Heat-Treatment Terms Used in Aircraft Processes

Heat-Treatment related terms used in aircraft processing

Critical Range:
Critical range, applied to steel, refers to the range of temperature between 1300'F and 1600'F. When steel passes through this temperature range, its internal structure is altered. Rapid cooling of the metal through this range of temperature will prevent the normal change of the structure, and unusual properties will be possessed by the material so treated. The heat treatment of steel is based on this phenomenon.

Annealing:
Annealing id the process of heating steel above the critical range, holding it at that temperature until it is uniformly heated and the grain is refined, and then cooling it very slowly. Other materials do not possess critical ranges, but all are annealed by a similar heating process which permits rearrangement of the internal structure, followed by cooling (either slowly or quickly), depending on the material. The annealing process invariably softens the metal and relieves internal strains.

Normalizing:

Normalizing is similar to annealing, but the steel is allowed to cool in still air - a method that is somewhat faster than annealing cooling. Normalizing applies only to steel. It relieves internal strains, softens the metal somewhat less than annealing, and at the same time increases the strength of the steel about 20% above that of annealed material.

Heat treatment:

Heat treatment consists of a series of operations which have as their aim the improvement of the physical properties of a material. In the case of steel these operations are hardening (which is composed of heating and quenching) and tempering.

Hardening:

Hardening of steel is done by heating the metal to a temperature above the critical range and then quenching it. Aluminum alloys are hardened by heating to a temperature above 900'F and quenching.

Quenching

Quenching is the immersion of heated metal in a liquid, usually either oil or water, to accelerate its cooling.

Tempering:

Tempering is the reheating of the hardened steel by heating to a temperature below the critical range, followed by cooling as desired. Tempering is sometimes referred to as drawing.

Carburizing:

Carburizing is the addition of carbon to steel by heating it at a high temperature whiles in contact with a carbonaceous material either in solid, liquid, or gaseous form. Carburizing is best performed on steels containing less than 0.25% carbon content.

Casehardening:

Casehardening consists of carburizing, followed by suitable heat treatment to harden the metal.

Posted in: Aircraft,Sciences,Technology