Case Hardening Steels

Case Hardening Steels

Case-hardening steels are special engineering steels—both alloyed and unalloyed—with relatively low carbon contents of roughly 0.10 to 0.25%.

Following carburising and hardening of the surface, case-hardening steels display high hardness and wear resistance in the region of the surface layer, while the strength and toughness of the base material are retained in the core. Consequently, case-hardening steels or case-hardened components are indispensable wherever high wear resistance, high fatigue strength and low notch sensitivity are required.

The choice of a steel grade is governed by the intended application, the types of stress involved, and the dimensions of the parts or the geometry of the components in question. Technical and economic aspects are likewise of decisive importance.

The ability to control the hardenability of the base material and the hardenability of the carburised surface layer is dependent upon the alloying elements in the steel. The hardenability of the base material is identified by means of the end-quench test according to DIN 50 191. This is an important parameter for determining hardness in the core, since case-hardened components are only tempered at low temperatures, up to approximately 180 °C, in order to ensure high surface hardness.

Hardness and the fatigue strength of the surface layer are achieved by the case-hardening treatment, i.e. by carburising, hardening, and tempering. If, for example, high strength is required in combination with high toughness of the core, the alloying elements must be matched in such a way that through-hardening is guaranteed at a given cross-section and with the given heat treatment.

Case-hardening steels are particularly advantageous for engines and gearboxes in automotive engineering. They can be used, for example, in piston pins, speed change gears, drive shafts, countershafts, synchroniser bodies, ring gears, differential bevel gears, bevel pinions, and differential side gears.

Alloying elements affect the hardenability of the base material and the hardenability of the carburised surface layer. The hardenability of the base material is identified by means of the end-quench test according to DIN 50 191 and is an important parameter for determining hardness in the core, since casehardened components are only tempered at low temperatures, up to approximately 180 °C, in order to ensure high surface hardness.

For reasons of toughness, the carbon content is limited to about 0.25%.

A surface hardness of 57 - 63 HRC has proved to be best for optimum wear resistance. This degree of hardness is achieved largely independently of the steel composition, with a carbon content at the surface of around 0.7%. Higher carbon contents in the surface layer provide only a slight increase in hardness. Super-carburisation in the surface layer may result in reduced toughness due to precipitation of secondary cementite and a hardness loss caused by increasing proportions of residual austenite.

The case depth, defined as the distance from the surface of a case-hardened workpiece to the point with a Vickers hardness of 550 HV1 (see DIN 50 190), is determined by the depth of carburisation, the heating and cooling conditions during hardening, and the hardenability in the carburised surface layer.