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Hydrogen embrittlement of high strength steels for fastener applications
Kent, Michelle N.
Kent, Michelle N.
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2025
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High-strength steel fasteners can be susceptible to hydrogen embrittlement when hydrogen is introduced cathodically during processing or in service conditions, leading to unexpected, brittle failure. Microalloying to precipitate fine alloy carbides can be used to trap hydrogen and mitigate hydrogen accumulation at susceptible microstructural features and stress concentrators. However, increasing trap density increases the driving force for hydrogen absorption during cathodic or gaseous hydrogen charging in microalloyed conditions compared to non microalloyed conditions of similar strength. In this study, the effects of V and Mo microalloying and alloy carbide precipitation on hydrogen embrittlement susceptibility are investigated for a range of cathodic hydrogen pre charging and in situ charging conditions to determine whether the increase in trapping capacity can increase resistance to hydrogen embrittlement despite increased hydrogen absorption. Additionally, hydrogen embrittlement is evaluated in notched and smooth specimens with continuous displacement and notched specimens with static loading to study the differences between hydrogen embrittlement behavior with different hydrogen embrittlement evaluation methods. Melt extraction and thermal desorption analysis were used to characterize the hydrogen concentration with different alloying, heat treatment, and charging parameters. For the same strength level, V microalloyed conditions were found to increase the resistance to embrittlement across a range of temperatures up to a point near saturation despite increased hydrogen absorption compared to a V-free condition. When tempered to the same strength level, the addition of Mo to the V-added condition further increased the resistance to hydrogen embrittlement with similar hydrogen absorption behavior. Alloy carbides are interpreted to increase resistance to hydrogen accumulation at stress concentrators by reducing hydrogen diffusion and trapping a portion of diffusible hydrogen. Notched specimens were sensitive to hydrogen accumulation at the notch, with embrittlement detected in conditions with little hydrogen charging. Smooth specimens were less sensitive to hydrogen charging without coordinated diffusion toward a large stress concentrator such as a notch during loading, and hydrogen accumulation and fracture were observed at several local stress concentrators assumed to be inclusions in the gauge section. Constant load testing led to fracture at lower stress compared to continuous displacement, and the difference between the two testing methods was larger between conditions with less total hydrogen. Conditions with high total hydrogen concentration are less time sensitive because hydrogen can quickly accumulate to critical concentrations when a stress concentrator is introduced.
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