Modelling hydrogen embrittlement in 316L austenitic stainless steel for the first wall of the Next European Torus

This paper presents the final results of the work which the authors have been performing for the European Fusion Technology Programme (the NET Team). The contribution deals with the modelling of hydrogen embrittlement in AISI type 316L solution-annealed austenitic stainless steel, to be used for the first wall of NET (Next European Torus). Numerical modelling of hydrogen diffusion in the samples was performed on the basis of a set of non-conventional diffusion equations, in which hydrogen diffuses not only to the points of minimum concentration, but also towards those places of maximum hydrostatic stress. The diffusion computer program was coupled with an elastic—plastic finite element program to allow the calculation of both the stress state and the distribution of hydrogen concentration at the sample points step by step. Hydrogen penetration rates due to diffusion were extremely low in the unaltered material, even under mechanical loading. However, a clear loss of load bearing capacity of the notched specimens in hydrogen environment was observed even for extremely short tests. A possible explanation is that hydrogen embrittlement might not be associated with bulk diffusion through the specimen (there was not enough time for this) but rather with a localized degrading action, due to hydrogen, on the area surrounding the notch. In particular, hydrogen enhanced multi-cracking is a possibility which must be taken into account.