Comparative evaluation of environment induced cracking of conventional and advanced steam turbine blade steels. Part 1: Stress corrosion cracking

A prerequisite for the introduction of a new high strength steel blade in steam turbines is to ensure that its resistance to environment assisted cracking is at least no worse than that of existing materials. To that end a series of stress corrosion 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 solution in order to determine the threshold stress intensity factor, KISCC, and the crack growth rate. Under normal water chemistry conditions, typically containing 300 ppb Cl- and 300 ppb SO42- but with the solution aerated, KISCC was significantly higher for the more highly alloyed PH13-8 steel (79-85 MPa m1/2 compared to 42-53 MPa m1/2 for FV566) but in aerated 35 ppm chloride solution KISCC was reduced dramatically for both alloys to near 20 MPa m1/2 with the FV566 steel showing a similar or slightly lower threshold value. However, the growth rate for the higher strength steel was about an order of magnitude greater. Also, the effect of an excursion in chloride concentration to 35 ppm was to reduce the KISCC value for the PH13-8 steel when returned to normal, but aerated, water chemistry whereas there was little change in the KISCC for the FV566 steel following an excursion. Importantly, for both steels, deaeration of the solution to reflect continuous on-load conditions in service caused the growing crack effectively to arrest.