Process Encyclopaedia

Transformation hardening

Overview

Transformation hardening is often used in addition to carburising or nitriding, and is primarily used to improve the mechanical properties of the surfaces of steel components. There are many ways to "transform" the surface microstructure, but all of them involve heating of the surface followed by a rapid quench (either in oil or water, or by a "self-quench" because the bulk of the component will still be cold).

• Flame hardening uses a flame gun to provide the heating. It is inexpensive and flexible; however it is quite slow, difficult to control accurately and not easily automated. Only external surfaces can be treated.
• Induction hardening works by placing the component in a high-frequency magnetic field. This "induces" a current in the surface and so heats it rapidly. It can be used to uniformly treat large components such as the rolls for a rolling mill. Although this process is expensive and requires some dedicated tooling, it is easily automated and can be applied accurately – e.g. to just the teeth on a gear cog.
• Laser hardening works by focusing a laser beam on to the surface to provide very rapid heating. As a result, a self-quench is usually sufficient. The equipment is very expensive and not economic for large surfaces, but automation is straightforward and very precise control can be achieved.