Alloy 690 is susceptible to hydrogen embrittlement where hydrogen reduces the ductility and causes the fracture morphology to change to predominantly intergranular. The role of carbide precipitation in the embrittlement behavior is not well defined. The objective of this work is to understand the effect of intergranular carbide precipitation on the hydrogen embrittlement of alloy 690. The work reported herein used tensile and compact-tension specimens in both the solution-annealed condition (minimal grain-boundary carbide precipitation) and in the solution-annealed condition followed by an aging treatment to precipitate grain-boundary carbides. By performing the mechanical tests on materials in both uncharged and hydrogen-charged conditions, it was possible to evaluate the degree of embrittlement as a function of the carbide precipitation. It is shown that the embrittlement due to hydrogen increased as the material was aged to allow grain-boundary carbide precipitation. It is proposed that the increase in embrittlement was caused by increased hydrogen at the carbide/matrix interface due to the trapping and increased stresses at the precipitate interface, which developed from strain incompatibility of the precipitate with the matrix. It is further shown that increasing the hydrostatic stress increased the tendency for intergranular fracture, as is consistent with other nickelbase alloys.

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NOTCE

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The present work quantifies the role of plastic deformation on the hydrogen-trapping behavior within the material and at the carbides. Isothermal desorption spectroscopy and thermal desorption spectroscopy (TDS) were performed on unstrained and strained alloy 600 (Ni-15Cr-8Fe) and alloy 690 (Ni30Cr-8Fe), in order to quantify the effect of strain on the trapping energy. The results show that the M 23 C 8 carbides in alloy 690 were stronger traps than the M 7 C 3 carbides in alloy 600. It was further shown that cold work tended to increase the binding energy of hydrogen to the trap sites associated with grain-boundary carbides, although this effect was small.