Achievement of the 9Mn steel weld with high strength and plasticity by friction stir welding

: To address the Mn evaporation and segregation encountered in fusion welding, friction stir welding (FSW) was performed on 9Mn steel at rotational rates of 100 and 200 rpm. The base metal (BM) consisted of lath ferrite together with lath and blocky austenite, with an austenite volume fraction of approximately 57%. At 100 rpm, the peak temperature of the weld was located within the α-γ dual-phase region, where the high temperature and severe plastic deformation during FSW promoted the partitioning of C and Mn and dynamic recrystallization of both ferrite and austenite, producing a uniform ultrafine duplex microstructure with grain sizes of 300–400 nm. The weld exhibited a tensile strength of 1435 MPa and an elongation of 19.5%, while the excessively high deformation stability of austenite suppressed the continuous activation of the transformation-induced plasticity (TRIP) effect during tensile deformation. In contrast, at 200 rpm, the peak temperature exceeded A c3 , resulting in complete austenitization during FSW. During cooling, most of the austenite transformed into refined martensite, while 26.1% austenite was preserved in the weld. Consequently, the weld achieved a tensile strength of 1894 MPa and an elongation of 24.2%. Compared with the 100 rpm weld, the refined martensitic microstructure provided higher strength, whereas the lower deformation stability of austenite promoted a more effective TRIP effect and sustained strain hardening, thereby achieving ultra-high strength while maintaining satisfactory ductility.