In the present study, Nd:YAG laser surface hardening of AISI 431 stainless steel was performed. The effect of process parameters was investigated on the geometrical dimensions of the hardened zone, micr ohardness distribution, and microstructure of AISI 431 stainless steel. The results reveal that increasing the laser frequency and the pulse width causes more heat input. Therefore, the geometrical dimensions of the hardened zone rise. On the other hand, more heat input causes to increase the prior austenite grain size which contains the martensitic phase. So, the hardness value of laser treated region decreases. When the scanning speed and the focal plane position increase, the heat input decreases. Hence, the geometrical dimensions of the hardened zone reduce. The average hardness value increases by reducing the scanning speed due to the increase of the martensitic phase and the decrease of retained austenitic phase in the microstructure. Also, decreasing the heat input induced by increasing the focal plane position causes the grain size reduces and the hardness value increases. The structure consists of three regions (hardened zone, heat affected zone, and base metal). The microstructure of the hardened region consists of delta ferritic phase, martensitic phase and a small amount of retained austenitic phase. The best laser hardening condition is at the laser frequency of 17 Hz, the pulse width of 18ms, the scanning speed of 20 mm/min, and the focal plane position of 7.5 mm. The hardness, the depth and the width of the hardened layer for the optimum sample are 650 Hv, 330 µm and 2020 µm, respectively.
