Predicting transient softening in the sub-critical heat-affected zone of dual-phase and martensitic steel welds

Abstract

To improve vehicle fuel economy and crash worthiness the automotive industry has been redesigning parts from advanced high strength steels such as dual-phase and martensitic steels. These steels have high strengths with the higher formability characteristics when compared to lower strength conventional steels of similar ductility. These steels derive their unique properties from their complex microstructures containing ferrite and martensite. During assembly welding, the martensite within the sub-critical region of the heat-affected zone tempers, which locally reduces mechanical properties. Although this phenomenon is well studied, it has yet to be quantified. The present work proposes a technique to measure the softening kinetics of dual-phase and martensite steels using rapid isothermal tempering. The resulting model was then validated by predicting the heat-affect zone softening that occurs in laser and resistance spot welds as well as by comparing the microstructures of the rapid tempered samples to the microstructures found in the sub-critical heat-affected zone of welded samples. © 2013 ISIJ.