Softening Kinetics in the Subcritical Heat-Affected Zone of Dual-Phase Steel Welds

Welds in dual-phase (DP) steels exhibit heat-affected zone (HAZ) softening in which the tempered or subcritical HAZ exhibits a lower hardness vs that of the parent material. The rate of this softening reaction with respect to welding heat input was determined for four DP steels by making several bead-on-plate laser welds using a variety of heat inputs and measuring the resulting minimum hardness. The reduction in hardness was then fit to the Avrami equation, enabling a comparison of the relative heat needed to soften each steel. It was found that the heat input required for HAZ softening decreased as the C content of the martensite within the DP structure increased. However, the presence of carbide forming alloying elements such as Cr and Mo was able to increase resistance to softening.