Injustice - Developers Among Us (SciFiDevCon 2024)
HEAT TREATMENT.pptx
1.
2. HEAT TREATMENT,Need of heat treatment
NUCLEATION
TTT Diagram(TIME-TEMPERATURE-TRANSFORMATION Diagram)
CCT Diagram(CONTINUOUS COOLING TRANSFORMATION CURVE)
Various Heat Treatment processes , its effect on material properties
3. Metals and alloys may not posses all the desired properties in the finished
product. Alloying and heat treatment are two methods which are extensively
used for controlling material properties.
In heat treatment, the microstructures of materials are modified. The
resulting phase transformation influences mechanical properties like
strength , ductility, toughness, hardness and wear resistance.
Most of the engineering components are made out of casting, forging,
rolling and welding and other processes.During processing, these
components may be subjected to severe stresses or it may have residual
stresses.
Heat treatments are the only solutions to recover and refine the grains by
removing the internal stresses from the materials
Non-uniform heat distribution and Dissimilar grain size
during welding
4. DEFINITION:HEAT TREATMENT
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).
5. Soaking
-Internal structural changes take place.
-soaking period depends on the chemical analysis of the metal and the
mass of the part.
Cooling Stage
- To cool the metal, you can place it in direct contact with a COOLING
MEDIUM composed of a gas, liquid, solid, or combination of these.
6. Some examples of Heat Treatment are
ANNEALING(Full annealing, Process annealing and Spheroidising)
Normalising
Hardening
Tempering
Case hardening
Surface hardening
ANY MANY MORE………
7. NEED OF HEAT TREATMENT
The primary need of heat treatment processes is to change the
properties of a material i.e,to obtain a desired microstructure to
achieve some predetermined properties so that it can be used in a
suitable purpose.
To relieve the internal stresses set up during hot / cold
working
To refine the grains and grain structure
To improve machinability
To improve ductility and toughness
To improve heat, wear and corrosion resistance
To improve red or hot hardness
To improve formability
FOR INSTANCE
to increase strength, hardness and wear resistance (bulk hardening, surface
hardening)
to increase ductility and softness (tempering, recrystallization annealing)
to increase toughness (tempering, recrystallization annealing)
to improve cutting properties of tool steels (hardening and tempering)
to improve surface properties (surface hardening, corrosion resistance-stabilising
treatment and high temperature resistance-precipitation hardening, surface
treatment)
8. NUCLEATION AND
GROWTH
Aggregation of molecules builds larger and
larger molecules – becomes a nucleus at some
point
Nucleus– size of this is either big enough to
continue growth or will re-dissolve (Critical size)
Overall rate of nucleus formation vs. crystal
growth determines crystal size/distribution
(a) nucleus of a metal
(b) nucleus of an organic
crystal
(c) nucleus of a long-chain
molecular liquid
9. The rate of nucleation(i.e, the rate at which
nuclei of critical size or larger appear) is the
result of two competing factors--------
At precise transformation temperature(for
example-melting point),the solid and liquid
particles are in equilibrium and there is no
net driving force for the transformation to
occur.As the liquid is cooled below
transformation temperature,it becomes
increasingly unstable.The driving force for
solidification increases and rate of
nucleation increases sharply.This increase
cannot continue indefinitely.
The clustering of atoms to form a nucleus is
a local-scale diffusion process.As such,this
step decrease in rate with decreasing
temperature.This rate is exponential in
nature(just like ARREHENIUS behaviour).
The overall nucleation rate is due to combined
effects of these ……….
10. Phase diagram and TTT
diagram
• Phase diagram :
– Describes equilibrium
microstructural
development that is
obtained at extremely
slow cooling or heating
conditions.
– Provides no information
on time taken to form
phase
• TTT diagram
– For a given alloy
composition, the
percentage completion
of a given phase
transformation on
temperature-time axes
is described.
Which information are obtained from phase diagram or
TTT diagram?
13. NUCLEATION RATE
• As temperature decreases below eutectoid temperature,
r* (critical size of nucleus) decreases increasing the
nucleation rate N.
• At very low temperature, nucleation rate decreases due
to large decrease in diffusion rate.
• At intermediate temperature, nucleation rate is
maximum
14. Growth of nuclei is a diffusion controlled process
Growth Rate
where QD : activation energy for self
diffusion
Growth rate decreases with decrease in temperature
RT
QD
ce
G
.
15. Transformation rate of a phase :
Transformation rate first increases, reaches a maximum and
then starts decreasing with decrease in temperature
.
.
G
N
16. TIME FOR
TRANSFORMATION
Time required for transformation as a function of
temperature follows a reverse trend than the rate of
transformation.
Time required for transformation fist decreases,
reaches a minimum and then starts increasing with
decrease in temperature.
17. Indicates the amount of transformation at a constant
temperature.
Samples are austenitised and then cooled rapidly to a lower
temperature and held at that temperature whilst the amount
of transformation is measured, for example by dilatometry.
Obviously a large number of experiments are required to
build up a complete TTT diagram
TIME-TEMPERATURE-TRANSFORMATION(TTT)
DIAGRAM
20. TTT DIAGRAM Indicates the amount
of transformation at a constant
temperature.
21.
22. Upper half of TTT Diagram(Austenite-
Pearlite Transformation Area)
Lower half of TTT Diagram (Austenite-
Martensite and Bainite Transformation
Areas)
TTT Diagram and microstructures
obtained by different types of cooling rates
23. Full TTT Diagram
The complete TTT
diagram for an iron-
carbon alloy of eutectoid
composition.
A: austenite
B: bainite
M: martensite
P: pearlite
24. PEARLITE CAN BE ALSO
FORMED FROM
AUSTENTITE BY THIS
PROCESS ALSO
25. If you don’t hold at one temperature and allow
temperature to change with time, you are
“Continuously Cooling”.
In continuous cooling, the constant temperature
basis of TTT diagram becomes obviously
unrepresentative.
More relevant information can, thus, be obtained
from a CCT diagram in which phase changes are
tracked for a variety of cooling rates.
Therefore, a CCT diagram’s transition lines will be
different than a TTT diagram.
Plotting actual cooling curves on such a diagram
will show the types of transformation product
formed and their proportions.
26. Effect of Cooling Rate on the Formation of Different Reaction
Products
Very slow cooling rate (furnace cooling),
typical of conventional annealing, will result
in coarse pearlite with low hardness.
(ANNEALING)
Air cooling is a faster cooling rate than
annealing and is known as normalizing. It
produces fine pearlite.
In water quenching, entire substance
remains austentic until the Ms line is
reached, and changes to martensite
between the Ms and Mf lines.
It is possible to form 100% pearlite or 100%
martensite by continuous cooling, but it is
not possible to form 100%
Bainite.(THEORITICALLY 100%)
To obtain a bainitic structure, cool rapidly
27. Critical Cooling Rate (CCR)
• If the cooling curve is tangent
to the nose of TTT curve, the
cooling rate associated with
this cooling curve is Critical
Cooling Rate (CCR) for this
steel.
• Any cooling rate equal to or
faster than CCR will form only
martensite.
NOSE OF
TTT
CURVE
29. Factors Affecting Critical Cooling Rate
Any thing which shifts the TTT diagram
towards right will decrease the critical
cooling rate
The following factor affect the critical
cooling rate
1. Grain size
2. Carbon content
3. Alloying elements
Increase in grain size, carbon content
or alloying elements shifts the TTT
diagram towards right and hence
reduces the
critical cooling rate as shown.
30.
31. Annealing involves heating the material to a predetermined temperature
and hold the material at the temperature and cool the material to the room
temperature slowly. The process involves:
Annealing: Annealing
1) Heating of the material at the
elevated or predetermined
temperature
2) Holding the material (Soaking) at
the temperature for longer time.
3) Very slowly cooling the material
to the room temperature.
32. The various purpose of these heat treatments is to:
Annealing:
1) Relieve Internal stresses developed during
solidification, machining, forging, rolling or
weilding,
2) Improve or restore ductility and toughness,
3) Enhance Machinability,
4) Eliminate chemical non-uniformity,
5) Refrain grain size, &
6) Reduce the gaseous contents in steel.
Applicatio
n
Annealing process is employed in
following application
Casting
Forging
Press work
and many more……
34. Heating the steel to a
temperature at or near the
critical point , holding there
for a time period and then
allowing it to cool slowly in
the furnace itself .
Example
In full annealing of hypoeutectoid steels
less than 0.77% is heated to 723 to 910 C
above AC3 line convert to single phase
austenite cooled slowly in room temperature .
Resulting structure is coarse pearlite
with excess of ferrite it is quite soft and more
ductile
• it is time consuming
• require considerable
energy to maintain the
elevated temperature
GIVES
Refined grains
Removes Stains
Induces softness
Relieves internal stresses
Improves formability
35. Process annealing is a heat treatment that is often used to soften and increase the
ductility of a previously strain hardened metal . Ductility is important in shaping
and creating a more refined piece of work through processes such
as rolling, drawing, forging, spinning, extruding and heading.
Example
it is extensively employed for steel wires and sheet products (especially low
carbon steels) AC1 temperature and cooled at any desired rate
The temperature range for process annealing ranges from 260 °C (500 °F) to 760 °C
(1400 °F), depending on the alloy in question.
This process is extensively used in the treatment of sheets and wires.
Parts which are fabricated by cold forming such as stamping, extrusion, upsetting and
drawing are frequently given this treatment as an intermediate step.
Scaling or oxidation can be prevented or minimized by this process specially if annealed
at lower temperatures or in non-oxidizing areas.
GIVES
Refined grains
Eliminates residual stresses
To facilitate further cold working
36. Internal stresses are those stresses which can exist within a body in the
absence of external forces. These are also known as residual stresses are
locked-in stresses.
These stresses are developed in operations like:
Solidification of castings, welding, machining, grinding, shot peening,
surface hammering, cold working, case hardening, electroplated coatings,
precipitation and phase transformation.
As the name suggests, this process is employed to relieve internal
stresses. No microstructural changes occur during the process.
These internal stresses under certain conditions can have adverse effects:
example: Steels with residual stresses under corrosive environment fail with stress
corrosion cracking.
These stresses also enhance the tendency
of steels towards warpage and dimensional
instability.
Fatigue strength is reduced considerably
when residual tensile stresses are present
in steel.
The problems associated with internal
stresses are more difficult in brittle
materials than in ductile materials.
Stress – Corrosion Cracking
37. The process of stress relieving consists of heating materials uniformly to a temperature below
the lower critical temperature, holding at this temperature for sufficient time, followed by uniform
cooling.
Uniform cooling is of utmost importance as non-uniform cooling will itself result in the
development of internal stresses. Thus the very purpose of stress relieving will be lost.
38. Objective :
To improve the machinability of hyper-eutectoid steels
If hyper-eutectoid steels are heated and cooled slowly
(as like in full annealing), it induces softness in the steels
and hence can’t be used for the manufacture of cutting
tools
39. Normalizing is similar to full annealing, except steel is generally cooled
in still air.
The normalizing consists of
heating steel to about 40-55 oC
above critical temperature
(Ac3 or Accm), and holding for
proper item and then cooling
in still air or slightly agitated
air to room temperature.
In some special cases,
cooling rates can be controlled
by either changing air
temperature or air volume.
After normalizing, the resultant micro-structure should be pearlitic.
Since the temperature involved
in this process is more than that
for annealing , the homogeneity
of austenite increases and it
results in better dispersion of
ferrite and Cementite in the final
structure.
The grain size is finer in normalized structure than in annealed structure.
40. Normalized steels are generally stronger and harder than fully annealed
steels.
Steels are soft in annealed
condition and tend to stick during
machining. By normalizing, an
optimum combination of strength
and softness is achieved, which
results in satisfactory level of
Machinability in steels.
Normalized treatment is frequently applied to steel in order to achieve
any one or more of the objectives, namely:
To refine the grain structure,
To obtain uniform structure,
To decrease residual stresses,
To improve Machinability.
41. -In practice, 0.80 % C is required for maximum hardness.
-When you increase the carbon content beyond 0.80 per cent, there is
no increase in hardness, but there is an increase in wear resistance.
-This increase in wear resistance is due to the formation of a substance
called hard cementite.
-Heating the steel to a set temperature and then cooling
(quenching) it rapidly by plunging it into oil, water, or brine.
-Hardening increases the hardness and strength of the steel,
but makes it less ductile.
42. To relieve the internal stresses and reduce brittleness, Tempering is done after Hardening.
Tempering is done by heating the metal to some temperature below the critical temperature
for a certain period of time, then allowed to cool in still air.
The exact temperature determines the amount of hardness removed, and depends on both
the specific composition of the alloy and on the desired properties in the finished product.
For instance, very hard tools are often tempered at low temperatures, while springs are
tempered to much higher temperatures. In glass, tempering is performed by heating the glass
and then quickly cooling the surface, increasing the toughness.
43. -Case hardening produces a hard, wear-resistant surface or
case over a strong, tough core.
-Only ferrous metals are case-hardened.
-The steels best suited for case hardening are the low-
carbon and low-alloy series.
1) CARBURIZING
-Carbon is added to the surface of low-carbon steel. Two
methods carburizing steel.
i ) Heating the steel in a furnace containing a carbon
monoxide atmosphere.
ii) Steel placed in a container packed with charcoal or some
other carbon-rich material and then heated in a furnace.
