Stainless steel brazing alloys and brazing joints

Abstract

This invention provides a stainless steel brazing alloy that has good wire drawing properties, is inexpensive, and possesses excellent brazing capabilities, as well as a brazing joint using the same. [Solution] A stainless steel brazing alloy comprising, by weight %, Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3%, with the remainder being Cu and other unavoidable impurities, and further containing Ni: 0.01-0.3% and Mn: 0.01-0.3%, characterized in that the brazing alloy is used when brazing between pipes of the same type of stainless steel or between pipes of different types of stainless steel and copper.

Description

Technical Field

[0001] The present invention relates to an alloy for brazing stainless steel and a brazed joint, and more particularly, to an alloy for brazing stainless steel and a brazed joint that can be suitably applied during brazing between pipes of the same type of stainless steel or between pipes of different types of stainless steel and copper.

Background Art

[0002] An air conditioner is a device that changes temperature through compression and expansion of a refrigerant, and is used in electronic products such as air conditioners, refrigerators, and washing machines that are widely used in daily life. In the case of a conventional pipe through which the refrigerant of an air conditioner flows, it is mostly made of Cu or a Cu-based alloy because the above Cu or Cu-based alloy is excellent in corrosion resistance and brazing properties. Also, when brazing a Cu pipe, an alloy (insert metal) of the CuP series (containing P and Ag at a level of 0 to 15% with Cu as the main component) that is generally relatively inexpensive is utilized, and this enables brazing without using flux.

[0003] In the case of the above Cu or Cu-based alloy (hereinafter also referred to as "Cu metal"), since the raw material price is high and the price fluctuation is intense, an alternative to stainless steel (hereinafter also referred to as "STS") that can guarantee corrosion resistance and strength and is relatively inexpensive is actively being carried out. However, although the corrosion characteristics of Cu metal and STS are almost similar, in the case of STS, since a thin Cr-based oxide is formed on the surface, the molten insert metal does not wet and spread on the STS surface during brazing, so it has the drawback that brazing is difficult. To solve this, a proposal has been made to use an insert metal of the Ag series (containing Cu and other alloys with Ag as the main component (about 30 to 70%)), but in the case of Ag, it is about 100 times or more expensive than Cu and other elements, so the brazing cost increases and the cost reduction effect by the above STS replacement is lost, and there is a limit to its practical application.

[0004] As mentioned above, in order to replace expensive Cu metal with inexpensive STS, the development of inexpensive brazing insert metals is essential. This requires the development of materials that ensure the reliability of the brazed area while the molten insert metal spreads well to the STS surface during brazing (torch, high frequency, vacuum, etc.). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-508361 [Patent Document 2] European Patent No. 2852695 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The object of the present invention is to provide a stainless steel brazing alloy and brazing joint that can be suitably applied when brazing between pipes of the same type of stainless steel or between dissimilar pipes of stainless steel and copper.

[0007] Another object of the present invention is to provide a stainless steel brazing alloy that has good wire drawing properties, is inexpensive, and has excellent brazing properties, as well as a brazing joint using the same. [Means for solving the problem]

[0008] The present invention is characterized by a stainless steel brazing alloy comprising, by weight, Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3%, with the remainder being Cu and other unavoidable impurities.

[0009] The alloy may further contain Ni: 0.01-0.3% and Mn: 0.01-0.3%.

[0010] The brazing alloy can be used when brazing pipes of the same type of stainless steel or pipes of different types, such as stainless steel and copper.

[0011] The present invention features a brazed joint between identical stainless steel pipes or between dissimilar stainless steel and copper pipes, characterized by a brazed joint having a wetting angle of 120° or more between the stainless steel and the brazed portion, with the composition being Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3% by weight, with the remainder being Cu and other unavoidable impurities, a brazed portion length of 5 mm or more, and the wetting angle between the stainless steel and the brazed portion or between the copper and the brazed portion.

[0012] The welded joint may further contain Ni: 0.01-0.3% and Mn: 0.01-0.3%. [Effects of the Invention]

[0013] According to the present invention, a stainless steel brazing alloy and a brazing joint can be provided.

[0014] According to the present invention, it is possible to provide a stainless steel brazing alloy that has good wire drawing processability and excellent brazing properties while being inexpensive, as well as a brazing joint using the same. [Brief explanation of the drawing]

[0015] [Figure 1] This photograph shows the results after torch brazing between identical stainless steel pipes using Invention Example 1 and Conventional Examples 1 and 2, followed by a torsional shear test. [Figure 2] This is a cross-sectional microstructure photograph of a brazed joint after torch brazing between identical stainless steel pipes according to Invention Example 1 of the present invention. [Figure 3] This is a cross-sectional microstructure photograph of a brazed joint after torch brazing between identical stainless steel pipes according to Conventional Example 1 of the present invention. [Figure 4] This is a cross-sectional microstructure photograph of the brazed joint after torch brazing between stainless steel pipes of Conventional Example 2 of the present invention. [Figure 5] This is a photograph after a torsional shear test was carried out after torch brazing between stainless steel and copper dissimilar pipes using Invention Example 1 and Conventional Example 1 of the present invention. [Figure 6] This is a cross-sectional microstructure photograph of the brazed joint after torch brazing between stainless steel and copper dissimilar pipes of Invention Example 1 of the present invention. [Figure 7] This is a cross-sectional microstructure photograph of the brazed joint after torch brazing between stainless steel and copper dissimilar pipes of Conventional Example 1 of the present invention.

Mode for Carrying Out the Invention

[0016] Hereinafter, the brazing alloy for stainless steel of the present invention will be described. The brazing alloy of the present invention has a brass alloy (Cu + Zn) as a basic component system and contains Ag. Further, the brazing alloy for stainless steel may further contain P, Al, Ni, and Mn.

[0017] In the case of commercial brass insert metals, although they are partially applied during the brazing of STS, they have the disadvantages of having a high melting point, insufficient wettability, taking an excessive brazing time, and being inferior in brazing properties and the corrosion resistance of the brazed part. On the other hand, the brazing alloy for stainless steel of the present invention contains Ag, which is excellent in the wettability of the STS surface, so that it has a lower melting point and improved wettability compared with conventional brass insert metals. Further, the brazing alloy for stainless steel of the present invention can supplement the insufficient wettability by containing Al. On the other hand, the brazing alloy for stainless steel of the present invention can further add Ni and Mn for improving other workability, joint strength, and corrosion resistance.

[0018] On the one hand, in the case of commercially available Cu-based brazing filler metals, P is contained at 5% or more to lower the melting point and improve the wettability. However, in the case of brazing filler metals for STS, it is necessary to extremely limit the P content. This is because STS contains Fe. When brazing STS using a brazing filler metal containing the P component, the P component generates an intermetallic compound layer of fragile Fe2P or Fe3P at the bonding interface, resulting in fracture at the interface during the brazing part workability test and thus unable to guarantee the soundness of the brazing part.

[0019] Hereinafter, the alloy for brazing stainless steel of the present invention will be specifically described. The content of the alloy composition described below means weight % unless otherwise specified.

[0020] Zn: 36 - 42% Zn alloyizes with Cu to lower the melting point and at the same time reduces the Cr oxide of STS and plays a role in improving the bonding force with Fe. In order to sufficiently obtain the above effects, the content of Zn is preferably 36% or more. On the other hand, when the content of Zn exceeds 42%, the wire drawing workability deteriorates due to the increase of the beta phase, the amount of zinc fume generated during brazing increases, and the quality of the brazing part may be reduced. The lower limit of the Zn content is more preferably 37%, and even more preferably 38%. The upper limit of the Zn content is more preferably 41%, and even more preferably 40%.

[0021] Ag: 3 - 6% Ag alloyizes with Cu to lower the melting point and at the same time plays a role in improving the wettability with the surface oxide of STS. In order to sufficiently obtain the above effects, the content of Ag is preferably 3% or more. On the other hand, since Ag is very expensive, when its content exceeds 6%, it is disadvantageous in terms of economy. The lower limit of the Ag content is more preferably 4%. The upper limit of the Ag content is more preferably 5%.

[0022] P: 0.2% or less (including 0%) P generally exhibits a significant effect in lowering the brazing temperature as the element that most strongly lowers the melting point in Cu-based intermetallic materials. However, even when added in very small amounts during the brazing of STS, P reacts with Fe in the STS to form very brittle intermetallic compounds such as Fe2P and Fe3P, thus failing to guarantee the workability of the joint. Therefore, in this invention, it is advantageous to keep the P content as low as possible, controlling it to a level of 0.2% or less.

[0023] Al: 0.01~0.3% Al, when alloyed with Cu, lowers the melting point and simultaneously reduces Cr oxide on the STS surface, thereby improving the wettability of the molten insert metal. To fully obtain the above effects, it is preferable that the Al content be 0.01% or higher. However, if the Al content exceeds 0.3%, it has the disadvantage of poor wire drawing processability, making wire manufacturing difficult. On the other hand, to confirm whether Al wetts and spreads across the surface of STS, it is advisable to calculate the Gibbs-free energy at 700°C as shown in [Equation 1] below. [Equation 1] 4 / 3Al + O2 = 2 / 3Al2O3: △G @700℃ ≈-920kJ / mol

[0024] As can be seen from Equation 1, the Gibbs-free energy for the formation of Al oxide appears to be approximately 1.6 times lower than the Gibbs-free energy for the formation of Cr oxide. In other words, when Cr2O3, which is Cr oxide formed on the surface of STS, is reduced to produce Al2O3, which is Al oxide, a wetting phenomenon occurs on the surface.

[0025] The remaining component is Cu. However, in normal manufacturing processes, unintended impurities from the raw materials or the surrounding environment are inevitably introduced and cannot be eliminated. Since these impurities are recognizable to any technician in normal manufacturing processes, not all of them are specifically mentioned in this specification.

[0026] On the other hand, the stainless steel brazing alloy of the present invention may further contain Ni: 0.01-0.3% and Mn: 0.01-0.3%.

[0027] Ni: 0.01~0.3% When added in small amounts to Cu-Zn-Ag alloys, nickel (Ni) plays a role in improving wire drawability, joint strength, and corrosion resistance. For these effects, a Ni content of 0.01% or more is preferable. However, if the Ni content exceeds 0.3%, wire drawability deteriorates, making wire manufacturing difficult.

[0028] Mn: 0.01~0.3% When manganese (Mn) is added in small amounts to Cu-Zn-Ag alloys, it plays a role in improving wire drawability, joint strength, and corrosion resistance. For the above effects, it is preferable that the Ni content be 0.01% or higher. However, if the Ni content exceeds 0.3%, it has the disadvantage of making wire drawability worse and wire manufacturing difficult.

[0029] As described above, the stainless steel brazing alloy of the present invention can be preferably used when brazing between pipes of the same type of stainless steel or between pipes of different types of stainless steel and copper. Furthermore, the stainless steel brazing alloy of the present invention has good wire drawability and offers excellent brazing properties while being inexpensive.

[0030] On the other hand, the stainless steel brazing alloy of the present invention can be manufactured by various methods, and the present invention does not particularly limit the manufacturing method. However, as one example, it can be manufactured using casting. The stainless steel brazing alloy of the present invention obtained in this way can be manufactured and used in various forms thereafter, and the present invention does not particularly limit the form. However, as one example, it can be manufactured and used in the form of a wire having an appropriate diameter through numerous wire drawing, annealing, skin pass, and pickling processes. As yet another example, the above wire can be manufactured and used in the form of a ring.

[0031] Another embodiment of the present invention provides a brazed joint using the above-described stainless steel brazing alloy. More specifically, it provides a brazed joint between identical stainless steel pipes or between dissimilar stainless steel and copper pipes, comprising, by weight %, Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3%, with the remainder being Cu and other unavoidable impurities, having a brazed portion length of 5 mm or more, and a wetting angle of 120° or more between the stainless steel and the brazed portion or between the copper and the brazed portion.

[0032] If the length of the brazing portion is less than 5 mm, or if the wetting angle between the stainless steel and the brazing portion, or between the copper and the brazing portion is less than 120°, good brazing performance cannot be ensured, which may make product application difficult. On the other hand, the brazing portion refers to the area formed when the brazing alloy is dissolved into the stainless steel during brazing.

[0033] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes to illustrate the present invention in more detail and do not limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. [Examples]

[0034] After producing an alloy with the alloy composition shown in Table 1 by casting, the alloy was manufactured in the form of a wire with a diameter of 9 mm. Subsequently, wire drawing, annealing, skin pass, and pickling processes were performed to produce a wire with a diameter of 1.2 mm. At this time, the wire drawability was evaluated, and the results are shown in Table 1 below. The wire drawability was evaluated as "poor" if cracks or breakage occurred during the wire manufacturing process, making wire manufacturing impossible, and as "good" if wire manufacturing was completed.

[0035] Subsequently, the wires according to Invention Example 1, Comparative Examples 1-10, and Conventional Examples 1-3 were fabricated into ring shapes and then applied to torch brazing between identical stainless steel pipes and torch brazing between dissimilar stainless steel and copper pipes. After evaluating the brazing performance, the results are shown in Table 1 below. The brazing performance was evaluated from two aspects: first, by observing the cross-sectional structure of the brazed joint to evaluate whether the wettability was good; and second, by evaluating whether the workability was good. For the wettability evaluation, if the length of the brazed portion was 5 mm or more and the wetting angle between the stainless steel and the brazed portion or between the copper and the brazed portion was 120° or more, it was evaluated as "good"; if the length of the brazed portion was less than 5 mm or the wetting angle was less than 120°, it was evaluated as "poor". The workability evaluation was performed by a torsional shear test, which involved cutting the brazed portion in a cross shape and bending it at 90 degrees, then gripping it with a cutting plier and twisting it to confirm the fracture position. If fracture occurred in the base material, it was evaluated as "good." If fracture occurred in a mixture of the base material and the bonding interface, or if fracture occurred only at the bonding interface, it was evaluated as "poor."

[0036] On the other hand, Conventional Example 1 is a commercially available Cu-Zn alloy, Conventional Example 2 is a commercially available CuP-5 alloy, and Conventional Example 3 is a commercially available Ag-20 alloy. In the case of Comparative Examples 1, 2, 3, 6, and 8, wire fabrication was not possible, and therefore evaluation of brazing properties was also not possible.

[0037] [Table 1]

[0038] As can be seen from Table 1 above, in the case of Invention Example 1, which satisfies the alloy composition proposed by the present invention, it can be confirmed that both brazing properties and wire drawing properties are good. On the other hand, in the case of Comparative Examples 1 to 10, which do not satisfy the alloy composition of the present invention, and Conventional Examples 1 and 2, it can be confirmed that brazing properties or wire drawing properties are poor. On the other hand, in the case of Conventional Example 3, both brazing properties and wire drawing properties are good, but it can be seen that the price competitiveness is low due to the addition of a large amount of the expensive element Ag.

[0039] Figure 1 shows photographs taken after torch brazing between identical stainless steel pipes using Invention Example 1 and Conventional Examples 1 and 2, followed by a torsional shear test. As can be seen from Figure 1, in Invention Example 1, fracture occurred in the base material, confirming good workability of the brazed section. On the other hand, in Conventional Example 1, while some fracture occurred in the base material, some fracture occurred at the joint interface, confirming poor workability of the brazed section. In Conventional Example 2, fracture occurred at the joint interface, confirming poor workability of the brazed section.

[0040] Figure 2 is a cross-sectional microstructure photograph of a brazed joint between identical stainless steel pipes after torch brazing according to Invention Example 1 of the present invention. Figure 3 is a cross-sectional microstructure photograph of a brazed joint between identical stainless steel pipes after torch brazing according to Conventional Example 1 of the present invention. Figure 4 is a cross-sectional microstructure photograph of a brazed joint between identical stainless steel pipes after torch brazing according to Conventional Example 2 of the present invention. As can be seen from Figures 2 to 4, in Invention Example 1, brazing was performed well for approximately 10 mm from the start to the end, and good wettability can be confirmed. Furthermore, the wetting angle was excellent at 120° or more, and no defects or brittle intermetallic compound layers were found.

[0041] On the other hand, in Conventional Example 1, brazing was performed for only about 4 mm at the start, and poor wettability was confirmed. Although no intermetallic compound layer was found, shrinkage defects were observed. In Conventional Example 2, brazing was performed well for about 10 mm from the start to the end, but it was found that a brittle intermetallic compound layer such as Fe2P or Fe3P was formed in the brazed area, which is judged to have induced interfacial fracture during the torsional shear test.

[0042] Figure 5 shows a photograph of a torsional shear test conducted after torch brazing between dissimilar stainless steel and copper pipes using Invention Example 1 and Conventional Example 1 of the present invention. As can be seen from Figure 5, in Invention Example 1, fracture occurs in the base material, confirming that the workability of the brazed portion is good.

[0043] On the other hand, in the case of Conventional Example 1, while some fracture occurred in the base material, some fracture occurred at the joint interface, confirming that the machinability of the brazed portion was poor.

[0044] Figure 6 is a cross-sectional micrograph of the brazed joint between stainless steel and copper dissimilar pipes after torch brazing in Invention Example 1 of the present invention. Figure 7 is a cross-sectional micrograph of the brazed joint between stainless steel and copper dissimilar pipes after torch brazing in Conventional Example 1 of the present invention. As can be seen from Figures 6 and 7, in Invention Example 1, it can be confirmed that no brittle intermetallic compound layer was formed in the brazed area, and no interfacial fracture occurred.

[0045] On the other hand, in the case of Conventional Example 1, it can be confirmed that not only was the brazing not performed properly, but a large defect occurred within the brazed area.

Claims

1. A stainless steel brazing alloy characterized by containing, by weight percent, Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3%, with the remainder being Cu and other unavoidable impurities.

2. The alloy for brazing stainless steel according to claim 1, further comprising Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

3. The brazing alloy for stainless steel according to claim 1, characterized in that it is used when brazing between pipes of the same type of stainless steel or between pipes of different types of stainless steel and copper.

4. As a brazed welded joint between pipes of the same type of stainless steel or between dissimilar pipes of stainless steel and copper, In weight percent, it contains Zn: 36-42%, Ag: 3-6%, P: 0.2% or less (including 0%), Al: 0.01-0.3%, with the remainder being Cu and other unavoidable impurities. The length of the brazing section is 5 mm or more. A brazing joint characterized in that the wetting angle between the stainless steel and the brazing portion, or between the copper and the brazing portion, is 120° or more.

5. The brazed joint according to claim 4, characterized in that the welded joint further comprises Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

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