Microstructure and softening of advanced high-strength steel QP1180 lap joints welded with CMT

Abstract QP1180 lap joints are welded by the cold-metal-transfer method. The microstructure and softening of the welded joints are investigated. The results show that the fusion zone is composed of acicular ferrite and as hard as the base metal. The hardness of the supercritical heat-affected-zone (HAZ), where martensite is the main phase, is much higher than that of the base metal. Softening takes place in part of the intercritical HAZ and the whole subcritical HAZ. In intercritical HAZs, fresh martensite forms, while the size and amount of austenite are similar to those in the base metal. The hardness in the subcritical HAZ decreases due to the tempering effect. Austenite starts to decompose in the subcritical HAZ, where the hardness drops from 380 to 390 HV. However, in the softest place of the subcritical HAZ, the austenite phase almost disappears, and the hardness drops to approximately 320 HV.