High strength and ductility of friction-stir-welded steel joints due to mechanically stabilized metastable austenite

Steel plates were friction-stir-welded together under conditions in which samples were first heated above the lower critical temperature of the alloy and subsequently cooled at approximately 100 K s−1. This method produced joints with an excellent balance between tensile strength and ductility. Severe plastic deformation during friction-stir-welding stabilized the austenite phase in the steel joints. The austenite phase was subsequently transformed through deformation into the martensite phase when the joints were actually used.