The effect of heat-to-heat variations in metallurgy and hydrogen-metal interactions on the hydrogen embrittlement of Monel K-500

The influence of heat-to-heat variations on the hydrogen embrittlement susceptibility of age-hardened Monel K-500 (UNS N05500) was evaluated through detailed characterization of metallurgical attributes and hydrogen interactions, coupled with notched tensile specimen embrittlement metrics. Four nominally peak-aged material heats of Monel K-500 were assessed using slow strain rate tensile (SSRT) testing while immersed in 0.6 M NaCl solution and exposed to cathodic polarization levels ranging from −0.850 to −1.1 VSCE. Despite each of the four heats meeting the US Federal Procurement Specification QQ-N-286G, the hydrogen embrittlement susceptibility was found to vary extensively between the tested material heats. Characterization of microstructural features, composition, and hydrogen-metal interactions were performed to facilitate correlation between material property and susceptibility trends. Results suggest that subtle differences in grain boundary chemistry and H uptake behavior may contribute to heat-to-heat variations in hydrogen embrittlement susceptibility of Monel K-500. Conversely, parameters including yield strength, hydrogen diffusivity, hydrogen production rate, grain boundary character, and grain size do not independently control the observed variations in susceptibility. Based on these experimental results, a macroscale framework for assessing the degradation in fracture stress as a function of applied potential is proposed and possible avenues for framework improvement are suggested.