The fatigue crack propagation behavior of an experimental fully ferritic high-chromium steel HiperFer 17Cr2 was investigated at elevated temperatures of 650 °C and 675 °C at loading frequencies of 20, 5, and 0.05 Hz, motivated by the demand for advanced high-temperature materials capable of improving the thermodynamic efficiency of future thermal energy conversion systems and reducing greenhouse gas emissions. The widely used 9Cr-1Mo-V-Nb ferritic-martensitic steel P91 was examined in parallel at 650 °C for benchmarking purposes. Complementary microstructural analyses were performed to characterize frequency- and temperature-dependent damage mechanisms. At 650 °C, the stress intensity required for the initiation of crack propagation was substantially higher in HiperFer 17Cr2 than in P91 across all tested frequencies. Furthermore, crack growth rates were up to half an order of magnitude lower in HiperFer 17Cr2. At 675 °C, frequency-dependent damage mechanisms were identified, including dynamic recovery, subgrain formation, and pipe diffusion-assisted redistribution of Cr and Nb, promoting formation of the metastable C14 Cr2Nb Laves phase at grain and sub-grain boundaries. These precipitates effectively impeded crack progression, while crack-tip blunting reduced the local driving force for crack propagation. The results indicate that HiperFer 17Cr2 is suitable for continuous service at 675 °C under high-cycle fatigue conditions in the frequency range from 5 to 20 Hz.
Dynamic thermo-mechanical stresses caused by sudden temperature changes and molten steel impact, etc., accelerate the degradation of Al2O3-C refractories during service. To investigate the dynamic degradation behavior, dynamic mechanical tests were conducted using the Split Hopkinson Pressure Bar (SHPB), systematically examining the effects of partial substitution of flake graphite by expanded graphite and thermal degradation. The results show that the Al2O3-C refractories exhibit a significant strain-rate hardening effect, with strength increasing with impact velocity and the failure mode progressively transitioning from crack propagation to pulverization. Cyclic prolonged thermal exposure to 1500 °C contributes to the SiC whiskers formation and densification, and results in the increase strength and brittleness. The phenomenon of specimen after 5 cycles having the optimal impact resistance proves the both the strength and energy dominated failure process. The introduction of expanded graphite effectively suppresses crack propagation and enhances energy dissipation capacity through interlayer sliding and stress buffering related to the myrmekitic texture, which provides a rationale for the development of low-carbon materials.
