The tendency towards strain ageing (SA) and hydrogenation (HD) of steel Kh80, manufactured by controlled rolling is studied. Fragments of pipes 1420 mm in diameter with a wall thickness of 18 mm are used for the study. It is established that SA for a metal pipe made of steel Kh80 leads to a reduction in ductility by 25%, and the critical brittleness temperature is increased by 35°C. During operation of steel Kh80 under the action of HD, the greatest embrittlement effect develops at -40°C.In domestic and world practice of pipeline construction, a tendency is noted towards an increase in the strength category for the steels used. Use of pipes made of high-strength steels will make it possible to increase the working pressure in pipelines without increasing the metal content of the structure and to reduce construction expenditure.Steels of strength categories Kh80 and Kh100 are considered as promising materials for use in the northern European and far eastern pipelines. Due to the optimum combination of alloying, and heat treatment, these steels exhibit not only high strength properties, but also high resistance to brittle failure [1].A considerable amount of work has been devoted to studying the structure, weldability, and mechanical properties of these steels [1][2][3][4]. However, in scientific and technical publications there is insufficient information about the possibility of providing metal mechanical property stability over the whole pipeline operating period. Damage of oil and gas pipelines may be due to development in pipe steels of strain ageing (SA) phenomena [5] and hydrogenation (HD) [6,7]. Ageing develops particularly intensely in steels prepared by controlled rolling [2].The aim of this work is to study the tendency towards SA and HD of high-strength pipe steel Kh80, prepared by controlled rolling.Fragments of pipe steel Kh80 1420 mm in diameter with a wall thickness of 18 mm of the following composition (%) were studied: 0.057 C, 0.16 Si, 1.74 Mn, 0.008 P, 0.004S, 0.14 Ni, 0.27 Mo, 0.034 Cr, 0.14 Cu, 0.032 Nb, 0.005 V, 0.011 Ti, and 0.002 Ca.The tendency of steel towards SA was studied in accordance with GOST 7268−82. Specimens were used with a size of 12 × 12 × 250 mm cut from pipe fragments that were subjected to 10% deformation and subsequent tempering at 250°C for 1 h.
Studies have been made on fragments of 17GS steel tube during strain ageing by optical and electron metallography, and also by x-ray spectral and XRD analyses. During such ageing (with strain of 5-15%), there are substantial changes in all the mechanical characteristics: the ultimate strength and the yield point increase, while the relative extension and toughness decrease, and there is virtually no change in the relative narrowing.Strain ageing is a cause of reliability loss in pipelines. There are highly conflicting views on it. Some researchers [1] give data on such ageing in the operation of constructions such as oil pipelines [2], while others assert [3] that strain ageing is hardly observed at all if working states are maintained, and the mechanical characteristics alter only slightly.Here we consider how such ageing affects the structure and mechanical properties of 17GS pipeline steel. Pipelines may be subject to static, cyclic, and dynamic loads, and also to temperature fluctuations, from elevated during operation to negative ones in repair. There may also be substantial strains in pipelines due to movements in the foundations associated with climatic fluctuations in soil temperature in parts of the Far North [4].We used fragments of backup pipe made of low-alloy 17GS steel. Chemical composition in %: 0.18 C; 0.41 Si; 1.20 Mn; 0.013 P; 0.016 S; 0.025 Cr; 0.080 Ni; 0.13 Cu; <0.005 V; 0.006 Al; 0.006 Ti; <0.001 Nb; <0.001 B; the mechanical characteristics are given in Table 1.Strain ageing was simulated in accordance with GOST 7268-82, for which we cut specimens of size 12 × 12 × 250 mm from the tube, which were used in stretching in an EU 40 machine (strain ε = 5%; 10%; and 15%), and then were annealed at 250°C (hold for 1 h, cooling in air). We estimated the mechanical characteristics in static loading [5], cyclic loading [6], and dynamic loading [7].We used optical and electron metallography and also x-ray spectral analysis and XRD. Tests showed ( Table 1) that the ageing substantially affects all the mechanical characteristics, and in particular the ultimate strength is increased from 11 to 18% and the yield point from 44 to 62% for changes in strain correspondingly from 5 to 15%.There was a considerable reduction in the relative extension with almost unaltered relative contraction, which indicates embrittlement [2]. This is confirmed by tests on brittle failure and by fractographic analysis of failed surfaces (Fig. 1).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.