ABSTRACT
Most pipelines are exposed to severe operational conditions and sour environments that might lead to hydrogen entry in steels. The existence of weld joints can affect the service life of these tubes, acting as preferential areas for incidence of hydrogen embrittlement, due to residual stress and microstructure. There is a great concern to understand the influence of each region of the weld joint on hydrogen induced crack initiation and propagation. Formation of iron sulphide film on the steel surface plays an important role on this process. This study investigates the behavior of an API X80 steel, base metal and weld joint, in relation to hydrogen absorption and embrittlement in H2S environments. Slow strain rate tests, and iron sulphide film analysis were conducted aiming to determine the influence of each region on the loss of ductility. Particularly, focusing on the reproducibility of tests carried out in weld joints due to different fracture susceptibility and film formation ability of the weld joint regions. It was observed that the loss of ductility and the iron sulphide film morphology, and the promoted barrier effect against hydrogen, are dependent on the weld joint area considered.
INTRODUCTION
Carbon steel pipelines are extremely important as a transporting system of oil and gas over long distances from their sources to ultimate consumers. Hence, these pipelines face various environments, onshore and offshore, throughout extreme conditions as corrosive mediums containing H2S, CO2, chloride ions, and severe operational conditions in terms of pressure and temperature.
Besides, long distance transportation pipelines are usually manufactured by welding. The weld thermal cycle process remarkably affects the microstructure heterogeneity of welded joint and in turn HE susceptibility that is dependant of the microstructure1.
Due to this high demand in the flow of gas, oil and its derivatives, rises a need to develop high strength steels allied to a great tenacity and good weldability. This development is needed as an alternative for costs reductions on building and maintaining the pipelines and offshore structures, making possible the reduction of its wall thickness without modifying the operational conditions2.
