High/very-high fatigue properties and microstructure evolutions of 9Cr3W3Co turbine rotor steel at room temperature and 650 °C

The study aimed to investigate the high and very-high fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature (RT) and 650 & DEG;C. The resulting S-N curves demonstrated a continuous downward trend, without exhibiting a conventional fatigue limit. The fatigue strength corresponding to 5 x 107 cycles is approximately 53.9% of yield strength at RT, increasing to around 55.4% at 650 & DEG;C. Meanwhile, compared to the fatigue strength at 5 x 107 cycles at RT, it at 650 & DEG;C decreased by 53.3%. Surface crack initiation was found to be the main fatigue fracture failure model, observed at both RT and 650 & DEG;C. Microscopic analysis revealed that severe dislocation motion and grain deformation occurred within the material matrix during the cyclic loading process at both RT and 650 & DEG;C, resulting in the formation of abundant low-angle grain boundaries, dislocation lines and sub-grains. At RT, the presence of the M23C6 precipitated phase imposed a strong pinning effect, while the MX phase within the martensite laths effectively impeded the movement of dislocation lines, thereby enhancing fatigue properties. However, at 650 & DEG;C, the emergence of newly precipitated Cu-rich phases was observed, which can reduce the free path of dislocations, hinder slip motion of dislocations, and thus compensate for the negative effect caused by high-temperature microstructural degeneration.

Automated summary

The study investigated the fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature and 650°C. The fatigue strength continuously decreased without a fatigue limit. Surface crack initiation was the main failure mode observed at both temperatures. Severe dislocation motion and grain deformation occurred during cyclic loading, resulting in the formation of low-angle grain boundaries, dislocation lines, and sub-grains. The presence of precipitated phases at room temperature enhanced fatigue properties, while newly precipitated phases at 650°C compensated for the negative effect of high-temperature microstructural degeneration.