Control of Stress-Accelerated Oxygen-Assisted Cracking of INCOLOY® Alloy 908 Sheath for Nb3Sn Cable-in-Conduit
INCOLOY® alloy 908 is a controlled thermal expansion precipitation hardenable Ni-Fe superalloy having outstanding cryogenic properties to 4 K. The alloy was developed for Nb3Sn cable-in-conduit sheathing and other cryogenic applications and has been specified for the ITER model superconducting magnet coils. INCOLOY alloy 908, like other superalloys, exhibits intergranular cracking under certain combinations of environment, stress, and material heat treatment conditions. For example, cracking may occur while material is exposed in air at temperatures between about 500 and 800°C if the total tensile stress is greater than some threshold tensile stress. This mechanism, known as stress-accelerated grain boundary oxygen-assisted cracking (or SAGBO), is analogous to intergranular stress corrosion cracking. INCOLOY alloy 908 SAGBO behavior is discussed in the general context of other superalloys. Using a variety of test methods, we examine the SAGBO characteristics under varying environmental-stress conditions. Some practical methods are available to eliminate or minimize SAGBO cracking in Nb3Sn reaction heat treatments.
KeywordsResidual Stress Tensile Residual Stress Intergranular Crack Shot Peening Corner Radius
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