2. INTRODUCTION TO HEAT TREATMENT
• The heat treatment is a very important tool. The heat
treatment is a very broad term and includes ‘any heating
and cooling operations or any sequence of two or more
such operations applied to any material in order to
modify its internal structure or to alter its physical,
mechanical or chemical properties.’
• Usually it consist of heating materials to certain specific
temperature, holding at this temperature for a definite
period and cooling to room temperature or below room
temperature with a definite rate.
4. 1. ANNEALING
• Annealing is the process of heating a metal which
is in a metastable or distorted structural state, to a
temperature which will remove the instability or
distortion and then cooling (usually at a slow rate)
so that the room temperature structure is stable
and/or strain free.
• The purpose of annealing is to reduce hardness,
internal stress and increase machinability and
ductility.
5. 2. NORMALIZING
• It is a type of heat treatment applicable to ferrous
metals only.
• It differs from annealing in that the metal is
heated to a higher temperature and then removed
from the furnace for air cooling.
• The purpose of normalizing is to remove the
internal stresses induced by heat treating, welding,
casting, forging, forming, or machining.
6. 3. HARDENING
• Hardening is that heat treatment of steel which
increases its hardness by quenching and tempering.
• Hardening can be defined as ‘the process of heating
the steel above the critical temperature, holding it at
this temperature for considerable time and cooling
suddenly’.
• The quenching media used is water, oil or brine
solution (i.e. mixture of ice and salt).
7. 4. TEMPERING
• Tempering may be defined as ‘the process of
heating the hardened steel below its lower critical
temperature, holding it at this temperature for 3 to
5 minutes for each mm of thickness or diameter,
cooling it either rapidly or slowly except in case of
steels susceptible to temper brittleness.’
• This process decreases hardness and brittleness of
steels.
• Tempering is used to produce components like
connecting rod, coil spring, hammer, gears, chisels
etc.
8. 5. AUSTEMPERING
• Austempering consists of cooling the austenitized
steel with the rate exceeding the critical cooling
rate in a molten bath held at some constant
temperature between the nose of TTT diagram
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.
• It produces structures and properties very much
similar to tempered martensites without involving
martensitic transformation.
9. 6. MARQUENCHING
• Marquenching is the interrupted quenching.
• In this method, the steel is heated to the
hardening temperature and then quenched in a
medium usually (from 200 to 350˚C) in a salt
bath.
• Since the component has to be held for some
time for equalization of temperature, the process
will be applicable to steels of slightly high
hardenability such as high carbon steels and low
alloy steels.
10. 7. CASE HARDENING
• It is the process of carburization i.e. saturating
the surface layer of steel with carbon or some
other substance by which outer case of the
object is hardened where as the core remains
soft. It is applied to very low carbon steel.
• It is performed for obtaining hard and wear
resistance on surface of metal and higher
mechanical properties with higher fatigue,
strength and toughness in the core.
11. 8. AUSFORMING
• Ausforming also known as Low and High
temperature thermomechanical treatments is a
method used to increase the hardness and
stubbornness of an alloy by simultaneously
tempering, rapid cooling, deforming and
quenching to change its shape and refine the
microstructure.
• In this process, the deformation over 50% of
metastable austenite in done in metastable bay
between the pearlite and bainite noses of the
pertinent TTT diagram (i.e., in range 600-400°C)
and is then quenched to martensite (or
bainite). It is then tempered.
12. CONCLUSION
• Here I have tried to show that the different means
of cooling rate are responsible to provide
significant change or effect on the microhardness
of steels depending on the carbon content of steel.
• The microhardness increases with the increasing
cooling rate and carbon content due to solid
solution hardening and formation of the martensite
phase.
• Thus heat treatment is used to obtain desired
properties of steels such as improving the
toughness, ductility or removing the residual
stresses.