Stress-relief annealing

Stress-relief annealing is necessary when existing residual stresses have a detrimental influence on distortion behavior during hardening. Corrections of the resulting changes in shape and size are taken into consideration by adding a certain machining allowance. The temperature must be below the Ac1 transition temperature but should nevertheless be as close as possible to this temperature so that it is not necessary to maintain the temperature after heating. Heating and cooling must be carried out in such a way that no additional or new residual stresses can develop. In the case of cold-formed tools, the normalizing process is preferred, whereas in the case of stress-relief annealing, coarse grain formation might occur as a result of re-crystallization.

Quenching and tempering

Hardening with subsequent tempering, mostly above 550°C, to reach a desired combination of mechanical properties, in particular to increase the toughness compared to the hardened condition.

Hardening

Hardening serves to achieve a high hardness in the component, preferably by martensite accumulation. It consists of the following two stages: austenitizing and cooling at an adequate speed.

Precipitation hardening

Process to increase stability by accumulation caused by precipitation. It consists of the solution treatment and precipitation heat treatment sub-processes.

Tempering

A single or repeated heating of a hardened component to a pre-determined temperature (<ac1), maintenance of the given temperature and subsequent adequate cooling.

Soft annealing

In the soft annealing process, heat treatment takes place to achieve the lowest possible hardness with high malleability at the same time. By heating and maintaining the temperature in the range of the Ac1 transition temperature for several hours, hardening structure constituents and strain hardening are eliminated and the cementite fins of the pearlite take on a spheroid form (this is also referred to as annealing to achieve spheroidal carbides). In most cases the soft annealed condition is the most suitable form for machining and cold forming as well as hardening. Exceptions are steels with lower carbon contents that tend to “smear” when being turned or milled.

Normalizing

Consists of heat treatment to austenitize the steel, together with subsequent cooling in calm air.

Structural heat treatment

Like stress-relief annealing, the purpose of structural heat treatment is to disperse stresses caused by machining the material; the process is normally carried out after rough machining. The treatment consists of hardening followed by soft annealing. As a result of the change in structure, reduced changes in shape and dimensions can be anticipated during subsequent heat treatment.

Steam tempering

A dark blue-grey-black oxide layer (magnetite) is deposited on the surface of the tools during steam tempering. As well as changing the optical appearance, steam tempering improves corrosion-resistance and minimizes the coefficient of friction.

Sub-zero cooling

Treatment after hardening to convert a large amount of the residual austenite to martensite. The treatment consists of a cooling process and the maintenance of a given temperature below room temperature.

Stress-relief annealing

Stress-relief annealing takes place at temperatures below the Ac1 transition temperature; it is followed by slow cooling to reduce the inner tension (residual stress).

Hardening

Hardening serves to achieve a high hardness in the component, preferably by martensite accumulation. It consists of the following two stages: austenitizing and cooling at an adequate speed.

Quenching and tempering

Hardening with subsequent tempering, mostly above 550°C, to reach a desired combination of mechanical properties, in particular to increase the toughness compared to the hardened condition.

Carburization

A thermo-chemical treatment of a workpiece in an austenitized condition; the carbon content of the surface layer is increased. The carbon content, when austenitized, is in a solid solution.

Case hardening

Case hardening consists of carburization, or carbonitriding with subsequent hardening – either directly afterwards or after an interim cooling and re-heating to achieve an adequate hardening temperature. Before hardening, the outer layer is enriched with carbon (carburization) or with carbon and nitrogen (carbonitriding). Compared to carburization, the additional nitrogen accumulation results in a higher temperability by changing the conversion behavior of the outer layer, and thus also resulting in a higher tempering stability after hardening. Depending upon the required characteristics or the requirements of the following process (e.g. grinding), after the hardening process the component is tempered or deep-frozen and then tempered. Case-hardening the outer layer of workpieces and steel tools (with C contents of 0.2 %) achieves a significant increase in the hardness of the outer layer and an enhancement of the mechanical properties of the workpiece/tool.

Carbon nitriding

This process is similar to case hardening. In addition to carbon, the surface is also enriched with nitrogen in the carbon nitriding process.

Artificial ageing through subzero cooling

A process to achieve dimensional stability of heat-treated parts by means of subzero cooling directly following the heat treatment. Its purpose is the conversion of the remaining austenite and the creation of a stable structure.