Thermal Embrittlement Risk Assessment of 17-4 PH Stainless Steel Main Steam Isolation Valve Components

Wednesday, September 15, 2021: 2:00 PM
240 (America's Center)
Mr. John H. Macha, P.E. , Southwest Research Institute, San Antonio, TX
Mr. W. Fassett Hickey , Southwest Research Institute, San Antonio, TX
Mr. Matthew L. Kirby , Southwest Research Institute, San Antonio, TX
Mr. David S. Riha , Southwest Research Institute, San Antonio, TX
Multiple failures of main steam isolation valve (MSIV) stems have occurred at U.S. nuclear power plants in recent history. These stems are typically composed of ASM SA564 Grade 630 PH steel in the H1100 condition. A number of past valve stem failures have been attributed to thermal embrittlement, a condition under which the stem base material exhibits a degradation of mechanical properties due microstructural changes induced by long-term exposure to a temperature range of 500-600oF. Failure mechanisms of embrittled MSIV stems include sudden shear fracture due to actuation loads during plant outages as well as in-service fatigue crack growth caused by vibrations from the steam flow. In this study, material from three MSIV stems removed from service after nine years in a pressurized water reactor was metallurgically and mechanically evaluated to determine the severity and extent of thermal embrittlement present. The findings from these evaluations were used to develop a condition-based projection of risk over time of the embrittlement-induced failure of the stem material. Results from the evaluations will be discussed, as well as the implications for extending the service life of the material.