Laser transformation hardening of steel surfaces

Sammanfattning: The process of laser transformation hardening is a complex one to model in that there are a large number of parameters to account for. These include parameters of the process itself, e.g. beam intensity, intensity distribution, beam size, velocity of movement, degree of absorptivity, etc., material parameters, e.g. the temperature dependent elastic constants, thermal conductivity and diffusivity, composition including form of starting microstructure, carbon content etc., diffusivity and activation energies of diffusion, etc., and finally the temperature dependent material phenomena such as phase transformations during heating and cooling. In the present work, kinetic equations describing each of these areas of application were developed and integrated into a single, allembracing model. The equations were processed on a micro-computer, since this afforded more flexibility when manipulating and evaluating the various parameters, than if the model had been based on a main-frame computer. However, this meant that equations had to be kept in a simple analytical form, although in fact the end result firmly justified this simplified approach. The read-out was achieved in the form of Laser Processing Diagrams, a new concept which maps out contours of hardness and martensite volume fraction as a function of laser input energy and depth below the surface. The model was developed for both hypo- and hypereutectoid steels and a nodular cast iron, and it was checked by laser experiments on each of these materials. In addition, a simplified analytical model of residual stress due to laser hardening was developed. It is shown that these process diagrams can help to optimise laser hardening, as well as give fairly accurate prediction of degree of hardening.