Phase, microstructure characterization and hardening efficiency of SKS3 tool steel laser surface hardened by a high-power diode laser

In this research, the surface hardening of SKS3 cold work tool steel has been performed by high power diode laser with a maximum power 1600 W. The applied powers of 1200 and 1450 W, the scanning speeds of 1.6 to 3 mm.s-1 and working distances of 55,, 70 and 75 mm were applied as test parameters. Microstructure and phase analysis and microhardness measurement were studied by optical microscopy, electron microscopy, X-ray diffraction and microhardness testing methods, respectively. The microstructure and phase analysis showed that the hardening process led to the formation of martensite and retained austenite dual-phase structure. The use of laser, created all three states of surface hardening, surface melting and non-hardening. The best result in terms of depth and width of hardening was obtained for samples with energy density of 208-250 J/mm2. The calculation of retained austenite phase percentage and heat input, indicated that an increase in the heat input raised the residual austenite percentage, so that for the sample with maximum heat input, the retained austenite phase percentage was calculated to be approximately 37%. Also, for the sample with the highest heat input (906/J/mm2), the lowest hardness was obtained (653 Vickers) and the sample with lower heat input (725 J/mm2) was owned the highest hardness (760 Vickers). Investigations of the values of hardening efficiency showed that the hardening conditions are not improved only by increasing the laser energy density, but to obtain highest hardness and appropriate microstructure, the optimum amount of power and scanning speed is needed.