Stress Corrosion Cracking of Steels in Low-Pressure Turbine Environments

In the past ten years, a number of in-service failures have occurred in forged, low-alloy steel rotor discs of power plant low-pressure (LP) steam turbines. The cause of these failures has been identified as stress corrosion cracking (SCC). In addition to the obvious safety hazards to personnel resulting from such unexpected failure of rotating machinery, the economic impact is enormous. Depending on the extent of the failure and subsequent damage done to the turbine and surrounding machinery, equipment replacement costs in the United States range from $2 to 6 million for replacement of an entire turbine. The cost of replacement power is potentially as costly. In May, 1979, it was estimated that the cost of power from oil-fired plants purchased to replace lost generating capacity of a 1,000 MW nuclear plant during a turbine outage was about $500,000 per day [1]^*. Subsequent increases in the cost of fuel oil make the current replacement power cost even greater. And, since replacement discs are normally not stocked by utilities and must be made to order by manufacturers, an LP turbine disc failure can remove a plant from service for a period ranging in length from weeks to years.