Microstructure and toughness of the simulated welding heat affected zone in X100 pipeline steel with high deformation resistance

Abstract

A single welding thermal-cycles with different heat inputs (8, 16, 20, 25, 30 and 50 kJ/cm) were simulated by Gleeble 3800 to study the correlation of toughness, hardness and microstructure in heat affect zone (HAZ) of the X100 pipeline steel with multi-phases and 0.10%Nb (mass fraction). The microstructures of the CGHAZ in HAZ were characterized by means of OM, SEM and EBSD, and mechanical properties were tested. The results show that for a low heat input of less than 20 kJ/cm, the microstructure is lath bainite or acicular ferrite structure with high-density of large-angle boundaries (≥15°), which exhibits good Charpy impact toughness. However, for a large heat input over 25 kJ/cm, the uniformity of prior austenite grains becomes worse, the M/A constituents and the granular bainite (GB) are coarsening, and the amount of large-angle boundaries decreases with the increase of heat input. The results of the instrumented Charpy impact test and the observation of fracture surfaces on the specimens indicate that the cracks are induced near the coarse M/A constituents and the large-angle boundaries can remarkably restrict crack propagations. Therefore, in order to ensure a strong match between the HAZ and the base metal, and the resistance to hydrogen induced delayed damage because of high hardness of HAZ, the heat input energy should be about between 15 and 20 kJ/cm. © right.