Gaseous Carburizing

GOVERNING SPECIFICATION(S):  AMS2759/7

Gaseous carburizing is the most widely employed carburizing process.   The carbon needed for diffusion is derived from a gaseous furnace atmosphere produced either from hydrocarbons such as methane, propane and butane or from vaporized hydrocarbon liquids.   Controlled carburizing atmospheres are produced by blending a carrier gas with an enriching gas, which serves as the source of carbon.   The carrier, an endothermic gas mixture, accelerates the gas carburizing reaction at the surface of the parts.   The amount of enriching gas required by the process depends primarily on the demand for carbon (i.e.) the rate at which carbon is absorbed by the work load.

The Carburizing process is carried out at a temperature between 1500 ° F. – 1750 ° F.
Depending on the specification requirements, the parts are then oil quenched either directly from the Carburizing temperature or may be reduced to the Austenitizing temperature, stabilized and then quenched.   Design considerations may be necessary to allow for some part growth or distortion.

The carbon rich surface produces a hard, wear resistant case.   Case depths range from .005% – .100%.   Carburizing is best suited for steels with low hardenability such as 1018, 1117, 12L14 and highly alloyed steels such as 8620 and 9310.   Typical as quenched case hardness is in the range of 58 – 62 HRC.   The parts are then tempered at a temperature sufficient to achieve the desired final case hardness.

The Carbonitriding process is basically identical to the Carburizing process, except that ammonia gas is introduced into the Carburizing atmosphere in order to form nitrides.   This is done in instances where carburizing alone will not produce a case hard enough to meet requirements.