HEAT TREATMENT

Heat treatment is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is in metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.

DEFINATION :-

Heat treatment is a controlled process used to alter the microstructure of metals and alloys such as steel and aluminium to impart properties which benefit the working life of a component, for example increased surface hardness, temperature resistance,magnetic properties etc.

There are five basic heat treating processes: hardening, case hardening, annealing, normalizing, and tempering. Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling.

Objective of the Heat treatment are:-

To increase or decrease the Mechanical properties like Hardness, toughness, resilience,Machinability, etc.,
To relieve internal stresses, which are set up in the metal due to cold or hot working .
To soften the metal by the process of annealing.
To improve hardness of the metal surface by the process of hardening.
To improve machinability.
To refine grain structure
To improve mechanical properties like tensile strength, ductility and shock resistance, etc.
To improve electrical and magnetic properties.
To increase the resistance to wear, tear, heat and corrosion, etc.

Objective of the Heat treatment process:-

Annealing - It helps relieve pre-cold worked stresses. It allows sufficient grain growth which in turn increases toughness and ductility.
Normalizing: In this air-cooled heat treatment process, advantages and disadvantages take a middle ground of annealing and hardening.
Hardening: In this air / water cooled heat treatment process, martensite is formed in steel which is the hardest form of steel. Thus, it increases hardness of specimen by decrease it ductility.

We can categorize heat treatment into two broad criteria.
1. Softening
2. Hardening

SOFTENING

1. Full anneal - The alloys are heat treated (Working temp depends on the type of alloy) to increase ductility. The final grains are coarse in size as the alloy is furnace cooled.

They alloy is then sent for secondary application like forming, stamping, shaping, bending.

2. Normalizing - Same as above process but air cooled. So has a fine grain structure.

3. Process Annealing - This process is used to reduce the effect of cold working and enable further working without failure on the metal.

4. Spherodization - Usually done in High carbon steels where the steel is heated for a long time and then slowly cooled to get spherical micro-structure.

5. Tempering - It is done immediately after quench hardening as the quench hardened metal is very hard and brittle. This imparts better toughness and recovers the strength a bit.

6. Austempering - Austenite to Bainite phase transformation.

7. Patenting - Used in 0.3 -0.5% Carbon steel to create very fine pearlite. Used for steel wires.

8. Martempering - Slowly cooled through the martensite region.

HARDENING

1. Carburizing - It's the process of adding carbon to the surface to impart strength and hardness.

2. Nitriding - Addition of nitrogen for achieving a hard case with low distortion. Plus this process doesn't require quenching.

3. Flame hardening - Normal quenching procedure followed after heating the outer face to get a hardened surface and a soft core.