Laser hardening of homogeneous or not-homogeneous PM steels

An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed, admixed or diffusion-bonded powders. The surface temperature of treated zone has been measured by pyrometer (to avoid local melting or surface damaging) and the travel speed on scanning the surface of test samples has been an investigated variable. A special attention has been given to specimen clamping, to avoid any misalignment with the beam movement. The independent variables investigated have been: surface temperature, T, travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth (HD), hardened width (HW) and overall cross-sectional hardened area (HA) were measured as well and used as responses for the ANOVA of the experimental data gathered. The microhardness profiles present a sharp drop at low distance from the hardened surface. The typical splitting between hardened zone and heat-affected zone (HAZ), well known from laser hardened fully dense steels, holds also for P/M (porous) steels. The research showed that Laser Transformation Hardening (LTH) is a suitable process for producing hard, wear resistant surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with induction hardening. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.