Local -brittle cracking in the heat-affected zone of lightweight steels

Abstract

According to previous studies, the presence of kappa-carbide typically has a significant effect on the microstructure and mechanical properties of austenitic and ferritic Fe-Mn-Al-C steels. However, research on the welding characteristics of lightweight Fe-Mn-Al-C steels is lacking, despite the kappa-carbide-related behavior being a critical factor in determining the performance of welds in lightweight Fe-Mn-Al-C steels. In this study, the cause for the cracking at heat-affected zone (HAZ) in lightweight steel was elucidated. With respect to this, varestraint test was performed, and the investigation was conducted with focus on the kappa-carbide. Moreover, the effect of Al (kappa-carbide creation promoting element) and Cr (kappa-carbide creation suppressing element) addition on crack susceptibility was elucidated to clearly confirm the effect of kappa-carbide. Varestraint tests showed that cracks were mainly found in the 750 degrees C-900 degrees C peak HAZ, 1000-1500 nn away from the fusion line. The cracks occurred at , the austenite/austenite and austenite/ferrite grain boundaries. The microstructural arid hardness evaluation revealed that the cracking along the grain boundaries at the 750 degrees C-900 degrees C peak HAZ Was induced by two factors: i) relative weakening of grain boundaries due to grain interior kappa-carbide and ii) grain boundary embrittlement due to grain boundary kappa-carbide. The effect of kappa-carbide on the crack susceptibility of HAZ was further clarified by investigating the cases of Al (kappa-carbide creation promoting element) and Cr (kappa-carbide creation suppressing element) addition.