The Initiation of Corrosion Fatigue Cracking from Pitting in 3.5Ni-Cr-Mo-V Low-Pressure Rotor Steel

Abstract

Corrosion and Corrosion fatigue properties of 3.5 Ni-Cr-Mo-V steam turbine 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 cartography showed that MnS inclusions were partly dissolved and corrosion pits were initiated from the edge of the dissolved MnS. Long-term corrosion fatigue tests were done in aqueous environment up to 5×10⁸ stress cycles. The S-N curve was consisted of two different curves and there appeared substantial reduction in fatigue strength over 5×10⁷ stress cycles. The fatigue crack initiation and growth from pits can be considered as the main factor of reduction of fatigue strength. Assuming the corrosion pit as a semi-elliptic surface crack, the critical stress intensity factor range ΔΚ_th for corrosion fatigue crack initiation was calculated. The average value of ΔΚ_th was around 1.50 MPam^{1 / 2}, about one half of the value obtained from conventional macroscopic fatigue crack growth rate tests.