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Advanced oxide metallurgy technique was adopted to produce 100-kg Y-bearing 12Cr ferritic/martensitic steel via vacuum induction melting and casting route. Subsequently, nine specimens at top, middle and bottom regions of the sheet were characterized to evaluate the homogeneity of chemical composition, microstructure and mechanical properties. The small vibration of hardness (200–220 HBW), ultimate tensile strength (672–678 MPa), yield strength (468–480 MPa), total elongation (26.2%–30.5%) and Charpy energy at room temperature (98–133 J) and at − 40 °C (12–40 J) demonstrated that mechanical properties’ homogeneity of Y-bearing steel was acceptable although slight Y segregation and inhomogeneous microstructure occurred at the bottom. Furthermore, the effect of Y content on microstructure characteristics and mechanical properties was explained and the comparison of failure mechanism for the dual-phase steel between tensile test (i.e., quasi-static loading) and Charpy test (i.e., dynamic loading) was discussed in detail.

9Cr ferritic/martensitic (F/M) steels with 0 and 0.3 wt.% yttrium (Y) addition were fabricated via vacuum casting technique and were creep tested at 650 °C and 120 MPa. The creep property of 9Cr F/M steel was improved by the addition of yttrium. Even though the initial creep rates of 9Cr-0Y and 9Cr-0.3Y were almost the same, the creep rupture life of 9Cr-0.3Y specimen was extended about 2˜3 times with the longer steady state creep stage and the lower minimum creep rate than 9Cr-0Y specimen. The coarsening of grains and martensitic laths occurred during creep, and M23C6 carbides, MX/M2X-type carbonitrides and Fe2W/Mo Laves phase were identified along the boundary in both specimens. For 9Cr-0.3Y, the nano-sized oxide clusters were formed during creep, because of the diffusion and local segregation of the dissolved yttrium and oxygen atoms at high temperature and with the promotion of creep stress. The softening of microstructure recovery is compensated by the dispersion strengthening of nano-particles and the balance of them was the reason of longer steady state creep stage of the creep curves. Keywords: 9Cr steel, Yttrium, Vacuum casting technique, Creep properties, Microstructure characteristics

Casting oxide dispersion strengthened (ODS) steel is difficult due to the coarsening and agglomeration of oxide particles during melting and solidification. In the present paper, a new method was developed to prepare 9Cr-ODS steel via the in situ reaction between elemental yttrium in molten Fe and activated oxygen from Fe2O3 oxygen carrier. To identify whether coarsening and aggregation of yttrium oxides can be suppressed, the cast steels did not suffer plastic deformation. Finally, tempered martensite matrix was obtained accompanied with about 0.2–1.1 μm yttrium oxides. Some yttrium oxides suffered coarsening and agglomeration due to the high oxygen content. 9Cr-ODS steel exhibited higher ultimate tensile strength, yield strength and slight lower elongation compared to 9Cr steel. The results indicated that yttrium oxides could be effectively introduced into the steel matrix via this casting method, which provided an alternative method for ODS steel fabrication.

Creep behaviors of CNS-2 steel have been investigated under different conditions at 600–650 °C and 105–180 MPa, and the microstructures and precipitations of crept and aged specimens have been characterized. It is showed that the CNS-2 steels have remarkable creep resistance at 600°C/105MPa until at least 1500 h, and the accelerated creep occurs at the elevated temperature of 650 °C and applied stress of 180MPa. The CNS-2 steels are toughened after creep without hardening and embrittlement. Their grain microstructures are stable but the sub-structures change during the creep. The subgrains coarsen with creep strain and the density of dislocations decreases due to the recovery at elevated temperature. Three kinds of precipitations including M 23 C 6 , MX-type carbonitride and Laves phase are identified in the crept steels by TEM, EDS and SEAD analysis. It is also found that the applied stress promotes the precipitations formed at the grain or subgrain boundary.

A kind of Y-bearing ferritic/martensitic fuel cladding tube with qualified dimensions, acceptable mechanical properties as well as good flattening and flaring performance was manufactured recently by our group. In this paper, to improve and further evaluate its oxidation resistance, the cladding tube was aluminized firstly and then subjected to high temperature steam oxidation at 1200 °C for 8 h. The results indicated that aluminizing coating with gradient content of Al was prepared on the tube successfully. And the matrix microstructure was transformed from tempered martensite into ferrite during aluminizing. Weight gains after high temperature steam oxidation were 72.7 and 1.48 mg cm−2 for the bare and aluminized tubes, respectively. The latter one exhibited better oxidation resistance due to the generated dense aluminum oxide film. Meanwhile, Kirkendall pores were formed near oxidation surface and should be eliminated for the real application of the aluminized tube in the future.

Rolling contact fatigue (RCF) is an important concern for the durability of railroad wheels and rails, especially when considering high-speed trains. In this work, the effect of microstructures on the RCF of high-speed-railway was studied under a simulated train speed of 500 km/h. Two different carbon steel microstructures: spheroidized pearlite and lamellar pearlite, were achieved by different heat treatment. Results of rolling contact fatigue test demonstrated that the fatigue resistance of lamellar steel considerably exceeds that of spheroidized steel. The lamellar steel presented a more fragmentary surface but lower weight loss during the entire fatigue tests. Analysis of cross-section profiles of worn specimens suggests that cracks propagated near the surface of lamellar steel and may strengthen the fatigue resistance of the steel. Based on micro-indentation hardness data, it is suggested that greater energy dissipation induced by larger plastic deformation may also improve RCF resistance.

CNS-I (9Cr) and modified CNS-II (12Cr) were specifically designed for supercritical water reactors. Chromium (Cr) coatings with thicknesses of about 1 μm were deposited on the CNS-I and modified CNS-II substrates by magnetron sputtering to improve their corrosion resistances. Corrosion behavior was investigated in supercritical water at 823 K and 25 MPa with an oxygen concentration of 200 ppb for 200, 400, 600, 800, and 1,000 h. The coated CNS-I and coated modified CNS-II showed negligible weight gains of 0.542 mg/dm2 and −13.351 mg/dm2, respectively, considerably lower than those of the corresponding bare samples (602.17 mg/dm2 and 459.42 mg/dm2, respectively) after 1,000 h of exposure. The remarkable improvement in corrosion resistance could be attributed to the formation of dense chromium oxide (Cr2O3) layers on the Cr coatings. Interestingly, the Cr-coated CNS-I consisting of 9 wt% Cr displayed an equally excellent corrosion resistance to the coated modified CNS-II consisting of 12 wt% Cr due to the b...

Thin wall cladding tubes of yttrium-bearing 9Cr ferritic-martensitic steel have been fabricated by cold-rolling process, with the final dimensions of 6 mm outer diameter, 0.5 mm thickness and 3 m length. The changes of microstructure of hot-rolled plate, forged rod and the final tubes were attributed to the cold-rolling process and the intermediate heat treatment. Mechanical properties were tested from room temperature to 600 °C. The final tubes showed the strength level between the traditional ferritic-martensitic steel and the metallurgy alloyed oxide dispersion strengthened steel. It was also found that the degradation of ductility at 400 °C was related to the dynamic strain ageing (DSA) effect. At last, ultra-sound test, flattening test and flaring test were carried out to evaluated the qualities of the final tubes.

Hardness matching between rail and wheel is important for the safe and economic operation of high speed train. In this work, various hardness of 39.5, 36.8, 33.4, 30.5, 28.1 HRC were achieved in the wheel steels tempered at 748, 773, 793, 813, 833 K, respectively. Then the twin-disc fatigue test under 1000 MPa of Hertzian contact stress, 1676 r min−1 of rotating speed (i.e., 300 km h−1) and 0.7% of creepage was conducted using these tempered wheel steels and U71MnK rail steel with the hardness of about 32 HRC. Thus various hardness ratio of Hr/Hw (Hr is the hardness of rail and Hw is the hardness of wheel) including 0.81, 0.89, 0.96, 1.07 and 1.17 were achieved. Finally, wear resistance and rolling contact fatigue performance of the wheels as the function of Hr/Hw were evaluated and the relevant failure mechanism was discussed basing on the worn morphology and mechanical properties. The results indicated that wear rate of wheels increased with Hr/Hw. Furthermore, the wheel/rail couple presented the longest fatigue lifespan of 150 000 cycles when Hr/Hw was 0.96. Thus it can be concluded that the optimal hardness ratio of Hr/Hw was 0.96 basing on the less wear rate and the longest fatigue lifespan of the wheel/rail couples under high speed of 300 km h−1 due to the reasonable strength, ductility, toughness and hardness of the wheel steel tempered at 793 K.