Fatigue life estimation of pitted 12% Cr steam turbine blade steel in different environments and at different stress ratios

The influence of corrosion pits on the endurable fatigue loading in different environments and at various stress ratios has been investigated for 12% Cr steam turbine blade steel. Very high cycle fatigue measurements were performed using ultrasonic fatigue testing technique with superimposed static load at stress ratios ranging from R=0.05 to R=0.9. Fatigue crack growth rate (FCGR) measurements in the near threshold regime and S–N tests were conducted at a temperature of 90°C in air, de-aerated 300ppbCl− solution and aerated 6ppmCl− solution. The influence of corrosion pits on the fatigue limit was determined with artificially generated corrosion pits. It was found that the FCGRs in solution are lower and the threshold stress intensity factor ranges ΔKth are higher than in air. Fracture surface investigation with scanning electron microscope and roughness measurement were made which suggest oxide and roughness induced crack closure as the most appropriate explanation. Data evaluation of fatigue life tests with pre-pitted specimens supports the applicability of treating pits as effective cracks. An estimation for the stress intensity factor range of pits which allows determining the endurable fatigue loading was empirically found.