Alloy for brazing of stainless steel, and brazed joint

Abstract

The present invention relates to an alloy for brazing of stainless steel, and a brazed joint, and, more specifically, to an alloy for brazing of stainless steel and a brazed joint, that can be preferably applied when pipes made of the same material such as stainless steel or pipes made of different materials such as stainless steel and copper are brazed.

Description

Alloys and brazing joints for stainless steel

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

[0002]

[0003] Air conditioners are devices that change temperature through the compression and expansion of refrigerant, and are widely used in electronic appliances such as air conditioners, refrigerators, and washing machines. Most existing pipes that carry the refrigerant in air conditioners are made of copper or copper-based alloys because of their excellent corrosion resistance and brazing properties. Furthermore, when brazing copper pipes, a relatively inexpensive CuP series alloy (containing copper as a main component, plus phosphorus and 0-15% silver) is typically used, which enables brazing without the use of flux.

[0004]

[0005] In the case of the above Cu or Cu-based alloy (hereinafter also referred to as 'Cu metal'), since the raw material price is expensive and the price fluctuates greatly, it is actively being replaced with stainless steel (hereinafter also referred to as 'STS') that can guarantee corrosion resistance and strength and is relatively inexpensive. However, although the corrosion characteristics of Cu metal and STS are almost similar, in the case of STS, since a thin layer of Cr-based oxide is formed on the surface, the molten insert metal during brazing does not wet the STS surface, making brazing difficult. To solve this problem, a method of using an insert metal of the Ag series (containing Cu and other alloys in addition to Ag as the main component (approximately 30-70%)) has been proposed. However, since Ag is about 100 times more expensive than Cu and other elements, the brazing cost increases, so the cost reduction effect of replacing STS mentioned above is lost, and there are limitations in practical application.

[0006]

[0007] As mentioned above, in order to change expensive Cu metal to inexpensive STS, it is essential to develop inexpensive brazing insert metal, and it is necessary to develop a material that can ensure the reliability of the brazing joint while ensuring that the insert metal melted on the STS surface during brazing (torch, high frequency, vacuum, etc.) is well wetted.

[0008]

[0009] [Prior Art Literature]

[0010] (Patent Document 1) Japanese Patent Publication No. 2018-508361

[0011] (Patent Document 2) EP Patent Publication No. 2852695

[0012]

[0013] One aspect of the present invention is to provide an alloy for brazing stainless steel and a brazing joint.

[0014] Another aspect of the present invention is to provide a stainless steel brazing alloy having good fresh processability, low cost, and excellent brazing properties, and a brazing joint using the alloy.

[0015]

[0016] One embodiment of the present invention provides an alloy for brazing stainless steel, which comprises, by weight %, Zn: 36 to 42%, Ag: 3 to 6%, P: 0.2% or less (including 0%), Al: 0.01 to 0.3%, the remainder Cu, and other unavoidable impurities.

[0017]

[0018] The above alloy may additionally contain Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

[0019]

[0020] The above brazing alloy can be used for brazing between stainless steel homogeneous pipes or between stainless steel and copper dissimilar pipes.

[0021]

[0022] Another embodiment of the present invention provides a brazed joint between stainless steel homogeneous pipes or between stainless steel and copper dissimilar pipes, the brazed joint comprising, in wt%, Zn: 36 to 42%, Ag: 3 to 6%, P: 0.2% or less (including 0%), Al: 0.01 to 0.3%, the remainder being Cu and other unavoidable impurities, the length of the brazed portion being 5 mm or more, and the wetting angle between the stainless steel and the brazed portion or between the copper and the brazed portion being 120° or more.

[0023]

[0024] The above weld joint may additionally contain Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

[0025]

[0026] According to one aspect of the present invention, an alloy for brazing stainless steel and a brazing joint can be provided.

[0027] According to another aspect of the present invention, a stainless steel brazing alloy having good fresh processability and excellent brazing properties while being inexpensive, and a brazing joint using the alloy can be provided.

[0028]

[0029] FIG. 1 is a photograph of a torsional shear test performed after torch brazing between stainless steel pipes of the same type using Invention Example 1 and Conventional Examples 1 and 2 according to one embodiment of the present invention.

[0030] FIG. 2 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel homogeneous pipes of Invention Example 1 according to one embodiment of the present invention.

[0031] FIG. 3 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel homogeneous pipes of a conventional example 1 according to one embodiment of the present invention.

[0032] FIG. 4 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel pipes of the prior art example 2 according to one embodiment of the present invention.

[0033] FIG. 5 is a photograph of a torsional shear test performed after torch brazing between stainless steel and copper dissimilar pipes using Invention Example 1 and Conventional Example 1 according to one embodiment of the present invention.

[0034] FIG. 6 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel and copper dissimilar pipes of Invention Example 1 according to one embodiment of the present invention.

[0035] FIG. 7 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel and copper dissimilar pipes of conventional example 1 according to one embodiment of the present invention.

[0036]

[0037] Hereinafter, a stainless steel brazing alloy according to one embodiment of the present invention will be described. The brazing alloy of the present invention comprises a brass alloy (Cu+Zn) as its basic component system and includes silver. In addition, the stainless steel brazing alloy according to one embodiment may further include P, Al, Ni, and Mn.

[0038] In the case of commercial brass insert metal, some are applied in the brazing of STS, but they have the disadvantage of high melting point and insufficient wettability, so that the brazing time is excessive and the brazing property and corrosion resistance of the brazed part are poor. On the other hand, the alloy for brazing stainless steel according to an embodiment of the present invention includes Ag, which has excellent wettability on the surface of STS, so that the melting point can be lowered and the wettability can be improved compared to the existing brass insert metal. In addition, the alloy for brazing stainless steel according to an embodiment of the present invention can supplement the insufficient wettability by including Al. Meanwhile, the alloy for brazing stainless steel according to an embodiment of the present invention may further include Ni and Mn to improve other workability, bonding strength, and corrosion resistance.

[0039]

[0040] Meanwhile, commercial Cu-based brazing insert metals contain more than 5% P to lower the melting point and improve wettability, but in the case of brazing insert metals for STS, the P content needs to be extremely limited. This is because STS contains Fe, so when brazing STS using a brazing insert metal containing P, the P component creates an intermetallic layer called Fe2P or Fe3P, which is vulnerable to the joining interface, and thus, when testing the workability of the brazing joint, destruction occurs at the interface, making it impossible to guarantee the soundness of the brazing joint.

[0041]

[0042] Hereinafter, a specific description will be given of an alloy for brazing stainless steel according to one embodiment of the present invention. The contents of the alloy composition described below are in wt% unless otherwise specified.

[0043]

[0044] Zn: 36~42%

[0045] Zn acts to lower the melting point by being alloyed with Cu, and at the same time, reduces Cr oxide of STS and improves the bonding strength with Fe. In order to sufficiently obtain the above effect, it is preferable that the Zn content is 36% or more. On the other hand, when the Zn content exceeds 42%, the fresh workability deteriorates due to an increase in the beta phase, and the amount of zinc fume generated during brazing increases, which may lower the quality of the brazed joint. The lower limit of the Zn content is more preferably 37%, and is even more preferably 38%. The upper limit of the Zn content is more preferably 41%, and is even more preferably 40%.

[0046]

[0047] Ag: 3~6%

[0048] Ag, when alloyed with Cu, lowers the melting point while also improving the surface oxide and wettability of STS. To fully achieve the above effects, the Ag content is preferably 3% or more. However, since Ag is very expensive, a content exceeding 6% is economically disadvantageous. The lower limit of the Ag content is more preferably 4%. The upper limit of the Ag content is more preferably 5%.

[0049]

[0050] P: 0.2% or less (including 0%)

[0051] Phosphorus (P) is generally the element that most strongly lowers the melting point in Cu-based insert metals, demonstrating a significant reduction in brazing temperature. However, even when added in very small amounts during STS brazing, P reacts with Fe in STS to form highly brittle intermetallic compounds such as Fe2P and Fe3P, thereby hindering the workability of the joint. Therefore, in the present invention, it is advantageous to reduce the P content as much as possible, and control it to a level of 0.2% or less.

[0052]

[0053] Al: 0.01~0.3%

[0054] Al is alloyed with Cu to lower the melting point and at the same time, it plays a role in reducing Cr oxide on the surface of STS, thereby improving the wettability of the molten insert metal. In order to sufficiently obtain the above effect, the content of Al is preferably 0.01% or more. However, if the content of Al exceeds 0.3%, there is a disadvantage in that the fresh workability is poor, making wire manufacturing difficult. Meanwhile, in order to confirm whether Al is wetted on the surface of STS, the Gibbs Free Energy at 700℃ can be calculated as in [Equation 1] below.

[0055]

[0056] [Equation 1] 4 / 3Al + O2= 2 / 3Al2O3: △G @700℃ ≒ - 920kJ / mol

[0057]

[0058] As can be seen from the above equation 1, the Gibbs free energy of formation of Al oxide is approximately 1.6 times lower than the Gibbs free energy of formation of Cr oxide. That is, when Cr2O3, which is a Cr oxide formed on the surface of STS, is reduced and Al2O3, which is an Al oxide, is formed, a wetting phenomenon occurs on the surface.

[0059]

[0060] The remaining component is copper. However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the normal manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0061]

[0062] Meanwhile, the alloy for brazing stainless steel of the present invention may additionally include Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

[0063]

[0064] Ni: 0.01~0.3%

[0065] When added in small amounts to Cu-Zn-Ag alloys, nickel improves the workability, bonding strength, and corrosion resistance. To achieve the above effects, the nickel content is preferably 0.01% or more. However, when the nickel content exceeds 0.3%, the disadvantage is that the workability is poor, making wire manufacturing difficult.

[0066]

[0067] Mn: 0.01~0.3%

[0068] When added in small amounts to Cu-Zn-Ag alloys, manganese improves the workability, bonding strength, and corrosion resistance. To achieve the above effects, the content of nickel is preferably 0.01% or more. However, when the content of nickel exceeds 0.3%, the disadvantage is that the workability is poor, making wire manufacturing difficult.

[0069]

[0070] As described above, the stainless steel brazing alloy of the present invention can be preferably used for brazing between stainless steel pipes of the same type or between stainless steel and copper pipes of different types. In addition, the stainless steel brazing alloy of the present invention has excellent drawability and excellent brazing properties while being inexpensive.

[0071]

[0072] Meanwhile, the stainless steel brazing alloy of the present invention can be manufactured by various methods, and the present invention does not specifically limit the manufacturing method. However, as an 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 specifically limit the form. However, as an example, it can be manufactured and used in the form of a wire having an appropriate diameter through a number of drawing, annealing, skin pass, and pickling processes. As another example, the wire can be manufactured and used in the form of a ring.

[0073]

[0074] Another embodiment of the present invention provides a brazing joint using the alloy for brazing stainless steel described above. More specifically, the present invention provides a brazing joint brazed between stainless steel homogeneous pipes or between stainless steel and copper dissimilar pipes, the brazing joint comprising, in wt%, Zn: 36 to 42%, Ag: 3 to 6%, P: 0.2% or less (including 0%), Al: 0.01 to 0.3%, the remainder being Cu and other unavoidable impurities, the length of the brazing portion being 5 mm or more, and the wetting angle between the stainless steel and the brazing portion or between the copper and the brazing portion being 120° or more.

[0075]

[0076] If the length of the brazing portion is less than 5 mm, or the wetting angle between the stainless steel and the brazing portion or between the copper and the brazing portion is less than 120°, it may be difficult to apply the product because good brazing properties are not secured. Meanwhile, the brazing portion refers to an area formed by the melting of a brazing alloy during the brazing of stainless steel.

[0077]

[0078] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0079]

[0080] (Example)

[0081] An alloy having the alloy composition shown in Table 1 below was manufactured through casting, and the alloy was then formed into a wire having a diameter of 9 mm. Thereafter, wire drawing, annealing, skin pass, and pickling processes were performed to manufacture a wire having a diameter of 1.2 mm. At this time, the wire drawing processability was evaluated, and the results are shown in Table 1 below. The wire drawing processability was evaluated as ‘poor’ when cracks or breaks occurred during the wire drawing process, making it impossible to manufacture the wire, and as ‘good’ when the wire drawing process was completed. Thereafter, the wires according to Inventive Example 1, Comparative Examples 1 to 10, and Conventional Examples 1 to 3 were manufactured into a ring shape, and then applied to torch brazing between stainless steel homogeneous pipes and between stainless steel and copper dissimilar pipes, and the brazing processability was evaluated, and the results are shown in Table 1 below. The brazing property was evaluated from two aspects: one was to evaluate whether the wettability was good by observing the cross-sectional structure of the brazed joint, and the other was to evaluate whether the workability was good. The wettability was evaluated as ‘good’ when the length of the brazing part was 5 mm or more and the wetting angel between the stainless steel and the brazing part or between the copper and the brazing part was 120° or more, and was evaluated as ‘poor’ when the length of the brazing part was less than 5 mm or the wetting angel was less than 120°. The workability was evaluated through a torsional shear test, which was conducted by cutting the brazing part into a cross shape, bending it at 90 degrees, and then twisting it with cutting pliers to check the fracture location. If a fracture occurred in the base material, it was evaluated as ‘good’, and if a fracture occurred mixedly in the base material and the bonding interface, or if a fracture occurred in the bonding interface, it was evaluated as ‘poor’. Meanwhile, conventional example 1 is a commercial product Cu-Zn alloy, conventional example 2 is a commercial product CuP-5 alloy, and conventional example 3 is a commercial product Ag-20 alloy.In addition, in the case of comparative examples 1, 2, 3, 6, and 8, since wire production was not possible, evaluation of brazing properties was also not possible.

[0082]

[0083] Classification Alloy composition (weight %) Brazing property Drawability ZnAgPAlNiMnCu Wetting property Workability Invention example 138.0500.30.30.3 Remainder Good Good Good Comparative example 135.04.70.310.50.5 Remainder--Poor Comparative example 233.050211 Remainder--Poor Comparative example 338.430.10.50.30.3 Remainder--Poor Comparative example 4404000.010.01 Remainder Poor Good Good Comparative example 53850.40.010.020.02 Remainder Poor Good Good Comparative Example 638.050.10.20.50.1 Balance--Bad Comparative Example 738.050.10.200.1 Balance Good Bad Good Comparative Example 838.050.10.10.10.6 Balance--Bad Comparative Example 938.050.10.10.20 Balance Good Bad Good Comparative Example 1038.01.500.10.10.1 Balance Bad Bad Good Conventional Example 140-----Balance Bad Bad Good Conventional Example 2-155---Balance Good Bad Good Conventional Example 33230----Balance Good Good Good

[0084]

[0085] As can be seen from Table 1 above, in the case of Inventive Example 1, which satisfies the alloy composition proposed by the present invention, it can be confirmed that both brazing properties and wire workability are good. On the other hand, in the case of Comparative Examples 1 to 10 and Conventional Examples 1 and 2, which do not satisfy the alloy composition of the present invention, it can be confirmed that the brazing properties or wire workability are poor. On the other hand, in the case of Conventional Example 3, although both brazing properties and wire workability are good, it can be seen that the price competitiveness is low due to the addition of a large amount of Ag, which is an expensive element.

[0086]

[0087] Fig. 1 is a photograph of a torsional shear test conducted after torch brazing between stainless steel pipes of the same type using Inventive Example 1 and Conventional Examples 1 and 2. As can be seen from Fig. 1, in the case of Inventive Example 1, it can be confirmed that the workability of the brazed portion is good because a fracture occurs in the base material. On the other hand, in the case of Conventional Example 1, it can be confirmed that some fractures occurred in the base material, but some fractures occurred at the joint interface, and thus the workability of the brazed portion is poor. In the case of Conventional Example 2, it can be confirmed that the workability of the brazed portion is poor because a fracture occurred at the joint interface.

[0088]

[0089] FIG. 2 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel pipes of the invention example 1 according to an embodiment of the present invention. FIG. 3 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel pipes of the prior art example 1 according to an embodiment of the present invention. FIG. 4 is a cross-sectional tissue photograph of a brazed joint after torch brazing between stainless steel pipes of the prior art example 2 according to an embodiment of the present invention. As can be seen from FIGS. 2 to 4, in the case of the invention example 1, brazing was performed well for about 10 mm from the start to the end, confirming good wetting properties. In addition, the wetting angle was excellent at 120° or more, and no defective portions or weak intermetallic compound layers were found. On the other hand, in the case of conventional example 1, brazing was performed only about 4 mm from the start portion, confirming poor wettability, and although no intermetallic compound layer was found, it was known that shrinkage defects existed. In the case of conventional example 2, brazing was performed well about 10 mm from the start portion to the end portion, but it was known that a weak intermetallic compound layer such as Fe2P or Fe3P was created in the brazing portion, and it is believed that this caused interface fracture during the torsional shear test.

[0090]

[0091] FIG. 5 is a photograph of a torsional shear test conducted after torch brazing between stainless steel and copper dissimilar pipes using Invention Example 1 and Conventional Example 1 according to an embodiment of the present invention. As can be seen from FIG. 5 , in the case of Invention Example 1, fracture occurred in the base material, confirming good workability of the brazed portion. On the other hand, in the case of Conventional Example 1, fracture occurred in some of the base materials, but in some of the cases, fracture occurred at the bonding interface, confirming poor workability of the brazed portion.

[0092]

[0093] Fig. 6 is a cross-sectional micrograph of a brazed joint after torch brazing between stainless steel and copper dissimilar pipes of Invention Example 1 according to an embodiment of the present invention. Fig. 7 is a cross-sectional micrograph of a brazed joint after torch brazing between stainless steel and copper dissimilar pipes of Conventional Example 1 according to an embodiment of the present invention. As can be seen from Figs. 6 and 7, in the case of Invention Example 1, it can be confirmed that a weak intermetallic compound layer was not formed in the brazing portion, and thus no interface fracture occurred. On the other hand, in the case of Conventional Example 1, it can be confirmed that not only was brazing not performed well, but also a large defect occurred in the brazing portion.

Claims

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

2. In claim 1, The above alloy is an alloy for brazing stainless steel, which additionally contains Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

3. In claim 1, The above brazing alloy is a stainless steel brazing alloy characterized in that it is used for brazing between stainless steel homogeneous pipes or between stainless steel and copper dissimilar pipes.

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

5. In claim 4, The above weld joint is a weld joint additionally containing Ni: 0.01 to 0.3% and Mn: 0.01 to 0.3%.

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