Case hardening

what is case hardening?

Case hardening is a thermo-chemical process. Within the framework of this process, the surface layer of components is enriched with carbon (carburization), in order to improve the mechanical properties of the component surface layer.

the process

As a rule, case hardening comprises three working steps. In the first step, the workpieces are exposed to a carbon rich environment at a temperature of 800 to 1,050 °C. In the second step, the quenching takes place either directly from the application temperature or after intermediate cooling and reheating to a material-specific hardening temperature. The third step, the tempering treatment serves essentially to relieve the highest stresses in the material structure and to reduce sensitivity to grinding cracks.

in short

workpiece before

Case hardening steel (C ≤ 0.25 %)

easy to machine
easy to weld
outstanding toughness

workpiece after

Core zone (C ≤ 0.25 %)

tough and ductile
enhanced service properties
(toughness and, if applicable, strength)

Surface layer (C = 0.70…0.90 %)

hard and wear resistant
improved fatigue strength under
vibration stress

fields of application

Case Hardening results in a very hard case of the component: specifically, this is useful in the case hardening of gears and similar components which are subject to a great deal of wear in hostile environments. This heat treatment process is especially useful if there is a requirement for a hard, wear resistant surface, overlying a much softer and tougher core. A good example for its application is a gear wheel. The surface of the teeth needs to be extremely hard so that they can withstand constant metal-to-metal contact, without undue wear. The underlying material needs to be tough so that the teeth can tolerate occasional impact loads, without the risk of fracture.

Case hardening, properly known as carbon case hardening, is used to give a hard, wear and indentation resisting surface to mild and low alloy steels, up to depths of 4-5 mm. Being one of the most common heat treatment processes used in the world, case hardening is commonly applied to gears, machinery, automotive and aerospace applications and many others.

Steels suitable for case hardening are unalloyed or alloyed steels with a carbon content below approx. 0.25%.

Partial case hardening is possible, thanks to suitable insulation techniques.