Résumé
The race to diversify energy sources at the turn of the century and the quest for green energies have made hydrogen an attractive alternative to fossil fuels. Using hydrogen as a new energy source raises the question of how to transport this gas, and to take into account all the constraints relating to its hazardous nature and its effect on materials. The pipelines currently used to transport natural gas are made of high-strength API X80 and have welds resulting from the assembly process. There are also plans to use these pipelines for hydrogen distribution. This study focuses on a new surface treatment likely to reduce the risk of hydrogen embrittlement on this X80 pipeline steel and more specifically on the girth weld. Recent studies have shown that surface treatments such as shot peening can mitigate hydrogen-induced effects in low-alloyed teels. The microstructural refinement induced by the treatment, and the generation of a high density of dislocations and defects near the surface, seem to act as hydrogen trapping sites. The aim of this study is to investigate the behaviour of the welded zone, ncluding the heat-affected and the melted zones under hydrogen atmosphere, and then to study the effect of Surface Mechanical Attrition Treatment (SMAT) on the weld bead. While an orientated ferrite-pearlite microstructure was observed in the base metal, bainite and traces of martensite were found in the heat-affected zone and the melted zone of the girth weld, respectively. This is accompanied by an increase in hardness which is accentuated by the SMAT treatment. To analyse the effect of SMAT on the highly heterogeneous microstructure in the welded area, microstructural characterisation was also performed using Scanning Electron Microscopy (SEM) and EBSD coupled with residual stress measurements using X-ray diffraction. In addition, gas permeation tests were carried out on untreated and SMATed samples to investigate the effect of SMAT on the diffusivity of hydrogen in the weld area. The relationship between the microstructural evolution during the treatment, the number and types of trapping sites and the diffusivity of hydrogen is discussed. The aim of this study is therefore to gain a better understanding of the hydrogen trapping mechanisms in the girth weld area of a X80 pipe steel after SMAT treatment. It also aims to determine whether SMAT has a different effect on the hydrogen gas permeation performance of X80 steel heat treated by the welding process.