A method for preventing high-speed brazing double-layer welded pipe from cracking during subsequent use

Abstract

The present invention provides a method for preventing cracking of a high-speed brazed double-layer welded pipe during subsequent use. By controlling the brazing temperature, brazing time, and cooling rate during the brazing process of the double-layer welded pipe, the resulting double-layer welded pipe is free of Widmanstätten structure, and the metallographic structure satisfies the following conditions: (1) n>N / 0.7*20, where n represents the number of grains extending through the entire wall thickness of the double-layer welded pipe, and N represents the wall thickness of the double-layer welded pipe; (2) the diameter of the grains is less than 35 μm; and (3) the total area of ​​large and small grains is less than 20% of the total metallographic structure area of ​​the double-layer welded pipe, wherein the diameter of the large grains is greater than 50 μm, and the diameter of the small grains is less than 10 μm. The present invention is used for high-speed manufacturing of double-layer welded pipes, wherein the brazing speed of the double-layer welded pipes is ≥60 m / min, and the brazing production efficiency is high. The double-layer welded pipe can avoid cracking caused by uneven structure during subsequent use, thereby ensuring the safety of the double-layer welded pipe.

Description

Technical Field

[0001] The present application relates to the technical field of double-layer welded pipes, and in particular to a method for preventing high-speed brazing double-layer welded pipes from cracking during subsequent use. Background Art

[0002] Double-walled welded pipe is a high-precision, thin-walled steel pipe manufactured from copper-plated steel strip through a unique rolling and copper brazing process. It is primarily used in automotive brake systems, fuel delivery lines, refrigerator lines, and air conditioning and refrigeration equipment piping. Due to the numerous bending and flaring processes involved in production and use, double-walled welded pipe can sometimes crack. Automotive piping components, particularly brake lines, are critical safety components and have a zero tolerance for cracks.

[0003] The main causes of cracking in double-layer welded pipes during use include aging of the steel pipe, poor brazing, and uneven microstructure. Brazing is performed at high temperatures, exceeding the melting point of copper (1083°C). The faster the brazing speed, the shorter the brazing process. The faster the heating and cooling rates during the brazing process, the more likely the pipe will develop uneven microstructure after brazing. This can lead to cracking during subsequent use, particularly subtle, hidden cracks that pose a safety hazard. Because high-speed brazing can lead to uneven microstructure and other quality issues, many production lines have been forced to slow down. Brazing speeds below 60m / min for double-layer welded pipes are relatively easy to maintain, but production efficiency is low. Therefore, maintaining high brazing speeds while ensuring microstructure uniformity during pipe manufacturing is a pressing issue to ensure safety. Summary of the Invention

[0004] The present application provides a method for preventing high-speed brazing double-layer welded pipes from cracking during subsequent use, so as to solve the technical problem of existing high-speed brazing double-layer welded pipes cracking due to uneven structure during subsequent use.

[0005] In a first aspect, an embodiment of the present application provides a method for preventing cracking of a double-layer welded pipe during high-speed welding, the method comprising:

[0006] Under the conditions of set temperature and set time, high-speed welding is performed on double-layer coiled pipes;

[0007] Cooling the high-speed welded double-layer coiled pipe under a set cooling rate to obtain a double-layer welded pipe with a set metallographic structure; wherein the set cooling rate is obtained based on the phase transition point of the double-layer coiled pipe;

[0008] The metallographic structure of the double-layer welded pipe satisfies at least one of the following conditions: In>N / 0.7*20, where n represents the number of grains passing through the double-layer welded pipe, and N represents the wall thickness of the double-layer welded pipe.

[0009] II. Average grain size <35μm,

[0010] III. The total area of ​​large grains and small grains is <20% of the total metallographic area, the diameter of the large grains is >50 μm, and the diameter of the small grains is <10 μm.

[0011] Optionally, the microstructure of the double-layer welded pipe satisfies:

[0012] N / 0.7*20<n<N / 0.7*40, and\or

[0013] The total area of ​​large grains and small grains is less than 15% of the total metallographic area.

[0014] Optionally, the metallographic structure of the double-layer welded pipe satisfies:

[0015] N / 0.7*20<n<N / 0.7*30, and\or

[0016] The total area of ​​large grains and small grains is less than 10% of the total metallographic area.

[0017] Optionally, the metallographic structure of the double-layer welded pipe satisfies the following requirements: no Widmanstätten structure.

[0018] Optionally, the set temperature is greater than 1100° C., and the set time is less than or equal to 33 seconds; wherein,

[0019] If the set temperature is greater than 1200°C, the set time is ≤ 20s;

[0020] If the set temperature is greater than 1100° C. and less than or equal to 1200° C., the set time is less than or equal to 33 seconds.

[0021] Optionally, the relationship for obtaining the set cooling rate according to the phase transition point of the double-layer coiled tube is:

[0022] The cooling rate of the double-layer coiled tube from Ar3 to Ar1 temperature stage is set, wherein the cooling rate is ≤40°C / s.

[0023] Optionally, the welding speed of the high-speed welding is ≥60m / min.

[0024] Optionally, the chemical composition of the steel matrix of the double-layer coiled tube is calculated by mass fraction as follows: C≤0.08%, Si≤0.03%, Mn≤0.5%, P≤0.03%, S≤0.03%, Alt≥0.02%, and the balance is Fe and unavoidable impurities;

[0025] The metallographic structure of the steel matrix of the double-layer coiled tube before welding includes ferrite, and the average grain diameter of the ferrite is less than 20 μm.

[0026] Optionally, the wall thickness of the double-layer welded pipe is 0.6 mm to 0.8 mm.

[0027] In a second aspect, an embodiment of the present application provides a double-layer welded pipe obtained by the method of the first aspect.

[0028] The above technical solutions provided by this application and embodiments have the following advantages over the prior art:

[0029] The method provided in the embodiment of the present application controls the brazing temperature, brazing time and cooling rate during the brazing process of the double-layer welded pipe, so that the double-layer welded pipe obtained does not have Widmanstätten structure, and the metallographic structure satisfies at least one of the following: (1) n>N / 0.7*20, where n represents the number of grains that penetrate the entire wall thickness of the double-layer welded pipe, and N represents the wall thickness of the double-layer welded pipe; (2) the diameter of the grains is <35μm; (3) the total area of ​​large grains and small grains is <20% of the total area of ​​the metallographic structure of the double-layer welded pipe, the diameter of the large grains is >50μm, and the diameter of the small grains is <10μm. The present application is used for high-speed manufacturing of double-layer welded pipes, the brazing speed of the double-layer welded pipes is ≥60m / min, and the brazing production efficiency is high. The double-layer welded pipe can avoid cracking problems caused by uneven structure in subsequent use, thereby ensuring the safety of the double-layer welded pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0031] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0032] Figure 1 Schematic diagram of the cross section of a double-layer welded pipe provided for this application;

[0033] Figure 2 A partial cross-sectional view of a double-layer welded pipe provided for this application;

[0034] Figure 3 Brazing process at different brazing heating temperatures at a pipe making speed of 80 m / min

[0035] Figure 4 Brazing process at different pipe making speeds

[0036] Figure 5 The metallographic structure of the double-layer welded pipe obtained in Example 1 of the present invention.

[0037] Figure 6The metallographic structure of the double-layer welded pipe obtained in Example 2 of the present invention.

[0038] Figure 7 The metallographic structure of the double-layer welded pipe obtained in Example 3 of the present invention.

[0039] Figure 8 The metallographic structure of the double-layer welded pipe obtained in Example 4 of the present invention.

[0040] Figure 9 Metallographic structure of the double-layer welded pipe obtained in Example 5 of the present invention.

[0041] Figure 10 The metallographic structure of the double-layer welded pipe obtained in Example 6 of the present invention. DETAILED DESCRIPTION

[0042] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0043] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0044] In a first aspect, the present application provides a method for preventing high-speed brazing double-layer welded pipes from cracking during subsequent use, the method comprising:

[0045] Under the conditions of set brazing temperature and set brazing time, the double-layer coiled tube is brazed at high speed;

[0046] During high-speed brazing, the double-layer coiled tube is heated and then cooled, and the cooling rate between the phase transition points Ar3 and Ar1 of the double-layer coiled tube is controlled to obtain a double-layer welded tube with a set metallographic structure;

[0047] The metallographic structure of the double-layer welded pipe after high-speed brazing satisfies the relationship: n>N / 0.7*20,

[0048] Where n represents the number of grains that penetrate the entire wall thickness of the double-layer welded pipe, N represents the wall thickness of the double-layer welded pipe,

[0049] The diameter of the grains of the double-layer welded pipe after high-speed brazing is less than 35 μm, the grains include large grains and small grains, and the total area of ​​the large grains and small grains is less than 20% of the total metallographic area of ​​the double-layer welded pipe.

[0050] When brazing speeds for double-layer welded pipes are less than 60 m / min, ensuring uniformity of the pipe's microstructure is relatively easy, but production efficiency is low. When brazing speeds are ≥60 m / min, the pipe is prone to uneven microstructure after brazing, leading to cracking during subsequent use. The double-layer welded pipe of this embodiment, capable of high-speed brazing at welding speeds ≥60 m / min, achieves high production efficiency and reduces production costs. This embodiment simultaneously balances both ensuring microstructure quality and high-speed pipe production, resolving the conflict between low cost and high quality.

[0051] In this embodiment, the preparation process of the double-layer welded pipe is mainly as follows: the steel coil is cut into narrow strips of a certain width after being uncoiled, the narrow strips are copper-plated on both sides, and the copper-plated narrow strips are rolled into double-layer coiled pipes on a rolling mill. The double-layer coiled pipes enter the welding equipment for brazing, and the temperature is heated to above the melting point of copper. After the copper melts, the two layers of the double-layer coiled pipe are welded together. The welded pipe is a double-layer welded pipe. After the surface of the double-layer welded pipe is galvanized and covered with an organic coating, it is processed into parts such as automobile brake pipes.

[0052] In this embodiment, the phase transformation point of the double-layer coil refers to the starting temperature Ar3 of the transformation from austenite to ferrite and the ending temperature Ar1 of the transformation from austenite to ferrite. The purpose is to determine the accurate temperature range for adjusting the cooling rate in the cooling stage during the welding process.

[0053] In this embodiment, the metallographic structure of the double-layer coiled pipe satisfies the following: n>N / 0.7*20, where n represents the number of grains penetrating the double-layer welded pipe, and N represents the wall thickness of the double-layer welded pipe in mm. The equation N / 0.7*20 means dividing N by 0.7 to obtain the quotient and then multiplying it by 20, where " / " represents a division sign and "*" represents a multiplication sign. The schematic cross-sectional view and partial cross-sectional view of the double-layer welded pipe obtained in this embodiment are shown in FIG. Figure 1 and Figure 2 As shown in the figure, the average grain size of the double-layer welded pipe after welding is less than 35μm, and the total area of ​​large grains exceeding 50μm and small grains less than 10μm must be less than 20% of the total metallographic area. These are all regulations for "organization uniformity". The more uniform the grains, the less likely the double-layer welded pipe is to crack.

[0054] Preferably, the metallographic structure of the double-layer welded pipe after high-speed brazing satisfies the relationship: N / 0.7*20<n<N / 0.7*40, and the total area of ​​the large and small grains is less than 15% of the total metallographic structure area of ​​the double-layer welded pipe. For example, when N is 0.7 mm, 20<n<40, meaning the number of grains extending through the entire wall thickness of the double-layer welded pipe is 20-40, and the average grain size is 17.5-35 μm. In this case, the grain size is more appropriate, with fewer large and small grains, and the uniformity of the structure is better. If n<20, meaning the number of grains extending through the entire wall thickness of the double-layer welded pipe is less than 20, then the average grain size is greater than 35 μm, and the pipe strength is relatively low. If n>40, meaning the number of grains extending through the entire wall thickness of the double-layer welded pipe is greater than 40, then the average grain size is less than 17.5 μm, and the pipe strength is relatively high.

[0055] Optimally, the metallographic structure of the double-layer welded pipe after high-speed brazing satisfies the relationship: N / 0.7*20 < n < N / 0.7*30, and the total area of ​​the large and small grains is less than 10% of the total metallographic area of ​​the double-layer welded pipe. Specifically, if N is 0.7 mm, 20 < n < 30, meaning the number of grains extending through the entire wall thickness of the double-layer welded pipe is 20-30, and the average grain size is 23.3-35 μm.

[0056] In the present invention and embodiments, the metallographic structure of the double-layer welded pipe does not include Widmanstatten structure. The present invention and embodiments avoid the generation of Widmanstatten structure in order to avoid cracking caused by the structure problem.

[0057] In the present invention and embodiments, the relationship between brazing temperature and brazing time is coordinated. When the brazing temperature is greater than 1100°C, the total time is ≤33s, and when the brazing temperature is greater than 1200°C, the total time is ≤20s. This is to ensure the uniformity of the microstructure of the high-speed brazed double-layer pipe and obtain the desired microstructure. Figure 3 The brazing process at different brazing heating temperatures when the pipe making speed is 80m / min. The brazing process includes a heating stage and a cooling stage. The higher the temperature in the brazing heating stage or the longer the time at high temperature, the coarser the grains of the double-layer pipe after brazing, and the more likely it is to have uneven structure.

[0058] In the present invention and embodiments, the cooling rate is ≤40°C / s, the purpose of which is to ensure the uniformity of the structure of the double-layer welded pipe during high-speed brazing, especially to avoid the formation of Widmanstätten structure. Figure 4Brazing process under different pipe making speeds. For a brazing production line of a certain length, the faster the pipe making speed, the shorter the time the double-layer pipe undergoes brazing heating and cooling, and the faster the heating rate and cooling rate. During the brazing cooling stage, if the cooling rate from Ar3 to Ar1 temperature is greater than 40℃ / s, Widmanstätten structure is likely to appear. For example, if the phase transformation points Ar3 and Ar1 of the steel for double-layer welded pipe are measured to be 906℃ and 871℃, the maximum cooling rate from 906℃ to 871℃ should be controlled to be ≤40℃ / s during the brazing cooling stage.

[0059] In the present invention and embodiments, the diameter of the large grains is greater than 50 μm, and the diameter of the small grains is less than 10 μm.

[0060] In the invention and embodiments of the present application, the chemical composition of the raw material steel strip used for the double-layer welded pipe is as follows by mass fraction: C≤0.08%, Si≤0.03%, Mn≤0.5%, P≤0.03%, S≤0.03%, Alt≥0.02%, and the balance is Fe and unavoidable impurities.

[0061] C≤0.08%. If the C content is too high, the strength of the steel strip will be too high, which is not conducive to coil forming.

[0062] The Si content seriously impairs the plasticity and formability of the cold-rolled steel strip, so Si is limited to 0.03%.

[0063] Mn can form MnS particles to eliminate the harmful effects of the inevitable S in steel. Mn is also a strengthening element, but its content should not be too high, otherwise the strength of the steel strip will be too high, which is not conducive to coiling. Therefore, the present invention limits the Mn content to Mn ≤ 0.5%.

[0064] Phosphorus and sulfur are both harmful elements in steel. In cold-rolled steel strips, it is desired to control these two elements at a low level. However, considering the actual process control capability, the present invention limits P to ≤ 0.03% and S to ≤ 0.03%.

[0065] Aluminum has the function of deoxidation, and aluminum can form aluminum nitride with free nitrogen to fix a portion of nitrogen atoms, reducing the number of solid-solution nitrogen atoms, thereby facilitating improvement of the aging resistance of the steel plate. The present invention limits Alt to ≥ 0.02%.

[0066] In the present invention and examples, the metallographic structure of the steel strip used as the raw material for the double-layer welded pipe is ferrite, with a ferrite grain diameter of less than 20 μm. If the grains in the steel strip are too large, the grains in the steel strip coiled pipe after brazing heat treatment may be too coarse. To achieve a double-layer welded pipe with a grain diameter of less than 35 μm, the grain diameter of the steel strip must be less than 20 μm.

[0067] In the present invention and embodiments, the wall thickness of the double-layer welded pipe is 0.6 mm to 0.8 mm.

[0068] In a second aspect, the present application provides a product comprising a double-layer welded pipe obtained by the method described in any one of the first aspects.

[0069] The double-layer welded pipe is realized based on the above-mentioned method for preventing cracking of the high-speed brazed double-layer welded pipe during subsequent use. The specific steps of the method for preventing cracking of the high-speed brazed double-layer welded pipe during subsequent use can be referred to the above-mentioned embodiment. Since the double-layer welded pipe adopts part or all of the technical solutions of the above-mentioned embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above-mentioned embodiments, which will not be repeated here.

[0070] The present application will be further described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present application only and are not intended to limit the scope of the present application. The experimental methods in the following examples where specific conditions are not specified are usually measured in accordance with national standards. If there are no corresponding national standards, then the methods are carried out in accordance with general international standards, conventional conditions, or according to the conditions recommended by the manufacturer.

[0071] Example 1

[0072] The double-layer welded pipe was manufactured at a speed of 60 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 26 seconds, and 1100°C for 33 seconds. The phase transition points Ar3 and Ar1 of the steel used in the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 35°C / s. The resulting double-layer welded pipe, brazed using the aforementioned method, exhibited ferrite microstructure with no Widmanstätten structure. The microstructure of the double-layer welded pipe was non-uniform, with an average grain size exceeding 35 μm. Large grains larger than 50 μm accounted for more than 60% of the total metallographic area. Due to the coarse and severely non-uniform grain structure, measuring the number of grains throughout the entire wall thickness of the double-layer pipe was meaningless. The double-layer welded pipe subsequently cracked during use.

[0073] Example 2

[0074] The double-layer welded pipe was manufactured at a speed of 70 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 20 seconds, and 1100°C for 40 seconds. The phase transition points Ar3 and Ar1 of the steel used for the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 40°C / s. The resulting double-layer welded pipe, brazed using the aforementioned method, exhibited ferrite microstructure with no Widmanstätten structure. The microstructure of the double-layer welded pipe was non-uniform, with large grains larger than 50 μm accounting for more than 30% of the total metallographic area. Due to the coarse and severely non-uniform grain structure, measuring the number of grains throughout the entire wall thickness of the double-layer pipe was meaningless. The double-layer welded pipe subsequently cracked during use.

[0075] Example 3

[0076] The double-layer welded pipe was manufactured at a speed of 100 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 17 seconds, and 1100°C for 27 seconds. The phase transition points Ar3 and Ar1 of the steel used in the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 50°C / s. The resulting double-layer welded pipe, brazed using the aforementioned method, exhibited a microstructure composed of ferrite and widmanstattenite. The microstructure of the double-layer welded pipe was non-uniform, with large grains larger than 50 μm accounting for more than 30% of the total metallographic area. Due to the presence of widmanstatten structure, measuring the number of grains extending through the entire wall thickness of the double-layer pipe was meaningless. The double-layer welded pipe subsequently cracked during use.

[0077] Example 4

[0078] The double-layer welded pipe was manufactured at a speed of 110 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 16 seconds, and 1100°C for 26 seconds. The phase transition points Ar3 and Ar1 of the steel used in the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 60°C / s. The resulting double-layer welded pipe, brazed using the aforementioned method, exhibited a microstructure composed of ferrite and widmanstattenite. The microstructure of the double-layer welded pipe was non-uniform, with large grains larger than 50 μm accounting for more than 20% of the total metallographic area. Due to the presence of widmanstatten structure, measuring the number of grains extending through the entire wall thickness of the double-layer pipe was meaningless. The double-layer welded pipe subsequently cracked during use.

[0079] Example 5

[0080] The double-layer welded pipe was manufactured at a speed of 90 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 18 seconds, and 1100°C for 30 seconds. The phase transition points Ar3 and Ar1 of the steel used for the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 40°C / s. The resulting double-layer welded pipe exhibited a uniform ferrite structure with an average grain size of less than 35 μm. The combined area of ​​large and small grains was less than 10% of the total metallographic area. Thirty grains were found to extend throughout the entire wall thickness of the double-layer pipe. The double-layer welded pipe exhibited no cracking during subsequent use.

[0081] Example 6

[0082] The double-layer welded pipe was manufactured at a speed of 80 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 20 seconds, and 1100°C for 33 seconds. The phase transition points Ar3 and Ar1 of the steel used for the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 35°C / s. The resulting double-layer welded pipe exhibited a uniform ferrite structure with an average grain size of less than 35 μm. The combined area of ​​large and small grains was less than 10% of the total metallographic area. Twenty-nine grains penetrated the entire wall thickness of the double-layer pipe. The double-layer welded pipe exhibited no cracking during subsequent use.

[0083] Example 7

[0084] The double-layer welded pipe was manufactured at a speed of 90 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 17 seconds, and 1100°C for 28 seconds. The phase transition points Ar3 and Ar1 of the steel used for the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 40°C / s. The resulting double-layer welded pipe exhibited a uniform ferrite structure with an average grain size of less than 35 μm. The combined area of ​​large and small grains was less than 15% of the total metallographic area. Thirty-one grains penetrated the entire wall thickness of the double-layer pipe. The double-layer welded pipe exhibited no cracking during subsequent use.

[0085] Example 8

[0086] The double-layer welded pipe was manufactured at a speed of 70 m / min. The total duration of the brazing process, with temperatures exceeding 1200°C for 20 seconds, and 1100°C for 33 seconds. The phase transition points Ar3 and Ar1 of the steel used for the double-layer welded pipe were measured to be 906°C and 871°C, respectively. The maximum cooling rate during the Ar3 to Ar1 temperature range was 40°C / s. The resulting double-layer welded pipe exhibited a uniform ferrite structure with an average grain size of less than 35 μm. The combined area of ​​large and small grains was less than 10% of the total metallographic area. Twenty-one grains penetrated the entire wall thickness of the double-layer pipe. The double-layer welded pipe exhibited no cracking during subsequent use.

[0087] Related experiments and effect data in Examples 1 to 8:

[0088]

[0089]

[0090] On the one hand, the present application provides a method for preventing cracking of a high-speed brazed double-layer welded pipe during subsequent use. The obtained high-speed brazed double-layer welded pipe has good microstructure uniformity and no Widmanstätten structure is produced, thereby avoiding the cracking problem caused by microstructure unevenness and ensuring the safety of the double-layer welded pipe. On the other hand, the present invention is used in situations where the manufacturing speed of the double-layer welded pipe is ≥60m / min. Since high-speed brazing can be performed, the pipe production efficiency is high and the production cost is reduced. On the other hand, the present invention takes into account both the microstructure quality of the double-layer welded pipe and the high-speed welding of the pipe, solving the contradiction between low cost and high quality.

[0091] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a hard limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numbers within the range. For example, the description of a range from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0092] In this application, unless otherwise specified, the directional words used, such as "upper" and "lower", refer specifically to the directions of the drawings in the accompanying drawings. In addition, in the description of the present application specification, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second" are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" refers to one or more, and "plurality" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple.

[0093] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A method for preventing cracking of a double-layer welded pipe during high-speed brazing, characterized in that: The method comprises: Under the conditions of set temperature and set time, the double-layer coiled tube is brazed at high speed; Cooling the high-speed brazed double-layer coiled pipe under a set cooling rate to obtain a double-layer welded pipe with a set metallographic structure; wherein the set cooling rate is obtained based on the phase transition point of the double-layer coiled pipe; The metallographic structure of the double-layer welded pipe satisfies at least one of the following conditions: I. n>N / 0.7*20, where n represents the number of grains passing through the double-layer welded pipe, and N represents the wall thickness of the double-layer welded pipe. II. Average grain size <35μm, III. The total area of ​​large grains and small grains is <20% of the total metallographic area, the diameter of the large grains is >50μm, and the diameter of the small grains is <10μm; The set temperature is greater than 1100°C, and the set time is less than or equal to 33s; wherein, If the set temperature is greater than 1200°C, the set time is ≤ 20s; If the set temperature is greater than 1100°C and less than or equal to 1200°C, the set time is less than or equal to 33s; The relationship for obtaining the set cooling rate according to the phase transition point of the double-layer coiled tube is: Set the cooling rate of the double-layer coil from Ar3 to Ar1 temperature stage, where the cooling rate is ≤ 40℃ / s; The welding speed of the high-speed brazing is ≥ 60m / min; The metallographic structure of the steel matrix of the double-layer coiled tube before welding includes ferrite, and the average grain diameter of the ferrite is less than 20 μm; The metallographic structure of the double-layer welded pipe satisfies the following requirements: no Widmanstätten structure is contained.

2. The method according to claim 1, characterized in that The microstructure of the double-layer welded pipe meets the following requirements: N / 0.7*20<n<N / 0.7*40, and\or The total area of ​​large grains and small grains is less than 15% of the total metallographic area.

3. The method according to claim 2, characterized in that The metallographic structure of the double-layer welded pipe satisfies: N / 0.7*20<n<N / 0.7*30, and\or The total area of ​​large grains and small grains is less than 10% of the total metallographic area.

4. The method according to claim 1, wherein The chemical composition of the steel matrix of the double-layer coiled tube is calculated by mass as follows: C≤0.08%, Si≤0.03%, Mn≤0.5%, P≤0.03%, S≤0.03%, Alt≥0.02%, and the remainder is Fe and unavoidable impurities.

5. The method according to claim 1, wherein The wall thickness of the double-layer welded pipe is 0.6mm-0.8mm.

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