Modification of Carbon Steel by Laser Surface Melting: Part II: Effect of Laser Beam Power on Microstructural Features and Surface Hardness

Abstract

The surface hardness has an important effect on the wear resistance of different materials. The present study aims to improve the surface hardness of carbon steel through the application of laser surface melting with suitable conditions. The laser beam power and travelling speed are the main factors that affect the properties of the treated zone. In this study, three different conditions of laser beam power (1800, 1500 and 1200 W) at fixed travelling speed of 1000 mm min^-1 were chosen to study the effect of laser beam power. The resulted laser treated specimens were investigated in macro and microscopically scale using optical and scanning electron microscope. Hardness measurements were also carried out through the thickness of the laser treated zones. The laser treated areas with all used powers results in melted and solidified zone on the surface of the steel. The laser power of 1800 W results in the deepest value of the laser treated zone (about 1.7 mm). Moreover, by increasing the laser power, the width of the treated zone was slightly increases. At areas near the free surface, large martensite plates were observed in higher laser power (1800 W), while longer acicular martensite was observed in lower laser power (1200 W). For laser power of 1800 W, the bainite structures in ferrite grains were more pronounced in larger areas and in closer areas to the free surface. On the other hand, the lower laser power shows higher hardness on the free surface than that of higher power. The sizes of Heat Affect Zone (HAZ) areas were increased by increasing the laser beam power. In all conditions, the heat affected zone areas were composed of partially decomposed pearlite in ferrite grains.