Observation of Hydrogen Diffusion, Retention, and Embrittlement in Nickel-base Alloy 718 and Alloy 945X

摘要

Nickel based alloys are of growing consideration in the application of deep-sea down-hole components. Of particular interest is their resistance to hydrogen embrittlement, a mechanism that has been found responsible for the premature failure of critical components in deep-sea oil well environments with a susceptible microstructure. In this paper, the hydrogen uptake and outgassing properties of precipitation-hardened Nickel alloy 945X and alloy 718 have been characterized in the standard mill produced condition. Tensile tests of cathodically hydrogen charged samples were conducted, and data about crack nucleation sites, crack propagation, and hydrogen diffusion coefficients obtained for both alloys. The rate of hydrogen ingress and properties of hydrogen retention of the alloys was explored, using a thermal melt hydrogen analyser. The hydrogen concentration was measured for both alloys following an array of charging and outgassing experiments. Alloy 718 was shown to be more sensitive to the ingress of hydrogen, and slower in the outgassing, when compared to alloy 945X. Accompanying this work is an array of tensile data from samples which were treated to the same charging and outgassing conditions, in order to explore the relationship between hydrogen ingress rate, hydrogen retention, and microstructure embrittlement. Results showed that an increase in charging time corresponded to greater hydrogen concentration in the samples and reduced strain-to-failure. The depth of hydrogen embrittlement in the materials was analysed, and H-diffusion coefficients for alloys 718 and 945X estimated. The microstructures of both alloys were also observed during straining using miniature-tensile tests of cathodically charged samples, in order to explore the degree to which the sub-surface hydrogen concentration affects the fracture mechanisms of the alloys. A detailed progression of slip band formation, crack initiation, and crack propagation has been obtained via in-situ optical imaging with EBSD correlation. Statistical analysis of these factors across various hydrogen concentration supports the role hydrogen plays in hydrogen embrittlement.