Hydrogen-Assisted Crack Propagation in Austenitic Stainless Steel Fusion Welds
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INTRODUCTION
AUSTENITIC stainless steels are well suited as structural materials in hydrogen environments, because this alloy class is one of the most resistant to hydrogenassisted fracture.[1–3] Although the hydrogen-assisted fracture of austenitic steels has been extensively studied,[3,4] few efforts have focused on welds. Fusion welds in austenitic steels are generally designed to solidify as primary ferrite, which results in the retention of a small amount of d ferrite following the solid-state transformations that occur during cooling.[5] Because ferritic steels can be highly susceptible to hydrogen-assisted fracture,[1–3] the presence of ferrite in austenitic stainless steel welds raises concerns about the fracture resistance of welds in hydrogen environments. Studies of the hydrogen-assisted fracture in austenitic stainless steel welds are limited[6–11] and almost none used fracture mechanics methods to quantify fracture resistance and assess fracture mechanisms. Efforts to characterize hydrogen-assisted fracture in B.P. SOMERDAY, Principal Member of Technical Staff, D.K. BALCH, Senior Member of Technical Staff, and K.A. NIBUR, Postdoctoral Research Associate, are with Sandia National Laboratories, Livermore, CA 94550. Contact e-mail: bpsomer@sandia. gov M. DADFARNIA, Postdoctoral Research Associate, and P. SOFRONIS, Professor, are with the Department of Mechanical Engineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801. C.H. CADDEN, formerly Manager with Sandia National Laboratories, is deceased. Manuscript submitted July 22, 2008. Article published online August 8, 2009 2350—VOLUME 40A, OCTOBER 2009
austenitic steel welds using tensile tests showed that tensile ductility was degraded and weld ferrite served as the preferred sites for fracture.[6–8] Hydrogen promoted the separation of the austenite/ferrite interface and fracture of the ferrite itself, although the predominant mechanism was not consistent among the studies and likely depended on variables such as the hydrogen concentration and volume fraction of ferrite. Although such tensile fracture studies provide some insight into hydrogen-assisted fracture, the results have limited use in understanding crack extension in structures exposed to hydrogen environments. Fracture toughness data are needed to assess the susceptibility of welds to hydrogen-assisted crack extension. In addition, the hydrostatic stress field ahead of a crack could activate mechanisms of hydrogen-assisted fracture that are not accessible in the uniaxial stress field of a tensile specimen. The objective of this study was to characterize hydrogen-assisted crack propagation in gas-tungsten arc (GTA) welds of the nitrogen-strengthened, austenitic stainless steel 21Cr-6Ni-9Mn (21-6-9) using fracture mechanics methods. The fracture initiation toughness and crack growth resistance curves were measured using fracture mechanics specimens that were thermally precharged in hydrogen gas. Mechanisms of hydrogenassisted crack propagation in the welds were identified from the examin
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