SENSITIVITY TO HYDROGEN EMBRITTLEMENT OF 2 PIPE STEELS G Pluvinage*1and J. Capelle2 *Fiabilité Mécanique. Conseils Silly sur Nied France ENIM Metz France * Corresponding author : pluvinage@cegetel.net Fracture toughness and fatigue résistance of two pipe steels (X52and X70) has been determined using curved specimens (roman tiles) directly extracted from pipes. In both case, hydrogen embrittlement is done by electrolytic method. The comparative assessment of local fracture initiation at notches have been done for conditions of electrochemical hydrogenating. The factor of hydrogen concentration in metal was taken into account. The relationship between hydrogen concentration in metal and work for initiation of the local fracture emanating from the notch has been derived. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown and discussed. The concept of K has been extended to fatigue initiation emanating from notch and under hydrogen embrittlement. Here, fatigue initiation is detected by acoustic emission. It has been found that the fatigue initiation decreases after hydrogen absorption. This can be explained by interaction of hydrogen and plasticity as can be seen for tensile and fracture behaviour steel after introduction of hydrogen. Keywords : pipe steel, hydrogen embrittlement, local fracture energy, fatigue initiation 1.INTRODUCTION General reason, which increases of attention to the problem hydrogen degradation of pipeline steels, is fact that hydrogen will play a decisive role in a future energy system, when fossil fuels have become scarce and thus expensive and/or unsuitable from ecological reasons. The number of aspects related to the technical feasibility and economics of developing a hydrogen energy infrastructure are presented and discussed in literature during last decades [1, 2]. The possible use of existing pipeline networks for mixtures of natural gas and hydrogen offers a unique and costeffective opportunity to initiate the progressive introduction of hydrogen as part of the development of a full hydrogen energy system [3]. From this reason, the safety, durability and integrity issues related to hydrogen/natural gas mixtures in the existing natural gas system become actual and important. First of all, there is a potential problem of so-called “hydrogen embrittlement” of pipeline material - the effects of transported hydrogen on material mechanical properties [4]. Moreover, the specific long-term exploitation of pipelines promotes the hydrogenating of steel. The external environmental conditions cause free corroding processes, where hydrogen can evaluate on metal surface as result of cathodic counterpart of the anodic dissolution reaction. This fact has been proved by several studies [5-7]. External interference incidents are characterised by potentially severe consequences: leaks and ruptures. In majority, these types of incidents are caused by initial damaging of the pipe external surface as result of third party interference e.g. by scratches and gouges and also pitting corrosion.
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