Thermal Fatigue Testing and Analysis of Stainless Steel Girth Butt Weld Piping

A piping thermal fatigue test loop has been constructed at the Bettis Laboratory and is being used by the Bechtel Marine Propulsion Corporation (BMPC) to obtain thermal fatigue data on 304 Stainless Steel (304SS) piping and piping girth butt welds. These specimens were subjected to alternating hot and cold forced flow, low oxygenated water every three minutes so that rapid changes in water temperature impart a thermal shock event to the inner wall of the girth butt welds. Thermal and structural piping analyses were conducted using the ASME Boiler and Pressure Vessel Code Section III NB-3600 piping analysis methods and a series of current and proposed design fatigue curves for 304 stainless steel that include water environment effects. These analyses were also used to assess the conservatism in the ASME Code Section III NB K₃ thermal stress index for girth butt welds. The results of this thermal fatigue testing and analysis assessment demonstrate that the role of fatigue crack initiation with respect to piping analysis calculations must be considered for systems subjected to high thermal-induced surface stresses. The ASME Code Section III NB-3600 piping design methods with both current and previous austenitic steel air design fatigue curves are potentially unconservative with respect to the estimated cycles to initiation, based on the test results. Use of the design fatigue curves including water environment effects yield more reasonable margins with respect to design cycles to fatigue crack initiation and through-wall leakage. Additionally, the results indicate that the current K₃ index is conservative and consideration should be given to a reduction from 1.7 to align the design margins when explicitly considering environment effects.