Effect of microstructure on fatigue crack propagation behavior in a steam turbine rotor steel

Fatigue crack growth behavior was investigated in a newly developed steel 25Cr2NiMo1V with different heat treatments to meet different property requirements of high-pressure (HP) and low-pressure (LP) parts in the combined steam turbine rotor. The load-shedding method and constant load amplitude techniques were adopted. Fractography including crack surface observation and fatigue crack growth path analyses was carried out. Results show that in the threshold regime, fatigue crack growth resistance of HP is clearly superior to that of LP. The distributed bainitic microstructures and larger prior austenite grain size in HP result in more tortuous crack propagation path than that in LP. Compared with ferritic blocks in HP, the tempered martensitic laths in LP do not play a dominant role in stopping the fatigue crack advance. Nevertheless, no clear difference in fatigue crack growth in HP and LP is observed in the Paris regime.