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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.

Due to their excellent creep resistance and good oxidation resistance, 9–12% Cr ferritic–martensitic stainless steels are widely used as high temperature construction materials in power plants. However, the mutual combination of different loadings (e.g., creep and fatigue), due to a “flexible” operation of power plants, may seriously reduce the lifetimes of the respective components. In the present study, low cycle fatigue (LCF) and relaxation fatigue (RF) tests performed on grade P92 helped to understand the behavior of ferritic–martensitic steels under a combined loading. The softening and lifetime behavior strongly depend on the temperature and total strain range. Especially at small strain amplitudes, the lifetime is seriously reduced when adding a hold time which indicates the importance of considering technically relevant small strains.

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic (RAFM) steels. High-angle grain boundaries, subgrain boundaries, nano-sized M 23 C 6 , and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength. M 23 C 6 carbides are easily coarsened under high temperatures, thereby weakening their ability to block dislocations. Creep properties are improved through the reduction of M 23 C 6 carbides. Thus, the loss of strength must be compensated by other strengthening mechanisms. This review also outlines the recent progress in the development of RAFM steels. Oxide dispersion-strengthened steels prevent M 23 C 6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength. Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel. The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries. This procedure increases the creep life of TMT(thermo-mechanical treatment) 9Cr-1W-0.06Ta steel by ∼20 times compared with those of F82H and Eurofer 97 steels under 550°C/260 MPa.

The activated corrosion product assessments of fusion structural materials are essential to designing components and evaluating workers’ radiation exposure. This paper first gives the R&D status of the high-temperature pressurized water corrosion study of reduced activation ferritic/martensitic steels and chromium–zirconium–copper (CuCrZr) alloys, which are the leading candidate materials of fusion reactor in-vessel components such as breeding blanket and divertor, which are utilized in high-temperature and high-pressure water, and the recent progress of corrosion test apparatus simulating the unique environment of a fusion reactor will also be presented.

The effect of the manganese content (0.5-2.0 wt.%) on the microstructure, mechanical properties and high temperature oxidation resistance of reduced activation ferritic/martensitic (RAFM) steels was studied. The examinations reveal that the microstructure of the steels shows a fully martensite structure. A large number of M 23 C 6 carbides and a small number of Zr(C, N) carbonitrides were detected in the steels. The results of mechanical experiments showed that the strength increases and the toughness decreases with increasing Mn content. The improvement of strength can be ascribed to solution strengthening by Mn and gain size strengthening by refining lath width. After oxidation at 650 °C for 400 h, both MnCr 2 O 4 oxides and Cr 2 O 3 oxides are founded on the surface of the steels. The steel with 1.2 wt.% Mn exhibit the optimum high temperature oxidation resistance, followed by the steel with 0.5 wt.% Mn. The increase of oxidation resistance can be ascribed to the addition of Mn promotes the formation of Mn oxides, which refined the diameter of MnCr 2 O 4 oxides. The decrease of oxidation resistance mainly due to the higher Mn content reduces the critical concentration of Cr to form Cr oxides (Cr 2 O 3 ), thus promoting the formation of large-sized Cr 2 O 3 oxides.

Thermomechanical processing (TMP) of ferritic–martensitic (FM) steels, such as HT9 (Fe–12Cr–1MoWV) steels, involves normalizing, quenching, and tempering to create a microstructure of fine ferritic/martensitic laths with carbide precipitates. HT9 steels are used in fast reactor core components due to their high-temperature strength and resistance to irradiation damage. However, traditional TMP methods for these steels often result in performance limitations under irradiation, including embrittlement at low temperatures (<~430 °C), insufficient strength and toughness at higher temperatures (>500 °C), and void swelling after high-dose irradiation (>200 dpa). This research aimed to enhance both fracture toughness and strength at high temperatures by creating a quenched and tempered martensitic structure with ultrafine laths and precipitates through rapid quenching and unconventional tempering. Mechanical testing revealed significant variations in strength and fracture toughness depending on the processing route, particularly the tempering conditions. Tailored TMP approaches, combining rapid quenching with limited tempering, elevated strength to levels comparable to nano-oxide strengthened ferritic alloys while preserving fracture toughness. For optimal properties in high-Cr steels for future reactor applications, this study recommends a modified tempering treatment, i.e., post-quench annealing at 500 °C or 600 °C for 1 h, possibly followed by a brief tempering at a slightly higher temperature.

Interfacial evolution and bonding mechanism of reduced activation ferritic/martensitic (RAFM) steel were systematically investigated through a series of hot compression tests conducted at various strains (0.15–0.8), strain rates (0.001–1 s−1), and temperatures (950–1050 °C). Interfacial microstructural analysis revealed that plastic deformation of surface asperities effectively removes interfacial voids, and the evolution of dynamic recrystallization (DRX) aids in achieving a joint characterized by homogeneously refined microstructure and adequate interfacial grain boundary (IGB) migration. Electron backscattered diffraction analysis demonstrated that the continuous dynamic recrystallization, characterized by progressive subgrain rotation, is the prevailing DRX nucleation mechanism in RAFM steel during hot compression bonding. During DRX evolution, emerging DRX grains in the interfacial region expand into adjacent areas, transforming T-type triple junction grain boundaries into equal form, and resulting in a serrated and intricate interface. Elevated temperatures and strains, coupled with reduced strain rates, augment DRX grain nucleation and IGB migration, thus enhancing RAFM joint quality with regard to the interface bonding ratio and the interface migration ratio.

Ferritic/martensitic (F/M) steels are considered optimal candidates for utilization as structural materials in Lead-Cooled Fast Reactor (LFR) due to their excellent mechanical properties at high temperatures and outstanding resistance to irradiation swelling. However, the combined effect of lead-bismuth eutectic (LBE) corrosion and irradiation damage degrades the performance of structural materials and reduces their safe service life limits. Accordingly, this study investigated the LBE corrosion behavior of 9Cr F/M steels with different Si contents, which had been irradiated with 6 MeV Au ions. Furthermore, the influence of Si content on the irradiation damage and corrosion behavior of 9Cr F/M steel was discussed. With increasing irradiation dose, both the size and density of dislocation loops increased, leading to irradiation hardening. Irradiation-accelerated LBE corrosion was observed in samples with different Si contents, and the sample with 0.7 wt% Si content exhibited superior LBE corrosion resistance under irradiation-corrosion synergy.

The EK-181 and Eurofer steel samples were saturated in gaseous deuterium at 200°C and a pressure of 5 atm for 25 h. The effect of sample storage conditions (in vacuum or in air) on the deuterium release from EK-181 and Eurofer steels was studied. The deuterium retention was examined by thermal desorption spectroscopy after storage from several days to one and a half years after the samples were saturated with deuterium. The EK-181 steel samples contained 3.5–6 times more deuterium than the Eurofer steel samples. After one and a half years of storage, the deuterium retention decreased by 1.5–3 times in EK-181 steel and by 3–4 times in Eurofer steel. For the EK-181 steel samples, no unambiguous conclusion can be made about the effect of storage conditions on the deuterium release from the material. Deuterium releases more slowly from the Eurofer steel samples if they were stored in a vacuum. Time dependences of the deuterium retention under different storage conditions were obtained.

The formation of δ -ferrite in advanced structural materials is well known to impair mechanical and corrosion properties. In the present work, high-temperature phase stability and phase transformation characteristics of the γ  →  δ phase transformation of ferritic/martensitic steel were studied by differential scanning calorimetry (DSC) and electron backscatter diffraction (EBSD). The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. The results show that γ -austenite completely transforms to δ -ferrite during austenitization beyond 1523 K. The absence of martensite transformation was observed in the DSC thermogram for the sample annealed at 1523 K for 5 h. A detailed EBSD study of the time-dependent evolution of δ -ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. There was no reverse transformation ( δ -ferrite →  γ -austenite) observed during heating in DSC. Further, the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the γ -austenite →  δ -ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol −1 and 2.1, respectively. Tis result has significant technological implications as it revealed an important fact that δ -ferrite, once formed in the material, does not dissolve by heat treatment. Graphical Abstract The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. It is observed that the γ -austenite completely transforms to δ -ferrite during austenitization beyond 5 h at 1523 K. A detailed EBSD study of the time-dependent evolution of δ -ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. For δ-ferrite , there was no reverse transformation ( δ-ferrite  →  γ-austenite) observed during heating in DSC which signifies the difficulty associated with its dissolution by heat treatment. Further, Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the  γ -austenite →  δ -ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol −1 and 2.1, respectively.