ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 22 The urgency to combat climate change is more evident than ever. Clean energy plays a vital role in reducing the impact of further climate change at a global scale. Hydrogen is a key element to the future of clean energy, yet hydrogen is notorious for adversely affecting material properties, most notably, causing ductility loss in almost all metallic materials, leading to what is commonly referred as hydrogen embrittlement. Through decades of research, it has become clear that hydrogen embrittlement in high-pressure hydrogen systems is likely inevitable. But the severity of property degradation varies in different materials. The practical solution to this challenge is to design hydrogen systems with a good understanding of hydrogen embrittlement. Type 316 stainless steel has emerged as a material with good resistance to hydrogen embrittlement. Appropriate material selections can increase the safety of high-pressure systems and help soften business impacts, making clean energy more affordable and accessible. EMBRITTLEMENT OF MATERIALS Although hydrogen embrittlement can be severe and may lead to catastrophic structure failures, hydrogen system safety can be achieved through MATERIAL SELECTION OF 316 STAINLESS STEEL FOR HIGH-PRESSURE HYDROGEN SYSTEMS Selecting materials for high-pressure hydrogen systems requires balancing technical understanding of hydrogen embrittlement and business considerations. Xiaoli Tang,* Swagelok Company, Solon, Ohio *Member of ASM International contact with hydrogen, especially under high pressure. It is also understood that each material is affected by hydrogen differently. In general, hydrogen embrittlement in austenitic stainless steels appears mild as compared with materials of many other types. And Type 316 stainless steels are among the best for resistance to hydrogen embrittlement. Figure 1 shows the hydrogen embrittlement results of several commercially available austenitic stainless steels, extracted from Thompson’s study[1]. Of this group, Type 304 stainless steel experienced the most ductility loss, while 316 stainless steel exhibited the best resistance. Austenite stability of these alloys, i.e., the resistance to martensitic phase transformation, has been thought to be related to hydrogen embrittlement susceptibility of austenitic materials. It is closely linked to the chemical composition of each individual alloy. One of the most often used methods to characterize the stability of austenite is through the calculation of MD30, the temperature at which 50% austenite is transformed into martensite when the material is subjected to 30% true strain. One widely used formula to calculate MD30 (in Celsius) by Angel[2] is: MD30 = 413 - 462 (C + N) - 9.5Ni - 13.7Cr - 8.1Mn - 18.5Mo - 9.2Si (Eq 1) where alloy elements are in weight proper understanding of and respect for hydrogen embrittlement. Embrittle- ment of materials is not unique in hydrogen systems. It can happen in different ways and under various circumstances. By broad definition, any ductility reduction of a material can be viewed as embrittlement. This is because, as the result of ductility loss, the material becomes more brittle. Engineers have been coping with controlled embrittlement successfully for generations. For example, when a material is cold worked, the strength of the material increases, resulting in subsequent ductility reduction. As long as the cold work is carefully controlled, ductility loss in a cold-worked material is not usually a concern. These property changes can be used to engineering advantage, and which are the principles of strain hardening of materials. It can be argued that hydrogen embrittlement is not as controllable as strain hardening. That makes it more important to understand hydrogen embrittlement and select suitable materials for engineered structures. DUCTILITY LOSS OF TENSILE PROPERTIES It is recognized now that hydrogen embrittlement in metallic materials is nearly inevitable; no metallic material is immune to degradation when in
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