Changes in microstructure and mechanical properties of ferritic high chromium steel and P91 induced by isothermal fatigue

The ferritic high chromium steel HiperFer-17Cr2 provides higher corrosion resistance than conventional 9–12 % Cr-steels and high strength at temperatures above 600 °C. This strength is mainly attributed to Laves phase precipitation, which is influenced by plastic deformation, temperature and loading duration. Previous work indicates a higher fatigue strength under low cycle (LCF) compared to high cycle fatigue (HCF) loads. Due to the incomplete understanding of the underlying microstructural changes, particularly how and when thermomechanically induced precipitates affect mechanical properties, this work examines the microstructural evolution of HiperFer-17Cr2 during LCF and HCF tests at 600 °C, 635 °C, and 650 °C. To provide a comparison with a benchmark material, similar tests were performed on the martensitic steel P91. The HiperFer-17Cr2 shows cyclic hardening and increasing hardness, initially driven by dislocation formation in the first cycles, while the further, more pronounced increase is attributed to precipitation formation. In contrast, P91 shows a reduction in hardness due to recovery processes. For HiperFer-17Cr2, a greater hardness increase was observed under LCF loads, which is attributed to stronger plastic deformation. This leads to a higher density of dislocations acting as nucleation sites for precipitates, enhancing precipitation and dislocation strengthening, and consequently improving fatigue strength.