Effect of Post-weld Heat Treatment on Microstructures and Properties of Laser-Welded FV520B Steel

Abstract For the laser-welded FV520B steel welds, the experiments of post-weld aging treatment have been conducted at different temperatures. The microstructural transformations under various heat-treatment conditions are investigated by optical microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Vickers hardness and impact toughness are also measured after the aging treatments. The results of microstructural observations show that before heat treatment the fusion zone (FZ) mainly includes low carbon martensite and δ-ferrite. With the increase of aging temperature and time, in FZ the δ-ferrite phase spheroidizes and disappears. When the aging temperature is lower than 400 ºC, the carbides grow with increasing aging time. After aging treatment at 550 ºC for 12 h, a small amount of reversed austenite appears in FZ. The volume fraction of reversed austenite increases with the further increase of aging temperature and time. However, in FZ all the reversed austenite disappears after aging at 650 ºC for 12 h. This indicates that at room temperature the amount of reversed austenite is related not only to the volume fraction of the reversed austenite obtained from aging, but also to the stability of reversed austenite during cooling. The Vickers hardness and impact toughness of the welds mainly depend on the balance between the dispersion strengthening effect of the Cu-rich precipitates and the weakening effect of tempered martensite. The presence of reversed austenite can also improve the toughness of welds. In addition, as the aging temperature and aging time increase, the hardness difference between the three zones of FZ, heat affected zone (HAZ) and base metal (BM) decreases.