Stainless steel connecting pipe as well as preparation method, application and brazing method thereof

Abstract

The invention relates to the technical field of brazing connection, and provides a stainless steel connecting pipe and a preparation method and a brazing method thereof. The stainless steel connecting pipe is firstly provided and comprises a stainless steel base body and a fused copper-steel layer compounded on the surface of the stainless steel base body, a steel room-temperature structure is distributed in copper of the fused copper-steel layer, and the steel room-temperature structure is partially connected with the stainless steel base body. The fused copper-steel layer is compounded on the surface of the stainless steel connecting pipe, so that the interface characteristic difference between the stainless steel connecting pipe and the stainless steel base body is weakened, the bonding strength of the stainless steel connecting pipe and the stainless steel base body is improved, the fused copper-steel layer serves as a brazing connection transition layer of the stainless steel connecting pipe and the copper connecting pipe, and rapid, high-strength and effective welding is conveniently achieved; and the brazing process of main engine plants such as air conditioners is economical, simple and easy to implement.

Description

Technical Field

[0001] The invention relates to the technical field of brazing connection, in particular to a stainless steel pipe, a preparation method thereof, an application thereof and a brazing method. Background Art

[0002] A large amount of brass and copper raw materials are used in air-conditioning refrigeration systems. Among them, brass materials are mainly lead brass, such as C3771 and C3604 in the Japanese standard JIS H3250. In many links such as forging, processing, assembly, welding and packaging, there are risks of exposure to lead and lead accumulation for employees. The EU ROHS2.0 exemption for lead content in brass will expire on July 21, 2026, and there will only be an 18-month transition period after the expiration. It is urgent to achieve lead-free raw materials for refrigeration accessories. On the other hand, with the application of new energy vehicles, the price of copper remains high, which makes the cost pressure of refrigeration accessories greater, and it is urgent to achieve copper removal in material selection. In the process of promoting lead-free and copper removal of refrigeration valve materials, stainless steel with excellent performance, moderate price and relative stability has become the preferred material. Austenitic stainless steel (such as SUS304) is mainly used, and ferritic stainless steel (such as SUS430) and martensitic stainless steel (such as SUS420) are used in small quantities. Carbon steel material is also available after rust protection.

[0003] On the other hand, air conditioner mainframe manufacturers still use copper pipes. It is difficult to connect stainless steel valve pipes and copper pipes through ordinary flame brazing. They often solve the welding problem by brazing a short section of copper pipe at the welding position of the stainless steel pipe (copper joint solution) or brazing a thin copper sleeve on the inner surface of stainless steel (lined copper sleeve solution) or brazing a thin copper sleeve on the outer surface of stainless steel (outer copper sleeve solution). Not only are the raw materials and process costs high, but there are also risks of falling off and external leakage. The above-mentioned brazing connection methods of stainless steel valve pipes and copper pipes are summarized as follows: Figure 1 As shown. In the process of pre-brazing copper sleeves and copper joints, it is also necessary to prevent stainless steel from "copper brittleness" failure. The so-called "copper brittleness" means that when there are residual copper scraps or cuprous oxide on the surface of stainless steel or in the furnace (smelting, hot processing, welding, etc.), or when the local temperature exceeds the melting point of copper 1083℃ or even higher, copper or cuprous oxide vapor will be formed, which will react with iron to reduce to free copper (Cu2O+Fe=FeO+2Cu). The molten free copper atoms at high temperature penetrate into the surface of the steel part, expand along the grain boundary, and form a brittle liquid film at the grain boundary. The strength of this liquid film is very low. In the subsequent cooling process, due to the difference in the thermophysical properties of stainless steel and copper, the shrinkage degree of the two is inconsistent, resulting in welding heat penetration cracks, which generally tend to crack along the grain boundary. On the surface, copper brittleness presents crack-like crack defects or densely distributed pitting surface defects.

[0004] Therefore, it is of great significance to provide a stainless steel pipe with appropriate performance to achieve effective welding of the stainless steel pipe and the copper pipe. Summary of the invention

[0005] The technical problem solved by the present invention is to provide a stainless steel pipe, which can realize effective welding of the stainless steel pipe and the copper pipe, and has good welding quality and high joint strength.

[0006] In view of this, the present application provides a stainless steel connecting pipe, comprising a stainless steel substrate and a fused copper-steel layer composited on the surface of the stainless steel substrate, wherein the room temperature structure of steel is distributed in the copper of the fused copper-steel layer, and the room temperature structure of the steel is partially connected to the stainless steel substrate.

[0007] Preferably, the room temperature structure of the steel is a single grain or a grain group composed of several grains, and the room temperature structure of the steel includes austenite or ferrite or a mixed structure of austenite and ferrite.

[0008] Preferably, the volume proportion of the room temperature structure of the steel in the fused copper-steel layer is 2-50%.

[0009] Preferably, in the cross section in the thickness direction of the fused copper-steel layer, there is a region with a length of 100 μm within the range of 5% to 50% of the thickness, and the volume proportion of the room temperature structure of the steel in the region is greater than 20%.

[0010] Preferably, the thickness of the fused copper-steel layer is 0.5-40 μm.

[0011] The present application also provides a method for preparing the stainless steel pipe, comprising the following steps:

[0012] A) sandblasting the stainless steel substrate;

[0013] B) preparing a copper layer on the surface of the stainless steel substrate obtained in step A);

[0014] C) fusing the stainless steel pipe obtained in step B).

[0015] Preferably, the roughness of the stainless steel substrate obtained by the sandblasting treatment is Ra0.5 to Ra1.5.

[0016] Preferably, the thickness of the copper layer is 2 to 40 μm; the raw material of the copper layer is pure copper powder with a size of 3 to 20 μm, or a mixed powder of copper powder and nickel powder with equivalent particle size, and the content of nickel powder in the mixed powder is 0.2 to 1.2 wt%; the preparation method is high-pressure cold spraying, the distance between the stainless steel substrate and the spray gun is 10 to 20 mm, the spraying pressure is 3 to 10 MPa, and the temperature of the Laval heating zone is 800 to 900°C.

[0017] Preferably, the protective atmosphere of the fusion treatment is hydrogen or a nitrogen-hydrogen mixture with a hydrogen volume percentage of not less than 10%, the temperature of the fusion treatment is 1000-1150°C, and the time is 10-20 minutes; the porosity of the fused copper-steel layer obtained after the fusion treatment is less than 5%.

[0018] The present application also provides a brazing method, comprising:

[0019] A stainless steel pipe and a copper pipe are brazed, wherein the stainless steel pipe is the stainless steel pipe or the stainless steel pipe prepared by the preparation method, and the fused copper-steel layer of the stainless steel pipe serves as a brazing connection.

[0020] The present application also provides a valve, comprising: a valve body (100) and a stainless steel pipe (200), wherein the valve body (100) and the stainless steel pipe (200) are welded and fixed, and the stainless steel pipe (200) is the stainless steel pipe described above or a stainless steel pipe prepared by the preparation method described above.

[0021] The present application also provides a heat exchanger, comprising: a heat exchanger body (300) and a stainless steel pipe (400), wherein the heat exchanger body (300) and the stainless steel pipe (400) are welded and fixed, and the stainless steel pipe (400) is the stainless steel pipe described above or a stainless steel pipe prepared by the preparation method described above.

[0022] The present application provides a stainless steel pipe, which includes a stainless steel substrate and a fused copper-steel layer compounded on the surface of the stainless steel substrate, wherein the fused copper-steel layer is mainly copper, and is distributed with the room temperature structure of steel, and the room temperature structure of the steel is partially connected to the stainless steel substrate. The fused copper-steel layer in the stainless steel pipe provided by the present application includes the room temperature structure of steel in its microstructure, so that the interface characteristic difference between it and the steel layer substrate is weakened, thereby significantly improving the bonding strength between the two, and the "copper brittleness" problem will not occur during the brazing fusion process. The fused copper-steel layer is used as a connecting layer for brazing the stainless steel pipe and the copper pipe, which is convenient for low-cost fast, high-welding-strength, and effective welding, making the brazing process of main engine manufacturers such as air conditioners simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a simplified diagram summarizing the connection methods of stainless steel valve pipes and copper pipes;

[0024] Figure 2 This is a real photo of a stainless steel pipe partially covered with a fused copper-steel layer according to the present invention;

[0025] Figure 3 This is a schematic diagram of the structure of the fused copper-steel layer of the present invention;

[0026] Figure 4 This is a scanning electron microscope photograph of the fused copper-steel layer prepared in Example 1 of the present invention;

[0027] Figure 5 This is an energy spectrum analysis photograph of the fused copper-steel layer prepared in Example 1 of the present invention;

[0028] Figure 6 This is a scanning electron microscope photograph of the nickel-containing fused copper-steel layer prepared in Example 2 of the present invention;

[0029] Figure 7 This is a scanning electron microscope photograph of the fused copper-steel layer prepared in Comparative Example 1 of the present invention;

[0030] Figure 8 A schematic diagram of the structure of the valve provided by the present invention;

[0031] Fig. 9 This is a schematic structural diagram of the heat exchanger provided by the present invention. DETAILED DESCRIPTION

[0032] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0033] In view of the technical problems existing in the prior art of welding copper pipes with stainless steel pipes, the present application provides a stainless steel pipe and a preparation method thereof, which comprises preparing a fused copper-steel layer on the surface of the stainless steel pipe, such as Figure 2 As shown, the layer has a high bonding strength with the steel matrix, and as a connecting transition layer for welding the stainless steel pipe and the copper pipe, the welding quality of the two can be improved. Specifically, the present application provides a stainless steel pipe, including a stainless steel matrix and a fused copper-steel layer composited on the surface of the stainless steel matrix, the room temperature structure of steel is distributed in the copper of the fused copper-steel layer, and the room temperature structure of the steel is partially connected to the stainless steel matrix.

[0034] The composite fused copper-steel layer on the surface of the stainless steel pipe of the present invention can cover the entire surface of the stainless steel pipe, or can cover part of the surface of the stainless steel pipe (e.g. Figure 2 As shown), that is, the present application has no particular restrictions on the position of the fused copper-steel layer in the stainless steel pipe, and it can be partially covered, fully covered, or internally covered (such as Figure 2 Right), or external coverage (such as Figure 2 Left), it can also cover both the inside and the outside at the same time.

[0035] For the composite fused copper-steel layer on the surface of the stainless steel pipe of the present application, its structural schematic diagram is as follows: Figure 3As shown. Specifically, the fused copper-steel layer is a composite structure formed by copper wrapping the room temperature structure of steel, wherein the room temperature structure of the steel can be a single grain or a grain group composed of multiple grains; depending on the different matrix materials of the stainless steel pipe, the room temperature structure of the steel is also different, which can be austenite, ferrite, or both austenite and ferrite. In the fused copper-steel layer, the room temperature structure of the steel is separated by copper into small pieces (such as pebbles) and distributed in the copper layer, which is completely or half-wrapped by copper, but part of the room temperature structure of the steel is still connected to the steel matrix. The bonding strength between the fused copper-steel layer and the stainless steel matrix can reach more than 100MPa.

[0036] In the present application, the volume proportion of the room temperature structure of the steel in the fused copper-steel layer is 2% to 50%. Specifically, the volume proportion of the room temperature structure of the steel in the fused copper-steel layer is 5% to 30%.

[0037] In the cross section of the fused copper-steel layer in the thickness direction, there is a region with a length of 100 μm in the range of 5% to 50% thickness, in which the volume proportion of the room temperature structure of the steel is greater than 20%.

[0038] In the present application, the thickness of the fused copper-steel layer is 0.5 to 40 μm, and specifically, the thickness of the fused copper-steel layer is 3 to 30 μm.

[0039] Furthermore, the present application also provides a method for preparing a stainless steel pipe, comprising the following steps:

[0040] A) sandblasting the stainless steel substrate;

[0041] B) preparing a copper layer on the surface of the stainless steel substrate obtained in step A);

[0042] C) fusing the stainless steel pipe obtained in step B).

[0043] In the preparation process of the stainless steel pipe, the present application first performs sandblasting on the stainless steel substrate to remove the inert passivation layer on the surface of the stainless steel substrate and ensure that the surface roughness is Ra0.5~Ra1.5. If the roughness is too low, it is not conducive to the basic bonding of the fused copper-steel layer. If the roughness is too high, the surface drop is large, and the copper layer may not be fully covered, resulting in poor density of the copper layer and insufficient bonding strength. The sandblasting improves the surface cleanliness of the stainless steel substrate, provides the basic bonding strength of the steel-copper interface, and is conducive to further improvement of the fusion process and bonding strength. The sandblasting site can be processed according to the specific fusion copper-steel layer site of the stainless steel pipe. The specific implementation method of the sandblasting is carried out in a manner familiar to those skilled in the art, and the present application does not impose any special restrictions on this.

[0044] After the sandblasting, the present application preferably cleans the stainless steel substrate to remove residual impurities such as lubricating oil or cleaning agent during the processing, so as to facilitate the improvement of the bonding strength.

[0045] The present application then prepares a copper layer on the surface of the obtained stainless steel substrate. In the present application, the raw material for obtaining the coating is pure copper powder with a size of 5 to 20 μm, or a mixed powder of copper powder and nickel powder, and the content of nickel powder in the mixed powder is 0.2 to 1.2 wt%. For the above-mentioned pure copper powder, the content of other impurity elements is not higher than 0.2 wt%, and in particular, it cannot contain elements such as aluminum and tin, otherwise the proportion of steel in the copper layer will be too large after the fusion treatment, which will affect the brazing performance; similarly, for the above-mentioned copper powder or nickel powder, the content of other impurity elements is not higher than 0.2 wt%, and in particular, it cannot contain elements such as aluminum and tin, otherwise, after the fusion treatment, the proportion of steel in the copper layer will also be too large, which will affect the brazing performance. The preparation method can adopt a spraying process well known to those skilled in the art, and this application has no special restrictions on this. For example, the spraying adopts a high-pressure cold spraying process, which requires the particle speed of the copper powder to reach more than 560m / s, and the mesh number of the copper powder is not less than 500 mesh; the distance between the stainless steel substrate and the spray gun is 10-20mm, the spraying pressure is 3-10MPa, and the temperature of the Laval heating zone is 800-900℃. If thermal spraying such as plasma spraying, supersonic flame spraying, arc spraying and other processes are adopted, the copper atoms reaching the stainless steel substrate are semi-molten or molten, which is very easy to oxidize. It is necessary to equip the spraying station of the equipment with corresponding oxygen blocking measures (the average oxygen content of the spraying station does not exceed 5wt%), which can ensure that the oxygen content of the prepared copper layer is low, for example, not higher than 0.25wt%, which is conducive to subsequent fusion processing. The thickness of the copper layer is 2-40μm.

[0046] The present application then carries out a fusion treatment on the obtained stainless steel pipe. During the fusion treatment, a dissolution reaction occurs on the scale of several grains located in the shallow surface layer of the steel, which can be further considered to be mainly grain boundary dissolution, accompanied by a small amount of grain dissolution; copper penetrates along the grain boundary of the steel, gradually causing a single grain or a grain group formed by multiple grains of the steel to separate from the steel matrix, and these separated or free grains or grain groups enter the copper layer to form a composite structure with copper, forming a fused copper-steel layer. The fused copper-steel layer is different from the element diffusion effect formed by general metallurgical diffusion, but is a tissue diffusion effect: in the composite structure, a single grain or a grain group is freed into the copper and wrapped by the copper, but the position in the copper is random; under a 1000x electron microscope, the proportion of the room temperature structure of the steel in the fused copper-steel layer is between 2% and 50%. In the present application, the fusion treatment can be carried out in a heat treatment furnace, a tunnel furnace or a mesh belt furnace at 1000-1150°C, specifically at 1060-1140°C; the fusion treatment time is 10-20 minutes, and the protective gas is hydrogen or a nitrogen-hydrogen mixture (the hydrogen ratio is not less than 10%).

[0047] During the fusion process, the temperature and duration of the fusion treatment will affect the room temperature structure ratio, distribution and distribution position of the steel in the fused copper-steel layer; specifically, the higher the temperature and the longer the time of the fusion treatment, the higher the room temperature structure ratio of the steel, but too high a ratio will lead to a decrease in the brazing performance with the copper tube; if the temperature is too low, the densification of the copper layer cannot be achieved, and there are more pores, which affects the strength of the copper layer itself; no fused copper-steel structure appears, which affects the connection strength as a brazing transition layer. After the fusion treatment, the porosity of the fused copper-steel layer is less than 5%, and densification is achieved inside the fused copper-steel layer.

[0048] On the basis of the above-mentioned stainless steel pipe surface being composited with a fused copper-steel layer, the present application provides a brazing method, comprising:

[0049] A stainless steel pipe and a copper pipe are brazed, wherein the stainless steel pipe is the stainless steel pipe described in the above scheme, and the fused copper-steel layer of the stainless steel pipe serves as a brazing connection.

[0050] During the brazing process, the fused copper-steel layer greatly weakens the interface effect between the stainless steel substrate of the stainless steel pipe and the copper substrate of the copper pipe, and significantly improves the bonding strength between the copper layer and the stainless steel substrate. For example, when brazing a stainless steel pipe with a fused copper-steel layer (generally using tin bronze solder) to a section of copper pipe, the pull-off strength is tested as the bonding strength of the fused copper-steel layer, which can reach more than 100MPa. The comprehensive components of the fused copper-steel layer are fused with the copper layer and the steel layer respectively, which reduces the difference in physical parameters between the copper layer and the steel layer, improves the physical compatibility between the two, and can be better used for brazing transition.

[0051] Furthermore, the present application also provides a valve, which includes a valve body and a stainless steel pipe, wherein the valve body and the stainless steel pipe are welded and fixed, and the stainless steel pipe is the stainless steel pipe described in the above solution.

[0052] In one embodiment, the structural schematic diagram of the valve is as follows: Figure 8 As shown, 100 is the valve body and 200 is the stainless steel pipe; as shown in the figure, four stainless steel pipes are arranged on the valve body, which are distributed on both sides of the valve body. In this application, there is no special restriction on the number of the stainless steel pipes, which can be set according to the requirements. This application has no special restriction on the valve body, which can be a valve body familiar to those skilled in the art.

[0053] Furthermore, the present application also provides a heat exchanger, comprising: a heat exchanger body and a stainless steel pipe, wherein the heat exchanger body and the stainless steel pipe are welded and fixed, and the stainless steel pipe is the stainless steel pipe described in the above solution.

[0054] In one embodiment, the structural schematic diagram of the heat exchanger is as follows: Fig. 9 As shown, 300 is the heat exchanger body, 400 is the stainless steel pipe; as shown in the figure, four stainless steel pipes are symmetrically arranged on the heat exchanger body. In this application, the heat exchanger body is a heat exchanger body well known to those skilled in the art, and this application does not impose any special restrictions on this.

[0055] The present invention proposes a stainless steel pipe partially or completely covered with a fused copper-steel layer, wherein the difference in interface characteristics between the stainless steel pipe and the steel layer substrate is weakened, thereby significantly improving the bonding strength between the two; the "copper brittleness" defect will not occur during the fusion process; the fused copper-steel layer serves as a transition layer for brazing stainless steel and copper pipes, facilitating rapid, high-strength, and effective welding, making the brazing process of main engine manufacturers such as air conditioners simple and easy to implement; and the thinner fused copper-steel layer reduces the use of raw material copper, and has greater economy.

[0056] The experimental results show that the bonding strength between the fused copper-steel layer and the stainless steel substrate exceeds 100MPa (welding drawing method), which greatly exceeds the strength of the copper coating formed by spraying (typically, the bonding strength of the cold-sprayed copper layer is 30-80MPa, the actual value is about 50MPa, and the thermal sprayed copper layer is equivalent to it). It avoids peeling during the subsequent processing and manufacturing process, and does not cause copper brittle failure. In the process of stainless steeling of refrigeration valve parts, this layer of fused copper-steel layer with a thickness of only 0.5 to 40μm is sufficient to help the air-conditioning main engine factory implement the brazing process, while improving the thermal conductivity of the stainless steel pipe fittings. It can be quickly heated during brazing, and the brazing efficiency is equivalent to the inner lining copper sleeve or outer copper sleeve scheme. The results are shown in Table 2. In order to further understand the present invention, the stainless steel pipe, its preparation method and brazing method provided by the present invention are described in detail in conjunction with the embodiments below. The protection scope of the present invention is not limited by the following embodiments.

[0057] Example 1

[0058] First, the brazing connection section of the SUS304 stainless steel pipe where the copper layer needs to be prepared needs to be mechanically sandblasted to remove the inert passivation layer on the surface and ensure that the surface roughness is between Ra0.5 and Ra1.5;

[0059] Table 1 SUS304 stainless steel composition data table

[0060] Brand C Si Mn P S Ni Cr other SUS304 ≤0.08 ≤1.00 ≤2.00 ≤0.045 ≤0.03 8.00～10.50 18.00～20.00 --

[0061] Subsequently, a thin layer of copper is prepared inside or / and outside the expanded or contracted position of the stainless steel pipe by a high-pressure cold spraying process; in the high-pressure cold spraying, the inlet pressure of the Laval nozzle (convergent-divergent type) is 3.5MPa, the working gas used is N2, and the temperature of the Laval heating zone is 850°C; the mesh size of the metal powder is controlled between 500-600 meshes, the copper content is above 99.8wt%, and the powder enters the nozzle through a powder feeder and is mixed with the working gas before being sprayed out at a speed of up to 600m / s, and the spraying distance is 10-20mm, so that the high-speed sprayed copper powder impacts the surface of the pipe to form a thin copper layer;

[0062] Finally, the stainless steel pipe with copper layer was fused in a heat treatment furnace for 20 minutes, with hydrogen as the protective gas and a temperature range of 1060°C. After fusion treatment, it was further densified with a porosity of less than 5%, and a stainless steel pipe with a fused copper-steel layer of about 30 μm on the surface was obtained.

[0063] Figure 3 This is a scanning electron microscope photo of the fused copper-steel layer prepared in this embodiment. Figure 4 for Figure 3 Energy spectrum analysis photos corresponding to the fused copper-steel layer; according to Figure 4The combination of the two can clearly show the specific distribution of elements in the fused copper-steel layer. The room temperature structure of the steel is divided into small pieces like pebbles by copper and distributed in the copper layer. It is completely wrapped by copper, but some of it is still connected to the steel matrix. The room temperature structure of the steel contains elements such as Fe, Cr, and Ni (the main components of SUS304), and its composition is basically consistent with the SUS304 steel matrix.

[0064] The fusion treatment of the above embodiment makes the stainless steel substrate and the copper layer become one, and the bonding strength between the fused copper-steel layer and the stainless steel substrate exceeds 100MPa (welding drawing method), which greatly exceeds the strength of the copper coating formed by spraying (typically, the bonding strength of the cold-sprayed copper layer is 30-80MPa, and the actual value is about 50MPa, which is equivalent to the thermal-sprayed copper layer). It avoids peeling during the subsequent processing and manufacturing process, and does not cause copper brittle failure. In the process of stainless steeling of refrigeration valve parts, this fused copper-steel layer with a thickness of only 2 to 40μm is sufficient to help the implementation of the brazing process of the air-conditioning main engine factory, and at the same time improves the thermal conductivity of the stainless steel pipe fittings, which can be quickly heated during brazing, and the brazing efficiency is equivalent to the inner lining copper sleeve or outer copper sleeve scheme, as shown in Table 2.

[0065] Table 2 Comparison of different solutions Reference to the main engine factory brazing using the same size of stainless steel parts and copper (refrigeration industry generally TP2 material) brazing performance data table

[0066]

[0067]

[0068] Example 2

[0069] First, the brazing connection section of the stainless steel pipe that needs to be prepared is first mechanically sandblasted to remove the inert passivation layer on the surface and ensure that the surface roughness is between Ra0.5 and Ra1.5;

[0070] Subsequently, a high-pressure cold spraying process is used to prepare a thin layer of copper + nickel inside or / and outside the expanded or contracted position of the stainless steel pipe, with a thickness between 2 and 40 μm; in the high-pressure cold spraying, the air inlet pressure of the Laval nozzle (convergent-divergent type) is 3.5 MPa, the working gas used is N2, and the temperature of the Laval heating zone is 850° C. The mesh size of the metal powder is controlled between 500 and 600 meshes, wherein the content of copper powder is about 99wt%, and the content of nickel powder is about 1wt%, and the powder enters the nozzle through a powder feeder and is mixed with the working gas before being sprayed out at a speed of up to 600 m / s, and the spraying distance is 10-20 mm, so that the high-speed sprayed metal powder impacts the surface of the pipe to form a thin copper + nickel layer;

[0071] Finally, the stainless steel pipe fittings with copper + nickel layer are subjected to fusion treatment in a heat treatment furnace for 15 minutes, the protective gas is hydrogen, and the temperature range is 1100-1140°C; after the fusion treatment, they are further densified, and the porosity is less than 5%, thereby obtaining stainless steel pipe fittings with a fused copper-steel layer on the surface.

[0072] like Figure 6 As shown, the structure of the fused copper-steel layer is inlay material, fused copper-steel layer and steel matrix from top to bottom; the introduction of 1wt% nickel powder makes the room temperature structure of steel in the fused copper-steel layer tend to gather toward the surface, but still above the straight line, and there is an obvious interface between the copper-steel layer and the steel layer, the fusion effect is weakened, and the bonding strength is about 90MPa (welding drawing method), which is lower than that of Example 1. In addition, as the room temperature structure of steel in the fused copper-steel layer gathers toward the surface, the characteristics of the steel at the surface are strengthened, and the brazing performance and brazing efficiency are reduced.

[0073] Comparative Example 1

[0074] The preparation method is the same as that in Example 1, except that the copper layer is prepared by low-pressure cold spraying, and the raw copper powder of the copper layer contains 10wt% alumina powder, which acts as a hard point to prevent nozzle clogging. The scanning electron microscope photo of the final fused copper-steel layer is as follows: Figure 7 As shown in the figure, it can be seen that the residual proportion of aluminum oxide in the copper layer is about 2% to 5%. The aluminum oxide in the copper layer causes excessive fusion of the copper layer and the steel layer. The proportion of steel organization is close to 90%, and the brazing property is relatively poor. It is difficult to braze and is even comparable to a stainless steel pipe.

[0075] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0076] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A stainless steel pipe, comprising a stainless steel substrate and a fused copper-steel layer composited on the surface of the stainless steel substrate, wherein the room temperature structure of steel is distributed in the copper of the fused copper-steel layer, and the room temperature structure of the steel is partially connected to the stainless steel substrate.

2. The stainless steel pipe according to claim 1, characterized in that: The room temperature structure of the steel is a single grain or a grain group composed of several grains, and the room temperature structure of the steel includes austenite or ferrite or a mixed structure of austenite and ferrite.

3. The stainless steel pipe according to claim 1, characterized in that: The room temperature structure of the steel accounts for 2 to 50% by volume in the fused copper-steel layer.

4. The stainless steel pipe according to claim 1, characterized in that: In the cross section of the fused copper-steel layer in the thickness direction, there is a region with a length of 100 μm within the range of 5% to 50% thickness, and the volume proportion of the room temperature structure of the steel in the region is greater than 20%.

5. The stainless steel pipe according to claim 1, characterized in that: The thickness of the fused copper-steel layer is 0.5-40 μm.

6. The method for preparing the stainless steel pipe according to claim 1, comprising the following steps: A) sandblasting the stainless steel substrate; B) preparing a copper layer on the surface of the stainless steel substrate obtained in step A); C) fusing the stainless steel pipe obtained in step B).

7. The preparation method according to claim 6, characterized in that: The roughness of the stainless steel substrate obtained by the sandblasting treatment is Ra0.5 to Ra1.

5.

8. The preparation method according to claim 6, characterized in that: The thickness of the copper layer is 2 to 40 μm; the raw material of the copper layer is pure copper powder with a size of 3 to 20 μm, or a mixed powder of copper powder and nickel powder with equivalent particle sizes, and the content of nickel powder in the mixed powder is 0.2 to 1.2 wt%; the preparation method is high-pressure cold spraying, the distance between the stainless steel substrate and the spray gun is 10 to 20 mm, the spraying pressure is 3 to 10 MPa, and the temperature of the Laval heating zone is 800 to 900°C.

9. The preparation method according to claim 6, characterized in that: The protective atmosphere of the fusion treatment is hydrogen or a nitrogen-hydrogen mixed gas with a hydrogen volume percentage of not less than 10%. The temperature of the fusion treatment is 1000-1150° C. and the time is 10-20 minutes. The porosity of the fused copper-steel layer obtained after the fusion treatment is less than 5%.

10. A brazing method comprising: A stainless steel pipe and a copper pipe are brazed, wherein the stainless steel pipe is the stainless steel pipe described in any one of claims 1 to 5 or the stainless steel pipe prepared by the preparation method described in any one of claims 6 to 9, and the fused copper-steel layer of the stainless steel pipe serves as the brazing connection.

11. A valve, comprising: A valve body (100) and a stainless steel pipe (200), wherein the valve body (100) and the stainless steel pipe (200) are welded and fixed, and the stainless steel pipe (200) is the stainless steel pipe according to any one of claims 1 to 5 or a stainless steel pipe prepared by the preparation method according to any one of claims 6 to 9.

12. A heat exchanger comprising: A heat exchanger body (300) and a stainless steel pipe (400), wherein the heat exchanger body (300) and the stainless steel pipe (400) are welded and fixed, and the stainless steel pipe (400) is the stainless steel pipe according to any one of claims 1 to 5 or a stainless steel pipe prepared by the preparation method according to any one of claims 6 to 9.

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