Fracture toughness of alloy 690 and EN52 welds in air and water
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DUCTION
ALLOY 690 is an attractive candidate for replacing alloy 600 components in primary water reactor applications when stress corrosion cracking (SCC) is a design concern. Indeed, testing to date has shown that this material is immune to high-temperature SCC in primary and pure water, regardless of microstructure and heat treatment condition.[1–5] The composition of alloy 690 is similar to that for alloy 600, except that its chromium content has been doubled from about 15 to 30 pct. As a result of its superior SCC resistance, alloy 690 has been used to replace alloy 600 pressurizer heater sleeves and instrument nozzles, as well as steam generator tubes.[6,7] Alloy 690 is typically welded using either EN82H or EN52 weld filler metals. The EN82H, which is also used to weld alloy 600 components, is rather susceptible to hightemperature SCC. While the SCC resistance for EN52 is far superior to that for EN82H,[1] its inferior weldability has limited its use. The expected failure mechanisms for welded alloy 600 and alloy 690 components involve crack initiation and propagation due to SCC or corrosion fatigue, followed by stable or unstable tearing when a critical crack size is reached. This study focuses on the final step in the cracking process, involving stable and unstable fracture. While these materials exhibit excellent toughness in air, fracture properties are degraded in low-temperature water.[1] Extensive testing of alloy 600 and EN82H welds showed that these materials have exceptionally high fracture toughness in air and hightemperature water.[8] In low-temperature water, alloy 600 showed a modest decrease in toughness, but retained sufficient cracking resistance to avoid fracture concerns. By contrast, EN82H welds tested in low-temperature (⬍149 ⬚C) water exhibited a dramatic reduction in toughness, as JIC and tearing moduli were reduced by one to two orders of C.M. BROWN, Senior Engineer, and W.J. MILLS, Consultant, are with the Bettis Atomic Power Laboratory, West Mifflin, PA 15122. Contact email: [email protected] Manuscript submitted April 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
magnitude. The reduction in toughness was associated with a hydrogen embrittlement mechanism, whereby hydrogen from the water weakened grain boundaries and promoted planar slip, which localized strain concentrations along grain boundaries. A preliminary study[1] indicated that alloy 690 and EN52 weld metal also exhibit a substantial loss of toughness in low-temperature water. Thus, the purpose of this work is to determine the effects of low- and high-temperature water with high hydrogen on the cracking resistance of these materials. Fracture properties were determined in 24 ⬚C to 338 ⬚C air and hydrogenated water using elastic-plastic JIC methodology, due to their ductile response. Scanning electron microscopy (SEM) examinations of fracture surfaces were performed to correlate trends in fracture-toughness behavior with operative cracking mechanisms. The role of hydrogen embrittlement in degrading fractur
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