Properties degradation of pipeline steels caused by long-term service in hydrogen enriched environments V.V. Panasyuk, H.M. Nykyforchyn 5, Naukova Str., Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Ukraine panasyuk@ipm.lviv.ua Keywords: pipeline steel, degradation of properties, deformation aging, dissipated damaging, crack growth resistance, corrosion, hydrogen cracking, fracture risk assessment. Abstract. Long-term service of steels causes the essential deterioration of their as-received properties, especially the parameters of brittle fracture, and the crack growth resistance first of all. The sensitivity of steels to hydrogen embrittlement increases, therefore, for the evaluation of its in- service degradation it is recommended to test the metal in the hydrogenation conditions, best of all – for the crack growth resistance under action of hydrogenating environment. The investigations of the properties of different parts of oil and gas pipelines (pipe top and bottom) showed that the metal mechanical properties are lower for the parts with more intensive corrosion damages of the internal surface (pipe bottom) than for the metal of the pipe top part. It indicates the essential effect of hydrogen on the processes of in-service degradation of steels. Two stages of in-service properties degradation of pipeline steels are considered: deformation aging and dissipated damaging. The role of hydrogen is revealed in the intensification of the dissipated damaging stage. The main regularities of steels properties degradation for the different stages of service are given. So, the strength and hardness increase, plasticity and brittle fracture parameters decrease for the stage of deformation aging according to the conventional point of view. At the same time the brittle fracture resistance parameters and the reduction of area decrease continually but (and it is untypical) the relative elongation increases, the strength and hardness decrease for the stage of dissipated damaging. The last peculiarity demonstrates the development of microdefects and their opening in-bulk material. The evaluation criteria of pipeline safe service with crack type defects in hydrogen enriched environments are proposed using the approaches of fracture mechanics. The analytical dependence for the description of the cyclic crack growth in steels in hydrogenating environments, which considers hydrogen concentration at the crack tip, is proposed and experimentally confirmed. Introduction An evaluation of the technical state of long-term exploited structures and calculation of their residual life time is impossible without an account of the change of the material physical and mechanical properties, which appear during service. Really these properties are changed mainly in worse direction, so “in-bulk” material degradation should be considered. It concerns, in particular, pipeline steels exploited at ambient and elevated temperatures. It is evident that diffusion factor can play a dominant role in high temperature degradation of steel properties. It causes changes of the metal microstructure which worsen its resistance to fracture. In particular, the main change for ferrite-pearlite microstructure of power steam pipeline steels consists in dissolution of pearlite and formation of grain boundary carbides [1]. For usual service temperatures change of steel structure is not so pronounced but, for example, for main gas pipeline steels, the changes on the dislocation substructure level, coagulation of grains, grain boundary carbide formation on nanoscale are considered [2].