Singh, Vijendra, Physical Metallurgy, Standard Publishers and Distributors (1999).
• Avner S.H., Introduction to Physical Metallurgy, ( McGraw Hill ).
• James F. Shakel, Introduction to Material Science for Engineering (Pearson, Prentice
Hall , New Jersey , 6th edition)
• Askeland, Fulay, Wright, Balani, The Science & Engineering of Materials (by
Cengage Learning)
• V. Raghvan, Physical Metallurgy , Principles & Practices ( PHI, New Delhi)
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49. References
• http://nptel.ac.in/downloads/112108150/ (Material Technology)
• http://nptel.ac.in/courses/113101003/23# (Heat Treatments)
• https://web.utk.edu/~prack/MSE%20300/FeC.pdf (Iron carbon Diagram)
• http://nptel.ac.in/courses/113105023/Lecture34.pdf (Heat Treatments)
• https://www.tf.uni-kiel.de/matwis/amat/iss/kap_6/illustr/s6_1_2.html (Iron
Carbon Diagram)
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50. Thanks
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