METHOD FOR PRODUCING PIPES WITH DIAMETER OF 114-1420 mm
By vertically cooling sheets to create a symmetrical microstructure gradient and welding them into pipes with a strength gradient, the method addresses the limitations of existing heat treatment methods, achieving enhanced crack resistance and mechanical properties in large-diameter pipes.
Patent Information
- Authority / Receiving Office
- RU · RU
- Patent Type
- Patents
- Current Assignee / Owner
- NOT PUBLISHED
- Filing Date
- 2025-10-10
- Publication Date
- 2026-06-30
AI Technical Summary
Existing methods for improving crack resistance in thick-walled plates and large-diameter pipes used in gas and oil pipelines are limited, as they often result in non-uniform heat transfer during quenching, leading to structural warping and incomplete crack inhibition, with cracks propagating over an undesirably long distance, necessitating multiple pipe replacements.
A method involving vertical cooling of thick sheets to create a symmetrical microstructure gradient across the sheet width, followed by bending and welding to form a pipe with a strength gradient around its circumference, redirecting crack propagation along the least resistant path.
This approach effectively reduces crack propagation to a single pipe segment, enhancing crack resistance and mechanical properties, ensuring compliance with industry standards by stopping cracks within a single pipe, rather than multiple segments.
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Abstract
Description
[0001] The proposed invention can be used in the heat treatment of thick rolled plates and in the manufacturing of large-diameter pipes (114 - 1420 mm according to GOST 31448-2012, Interstate Standard for Steel Pipes with Protective External Coatings for Main Gas and Oil Pipelines). Pipes manufactured according to this standard are used for the construction of main gas and oil pipelines, oil product pipelines, process and field pipelines.
[0002] The main criterion for crack resistance in main gas and oil pipelines is the crack propagation length. Reducing this length is a pressing issue. Existing methods for improving the crack resistance of thick-walled plates rely on increasing the energy intensity of crack propagation.
[0003] When transporting liquid or gas under pressure through a pipe, a stress state arises in the pipe walls [1]. Provided that the pipe is thin-walled (the ratio of the pipe radius to the wall thickness is more than 20), a plane stress state arises in the pipe wall: circumferential tensile stress ( ) and meridional tensile stress ( ) (Fig. 1). If the internal pressure in the pipe is q, then the meridional stress is calculated using the formula:
[0004] (1)
[0005] whereq internal pressure in the pipe,
[0006] R - pipe radius,
[0007] h - pipe wall thickness.
[0008] Moreover, the following condition is met:
[0009] = 2 , (2)
[0010] From this condition, it follows that if the hoop tensile stress is greater than the meridional stress, the resulting crack will propagate along the length of the pipe under the action of the hoop tensile stress. For a homogeneous isotropic material, this is a necessary and sufficient condition for determining the crack propagation direction. The crack propagation length can be tens or hundreds of meters, requiring the replacement of a large number of pipes.
[0011] Thermal and thermo-deformational treatment of rolled products (controlled rolling with accelerated cooling, quenching and tempering) increases the crack resistance of steel. [2 - 4].
[0012] During thermal and thermal deformation treatment of steel, it is necessary to obtain a more dispersed structure with high energy capacity. Thus, for steels of class K65 (X80) [5], with a ferrite-bainite structure (lower bainite structure), the crack propagation work is 1000-1400 J / cm², and with a ferrite-pearlite structure 500 - 800 J / cm² at test temperature - Ferrite-bainite structures have high energy capacity of destruction due to the dispersion of bainite laths and optimization of secondary phases.
[0013] STO Gazprom 2-4.1-741-2013 includes an optional requirement for the crack propagation work value, which ensures sufficient resistance to extended ductile fracture to stop a crack within three pipes. Currently, most thick plate for large-diameter pipes for main gas and oil pipelines is used after controlled rolling with accelerated cooling. It is also used after heat treatment of normalization, normalization with accelerated cooling, quenching and tempering, and accelerated cooling after rolling. In all cases, quenching or accelerated cooling is carried out in the horizontal position of the sheet by applying a cooling medium (usually industrial water) to the upper and lower sides of the sheet. Cooling water is supplied through special nozzles and sprayed onto the surface of the sheet. On the upper side of the sheet, the liquid spreads over the surface, and on the lower part Liquid separation upon contact with the underside of the sheet. Consequently, with equal amounts of water supplied to the top and bottom of the sheet, heat transfer to the top side is greater than to the bottom. Therefore, to ensure equal heat transfer on both sides of the sheet, 1.5 to 2 times more water is supplied to the underside than to the top. However, achieving equal (symmetrical) heat transfer on both sides of the sheet is not always possible, resulting in warping of the sheet, which is straightened in straightening machines. As a result of quenching or accelerated cooling, a uniform structure is formed throughout the sheet: a ferrite-cementite structure after quenching and subsequent tempering, or a ferrite-bainitic or ferrite-pearlite structure with varying degrees of dispersion during accelerated cooling. In this process, an attempt is made to achieve a homogeneous structure throughout the sheet.To produce a large-diameter straight-seam welded pipe, two sheets are bent along their lengths relative to the centerline and have a circular cross-section. Welding is then performed along the facing longitudinal edges of the bent sheets, producing a pipe with two continuous longitudinal welds located diametrically opposite each other.
[0014] Thus, improving the microstructure through heat and other treatments allows (and even then optionally) crack propagation over a distance of up to three pipes. This is a drawback of existing heat and other steel treatment methods for the production of large-diameter pipes.
[0015] The above-mentioned methods for increasing metal crack resistance only allow for a reduction in the length of a damaged section, such as a gas or oil pipeline, to a certain physically possible level (for example, by achieving the minimum possible structural dispersion). Methods for reducing the microstructure dispersion of steels through various types of thermal deformation treatment of rolled products have practically exhausted their potential.
[0016] A fundamental reduction in the length of pipeline destruction is possible with a controlled change in the crack trajectory.
[0017] A number of prerequisites for the inhibition and change of the trajectory of propagating cracks are known [6, 7]:
[0018] - creating conditions for branching of the main crack;
[0019] - taking into account the stress-strain state of the product operation.
[0020] For example [8], to purposefully change the crack trajectory in sheet metal, hardened sections are made in the form of concentric rings, the largest diameter of which is equal to the sheet width. To simulate the fracture process, varnish was applied to a sheet of dense Whatman paper on the concentric rings, simulating hardened strips. The Whatman sheet was then rolled into a tube and tested on a special rig under internal pressure until failure (Fig. 2a). It is evident that the moving main crack, passing through the first hardened strip, changes its trajectory, and passing through the second, branches into three secondary cracks. At points D, E, and D, the cracks stop. As the main crack branches, the energy is distributed among the secondary cracks. Therefore, the energy and velocity of the secondary crack are less than that of the main crack, and its movement quickly attenuates. In this method, crack inhibition and stopping is achieved by creating conditions for the branching of the main crack.The disadvantage of this method is the difficulty of obtaining reinforced areas in the form of rings on sheet metal.
[0021] Similar results on the branching of the main crack when passing through strengthened sections in the form of inclined strips were obtained in the work [9]. In a homogeneous material, a crack in a pipe, under the action of internal pressure, propagates almost in a straight line [9] (Fig. 2b).
[0022] The closest to the claimed technical solution is the patent “Method for increasing crack resistance of thick sheet metal”
[10] .
[0023] The technical result is achieved by rotating the crack trajectory by an angle of up to 90° relative to the original direction. This is achieved by selectively hardening the sheet metal in the form of strips bounded by lines according to the equation (Fig. 3):
[0024] (3)
[0025] where b is the sheet width, m;
[0026] c - width of reinforced and non-reinforced strips, m;
[0027] k=1,2,3,…,N, whereN=b / c=(6-7);
[0028] a- parameter determined in the range (0.3 - 0.4) of the sheet length, m.
[0029] Hardened metal strips alternate with non-hardened ones, i.e. with the original structure.
[0030] Reinforced strips are applied to any part of the sheet relative to its length.
[0031] After the first group of reinforced strips (I, Fig. 3), a second group of the same reinforced strips is applied without a gap, shifted across the width of the sheet by a value relative to the strips of the first group (II, Fig. 3).
[0032] Strengthening is achieved by heating these areas of the sheet to the austenitization temperature (A С3 + (30 - 50)°C), and then subjected to accelerated cooling at a rate sufficient to obtain a finer-grained ferrite-pearlite structure compared to the matrix
[0033] The finer microstructure is characterized by greater strength and crack resistance than the matrix (original) one.
[0034] The essence of the method is that a main crack, having formed and initially moving in a homogeneous steel structure, propagates rectilinearly along the generatrix of the pipe, entering, for example, an unhardened strip. An elastic stress wave (acoustic wave) propagates ahead of the moving crack
[11] . Stronger microstructures have a more distorted crystalline structure (higher density of dislocations and other imperfections) than weaker ones. In more distorted crystalline structures, the propagation velocity of elastic stress waves is lower than in the original. Waves, regardless of their nature (acoustic, electromagnetic, light, etc.) propagate in the direction of least resistance (where their speed is greater)
[12] .
[0035] As a result, most of the stress waves will propagate in the unhardened strip. The crack, following the elastic wave, will deviate from its original direction, with its trajectory turning at an angle α of up to 90°. If the main crack initially enters the hardened strip, then, after passing through the first group of strips (without significantly changing direction, but at a lower speed), it enters the unhardened strip of the second group of strips. After this, the crack propagation mechanism was described above.
[0036] The disadvantage of the method under consideration is that the crack, before changing its trajectory by 90°, propagates part of the way in a straight line, that is, it destroys part of the pipe, as well as the difficulty of creating reinforced strips.
[0037] In order to eliminate the rectilinear propagation of a crack, this invention is proposed. The technical objective of the invention is to change the crack trajectory by 90 that is, around the circumference of the pipe.
[0038] The problem is solved as follows. A thick sheet is placed vertically on a plane formed by the length edge and the thickness edge and is rapidly cooled in this position (Fig. 4).
[0039] Designations in Figs. 4-7: 1 - thick sheet, 2 - roller table for moving the sheet, 3 - electric motor of the roller table drive, 4 - vertical guide rollers, 5 - collectors for supplying water during accelerated cooling (arrow - direction of water supply), 6 - nozzles for spraying water, 7 - weld seam after welding, 8 - sprayed water from the nozzle.
[0040] The cooling rate across the sheet width varies to produce a continuously changing spectrum of microstructures, from ferrite-bainitic at the top to ferrite-pearlite at the bottom. Since the sheet is vertical, this allows for symmetrical, accelerated cooling on both sides (Fig. 5). This results in a symmetrical set of microstructures on both sides of the sheet. After heat treatment, the sheet is bent, producing a semicylinder with a semicircle at its base. A similar heat treatment is performed on the second sheet, which is also bent. The ends of the sheets are then joined and welded, producing a pipe. The ends are joined so that the ferrite-pearlite joint of one bent sheet is connected to the ferrite-bainitic joint of the other bent sheet (Fig. 6). Thus, we obtain a pipe with a regular distribution of microstructure around its circumference.As a result, mechanical properties, such as strength (yield strength and tensile strength), will increase in the direction from a ferrite-pearlite structure to a ferrite-bainitic structure. The magnitude of the change in strength will be characterized by a strength gradient (grad). ).
[0041] Figure 7 shows a pipe scan and the direction of the strength gradient. That is, when viewed from the end of the pipe, the strength gradient is directed along the pipe's circumference in one direction (Figure 6).
[0042] In the wall of a pipe manufactured using the proposed technology, loaded with internal pressure, a plane-stress state arises (Fig. 1) with circumferential and meridional stresses, with the condition of formula (2) being met.
[0043] A crack in a pipe made of homogeneous material propagates under the action of circumferential tensile stresses that is, along the pipe. A number of studies have established that in pipes made of heterogeneous material, a crack propagates in the direction of least resistance to its movement [12, 13].
[0044] In
[13] , eccentric tensile tests were performed on St3sp steel specimens with a sharp notch to initiate crack formation under the action of tensile forces P (Fig. 8). The sharp notch was located in the part of the specimen with a coarse-grained ferrite-pearlite structure. During tension, the crack initiated and initially propagated in this structure in a direction perpendicular to the applied forces. Upon approaching the interface between the coarse-grained ferrite-pearlite structure and the fine-grained ferrite-pearlite structure (indicated by the dotted line in Fig. 8), the crack changed its direction of motion, continuing to propagate in the coarse-grained ferrite-pearlite structure.
[0045] The work of crack propagation in a fine-grained ferrite-pearlite structure is greater than in a coarse-grained ferrite-pearlite structure. Therefore, a crack propagating from the coarse-grained ferrite-pearlite structure, upon approaching the boundary with the fine-grained ferrite-pearlite structure, turns and continues to propagate within the coarse-grained ferrite-pearlite structure (Fig. 9).
[0046] That is, the crack chooses a path where the work of propagation is less. This is consistent with a fundamental physical principle—the second law of thermodynamics
[14] .
[0047] For a steel sheet with a strength gradient across its width, a crack will propagate opposite to the gradient. However, since the pipe has a strength gradient along its circumference, the crack propagates in the direction of the least crack propagation work. The direction of decreasing crack propagation work is opposite to the strength gradient, i.e., along the pipe circumference. Crack propagation in this direction will also be facilitated by meridional tensile stresses. As a result of these factors, the crack will propagate along the pipe circumference. After completing one revolution around the pipe circumference, the crack will close in on itself and fracture will stop. Fracture resulted in the pipe dividing into two parts. One pipe failed, not three, as optionally allowed by STO Gazprom 2-4.1-741-2013.
[0048] Thus, the use of the proposed invention “Method for improving crack resistance of a large diameter pipe” is more effective than the prototype.
[0049] We will consider an example of the implementation of the proposed invention on low-alloy pipe steels 05KhGB, 08KhMFA, 08KhFA. Based on customer requirements, we set the maximum and minimum values of the strength characteristics of the steel across the sheet width (yield strength ( ) or temporary tensile strength ( )). In accordance with the terms of delivery of pipe steel of category K60 (“Requirements for mechanical properties of sheets made of steel of strength K60”), the yield strength is =480 - 590 MPa, tensile strength = 590 - 700 MPa. According to the relationships
[15] between strength characteristics and hardness, we find the extreme values of hardness:
[0050] , (4)
[0051] (5)
[0052] We will calculate the hardness based on the tensile strength. According to formula (5), the hardness range across the width is (1517 - 1864) MPa or (152 - 186) .
[0053] According to the thermokinetic diagram of the decomposition of supercooled austenite of 05KhGB steel (Fig. 10)
[16] , we obtain that the hardness range (152 - 186) obtained at a cooling rate in the range (20 - 100) The microstructure changes from ferrite-pearlite to ferrite-bainitic.
[0054] According to the thermokinetic diagram of the decomposition of supercooled austenite for 08KhMFA steel (Fig. 11)
[16] , we obtain that the hardness range is (152 - 186) obtained at a cooling rate in the range (7 - 20) The microstructure changes from ferrite-pearlite to ferrite-bainitic.
[0055] According to the thermokinetic diagram of the decomposition of supercooled austenite for 08KhFA steel (Fig. 12)
[16] , we obtain that the hardness range is (152 - 186) obtained at a cooling rate in the range (5 - 30) The microstructure changes from ferrite-pearlite to ferrite-bainitic.
[0056] Intermediate values of cooling rates can also be determined from the thermokinetic diagrams of the decomposition of supercooled austenite for the corresponding steel grade.
[0057] The distribution of the amount of cooling water for accelerated cooling to ensure a given cooling rate is calculated according to the formulas in the reference book
[17] .
[0058] List of references
[0059] 1. Darkov A.V., Shpiro G.S. Strength of materials. - M., Higher School, 1989 - 623 p.
[0060] 2. Energy intensity of crack propagation. https: / / studexpo.net / 873093 / materialovedenie / energoemkost_rasprostraneniya_treschiny
[0061] 3. Shmakov A.V. Development of an effective technology for controlled rolling of high-strength pipe steels based on modeling of temperature conditions of the process. Diss. Cand. Tech. Sciences. Magnitogorsk: 2011.159p.
[0062] 4. Matrosov M.Yu. Effect of accelerated cooling after thermomechanical treatment on the structure formation and properties of steels for large-diameter pipes. Diss. Cand. Tech. Sciences. Moscow: 2007. 195 p.
[0063] 5. Mishetyan N.R. Features of the mechanisms of destruction and strain aging depending on the structural state of low-alloy pipe steels. Diss. Cand. of Technical Sciences, Moscow Central Research Institute of Black Sea Mechanics. 2021. 145 p.
[0064] 6. Lopukhin P. I., Gorelik S. S., Vorontsov V. K. Physical principles of plastic deformation. M. Metallurgy. 1982. - 584 p.
[0065] 7. Finkel V. M. - Physical principles of destruction inhibition. - M.: Metallurgy, 1977 -360 p.
[0066] 8. Patent for Utility Model No. 63929 (Ukraine) Method of Strengthening Sheet Metal. Maksimov A.B. Published October 25, 2011. Bulletin No. 20.
[0067] 9. Maksimov A. B. Propagation of cracks in pipes made of heterogeneous material. News of universities. Ferrous metallurgy. No. 7. 2013. P. 53-56.
[0068] 10. Russian Federation Patent No. 2688786. Method for Improving Crack Resistance of Thick Plate Products. Maksimov A.B., Dymnichenko L.G., Erokhina I.S. Published. Bulletin No. 9. Date: 25.03.2019.
[0069] 11. Stanchits S.A. Radiation of elastic waves by growing cracks. Abstract of Cand. Sci. (Phys. and Mathematics): 01.04.07. Leningrad. USSR Academy of Sciences. A.F. Ioffe Phys. and Tech. Institute. Leningrad. 1990. 16 p.
[0070] 12. Maksimov A.B. Thermally reinforced thick-sheet rolled products from low-alloy steels. Materials Science Issues. 2010. No. 3, pp. 40-44.
[0071] 13. Maksimov A.B. Destruction of thermally reinforced steel. News of universities. Ferrous metallurgy. 2014. No. 5. P. 12-14.
[0072] 14. Sivukhin D. V. General course of physics. - 5th ed. Moscow: Fizmatlit, 2005. V. 2. 544 p.
[0073] 15. EFIMENKO L.A., PRYGAEV A.K. Determination of actual mechanical properties of pipeline metal based on hardness measurements. https: / / www.gubkin.ru / faculty / mechanical_engineering / chairs_and_departments / welding_and_oil-field_facility_monitoring / files / 2019-06-11 / uch_pos_2.pdf
[0074] 16. Vyboishchikov M. Ya. Corrosion-mechanical destruction of low-carbon bainite. Tolyattin State University. https: / / docs.yandex.ru / docs / view?tm=1758975611&tld=ru&lang=ru&name=4c3917b8deb8722c5e135fdf4ab47dfe.pdf&text=
[0075] 17. Thermotechnical reference book. Ed. Yureneva V.N. and Lebedeva P.D. -M.: Energy. 1976. T2. - 896 s.
Claims
A method for producing a pipe with a diameter of 114-1420 mm, which includes, after hot rolling or heating to the austenitizing temperature, accelerated cooling of two thick sheets in such a way that the microstructure of the steel changes from ferrite-bainitic to ferrite-pearlite across the width of each sheet, characterized in that two-sided symmetrical accelerated cooling is carried out across the width of each sheet in a vertical position, wherein the cooling rate is reduced from one edge of the sheet to the other, then each sheet is bent to obtain a semi-cylinder, the ends of the bent sheets are connected and welded to obtain a pipe, wherein the ends are connected so that the joint with the ferrite-pearlite structure of one bent sheet is connected with the ferrite-bainitic structure of the other bent sheet.