P. Kathayat, L. Cho, J. Speer, J. Kong, K. Findley
Colorado School of Mines,
Keywords: Hydrogen Storage, Low Cost Steel
Summary:Hydrogen is a potential fuel source for automotive and heavy equipment applications, with the only byproduct being water. One challenging task for implementing this technology is the infrastructure cost. The alloys widely used for hydrogen fueling infrastructure are austenitic stainless steels because of their resistance against hydrogen embrittlement. These alloys have a high nickel (Ni) content, the cost of which is relatively high. It is essential to develop lower-cost options without significantly lowering the hydrogen embrittlement resistance and mechanical properties. In this study, high manganese (Mn) austenitic alloys were designed as an alternative to high Ni austenitic stainless alloys due to the substantially lower cost of Mn compared to Ni. Mn, similar to Ni, is a powerful austenite stabilizer. The alloy compositions were designed to achieve a stacking fault energy (SFE) above 39 mJ·(m^-2) to avoid planar slip deformation mechanisms, including deformation induced twinning and martensite formation. Vanadium (V) was added to one of the high Mn alloys for thermomechanical processing and potential precipitate strengthening considerations. Hydrogen embrittlement characteristics of the designed high Mn steels having different SFE values were compared against 316L stainless steel. Hydrogen embrittlement sensitivity was investigated by cathodically charging circular notch tensile specimens with hydrogen in a 0.05M NaOH electrolytic solution. The notch strength loss (in pct.) of alloys in hydrogen was calculated using the difference in notch tensile strength in air and hydrogen to notch tensile strength in air. The 316L stainless steel exhibited no notch strength loss and ductile fracture features regardless of the testing environment, which further validated its high hydrogen embrittlement resistance. The high Mn alloys with SFE of ̴ 29 mJ·(m^-2) and 49 mJ·(m^-2) had notch strength losses of 11 pct. and 6 pct., respectively. The high Mn steel with low SFE exhibited transgranular (brittle) fracture, whereas in the high Mn steel with high SFE, only ductile fracture was observed, similar to 316L stainless steel. Martensite was detected in the high Mn steel with SFE of ̴ 29 mJ·(m^-2) condition after tensile deformation, which is consistent with its lower stacking fault energy. The V-microalloyed high Mn steel in the as hot-rolled condition had a notch strength loss of 17 pct., and intergranular (brittle) fracture with some features of ductility was observed near the notch root.