Case hardening may be defined as a process for hardening a ferrous material in such a manner that the surface layer, known as the case, is substantially harder than the remaining material, known as the core. The process embraces carburizing, nitriding, carbonitriding, cyaniding, induction, and flame hardening. In every instance, chemical composition, mechanical properties, or both are affected by such practice.

This testing procedure describes various methods for measuring the depth to which change has been made in either chemical composition or mechanical properties. Each procedure has its own area of application established through proved practice, and no single method is advocated for all purposes.

Methods employed for determining the depth of case are either chemical, mechanical, or visual, and the specimens or parts may be subjected to the described test either in the soft or hardened condition. The measured case depth may then be reported as either effective or total case depth on hardened specimens, and as total case depth on unhardened specimens.

It should be recognized that the relationship between case depths as determined by the different methods can vary extensively. Factors affecting this relationship include case characteristics, parent steel composition, quenching conditions, and others. It is not possible to predict, in some instances for example, effective case depth by chemical or visual means. It is important, therefore, that the method of case depth determination be carefully selected on the basis of specific requirements, consistent with economy.