ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 25 ANSI/CSA CHMC 1[17], and SAE J2579[18]. They all require tests in a hydrogen environment that may not be easily accessible for engineering purposes. As an alternative, the approach of a material selection criteria based on material chemical composition has been explored. Different from the earlier discussed MD30, which does not appear to be reliably linked to the hydrogen embrittlement resistance of austenitic stainless steels, a new term known as nickel equivalent, Nieq, has emerged. A widely used formulate from Hirayama and Ogirima[19] states: Nieq=12.6 × C + 0.35 × Si + 1.05 × Mn + Ni + 0.65 × Cr + 0.98 × Mo (Eq 2) where all elements are in weight percentage. In Eq 2, there are other elements in austenitic stainless steel that are potentially more potent than nickel in benefiting hydrogen resistance. But those elements are in significantly lower quantity in austenitic stainless steels. Overall, nickel is still the most beneficial alloy element in these alloys. Austenitic stainless steel with different values of Nieq have been studied and corelated to hydrogen effect on mechanical properties in several studies[4,5,20]. While the results from Yamada[5] seemed to suggest that an Nieq value as high as 28.5 could prevent ductility loss in hydrogen almost completely, Zhang et al.[4] and Ueno and Benjamin[20] observed nearly no ductility loss in alloys with Nieq around 27. When fatigue behavior was studied, Ueno and Benjamin[20] compared two lots of SUS 316L alloys with Nieq values of 25.7 and 27.0, respectively. Figures 5a and 5b are from their study and comparison of the two indicated that fatigue cycle number reduction in the material with an Nieq value of 25.7 was more significant than the material with an Nieq value of 27.0. More interesting is that when fatigue endurance limits were examined, hydrogen had almost no measurable effect on the material with an Nieq of 27.0. Similar conclusions could be drawn from the crack growth results of Ohmiya and Fujii[8] in Fig. 3. When calculated using Eq 2, the SUS 316 with 11.7 wt% nickel used in that study had a Nieq value of 26.4. That material showed very limited crack growth acceleration in hydrogen as compared in air. All these results suggest that once the alloy composition reached an Nieq value of around 27, it achieved its most potential in offsetting the effect of hydrogen degradation. A further increase in alloy content likely would not bring meaningful additional benefits. There has been a call for higher Nieq values as the criteria. Although not harmful technically, higher than necessary alloy contents can increase the overall cost of hydrogen systems. The balance between technical benefits and business impacts must be considered when designing engineered structures. Overly high alloyed 316 stainless steel could lead to noticeable increase in material and consequently system costs. In addition, materials with alloy contents that are higher than market normal are less readily available which would add another logistic huddle to procurement, while 316 stainless steel with an Nieq of 27 is much more widely available. It is worthy to point out that the necessity of material selection criteria is also open for debate. No metallic material is immune from hydrogen embrittlement, even those with high Nieq values. As long as the hydrogen impact on materials is appropriately understood, many materials including those with an Nieq below 27 can be and are used with appropriate design and maintenance cautions. On the other hand, simple and easy to use methods, such as material selection criteria like Nieq, can provide needed assistance for engineering design and applications, if they are set appropriately. An Nieq value of 27 can provide a good balance between technical benefit and business impact. SUMMARY Material selection for high- pressure hydrogen systems requires a technical understanding of hydrogen embrittlement impact on material performance as well as business acumen. It is not practical, or even possible, to seek materials that are unaffected by hydrogen especially in high-pressure hydrogen systems. While the direct effect of hydrogen on materials is usually manifested as ductility loss under tension stress, the most concerning failure in hydrogen system is fatigue. Although no material is immune to property degradation caused by hydrogen, Type 316 stainless steel is among the best regarding resistance to hydrogen embrittlement, although material response to hydrogen can still vary depending on the chemical composition of specific materials. Higher alloy contents are beneficial for hydrogen embrittlement resistance to a certain Fig. 5 — Fatigue behavior of two 316 stainless steels with different Nieq values (Source: Ueno and Benjamin[20]), (a) Nieq = 27.0 %, R = -1.0. (b) Nieq = 25.7 %, R = -1.0. (a) (b)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==