Narrow Results

This paper presents the research results on the microstructure changes of SKD61 steel after quenching at 1050${}^{\circ }$C and then tempering at 580${}^{\circ }$C. After quenching and tempering, the microstructure of this steel is determined by various moieties, including tempering martensite, residual austenite, and carbides dispersed within the matrix. Scanning electron microscopy, X-ray, and energy-dispersive spectroscopic analysis reveal that the presence of the carbides Cr₇C₃, Mo₃C₂ and MoC determines the mechanical properties of steel during high-temperature operations.

Carbides of Fe₃AlC_x precipitated from iron aluminides can strengthen the matrix; the empirical electron theory (EET) was applied to analyze the attribute of carbides in the paper, giving theoretical explanation on the matrix and precipitation. Valence electron structure (VES) of Fe₃AlC_x was studied in detail, comparison with the iron aluminides matrix, the hard and brittle phase of Fe₃AlC_x can be interpreted form the viewpoint of valence electron structure.

The WC–Co nanocomposites were synthesized by using a polymer precursor such as polyacrylonitrile, which severs as an in situ carbon source. The WC–Co nanocomposites formed are characterized by X-ray diffraction and electron microscopy. Nearly pure WC–Co nanocomposites with a particle size in the range of 60–80 nm have been obtained. The use of H₂ atmosphere enhanced the carbide formation and in turn reduced the reaction time (at high temperature). The phase purity of the products is strongly influenced by the processing conditions such as the firing temperature, time and atmosphere. This suggests that the process parameters have to be optimized before scaling up the process for commercial production.

Low alloy Cr-Mo-V steels are usually used in steam power generation units. The evolution of the carbides often leads to embrittlement of the components during elongated service. Therefore, the determination of carbide evolution mechanism during long-time service is important to understand and prevent premature failures such as temper embrittlement. In this study, low alloy Cr-Mo-V steels used as main steam pipes in a thermal power plant were studied after various service times as well as in the as-fabricated condition. Electron microscopic analyses were carried out on extraction replicas to observe and analyze the morphology and composition of the carbides. Predominant plate-like vanadium-rich carbides were observed in the as-fabricated condition. When exposed to on-site service, the V-rich carbides transformed to Mo-rich carbides which have a typical H morphology. The change of morphology and composition of the carbide is mainly due to the gradual depletion of Mo from the solid solution. In addition, a non-destructive carbide extraction method was established for examination of the precipitates in the working turbine rotor.