Ferritic-martensitic ODS steels are one of the candidate materials for Gen-IV nuclear fission and fusion reactors. Residual ferrite was often found in the microstructure of 9Cr ODS steels. This constituent was reported to be responsible for the superior creep and high-temperature strength. Using optical microscopy of an air-cooled batch of ODS EUROFER, inhomogeneous regions in the microstructure have been found with similar appearance to previously reported residual ferrite. In order to avoid a potential misinterpretation of inhomogeneous regions as residual ferrite, detailed microstructural investigations have been carried out on the inhomogeneous regions using site-specific nanoindentation, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. It is demonstrated that the inhomogeneous regions are free of oxide nanoparticles, which possibly form due to imperfect mechanical alloying. These regions also exhibit lower hardness which is attributed to the absence of nanoparticles and a lower dislocation density. It is concluded that optical microscopy alone is insufficient to distinguish beneficial residual ferrite from undesired particle-free regions. Our findings are underpinned by the consistency between the calculated theoretical yield strength, the yield strength converted from the indentation hardness and the yield strength obtained from tensile testing.

铁素体-马氏体ODS钢是第四代核裂变和聚变反应堆的候选材料之一。9Cr ODS钢的显微组织中经常发现残留的铁素体。据报道，该成分是导致优异的蠕变和高温强度的原因。使用空气冷却的ODS EUROFER批次的光学显微镜，发现微观结构中的不均匀区域外观与先前报道的残留铁氧体相似。为了避免将不均匀区域潜在地误解为残留铁素体，已使用位点特定的纳米压痕，扫描电子显微镜（包括电子背散射衍射）和透射电子显微镜对不均匀区域进行了详细的微观结构研究。证明不均匀区域没有氧化物纳米颗粒，其可能由于不完善的机械合金化而形成。这些区域还表现出较低的硬度，这归因于不存在纳米颗粒和较低的位错密度。结论是，仅光学显微镜不足以将有益的残留铁素体与不需要的无颗粒区域区分开。计算得出的理论屈服强度，从压痕硬度转换的屈服强度与从拉伸试验获得的屈服强度之间的一致性为我们的发现提供了支持。结论是，仅光学显微镜不足以将有益的残留铁素体与不需要的无颗粒区域区分开。计算得出的理论屈服强度，从压痕硬度转换的屈服强度与从拉伸试验获得的屈服强度之间的一致性为我们的发现提供了支持。结论是，仅光学显微镜不足以将有益的残留铁素体与不需要的无颗粒区域区分开。计算得出的理论屈服强度，从压痕硬度转换的屈服强度与从拉伸试验获得的屈服强度之间的一致性为我们的发现提供了支持。