These are fundamental heat treatment processes dealing with dramatic variations of the mechanical properties of metals. Both techniques allow specified properties to be obtained for particular applications by control of the metal's microstructure.

Hardening

Hardening is a process to increase metal's hardness, strength, and resistance to wear. The metal is heated above the austenitic temperature and, after that, rapidly cooled or quenched. This fast cooling triggers the phase change, which traps the carbon atoms into a structure in the crystal lattice of metal. This modified microstructure often includes martensite and leads to very high hardness and strength.

There are many variations on hardening to achieve desired final properties:

The following are the most used quenching techniques:

Quenching:This is the most common method, involving rapid cooling in water, oil, or other quenching media.

Martempering: Martempering is a process in which the metal's controlled cooling is achieved by first quenching to some temperature above Ms, followed by isothermal holding and subsequent cooling to room temperature. This refines the microstructure and reduces distortion.

Austempering: It is a type of controlled cooling process wherein the metal is cooled to a temperature lower than the start temperature of martensite, held isothermally to form bainite, and finally cooled to room temperature. This provides a good combination of strength with bainite and toughness.

Applications

Automotive industry:

• Gear, shaft, and axle hardening to enhance wear-resistance and durability

• Components for case hardening where surface is required to have a certain level of hardness while the toughness of the core is to be maintained – camshafts, crankshafts.

• Hardening springs to enhance elasticity and bearing loads.

Tool and Die Industry:

• Hardening cutting tools like drills, milling cutters, and taps to extend tool life.

• Die and punches hardening for the enhancement of wear-resistance.

Manufacturing Industry:

• Hardening components subjected to maximum abrasion and wear and tear, like bearings and piston rings.

• Enhancement in the strength and durability of machine parts.

Annealing

Annealing, unlike hardening, seeks to soften a metal, hence making it ductile, malleable, and easily machinable. It includes heating the metal at a selected temperature, holding for an adequate period, followed by cooling at a very slow rate. Such controlled cooling is meant to recover the microstructure of the metal from the stresses introduced through previous working like cold working.

The annealing processes are different according to the end product:

Full annealing: This involves heating above the upper critical temperature, holding, and slowly cooling. Through this process, a completely new grain structure is obtained, and maximum softness is achieved.

Process annealing: It is a process of heating below the lower critical temperature, holding and cooling slowly. It's a technique that is mostly used to make cold-worked metals softer without complete recrystallization.

Stress relief annealing: This process involves heating to a temperature below the lower critical temperature, holding, and slow cooling. It relieves internal stresses without appreciably modifying the microstructure.

Applications:

Metal working:

• Softening of metals for easier forming, bending and drawing, e.g., wire drawing, sheet metal forming.

• Improving the machinability of cast and forged parts.

• Relieving stresses, which have developed because of cold working or welding.

Heat treatment:

• To prepare metals for further heat treatments such as normalizing or hardening.

• Improvement of grain structure for improved mechanical properties.

Electrical industry:

• To increase electrical conductivity and magnetic properties of metals.