Surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. For iron or steel with low carbon content, which has poor to no hardenability of its own, the case hardening process involves infusing additional carbon into the case. Case hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process.

Because hardened metal is usually more brittle than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for applications where the metal part is subject to certain kinds of stress. In such applications, case hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface.

Engineering methods for surface hardening of steels

Carburizing

Carburizing, also referred to as Case Hardening, is a heat treatment process that produces a surface which is resistant to wear while maintaining toughness and strength of the core. This treatment is applied to low carbon steel parts after machining as well as high alloy steel bearings, gears and other components.

Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core. This treatment is applied to low carbon steels after machining.

Most carburizing is done by heating components in either a pit furnace, or sealed atmosphere furnace and introducing carburizing gases at temperature.

Gas carburizing allows for accurate control of both the process temperature and carburizing atmosphere (carbon potential). Carburizing is a time/temperature process; the carburizing atmosphere is introduced into the furnace for the required time to ensure the correct depth of case. The carbon potential of the gas can be lowered to permit diffusion, avoiding excess carbon in the surface layer.

Carburizing cannot be done in ferrite phase as it has very low solid solubility for carbon at room temperature. It is done in the Austenite region above 727°C in carbon rich atmosphere.

Types of carburizing:

i. Pack carburizing

ii. Gas carburizing

iii. Liquid carburizing