For decarbonated flights, Airbus investigates the use of Liquid Hydrogen (LH2) as an alternative to hydrocarbon fuel. This rises many challenges about the on-board storage of this new fuel. One of the points of intention is the potential absorption and diffusion of atomic hydrogen with the associated embrittlement of metallic materials used to manufacture the tank. A resistant alloy commonly considered for this purpose is the 316L austenitic stainless steel, mainly due to its superior microstructural stability even at low temperature. In the present PhD thesis project, two alternative grades similar to 316L are investigated by means of Slow Strain Rate Tensile Testing (SSRTT), fatigue and fatigue crack growth tests. The objective is to characterize each grade and its associated welds in their most critical conditions. Comparing the performance of these materials with the widely used 316L will be the key to ensure a good properties-over-cost ratio. Particular attention will be given on welded joints due to their complex microstructures.
Firstly, the hydrogen uptake of a tank during the service life, including in a worst-case scenario, is estimated, so as to assess the risks of embrittlement. In this aim, thermal and mass diffusion simulations through the wall thickness of an insulated tank were performed. In normal flight conditions, a very low hydrogen uptake and diffusion due to the LH2 low temperature (20K). However, in the case of the insulation being damaged, temperature of the gas part increases up to room temperature, leading to greater intake at the surface (11 wppm.) and faster diffusion. In consequence, long exposition with damaged insulation may increase the total hydrogen uptake and penetration depth, which may embrittle the materials over the years, leading to eventual failure.
Secondly, to simulate a critical hydrogen embrittlement case, the materials have been pre-charged at elevated temperature in order to ensure hydrogen concentration profile homogeneity. Thermal Desorption Spectroscopy analysis, performed in LaSIE, revealed a total hydrogen concentration of around 80 wppm. for each material. Then SSRT tests were performed at 293K and 203K. Both materials exhibit improved strength and ductility properties with hydrogen content, which was unexpected due to the high hydrogen concentration. As seen in figure 1, at low temperatures, the embrittlement index, expressed as the Relative Reduced Area (RRA), gets closer to 1, meaning no significant change in ductility with hydrogen. [1] However the temperature corresponding with the maximum embrittlement [2] might be lower than 203K. In the future, additional tests with different hydrogen conditions will be performed to assess the impact of the hydrogen environment. Fatigue and surface crack growth tests will also be carried out to check fatigue life performance of the stainless steels in various hydrogen conditions.
A particular attention will be paid to the microstructure, especially for welded materials, due to microstructural evolutions and heterogeneity which may affect properties or hydrogen embrittlement susceptibility, such as grains growth, the apparition of brittle σ-phase, δ-ferrite, Z phase or Laves phases.
Hydrogen embrittlement, austenitic stainless steels, welds, SSRT, fatigue, crack growth