Stainless clad steel pipe for heat exchanger and method for manufacturing the same

Abstract

The application discloses a stainless steel composite steel pipe for a heat exchanger, belongs to the field of composite material preparation, and further discloses a preparation method of the stainless steel composite steel pipe for the heat exchanger. The preparation method comprises seven steps of stainless steel composite plate / belt preparation, steel / belt edge cutting, pipe rolling, welding, heating, hot working and heat treatment. Through special design of the welded pipe material, the stainless steel layers on the inner and outer sides of the welded pipe can effectively guarantee the corrosion resistance of the pipe material, the middle layer is copper and carbon steel (low alloy steel), the heat conduction performance of the welded pipe can be obviously improved, meanwhile, the carbon steel (low alloy steel) in the middle part can also well guarantee the pipe strength and reduce the material cost, welding defects can be obviously eliminated after the hot working and heat treatment, the thickness of the inner and outer stainless steel corrosion resistant layers is ensured, and the comprehensive performance of the welded pipe is improved.

Description

Technical Field

[0001] This invention belongs to the field of composite material preparation, specifically relating to a stainless steel composite pipe for heat exchangers and its preparation method. Background Technology

[0002] Straight seam welded pipe is a type of pipe made by rolling hot-rolled or cold-rolled steel plates or strips into tubular shapes and then welding them together with straight seams on welding equipment. It gets its name from the straight line formed by the weld joint. Depending on its intended use, it undergoes different subsequent production processes. The standard for austenitic stainless steel welded pipes used in heat exchangers is GB / T24593-2018.

[0003] Currently, most composite pipes on the market are carbon steel-stainless steel composite pipes, with stainless steel inside that is corrosion resistant, while carbon steel outside is not corrosion resistant. Most stainless steel pipes used in heat exchangers are pure stainless steel pipes. Due to the high price of stainless steel and its poor thermal conductivity, the research and development of new composite steel pipes has broad prospects.

[0004] This invention uses copper and carbon steel (low alloy steel) as the intermediate layer, which can improve the thermal conductivity of the pipe and increase the thermal efficiency; at the same time, the inner and outer stainless steel layers can also ensure that the pipe has good corrosion resistance. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a stainless steel composite steel pipe for heat exchangers and its preparation method. Through special design of the welded pipe material, the inner and outer layers of the welded pipe are made of stainless steel, which can effectively ensure the corrosion resistance of the pipe. The middle layer is made of copper and carbon steel (low alloy steel), which can significantly improve the thermal conductivity of the welded pipe. At the same time, the carbon steel (low alloy steel) in the middle can also ensure the strength of the pipe and reduce the material cost. After hot working and heat treatment, welding defects can be significantly eliminated, ensuring the thickness of the inner and outer stainless steel corrosion-resistant layers and improving the overall performance of the welded pipe.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A stainless steel composite steel pipe for heat exchangers consists of a five-layer structure: an outer layer of stainless steel 1, a second layer of copper 2, a third layer of carbon steel or low alloy steel 3, a fourth layer of copper 4, and an inner layer of stainless steel 5.

[0008] Furthermore, the outer stainless steel layer 1 has a thickness of 0.2-1.5 mm, the inner stainless steel layer 5 has a thickness of 0.3-2.0 mm, the third carbon steel or low alloy steel layer 3 has a thickness of 0.3-8.0 mm, and the total thickness of the second copper layer 2 and the fourth copper layer 4 is 0.3-3.0 mm.

[0009] Furthermore, the ratio of the thickness of the third layer of carbon steel or low alloy steel 3 to the total thickness of the second layer of copper 2 and the fourth layer of copper 4 is 1-8:1.

[0010] A method for preparing a stainless steel composite steel pipe for heat exchangers, the method comprising the following steps:

[0011] Step 1, Preparation of stainless steel composite plate / strip: Stainless steel plate, copper plate, carbon steel or low alloy steel plate are assembled into stainless steel composite billet according to the design thickness, and then the composite billet is prepared into composite steel plate / strip of the required thickness through rolling process.

[0012] Step 2, Steel / Strip Edge Trimming: Remove the oxide scale and impurities from the edges of the stainless steel composite plate / strip, and cut the plate / strip to the required width;

[0013] Step 3, Tube Rolling: Use a plate rolling machine to roll the stainless steel composite plate / strip into a tube shape;

[0014] Step 4, Welding: The pipe is welded into shape using high-frequency welding or resistance welding.

[0015] Step 5, Heating: Heat the welded pipe to 500-650℃ and hold for 0.5-2 hours to ensure the pipe is thoroughly heated;

[0016] Step 6, Hot working: The heated pipe is processed to the required size by drawing and tension reduction to obtain the composite pipe;

[0017] Step 7, heat treatment: The obtained composite pipe is normalized by heating to 850-950℃ and holding for 0.5-3 hours, and then rapidly cooled after being taken out of the furnace.

[0018] Furthermore, in step four, nitrogen or an inert gas is used to protect the weld during welding.

[0019] Furthermore, in step six, the drawing and tension reduction methods employ drawing dies and reducing rollers, and the drawing dies and reducing rollers need to be cooled by a medium to control the temperature below 100°C.

[0020] Furthermore, in step six, the total wall thickness reduction of the composite pipe after cold working is greater than 0.5 mm, and the wall thickness reduction is greater than 25%.

[0021] Furthermore, in step seven, the rapid cooling method adopts a jet cooling method, which means that the cooling medium is sprayed out from the nozzle to impact the inner and outer surfaces of the pipe, so that the inner and outer surfaces of the pipe are cooled rapidly at the same time.

[0022] The beneficial effects of the present invention are: (1) The present invention has a special design for the welded pipe material. The inner and outer layers of the welded pipe are stainless steel, which can effectively ensure the corrosion resistance of the pipe. The middle layer is copper and carbon steel (low alloy steel), which can significantly improve the thermal conductivity of the welded pipe. At the same time, the carbon steel (low alloy steel) in the middle can also ensure the strength of the pipe and reduce the material cost. After hot working and heat treatment, welding defects can be significantly eliminated, ensuring the thickness of the inner and outer stainless steel corrosion-resistant layers and improving the comprehensive performance of the welded pipe.

[0023] (2) The intermediate layer in this invention uses copper and carbon steel or low alloy steel, which increases the thermal conductivity of the composite steel pipe and significantly improves its heat exchange performance while ensuring the corrosion resistance of the pipe. The thickness of the third layer of carbon steel or low alloy steel is 0.3-8.0mm, which enables the joint to reach a certain strength during the welding process, significantly eliminates stress, and improves the weld strength.

[0024] (3) In this invention, the composite welded pipe is heated to 500-650℃ and then subjected to diameter reduction and drawing. Since the inner and outer surfaces are in contact with the mold or air, the temperature of the inner and outer layers drops faster, while the core temperature is higher. During the diameter reduction and drawing process, the stainless steel deformation of the inner and outer surfaces is relatively small, thus better ensuring the thickness of the inner and outer stainless steel layers. At the same time, the medium temperature deformation can also better eliminate welding defects and improve the weld strength.

[0025] (4) The present invention uses rapid cooling after heat treatment. Because copper is softer than other metals, it will begin to soften at temperatures above 280°C. During the heating process, it will gradually penetrate into the surface of the carbon steel layer in contact. Since the expansion coefficients of copper and steel are very different, so-called penetration cracks are easily formed at the weld. After high-temperature heating, rapid cooling of both inner and outer layers is carried out to quickly harden the softened copper layer and avoid the formation of penetration cracks during slow cooling. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the cross-sectional structure of a stainless steel composite steel pipe used in heat exchangers.

[0027] In the diagram: 1. Outer layer stainless steel; 2. Second layer copper; 3. Third layer carbon steel or low alloy steel; 4. Fourth layer copper; 5. Inner layer stainless steel. Detailed Implementation

[0028] To better understand the present invention, the embodiments of the present invention will be described in detail below with reference to the examples. However, those skilled in the art will understand that the following examples are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention.

[0029] Example 1:

[0030] A method for preparing a stainless steel composite steel pipe for heat exchangers, the method comprising the following steps:

[0031] 1. Use 304 stainless steel as the outer layer, copper and Q420N as the core middle layer, and 316 stainless steel as the inner layer to make multi-layer stainless steel composite steel pipe.

[0032] 2. Clean the surfaces of stainless steel plates, copper plates, and Q420N steel plates, stack them together in sequence, and vacuum seal them to prepare a composite steel billet. The 304 stainless steel plate is 20mm thick, the two copper plates are both 20mm thick, the Q420N steel plate is 100mm thick, and the 316 stainless steel plate is 40mm thick. Heat the composite steel billet to 1000℃ and hot roll it to prepare a 6mm thick composite steel plate, and then pickle it. The outer layer of stainless steel 1 is 0.6mm thick, the inner layer of stainless steel 5 is 1.2mm thick, the third layer of carbon steel or low alloy steel 3 is 3mm thick, and the total thickness of the second layer of copper 2 and the fourth layer of copper 4 is 1.2mm.

[0033] 4. Trim the edges of the composite steel plate and cut it into narrow strips with a width of 471mm;

[0034] 5. Use a plate rolling machine to roll the composite steel plate into a tubular shape;

[0035] 6. Use straight seam welding equipment to weld the pipes into Φ150mm straight seam welded pipes;

[0036] 7. A straight seam welded pipe with a diameter of Φ150mm and a wall thickness of 6mm is heated to 600℃ and held at that temperature for 1 hour. After being taken out of the furnace, it is processed into a pipe with a diameter of Φ100 and a wall thickness of 3mm through two drawing passes and three tension reduction passes. The drawing or tension reduction method uses a drawing die or a reducing roller. The drawing die or reducing roller needs to be cooled by a medium to control the temperature below 100℃.

[0037] 8. The stainless steel composite pipes processed to the finished product specifications are subjected to normalizing treatment at a temperature of 900℃. After holding at this temperature for 1 hour, the pipes are removed from the furnace and rapidly cooled. The rapid cooling method is a jet cooling method, which means that the cooling medium is sprayed from the nozzle to impact the inner and outer surfaces of the pipe, so that the inner and outer surfaces of the pipe are cooled rapidly at the same time. The resulting stainless steel composite pipes have complete weld surfaces without surface cracks, pores, or fusion spatter.

[0038] Example 2:

[0039] A method for preparing a stainless steel composite steel pipe for heat exchangers, the method comprising the following steps:

[0040] 1. Use 316 stainless steel as the outer layer, copper and 45 steel as the core middle layer, and 316 stainless steel as the inner layer to make multi-layer stainless steel composite steel pipe.

[0041] 2. Clean the surfaces of stainless steel plates, copper plates, and 45 steel plates, stack them together in sequence, vacuum seal them, and prepare a composite steel billet. The inner and outer layers of 316 stainless steel plates are 30mm thick, the two copper plates are 20mm thick, and the 45 steel plate is 100mm thick. Heat the composite steel billet to 1000℃ and hot roll it to prepare a 5mm thick composite steel plate, and then pickle it. The outer layer of stainless steel 1 is 0.75mm thick, the inner layer of stainless steel 5 is 0.75mm thick, the third layer of carbon steel or low alloy steel 3 is 2.5mm thick, and the total thickness of the second layer of copper 2 and the fourth layer of copper 4 is 1mm.

[0042] 4. Trim the edges of the composite steel plate and cut it into narrow strips with a width of 376.8mm;

[0043] 5. Use a plate rolling machine to roll the composite steel plate into a tubular shape;

[0044] 6. Use straight seam welding equipment to weld the pipes into Φ120mm straight seam welded pipes;

[0045] 7. A straight seam welded pipe with a diameter of Φ120mm and a wall thickness of 6mm is heated to 500℃ and held at that temperature for 2 hours. After being taken out of the furnace, it is processed into a pipe with a diameter of Φ60 and a wall thickness of 2mm by one drawing pass and five tension reduction passes. The drawing or tension reduction method uses a drawing die or a reducing roller. The drawing die or reducing roller needs to be cooled by a medium and the temperature is controlled to be below 100℃.

[0046] 8. The stainless steel composite pipes processed to the finished specifications are subjected to normalizing treatment at a temperature of 850℃. After holding at this temperature for 3 hours, the pipes are removed from the furnace and rapidly cooled. The rapid cooling method is jet cooling, which means that the cooling medium is sprayed from the nozzle to impact the inner and outer surfaces of the pipe, so that the inner and outer surfaces of the pipe are cooled rapidly at the same time. The resulting stainless steel composite pipes have complete weld surfaces without surface cracks, pores, or fusion spatter.

[0047] Example 3:

[0048] A method for preparing a stainless steel composite steel pipe for heat exchangers, the method comprising the following steps:

[0049] 1. Use 321 stainless steel as the outer layer, copper and 45 steel as the core middle layer, and 316 stainless steel as the inner layer to make multi-layer stainless steel composite pipe.

[0050] 2. Clean the surfaces of stainless steel plates, copper plates, and 45 steel plates, stack them together in sequence, and vacuum seal them to prepare a composite steel billet. The outer layer is 321 stainless steel plate with a thickness of 30mm, two copper plates are each 20mm thick, the 45 steel plate is 100mm thick, and the inner layer is 316 stainless steel with a thickness of 40mm. The composite steel billet is heated to 1000℃ and hot-rolled to prepare a 7mm thick composite steel plate, and then pickled. The outer layer of stainless steel 1 has a thickness of 1.0mm, the inner layer of stainless steel 5 has a thickness of 1.33mm, the third layer of carbon steel or low alloy steel 3 has a thickness of 3.34mm, and the total thickness of the second layer of copper 2 and the fourth layer of copper 4 is 1.33mm.

[0051] 4. Trim the edges of the composite steel plate and cut it into narrow strips with a width of 497mm;

[0052] 5. Use a plate rolling machine to roll the composite steel plate into a tubular shape;

[0053] 6. Use straight seam welding equipment to weld the pipes into Φ160mm straight seam welded pipes;

[0054] 7. A straight seam welded pipe with a diameter of Φ160mm and a wall thickness of 7mm is heated to 650℃ and held at that temperature for 0.5h. After being taken out of the furnace, it is processed into a pipe with a diameter of Φ120 and a wall thickness of 4mm through three drawing passes and three tension reduction passes. The drawing or tension reduction method uses a drawing die or a reducing roller, and the drawing die or reducing roller needs to be cooled by a medium to control the temperature below 100℃.

[0055] 8. The stainless steel composite pipes processed to the finished specifications are subjected to normalizing treatment at a temperature of 950℃. After holding at this temperature for 0.5 hours, the pipes are removed from the furnace and rapidly cooled. The rapid cooling method is jet cooling, which means that the cooling medium is sprayed from the nozzle to impact the inner and outer surfaces of the pipe, so that the inner and outer surfaces of the pipe are cooled rapidly at the same time. The resulting stainless steel composite pipes have complete weld surfaces without surface cracks, pores, or fusion spatter.

[0056] Comparative Example 1:

[0057] The difference between Comparative Example 1 and Example 1 is that in the preparation method, the cooling method used in step 8 is air cooling. The other technical contents are the same as those in Example 1. The copper layer of the obtained stainless steel composite pipe has microcracks.

[0058] Comparative Example 2:

[0059] The difference between Comparative Example 2 and Example 1 is that the second and fourth copper layers were removed from the multi-layer stainless steel composite pipe material, leaving only Q420N as the core intermediate layer. Other technical contents are the same as in Example 1. The thermal conductivity of the resulting stainless steel composite pipe is significantly reduced.

[0060] The stainless steel composite pipes prepared in Examples 1-3 and Comparative Examples 1-2 were tested for corrosion resistance and thermal conductivity. The test results are shown in Table 1.

[0061] Table 1

[0062]

[0063]

[0064] The above description of the embodiments is intended to facilitate understanding and use of the present invention by those skilled in the art. Those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the principles of the present invention, without departing from the scope of the invention, should be within the protection scope of the present invention.

Claims

1. A method for preparing a stainless steel composite steel pipe for heat exchangers, characterized in that, It consists of a five-layer structure, namely, an outer layer of stainless steel (1), a second layer of copper (2), a third layer of carbon steel or low alloy steel (3), a fourth layer of copper (4), and an inner layer of stainless steel (5); the preparation method includes the following steps: Step 1, Preparation of stainless steel composite plate / strip: Stainless steel plate, copper plate, carbon steel or low alloy steel plate are assembled into stainless steel composite billet according to the design thickness, and then the composite billet is prepared into composite steel plate / strip of the required thickness through rolling process. Step 2, Steel / Strip Edge Trimming: Remove the oxide scale and impurities from the edges of the stainless steel composite plate / strip, and cut the plate / strip to the required width; Step 3, Tube Rolling: Use a plate rolling machine to roll the stainless steel composite plate / strip into a tube shape; Step 4, Welding: The pipe is welded into shape using high-frequency welding or resistance welding. Step 5, Heating: Heat the welded pipe to 500-650℃ and hold for 0.5-2 hours to ensure the pipe is thoroughly heated; Step 6, Hot working: The heated pipe is processed to the required size by drawing and tension reduction to obtain the composite pipe; Step 7, heat treatment: The obtained composite pipe is normalized by heating to 850-950℃ and holding for 0.5-3 hours. Finally, it is taken out of the furnace and rapidly cooled. The rapid cooling method adopts the jet cooling method, which means that the cooling medium is sprayed out from the nozzle to impact the inner and outer surfaces of the pipe, so that the inner and outer surfaces of the pipe are cooled rapidly at the same time.

2. The method for preparing a stainless steel composite steel pipe for heat exchangers according to claim 1, characterized in that, The outer stainless steel (1) has a thickness of 0.2-1.5 mm, the inner stainless steel (5) has a thickness of 0.3-2.0 mm, the third carbon steel or low alloy steel (3) has a thickness of 0.3-8.0 mm, and the total thickness of the second copper (2) and the fourth copper (4) is 0.3-3.0 mm.

3. The method for preparing a stainless steel composite steel pipe for heat exchangers according to claim 1, characterized in that, The ratio of the thickness of the third layer of carbon steel or low alloy steel (3) to the total thickness of the second layer of copper (2) and the fourth layer of copper (4) is 1-8:

1.

4. The method for preparing a stainless steel composite steel pipe for a heat exchanger according to claim 1, characterized in that, In step four, nitrogen or inert gas is used to protect the weld during welding.

5. The method for preparing a stainless steel composite steel pipe for a heat exchanger according to claim 1, characterized in that, In step six, the drawing and tension reduction methods use drawing dies and reducing rollers, and the drawing dies and reducing rollers need to be cooled by a medium to control the temperature below 100°C.

6. The method for preparing a stainless steel composite steel pipe for a heat exchanger according to claim 1, characterized in that, In step six, the total wall thickness reduction of the composite pipe after cold working is greater than 0.5 mm, and the wall thickness reduction is greater than 25%.

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