Case Hardening

Knowledge Base > Case Hardening

It is a process of thermo-chemical diffusion within which alloying element, mostly nitrogen or carbon diffuse in to surface of monolithic metal. It results in an interstitial solid-solution that is harder as compared to base metal. This improves wear-resistance without any loss of toughness. Laser surface-engineering is surface treatment through high selectivity, versatility & novel properties. As rate of cooling is much higher in case of laser treatment, meta stable and metallic glasses are also possible through this process.

It is specified by case depth and hardening. There are two ways to specify the depth of the case:

  • Effective case-depth
  • Total case-depth

It is total case-depth that is true case depth. In case of majority of alloys, effective case-depth is the case depth having hardness equal to the HRC50. However, few of the alloys specify varying hardness (40 to 60 HRC) at the effective-case depth. The tester used to check this is Tukon microhardness. Value for this is roughly approximated to be 65 percent of total case-depth. This approximation is however affected by the hardenability and chemical composition. If there is no specification of the case depth type, it is assumed to be total case-depth. In case of hardened parts, specification is to have tolerance of at-least ±0.005 in (0.13mm). if part is to-be ground after the heat treatment, it is assumed that case depth will be the one taken after grinding.

For the purposes of specification, Rockwell Hardness-scale is used that depends upon total-case depth`s depth. Hardness is usually measured on Rockwell-C scale but load used upon scale penetrates through case if case is lower than the 0.030 (0.76mm). In this case use of Rockwell-C scale for thinner case brings false readings. Cases having less than 0.015” (0.38mm) thickness, for them Rockwell scale is not a reliable source for taking readings. In this case use of file hard tool is specified. It is equal to almost 58 HRC. When hardness is specified, it is recommended that minimum hardness should be specified or range given. In case of range specified, there should be at-least 5 points.