case-harden Definition

1. metallurgy.
To harden the surface layer of (steel) by diffusing carbon into it at a high temperature, so that the outer surface is tough and resistant to wear, while the inner core remains resistant to fracture.
2. geol.
Said of a mineral: to form a coating on the surface of porous rock as a result of the evaporation of a solution containing that mineral.
3. To make someone insensitive or callous.