Improvement of low-cycle fatigue behavior of modified 9Cr-1Mo steels at 450 °C in liquid LBE environment by the addition of Si element

Abstract

This study examined the low-cycle fatigue performance of two modified 9Cr-1Mo steels containing Si at levels of 0.38% and 1.2%, respectively. The investigation was conducted under conditions of 450 °C oxygen-saturated and oxygen-poor lead–bismuth. The elevated oxidative corrosion behavior at this temperature aligns more closely with operational conditions. Comparing the steels with differing Si contents offers a clearer insight into the impact of Si element incorporation on the material's mechanical properties. The results indicated that the high-Si steels had a higher fatigue life, and their fatigue fractures exhibited quasi-dissociation. The high-Si steel displayed elevated oxidation resistance, as indicated by the slower growth of the oxide layer. Owing to significant oxidative corrosion, the fatigue life of the low-Si steel was shorter in oxygen- saturated LBE compared to oxygen-poor LBE. The LME-induced cracking of modified T91 steel in LBE initiated at the grain boundaries. During the expansion phase, a blend of ductile and brittle behaviors was observed, occurring through and alongside grain expansion patterns. Due to the elevated Si elemental content, high-Si steel exhibited weakened susceptibility to LME in oxygen-saturated LBE, owing to the creation of SiO2 through the reaction between Si and dissolved oxygen in the LBE, which shielded the steel matrix.