A methodology for the numerical assessment of autofrettage influence on hydrogen content near a notch in a 4130 steel pressure vessel

The introduction of compressive residual stresses, e.g. by means of an autofrettage process, is a common strategy in industry for the improvement in fatigue life of high-pressure vessels. The application of autofrettage process in a vessel, storing hydrogen at 70 MPa, is evaluated here. Compressive stresses make difficult the entry of hydrogen in the metal lattice, however the produced plastic strain might be related to the creation of trapping sites for hydrogen. The balance between these two phenomena is analysed. In addition, the likely presence of notches is considered since stress concentrators increase the level of hydrostatic stress and plastic deformation, thus rising hydrogen content in the vicinity of the notch. A pressure vessel is simulated with the Finite Element method combining a static stress-strain analysis with a steady-state hydrogen diffusion analysis. Both autofrettage process and service life are simulated, and a notch is introduced in the inner surface of the vessel. Results indicate that, for each of the considered notch depths, autofrettage pressure must be limited in order to avoid high hydrogen concentrations, thus reducing the efficiency of the autofrettage process. This phenomenon is strongly dependent on the binding energy of trapping sites created by plastic strain.