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Table of Contents:
Introduction
The heat treatment processes are done on the metals to achieve certain properties or characteristics for a particular metal. One might want to achieve very strong or brittle or conductive or all of them. Depending on the applications one can use different heat treatment processes.
Different heat treatment processes can be achieved by varying the time of heating, holding at a temperature, rate of cooling, cooling agent, etc.
The microstructure of the metal is different when undergoing different heat treatment processes, hence changing the properties of the metal.
The heat treatment process can be performed on any metal or substance. Generally, it is common to perform these processes on iron with varying carbon percentages. If you are not familiar with the Iron-Iron Carbide diagram we strongly recommend you understand it before diving in here.
The iron-iron carbide diagram is a diagram between iron and carbon at different temperatures, compositions, and phases of iron. There are 3 horizontal lines at 727 deg, 910 deg, and 1147 deg Celcius. Each horizontal line represents an isothermal reaction. i.e. Peritecticc, Eutectic, Eutectoid reactions.
Some temperatures to remember throughout:
A1: The upper limit of the cementite/ferrite phase
A2: The temperature at which a metal loses its magnetism is also called Curie temperature.
A3: The interface that separates Austenite + Ferrite phase from the γ (Gamma) austenite phase.
Acm: The interface that separates γ Austenite from the Austenite + Cementite field.
Annealing:
In this type of heat treatment, the metal is heated to its recrystallization temperature and cooled down slowly in the furnace itself.
For an Iron-Carbide the metal is heated to A3/Acm and cooled down slowly.
What is achieved?
Reduce/eliminate internal stresses, remove defects.
Increase the ductility.
Increase toughness and decrease hardness.
Improves magnetic properties.
Types of annealing (Iron Carbide)
Full annealing: The steel is heated above A3 temperature and cooled in a furnace without any disturbance. Hence, very coarse grains of austenite are formed.
Partial annealing (inter-critical annealing): Steel is heated just above the A1 line and, then, slow cooling is carried out, which results in fine pearlite and martensite. It is preferable for hypo-eutectoid steels.
Subcritical annealing (Recrystallization Annealing): Steel is heated below the A1 line, there is no phase transformation.
Iso-thermal annealing: The steel is heated above A3 temperature allowing for uniform austenitization of the whole steel part. After that, the steel part is cooled rapidly below A1, for economical reasons.
Stress Relieve annealing: The steel is heated below the lower critical line without altering the microstructure of the material.
Normalizing:
When normalizing the metal is heated to a certain temperature and cooled down faster than in annealing but slower than that quenching. The metal is taken out of the furnace and cooled in the air, out of the furnace.
For an Iron-Carbide the metal is heated to A3/Acm and cooled down outside the furnace.
What is achieved?
Improves the toughness of steel and reduces its hardness
Improves plasticity
Reduces cracking
Quenching:
Heating the metal to a certain critical (recrystallization) temperature and holding it for some time, later cooling it down rapidly by a fluid is called quenching. Rapid cooling can be done in water, oil, salts, etc depending on the application of the metal.
What is achieved?
After quenching the metal becomes hard and brittle.
It has high internal stresses.
Improves the wear, vibration, and corrosion resistance of stainless steel.
Improves ferromagnetism in steel.
The elastic limit is improved.
Application (Iron Carbide):
For an Iron-Carbide the metal is heated to A3/Acm and cooled down rapidly. When following the process, martensite is formed. Martensite is a ferrite that has too much carbon trapped inside. When we quench the steel, it cools rapidly and wants to transform from BCC austenite to FCC ferrite. However, the cooling rate is too fast for the carbon atoms to move out of the way, so they essentially get trapped in the FCC phase, which is the martensite (bainite).
Tempering:
Quenching is generally followed by tempering, by heating the metal again to a certain temperature (below recrystallization) and cooling it down. The cooling of the metal is done with the help of air, hot water, etc. The cooling rate of tempering is generally lower than that of quenching.
What is achieved?
Reduces brittleness and increases ductility
Reduces internal stresses, improves tensile strength
Reduce deformation and cracking
Other heat treatment processes:
Cold treatment: The steel is heated to -60 to -80 deg Celcius, later allowed to be at room temperature normally. It is done to convert austenite to martensite.
Carburizing/Nitriding: To improve the external hardness, wear resistance, etc the steel parts are put in carburizing/ammonia gas medium and heated.
Aging (precipitation hardening): It is similar to tempering. The medium to cool is a heat treatment method mostly used to increase the yield strength of malleable metals.
Summary
Different heat treatment processes can be achieved by varying the time of heating, holding at a temperature, rate of cooling, cooling agent, etc.
In Annealing, the metal is heated to its recrystallization temperature and cooled down slowly in the furnace itself.
When normalizing the metal is heated to a certain temperature and cooled down faster than in annealing but slower than that quenching.
Heating the metal to a certain critical (recrystallization) temperature and holding it for some time, later cooling it down rapidly by a fluid is called quenching.
Quenching is generally followed by tempering, by heating the metal again to a certain temperature (below recrystallization) and cooling it down.
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