Effect of refined multi-scale microstructures on the ultra-high impact toughness of bainitic steel austempered below Ms temperature

作者：Wang, Kun; Hu, Feng^*; Zhou, Wen; Zhang, Dong; Yin, Chaochao; Yershov, Serhii; Wu, Kaiming^*

来源：Journal of Materials Research and Technology-JMR&T, 2023, 26: 5773-5785.

DOI：10.1016/j.jmrt.2023.09.003

• Bainitic steel
• Austempering process
• Multi-scale microstructures
• Toughening mechanism

In this study, the multi-scale microstructures of bainitic steel were refined to different degrees by austempering below Ms (martensite start temperature). The influence of multi scale microstructures and their boundaries on impact toughness was analyzed. The results revealed that the impact toughness of bainitic steel significantly improved as the austempering temperature decreased. The block structure and Bain groups are refined by austempering below Ms, thereby introducing high-density HAGBs (high-angle grain boundaries) and increasing crack propagation resistance. The proportion of V2, V3, and V5 variants increased and formed HAGBs with the V1 variant, which regulated phase transformation strain and delayed crack formation. The improvement in impact toughness can be attributed to the formation of more HAGBs. This is mainly caused by the refinement of the block structure. Therefore, the block structure can be considered as the dominating and effective controlling unit of impact toughness.