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

• Published in: Materials letters 2021-03, Vol.287, p.129282, Article 129282
• Main Authors: Wang, Xuerou, Wu, Yue, Pan, Hua, Yao, Chengwu, Huang, Jian
• Format: Article
• Language: English
• Subjects: Austenite; Base metal; CMT; Cold welding; Hardness; Heat affected zone; Heat treating; High strength steels; Lap joints; Martensite; Materials science; Microstructure; QP steel; Softening; Welded joints; Welding
• ISSN: 0167-577X; 1873-4979
• DOI: 10.1016/j.matlet.2020.129282

•Softening occurs in the intercritical HAZ partially and whole subcritical HAZ.•Fresh martensite forms in the intercritical HAZ.•Quantity and size of austenite in the intercritical HAZ are similar to base metal.•Tempering of martensite occurs in the subcritical HAZ.•Austenite disappears in the softest zone. 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.