Effect of tempering heat treatment on the microstructure and impact toughness of a Ni–Cr–Mo–V steel weld metal

Microstructure evolution and impact toughness of Ni–Cr–Mo–V weld metal in as-welded (AW) and different post-weld heat treatment (PWHT) conditions were investigated using Transmission Electron Microscopy (TEM), Transmission Kikuchi Diffraction (TKD), and Charpy impact test. The enrichment of Mn and Ni in inter-dendritic (ID) regions caused by welding solidification segregation lowered the Ac1 temperature, making partial untempered martensite transform into reversed austenite during PWHT at 580 °C, which was confirmed directly by Laser Scanning Confocal Microscope (LSCM). The transformation kinetics and element distribution during austenite reversion were revealed by DICTRA simulation, suggesting the enrichment of C, Mn and Ni in reversed austenite. The microstructure at room temperature of ID regions depended on the tempering cooling rates. When weld metal was cooled by water after tempering, the microstructure in ID regions consisted of finer fresh martensite and over tempered martensite, which was a major contributor to the higher impact toughness. While weld metal was cooled at a rate of 5 °C/h, the reversed austenite in ID regions transformed into fresh ferrite and cementite, which was difficult to resist crack propagation, and resultantly the impact toughness was deteriorated. The impact absorbed energy of weld metal increased with the increase of tempering cooling rates, which was attributed to the microstructure change in ID regions from ferrite to martensite. The inherited substructures of martensite in ID regions had a hindering effect on crack propagation, and resultantly more energy was consumed.