Continuous cooling transformation diagram and mechanical properties in weld coarse-grain heat-affected zone of API X70 steel

In the present work, continuous cooling transformation (CCT) of coarse-grained heat-affected zone (CGHAZ) and simulation of Charpy-sized impact specimens were performed using a Gleeble 3800 thermomechanical simulator. Results obtained from the dilation studies show significant effect of cooling rates on microstructure and low-temperature (–20 °C) Charpy impact toughness. Phase transformation temperatures (Ar3 and Ar1) and impact toughness decreased while hardness and amount of bainite increased with increasing cooling rates. At slow cooling condition (< 5 °Cs–1) quasi-polygonal ferrite and pearlite phases were observed in the microstructure. At medium cooling rate (5–25 °Cs–1), bainite and quasi-polygonal ferrite were obtained in the microstructure. For still faster cooling rates, microstructure was completely bainitic in nature. The microstructures were confirmed by hardness measurement where the hardness value for lower, medium and high cooling rates were 191–196, 213–214 and 234–253 HV, respectively. Charpy impact toughness increased with decrease in cooling rate due to the presence of softer ferrite phase.