Laser Surface Modification of Various Tool Steels for Improving Hardness and Corrosion Resistance

Laser surface modification of nine tool steels, namely, plastics mold steels (PMSs), high-speed steels (HSSs) and cold/hot-work steels (CHWSs), was achieved by means of a CW Nd:YAG laser. The microstructure and the phases present in the surface of the specimens were analyzed by optical microscopy, scanning-electron microscopy and X-ray diffractometry. The surface hardness of the specimens was measured using a Vickers microhardness tester. The corrosion characteristics of the laser surface-melted steels in 3.5 wt% NaCl solution at 25 oC were studied by potentiodynamic polarization technique. The microstructures of the surface of the steels were changed completely after laser surface melting. Some steels showed improved corrosion resistance compared with the conventionally hardened specimens due to dissolution of the alloying elements in solid solution. The hardness and corrosion characteristics of all the laser surface-melted specimens are strongly dependent on the amount of passivating elements in solid solution and also on the morphology of the re-precipitated carbides. Both these factors depend on the laser processing parameters and the substrate compositions.