10288 Environment Induced Cracking of Advanced Steam Turbine Blade Steels

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Product Number: 51300-10288-SG

ISBN: 10288 2010 CP

Author: A Turnbull and S Zhou

Publication Date: 2010

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A prerequisite for the introduction of a new high strength steel blade in steam turbines is to ensure that its stress corrosion and corrosion fatigue performance is at least no worse than that of existing materials. To that end a series of tests has been conducted to assess the performance of the candidate material, PH13-8, relative to that of conventional blade steel, represented typically by FV566. Fracture mechanics tests using compact tension specimens have been undertaken in aerated and deaerated simulated condensate with different chloride concentrations. The threshold stress intensity factor for stress corrosion cracking (K_ISCC) in aerated 300 ppb Cl- and 300 ppb SO4 ^2- (normal water chemistry, albeit aerated) at 90 °C was significantly greater for the PH13-8 but under upset conditions with a higher chloride concentration of 35 ppm the differences from the FV566 steel were marginal. The key observation was that stress corrosion crack growth was not sustained for either steel in deaerated solution, corresponding to on-load service conditions. Corrosion fatigue testing to simulate two-shifting gave high cyclic crack growth rates in both aerated and deaerated solution for both steels but when account is taken of the limited number of cycles per year this corresponds to a maximum crack growth rate of 0.25 mm/y based on a DK of 40 MPam^1/2. There is nothing in the data generated here to limit the introduction of the PH13-8 steel within the constraints of well-managed water chemistry.

Keywords: steam turbines, stress corrosion cracking, corrosion fatigue, environment induced cracking, steel