Improvement of low-temperature impact toughness for 304 weld joint produced by laser-MIG hybrid welding under magnetic field

A process method with steady magnetic field assist laser-MIG hybrid welding was proposed, grain refinement and ferrite variation were investigated through Electron Backscattered Diffraction test. The related low-temperature (−40 °C) impact toughness of weld metals was found to be improved by 36%. Stirring effect in weld pool brought by external magnetic field promoted grain refinement and ferrite reduction. Brittle fracture was found to appear at interface between ferrite and stress-induced martensite, thus decrease of ferrite contributed to the increase of impact crack initiation energy. Grain refinement and random distribution of ferrite increased crack propagation barrier. The remarkable increase in the low-temperature impact toughness of weld metal was attributed to the decreased volume of ferrite, random distribution of ferrite and refinement of microstructure.