Microstructure and microhardness evolution of thermal simulated HAZ of Q&P980 steel

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. A c1 and A c3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450–460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.