In this paper laser surface hardening of martensitic stainless steel AISI 420 is conducted using a 1600 Watts 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 was conducted in order to study the microstructure of hardened zone. Macrography was also performed to measure the geometrical dimensions of hardened zone (width and depth). Microstructure evaluation investigated through optical microscopy and field emission scanning electron microscopy (FE-SEM). Microstructure observation of laser treated zone indicated that the higher surface hardness created the finer and more uniform martensitic phase. Results show that by increasing the laser power and decreasing the focal plane position, depth of penetration and microhardness of hardened zone increase. By increasing the scanning speed and focal plane position, penetration depth decreases while width of hardened zone increases. In 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 Vickers from 210 Vickers obtained. 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 show that the laser hardening process is more effective and precise that conventional processes.
