Microstructural evolution and mechanical properties of ultrafine-grained 9Cr2WVTa ferritic-martensitic steel via isothermal aging at 823 K to 923 K was studied. Fine dispersed carbides with a low coarsening rate could effectively impede migration of grain boundaries via the Zener pinning effect in the ultrafine-grained 9Cr2WVTa during thermal aging at 823 K, which is responsible for the good microstructural stability. Higher strength and higher ductility are present in the ultrafine-grained 9Cr2WVTa than those of tempered 9Cr2WVTa after thermal aging at 823 K up to 5000 hours. Laves phase precipitating adjacent to carbides at subgrain/grain boundaries grow into fine rod-like shape along grain boundaries in ultrafine-grained 9Cr2WVTa, while granular-like Laves phase is resulted in tempered 9Cr2WVTa. Newly formed nanometer-size Laves phase could provide precipitation strengthening to compensate loss of solid solution hardening, and relatively homogenous precipitation of Laves phase could considerably suppress microcrack formation in ultrafine-grained 9Cr2WVTa. However, abnormal grain growth is induced by the non-uniformity of pinning efficiency in the ultrafine-grained 9Cr2WVTa during thermal aging at 923 K. The abnormal grain growth leads to a significant reduction in the strengthening effect from subgrain hardening, which is more than three times larger than the Orowan stress from particle hardening, thus a dramatic decrease in the micro-hardness is resulted.

组织演变和力学性能的超细晶9Cr2WVTa铁素体-马氏体钢通过等温在823K老化到923 K的影响。精细的分散的碳化物具有低粗化率的晶界可有效阻碍移民通过在823 K的热时效过程中，超细晶粒9Cr2WVTa中的齐纳钉扎效应，具有良好的微观结构稳定性。在823 K高达5000小时的热时效之后，超细晶粒9Cr2WVTa的强度和延展性高于回火9Cr2WVTa。在亚晶粒/晶粒边界处碳化物附近析出的拉夫斯相沿超细晶粒9Cr2WVTa的晶界生长成细棒状，而颗粒状拉夫斯相则导致回火的9Cr2WVTa。新形成的纳米级Laves相可提供沉淀增强作用，以补偿固溶体硬化的损失，而相对均匀的Laves相沉淀可显着抑制超细晶9Cr2WVTa中的微裂纹形成。然而，