Creep fatigue interaction at high temperature in C630R ferritic/martensitic heat-resistant steel

The study investigated the creep-fatigue interaction behavior, fracture mechanism, and microstructure evolution of C630R ferritic/martensitic heat-resistant steel at 630 °C and 1% strain amplitude. The results indicate that C630R ferritic/martensitic heat-resistant steel exhibits noticeable cyclic softening behavior, with a decrease in the softening ratio from 0.46 to 0.40 as loading time increases. Microstructure evolution was examined using optical and scanning electron microscopes, revealing the appearance of secondary cracks on the longitudinal section of the sample. These cracks exhibited a curved expansion trend with multiple deformations and highly bifurcated micro-cracks at a low loading time of 30s. However, when the load holding time was increased to 300s, the fatigue cracks tended to expand in a straight line with fewer branches. Furthermore, the observed crack growth path under different loading times showed enrichment in Fe and Cr, with the crack tip containing Cr oxide, which helped prevent fatigue crack propagation. As the loading time increases, the rise in crack closure resulting from oxidation leads to a decrease in crack propagation. Electron backscatter diffraction observation revealed the formation of numerous substructures and recrystallized grains after varying holding times, enhancing the material's resistance to crack propagation. Moreover, the study found that with increasing load dwell time, the deformed grains decreased, and substructural grains became dominant (57.68% and 62.77% under 30s and 300s, respectively). This increased crack resistance resulted in shorter secondary cracks under prolonged load retention, with the secondary crack length decreasing from 146.36 μm at 30s loading time to 109.09 μm at 300s loading time. Additionally, the kernel misorientation angle value decreased significantly after different loading times, leading to a higher likelihood of cracks or holes forming in regions with high KAM values, where low-angle grain boundaries were formed.