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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.

An estimate is given of the effect of strain ageing on the corrosion resistance of pipe steel 17GS in a corrosive operating medium. Metal structure and mechanical properties after strain ageing are considered in two conditions, i.e., after controlled rolling and after normalizing. It is shown that "aged" steel after controlled rolling exhibits greater corrosion resistance than normalized steel.As is well known [1], the majority of pipe steels of grades for oil and gas are subject during operation to strain ageing, which is reflected, on the one hand, by an increase in strength properties, and, on the other hand, by a marked reduction in steel resistance to failure (embrittlement). The main strain ageing mechanisms are known, whereas the connection of this phenomenon with metal corrosion resistance has not been studied sufficiently.The aim of this work is to evaluate the effect of strain ageing on corrosion resistance of pipe steel in an operating medium.Tests were performed on pipe steel 17GS (composition, %: 0.18 C, 0.41 Si, 1.2 Mn, 0.013 P, 0.016 S, 0.025 Cr, 0.080 Ni, 0.013 Cu). The steel was studied in two original conditions: after controlled rolling and after normalizing. Metal ageing was carried out by strain in tension (10%) followed by tempering with T = 250°C for 1 h and air cooling (GOST 7268).Specimens of "aged" steel had a prismatic shape with a size of 12 × 12 × 60 mm with a working face in the form of a microsection (11 × 12 mm) in contact with the corrosive medium.Before the start of tests, the working face was degreased and subjected to acid etching in 5M HCl in order to remove surface damage after machining. The nonworking surface of specimens was coated with chemically resistant insulating material.The corrosive agent used was 30% NaCl solution in distilled water with pH = 4. Experiments were performed in two stages: corrosion tests; coulometric recording of the corrosion products. During corrosion tests in a corrosive medium (volume V = 60 ml; stirring rate v = 20-25 cm/sec), the working face of a specimen was immersed and held successively for 0.5, 1, 1.5, 2, 3, and 4 h. A "one-time" specimen was used for each time point.The corrosion products formed at the working face during tests were removed in a solution of specially remover with pH = 3.8. In parallel, in order to determine background corrosion indices in the removal solution, a control "aged" specimen was exposed to it for 1 h without mixing. The solutions obtained for the test medium and remover with "aged" specimen corrosion products were stored in 0.1M HCl, after which coulometric analysis was carried out.

Features of the change in properties of pipe steel of strength category Kh80 during strain ageing and its effect on weldability are studied. It is shown that depending on the degree of plastic deformation with ageing of basic metal there is a change in the rational range of cooling rate in the temperature range of diffusion transformation of austenite (w 8-5 ) of the area near the seam (ANS) of the heat affected zone (HAZ), providing the required set of mechanical properties and in particular failure resistance. This should be taken into account in developing welding repair and restoration technology for main gas pipelines made of high-strength steels, previously in operation.Performance of repair work in gas pipelines is connected with the use of welding (surfacing) both in metal not previously in operation, and in metal after prolonged use.It is well known that prolonged operation of pipeline steel may change the mechanical properties. This is expressed as an increase in strength properties with a reduction in ductility, and particularly brittle failure resistance [1][2][3][4].In addition, questions of the weldability of this metal are not reflected in the development of repair technology both for traditional pipe steels and for the new generation of high-strength steels.Normally in developing a repair procedure the focus is on the traditional properties of weldable pipe steels, whereas a change in metal properties during operation may require correction of the welding and surfacing regimes during pipeline repair work.Thus, in developing or approving technology for carrying out repair work on main pipelines it is necessary to delineate clearly when this work is carried out: during construction or during operation.As applied to high-strength steels of strength category K65 (Kh80) the regimes adopted for welding technology during repair of new metal do not differ from recommendations proposed previously [5].There is almost no experience of welding metal of strength category K65 (Kh80), after prolonged operation, and therefore in this work features are considered for the change in properties of the basic metal before welding and the effect of this process on the weldability of these steels [6,7].Pieces of pipe used for this study were of low-carbon microalloyed steel of strength category K65 (Kh80) with a preferred bainitic structure of the following chemical composition, %: 0.084 C; 0.22 Si; 1.90 Mn; 0.012 P; <0.002 S; 0.21 Ni; 0.25 Mo; 0.062 Cr; 0.05 Cu; 0.044 Al; <0.005 V; 0.018 Ti; 0.048 Nb.