Transformation in stir zone of friction stir welded carbon steels with different carbon contents

Five types of ferrite-pearlite structure carbon steels with different carbon contents (IF steel, S12C, S20C, S35C, S50C) were friction stir welded under various welding conditions, and the mechanical properties and microstructures of the FSW carbon steel joints were evaluated. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. When the carbon content is less than or equal to 0.12 mass%, the welding produces ferrite-pearlite structures, and the strength slightly increases compared to the base metal due to the refined microstructure; when the carbon content is above 0.2 mass%, the welding produces ferrite-pearlite plus harder phases like the martensite and bainite microstructures, resulting in a significantly increased strength of the joints. These are dependent on each of the thermal-mechanical cycles.