Could you be more specific with your question? Heat treatment (austenitize/quench) is the most common method to thru-harden certain grades of steel. The thru-hardening capacity of a steel is highly dependent on primary alloying elements when the steel is made.
If you are looking at nitriding or carburizing, these are surface hardening processes which are diffusion controlled and dependent on exposure of the part to time and temperature. Carbon or nitrogen is introduced at the surface and diffuses into the steel to increase hardness.
