Microstructural Evolution and Mechanical Properties of Ultrafine-Grained Ferritic-Martensitic Steel During Thermal Aging

Microstructural evolution and mechanical properties of ultrafine-grained 9Cr2WVTa ferritic-martensitic steelviaisothermal aging at 823 K to 923 K was studied. Fine dispersed carbides with a low coarsening rate could effectively impede migration of grain boundariesviathe 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.