Top 5 Hardening Process

In the case of suitable ferrous materials, hardening results in a martensitic structure and consequently an increase in hardness and strength.
The most important heat treatment processes in the field of “hardening” are presented below.

Tempering is a heat treatment process which is applied in order to make the hardened and relatively brittle material tougher. This involves increasing the temperature in the region of +160 °C to +650 °C with an adequate holding period and subsequent cooling to room temperature.
Tempering reduces the hardness, the strength decreases and the ductility and toughness increase. Depending on the type of steel, any residual austenite present is converted at temperatures in excess of +230 °C. During the tempering of steels, different temper colours are produced as a function of the tempering temperature, see table, Annealing and temper colours, Page 296.

The combination of hardening and tempering at a temperature in excess of +500 °C is referred to as quenching and tempering. Quenching and tempering is intended to produce an optimum ratio between strength and toughness.

In the case of surface layer hardening, austenitisation and hardening are restricted to the surface layer of the workpiece. In most cases, the material is heated by electric induction (medium or high-frequency alternating current) or by gas burners. The material is quenched by dipping or spraying.
Through the surface layer hardening of components that have already been quenched and tempered, a high basic strength can be combined with a high surface hardness in areas that are subjected to particularly high loads. The thickness of the hardened surface layer results from the hardness profile as the surface hardening depth SHD in mm (synonym: surface layer hardening depth); cf. standard DIN EN 10328:2005.

Case hardening (carburising, carbonitriding) involves carburising or carbonitriding followed by hardening. This hardening either takes place directly afterwards, or after the material concerned has been subjected to intermediate cooling and re-heated to an appropriate hardening temperature.

Depending on the required use characteristics and the subsequent machining requirements, the material is tempered, or deep-cooled and tempered, after hardening.

Case hardening gives the surface layer of workpieces a significantly higher level of hardness and the entire workpiece improved mechanical properties. To this end, the surface layer is enriched with carbon (carburising) or carbon and nitrogen (carbonitriding) before hardening. In comparison with carburisation, additional enrichment with nitrogen results in increased hardenability (by changing the transformation behaviour in the surface layer) and, after hardening, in improved annealing resistance.

During bainitic hardening (isothermal transformation in the bainite stage), the material is first heated to and maintained at the austenitising temperature. The material is cooled to a temperature of between +200 °C and +350 °C, depending on the steel, and kept at this temperature until the steel structure has transformed to bainite. The material is then cooled to room temperature. In this state, the hardness is less than that of martensite, but toughness is increased.
This process is regarded as an alternative to hardening if a high level of toughness is required without a high level of hardness, and if distortion and dimensional changes have to be minimised.

Besides, this is precipitation hardening process.

Precipitation hardening consists of solution annealing, quenching and ageing above room temperature (hot ageing). Ageing results in the segregation and separation of intermetallic compounds made up of specific solution elements dissolved in the base material. This changes material properties, for example hardness, strength, ductility and toughness.