STRESS-CORROSION CRACKING OF STEAM-TURBINE ROTORS

In the wake of the catastrophic failure of a low-pressure (LP) turbine disk at the Hinkley Point Nuclear Station in 1969, considerable resea rch and development has been devoted to the problem of stress corrosio n cracking (SCC) in steam turbine rotors. Principle factors affecting the susceptibility of rotors to SCC have been identified as disk yield strength applied stress level, and surface film/crevice chemistry. Mi crostructure and cleanliness of the steel have been found to have rela tively little effect. Advances in steel making and forging over the la st 20 years have provided manufacturers with additional design and mat erial options to mitigate the problem Increases in forging size capabi lities of steel companies and the welded construction of rotors now pe rmit designing with integral and partial integral rotors that use mate rials with lower yield strength (more SCC resistant) as well as elimin ating the SCC problem in bores and keyways. However a recent survey of U.S. utilities has shown that SCC in the blade attachment legion of L P rotors is an increasing concern This problem has led to development of repair and refurbishment methods for rim attachments, especially we ld buildup of rims with corrosion-resistant alloys. Life prediction of rotors under SCC conditions currently involves estimating crack growt h time from assumed defects to critical size. Factors that govern the location and time of crack initiation are not understood adequately.