An accident is
* Unplanned event
* Non-controlled event
* Undesirable event

Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface.
Normally Low carbon steel parts are subjected to case hardening process.

List of case hardening or surface hardening methods are
Carburising
Cyaniding
Nitriding
Induction hardening
Flame hardening