Steel pipe and aluminum fin cluster brazing method

Abstract

The invention discloses a technical method for brazed connection of a steel pipe and an aluminum fin, which sequentially comprises the following steps of: (1) carrying out surface treatment on a steel pipe raw material, and sequentially carrying out degreasing treatment, acid pickling and water washing; (2) electroplating the outer surface of the steel pipe to form a copper layer; (3) coating the outer layer of the copper-plated steel pipe with a brazing material; (4) the copper-plated steel pipe coated with the brazing material and the aluminum fin cluster are positioned, fixed and combined into a to-be-brazed assembly; and (5) the to-be-brazed assembly is subjected to vacuum brazing connection, and the welded finned tube is obtained. According to the technical method, the welding connection problem of the steel pipe and the thin aluminum fin cluster is solved, a layer of copper is electroplated on the surface of the steel pipe, then the gaps between the steel pipe plating layer and the aluminum fins are filled with the brazing material, it is guaranteed that the finned tube and the base tube have high bonding quality, and therefore the heat transfer performance of the finned tube is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of metal welding, and in particular to a brazing method between a steel pipe and an aluminum alloy dissimilar metal. Background Art

[0002] In response to the performance problems of large flow resistance, large scale resistance and low efficiency of round fin tube air coolers widely used in the energy, petrochemical, metallurgical and other industries, the inventor proposed a fin tube with gas partitioned flow. The applicant previously applied for a patent named "CN115096124A H-type fin tube with airflow partitioning grooves suitable for high-temperature or / and dust-laden gas heat exchange occasions", and announced an H-type fin tube with gas partitioning grooves suitable for high-temperature and / or dust-laden gas heat exchange occasions. The applicant previously applied for a patent named "CN218210940U A serpentine fin cluster with a circular arc concave on the wing top on one side", and announced a serpentine fin cluster for airflow partitioned fin tube bundles.

[0003] To improve the applicability of finned tubes, heat exchange tubes are often made of stainless steel or carbon steel round tubes to increase the pressure-bearing capacity of the finned tubes. Fins are often made of aluminum alloys with good thermal conductivity, which facilitates both the formation of the fin cluster and heat transfer. However, due to the significant differences in the properties of the two dissimilar metals, welding steel and aluminum alloys is difficult and challenging.

[0004] CN102794561A discloses a reactive friction stir welding method suitable for joining aluminum alloys and stainless steel. After forming a Ni-Cu coating on the welding surface of the stainless steel base material, the friction stir effect forms metallic bonds or strong chemical bonds at the layer interfaces, resulting in strong bonding between the layers and improving the strength of the joint surface. CN106271069A discloses a laser welding method for stainless steel and aluminum alloys. The process involves treating the plate-shaped stainless steel and aluminum alloy workpieces to be welded, removing impurities and drying the surfaces. The treated stainless steel and aluminum alloy workpieces are then tightly stacked together. A nanosecond infrared laser is used to output laser light and scan the surface of the stainless steel or aluminum alloy workpiece along a welding path to weld the workpieces. The thickness of the scanned stainless steel or aluminum alloy workpiece is 0.05-0.5 mm. During welding, the laser light output by the nanosecond infrared laser is perpendicular to the surface of the scanned stainless steel or aluminum alloy workpiece. Due to the thinness of the fins, fusion welding or stir welding cannot be used for the fins and steel pipes. Laser welding methods also suffer from high costs and low weld bond rates.

[0005] Exhaust steam condensers, widely used in the power industry, use a base tube made by rolling and laminating steel plates and aluminum strips to form a transitional bonding layer on the outer surface of the flat tube. This manufacturing method is difficult to apply to seamless steel pipes for material lamination.

[0006] The energy industry also uses aluminized and aluminized steel pipes to improve corrosion resistance and temperature resistance. Aluminized steel pipes have a pure aluminum surface layer and an aluminum-iron alloy (Fe2Al5) transition layer. During brazing, the pure aluminum diffuses into the transition layer, forming a brittle transition layer and a weak connection between the fins and the pipe. Therefore, a suitable method is needed to achieve a reliable connection between aluminum fins and steel pipes. Summary of the Invention

[0007] The present invention aims to provide a method for connecting steel tubes to aluminum fin clusters using brazing. By electroplating a layer of copper on the steel tube surface, the brazing method connects the steel tube to the aluminum alloy fin cluster, achieving high welding efficiency and low cost. The brazing filler fills the gap between the heat exchange tube and the fin, achieving a perfect fit between the heat exchange tube and the fin.

[0008] The technical solution of the present invention is a method for welding a steel pipe and an aluminum fin cluster, and a method for brazing a steel pipe and an aluminum fin cluster, which sequentially performs the following steps: Step (1), surface treatment of the steel pipe raw material, sequentially performing degreasing, pickling, and water washing; Step (2), electroplating the outer surface of the steel pipe to form a copper layer; Step (3), coating the outer layer of the copper-plated steel pipe with brazing material; Step (4), positioning and fixing the copper-plated steel tube coated with brazing material and the aluminum fin cluster to form an assembly to be brazed; Step (5) is to connect the components to be brazed by brazing to obtain welded fin tubes.

[0009] Preferably, the raw material of the steel pipe in step (1) includes a stainless steel pipe or a carbon steel pipe, and the straightness of the stainless steel pipe or the carbon steel pipe is 0.5 / 1000.

[0010] Preferably, the degreasing liquid used in the degreasing treatment in step (1) comprises the following components: 2-3% by mass of sodium hydroxide, 3.5-5% of sodium nitrate, and 0.3-0.5% of sodium silicate; the degreasing treatment temperature is 70-80° C.; the degreasing soaking time is 10-40 minutes; and the degreasing treatment is performed by a tank immersion method or a wiping method.

[0011] Preferably, the pickling solution used in the pickling of step (1) includes: sulfuric acid or hydrochloric acid solution with corrosion inhibitor added for non-alloy or low-alloy steel pipes; the sulfuric acid solution has the following components: 5% to 10% sulfuric acid and 0.5% to 1% urotropine by weight; the hydrochloric acid solution has the following components: 5% to 20% hydrochloric acid and 0.5% to 1% urotropine by weight; nitric acid solution or hydrofluoric acid-nitric acid mixture is used for stainless steel pipes; the nitric acid solution has the following components: 5% to 10% nitric acid by weight; the hydrofluoric acid-nitric acid mixture has the following components: 5% to 10% nitric acid and 0.5% to 1% hydrofluoric acid by weight; For non-alloy or low-alloy steel pipes, the sulfuric acid pickling temperature is 60~80℃, and the pickling time is 5~20 minutes; the hydrochloric acid pickling temperature is 20~50℃, and the pickling time is 5~20 minutes; for stainless steel pipes, the nitric acid pickling temperature is room temperature, and the pickling time is 15~60 minutes; When washing with water, rinse with 0.8MPa clean water to pH 6~7 or soak in ammonia water with pH 10~11 for 3 minutes to neutralize the residual acid, and then wash with water. Rinse with water to make the pH value close to neutral to pH 7~7.5.

[0012] Preferably, the electroplating method in step (2) is acid copper plating, and the current density during electroplating is 1-5 A / dm 2 The electroplating temperature is 20-50°C. The high concentration of sulfuric acid (190-220 g / L) in the acidic copper plating solution significantly improves conductivity. By adjusting the current density and controlling the electrode polarization process, a nanocrystalline structure with grain sizes less than 0.5 μm is formed. This microstructure ensures a bond strength of 300-400 MPa between the coating and the substrate, exceeding the 250 MPa level achieved with traditional cyanide copper plating.

[0013] Preferably, the thickness of the electroplated copper layer in step (2) is 10-20 μm.

[0014] Preferably, the brazing material in step (3) is a Zn-Al brazing material, and the brazing connection process in step (5) is vacuum brazing, and the conditions are set as follows: the brazing temperature is 420-500°C, the holding time is 25-35 minutes, the heating rate is controlled at 12-15°C / min, and the vacuum degree is ≥1×10 -3 Pa. Zn-Al solder has good wettability, can match the thermal expansion coefficient of the aluminum substrate, and reduce welding stress.

[0015] Preferably, the brazing material in step (3) is a Sn-based brazing material, and the brazing connection process in step (5) is vacuum brazing, and the conditions are set as follows: the brazing temperature is 240-270°C, the holding time is 5-10 minutes, the heating rate is controlled at 5-15°C / min, and the vacuum degree is ≥1×10 -3Pa. Sn-based solder matches the difference between copper and aluminum, reduces the risk of thermal stress cracking, and has good thermal conductivity.

[0016] Preferably, the aluminum fin cluster in step (4) is a fin cluster with a concave arc on one side of the fin top, and the outer surface of the heat exchange tube contacts the arc opened on the one side of the fin top of the aluminum fin cluster. The assembly gap is 0.1~0.3mm, which optimizes the capillary effect and avoids the overflow of the solder. Two groups of fin clusters are clamped with a group of heat exchange tube bundles, and then clamped by a special clamp to form a component to be brazed. The welded component is as follows Figure 1 As shown, multiple heat exchange tubes are arranged in parallel with equal spacing. Clusters of aluminum alloy fins are installed on both sides of the heat exchange tube bundle. These serpentine fins conform to the requirements of the applicant's previous patent application, "Serpentine Fin Cluster with a Circular Indentation on One Side of the Fin Top," CN218210940U. Each fin in the cluster is perpendicular to the axis of the heat exchange tube bundle. The heat exchange tubes are embedded in the circular indentations of the fins, forming an arc-shaped contact between the outer surface of the heat exchange tubes and the fins.

[0017] Preferably, the brazing connection process in step (5) is vacuum brazing, using gas shielded welding, and the shielding gas is 99.9999% high-purity inert gas with a dew point of ≤-40°C.

[0018] The beneficial effects of the present invention are as follows: (1) The present invention uses brazing to connect the steel tube and the aluminum alloy fin cluster, which has high welding efficiency and low cost. The gap between the heat exchange tube and the fin is well filled with brazing material, achieving a good bonding between the heat exchange tube and the fin. The bonding quality between the fin tube and the base tube is guaranteed, thereby ensuring the heat transfer performance of the fin tube and can be used at high temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the components to be brazed in a method for brazing a steel tube and an aluminum fin cluster according to the present invention.

[0020] 1 is a steel pipe; 2 is an aluminum alloy fin cluster with a circular arc concave on the fin top on one side; 3 is the circular arc concave on the fin cluster; 4 is a special fixture. DETAILED DESCRIPTION

[0021] The present invention will be described in more detail below with reference to the accompanying drawings and specific embodiments. The following examples are intended to facilitate a better understanding of the present invention by those skilled in the art, but are not intended to limit the present invention in any way. Furthermore, it should be noted that any equivalent modifications based on the present invention, as known to those skilled in the art, fall within the scope of protection of the present invention.

[0022] Example 1 This embodiment provides a method for welding a low alloy steel tube to an aluminum alloy fin cluster, the welding method comprising the following steps: (1) Low alloy steel pipes (such as carbon steel pipes with a straightness of 0.5 / 1000) are degreased, pickled, and washed in sequence. The degreasing process is performed by tank immersion, and the degreasing solution used (by mass) is: 2.5% sodium hydroxide, 4% sodium nitrate, and 0.4% sodium silicate; the degreasing process temperature is 75°C, and the degreasing soaking time is 20 minutes; the acid solution used for pickling is 7.5% sulfuric acid and 0.75% hexamethylenetetramine, the pickling temperature is 70°C, and the pickling time is 10 minutes. After pickling, rinse with water, using 0.8MPa clean water to neutrality (pH 6~7).

[0023] (2) The steel pipe is immediately subjected to electro-copper treatment, with a copper plating thickness of 20 μm. Acid copper plating method, the electroplating current density is 3A / dm 2 , the electroplating temperature is 30°C.

[0024] (3) The outer surface of the copper-plated steel pipe is coated with Zn-Al solder with a thickness of 0.1 mm; the solder model is: Zn72.5Al.

[0025] (4) combining the electroplated copper steel tube and the aluminum fin cluster into an assembly to be brazed; (5) Vacuum brazing was performed. The brazing process was as follows: the brazing temperature was 450 °C, the holding time was 30 minutes, the heating rate was controlled at 15 °C / min, and the vacuum degree was 1×10 -3 Pa. Gas shielded welding is adopted, and the shielding gas is 99.9999% high-purity argon with a dew point of ≤-40℃.

[0026] Example 2 This embodiment provides a method for welding a stainless steel tube to an aluminum alloy fin cluster, the welding method comprising the following steps: (1) The stainless steel pipe is degreased, pickled, and washed in sequence. The degreasing process is performed by tank immersion, and the degreasing solution used is composed of 3% sodium hydroxide, 5% sodium nitrate, and 0.5% sodium silicate (by weight). The degreasing process temperature is 75°C, and the degreasing soaking time is 20 minutes. The acid solution used for pickling is 7.5% nitric acid, the pickling temperature is room temperature (originally 70°C), and the pickling time is 15 minutes. After pickling, the pipe is neutralized and rinsed by soaking in ammonia water (pH 10-11) for 3 minutes to neutralize the residual acid. A second rinse is then performed by rinsing with 0.8MPa clean water until the pipe reaches a neutral pH of 7-7.5.

[0027] (2) The steel pipe is immediately subjected to electro-copper treatment, with a copper plating thickness of 10 μm. Acid copper plating method, the electroplating current density is 4A / dm 2 , the electroplating temperature is 30°C.

[0028] (3) The outer surface of the copper-plated steel pipe is coated with Zn-Al solder with a thickness of 0.1 mm; the solder model is: Zn95Al.

[0029] (4) combining the electroplated copper steel tube and the aluminum fin cluster into an assembly to be brazed; (5) Vacuum brazing was performed. The brazing process was as follows: the brazing temperature was 450 °C, the holding time was 30 minutes, the heating rate was controlled at 15 °C / min, and the vacuum degree was 1×10 -3 Pa. Gas shielded welding is adopted, and the shielding gas is 99.9999% high-purity argon with a dew point of ≤-40℃.

[0030] Example 3 The difference from Example 2 is that the coating is Sn-based solder. The vacuum brazing process is as follows: the brazing temperature is 250°C, the holding time is 8 minutes, the heating rate is controlled at 10°C / min, and the vacuum degree is ≥1×10 -3 Pa.

Claims

1. A method for brazing a steel tube and an aluminum fin cluster, characterized in that: The method performs the following steps in sequence: Step (1), surface treatment of the steel pipe raw material, sequentially performing degreasing, pickling, and water washing; Step (2), electroplating the outer surface of the steel pipe to form a copper layer; Step (3), coating the outer layer of the copper-plated steel pipe with brazing material; Step (4), positioning and fixing the copper-plated steel tube coated with brazing material and the aluminum fin cluster to form an assembly to be brazed; Step (5) is to connect the components to be brazed by brazing to obtain welded fin tubes.

2. The method for brazing a steel tube and an aluminum fin cluster according to claim 1, wherein: The raw material of the steel pipe in step (1) includes a stainless steel pipe and a carbon steel pipe, and the straightness of the stainless steel pipe or the carbon steel pipe is 0.5 / 1000.

3. The method for brazing a steel tube and an aluminum fin cluster according to claim 1, wherein: The degreasing liquid used in the degreasing treatment of step (1) comprises the following components: 2-3% by mass of sodium hydroxide, 3.5-5% by mass of sodium nitrate, and 0.3-0.5% by mass of sodium silicate. The degreasing treatment temperature is 70-80° C., and the degreasing soaking time is 10-40 minutes. The degreasing treatment is performed by a tank immersion method or a wiping method.

4. The method for brazing a steel tube and an aluminum fin cluster according to claim 1, wherein: The pickling solution used in the pickling of step (1) includes: sulfuric acid or hydrochloric acid solution with corrosion inhibitor added for non-alloy or low-alloy steel pipes; the sulfuric acid solution has the following components: 5% to 10% sulfuric acid and 0.5% to 1% urotropine by weight; the hydrochloric acid solution has the following components: 5% to 20% hydrochloric acid and 0.5% to 1% urotropine by weight; nitric acid solution or hydrofluoric acid-nitric acid mixture is used for stainless steel pipes; the nitric acid solution has the following components: 5% to 10% nitric acid by weight; the hydrofluoric acid-nitric acid mixture has the following components: 5% to 10% nitric acid and 0.5% to 1% hydrofluoric acid by weight; For non-alloy or low-alloy steel pipes, the sulfuric acid pickling temperature is 60~80℃, and the pickling time is 5~20 minutes; the hydrochloric acid pickling temperature is 20~50℃, and the pickling time is 5~20 minutes; for stainless steel pipes, the nitric acid pickling temperature is room temperature, and the pickling time is 15~60 minutes; When washing with water, rinse with 0.8MPa clean water to pH 6~7 or soak in ammonia water with pH 10~11 for 3 minutes to neutralize the residual acid, and then wash with water. Rinse with water to make the pH value close to neutral to pH 7~7.

5.

5. A method for brazing a steel tube and an aluminum fin cluster according to any one of claims 1 to 4, characterized in that: The electroplating method in step (2) is an acid copper plating method, and the current density during electroplating is 1-5 A / dm 2 , electroplating temperature: 20-50℃.

6. A method for brazing a steel tube and an aluminum fin cluster according to any one of claim 5, characterized in that: The thickness of the electroplated copper layer in step (2) is 10-20 μm.

7. A method for brazing a steel tube and an aluminum fin cluster according to any one of claims 1 to 4, characterized in that: The brazing material in step (3) is a Zn-Al brazing material. The brazing connection process in step (5) is vacuum brazing. The conditions are as follows: the brazing temperature is 420-500°C, the holding time is 25-35 minutes, the heating rate is controlled at 12-15°C / min, and the vacuum degree is ≥1×10 -3 Pa.

8. A method for brazing a steel tube and an aluminum fin cluster according to any one of claims 1 to 4, characterized in that: The brazing material in step (3) is Sn-based brazing material. The brazing connection process in step (5) is vacuum brazing. The conditions are as follows: the brazing temperature is 240-270°C, the holding time is 5-10 minutes, the heating rate is controlled at 5-15°C / min, and the vacuum degree is ≥1×10 -3 Pa.

9. A method for brazing a steel tube and an aluminum fin cluster according to any one of claims 1 to 4, characterized in that: The aluminum fin cluster in step (4) is a fin cluster with a concave arc on one side of the fin top, the outer surface of the heat exchange tube is in contact with the arc opened on the one side of the fin top of the aluminum fin cluster, and the assembly gap is 0.1~0.3mm.

10. A method for brazing a steel tube and an aluminum fin cluster according to any one of claims 1 to 4, characterized in that: The brazing connection process in step (5) is vacuum brazing, using gas shielded welding, and the shielding gas is 99.9999% high-purity inert gas with a dew point of ≤-40°C.

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