Toughness Behavior of Through-Wall Cracks in Dissimilar Metal Welds

Abstract

Over the years, J-estimation scheme procedures have been develop to predict the load-carrying capacity of through-wall cracks in nuclear grade piping materials. These procedures employ analytical or numerical procedures coupled with the fracture toughness of the material to predict the pipe response. For cracks in welds, the behavior has been shown to be related to the toughness of the weld and strength of the typically lower-strength base metal. However, with the advent of primary water stress corrosion cracking (PWSCC), flaws in dissimilar metal (DM) welds have occurred. These welds consist of a nickel-based weld joining stainless steel and carbon steel base metals. Prior research has demonstrated that crack-driving force for through-wall circumferential cracks in DM welds is highly influenced by both the stainless steel and carbon steel base metals, and is related to the axial position of the flaw within the weld. This effect brings into question the accuracy of typical J-estimation scheme procedures for calculation of load carrying capacity for circumferential through-wall flaws in DM welds. In addition, it is unknown if the apparent toughness of the cracked pipe is also affected by the position of the crack in the weld. Work is currently underway, sponsored by the NRC Office of Nuclear Regulatory Research, to investigate the behavior of through-wall and complex cracks in DM welds. In a prior paper, a series of full-scale pipe bend and laboratory-sized fracture experiments were documented. Initial analyses of those test results suggest that typical J-estimation scheme procedures could be used to predict the response if the weld toughness from a compact tension specimen and the appropriate material strength was used. In this paper, the fracture toughness from the DM weld is estimated from the pipe experiments using an η-factor solution and numerical techniques and these results are compared to the compact tension results. The results from this paper add insight into the effect of crack size and location within the DM weld on the apparent pipe toughness for through-wall cracks in DM welds.