Welding Parameter Dependent Microstructure and Mechanical Properties of Friction Stir Welded High Carbon Steel Plates

SK4 high carbon steel plates (0.95% C) with a thickness of 2 mm were subjected to the friction stir welding (FSW) process. After welding, three different types of microstructures were obtained in the stir zone depending on the different welding conditions. When FSW was carried out at 120 rpm, a refined ferrite matrix distributed with small cementite particles was formed in the entire stir zone. At 200 rpm, a refined pearlite phase was formed in the stir zone, together with a small amount of the martensite phase. However, at 400 rpm, a twin martensite structure became the dominate phase in the stir zone, accompanied by a small amount of the pearlite phase. The tensile tests revealed that the tensile strengths of the weld metals were higher than that of the base metal and increased with the increasing rotation speed. However, the elongations decreased. This study showed that the properties of that friction stir welded high carbon steel can be easily controlled by simply changing the welding conditions