Kai Yan

Analyses of the oxidation characteristics of HR3C austenitic steel exposed to supercritical carbon dioxide were carried out at temperatures ranging from 600 to 650 °C under 25 MPa. It was observed that the weight gain increased with increases in temperature and time. The oxide morphology and phase were characterized using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Furthermore, the three-dimensional morphology and chemical composition of the surface oxide were inspected using a secondary ion mass spectrometer (SIMS). The majority of the oxide formed on HR3C at 600–650 °C was Cr₂O₃. Carbon enrichment occurs on the surface of the oxide scale and the oxide–substrate interface due to a carbonization reaction. The corrosion mechanism is also discussed in this paper.

The corrosion behavior of austenitic steel HR3C in supercritical CO₂ at 550–600 °C under 25 MPa for 1000 h was investigated. The corrosion kinetics of HR3C were evaluated using weight change measurements. The microstructure and phase composition of HR3C were studied via scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and secondary ion mass spectroscopy. Weight gain data showed that the HR3C exhibited excellent corrosion resistance and that the corrosion kinetics followed a near-parabolic law. The surface of the sample is composed of fine granular oxides, with the main elements including C, O, Cr, Fe and Ni. The oxide phase analysis indicated that protective Cr₂O₃ formed, and a small amount of Fe₂O₃ was also detected. Carbon enrichment was observed on the surface of the outmost layer and the interface of the oxide layer and substrate. The corrosion mechanism and carbon diffusion process are furthermore discussed.

Corrosion behavior of austenitic steel HR3C in supercritical CO2 at 550-600 °C under 25 MPa for 1000 h was investigated. The corrosion kinetics of HR3C was evaluated by weight change measurements. Microstructure and phase composition of HR3C was studied by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy. Weight gain data showed that the HR3C exhibited the excellent corrosion resistance and corrosion kinetics followed near-parabolic law. The surface of the sample is composed of fine granular oxides, with the main elements including C, O, Cr, Fe and Ni. The oxide phase analysis indicated that protective Cr2O3 formed and a small number of Fe2O3 was also detected. Carbon enrichment was observed at the outmost layer surface and the interface of oxide layer and substrate. The corrosion mechanism and carbon diffusion process were furthermore discussed.

This paper reports the effects of different levels of tensile stress caused by quasi-static loading on the corrosion behavior of T91 steel in a molten salt environment. Corrosion tests were carried out in a molten salt environment with a NaCl:K₂SO₄:Na₂SO₄ ratio of 1:1:8 under different applied stresses. The corrosion behavior was investigated through measurements of the phase composition, oxide morphology, and elementary composition. The results indicated that a low tensile stress promotes the growth of chromium oxides near the substrate and enhances the corrosion resistance, but with an increase in stress, the chromium oxides that formed on the T91 steel are destroyed, accelerating the inward diffusion of sulfur into the substrate to increase corrosion. The mechanism underlying the effects of applied stress and temperature on the corrosion behavior of T91 steel is discussed.

The microstructure and impact toughness evolution of G115 steel after long-term (ranging from 500 h to 10,000 h) aging at 700 °C were investigated in this study. The results showed that the microstructure of the G115 steel evolved from a finer-grained matrix with minor precipitates to a coarse-grained matrix with more precipitate with aging time, presenting a decrease in the local deformation degree in the matrix. The impact toughness of the steel decreased with aging time, presenting the largest decline at the initial aging times. The decrease in impact toughness was attributed to the coarsening of precipitates (M₂₃C₆ and Laves phase) in the steel matrix. The stable impact toughness during the whole aging process (from 500 h to 10,000 h) should be related to the comprehensive effects, including the precipitation of the Laves phase, the increase in high-angle grain boundaries, and the softening of the metal matrix.

P91 steel and P91 steel joints experience performance degradation after serving for 30,000 h in working conditions. To clarify the damage and failure behavior and remaining life of the joints during subsequent service, further creep testing was conducted on the welded joints of P91 steel that had been in service for 30,000 h at three temperatures: 550 °C, 575 °C, and 600 °C. The fracture surface and the cross-section damage behavior were characterized by SEM and EBSD methods. The results show that there are two types of fracture modes in the joints at different temperatures: ductile cracking occurring at the BM, and type IV cracking occurring in the FGHAZ. The threshold stress for fracture mode transition decreases with an increase in working temperature. Type IV cracking near the HAZ is the main reason for the premature failure of joints during service. And based on the fracture mode, the dual-constant L-M method was proposed to predict the strength of in-service joint materials. The testing data are discussed and classified based on the fracture mode in this method, which has high accuracy and can prevent the premature failure of joints.

In this study, we conducted a preliminary examination of the microstructure and properties at room and high temperatures of G115 martensite heat-resisting steel following exposure at temperatures of 650℃ and 700℃ for a duration of 8000 hours. Our findings indicate that after aging at 650℃, the martensitic microstructure remained intact, with no significant change in the width of the martensitic laths. Additionally, the precipitation of dense and small-sized Laves phases was observed. Notably, the strength of G115 steel did not exhibit any substantial deterioration, except for a decrease in impact toughness. These results provide evidence of the favorable microstructural stability of G115 steel at 650℃. However, upon aging at 700℃, we observed the coarsening of high-density and large-size Laves phases, accompanied by the transformation of the majority of martensitic laths into sub-grains. Consequently, both the strength and impact toughness of the material experienced a sharp decline.

Impact toughness is a critical property that determines the service safety of components in martensite heat-resisting steel. In this study, we examined the evolution of the toughness in G115 material after aging at temperatures of 650℃ and 700℃ for 8000 hours. Through experimental analysis, it was observed that G115 steel exhibited favorable impact toughness (78 J) following the application of normalizing and tempering heat treatment. However, the impact toughness experienced a rapid and significant decline after aging at both 650℃ and 700℃ for a mere 500 hours. Microstructural investigations revealed a correlation between the development of Laves phase and the sudden drop of toughness. Notably, the detection of Laves phase near the initial crack source substantiates its role as the primary factor contributing to the reduction in impact toughness.