Method for welding a metal pipe

Abstract

This invention discloses a method for welding metal pipelines. The metal pipelines to be welded include a main pipe and branch pipes. The main pipe has a joint on its side wall, which includes a flared section and a straight section. The thermal expansion coefficient of the branch pipe material is greater than that of the joint. The welding method includes: inserting the branch pipe into the straight section to form an assembly gap; heating the flared section, with heat conducted to the main pipe, resulting in a lower temperature at the bottom of the straight section; the solder melting and penetrating into the gap; under continuous heating, the large radial thermal expansion of the branch pipe causes the assembly gap to contract and compress the liquid solder, while the liquid solder at the bottom solidifies to prevent overflow into the main pipe; when heating stops and cooling occurs, the large radial contraction of the branch pipe causes the gap to widen, creating a suction effect, and the remaining liquid solder continues to penetrate into the gap until the entire structure solidifies. This invention utilizes the thermal expansion difference between dissimilar metals to convert thermal deformation into a driving force for deep and dense penetration of the solder, effectively preventing incomplete welds and pipeline blockage.

Description

Technical Field

[0001] This invention relates to the field of metal pipe welding technology, and more specifically, to a method for welding metal pipes. Background Technology

[0002] In refrigeration, air conditioning, heat pump, and fluid transport piping systems, it is often necessary to connect metal pipes of different materials. Considering corrosion resistance, cost, and manufacturing processes, the main pipes of the piping system are mostly made of stainless steel, while branch pipes used to connect system components such as fittings are mostly made of copper. In the brazing process for these dissimilar metal pipes, due to the difference in thermal expansion coefficients between stainless steel and copper, conventional capillary brazing methods are prone to welding defects.

[0003] Traditional capillary brazing relies primarily on static assembly gaps. During the heating phase, the liquid solder penetrates the gap solely through gravity and capillary forces between the solder and the pipe wall. However, in actual welding, the radial deformation of dissimilar metals upon heating is asynchronous, leading to uncontrollable changes in the assembly gap size during heating and cooling. This asynchronous thermal deformation often disrupts the original capillary action conditions, making it difficult for the solder to overcome surface tension and achieve deep penetration. This easily results in shallow surface welds or even incomplete welds at the pipe joint openings, affecting the pressure-bearing and sealing performance of the pipeline.

[0004] Furthermore, in traditional brazing processes, the penetration depth of molten solder is difficult to control effectively through physical means. To ensure a full weld, a large amount of solder is often supplied to the joint during construction. When excessive molten solder flows downwards along the gaps in the straight pipes, it can easily penetrate the assembly gaps and overflow into the main pipe, causing problems such as internal pipe blockage, reduced flow cross-sectional area, and internal wall contamination. How to achieve deep and dense solder penetration while preventing overflow into the main pipe, while utilizing the thermal deformation characteristics of dissimilar metals, is a technical challenge that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The present invention aims to provide a metal pipeline welding method to solve the technical problems in the prior art of uncontrollable assembly gaps caused by heat deformation during brazing of dissimilar metal pipelines, difficulty in deep penetration of solder, and easy overflow of liquid solder into the main pipe causing pipeline blockage.

[0006] To achieve the above objectives, the present invention provides a metal pipeline welding method, wherein the metal pipeline to be welded includes a main pipe and a branch pipe. The main pipe has an outwardly extending joint on its side wall, and the joint forms a thermally conductive connection with the main pipe. The branch pipe has a larger thermal expansion coefficient than the joint. The joint includes a flared section and a straight pipe section that are connected sequentially in the direction toward the main pipe. The inner diameter of the flared section gradually decreases in the direction toward the main pipe, and the straight pipe section is connected to the interior of the main pipe. The welding method includes: S1. Pass the branch pipe through the flared section and insert it into the straight pipe section so that an assembly gap is formed between the outer wall of the branch pipe and the inner wall of the straight pipe section. S2. The flared section is continuously heated, and the heat is conducted to the main pipe through the straight pipe section, so that the temperature of the straight pipe section near the main pipe is lower than that of the flared section. S3. Provide solder, so that the solder is heated and melted in the flared section, and the liquid solder penetrates into the straight pipe section along the assembly gap; S4. Under the condition of continuous heating of the flared section, the radial thermal expansion of the branch pipe is greater than that of the straight pipe section, so that the assembly gap contracts and squeezes the liquid solder inside it; at the same time, the viscosity of the liquid solder that has penetrated into the assembly gap to the end of the straight pipe section near the main pipe increases and condenses into a solid, so as to prevent the liquid solder in the assembly gap from flowing into the main pipe. S5. Stop heating the flared section. During the cooling process, the radial shrinkage of the branch pipe is greater than that of the straight pipe section, so as to expand the assembly gap. At this time, the remaining liquid solder in the flared section continues to seep into the assembly gap in the direction closer to the main pipe until the solder cools and solidifies as a whole.

[0007] Preferably, the main pipe and the connector are made of stainless steel, and the branch pipe is made of copper.

[0008] Preferably, the connector and the main pipe are integrally formed.

[0009] Preferably, a ring of protrusions is fixedly provided on the outer wall of the branch pipe; In S1, when the branch pipe is inserted into place, the annular protrusion abuts against the open end of the flared section to seal the open end of the flared section.

[0010] Preferably, the welding material is an annular welding rod pre-fixed to the outer wall of the branch pipe, and the annular welding rod abuts against the side of the annular protrusion near the straight pipe section; In S1, when the branch pipe is inserted into place, the annular welding electrode extends into the flared section along with the branch pipe; In S3, the annular welding electrode is heated and melted to form the liquid welding material that penetrates into the straight pipe section along the assembly gap.

[0011] Preferably, in step S2, an induction heating device is used to continuously heat the flared section; The induction coil of the induction heating device is arranged around the periphery of the flared section, and the effective heating area of ​​the induction coil avoids the straight pipe section.

[0012] Preferably, the outer wall of the main tube is clamped with a heat dissipation clamp, which is arranged adjacent to the connector.

[0013] Preferably, the wall thickness of the straight pipe section is greater than the wall thickness of the flared section.

[0014] One or more technical solutions provided in this invention have at least the following technical effects or advantages: During the heating process of the flared section, heat is conducted from the straight section of the joint towards the main pipe. The main pipe's larger volume and surface area facilitate heat dissipation, ensuring that the end of the straight section closest to the main pipe maintains a relatively low temperature. When the liquid solder, seeping downwards along the assembly gap, reaches this relatively low-temperature region, its temperature drops rapidly, causing its viscosity to increase dramatically and solidify. This solidified solder, forming directly at the bottom of the assembly gap, acts as a robust barrier, directly blocking the path of the liquid solder dripping further into the main pipe, thus ensuring the cleanliness and smooth flow of fluids within the main pipe.

[0015] During the continuous heating phase, the assembly gap shrinks because the thermal expansion coefficient of the branch pipe is greater than that of the straight pipe section. This radial shrinkage forces the liquid solder deeper into the straight pipe section, while simultaneously squeezing out air bubbles and impurities from within the liquid solder, reducing weld porosity. During the cooling phase after heating stops, the branch pipe with the larger thermal expansion coefficient experiences a relatively larger radial shrinkage, causing the previously narrowed assembly gap to widen again. This widening gap creates a suction effect, drawing the remaining liquid solder trapped in the upper flared section downwards to fill the internal pores created by the pipe's cooling shrinkage. This synergistic effect of heating compression and cooling suction results in a dense and deep weld connection at the joint, significantly improving the pressure resistance and structural strength of the metal pipe connection. Attached Figure Description

[0016] Figure 1 This is a cross-sectional structural diagram of the main pipe and branch pipe in the assembly and positioning state in an embodiment of the present invention.

[0017] Figure 2 This is a cross-sectional structural diagram of the main pipe and branch pipe in the state of solder melting and penetration in an embodiment of the present invention.

[0018] Explanation of reference numerals in the attached drawings: 10, main pipe; 20, branch pipe; 21, annular protrusion; 22, annular welding rod; 221, liquid solder; 30, joint; 31, flared section; 32, straight pipe section; 40, induction coil; 50, heat dissipation clamp; 60, assembly gap. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the specific embodiments of this invention will be described in detail and rigorously below with reference to the accompanying drawings. It should be noted that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this invention. The physical coordination relationships, thermodynamic mechanisms, and specific process parameters used throughout the description are all intended to provide clear engineering implementation guidance for those skilled in the art.

[0020] Combination Figure 1 and Figure 2 As shown, this invention provides a method for welding metal pipes. The metal pipe to be welded includes a main pipe 10 and a branch pipe 20 that is plugged into and welded to the main pipe 10. A connector 30 extending outwards is provided on the side wall of the main pipe 10, forming a good thermally conductive connection with the main pipe 10. Here, the connector 30 includes a flared section 31 and a straight pipe section 32 that are sequentially connected in the direction toward the main pipe 10. The flared section 31 has an outwardly expanding cup-shaped or conical structure, and its inner diameter gradually decreases in the direction toward the main pipe 10, serving a guiding and accommodating function. The straight pipe section 32 is a pipe section of equal diameter, and its bottom end directly connects to the interior of the main pipe 10.

[0021] Regarding material selection, to spontaneously generate the required thermal expansion difference during heating and cooling, the thermal expansion coefficient of the branch pipe 20 is designed to be greater than that of the connector 30. In this embodiment, both the main pipe 10 and the connector 30 are made of stainless steel, such as austenitic stainless steel 304 or 316L, or ferritic stainless steel 430. The branch pipe 20 is made of copper, such as copper with excellent thermal conductivity and ductility, or brass, bronze, etc. Since the linear expansion coefficient of copper is significantly greater than that of stainless steel, this provides a solid physical basis for the radial mechanical extrusion that occurs during subsequent heating. In alternative solutions, depending on different industrial applications, the main pipe 10 and connector 30 can also be made of low-carbon steel, medium-carbon steel, or nickel-based alloys, while the branch pipe 20 can be made of aluminum or aluminum alloys, which have higher thermal expansion coefficients, as long as there is a significant step difference in the thermal expansion coefficients between the two.

[0022] Furthermore, the connector 30 and the main pipe 10 are preferably integrally formed. In specific processing, the connector 30 is formed by extending the side wall of the main pipe 10 outward through punching and flanging processes, which is a commonly used stretch forming process in the art. When using this process, because the metal pipe wall undergoes progressive plastic elongation during the outward pulling process, the material closer to the top of the stretch is stretched and thinned more drastically, resulting in the wall thickness of the straight pipe section 32 after forming being naturally greater than that of the flared section 31. This difference in wall thickness is of great thermodynamic significance. The thinner flared section 31 has a smaller heat capacity and can heat up rapidly under external high-frequency heating, while the thicker straight pipe section 32 has a smaller thermal resistance and can efficiently conduct heat downward to the main pipe 10, thereby creating a temperature field with a steep gradient in the axial direction of the straight pipe section 32. In another alternative, the joint 30 can also be a separate component that is independently processed and fixed to the side wall of the main pipe 10 by laser welding or argon arc welding. In this case, the difference in wall thickness between the straight pipe section 32 and the flared section 31, which is thinner at the top and thicker at the bottom, can be directly cut by machining.

[0023] In welding operation S1, the branch pipe 20 is passed through the flared section 31 of the connector 30 and axially inserted into the straight pipe section 32. At this time, since the inner diameter of the straight pipe section 32 is slightly larger than the outer diameter of the branch pipe 20, a ring-shaped assembly gap 60 will naturally form between the outer wall of the branch pipe 20 and the inner wall of the straight pipe section 32. The initial single-sided gap of this assembly gap 60 can be designed between 0.05 mm and 0.15 mm according to the pipe diameter. This not only ensures that the branch pipe 20 can be smoothly inserted during manual or mechanical assembly, but also reserves a channel for subsequent capillary penetration.

[0024] During this process, a ring-shaped protrusion 21 is pre-fixed on the outer wall of the branch pipe 20. This ring-shaped protrusion 21 can be formed by local upsetting and extrusion rolling on the copper pipe, or by independently fitting a ring-shaped copper component and laser-fixing it. When the branch pipe 20 is inserted into place, the lower surface of the ring-shaped protrusion 21 tightly abuts against the edge of the opening end of the flared section 31, thus physically sealing the entire upper opening end of the flared section 31. Simultaneously, the welding material used for this welding is a ring-shaped welding rod 22 pre-fixed to the outer wall of the branch pipe 20. This ring-shaped welding rod 22 can be selected according to the welding requirements of the base material, such as silver-based, copper-based, or aluminum-based brazing filler metals, for example, conventional phosphor bronze welding rods or brazing filler metals containing high silver content. The ring-shaped welding rod 22 tightly abuts against the side of the ring-shaped protrusion 21 closest to the straight pipe section 32. After the branch pipe 20 is axially inserted into place, the annular welding electrode 22 is fed into the internal space of the flared section 31 along with the branch pipe 20, and is tightly sealed from above by the annular protrusion 21 in the annular cavity formed between the flared section 31 and the outer wall of the branch pipe 20. This cap-like sealing structure can effectively isolate the external high-temperature air, reduce metal oxidation during the welding process, and constrain the flow of the molten welding material to prevent it from overflowing and contaminating the pipeline surface.

[0025] In step S2, a local heating device is used to continuously heat the flared section 31. To achieve precise local temperature control, this embodiment employs a high-frequency or medium-frequency induction heating device. For example... Figure 2 As shown, the induction coil 40 of the induction heating device is arranged around the periphery of the flared section 31. The number of turns and geometry of the induction coil 40 are optimized so that its effective magnetic field heating area is concentrated on the flared section 31, while its effective magnetic field distribution area completely avoids the straight pipe section 32 below. Since the induction heating is concentrated only on the flared section 31, the flared section 31 heats up rapidly due to its thin wall thickness and direct magnetic field induction. Subsequently, the heat begins to be conducted unidirectionally downwards through the straight pipe section 32 towards the main pipe 10. At this time, the main pipe 10, due to its large volume and surface area, plays the role of a natural heat sink during the welding process. To further enhance this heat dissipation effect, a heat dissipation clamp 50 is clamped and fixed on the outer wall of the main pipe 10 adjacent to the joint 30 throughout the heating and welding process from S2 to S4. This heat dissipation clamp 50 can be made of a solid copper block or aluminum alloy block with excellent thermal conductivity, or a forced cooling clamp with internal circulating cooling water or cryogenic liquid nitrogen. The heat dissipation clamp 50 can quickly remove the heat conducted from the main pipe 10, thereby forcibly lowering the temperature of the end of the straight pipe section 32 closest to the main pipe 10, thus creating a temperature gradient between it and the heated flared section 31 above.

[0026] In S3, as induction heating continues, the annular welding rod 22, enclosed inside the flared section 31, reaches its melting point and begins to melt within the flared section 31. The melted annular welding rod 22 transforms into liquid solder 221. Since the annular protrusion 21 has sealed the upper opening of the flared section 31, the liquid solder 221, under its own weight and the downward pressure of the annular protrusion 21, cannot overflow or splash outwards. Instead, it flows entirely into the assembly gap 60 between the straight pipe section 32 and the branch pipe 20 in the direction towards the main pipe 10, and penetrates deep into the interior of the straight pipe section 32 by relying on strong capillary suction.

[0027] In step S4, while the induction coil 40 continuously heats the flared section 31, the inner copper branch pipe 20 also gradually heats up at high temperatures. Since the thermal expansion coefficient of copper is significantly greater than that of the stainless steel connector 30, under the same ambient temperature, the radial thermal expansion of the branch pipe 20 is significantly greater than that of the straight pipe section 32. This causes the originally small assembly gap 60 to begin to shrink dramatically in the radial dimension, even tending towards an interference fit. This spontaneous shrinkage of the assembly gap 60 exerts a strong mechanical squeezing effect on the liquid solder 221 that is penetrating downwards inside. This microscopic radial extrusion force not only greatly increases the hydrodynamic pressure of the liquid solder 221 in the gap, driving it to be powerfully pushed into the deeper part of the straight pipe section 32 to achieve ultra-deep brazing, but also acts like a piston to force the flux residue, microbubbles and volatile gases trapped inside the liquid solder 221 to be squeezed upward or downward, thereby significantly eliminating porosity and slag inclusion defects in the brazing seam and making the weld structure extremely dense.

[0028] Meanwhile, as the squeezed liquid solder 221 flows rapidly downwards and penetrates to the edge of the straight pipe section 32 near the main pipe 10, the latent heat of phase change and sensible heat of the flowing liquid solder 221 are instantly dissipated because this area is adjacent to the low-temperature zone jointly constructed by the heat dissipation clamp 50 and the main pipe 10. This causes the temperature of the liquid solder 221 in this area to drop below the solidus line in a very short time, causing its viscosity to increase sharply and rapidly condense into a solid. This solid solder ring, which is formed in situ at the bottom of the assembly gap 60, is equivalent to creating a physical flow barrier at the junction of the straight pipe section 32 and the main pipe 10, completely blocking the outlet below. Thus, under continuous squeezing, it successfully prevents any excessive liquid solder 221 from flowing into the interior of the main pipe 10, effectively eliminating the pipe blockage and contamination caused by dripping inside the main pipe cavity in traditional processes.

[0029] After deep penetration and bottom blocking are completed, proceed to S5, where the heating of the flared section 31 by the induction coil 40 is stopped. During the overall cooling process of the pipeline, because the linear shrinkage rate of copper is significantly greater than that of stainless steel, the radial shrinkage rate and amount of the inner branch pipe 20 are significantly greater than those of the outer straight pipe section 32. This causes the assembly gap 60, which had narrowed in S4, to expand again. When the assembly gap 60 expands outward instantaneously, a microscopic local negative pressure suction effect is generated inside. At this time, the remaining liquid solder that was squeezed in the upper flared section 31 in S4, under the strong pull of this negative pressure suction, will continue to seep downwards along the direction closer to the main pipe 10 into the gap connecting the flared section 31 and the straight pipe section 32. Because the annular protrusion 21 has a large local mass, it acts as a heat storage unit after welding stops, continuously releasing its stored heat into the flared section 31. This effectively slows down the cooling rate of the remaining liquid solder and prolongs its liquid state time. This ensures that even at the critical moment when the assembly gap 60 widens due to cooling, the remaining solder above maintains good fluidity, continuously replenishing and filling the gap, thus perfectly filling the microscopic shrinkage pores caused by the overall cooling of the pipe body and the initial solidification and shrinkage of the bottom solder. As the temperature finally drops to room temperature, the solder completely cools and solidifies, forming a non-porous, non-fractured, deeply bonded, and highly pressure-resistant dissimilar metal brazed joint between the flared section 31, the straight pipe section 32, and the branch pipe 20.

Claims

1. A method for welding a metal pipeline, wherein the metal pipeline to be welded includes a main pipe (10) and a branch pipe (20), the side wall of the main pipe (10) is provided with an outwardly extending joint (30), the joint (30) and the main pipe (10) form a thermally conductive connection, and the branch pipe (20) has a greater coefficient of thermal expansion than the joint (30), characterized in that, The connector (30) includes a flared section (31) and a straight section (32) connected sequentially in the direction toward the main pipe (10). The inner diameter of the flared section (31) gradually decreases in the direction toward the main pipe (10), and the straight section (32) connects to the interior of the main pipe (10). The welding method includes: S1. Pass the branch pipe (20) through the flared section (31) and insert it into the straight pipe section (32) so that an assembly gap is formed between the outer wall of the branch pipe (20) and the inner wall of the straight pipe section (32); S2. The flared section (31) is continuously heated, and the heat is conducted to the main pipe (10) through the straight pipe section (32) so that the temperature of the end of the straight pipe section (32) closer to the main pipe (10) is lower than that of the flared section (31); S3. Provide solder, so that the solder is heated and melted in the flared section (31), and the liquid solder penetrates into the straight pipe section (32) along the assembly gap; S4. Under the condition of continuous heating of the flared section (31), the radial thermal expansion of the branch pipe (20) is greater than that of the straight pipe section (32), so that the assembly gap shrinks and squeezes the liquid solder inside it; at the same time, the viscosity of the liquid solder that has penetrated into the straight pipe section (32) near the main pipe (10) increases and condenses into a solid, so as to prevent the liquid solder in the assembly gap from flowing into the main pipe (10). S5. Stop heating the flared section (31). During the cooling process, the radial shrinkage of the branch pipe (20) is greater than that of the straight pipe section (32) so that the assembly gap is expanded. At this time, the remaining liquid solder in the flared section (31) continues to seep into the assembly gap in the direction closer to the main pipe (10) until the solder cools and solidifies as a whole.

2. The welding method according to claim 1, characterized in that, The main pipe (10) and the connector (30) are made of stainless steel, and the branch pipe (20) is made of copper.

3. The welding method according to claim 1 or 2, characterized in that, The connector (30) and the main pipe (10) are integrally formed.

4. The welding method according to claim 1, characterized in that, A ring-shaped protrusion (21) is fixedly provided on the outer wall of the branch pipe (20); In S1, when the branch pipe (20) is inserted into place, the annular protrusion (21) abuts against the opening end of the flared section (31) to block the opening end of the flared section (31).

5. The welding method according to claim 4, characterized in that, The welding material is an annular welding rod (22) pre-fixed to the outer wall of the branch pipe (20), and the annular welding rod (22) abuts against the side of the annular protrusion (21) near the straight pipe section (32); In S1, when the branch pipe (20) is inserted into place, the annular welding rod (22) extends into the flared section (31) along with the branch pipe (20); In S3, the annular welding electrode (22) is heated and melted to form the liquid welding material that penetrates into the straight pipe section (32) along the assembly gap.

6. The welding method according to claim 1, characterized in that, In S2, an induction heating device is used to continuously heat the flared section (31); The induction coil (40) of the induction heating device is arranged around the periphery of the flared section (31), and the effective heating area of ​​the induction coil (40) avoids the straight pipe section (32).

7. The welding method according to claim 1, characterized in that, The outer wall of the main tube (10) holds a heat dissipation clamp (50), which is arranged adjacent to the connector (30).

8. The welding method according to claim 1, characterized in that, The wall thickness of the straight pipe section (32) is greater than the wall thickness of the flared section (31).

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