ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 23 high-pressure hydrogen atmosphere, as in the studies by Caskey[3] and Yamada et al.[5] As pointed out in San Marchi et al.,[6] with the low diffusivity of hydrogen in austenitic steels, even using the slowest strain rates commonly achievable in tensile testing, hydrogen apparently was not able to enter/diffuse into these materials in the needed quantity to manifest its degradation potential on tensile properties as it did in the hydrogen saturated specimens. Therefore, tensile testing in high-pressure hydrogen gas alone may not reflect the full effect of hydrogen on devices that are expected to have extended contact with hydrogen and take up large quantities of hydrogen. In those conditions, the study conducted by San Marchi et al.[6] suggested that ductility reduction seemed to be unavoidable. Regardless of the testing procedure variation and somewhat different test result outcomes, all the tensile test results from nickel-containing austenitic stainless steels, particularly, 316 stainless steel, suggested that because of their inherently high ductility, even with degradation due to high pressure hydrogen, these materials still retained sufficient ductility needed for engineered structures. In most cases, the strengths of these materials were not affected. This could be why component failures in a hydrogen environment under steady tension load rarely happened regardless of nickel content in austenitic stainless steels. percentages. Typically, it was thought that the lower the value of MD30, the more stable the austenite against martensitic phase transformation. If austenite stability is a deciding factor for hydrogen embrittlement resistance, MD30 should have a reasonable correlation with the ductility loss of materials in hydrogen. Interestingly, in Thompson’s study[1], 316 stainless steel, which does not have the lowest value of MD30 of the group shown in Fig. 1, exhibited the highest resistance to hydrogen embrittlement, more so than 310 stainless steel, which has much lower calculated MD30. Clearly, there are other factors that have a more significant effect on the resistance of a material to hydrogen embrittlement. One often discussed is the stacking fault energy (SFE) of materials. Higher SFE promotes cross slip and is believed to benefit the resistance to hydrogen enhanced localized plasticity (HELP), one of the possible mechanisms of hydrogen embrittlement. Some alloy elements stabilize austenite and also raise SFE of the material. Those alloy elements will have more consistent benefit on the hydrogen resistance of materials, nickel being one of them. There are also others that affect the austenite stability and SFE in opposite ways, such as chromium and nitrogen. Evidently, MD30 alone is not a reliable parameter as a material selection criterion or qualifier. Overall, the effect of nickel on the response of materials to hydrogen is more consistent. Studies indicated that within the chemical composition range of austenitic stainless steels, the beneficial effect of nickel on resistance to degradation in hydrogen gas is very reliable[3-6]. Research results on the degree of nickel benefit seemed to vary, depending on testing conditions. Using tensile testing in high pressure hydrogen, Caskey[3] studied Fe-Cr-Ni alloys and discovered that 10 wt% or more nickel in those alloys provided significant resistance to ductility loss in hydrogen. In that same study, it was also observed that hydrogen appeared to have very little effect on the ductility of Fe-Cr-Ni alloys containing 12 wt% and up to 30% nickel. Similar results were also reported by Yamada et al.[5] On the other hand, when using specimens saturated with hydrogen through hydrogen precharging method, San Marchi et al.[6] observed ductility loss that decreased with increased nickel content. But the effect of hydrogen on ductility was persistent and not eliminated even as nickel content was above 13%, approaching its upper limit for 316 stainless steel. Ductility changes of a group of annealed 316 stainless steels with different nickel content from that study[6] are shown in Fig. 2. It is believed that the hydrogen-saturated specimens through pre-charging experienced a more significant hydrogen impact than the specimens without pre-charging that were tested in a Fig. 1 — Ductility loss of austenitic stainless steels in high pressure hydrogen (Data extracted from Thompson study[1]). Fig. 2 — Nickel effects on resistance to hydrogen induced ductility loss at room temperature in hydrogen charged AISI 316 stainless steel.
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