Heat treatment

1. Heat
Treatment of
Steel
Lecture 9

2. Heat-Treatment
Heat treatment is a method used to alter the
physical, and sometimes chemical properties
of a material. The most common application
is metallurgical
It involves the use of heating or chilling,
normally to extreme temperatures, to achieve
a desired result such as hardening or
softening of a material
It applies only to processes where the heating
and cooling are done for the specific
purpose of altering properties intentionally

4. Types of Heat-Treatment (Steel)
Annealing / Normalizing,
Case hardening,
Precipitation hardening,
Tempering, and Quenching

5. Time-Temperature-
Transformation (TTT)Curve
TTT diagram is a plot of temperature versus the
logarithm of time for a steel alloy of definite
composition.
It is used to determine when transformations
begin and end for an isothermal heat treatment
of a previously austenitized alloy
TTT diagram indicates when a specific
transformation starts and ends and it also shows
what percentage of transformation of austenite
at a particular temperature is achieved.

6. Time-Temperature-
Transformation (TTT)Curve
The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at
900°C

7. Decarburization during Heat
Treatment
Decrease in content of carbon in metals is
called Decarburization
It is based on the oxidation at the surface of
carbon that is dissolved in the metal lattice
In heat treatment processes iron and carbon
usually oxidize simultaneously
During the oxidation of carbon, gaseous
products (CO and CO2) develop
In the case of a scale layer, substantial
decarburization is possible only when the
gaseous products can escape

8. Decarburization Effects
The strength of a steel depends on the
presence of carbides in its structure
In such a case the wear resistance is
obviously decreased
In many circumstances, there can be a
serious drop in fatigue resistance
To avoid the real risk of failure of
engineering components, it is essential to
minimize decarburization at all stages in the
processing of steel

9. Annealing
It is a heat treatment wherein a material is
altered, causing changes in its properties
such as strength and hardness
It the process of heating solid metal to
high temperatures and cooling it slowly so
that its particles arrange into a defined
lattice

10. Types of Annealing
1. Stress-Relief Annealing (or Stress-relieving)
2. Normalizing
3. Isothermal Annealing
4. Spheroidizing Annealing (or Spheroidizing )

11. 1. Stress-Relief Annealing
It is an annealing process
below the transformation
temperature Ac1, with
subsequent slow cooling,
the aim of which is to
reduce the internal residual
stresses in a workpiece
without intentionally
changing its structure and
mechanical properties

12. Causes of Residual Stresses
1. Thermal factors (e.g., thermal stresses
caused by temperature gradients within the
workpiece during heating or cooling)
2. Mechanical factors (e.g., cold-working)
3. Metallurgical factors (e.g., transformation
of the microstructure)

13. How to Remove Residual Stresses?
R.S. can be reduced only by a plastic
deformation in the microstructure.
This requires that the yield strength of the material
be lowered below the value of the residual
stresses.
The more the yield strength is lowered, the greater
the plastic deformation and correspondingly the
greater the possibility or reducing the residual
stresses
The yield strength and the ultimate tensile
strength of the steel both decrease with
increasing temperature

14. Stress-Relief Annealing
Process
For plain carbon and low-alloy steels the
temperature to which the specimen is heated
is usually between 450 and 650˚C, whereas for
hot-working tool steels and high-speed steels it
is between 600 and 750˚C
This treatment will not cause any phase
changes, but recrystallization may take place.
Machining allowance sufficient to
compensate for any warping resulting from
stress relieving should be provided

15. Stress-Relief Annealing – R.S.
In the heat treatment of metals, quenching or
rapid cooling is the cause of the greatest residual
stresses
To activate plastic deformations, the local
residual stresses must be above the yield strength
of the material.
Because of this fact, steels that have a high yield
strength at elevated temperatures can withstand
higher levels of residual stress than those that
have a low yield strength at elevated
temperatures
Soaking time also has an influence on the effect
of stress-relief annealing

16. Relation between heating
temperature and Reduction
in Residual Stresses
Higher temperatures and
longer times of annealing
may reduce residual
stresses to lower levels

17. Stress Relief Annealing -
Cooling
The residual stress level after stress-relief annealing will
be maintained only if the cool down from the
annealing temperature is controlled and slow enough
that no new internal stresses arise.
New stresses that may be induced during cooling
depend on the (1) cooling rate, (2) on the cross-
sectional size of the workpiece, and (3)on the
composition of the steel

18. 2. Normalizing
A heat treatment process consisting of
austenitizing at temperatures of 30–80˚C
above the AC3 transformation
temperature followed by slow cooling
(usually in air)
The aim of which is to obtain a fine-
grained, uniformly distributed, ferrite–
pearlite structure
Normalizing is applied mainly to
unalloyed and low-alloy hypoeutectoid
steels
For hypereutectoid steels the
austenitizing temperature is 30–80˚C
above the AC1 or ACm transformation
temperature

19. Normalizing – Heating and
Cooling

20. Normalizing – Austenitizing
Temperature Range

21. Effect of Normalizing on Grain Size
Normalizing refines the grain of a steel that has
become coarse-grained as a result of heating
to a high temperature, e.g., for forging or
welding
Carbon steel of 0.5% C. (a) As-rolled or forged;
(b) normalized. Magnification 500

22. Need for Normalizing
Grain refinement or homogenization of the
structure by normalizing is usually performed
either to improve the mechanical properties
of the workpiece or (previous to hardening)
to obtain better and more uniform results
after hardening
Normalizing is also applied for better
machinability of low-carbon steels

23. Normalizing after Rolling
After hot rolling, the
structure of steel is
usually oriented in the
rolling direction
To remove the oriented
structure and obtain the
same mechanical
properties in all
directions, a normalizing
annealing has to be
performed

24. Normalizing after Forging
After forging at high temperatures,
especially with workpieces that vary
widely in crosssectional size, because
of the different rates of cooling from
the forging temperature, a
heterogeneous structure is obtained
that can be made uniform by
normalizing

25. Normalizing – Holding Time
Holding time at austenitizing temperature
may be calculated using the empirical
formula:
t = 60 + D
where t is the holding time (min) and D is the
maximum diameter of the workpiece (mm).

26. Normalizing - Cooling
Care should be taken to ensure that the cooling
rate within the workpiece is in a range
corresponding to the transformation behavior of
the steel-in-question that results in a pure ferrite–
pearlite structure
If, for round bars of different diameters cooled in
air, the cooling curves in the core have been
experimentally measured and recorded, then by
using the appropriate CCT diagram for the steel
grade in question, it is possible to predict the
structure and hardness after normalizing

27. 3. Isothermal Annealing
Hypoeutectoid low-carbon steels as well as
medium-carbon structural steels are often
isothermally annealed, for best machinability
An isothermally annealed structure should have
the following characteristics:
1. High proportion of ferrite
2. Uniformly distributed pearlite grains
3. Fine lamellar pearlite grains

28. Principle of Isothermal
Annealing
Bainite formation
can be avoided
only by very slow
continuous cooling,
but with such a
slow cooling a
textured
(elongated ferrite)
structure results
(hatched area)

29. Process - Isothermal
Annealing
Austenitizing followed by a fast cooling to the
temperature range of pearlite formation (usually
about 650˚C.)
Holding at this temperature until the complete
transformation of pearlite
and cooling to room temperature at an arbitrary
cooling rate

30. 4. Spheroidizing Annealing
It is also called as Soft
Annealing
Any process of heating and
cooling steel that produces
a rounded or globular form
of carbide
It is an annealing process at
temperatures close below or
close above the AC1
temperature, with
subsequent slow cooling

31. Spheroidizing - Purpose
The aim is to produce a soft structure by changing all
hard constituents like pearlite, bainite, and
martensite (especially in steels with carbon contents
above 0.5% and in tool steels) into a structure of
spheroidized carbides in a ferritic matrix
(a) a medium-carbon low-alloy steel after soft annealing at 720C;
(b) a high-speed steel annealed at 820C.

32. Spheroidizing - Uses
Such a soft structure is required for good
machinability of steels having more than
0.6%C and for all cold-working processes
that include plastic deformation.
Spheroidite steel is the softest and most
ductile form of steel

33. Spheroidizing - Mechanism
The physical mechanism of soft annealing is
based on the coagulation of cementite
particles within the ferrite matrix, for which the
diffusion of carbon is decisive
Globular cementite within the ferritic matrix is
the structure having the lowest energy
content of all structures in the iron–carbon
system
The carbon diffusion depends on temperature
and time

34. Spheroidizing - Mechanism
The solubility of carbon in ferrite, which is
very low at room temperature (0.02% C),
increases considerably up to the Ac1
temperature
At temperatures close to Ac1, the diffusion of
carbon, iron, and alloying atoms is so great
that it is possible to change the structure in
the direction of minimizing its energy
content

35. Spheroidizing - Process
Prolonged heating at a temperature just bel
ow the lower critical temperature, usually foll
owed by relatively slow cooling
In the case of small objects of high C steels,
the spheroidizing result is achieved more ra
pidly by prolonged heating to temperatures
alternately within and slightly below the critical
temperature range
Tool steel is generally spheroidized by heating
to a temperature of 749°-804°C and higher for
many alloy tool steels, holding at heat from 1 to
4 hours, and cooling slowly in the furnace

36. CASE HARDENING
Case hardening or surface hardening is the
process of hardening the surface of a
metal, often a low carbon steel, by infusing
elements into the material's surface,
forming a thin layer of a harder alloy.
Case hardening is usually done after the
part in question has been formed into its
final shape

37. Case-Hardening - Processes
Flame/Induction Hardening
Carburizing
Nitriding
Cyaniding
Carbonitriding

38. Flame and induction hardening
Flame or induction hardening are processes in
which the surface of the steel is heated to high
temperatures (by direct application of a flame,
or by induction heating) then cooled rapidly,
generally using water
This creates a case of martensite on the
surface.
A carbon content of 0.4–0.6 wt% C is needed
for this type of hardening
Application Examples - Lock shackle and
Gears

39. Carburizing
Carburizing is a process used to case harden
steel with a carbon content between 0.1 and
0.3 wt% C.
Steel is introduced to a carbon rich
environment and elevated temperatures for a
certain amount of time, and then quenched
so that the carbon is locked in the structure
Example - Heat a part with an acetylene
torch set with a fuel-rich flame and quench it
in a carbon-rich fluid such as oil

40. Carburizing
Carburization is a diffusion-controlled
process, so the longer the steel is held in
the carbon-rich environment the greater
the carbon penetration will be and the
higher the carbon content.
The carburized section will have a carbon
content high enough that it can be
hardened again through flame or
induction hardening

41. Carburizing
The carbon can come from a solid, liquid or
gaseous source
Solid source - pack carburizing. Packing low
carbon steel parts with a carbonaceous material
and heating for some time diffuses carbon into
the outer layers.
A heating period of a few hours might form a
high-carbon layer about one millimeter thick
Liquid Source - involves placing parts in a bath
of a molten carbon-containing material, often a
metal cyanide
Gaseous Source - involves placing the parts in a
furnace maintained with a methane-rich interior

42. Nitriding
Nitriding heats the steel part to 482–621°C in an
atmosphere of NH3 gas and broken NH3.
The time the part spends in this environment
dictates the depth of the case.
The hardness is achieved by the formation of
nitrides.
Nitride forming elements must be present in the
workpiece for this method to work.
Advantage - it causes little distortion, so the part
can be case hardened after being quenched,
tempered and machined

43. Cyaniding
Cyaniding is mainly used on low carbon steels.
The part is heated to 870-950°C in a bath of
sodium cyanide (NaCN)and then is quenched
and rinsed, in water or oil, to remove any
residual cyanide.
The process produces a thin, hard shell (0.5-
0.75mm) that is harder than the one
produced by carburizing, and can be
completed in 20 to 30 minutes compared to
several hours.
It is typically used on small parts.
The major drawback of cyaniding is that
cyanide salts are poisonous

44. Carbonitriding
Carbonitriding is similar to cyaniding except
a gaseous atmosphere of ammonia and
hydrocarbons (e.g. CH4)is used instead of
sodium cyanide.
If the part is to be quenched then the part is
heated to 775–885°C; if not then the part is
heated to 649–788°C

45. PRECIPITATION HARDENING
Precipitation hardening (or age hardening), is
a heat treatment technique used to increase
the yield strength of malleable materials
Malleable materials are those, which are
capable of deforming under compressive
stress
It relies on changes in solid solubility with
temperature to produce fine particles of an
impurity phase, which blocks the movement of
dislocations in a crystal's lattice

46. Precipitation Hardening
Since dislocations are often the dominant
carriers of plasticity, this serves to harden
the material
The impurities play the same role as the
particle substances in particle-reinforced
composite materials.
Alloys must be kept at elevated
temperature for hours to allow
precipitation to take place. This time
delay is called aging

47. Precipitation Hardening
Two different heat treatments involving
precipitates can change the strength of a
material:
1. solution heat treating
2. precipitation heat treating
Solution treatment involves formation of a
single-phase solid solution via quenching
and leaves a material softer
Precipitation treating involves the addition of
impurity particles to increase a material's
strength

48. Precipitation Mechanism –
Aluminum Alloy

49. Effect of Aging Time on
Precipitates

50. QUENCHING and TEMPERING
In quench hardening, fast cooling
rates, depending on the
chemical composition of the steel
and its section size, are applied to
prevent diffusion-controlled trans
formations in the pearlite range
and to obtain a structure
consisting mainly of martensite
and bainite
However, the reduction of
undesirable thermal and
transformational stresses usually
requires slower cooling rates

51. Quenching
To harden by quenching, a
metal must be heated into
the austenitic crystal phase
and then quickly cooled
Cooling may be done with
forced air, oil, polymer
dissolved in water, or brine
Upon being rapidly cooled, a
portion of austenite
(dependent on alloy
composition) will transform to
martensite

52. Quenching
Cooling speeds, from fastest to slowest, go
from polymer, brine, fresh water, oil, and
forced air
However, quenching a certain steel too fast
can result in cracking, which is why high-tensile
steels such as AISI 4140 should be quenched in
oil, tool steels such as H13 should be quenched
in forced air, and low alloy such as AISI 1040
should be quenched in brine
Metals such as austenitic stainless steel (304,
316), and copper, produce an opposite effect
when these are quenched: they anneal

53. Tempering
Untempered martensite, while very hard, is
too brittle to be useful for most applications.
In tempering, it is required that quenched
parts be tempered (heat treated at a low
temperature, often 150˚C) to impart some
toughness.
Higher tempering temperatures (may be up
to 700˚C, depending on alloy and
application) are sometimes used to impart
further ductility, although some yield strength
is lost

54. Tempering
Tempering is done to toughen the metal by
transforming brittle martensite or bainite into a
combination of ferrite and cementite or
sometimes Tempered martensite
Tempered martensite is much finer-grained
than just-quenched martensite
The brittle martensite becomes tough and
ductile after it is tempered.
Carbon atoms were trapped in the austenite
when it was rapidly cooled, typically by oil or
water quenching, forming the martensite

55. Tempering
The martensite becomes tough after being
tempered because when reheated, the
microstructure can rearrange and the
carbon atoms can diffuse out of the
distorted body-centred-tetragonal (BCT)
structure.
After the carbon diffuses out, the result is
nearly pure ferrite with body-centred
structure.