Heat Treatment of Metals Review

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Heat Treating Metals - Basically, heat treatment of steel consists of raising the material some specified temperature. The process is performed to change certain characteristics of steel to make them more suitable for a particular kind of service. Some of the reasons for heat treating are:

1. To soften a part so that it can be machined more easily

2. To relieve internal stresses so that a part will maintain its dimensional stability (i.e., not warp or fail prematurely due to locked-in stresses)

3. To refine the grain structure so that the part will be less apt to fracture abruptly (or to toughen)

4. To thoroughly harden a part so that it will be stronger

5. To case harden a part so that it will be more wear resistant

Heat Treatment Processes:

Annealing: Generally refers to the heating and controlled cooling of a material to remove stresses, make it softer, refine its structure, or change its ductility, toughness, or other properties.

Carburizing: Adding carbon to iron-base alloys by absorption through heating the metal at a temperature below its melting point in contact with carbonaceous materials. Such treatment followed by appropriate quenching hardens the surface of the metal. This is the oldest method of quenching.

Flame hardening: In this method of hardening, the surface layer of a medium- or high-carbon steel is heated by a high-temperature torch and then quenched.

Induction hardening: A hardening process in which the part is heated above the transformation range by electrical induction.

Nitriding: Adding nitrogen to solid iron-base alloys by heating at a temperature below the critical temperature in contact with ammonia or other nitrogenous material.

Hardening (as applied to the heat treatment of steel): Heating and quenching to produce increased hardness and increased strength, respectively.

Normalizing: Heating to about 100°F above the critical temperature and cooling to room temperature in still air. Provision is often made in normalizing for controlled cooling at a slower rate, but when the cooling is prolonged, the term used is annealing.

Stress relieving: Reducing residual stresses in a metal by heating to a suitable temperature for a certain period. This method relieves stresses caused by casting, quenching, normalizing, machining, cold working, or welding.

Tempering: Reheating after hardening to a temperature below the critical temperature and then cooling.

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