The mechanical properties and high temperature oxidation behaviors of 9Cr2WVTa steels with Mn contents in the range of 0.04–0.93 wt% were investigated. There are no obvious differences in the tensile properties at room temperature and high temperature, only a slight reduction in the impact toughness when Mn content reaches 0.93 wt%. Remarkably, the high temperature oxidation resistance is significantly improved with an increase of Mn content. After 500 h of oxidation, a (Fe0.6Cr0.4)2O3 oxide scale is developed on the steel with 0.04 wt% Mn, Mn1.5Cr1.5O4 oxides are occasionally detected when Mn content reaches 0.47 wt%, while a thin compact scale with a mixture of Mn1.5Cr1.5O4 and Cr1.3Fe0.7O3 oxides is formed on the steel containing 0.93 wt% Mn. Addition of Mn promotes the formation of Mn-oxides, which lowers oxygen partial pressure and accelerates external oxidation of Cr. At last, the presence of Mn-Cr spinels and Cr-rich oxides improves the oxidation resistance.

The high-temperature oxidation behavior of coarse-grained (CG) and ultrafine-grained (UFG) 9 %Cr ferritic-martensitic steel in air at 923 K up to 500 h was investigated. The UFG sample showed considerably greater oxidation resistance than the CG sample due to the fact that the outward diffusion of Mn was enhanced and the formation of Mn-rich oxide favored in the former. A duplex-layered scale structure consisting of an outer Fe-rich (Fe, Cr)2O3 layer and an inner Cr-rich (Fe, Cr)2O3 layer was identified on the CG sample, while a thin compact scale with a mixture of (Fe, Cr)2O3, MnCr2O4 and Mn2O3 oxides developed on the UFG sample. A continuous and stable scale composed of Cr-rich (Fe, Cr)2O3 and MnCr2O4 on the UFG sample in the early stage served as a protective barrier between the matrix and environment. With increased oxidation time, formation of Mn2O3 with low growth rate improved the compactness of oxide scale.

The effect of Si in the range of 0.05–0.77 wt.% on the microstructure, tensile properties and impact toughness of reduced activation ferritic/martensitic (RAFM) steels has been investigated. An increase in Si content affected the prior austenite grain size resulting in an increase in the tensile strength at room temperature. The tensile strength of steels tested above 773 K did not change significantly with the addition of Si, which was due to the diminished carbide hardening effect and boundary strengthening effect. Detailed fractographic analysis revealed that tear fractures occurred in the samples tensile tested at room temperature, while cup and cone fractures were found in samples tensile tested at temperatures above 773 K, which were induced by the easing of dislocation pile-ups. The ductile-to-brittle transition temperature (DBTT) decreased when the Si content increased to 0.22 wt.%. However, the DBTT increased when the Si content reached 0.77 wt.% and this was due to the precipitation of Laves phase. The RAFM steel with approximately 0.22 wt.% Si content was found to possess an optimized combination of microstructure, tensile properties and impact toughness.

A new process to produce ultrafine-grained reduced activation ferritic/martensitic steel was developed by cold-swaging and post-annealing. Microstructural examinations showed that the martensitic structures were broken into subgrain structures, and the rod-like carbides were fragmented after cold-swaging process with the cumulative strain of ~2.8. The tensile strength significantly increased accompanied by a reduction in the ductility in the cold-swaged samples, which was due to the insufficient strain hardening ability. After post-annealing at 973 K, a stable ultrafine-grained structure with uniformly distributed nanoprecipitates was obtained. The presence of uniformly distributed nanoprecipitates not only provided an effective precipitation strengthening but also improved the strain hardening ability, resulting in higher strength and better ductility than their as-tempered counterparts.

The hydrogen embrittlement (HE) sensitivity of a precipitation strengthened Fe–Ni based alloy with various grain sizes was investigated by hot hydrogen charging experiment and tensile test. The results showed that HE sensitivity was reduced with decreasing grain size, even though the hydrogen concentration in the small-grained specimen was higher than the large-grained specimen. Fractographic features showed that intergranular fracture was less pronounced with decreasing grain size, which was related to the transport of hydrogen to the grain boundary by moving dislocations. During deformation, the number of dislocations expected in the slip bands was proportional to the grain size, leading to that a lower hydrogen concentration being accumulated at the grain boundary in the small-grained specimen.