Low-Pressure Steam Turbine Disc Cracking—An Update

This paper summarizes results of studies conducted in the USA over the past five years to identify factors responsible for stress corrosion cracking (SCC) of forged, shrunk-on, low-alloy steel discs in low-pressure (LP) steam turbines used in nuclear power plants. Experience in the UK and subsequent research findings are reviewed, and results of a survey of US experience and of comprehensive experimental programs are reported. Recommendations are presented to minimize SCC of LP turbine discs in existing and future power plants. In general, the work performed revealed: (i) no specific chemical contaminant is required for cracking; crack initiation and growth occur in pure condensed steam; (ii) the yield strength of a disc steel is the dominant factor governing crack growth rates; (iii) other factors being equal, a higher incidence of cracking may be expected in rotor designs having higher applied stress levels. Crack initiation and growth in LP turbines may be minimized by control of the environment, by use of low-strength alloys, and by designs which eliminate keyways and minimize applied stress levels.