In this study, we address the effect of vacuum heat treatment on the morphology of Al₂O₃-3wt.%TiO₂ coating, element diffusion behavior, coating hardness, and corrosion resistance. The pores, cracks, and non-liquefied particles on the as-heat treated coating surface of the vacuum-heat-treated coating were observed and compared with the as-sprayed coating using a scanning electron microscope. The diffusion behavior of the elements in the coating was demonstrated by using a line scanning of a cross-section of the coating. Hardness and corrosion-resistance test results were used to judge the effect of a vacuum heat treatment on the coating. The research results show that compared with atmospheric heat treatment, the vacuum heat treatment had less effect on the pores, cracks, and non-liquefied particles on the surface of the coating. However, in the absence of new oxide formation, the pores and cracks in the cross-section of the coating were significantly improved by the vacuum heat treatment. The surface hardness and corrosion resistance of the coating were significantly improved. The crack defects were eliminated, and the uniformity of TiO₂ distribution was improved, which are the main factors that improved the coating performance after vacuum heat treatment. The combination of the coating and the substrate is strengthened, and an Al₂O₃ and TiO₂ interdiffusion zone is formed when the coating undergoes vacuum heat treatment, which is the main mechanism improving the performance of the AT3 coating.

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M₂₃C₆ phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings. The 9Cr-1.5Mo-1Co cast steel, referred to as CB2, is one of the most promising alloys for various cast components to be used under ultra-supercritical conditions. In this study, HRTEM, SEM, and XRD methods were used for qualitative and quantitative analyses of contents, phases, and sizes of the inclusions and precipitates, as well as microstructure observation of the tempered martensitic steel at different cooling rates. Traditional tensile tests were conducted to characterize the material mechanical properties. Results show that the size of the boron nitride and precipitate, the width of the martensite lath and the content of the S-ferrite are reduced as the cooling rate increases from 5-8 °C-min^-1 (CB2-S steel) to 50-60 °C-min^-1 (CB2-F steel). The precipitates are mainly composed of M₂₃C₆ and a small amount of M₃C. The average diameters of the M₂₃C₆ precipitates in CB2-F and CB2-S are 202 nm and 209 nm, respectively. The inclusions are mainly composed of BN, Al₂O₃ and MnO₂, and the inclusion ratios are 0.1969% for the CB2-F and 0.06556% for CB2-S steel. The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm, respectively. Furthermore, the M₃C having a diameter of about 150 nm and a thickness of 20 nm is observed in the 8-ferrite of the tempered CB2-S steel. The presence of the 8-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel. In addition, the lower cooling rate stabilizes the 8-ferrite structure at room temperature.