The Effect of Peak Temperature on HAZ Microstructure and Toughness of V- and Nb-Bearing Microalloyed Steels

Welding heat cycle processes of V- and Nb-bearing microalloyed steels at different peak temperatures (1350, 1100, 950, 800 and 650 °C) were simulated by Gleeble 3500. The microstructure and toughness of two steels were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and low-temperature impact toughness test (− 40 °C). The results indicated that the microstructure of both V and Nb steels consisted of the lath bainite (LB), granular bainite (GB), polygonal ferrite (PF) and a small amount of martensite austenite (MA) islands at 1350 °C, and acicular ferrite (AF) was also observed in V steel. Ferrite (F) phase became uniform, and hard phase LB decreased with the decrease in peak temperature. Impact tests indicated that both the two steels exhibited the best impact toughness (32.23 and 30.90 J) at the peak temperature of 950 °C. The impact toughness of V steel was better than of Nb steel all the time, which was consistent with the analysis of microfracture.