Influences of blocky retained austenite on the heat-affected zone softening of dual-phase steels

The purpose of the present study is to mitigate the effect of heat-affected zone (HAZ) softening on laser welded dual-phase (DP) steel by modifying the steel microstructure. It was shown that DP steels composed of ferrite, martensite, and blocky retained austenite provide enhanced joint toughness, which could match that of the un-welded base metal. Conversely, a DP steel containing only ferrite and martensite phases exhibited severe reduction in ductility due to HAZ softening. The underlining mechanism relates to the presence of retained austenite to maintain high joint strength and ductility was revealed by investigating the work hardening behaviour under tensile deformation and microhardness distributions across the welds as well as by examining the microstructure using scanning and transmission electron microscopy.