Enhanced strength-ductility-toughness synergy of friction stir-welded ultrahigh-strength medium-Mn steel joints via post-weld annealing

Ultrahigh-strength medium-Mn steels are one of the promising third-generation advanced high-strength steels with strength-ductility-toughness synergy. However, it has been a challenge to preserve the superior mechanical properties of ultrahigh-strength medium-Mn steels after fusion welding due to the high heat input-induced transformation of metastable microstructures. In this work, ultrahigh-strength medium-Mn steel plates with 1 GPa strength were joined by a solid-state welding technique—friction stir welding. Defect-free joints were fabricated under a specific parameter window. Transformation of austenite to quenched martensite with high hardness occurred in the nugget zones (NZs). All the as-welded joints exhibited equal strengths but significant losses in ductility compared to the base metal (BM). Moreover, the impact energies of the NZs were greatly reduced to less than 6 J, which induced premature failures of the joints. After post-weld annealing at an intercritical temperature, reverse transformation of austenite occurred in the NZs, producing a composited structure of ultrafine ferrite, martensite, and austenite. The impact energies of the annealed NZs increased to over 23 J, which was much higher than the 2.2 J measured in the as-welded counterparts. The hardness of the NZs was significantly reduced, enabling sizeable tensile elongations of the joints close to that of the BM. Consequently, enhanced strength-ductility-toughness synergy of ultrahigh-strength medium-Mn steel joints was achieved by post-weld annealing. This work demonstrates a viable method to fabricate ultrahigh-strength medium-Mn steel joints with high performance. • Ultrahigh-strength medium-Mn steel joints with 1 GPa strength were successfully fabricated by friction stir welding. • Reverse transformation of austenite occurred in the NZs after intercritical annealing to produce composite structures of ferrite/martensite and austenite. • The annealed joints exhibited comparable strength and ductility to the base metal. • The impact energies of the annealed NZs increased to over 23 J, which was ~4 times that of the as-welded NZs. • Enhanced strength-ductility-toughness synergy was achieved in 1 GPa medium-Mn steel joints.