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<p>Understanding the thermal stability of austenite is critical for designing high-performance steels. Grain size is a key factor controlling thermal stability, but the underlying mechanisms remain partially unclear. For instance, the grain boundary nucleation theory fails to explain why small-grained austenite exhibits higher thermal stability. While strong martensite variant selection in small-grained austenite has been proposed as a contributing factor, the mechanistic role of grain size in this selection process remains unresolved. In this study, quasi-<i>in-situ</i> ECCI and EBSD characterization techniques were employed to investigate these mechanisms. The results demonstrate that grain size influences martensitic transformation nucleation behavior, thereby affecting austenite thermal stability. Compared to fine-grained austenite, coarse-grained austenite more readily forms nucleation sites (<i>e.g.</i>, faulted bands) during cooling. Furthermore, the diversity of martensite packet variants is determined by the variant types of faulted bands. The variant types of faulted bands are themselves governed by austenite grain size. As grain size decreases, faulted bands transition from multiple variants to a single variant. Consequently, martensite in small-grained austenite forms <i>via</i> a single-variant mode, enhancing austenite stability. This study is the first to elucidate the grain size effect on austenite thermal stability through the lens of martensitic transformation nucleation and crystallography.</p>

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New Insights on the Effect of Grain Size on Martensitic Transformation in Metastable Austenite Steel

  • 期刊论文

  • 发表日期：

    2025年6月12日

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  • Jinliang Wang,
  • Xing Liu,
  • Zhengrong Ai,
  • Guangqi Dong,
  • Minghao Huang,
  • Chunguang Shen,
  • Zhen Zhang,
  • Xiaohui Xi

摘要

Understanding the thermal stability of austenite is critical for designing high-performance steels. Grain size is a key factor controlling thermal stability, but the underlying mechanisms remain partially unclear. For instance, the grain boundary nucleation theory fails to explain why small-grained austenite exhibits higher thermal stability. While strong martensite variant selection in small-grained austenite has been proposed as a contributing factor, the mechanistic role of grain size in this selection process remains unresolved. In this study, quasi-in-situ ECCI and EBSD characterization techniques were employed to investigate these mechanisms. The results demonstrate that grain size influences martensitic transformation nucleation behavior, thereby affecting austenite thermal stability. Compared to fine-grained austenite, coarse-grained austenite more readily forms nucleation sites (e.g., faulted bands) during cooling. Furthermore, the diversity of martensite packet variants is determined by the variant types of faulted bands. The variant types of faulted bands are themselves governed by austenite grain size. As grain size decreases, faulted bands transition from multiple variants to a single variant. Consequently, martensite in small-grained austenite forms via a single-variant mode, enhancing austenite stability. This study is the first to elucidate the grain size effect on austenite thermal stability through the lens of martensitic transformation nucleation and crystallography.