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The study investigated the creep-fatigue interaction behavior, fracture mechanism, and microstructure evolution of C630R ferritic/martensitic heat-resistant steel at 630 °C and 1% strain amplitude. The results indicate that C630R ferritic/martensitic heat-resistant steel exhibits noticeable cyclic softening behavior, with a decrease in the softening ratio from 0.46 to 0.40 as loading time increases. Microstructure evolution was examined using optical and scanning electron microscopes, revealing the appearance of secondary cracks on the longitudinal section of the sample. These cracks exhibited a curved expansion trend with multiple deformations and highly bifurcated micro-cracks at a low loading time of 30s. However, when the load holding time was increased to 300s, the fatigue cracks tended to expand in a straight line with fewer branches. Furthermore, the observed crack growth path under different loading times showed enrichment in Fe and Cr, with the crack tip containing Cr oxide, which helped prevent fatigue crack propagation. As the loading time increases, the rise in crack closure resulting from oxidation leads to a decrease in crack propagation. Electron backscatter diffraction observation revealed the formation of numerous substructures and recrystallized grains after varying holding times, enhancing the material's resistance to crack propagation. Moreover, the study found that with increasing load dwell time, the deformed grains decreased, and substructural grains became dominant (57.68% and 62.77% under 30s and 300s, respectively). This increased crack resistance resulted in shorter secondary cracks under prolonged load retention, with the secondary crack length decreasing from 146.36 μm at 30s loading time to 109.09 μm at 300s loading time. Additionally, the kernel misorientation angle value decreased significantly after different loading times, leading to a higher likelihood of cracks or holes forming in regions with high KAM values, where low-angle grain boundaries were formed.

The evolution of tensile properties and microstructure in three 9% Cr martensitic rotor steel with varying contents of W and Mo elements is investigated after aging at 630 °C for different durations, with a primary focus on the characteristics of martensite laths and blocks, grain boundaries, dislocations, and precipitates. The results indicate that the presence of W and Mo elements not only influences the precipitation and coarsening behavior of Laves phases but also that of M23C6 carbides, which differs from conventional beliefs. The elevated content of Mo and reduced content of W in the steel leads to a reduction in the equilibrium Cr concentration, promoting enhanced precipitation of Cr atoms and increases nucleation sites of M23C6 carbides. During the aging treatment, the M23C6 carbides in the 2.0W0.5Mo steel exhibits the highest content and the lowest coarsening rate. The strength of 9% Cr martensitic rotor steel is primarily enhanced through dislocation strengthening and grain boundary strengthening. The presence of fine M23C6 carbides effectively immobilizes dislocations and interfaces, resulting in the exceptional stability of the microstructure in the 2.0W0.5Mo steel and a minimal reduction in yield strength.

Based on the simulation results of ProCAST software for the filling and solidification process, the formation mechanism of the VIM electrode ingot's pipe and porosities and the influence of the casting process on the formation of electrode shrinkage were studied in detail. The research results indicate that the critical condition for the formation of 200 kg grade C650R heat-resistant alloy is verified to be G/R≤5.0 (K·s)0.5/cm. In addition to the alloy's inherent characteristics, the substantial height to diameter ratio (H/D) of the VIM electrode ingot is the primary factor contributing to the formation of deep pipe and severe porosities. Simply adjusting the casting time or casting speed alone does not significantly improve the solidification quality of the ingot. While the use of a hot top can eliminate pipe, its effectiveness is limited, as the alloy melt at the hot top location has minimal influence on the solidification process in the lower regions of the electrode ingot, thereby failing to effectively reduce the formation of porosities. Properly reducing the casting speed and casting superheat lead to the formation of a V-shaped melt at localized positions during the final stage of solidification. This enhances the ability of the liquid phase for further feeding, ultimately significantly decreasing both the depth of pipe and the porosity rate. The optimal casting process optimization solution for C650R alloy electrode ingot with a size of Φ130 mm × 1240 mm is the casting speed of 60 kg/min and the superheat of 40 °C.

The hot deformation behavior of Ni–Cr–Fe-based ChS57 alloy was investigated by isothermal compression tests in the temperature range of 1000–1200 °C with strain rates covering 0.001–10 s−1. Based upon the hyperbolic sine function and dynamic material model, the constitutive model and processing maps of the alloy were established, the microstructure evolution was systematically analyzed and the optimization of the hot working window was completed. The results indicate that the flow stress increases remarkably with increasing strain rate and decreasing deformation temperature, while the amount of strain required to reach peak stress also gradually increases. The hot deformation process of this alloy is the result of the competition between dynamic softening and work hardening, with a deformation activation energy of 451.34 kJ/mol; dynamic recrystallization (DRX) is the main softening mechanism, while both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) exist; the low temperatures and medium strain rates (1000–1025 °C, 0.045–0.37s−1) are the instability zone parameters of the alloy when flow localization and deformation bands are the microstructural manifestations of unstable hot working. The optimum hot working window for ChS57 alloy is the medium-low temperatures and low strain rates（1050–1100 °C, 0.001–0.003s−1）and the medium-high temperatures and high rates（1125–1175 °C, 1-10s−1 or 1100 °C/10s−1）, where the microstructure appears as randomly oriented and uniformly distributed full DRX grains.

Fe40Mn20Cr20Ni20 medium-entropy alloy (MEA) has a single-phase crystal structure with high strength and good ductility at room temperature. It is important to study the hot deformation behavior for this alloy at a partially recrystallized state for possible high-temperature applications. In this investigation, the tensile tests were conducted on sheet materials treated via cold rolling combined with annealing at strain rates of 1 × 10−3–1 × 10−1 s−1 and deformation temperatures of 573–873 K. And the hyperbolic sine model was used to study the relationship between the peak stress, deformation energy storage and Zener–Hollomon parameter (Z parameter) of Fe40Mn20Cr20Ni20 medium-entropy alloys under high-temperature tension. According to the Arrhenius-type model, the constitutive equation of the alloys based on the flow stress was constructed, and the deformation activation energy and material parameters under different strain conditions were obtained. Based on the power dissipation theory and the instability criterion of the dynamic material model, the power dissipation diagram and the instability diagram were constructed, and the hot working map with a strain of 0.1 was obtained. The results show that the hyperbolic sine relation between the peak stress and Zener–Hollomon parameters can be well satisfied, and the deformation activation energy Q is 242.51 KJ/mol. Finally, the excellent thermo-mechanical processing range is calculated based on the hot working map. The flow instability region is 620–700 K and the strain rate is 2 × 10−3–4 × 10−3 s−1, as well as in the range of 787–873 K and 2 × 10−3–2.73 × 10−2 s−1. The optimum thermo-mechanical window is 850–873 K, ε˙ = 1 × 10−3–2 × 10−3 s−1.

The 9–12% Cr ferritic–martensitic heat-resistant steel is the main candidate structural material for the Lead-cooled Faster Reactor. The lower Gibbs free energy change of Si oxide can promote the formation of a stable oxide layer, which can improve the corrosion resistance of the material. Therefore, it is of great significance to study the effect of silicon (Si) on the corrosion resistance of T91 steel in lead–bismuth eutectic (LBE). The corrosion resistance of T91 steel with Si contents of 0.5 wt.%, 1.3 wt.%, and 2.0 wt.%, both in dynamic and static LBE at 550 °C, was investigated. The microstructure was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), while the oxide films were characterized by electron probe microanalysis (EPMA). Results show that the addition of Si is conducive to improving the corrosion resistance of T91 steel in LBE. T91 steel with high Si content has a thinner and more stable oxide film. The change of Si content can change the contact angle between the steel and LBE, and the contact angle is the largest when the Si content is 1.3 wt.%. The Si-rich oxide layer is usually located in the inner oxide layer, which promotes the formation of a Cr oxide layer located in the internal oxidation zone (IOZ). Si will not enter the precipitated phase, but only change the ferrite content. The oxidation model of T91 steel containing Si in LBE was also proposed.