Improvement in toughness and ductility of friction stir welded medium-Mn steel joint via post-welding annealing

The medium-Mn steels have a great application potential in the engineering fields due to its excellent strength and toughness. However, the presence of coarse martensite and segregation of Mn in the fusion-welded medium-Mn steel joint severely deteriorated toughness and ductility. In this work, 6 mm thick medium-Mn steel plates were joined by friction stir welding, and then were intercritically annealed to enhance the toughness and ductility of the joint. The as-welded nugget zone (NZ) contained lath martensite with low proportion of high angle boundaries (HABs), leading to the joint with lower elongation of 22.5% and the impact energy of 73.6 J/cm2. However, the as-annealed NZ consisted of ferrite and austenite because partial martensite changed to reversed austenite, while other martensite became ferrite during annealing. In addition, the proportion of HABs in the as-annealed NZ increased obviously. In comparison with as-welded joint, the impact energy and elongation of the as-annealed joint was increased by 70.7% and 44.4%, respectively. The abovementioned excellent mechanical properties were attributed to the high proportion of HABs and a large amount of ultra-fine reversed austenite with high thermal and mechanical stability, which provided a significant transformation induced plasticity effect, obviously inhibiting crack initiation and propagation. This systematic work provided a reference for welding similar materials containing retained austenite.