An Introduction to the Heat Treatment of Metallic Alloys

General treatments

Homogenization

This treatment is used prior to hot working processes and is performed to equalise temperatures throughout an alloy or to reduce the coring effect caused by the non-uniform chemical composition.

Annealing 

Annealing covers a variety of heat treatment processes used to soften alloys and increase their ductility as an aid to cold working.

Normalizing Stress Relieving

Thermal treatments performed to remove internal stresses within components following welding, casting or rapid cooling.

Treatments which alter the surface chemistry of an alloy

Carburising, nitriding, carbontriding and nitrocarburizing

In these processes the surface layers of the alloy are both hardened and strengthened by exposing the component to an enriched gaseous atmosphere of species carbon or nitrogen while the material is subjected to an elevated thermal profile prior to quenching. Similar component properties can be achieved using other surface molecular components with processes such as Ion Implantation – Chemical Vapour Deposition (CVD), Physical Vapor Deposition (PVD), Boriding and Aluminizing.

Treatments which change the phase structure of an alloy

Hardening 

Metallic alloys can all be work hardened but specifically steel-alloys can also be hardened through heat treatment. The harden-ability of a steel-alloy is dependent on its carbon or other alloys content. The higher percentage carbon alloys can achieve a greater degree of hardness.
The hardening process is achieved by heating an alloy to a predefined temperature and then quenching in oil, water, air or a special polymer quenchant. The temperature and quenching parameters are dependent on the type of steel being processed.

Tempering 

Tempering usually follows the hardening process and is used to remove much of the brittleness of the alloy while retaining the components hardness.

To understand how successful these processes may be and the temperatures at which the treatments should be performed, it is necessary to study the phase diagram of the particular alloy.

Induction hardening

Rapidly heating using an induction coil immediately followed by quenching in a quench jet can also harden medium and high carbon steels. This process can also be carried out using alternative hot flame impingement or laser technology heating techniques.

Other specialist processes

Hot Iso Static processing (HIP ping)

This process is used for the densification of castings and pre-sintered components as well as in the diffusion bonding of alloys. The process usually uses very high temperatures and pressures within a specially designed vessel.

Sintering

Many products with complex cross sectional forms are manufactured from powdered, core material, which is pressed or moulded into the component shape. Sintering takes place in an atmosphere controlled environment and is used to strengthen the bonding of powder compacted components over a timed temperature process cycle.