12) The external surface of the part made of mild steel can be hardened by

Answer is:

Case hardening

Explanation:

Parts to be hardened by case hardening process are made from a steel with a carbon content of 0.15% so that they will not respond to direct hardening.
The steel is subjected to treatment in which the carbon content of the surface layer is increased to about 0.9%.
When the carburised steel is heated and quenched, only the surface layer will respond, and the core will remain soft and tough as required.