Microstructure and corrosion behavior of laser surface-melted high-speed steels

Three high-speed steels (HSSs) M2, ASP23, ASP30 were surface-melted by a CW 2.5-kW Nd:YAG laser. The microstructure of the laser surface-melted HSSs was investigated by optical microscopy, scanning electron microscopy and X-ray diffractometry, and the hardness profiles of the laser surface-melted layers were determined by a Vickers hardness tester. The corrosion behavior in 0.6 M NaCl and 0.5 M NaHCO3 solutions at 25 °C was studied by potentiodynamic polarization technique. Metallographical as well as electrochemical corrosion studies illustrated the beneficial effects of laser surface melting (LSM) in refining the microstructure and in enhancing the corrosion resistance of the HSSs. The large carbide particles of annealed HSSs were dissolved after LSM and ultrafine dendrites of austenite and martensite with submicroscopic carbide precipitation were formed in the melt zones of the laser surface-melted HSSs. LSM of M2, ASP23 and ASP30 produced surface layers of hardness 615, 580 and 665 Hv, respectively. The hardness of the laser surface-melted ASP23 and ASP30 reached about 0.75 to 0.80 that of the conventionally hardened ones, while the hardness of laser-melted M2 was comparable to that of conventionally hardened M2. The corrosion resistance of all laser surface-melted HSSs in both solutions was significantly improved, as evidenced by a noble shift of the corrosion potential and a reduction in the corrosion current density. Among the HSSs, laser surface-melted ASP23 possessed the highest corrosion resistance in both solutions. The presence of cobalt in ASP30 has no beneficial effect on enhancing its corrosion resistance. The enhancement in the corrosion resistance of the laser surface-melted HSSs is attributable to the combined effects of dissociation and refinement of large carbides and the increase of the passivating alloying elements such as Cr, Mo and W in solid solution.