Surface hardening is a process of increasing the hardness of only surface while the inner core metal remains soft.
As the carbon steels are poor for hardness, case hardening is a process of increasing the carbon infusion to the surface and increasing the hardness of surface.
Due to hardening of metal it becomes brittle, but case or surface hardening imparts only hardness to surface and inner core remains ductile.
Due to which certain kind of stresses can be neglected and such objects can be used for respective application with less stress and high hardness and reduces the chances of fracture.
Various methods are listed below for surface hardening process
Electron beam hardening