Brazing method for stainless steel

Abstract

We propose a relatively easy method for brazing stainless steel materials formed from aluminum-containing stainless steel represented by 18Cr-1.5Al-Nb. [Solution] A brazing method for stainless steel materials formed from aluminum-containing stainless steel represented by 18Cr-1.5Al-Nb comprises a thermal oxidation treatment step of applying a thermal oxidation treatment to at least the joint surface of the stainless steel material, a joint surface polishing step of applying a polishing treatment to the joint surface of the thermal oxidation-treated stainless steel material, and a brazing step of brazing the polished joint surface of the stainless steel material using a copper, manganese, and nickel brazing material.

Description

Technical Field

[0001] The present invention relates to a brazing method for stainless steel materials, and more particularly to a brazing method for stainless steel materials formed of aluminum-containing ferritic stainless steel represented by 18Cr-1.5Al-Nb.

Background Art

[0002] Conventionally, as a brazing method for this type of stainless steel material, there are those using a brazing filler metal which is copper powder with a mass ratio of particles passing through a sieve with a mesh size of 45 μm of 5% by mass or more and 40% by mass or less, and the oxygen content of the particles passing through a sieve with a mesh size of 45 μm and remaining on a sieve with a mesh size of 20 μm among the particles constituting the copper powder is 0.02% by mass or less (see, for example, Patent Document 1), those using stainless steel suitable for nickel brazing (see, for example, Patent Document 2), those using stainless steel suitable for phosphorus-copper brazing (see, for example, Patent Document 3), etc.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In heat exchangers used in solid oxide fuel cells and heat exchangers used for recovering waste heat from exhaust gases, the operating environment is high temperature and high humidity. If the heat exchanger is made of stainless steel containing chromium, chromium poisoning will reduce the power generation efficiency. For this reason, it is being considered to construct the heat exchanger using ferritic stainless steel containing aluminum, which has excellent resistance to high temperature oxidation and chromium evaporation by forming aluminum oxide on the surface. When considering miniaturization and mass production, it is preferable to construct the heat exchange components of the heat exchanger by brazing, but with existing nickel brazing material used for stainless steel, it is difficult to create thin films and it is also difficult to prevent chromium poisoning.

[0005] The primary objective of the present invention is to propose a method for relatively easily brazing stainless steel materials formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb. [Means for solving the problem]

[0006] The present invention employs the following means to achieve the main objective described above for brazing stainless steel materials.

[0007] The present invention provides a method for brazing stainless steel materials. A method for brazing stainless steel materials formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb, A thermal oxidation treatment step in which at least the joint surface of the stainless steel material is subjected to thermal oxidation treatment, A joint surface polishing step in which the joint surface that has been subjected to the thermal oxidation treatment is polished, A brazing step in which the polished joint surface is brazed using a copper, manganese, and nickel brazing material, It is characterized by having the following features.

[0008] In this stainless steel brazing method of the present invention, at least the joint surface of a stainless steel material formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb is subjected to a thermal oxidation treatment, the joint surface of the thermally oxidized stainless steel material is polished, and the polished joint surface is brazed using a copper-manganese-nickel brazing material. Since only thermal oxidation treatment and polishing treatment are applied to the joint surface of the stainless steel material, brazing of stainless steel materials formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb can be performed relatively easily and successfully.

[0009] In the stainless steel brazing method of the present invention, the thermal oxidation treatment step is preferably a step of heating the stainless steel at 950°C for 10 hours or more. Furthermore, the brazing step is preferably a step of heating at 995°C to 1005°C for 14 to 16 minutes in a low vacuum argon atmosphere. Moreover, the brazing material is preferably Cu-23.5Mn-9Ni. [Brief explanation of the drawing]

[0010] [Figure 1] This is a process diagram showing an example of the process for brazing stainless steel material according to the embodiment. [Figure 2] This is a schematic diagram illustrating an example of the state in which stainless steel material is brazed by the brazing method of the embodiment. [Figure 3] This is an explanatory diagram showing the brazing method for stainless steel material in an experimental example. [Figure 4] This is a table showing experimental examples and comparative examples. [Figure 5] The following are photographs showing the fracture of the joints of the specimens that tested at 363 MPa in the tensile test of Experimental Example 1 and 454 MPa in the tensile test of Experimental Example 2. [Figure 6] This is an explanatory diagram showing the relationship between the holding time during the thermal oxidation treatment and the presence or absence of the formation of an aluminum oxide (α-Al2O3) coating 22. [Modes for carrying out the invention]

[0011] Next, embodiments for carrying out the present invention will be described. Figure 1 is a process diagram showing an example of the process for brazing stainless steel material according to the embodiment, and Figure 2 is an explanatory diagram schematically showing an example of the state after brazing by the brazing method for stainless steel material according to the embodiment. In the brazing method for stainless steel material according to the embodiment, first, a thermal oxidation treatment is applied to the portion of the stainless steel material including the joint surface, which is formed of aluminum ferrite stainless steel represented by 18Cr-1.5Al-Nb (step S100). When the stainless steel material 20 is subjected to thermal oxidation treatment, a film 22 of aluminum oxide (α-Al2O3) (hatched portion in Figure 2(b)) is formed on the surface of the stainless steel material 20, as shown in Figures 2(a) to (b). In this embodiment, the thermal oxidation treatment was performed by holding at 950°C for 9 hours or more.

[0012] Next, the joining surfaces of the heat-oxidized stainless steel material 20 to be joined by brazing are polished (step S110). In Figure 2(c), the coatings 22a and 22b on the respective joining surfaces of the two stainless steel materials 20a and 20b are polished to remove the aluminum oxide (α-Al2O3) coating 22, and the two stainless steel materials 20a and 20b are arranged so that their respective joining surfaces face each other.

[0013] Next, brazing material is placed near the joining surface of the stainless steel material 20 and brazing is performed (step S120), completing the brazing method for the stainless steel material. As shown in Figures 2(d) to (e), brazing material 24 is placed near the joining surface of the two stainless steel materials 20a and 20b and brazing is performed, allowing the brazing material to penetrate the joining surface of the two stainless steel materials 20a and 20b and completing the brazing. In this embodiment, the brazing was performed in a low vacuum argon atmosphere and held at a temperature of 955°C to 1005°C for 14 to 16 minutes.

[0014] Next, experimental examples will be described. In the experiment, test pieces formed from aluminum-containing ferritic stainless steel represented by 18Cr-1.5Al-Nb with a width of 20 mm, a length of 20 mm, and a thickness of 1.5 mm were used. As the thermal oxidation treatment, the temperature was raised to 950°C in 1 hour in the atmosphere, and then held at 950°C for 24 hours. As the polishing treatment, emery paper (#320, #1000) was used for polishing. As shown in the explanatory drawing of FIG. 3, a clearance A of 0.03 mm was maintained, and the joint surfaces (polished end faces) of the two stainless steel plates 20a and 20b as test pieces were abutted, and both ends were fixed by spot welding, and a linear brazing material 24 of Cu-23.5Mn-9Ni was arranged on the upper part of the joint surface and brazed. The brazing was performed by holding in a furnace at 1000°C for 15 minutes in a low-vacuum argon atmosphere. Then, the brazed test pieces were cut to a width of 10 mm by wire cutting, and a tensile test was performed. The tensile test was performed at room temperature with a crosshead speed of 10 mm / min.

[0015] FIG. 4 is a list showing experimental examples and comparative examples. (1) In Experimental Example 1, four pairs of test pieces were used, subjected to thermal oxidation treatment, polished with #320 emery paper, chamfered on the end faces, and brazed. In Experimental Example 1, it had sufficient penetrability, and in the tensile test, it showed 409 MPa, 231 MPa, 118 MPa, and 363 MPa, indicating a joint strength above a certain level.

[0016] (2) Experimental Example 2 used three pairs of test pieces, subjected to thermal oxidation treatment, polished with #1000 emery paper, chamfered on the end faces, and brazed. In Experimental Example 3, all three test pieces had sufficient penetrability, and in the tensile test, they showed 454 MPa, 341 MPa, and 173 MPa, indicating a joint strength above a certain level. (3) Experimental Example 3 used one pair of test pieces, subjected to thermal oxidation treatment, and polished with #320 emery paper, but did not perform chamfering on the end faces and brazed. In the test piece of Experimental Example 3, the penetrability was slightly poor, and in the tensile test, it showed 293 MPa, indicating a slightly lower joint strength than Experimental Example 2 where chamfering of the end faces was performed.

[0017] (4) As a comparative example, in Experimental Example 4, two pairs of test pieces were used. They were brazed without thermal oxidation treatment, polishing treatment, and chamfering of the end faces. In Experimental Example 4, the permeability of both test pieces was extremely poor, but in the tensile test, they showed 316 MPa and 243 MPa, indicating a certain degree of joint strength.

[0018] From the above experimental examples, it can be seen that in Experimental Examples 1 to 3, although the permeability of the brazing filler metal in Experimental Example 3 is slightly poor, they have better brazing properties compared to the comparative example (Experimental Example 4).

[0019] Fig. 5 shows photographs of the fracture states of the joints of the test piece that was 363 MPa in the tensile test of Experimental Example 1 and the test piece that was 454 MPa in the tensile test of Experimental Example 2. The upper photograph in the figure is a photograph showing the fracture surface of the joint of the test piece, the second photograph is a secondary electron image (SE image) of the fracture surface of the joint of the test piece, the third photograph is a backscattered electron image (BSE image) of the surface of the joint of the test piece cut from the surface to approximately the center, and the lowermost photograph is a backscattered electron image (BSE image) of the cross section of the joint of the other test pieces of Experimental Example 1 and Experimental Example 2. In the secondary electron images of Experimental Examples 1 and 2, it seems that the brazing filler metal has not penetrated, but as can be seen from the backscattered electron images, it can be understood that the brazing filler metal has penetrated sufficiently. That is, effective brazing properties are recognized in both Experimental Examples 1 and 2.

[0020] Figure 6 is an explanatory diagram showing the relationship between the holding time during thermal oxidation treatment and the presence or absence of the formation of an aluminum oxide (α-Al2O3) film 22. In the figure, (a) shows the unheated sample, (b) shows the sample heated to 950°C for 1 hour and then held at 950°C for 1 hour, (c) shows the sample heated to 950°C for 1 hour and then held at 950°C for 9 hours, (d) shows the sample heated to 950°C for 1 hour and then held at 950°C for 10 hours, (e) shows the sample heated to 950°C for 1 hour and then held at 950°C for 12 hours, and (f) shows the sample heated to 950°C for 1 hour and then held at 950°C for 24 hours. In the figure, circles indicate the peak of iron, triangles indicate the peak of chromium, and squares indicate the peak of aluminum oxide (α-Al2O3). As shown in the diagram, since the thermal oxidation treatment only needs to form a stable film of aluminum oxide (α-Al2O3) on the surface of the stainless steel material, it can be seen that the holding time at 950°C should be 10 hours or more.

[0021] In the stainless steel brazing method of the embodiment described above, the joint surface of the stainless steel material formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb is subjected to a thermal oxidation treatment, the thermal oxidation treatment is applied to the joint surface, and the polished joint surface is brazed using a copper-manganese-nickel brazing material. Since the brazing is performed only by applying thermal oxidation treatment and polishing treatment to the joint surface of the stainless steel material, it is possible to braze stainless steel material formed from aluminum-containing ferrite stainless steel represented by 18Cr-1.5Al-Nb relatively easily.

[0022] Although embodiments of the present invention have been described above, the present invention is not limited in any way to these embodiments, and can be implemented in various forms without departing from the spirit of the invention. [Industrial applicability]

[0023] This invention can be used in industries such as stainless steel processing. [Explanation of symbols]

[0024] 20, 20a, 20b Stainless steel material; 22, 22a, 22b Coating; 24 Brazing material.

Claims

1. A method for brazing stainless steel materials formed from aluminum-containing stainless steel represented by 18Cr-1.5Al-Nb, A thermal oxidation treatment step in which at least the joint surface of the stainless steel material is subjected to thermal oxidation treatment, A joint surface polishing step in which the joint surface that has been subjected to the thermal oxidation treatment is polished, A brazing step in which the polished joint surface is brazed using a copper, manganese, and nickel brazing material, A method for brazing stainless steel materials, characterized by having [a certain feature].

2. A method for brazing stainless steel according to claim 1, The aforementioned thermal oxidation treatment step is a step of heating the stainless steel material at 950°C for 10 hours or more. Brazing method for stainless steel materials.

3. A method for brazing stainless steel according to claim 1, The brazing process involves heating in a low-vacuum argon atmosphere at 995°C to 1005°C for 14 to 16 minutes. Brazing method for stainless steel materials.

4. A method for brazing stainless steel according to claim 1, The brazing material is Cu-23.5Mn-9Ni. Brazing method for stainless steel materials.

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