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Environmental assisted cracking of turbine blade steels - a review.

Zhou, S (2007) Environmental assisted cracking of turbine blade steels - a review. NPL Report. DEPC-MPE 033

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This short report provides an overview of environment assisted cracking (EAC) of blade steels. The effect of material composition, chloride concentration and temperature on pitting is assessed. Critical pitting potential measurement using a potentiodynamic scan shows that the threshold chloride concentration for pitting of a 12% Cr steel in the absence of dynamic stain is in excess of 500 ppm, much higher than that in the condensate on the steam turbine under normal operating conditions. There is a significant effect of environment on the fatigue strength, with no evidence of a fatigue limit in aggressive environments, e.g., concentrated NaCl and NaOH solutions. In less aggressive environments pitting may still influence the development of cracks but only acting in concert with fatigue loading, the latter assisting initial pit development either by film breakdown or microcrevice formation at inclusions and sustaining propagation of the subsequent pit. In this case, a fatigue limit is still observed because pitting is contingent upon fatigue loading but it is lower than that in air.
There is generally little effect of environment on the growth rate of fatigue cracks under high frequency cyclic loading. However, in aggressive environments, such as NaCl solution, the threshold stress intensity factor range for fatigue cracking can be reduced relative to air, probably associated with dissolution of microstructural barriers to crack propagation. There is a marked effect of environment on the crack growth rate under load waveforms simulating two shifting operating conditions. Measurements of stress corrosion cracking (SCC) velocities are sparse. Significant crack growth rates have been reported but seem tied in with steels of high S and P impurity levels or very aggressive environments. In more relevant environments with modern steels the growth rate is very low, typically about 0.2 mm/y.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DEPC-MPE 033
Keywords: steam turbines, pitting, fatigue
Subjects: Advanced Materials
Advanced Materials > Corrosion
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/3813

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