Study of the microstructure in a thermally altered zone of a S355 microalloyed steel with vanadium

The heat-affected zone in the intercritical temperature range (ICCGHAZ) is one of the most fragile sections in high-strength microalloy steels welding. Following multiple heating cycles in temperature range between Ac1 and Ac3, the heat-affected zone with coarse grain undergoes a strong loss of toughness and fatigue strength, mainly caused by the formation of residual austenite (RA). The presence of microalloys in HSLA steels induces variations in the microstructure, more or less beneficial, which improve the mechanical performance of welded joints. In order to highlight the influence of vanadium on the microstructural variations in the ICCGHAZ area of a C-Mn steel, the microalloyed S355 grade with a V content of 0.1 wt.% was examined. For this reason, five double pass welding thermal cycles were simulated, using a dilatometer, with the maximum temperature of the secondary peak in the intercritical zone, in the range between 720 °C and 790 °C. The paper reports the microstructural evolution analyses with main attention to the formation of RA and to the precipitation state with consequent influence on the Vickers hardness values. © 2023 Instituto Italiano della Saldatura. All rights reserved.