Pit-to-crack transition under cyclic loading in 12% Cr steam turbine blade steel

• The minimum pit size leading to fatigue failure is dependent on R-ratio.
• Fatigue cracks initiate at the mouths of deep corrosion.
• A two-step model is developed to calculate ΔK for cracks emanating from pits.
• Similarity for small and long cracks is obtained in da/dN vs. ΔK/ΔKth diagrams.
• A formerly developed prediction model is supported by the normalised FCGR curves.

Fatigue measurements were performed up to the very high cycle fatigue regime in order to investigate pit-to-crack transition in 12% Cr steam turbine blade steel. Pre-pitted and smooth specimens were tested in air and aerated 6 ppm Cl− solution. S–N curves for different stress ratios were determined and a stress ratio dependent critical pit size for pit-to-crack transition with subsequent failure was found. Early crack initiation and small crack growth were observed in the process of development and by fractography using field emission scanning electron microscopy. Fatigue crack growth rates (FCGR) for cracks emanating from pits were determined. Good similarity of FCGR curves for short and long cracks was obtained by normalising the stress intensity factor ranges with the threshold values.