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

Abstract

According to previous studies, the presence of κ-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 κ-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 κ-carbide. Moreover, the effect of Al (κ-carbide creation promoting element) and Cr (κ-carbide creation suppressing element) addition on crack susceptibility was elucidated to clearly confirm the effect of κ-carbide. Varestraint tests showed that cracks were mainly found in the 750 °C–900 °C peak HAZ, 1000–1500 μm away from the fusion line. The cracks occurred at the austenite/austenite and austenite/ferrite grain boundaries. The microstructural and hardness evaluation revealed that the cracking along the grain boundaries at the 750 °C–900 °C peak HAZ was induced by two factors: i) relative weakening of grain boundaries due to grain interior κ-carbide and ii) grain boundary embrittlement due to grain boundary κ-carbide. The effect of κ-carbide on the crack susceptibility of HAZ was further clarified by investigating the cases of Al (κ-carbide creation promoting element) and Cr (κ-carbide creation suppressing element) addition.