Example Application of Code Case N-894 Rules for Weld Overlay of Thermal Fatigue Cracking

Abstract

Abstract The ASME Code, Section XI is working on guidance for application of weld overlay repairs to repair thermal fatigue cracking in nuclear piping systems. This new guidance will eventually be published as Code Case N-894 with the original technical basis in PVP2019-93360. Weld overlays have been extensively used in boiling water reactors (BWRs) and pressurized water reactors (PWRs) to mitigate stress corrosion cracking (SCC). The weld overlays applied to date mitigate SCC by putting the flaw into compression and they use materials that are resistant to SCC. Mitigation of thermal fatigue requires the crack to be in compression so that it does not achieve tensile cycling under the thermal fatigue loading condition. Code Case N-894 allows for the use of either stainless steel or nickel alloy filler metals to repair thermal fatigue flaws. This paper will evaluate the use of both filler metals for the weld overlay process to assess the performance difference between the two filler metals such that the welding advantages of stainless steel over nickel alloys can be quantified. Specifically, this paper assesses the residual stress state difference for nominal sized weld overlays on a six-inch pipe. Various cyclic thermal loading conditions are postulated, and the stress intensity factors are determined for both filler metals to assess the difference in mitigation of thermal fatigue flaws.