Laser hardening offers many advantages over conventional heat treatment processes - the economical aspects include a high throughput, reproducibility and product quality. In many applications, localized treatment and minimal heat input results in reduced distortion and the rapid quench rates produce a fine microstructure. Laser surface treatments can be divided into processes involving solid state transformations and melting processes. The former category includes martensitic hardening, tempering and shock hardening while the latter includes re-melting, alloying, cladding and dispersion hardening.
The most commonly employed process is martensitic transformation hardening and is applied to carbon steels and cast irons. Application of the laser beam rapidly raises the surface temperature (at up to 1000K/s), resulting in a thin layer that is converted into austenite. Subsequent removal of the energy results in self-quenching caused by the conduction of the heat into the relatively cool bulk. This produces a rapidly cooled surface layer and causes a transformation of the austenite into martensite. It is therefore important that the steel is in appropriate conditions (quenched and tempered) and that the process conditions are carefully selected.
The primary advantage of laser hardening is the improvement of wear resistance. As a result, abrasive wear is reduced to a great extent. Laser hardened surfaces exhibit higher hardness than the abrasive medium, while adhesive wear can also be influenced by means of a reduction in the coefficient of friction. In addition, laser hardening can improve the fatigue characteristics of surfaces thanks to an increase in the compressive stress, which shifts the load bearing capacity to a level higher than the applied Hertzian stress.