High Temperature Fatigue Crack Growth Kinetics of a High Performance Ferritic (HiperFer) Steel

ABSTRACT: 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.