Page "Differential heat treatment" Paragraph 2

Differential hardening methods consist either of heating the metal evenly to a red-hot temperature, changing the steel into austenite, and then cooling it at different rates, turning part of the object into very hard martensite, while the rest cools slower and becomes softer pearlite.

It may also consist of heating only a part of the object very quickly to red-hot and then rapidly cooling ( quenching ), turning only part of it into hard martensite, but leaving the rest unchanged.

Conversely, differential tempering methods consist of heating the object evenly to red-hot and then quenching the entire object, turning the whole thing into martensite.

The object is then heated to a much lower temperature to soften it ( tempering ), but is only heated in a localized area, softening only a part of it.