The microstructure evolution in nugget zone of Q&P980 steel joints under the condition of friction stir welding thermal-mechanical couple effects

The friction stir welding technique was used to weld the Q&P980 steel and the microstructure evolution in the nugget zone was investigated under the conditions of high temperature and severe plastic deformation. The results indicated that, the microstructure was determined by the synthetic effects of peak temperature, the severe plastic deformation and cooling rate. The peak temperature was proportional to the rotation speed. The higher the rotation speed, the higher the peak temperature. The cooling rate was determined by the welding speed. The higher the welding speed, the higher cooling rate and thus, the effect of plastic deformation was not obvious. When the rotation speed kept at the level of 400 r/min, the microstructure evolution was from mixed microstructure of martensite/ferrite/retained austenite and finally evolved into martensite with the welding speed from 50 mm/min to 400 mm/min. The grain size was coarsened correspondingly. When the welding speed was 100 mm/min, the microstructure evolution sequence was changed from martensite/ferrite/retained austesite to matensite, then changed into martensite/bainite. And the grain size was fined correspondingly.