Experimental Study of Surface Hardening of AISI 420 Martensitic Stainless Steel Using High Power Diode Laser

In this paper laser surface hardening of martensitic stainless steel AISI 420 was conducted using a 1600 W semiconductor diode laser. Focal plane position, laser power and scanning speed were considered as process variables. Microhardness was measured in depth and surface of the hardened layer and metallography of samples were conducted in order to study the microstructure of the hardened zone. Macrography was also performed to measure the geometrical dimensions of hardened zone (width and depth). Microstructure evaluation was investigated through optical microscopy and field emission scanning electron microscopy. Microstructure observation of laser treated zone indicated that the higher surface hardness created the finer and more uniform martensitic phase. Results showed that by increasing the laser power and decreasing the focal plane position, depth of penetration and microhardness of hardened zone increased. By increasing the scanning speed and focal plane position, penetration depth decreased while width of hardened zone increased. Under desired conditions resulting from this research (laser power 1400 W, scanning speed 5 mm/s and focal plane position 65 mm), surface hardness of AISI 420 martensitic steel increased to 720 from 210 Vickers. The dimension of hardened layer was 1.2 mm in depth and 6.1 mm in width. Comparing the results with the furnace hardening heat treatment showed that the laser hardening process was more effective and precise than conventional processes.