Effects of heat input and martensite on HAZ softening in laser welding of dual phase steels

Laser welds were made in three dual-phase (DP) alloys with ultimate tensile strengths ranging from 450-980 MPa and varying microstructures to investigate effects of heat input on heat affected zone (HAZ) softening. To compare the total heat transferred into the HAZ of all the welds, heat input was normalized using the Rosenthal Equation. It was found that HAZ softening experienced in a DP steel was a function of both martensite content and heat input. Maximum HAZ softening was proportional to the martensite content, and the heat input controlled the completion of softening. Material softening was normalized by martensite content, which showed that the contribution of martensite to material hardness from the three materials is the same: however the materials had different transformation kinetics.