Microsoft PowerPoint - HT2011 [Compatibility
Mode]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
Types of Heat-Treatment (Steel)
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.
Time-Temperature- Transformation (TTT)Curve
The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised
at 900°C
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
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
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
Types of Annealing
2. Normalizing
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
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)
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
Stress-Relief Annealing Process
For plain carbon and low-alloy steels the temperature to which the
specimen is heated is usually between 450 and 650C, whereas for
hot-working tool steels and high-speed steels it is between 600 and
750C
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
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
Relation between heating temperature and Reduction in Residual
Stresses
Higher temperatures and longer times of annealing may reduce
residual stresses to lower levels
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
2. Normalizing A heat treatment process consisting of
austenitizing at temperatures of 30–80C 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 30–80C
above the AC1 or ACm transformation temperature
Normalizing – Heating and Cooling
Normalizing – Austenitizing Temperature Range
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
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
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
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
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).
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
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
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)
Process - Isothermal Annealing Austenitizing followed by a fast
cooling to the
temperature range of pearlite formation (usually about 650C.)
Holding at this temperature until the complete transformation of
pearlite
and cooling to room temperature at an arbitrary cooling rate
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
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.
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
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
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
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
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
Case-Hardening - Processes
Flame/Induction Hardening
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
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
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
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
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
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
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
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
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
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
Precipitation Mechanism – Aluminum Alloy
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
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
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
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 150C) to impart some
toughness.
Higher tempering temperatures (may be up to 700C, depending on
alloy and application) are sometimes used to impart further
ductility, although some yield strength is lost
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
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.