The study on martensite morphology in the stir zone and its influence to impact toughness during friction stir welding medium–Mn ultrahigh strength steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2020-11, Vol.798, p.140102, Article 140102
• Main Authors: Cui, H.B., Lu, Y., Xie, G.M., Luo, Z.A., Wang, C.X., Kabwe, F.B., Liu, Z.G., Tang, X.
• Format: Article
• Language: English
• Subjects: Austenite; Base metal; Friction stir welding; Grain size; Heat treating; High strength steels; Impact strength; Impact tests; Impact toughness; Martensitic block; Medium-Mn ultrahigh Strength steel; Morphology; Plastic deformation; Rotation; Stress concentration; Tempered martensite; Toughness; Well tempered martensite
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2020.140102

A 6 mm thickness medium-Mn ultrahigh strength steel was friction stir welded (FSW) with various rotation speed at constant of welding speed in this study. The microstructural evolution and impact toughness of the stir zone were investigated in a systematic manner. The microstructural observation results suggested that the metastable austenite in base metal disappeared after FSW, and the stir zone of FSW samples contained various morphology of martensite (i.e., well tempered martensite, less tempered martensite and common martensite). The content of well tempered martensite and less tempered martensite in the stir zone was decreased with the increase of rotation speed, and the martensite blocks and prior austenite grain size were gradually coarsened. It is suggested that the plastic deformation and the small prior austenite grain size promoted the formation of well tempered martensite in the stir zone at low rotation speed. The well tempered martensite could effectively release the stress concentration by interfaces separation with other martensite of different morphology during impact test, which greatly benefited the impact toughness. Furthermore, the small martensite blocks could also improve the impact toughness. The impact toughness of the stir zone was decided by the martensite blocks size and the content of different morphology of martensite phase. •The stir zone of FSW samples contained various morphology of martensite.•The martensite morphology varies greatly at different welding parameters.•Massive well tempered martensite formed in the stir zone at low rotation speed.•The well tempered martensite can effectively release the stress concentration.