THE INITIATION OF CORROSION-FATIGUE CRACKING FROM PITTING IN 3.5NI-CR-MO-V LOW-PRESSURE ROTOR STEEL

Corrosion and Corrosion fatigue properties of 3.5 Ni-Cr-Mo-V steam tur bine low-pressure rotor steel were investigated. Immersion tests were performed on 3.5Ni-Cr-Mo-V steel by dipping the specimen into deionzed water and 10 ppm NaCl solution. Observation of SEM and X-ray cartogra phy showed that MnS inclusions were partly dissolved and corrosion pit s were initiated from the edge of the dissolved MnS. Longterm corrosio n fatigue tests were done in aqueous environment up to 5x10(8) stress cycles. The S-N curve was consisted of two different curves and there appeared substantial reduction in fatigue strength over 5X10(7) stress cycles. The fatigue crack initiation and growth from pits can be cons idered as the main factor of reduction of fatigue strength. Assuming t he corrosion pit as a semi-elliptic surface crack, the critical stress intensity factor range Delta K-th for corrosion fatigue crack initiat ion was calculated. The average value of Delta K-th was around 1.50 MP am(1/2), about one half of the value obtained from conventional macros copic fatigue crack growth rate tests.