This will explain the basics of material crystal structures,Iron Carbon diagram,TTT diagram and various heat treatment process.

1. Basics of Heat treatment
process

2. Agenda:
• Basics of crystals
• Material atomic structure
• Fe-C,TTT and CCT diagrams
• Introduction of HT
• Heat treatment process
• Examples

3. Material atomic structure
basics

4. Crystal structure
• The properties of material depend not only the bond strength but also the
arrangement of atoms.
• Examples of Crystalline snowflakes, diamond, and table salt.
• Most inorganic solids are not crystals but polycrystal, i.e. many microscopic
crystals fused together into a single solid. Most of Metals,Rocks,Ceramics and
Ice.
• A third category of solids is amorphous solids, where the atoms have no
periodic structure whatsoever. Glass, Wax and Many plastics.

5. Some common unit cells
• In a crystal, the repetition of unit cell forms the whole crystal. The
structure is identified and described by the unit cell.

6. BCC(Harder) FCC( High
ductile)
CPH( Low
ductile)
Cr Al Ti
W Cu Zn
V Ni Mg
Mo Ag Zr
Iron(Except
in the range
of FCC)
Iron(
910-1400
deg C)
Co

7. Crystal imperfections:
• These imperfection governs most of the mechanical properties of
solids.
Property dependent on basic
crystal structure
Property dependent on crystal
imperfection
Density Yield stress
Specific heat Creep
Coefficient of Thermal
expansion
Fracture strength
Melting point Work hardening
Elastic constants Fatigue strength
Hardness and ductility Electrical conductivity

8. • Crystal imperfections affect the stress–strain diagram in a number of
ways, often increasing the tensile strength as the density of
imperfections increase or their geometry creates partitioning of the
crystal structure.
• Work hardening and strain hardening are also dependent upon
imperfections. Hardness and yield stress for many metals and alloys
are related by a simple relationship, σy = H/3.

9. Types of defects
• Line Defects
• Edge dislocations, screw
dislocations
Surface Defects
Grain boundaries
Point Defects
Vacancy, substitutional (atom replaces host), interstitial
(atom squeezes in between host), impurity

11. • Ferrite- pure steel with very less
percentage of carbon ( 0.008% C at
room temperature and the rest is
Iron).Ex.Ferro magnets
• Cementite- It is relatively harder phase
which contains 6.67% C at room
temperature.
• Pearlite- It is a microstructure which
contains both ferrite and cementite.
This microstructure is formed after
Eutectoid Reaction at 723°C.
• Austenite- It is a phase of FCC
structure which exists above 723°C.

12. TTT-Diagram(Isothermal transformation)
• Eutectoid carbon steel
• Below A1 , austenite is unstable, i.e.,
it can transform into pearlite, bainite
or martensite.
• Below 6000C,transformation starts
with min lapse time.(Nose)
• Bainite is mixture of ferrite and
cementite.
• Beginning of transformation does not
exist below 2200C

13. Decomposition of Austenite

14. CCT diagram for Eutectoid steel:

15. Heat Treatment

16. Definition
• Heat treatment is an operation or combination of operations
involving heating at a specific rate, soaking at a temperature for a
period of time and cooling at some specified rate. The aim is to obtain
a desired microstructure to achieve certain predetermined properties
(physical, mechanical, magnetic or electrical).

17. Objectives of heat treatment:
 To increase strength, hardness and wear resistance
To increase ductility and toughness.
To obtain fine grain size by recrystallization annealing, full annealing or
normalising
To remove residual internal stresses formed due to cold working by
stress relief annealing
To improve machinability of forged components
To improve cutting properties of tool steels
To improve surface hardening, corrosion resistance, high temperature
resistance by precipitation hardening
To improve electrical properties
To improve magnetic properties by phase transformation, of the forged
components

18. Treatment of Steels:
• In general three kinds of treatments are:
• Thermal (heat treatment),
• Mechanical (working),
• Chemical (alteration of composition). A combination
of these treatments are also possible (e.g. thermo-
mechanical treatments, thermo-chemical treatments).
• The treatment may affect the whole sample or
only the surface.
• A typical industrial treatment cycle may be
complicated with many steps (i.e. a combination
of the simple steps)
• Annealing
Normalizing
Stress Relieving
Precipitation hardening
Quenching
Tempering
Case Hardening

19. Effects of heat treatment
Annealing & Normalizing
(Furnace cooling)
Hardening or Quenching
(Air/Oil/Water)
Softer, Less strong Harder and stronger
More ductile More brittle
Less internal stress More internal stress
Less distortion, Cracking More distortion, Cracking

20. • The heat treatment to form martensite consists of two steps:
Austenitizing and Quenching.
These steps are frequently followed by tempering to produce tempered
martensite.
• Austenitizing temperature can be determined from the phase diagram for
the particular alloy composition.
• General Steps to be followed in HT
• Heating above lower critical temperature
• Sufficient time to form new phase of structure
• Cooling and Quenching

21. Heat treatment of steels
Hardening Softening Conditioning
Direct hardening
Austenitize and
Quench
Selective austenitize
and Quench
• Flame
• Induction
• Laser
• Electron Beam
Diffusion treatment
Carburize(Diffuse C)
Nitride(Diffuse N)
Boronize(Diffuse B)
Carbonitriding (Diffuse C
+N)
Ferritic Nitro carburizing
(Diffuse C +N)
Recrystallization
Annealing
• Process
• Full
• Spherodizing
• Normalizing
Tempering
• Austempering
• Martempering
Heating for Hot
working
Stress relieving
Spring aging
Steam treating
Cryogenic treatment

22. • Hardening:
Hardening of steels is done to increase the strength and wear
properties.
• Softening:
Softening is done to reduce strength or hardness, remove residual
stresses, improve toughness, restore ductility, refine grain size or change the
electromagnetic properties of the steel.
• Restoring ductility or removing residual stresses is a necessary operation when a
large amount of cold working is to be performed, such as in a cold-rolling operation
or wiredrawing.
• Material Modification:
Heat treatment is used to modify properties of materials in addition to
hardening and softening. These processes modify the behaviour of the steels
in a beneficial manner to maximize service life.

23. Direct hardening
Hardness
• is a function of the Carbon content of
the steel. (0.6% or above C)
• Hardening of a steel requires a change
in structure from the BCC to FCC
structure found in the Austenitic
region.
• When suddenly quenched, the
Martensite is formed. This is a very
strong and brittle structure.
• When slowly quenched it would form
Austenite and Pearlite which is a partly
hard and partly soft structure.
• When the cooling rate is extremely
slow then it would be mostly Pearlite
which is extremely soft.

24. • Quenching is the act of rapidly cooling the hot steel to harden the steel.
• Water:
• Good rapid quenching medium,
• Corrosive with steel,
• Cause distortion or cracking.
• Salt Water:
• More corrosive than plain water
• Must be rinsed off immediately.
• Oil:
• Slower cooling rate
• Reduces the likelihood of cracking.
• Results in fumes, spills, and sometimes a fire hazard.
• Polymer quench:
• Cooling rate in between water and oil, it can vary by alter the mixture of water and glycol
polymers
• Repeatable results with less corrosion and fire hazard
• Cryogenic Quench:
• Function of the lowest temperature
• Fully martensite to avoid cracking and dimensional instability
• To eliminate retained austenite in high carbon and high alloy steel

25. Effect of cooling rate on microstructure
• Water quench
• Tensile stress=175e4 N/mm2
• No yield
• Low reduction of area
• Rc= 65
• Furnace cooled
• Tensile stress=52e4 N/mm2
• Yield stress=14e4 N/mm2
• Low reduction of area
• Rc= 15
• Air cooled
• Tensile stress=88e4 N/mm2
• Yield stress=28e4 N/mm2
• Low reduction of area
• Rc= 25
Tempering
Temp>723
deg C

26. Selective hardening
• Carbon steels that have minimum carbon content of 0.4%, or alloy
steels with a lower carbon content (hardenable stainless steels with
only 0.1% Carbon),
• Selectively hardenened in specific regions by applying heat and
quench only to those regions.
• These techniques are best suited for medium carbon steels with a
carbon content ranging from 0.4 to 0.6%.

27. • Flame Hardening: A high intensity oxy-acetylene
flame is applied to the selective region.
Tempering can be done to eliminate brittleness.
• The depth of hardening can be increased by
increasing the heating time. As much as 6.3 mm
(0.25 in) of depth can be achieved.
• In addition, large parts, which will not normally
fit in a furnace, can be heat-treated.

28. • Induction Hardening: In Induction hardening,
the steel part is placed inside a electrical coil
which has alternating current through it.
• This energizes the steel part and heats it up.
Depending on the frequency and amperage,
the rate of heating as well as the depth of
heating can be controlled.
• The details of heat treatment are similar to
flame hardening.
• Hardness to depth of 0.8mm in 1~5 sec
Typical applications of induction hardening include gears, shafts, axles, cam lobes, stampings, and
spindles, mostly symmetrical parts. Induction hardening is used to strengthen a specific area of a part.
Single piece, surface hardening of selective areas.
•Carbon steels,Alloy steels,Stainless steels(martensitic),Powder metal,Cast iron,Gray iron,Ductile iron

29. Laser Beam Hardening:
• Laser beam hardening is another variation of flame hardening. A phosphate
coating is applied over the steel to facilitate absorption of the laser energy.
• By varying the power of the laser, the depth of heat absorption can be
controlled.
• The parts are then quenched and tempered. This process is very precise and
can be run at high speeds, produces very little distortion.
• Case depth 0.75mm

30. Electron Beam Hardening:
• Electron Beam Hardening is similar to laser beam hardening. The heat
source is a beam of high-energy electrons.
• The beam is manipulated using electromagnetic coils. The process can
be highly automated, but needs to be performed under vacuum
conditions since the electron beams dissipate easily in air.
• As in laser beam hardening, the surface can be hardened very precisely
both in depth and in location.
• Case depth is 0.5~1.5mm
• Automotive transmission clutch

31. Diffusion treatment
• Carbon content is low (less than
0.25% for example) then an alternate
means to increase the Carbon content
of the surface.
• The part then can be heat treated by
either quenching in liquid or cooling
in still air depending on the properties
desired.

32. • Carburizing also referred to as Case Hardening, is a heat treatment
process that produces a surface which is resistant to wear, while
maintaining toughness and strength of the core.
• This treatment is applied to low carbon steel parts after machining, as
well as high alloy steel bearings, gears, and other components
• This is done by exposing the part to a Carbon rich atmosphere at an
elevated temperature and allows diffusion to transfer the Carbon atoms
into steel.

33. • Pack Carburizing: Parts are packed in a high
carbon medium such as carbon powder or
cast iron shavings and heated in a furnace
for 12 to 72 hours at 900 ⁰ C (1652 ⁰ F).
• CO gas is produced which is a strong
reducing agent. The reduction reaction
occurs on the surface of the steel releasing
Carbon, which is then diffused into the
surface due to the high temperature.
• The Carbon on the surface is 0.7% to 1.2%
depending on process conditions. The
hardness achieved is 60 - 65 RC. The depth of
the case ranges from about 0.1 mm (0.004
in) upto 1.5 mm (0.060 in).
• Difficult to control and inefficient heating

34. • Gas Carburizing: Gas Carburizing is conceptually the same as pack
carburizing, except that Carbon Monoxide (CO) gas is supplied to a
heated furnace and the reduction reaction of deposition of carbon takes
place on the surface of the part.
• This processes overcomes most of the problems of pack carburizing.
The temperature diffusion is as good as it can be with a furnace.
• A variation of gas carburizing is when alcohol is dripped into the furnace
and it volatilizes readily to provide the reducing reaction for the
deposition of the carbon.
• The carbon diffuses into the metal surface usually to a depth between 1
and 3mm

35. • Liquid Carburizing: The steel parts are immersed in a molten
carbon rich bath. In the past, such baths have cyanide (CN) as the
main component.
• However, safety concerns have led to non-toxic baths that achieve the
same result.

36. • Nitriding The Nitrogen forms Nitrides with
elements such as Aluminum, Chromium,
Molybdenum, and Vanadium.
• The parts are heat treated and tempered before
nitriding.
• The parts are then cleaned and heated in a
furnace in an atmosphere of dissociated
Ammonia (containing N and H) for 10 to 40 hours
at 500-625 ⁰C (932 - 1157 ⁰ F).
• Nitrogen diffuses to a depth of upto 0.65 mm
(0.025 in).
• The case is very hard and distortion is low. No
further heat treatment is required; in fact, further
heat treatment can crack the hard case.
• Since the case is thin, surface grinding is not
recommended.
Typical applications
include gears, crankshafts, camshafts, cam
followers, valve parts, extruder screws, die-
casting tools, forging dies, extrusion dies,
firearm components, injectors and plastic-
mold tools

37. • Carbonitriding process is most suitable for low carbon and low carbon
alloy steels. In this process, both Carbon and Nitrogen are diffused into the
surface.
• The parts are heated in an atmosphere of hydrocarbon (such as methane or
propane) mixed with Ammonia (NH3). The process is a mix of Carburizing
and Nitriding.
• Carburizing involves high temperatures (around 900 ⁰ C, 1652 ⁰ F) and
Nitriding involves much lower temperatures (around 600 ⁰ C, 1112 ⁰ F).
• Carbonitriding is done at temperatures of 760 - 870 ⁰ C (1400 - 1598 ⁰ F),
which is higher than the transformation temperatures of steel that is the
region of the FCC Austenite.

38. • Quenched in a netural gas (Oxygen free) atmosphere. This quench is less drastic than
water or oil,thus less distortion. This process is not suitable for high precision parts due
to the distortions that are inherent.
• The hardness achieved is similar to carburizing (60 - 65 RC) but not as high as Nitriding
(70 RC). The case depth is from 0.1 to 0.75 mm (0.004 to 0.030 in).

39. Comparison

40. Annealing
• Material is exposed to an elevated temperature but below critical
temperature for an extended time period and then slowly cooled, allowing
phase changes.
• Purpose
• Alteration of ductility and toughness
• Induction of softness
• Refinement of grain size and
• Removal of gases and stresses
• Utilized for low- and medium-carbon steels.
• Full Annealing
• Process Annealing or Stress Relief Annealing
• Spheroidising
• Normalizing

41. • Full annealing is the process of slowly raising the
temperature about 50 0C (122 0F) above
the Austenitic temperature line A3 , or above A1
line into the Austenite-Cementite region.
• It is then slowly cooled at the rate of about 20
0C/hr (360F/hr) in a furnace to about 50 0C (122 0F)
into the Ferrite-Cementite range.
• The grain structure has coarse Pearlite with ferrite
or Cementite (depending on whether hypo or
hyper eutectoid). The steel becomes soft and
ductile.
A1
A3
Acm

T
Wt% C
0.8 %
723C
910C
Full Annealing
Full Annealing

42. • Normalizing is the process of raising the
temperature to over 60 0C (140 0F), above line
A3 or line ACM fully into the Austenite range. It is
held at this temperature to fully convert the
structure into Austenite, and then removed from
the furnace and cooled at room temperature under
natural air convection.
• This results in a grain structure of fine Pearlite with
excess of Ferrite or Cementite. The resulting
material is soft; This process is considerably
cheaper than full annealing since there is not the
added cost of controlled furnace cooling.
• The main difference between full annealing and
normalizing is that fully annealed parts are uniform
in softness because of exposed to the controlled
furnace cooling.
• In the case of the normalized part, the cooling is
non-uniform resulting in non-uniform material
properties across the part.
• This may not be desirable if further machining is
desired, since it makes the machining job
somewhat unpredictable. In such a case it is better
to do full annealing.
A1
A3
Acm

T
Wt% C
0.8 %
723C
910C
Normalization
Normalization

43. • Process Annealing is used to treat work-
hardened parts made out of low-Carbon
steels (< 0.25% Carbon). This allows the parts
to be soft enough to undergo further cold
working without fracturing.
• Process annealing is done by raising the
temperature to just below the Ferrite-
Austenite region, line A1on the diagram. This
is held long enough to allow recrystallization
of the ferrite phase, and then cooled in still
air.
• The only change that occurs is the size, shape
and distribution of the grain structure.
• This process is cheaper than either full
annealing or normalizing since the material is
not heated to a very high temperature or
cooled in a furnace
A1
A3
Acm

T
Wt% C
0.8 %
723C
910C
Recrystallization Annealing

44. • Stress Relief Anneal is used
to reduce residual stresses in large
castings, welded parts and cold-
formed parts. Such parts tend to
have stresses due to thermal
cycling or work hardening.
• Parts are heated to temperatures
of up to 600 - 650 0C (1112 - 1202
0F), and held for an extended time
(about 1 hour or more) and then
slowly cooled in still air.
A1

T
Wt% C
0.8 %
723C
910C
Stress Relief Annealing

45. Spheroidization is an annealing process used for
high carbon steels (Carbon > 0.6%) that will be
machined or cold formed subsequently. This is
done by one of the following ways:
• Heat the part to a temperature just below
the Ferrite-Austenite line, line A1 or below
the Austenite-Cementite line
• Hold the temperature for a prolonged time
and follow by fairly slow cooling.
• The main purpose of the treatment is to
increase the ductility of the sample.
• Spheroidization also improves resistance
to abrasion
A1
A3
Acm

T
Wt% C
0.8 %
723C
910C
Spheroidization

46. Tempering
• Steel is usually harder than necessary and too brittle for practical use after
being hardened. Severe internal stresses are set up during the rapid cooling
of the metal.
• Steel is tempered after being hardened to relieve the internal stresses and
reduce its brittleness.
• Tempering consists of heating the metal to a specified temperature and
then permitting the metal to cool. The rate of cooling usually has no effect
on the metal structure during tempering. Therefore, the metal is usually
permitted to cool in still air.
• Temperatures used for tempering are normally much lower than the
hardening temperatures. The higher the tempering temperature used, the
softer the metal becomes.
• High-speed steel is one of the few metals that becomes harder instead of
softer after it is tempered.

47. • The mechanism of tempering depends on the
steel and the tempering temperature. The
prevalent Martensite is a somewhat unstable
structure.
• When heated, the Carbon atoms diffuse from
Martensite to form a carbide precipitate and
the concurrent formation of Ferrite and
Cementite, which is the stable form.
• Tool steels for example, lose about 2 to 4
points of hardness on the Rockwell C scale.
• Toughness (as measured by impact strength)
is increased substantially.
• Springs and such parts need to be much
tougher, these are tempered to a much lower
hardness.

48. • Austempering is a quenching technique that
is applied to ferrous metals, most notably
steel and ductile iron
• The part is not quenched through the
Martensite transformation. Instead the
material is quenched above the temperature
when Martensite forms MS, around 315 0C
(600 0F). It is held till at this temperature till
the entire part reaches this temperature.
• As the part is held longer at this temperature,
the Austenite transforms into Bainite.
• Bainite is tough enough so that further
tempering is not necessary, and the tendency
to crack is severely reduced.

49. • Martempering is a heat treatment for
steel involving austenitisation followed
by step quenching, at a rate fast enough
to avoid the formation of ferrite, pearlite
or bainite to a temperature slightly
above the martensite start (Ms) point.
• The drawback of this process is that the
large section cannot be heat treated by
this process

50. Stress relieving
• Machining induces stresses in parts. The bigger and more complex the
part, the more the stresses. These stresses can cause distortions in the part
long term.
• The treatment is not intended to produce significant changes in material
structures or mechanical properties
• If the parts are clamped in service, then cracking could occur. Also hole
locations can change causing them to go out of tolerance. For these
reasons, stress relieving is often necessary.
• Typically, the parts that benefit from stress relieving are large and complex
weldments, castings with a lot of machining, parts with tight dimensional
tolerances and machined parts that have had a lot of stock removal
performed.

51. • Stress relieving is done by subjecting
the parts below the transformation
temperature line A1 on the diagram,
which is about 723 0C (1340 0F) of
steel.
• Stress relieving is done at about
650 0C (1202 0F) for about one hour
or till the whole part reaches the
temperature.
• This removes more than 90% of the
internal stresses. Alloy steels are
stress relieved at higher
temperatures.
• After removing from the furnace, the
parts are air cooled in still air.

52. Steam treating:
• Steam treating is a controlled oxidation
treatment of metals to produce a thin layer
of oxide on the surface of the component.
The process is used to impart increased
corrosion and wear resistance, to increase
surface hardness, and to provide an
aesthetically pleasing surface finish.
• In the case of porous materials, such as
powder metallurgy parts, the process seals
porosity and increases the density

53. • Fe3O4 is an oxide of iron that is blue to black in
color and has a microhardness of ~50 HRC.
• Below diagram describes the process

54. Spring aging:
• A wound spring can lose its spring tension due to anelastic behavior,
which causes the spring to unwind or change its shape over time.
• To avoid this springs are placed in an oven at 315 - 3750 C for 2 hours
for spring aging. This will allow the spring to change shape or unwind.
• This unwinding or changing shape can be accommodated during the
design of the spring and be compensated. Once the springs are
treated to spring aging, they do not usually change shape.
• This heat treatment is recommended on any springs and formed parts
that need good dimensional accuracy in the formed shape.

55. Cryogenic treatment
• Cryogenics, or deep freezing is done to
make sure there is no retained Austenite
during quenching. When steel is at the
hardening temperature, there is a solid
solution of Carbon and Iron, known as
Austenite.
• The amount of Martensite formed at
quenching is a function of the lowest
temperature encountered. At any given
temperature of quenching there is a
certain amount of Martensite and the
balance is untransformed Austenite.
• This untransformed austenite is very brittle
and can cause loss of strength or hardness,
dimensional instability, or cracking.

56. • Quenches are usually done to room temperature. Most medium
carbon steels and low alloy steels undergo transformation to 100 %
Martensite at room temperature.
• However, high carbon and high alloy steels have retained Austenite at
room temperature. To eliminate retained Austenite, the temperature
has to be lowered.
• In Cryogenic treatment the material is subject to deep freeze
temperatures of as low as -185°C (-301°F), but usually -75°C (-103°F)
is sufficient.
• The Austenite is unstable at this temperature, and the whole
structures becomes Martensite. This is the reason to use Cryogenic
treatment.

57. Summary:
• Annealing(Quench medium-Furnace)
• Full annealing-Ductility inc toughness inc
• Process annealing-Stress relieving
• Spherodise annealing-Machinability inc
• Diffusion annealing-Homogenize the
chemistry of material
• Normalizing(Air)
• Hard surface and tough core
• Hardening
• Produce the Martensite
• Water-Vapour blanket,non uniform
cooling
• Oil-Uniform cooling
• Tempering
• Introducing toughness
• Case hardening
• Surface hardness by diffusion of C and N
• Carburising
• Pack carburising-50% charcoal
• Liquid
• 20% NaCN
• Gas
• CH4 gas
• Cyaniding
• 30% NaCN,NaCl
• Flame hardening
• Carburising flame-2900 deg C
• Neutral flame-3150 deg C
• Oxidizing flame-3500 deg C
• Induction hardening
• Fast method
• Suitable for medium carbon steel