Microstructural and Stress Corrosion Cracking Characteristics of Austenitic Stainless Steels Containing Silicon
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INTRODUCTION
AUSTENITIC stainless steels (SSs) core internal components are susceptible to irradiation-assisted stress corrosion cracking (IASCC) during service in light water reactors (LWRs).[1] One of the effects of irradiation is the hardening of the SS and a change in the dislocation distribution in the alloy.[2–4] Irradiation also alters the local chemistry of these austenitic alloys, for example, in the vicinity of the grain boundaries. This is called radiation-induced segregation. The segregation or depletion phenomena at near-grain boundaries may enhance the susceptibility of these alloys to environmentally assisted cracking (EAC). As the amount of damage by radiation increases, austenitic SSs become increasingly vulnerable to EAC or IASCC. For example, it has been suggested that a thermally nonsensitized irradiated SS becomes depleted in chromium (Cr) at the grain boundaries and subsequently becomes more susceptible to EAC.[3,4] It is also apparent that the susceptibility of SS to IASCC is highly dependent on small chemistry and process variations that are typical of different heats of the same material (heat-to-heat variability). It has been claimed many times that the segregation to the PETER L. ANDRESEN and RAUL B. REBAK, Research Scientists, and MARTIN M. MORRA and J. LAWRENCE NELSON, Laboratory Managers, are with the GE Global Research Center, Schenectady, NY 12309. Contact e-mail: [email protected] PETER H. CHOU, Program Manager, is with the Electric Power Research Institute, Palo Alto, CA 94304. This article is based on a presentation given in the symposium ‘‘Materials for the Nuclear Renaissance,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of the Corrosion and Environmental Effects and the Nuclear Materials Committees of ASM-TMS. Article published online September 3, 2009 2824—VOLUME 40A, DECEMBER 2009
grain boundaries of elements such as carbon (C), sulfur (S), phosphorus (P), oxygen (O), nitrogen (N), and silicon (Si) is an important factor in the resistance of SS to IASCC. However, the role of the segregation of these minor alloying elements on EAC-IASCC is still poorly understood.[1] Under irradiation, Si may enrich at the grain boundary up to 10 times its bulk composition.[2,4] The role of Si and its grain-boundary segregation in IASCC remains contradictory. Bruemmer has argued that Si does not play a role, because high-purity materials (with no Si) suffer degradation under irradiation similar to that suffered under commercial heats containing up to 1 pct Si.[4] Other researchers claimed a detrimental effect of Si on the resistance to cracking.[5–8] Li et al. reported a negative effect by Si and listed three mechanisms by which Si could make the SS less resistant to cracking in high-temperature water.[5] The three mechanisms proposed by Li et al. are the following: (1) Si may lower the stacking fault energy and therefore facilitate the coplanar slip and rupture the protective oxide film; (2) Si may decrease the strength of the oxide film,
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