Heat Treatment Glossary of Terms

Heat Treatment Definition

A combination of heating and cooling operations applied to metals and alloys in the solid state to obtain desired conditions or properties.

Thermal Treatments can be classified by their purpose:

Heat Treatments, which modify the microstructure of the material or change the phase structure to improve the mechanical properties for specific applications or further work processes:

Annealing

A term used to describe a variety of softening heat treatments by changing the microstructure of an alloy.

Homogenisation

Primarily used to equalise temperature in stock material prior to hot working, or to reduce excessive coring, which can occur in ingots and continuously cast alloys.

Stress Relieving

A process used to remove stresses in welded, rapidly cooled components or cold work products.

Normalising

A treatment used to remove undesired microstructure effects of previous heat treatments and performed to produce a uniform grain structure.

Hardening

One of a number of processes used to improve the hardness of an alloy superior to that normally present in the core stock. Usually achieved by quenching material from above its upper critical temperature.

Solution Heat Treatment

A process in which certain alloys, mainly some stainless steels and copper based alloys, are heated to a suitable temperature to allow their constituents to enter into solid solution. The process is then held at the defined temperature for a specified length of time, dependent on the alloy mix, to allow the various components to form a coherent solid but soluble mass before rapid cooling (precipitation).

Precipitation Hardening (Age Hardening)

A process in which the hardness and stressrupture strength can be improved of hardenable high-temperature steels, as well as titanium, nickel and cobalt alloys. The process consists of a solution heat treatment followed by aging, during which the constituents of the structure form a

solid solution that is frozen during rapid cooling. The hardness increase is caused by the aging cycle.

Age Hardening (Precipitation Hardening)

A process in which certain nonferrous and ferrous alloys are heated, quenched and then aged at a relatively low temperature above room temperature, to allow precipitation hardening to occur. Can significantly increase work-piece strength without affecting work-piece ductility. The

hardening process can be halted by refrigeration of the component.

Quench Hardening

The objective of this treatment is to produce a fully Martensitic micro-structure in the steel. To achieve this the steel must be cooled rapidly from the austenitic condition. The process is suitable for ferrous metal and alloys in which steel and cast iron alloys are heated above a certain critical temperature and rapidly cooled to produce a hardened structure. Either surface hardening or full-hardening can result, depending on the cooling rate. The process requires close control of temperature during heating and quenching.

Tempering

A treatment used to remove brittleness from mainly quench hardened steels and achieved by thoroughly soaking the material, at an alloy dependent temperature, prior to cooling. Also referred to as the Draw Process.

Case Hardening

One of a number of heat treatment processes which improve the surface hardness of a steel alloy without affecting the properties of the core material.

Flame Hardening

A localised hardening process where components are subject to mainly an acetylene flame and then spray quenched.

Induction Hardening

A widely used process particularly in the automotive and tools market for the surface hardening of steel. The components are heated by means of an induction set which applies an alternating magnetic field to the work-piece. The temperature of the components increases to the transformation range or just above before being immediately quenched. The physical properties of the core material remain unaffected by the process.

Micro Structure States

Heat treatments are used to change the micro structural state of steels and alloys. Each of the states holds advantages in different applications and metals may be produced which exhibit combinations of the states.

Treatments, which alter the surface chemistry of an alloy:

Typically Gas Carburising, Nitriding, Carbonitriding and Nitrocarburising. In these processes the surface layers of the alloy are hardened and strengthened by subjecting the component to an enriched gaseous atmosphere of carbon or nitrogen while the material is taken through an

elevated thermal profile. Similar material properties containing other surface molecular components can be obtained in processes such as Ion Implantation – Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD), Boriding and Diffusion Alloying Aluminising, and Salt Baths Plasma Nitriding is particularly popular with common batch styles. In this process nitrogen atoms are diffused off into the metal surface in the presence of a plasma environment. This process is also called Ion Nitriding-where a differential potential of minimum voltage is applied to two electrodes held in a gas at reduced pressure. The work piece is maintained within the abnormal glow discharge region and as an increasing voltage and current are applied to the electrodes, the work pieces becomes heated through the action of ionic bombardment. As a result nitrogen is transferred to the work pieces, which then penetrates the surface by diffusion.

Salt Baths

A method of providing thermal processing of steels using a bath of molten salts. The process prevents oxidation and provides a very uniform heating environment for hardening and tempering.

Hot Iso Static Processing (HIPping)

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

Treatments which strengthen the molecular bonding of powder compacted components. 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.