Laser surface melting of tool steels H13, O1 and D6

In the present study, the surface of three hot-work / cold-work tool steels (WTSs) H13, O1 and D6 were melted by a CW 2.5-kW Nd:YAG laser at a power density of 71.9 W/mm2. Their microstructures were investigated by optical microscopy and X-ray diffractometry and their corrosion behavior in 3.5 wt% NaCl solution at 25 °C was studied by potentiodynamic polarization technique. After LSM, the ferritic matrix with fine spheriodal carbides of annealed H13 and O1 was transformed to martensite as the main phase with submicron-sized carbides. For D6, with high Cr and C contents, the pearlitic matrix with large carbide particles of annealed D6 was converted to mainly austenite with some submicron-sized carbides. The maximum hardness of the melt zones of laser-melted H13, O1 and D6 reached 490, 625 and 410 Hv, respectively. Compared with the conventionally hardened WTSs, the corrosion resistance of the laser surface-melted WTSs was improved, as evidenced by decrease in the corrosion current density and noble shift in the corrosion potential. The enhancement in corrosion resistance is attributed to the combined effects of the refinement of large alloy carbides and the enrichment of the passivating alloying elements such as Cr, Mo, W and V in solid solution. Among the WTSs, laser surface-melted H13 possesses the highest corrosion resistance but its maximum hardness achieved is only 0.88 that of conventionally hardened H13.