OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology where e is the minimum elongation within the estimated length of 50.8 mm (2 in) as a percentage, rounded up to 0.5 percent when it is less than 10 % and up to one percent when it is 10 % or higher; k is a constant equal to 1942.57 (625,000 when calculated in inches); A is the cross-sectional area of the tensile test specimen in mm2 (in2), based on the specified outer diameter or nominal width of the specimen and the specified wall thickness, rounded to an accuracy of 10 mm2 (0.01 in2) or 490 mm2 (0.75 in2) (whichever is less); U is the minimum specified tensile strength in MPa (psi). Impact strength requirements In accordance with API 5CT [3], the impact test is carried out using the Charpy method for V-notched specimens. The requirements for the absorbed impact energy of the tested specimens (at least 3 pieces) should be: – for transverse specimens KV+21 ≥ 20 J; – for longitudinal specimens KV+21 ≥ 27 J. The result less than the required absorbed energy can be obtained on no more than one specimen, and the absorbed energy value should be less than two-thirds of the required. The permissible dimensions of the impact test specimens and the reduction coefficients of the absorbed impact energy are presented in the standards (table 6). Requirements for heat treatment The API 5CT standard does not contain specific requirements for the heat treatment of pipes of strength class K55, it is allowed to be supplied in a state after normalization, normalization with subsequent tempering or after quenching and tempering along the entire length and throughout the pipe body at the manufacturer’s choice or in accordance with the requirements of the supply contract. However, the weld of electric-welded pipes should be heat-treated after welding at a temperature not lower than 540 °C (1,000 °F) or treated in such a way that there is no untempered martensite. This is due to the requirements for testing pipes for crumpling. Production of pipes for oil and gas pipelines Currently, two main technologies are used for the production of rolled products for large diameter pipes: controlled rolling followed by air cooling and controlled rolling followed by accelerated cooling. The basic concept of thermal and mechanical treatment (TMT) or thermal and mechanical controlled treatment (TMCT) underlies the development of many advanced steel grades with improved mechanical properties over the past 50 years. At TMCT, cooling rates and deformation models affect the heterogeneity of the microstructure and crystallographic texture of thick-walled rolled plates. It led to heterogeneity of the mechanical behavior in thickness and affected the properties of the plate. An increase in the thickness of the steel plate leads to significant differences in the plastic ability of the material to deform in the direction of thickness at different stages of forming [1–3]. Tests of the mechanical properties of thick-walled pipeline steel K60 at TMCT demonstrated these differences in thickness [1, 2]. Thick-walled steel plate K60 undergoes a longer holding time in thickness near the center during rapid cooling; cooling occurs at a lower rate and promotes grain growth [8–13]. On the other hand, changes in the deformation mode also affect the microstructure along the thickness of the rolled metal. In the process of hot rolling, the surface layer undergoes severe shear deformation due to friction between the surface and the rolls, which leads to the appearance of many dislocations in the ferrite [10, 11]. Moving dislocations weave, forming new grain boundaries, as a result of which the initial ferrite grains break up into many subcrystals [13, 25, 26]. Crystal fragmentation leads to more significant deformation and an increase in the internal stored energy of the grain, contributing to the rapid formation of ferrite in the surface layer [25, 26]. This combination (rapid cooling and shear deformation) leads to a decrease in the grain size in the surface layer. Hardening during grain refining often improves mechanical properties. Reducing the grain size increases the plasticity of the surface layer, so that
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