An Introduction to the Heat Treatment of Metallic Alloys

Heat treatment can be defined as a combination of heating and cooling operations applied to a metal or alloy in its solid state to obtain desired conditions or properties.

Heat treatments can be used to homogenize cast metal alloys to improve their hot workability, to soften metals prior to, and during hot and cold processing operations, or to alter their microstructure in such a way as to achieve the desired mechanical properties.

Thermal treatments of metallic alloys are also used to alter the surface chemistry of a material. This is achieved by diffusing Carbon, Nitrogen and other gaseous or solid material in to the surface of the component. These processes are used to give defined surface hardness and to improve wear, corrosion and fatigue resistance.

The parameters and processes that can effect the composition and material properties of metal components include the following: 

  • Alloy type
  • Heating
  • Cooling
  • Work input
  • Time
  • Atmosphere
  • Surface plating
  • Surface diffusion

To ensure that any metal component is suitable and adequate for the designed purpose, it may need to be exposed to a selected range of conditioning and finishing treatments. The treatments are conducted in such a way so as to ensure that the required combination of these parameters are carefully controlled to achieve the desired finished component.

The heat treatment of metals involves raising the temperature of an alloy, often through a prescribed thermal profile, to a defined temperature. The material is then held at this temperature for a period of time before being cooled either at a prescribed rate or under rapid quenching conditions to a fixed temperature.

Treatments are carried out in furnaces and ovens where, in addition to the changes in temperature, gases are used to control the atmosphere for the process. Controlled atmospheres are either used to reduce the effects of oxidization or to provide an enriching atmosphere for surface chemistry effects, on the component being treated.

Heat Treatments are classified by their purpose:

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.

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