Laser surface melting of high-speed steels for enhancing surface properties

Two high-speed steels (HSS), ASP23 and ASP30 formed by powder metallurgy, have similar chemical compositions (Fe-4.2wt%Cr-6.4wt%W-5wt%Mo-3.1wt%V-1.3wt%C), but with ASP30 containing 8.5% Co, were surface remelted by a 2.5-kW Nd:YAG laser. Metallographical, X-ray diffraction, microhardness, as well as electrochemical corrosion studies illustrate the beneficial effects of laser surface melting (LSM) in enhancing the surface properties of the HSS. The laser-melted zone of the HSS consists of fine dendrites of austenite crossed by a very fine carbide precipitation network or the eutectic without the primary large carbides and with high chemical homogeneity. LSM of ASP23 and ASP30 produce hardened layers (640 Hv and 680 Hv respectively) at the same levels as the conventionally quenched HSS whilst retaining tough cores. Compared to the annealed and conventionally hardened HSS, the corrosion resistance of the laser-melted HSS was found to be improved, as evidenced by a shift of the corrosion potentials in the noble direction and a reduction of corrosion current densities by 5.5 times. The increase in microhardness and corrosion resistance of the laser-melted HSS is probably attributed to formation of the fine carbides and the retained austenite, and a higher Cr content in the matrix.