Achieving equal strength joint to parent metal in a friction stir welded ultra-high strength quenching and partitioning steel

The weldability of an advanced ultra-high strength quenching and partitioning (Q&P) steel with gigapascale yield strength was investigated via friction stir welding technique. The equal strength to parent metal was for the first time achieved in the friction stir welded joint although microhardness loss in heat affected zone (HAZ) occurred. The increased volume fraction of ferrite and dissolution of martensite and retained austenite were found in the softened HAZ. Nevertheless, the strength of the welded joint was unchanged due to the enhanced strain hardening ability achieved by the formation of ultra-fine dual phase structures and nano-carbide particles in the HAZ.