Intergranular stress corrosion cracking of alloy 600 and x-750 in high-temperature deaerated water/steam
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I.
INTRODUCTION
IT is
well known that stress corrosion cracking occurs with a critical combination of stress, environment, and a susceptible material. Intergranular stress corrosion cracking (IGSCC) of nickel-based alloys in pressurized water reactors (PWRs) occurs in cold-worked and stressed regions of various components which have not been optimally heat-treated. The main cracking path is along grain boundaries. During the past two decades, numerous IGSCC failures of both alloys 600 and X-750 in PWR systems have been reported. The alloy 600 cracking of interest here has occurred most frequently on the primary side of steam generators, primarily in the heavily coldworked areas. I1,21 Intergranular stress corrosion cracking has also been observed in alloy X-750 in high-strength components, such as bolts, springs, and guide pins, again in the primary (deaerated) water.[3.4] The char/~cteristics of IGSCC in high-temperature deaerated water can be summarized as follows: (1) Microstructure plays a dominant role. A microstructure with chromium carbides on grain boundaries is resistant to IGSCC, while a microstructure with no grain boundary chromium carbides is susceptible. [1-3,5.6] (2) Applied or residual stresses must be present to produce cracking. [2,3,7] Strain by plastic deformation also aids IGSCC.[2, 8] (3) The cracking rate increases rapidly with temperature, t2] (4) With a small overpressure of hydrogen, the cracking is enhanced. [9] Furthermore, driving the potential to the cathodic side generates more hydrogen and enhances the IGSCC of alloy 6 0 0 . [10] Several of these characteristics indicate that hydrogen plays an important role in the IGSCC of these alloys. A hydrogen-induced void-linkage model proposed by Shewmon [1~] can explain the characteristics of the IGSCC of alloy 600. He suggested that the IGSCC is aided by YULIN SHEN, formerly Graduate Student, The Ohio State University, is Research Engineer, Bethlehem Steel Corporation, Bethlehem, PA 18016. PAUL G. SHEWMON, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted August 27, 1990. METALLURGICAL TRANSACTIONS A
the nucleation and growth of a fine array of grain boundary microvoids in front of an advancing crack. Nucleation of the voids is aided by high-pressure methane, which can be formed through the reaction of hydrogen generated by corrosion near the crack tip and carbon put in supersaturated solution by the heat treatment. The stress then drives the growth of voids which finally join to form fissures. In a microstructure with grain boundary carbides, the carbon activity at the grain boundary is lower, the equilibrium methane pressure is reduced, and IGSCC is much slower. Thus, IGSCC is usually not observed in a microstructure with heavy grain boundary precipitation of chromium carbides. Shewmon [11[ has argued that the combination of the methane pressure and applied stress could give homogeneous nucleation of bubbles along the grain boundary. However, the
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