Study of microstructure and mechanical properties of friction stir welded ferrite-martensite DP700 steel

In the present study, a 2 mm thick ferrite-martensite dual phase steel was subjected to friction stir welding. The welding was conducted by a tungsten carbide tool at a constant rotational speed of 800 rpm and various feed rates of 50, 100 and 150 mm/min. The microstructural features of friction stir welded joints were characterized by field emission - scanning electron microscopy as well as by transmission electron microscopy. The relationship between microstructure and tensile properties of the joints was investigated. Results showed that the stir zone of the welds consisted of bainite packets, exhibiting a different morphology compared to the ferrite phase and to the martensite phase. Microstructural examination of the heat affected zone showed that there is a softened region in the heat affected zone in all joints, irrespective of the welding speed. Decomposition of the martensite phase during tempering of the initial martensite of the base material was responsible for the observed