Microstructural control and mechanical properties in friction stir welding of medium carbon low alloy S45C steel

Critical control of the welding conditions produced a fine ferrite-martensite duplex structure with the martensite volume fraction of around 33% in friction stir welding (FSW) of medium carbon (0.45 wt% C) low alloy steel sheets. This microstructure provides the preferable combination of tensile strength and ductility. The findings were achieved to clarify the effect of the welding speed on the weldability. The fraction of low angle grain boundaries (LAGBs) decrease with increasing welding speed. At the welding speed of 400 mm/min, the top region microstructure consists of lath martensite, while the bottom region shows a fine ferrite-pearlite structure. This microstructural variation can be linked with the distribution of the peak temperatures along the thickness of the weld. Additionally, it was shown that a non-contact thermal imaging system can be used as an effective tool for the online monitoring of several kinds of weld decays.