Résumé
This work presents a comparative study between the effect of ‘classical’ sandblasting and a severe plastic deformation process such as Surface Mechanical Attrition Treatment (SMAT) considered to prevent Hydrogen Embrittlement (HE) on an API X80 high strength pipeline steel. Recent studies have shown that surface treatment processes such as shot peening can mitigate hydrogen-induced effects in low-alloyed steels. This effect seems to be linked to the level of residual stresses and work hardening generated, but also to the microstructural state of the material after treatment. Thus, the microstructural refinement induced by the treatment, as well as the generation of a high density of dislocations and defects close to the treated surface, seem to act as hydrogen trapping sites. In order to assess the ability of the SMAT treatment to block hydrogen diffusion compared to other processes such as sandblasting, a comparative study was carried out. Hydrogen gas permeation tests were performed on flat samples with different surface conditions: sandblasted, SMAT-treated and mirror-polished. In addition to characterize the severity of plastic deformation and the thicknesses of the deformation layers resulting from SMAT, different measurements have been performed: Surface roughness, microhardness profiles, residual stresses by X-ray diffraction and grain size from micrographic examinations. It appears that, depending on the process parameters, SMAT can significantly reduce hydrogen diffusion in X80 grade steel compared to sandblasting. Therefore, the aim of this study is to gain a better understanding of the hydrogen trapping mechanisms in X80 steel treated by SMAT.