Estimation and prediction of HAZ softening in thermomechanically controlled-rolled and accelerated-cooled steel

Abstract

In order to evaluate the degree of HAZ softening in fine-grained ferritic-pearlitic steel manufactured by TMCP, variations of HAZ microhardness and tensile strength with different heat inputs were investigated. The degree of softening was also predicted using an established microstructural evolutional model and a rule of mixtures. All specimens welded with higher than 1 kJ/mm heat input showed a softened zone that had a lower hardness than that of the base metal. Even though softened zone width increased continuously 10 kJ/mm, the minimum hardness in a softened zone decreased slightly after a rapid decrease of up to 6 kJ/mm. Due to the softening effect, welded-joint tensile specimens were broken at the HAZ instead of the base metal. The reduction in tensile strength was similar to that for hardness and showed a maximum of 20% at 6 kJ/mm heat input. The degree of HAZ softening calculated with a modified microstructural evolution model and a rule of mixtures showed reasonably good agreement with a measured one.