🔬 Technological Frontier 🔬 High-strength Austenitic Stainless Steel with Excellent Low-temperature Toughness for Liquid Hydrogen Environment

Hydrogen is one of the key solutions to the problem of fossil fuels, and it is expected that the global demand for hydrogen will increase. Therefore, a large amount of hydrogen will be transported by sea and stored in ports between continents. In terms of hydrogen storage and transportation, liquid hydrogen with high volume density is more suitable for transportation or storage than gaseous hydrogen. Especially, to store liquid hydrogen, the tank material must be able to withstand low-temperature conditions (-253℃) and have good hydrogen embrittlement resistance. It is well known that stable austenite has better resistance to hydrogen embrittlement than metastable austenite. Therefore, the austenite stability of steel should be considered in the design of the alloy.

For use under low-temperature conditions, POSCO (Pohang Iron and Steel Company) in South Korea has designed a new alloy with added nitrogen, as nitrogen is the best alloying element that can improve both austenite stability and the strength of stainless steel. However, the addition of nitrogen will reduce low-temperature toughness because nitrogen will reduce the stacking fault energy (SFE).

Therefore, POSCO has newly developed a high-nitrogen austenitic stainless steel with optimized composition design, which has high strength, hydrogen embrittlement resistance, and good low-temperature toughness. In addition, welding materials that match the steel type for liquid hydrogen storage tanks should also be developed.

Mechanical Properties Index
Tensile Strength 📈 | Hydrogen Embrittlement Resistance 🛡️ | Low-temperature Ductility ❄️
400 MPa 📊 | RRA > 0.8 🌐 | Side Expansion at -253°C > 1.0 📏

Compared with ordinary austenitic stainless steels such as 304L and 316L, the new liquid hydrogen stainless steel has higher strength, low-temperature toughness, and hydrogen embrittlement resistance. The manufacturing process has been optimized for the production of liquid hydrogen storage tanks, and the development took two years.

Generally, the maximum operating pressure of liquid hydrogen storage tanks is 2 MPa, and the new stainless steel has excellent hydrogen embrittlement resistance in the state of high-pressure gaseous hydrogen. Therefore, it is expected to be used as a material for high-pressure facilities and pipelines at hydrogen stations in the future.

The wall thickness of liquid hydrogen storage tanks is large, and the strength is high, which can be reduced. Due to the new developed stainless steel with high low-temperature toughness and good hydrogen embrittlement resistance, the service life of the hydrogen storage tank can be extended. In addition, welding materials suitable for high-nitrogen stainless steel have also been developed.

Through the optimization of alloy design, controlling alloy elements such as nitrogen, manganese, and nickel, the newly developed stainless steel has excellent properties such as high strength, low-temperature toughness, and hydrogen embrittlement resistance, and is the most ideal material in the liquid hydrogen environment. Although the newly developed stainless steel has a high nitrogen content, it still has sufficient low-temperature toughness in the liquid hydrogen environment due to the high stacking fault energy.

TEM Image Comparison
Planar Slip Mode 🔄 | Wavy Slip Mode 🌊
Low Stacking Fault Energy 📉 | High Stacking Fault Energy 📈
Ordinary High-nitrogen Stainless Steel 📌 | Newly Developed Stainless Steel 🚀