Ni brazing material

Abstract

The present invention provides an Ni brazing material which exhibits excellent corrosion resistance in a brazed part, while having excellent brazability.　This Ni brazing material has a component composition that contains, by mass%, 8.0-25.0% of Cr, 3.0-20.0% of Fe, 2.0-7.0% of Mo, 1.50-5.00% of B, and 0.10-1.00% C, with the balance being made up of Ni and inevitable impurities, while satisfying formula (1). (1): 2.00 ≤ B + C ≤ 5.50 In formula (1), B and C indicate the content (mass%) of each element.

Description

Ni brazing material

[0001] The present invention relates to a Ni brazing filler metal that has excellent brazeability and can suppress a decrease in corrosion resistance at the brazed portion, particularly when brazing to ferritic stainless steel at high temperatures.

[0002] In recent years, from the standpoint of protecting the global environment, there has been a demand for further improvements in fuel efficiency and enhanced exhaust gas purification for automobiles. For this reason, the application of automotive heat exchangers such as exhaust heat recovery devices and EGR (Exhaust Gas Recirculation) coolers is expanding.

[0003] Here, a waste heat recovery device is a device that improves fuel efficiency by warming engine coolant with the heat of exhaust gas and shortening the warm-up time at engine start. Generally, a waste heat recovery device is installed between the catalytic converter and the muffler and is composed of a heat exchanger section that combines pipes, plates, fins, side plates, etc., an inlet pipe section, and an outlet pipe section. Exhaust gas enters the heat exchanger section from the inlet pipe, where it transfers its heat to the coolant via heat transfer surfaces such as fins, and is then discharged from the outlet pipe. Furthermore, brazing, rather than welding, is mainly used to bond and assemble the plates and fins that make up the heat exchanger section of such a waste heat recovery device.

[0004] In addition, the EGR cooler recirculates exhaust gas with low oxygen concentration to the intake side of the engine, lowering the fuel combustion temperature and reducing nitrogen oxides (NO Ｘ) is a device for suppressing the temperature rise. If high-temperature exhaust gas is directly returned to the engine, fuel will burn at an inappropriate time, resulting in abnormal vibrations known as knocking. For this reason, EGR coolers are composed of a pipe that takes in some of the exhaust gas, a heat exchanger that cools the taken-in exhaust gas, and a pipe that returns the cooled exhaust gas to the intake side of the engine. The temperature of the exhaust gas is over 600°C at the inlet side of the EGR cooler, but is cooled by the heat exchanger to below 100°C at the outlet side. In addition, the heat exchanger portion of the EGR cooler is composed of thin plates stacked in a fin-like shape for reasons of weight reduction, compactness, and cost reduction, and brazing is also mainly used for joining and assembling these parts.

[0005] As described above, the heat exchanger parts of the exhaust heat recovery device and the EGR cooler are bonded and assembled by brazing, so the brazing material is required to have excellent brazing properties. Ｘ In addition to sulfur oxides (SO Ｘ These contain hydrocarbons (HC), which condense inside the heat exchanger, turning into highly corrosive, acidic condensed water. For this reason, the brazing filler metals used in these heat exchanger parts must also have excellent corrosion resistance.

[0006] Therefore, the use of Ni brazing filler metal, which has excellent brazing properties and corrosion resistance, for exhaust heat recovery devices and EGR coolers has been considered.

[0007] For example, Patent Document 1 discloses a Ni brazing filler metal for brazing gas turbines and heat exchangers, which has a large amount of Cr added to ensure corrosion resistance and also has excellent brazability by adding P and Si in amounts that satisfy a certain relationship.

[0008] Patent Document 2 discloses a Ni brazing filler metal for brazing general-purpose heat exchangers and water heaters, which can be brazed at a low melting point by controlling the amounts of P and B added.

[0009] Patent Document 3 discloses a Ni brazing filler metal that ensures high-temperature strength by controlling the amounts of W and Mo added, particularly for brazing high-temperature heat exchangers.

[0010] In addition, Patent Document 4 discloses a Ni brazing filler metal that has excellent brazing properties when used for brazing general-purpose heat exchangers, EGR coolers, and water heaters, by adding P and Si in amounts that satisfy a certain relationship and also adding Sn.

[0011] Furthermore, Patent Document 5 discloses a Ni brazing filler metal for brazing EGR coolers and exhaust manifolds, which is cost-effective because part of the Ni is replaced with Fe or the like.

[0012] JP 2002-144080 A JP 2022-182396 A International Publication No. 2012 / 081346 International Publication No. 2015 / 156066 Japanese Patent No. 6116795 A

[0013] Exhaust heat recovery devices and EGR coolers have typically been made of austenitic stainless steels such as SUS316L or SUS304L, which have a reduced carbon content to prevent sensitization. However, austenitic stainless steels have problems, such as high cost due to their large Ni content, and poor fatigue properties in operating environments where they are subjected to high-temperature restraints due to their large thermal expansion. For these reasons, austenitic stainless steels are increasingly being replaced by ferritic stainless steels such as SUS444.

[0014] However, when the Ni brazing filler metals disclosed in Patent Documents 1 to 5 are applied to ferritic stainless steel, there is a problem that the corrosion resistance of the brazed portion is reduced, causing corrosion.

[0015] The present invention has been made in view of the above-mentioned current situation, and an object of the present invention is to provide a Ni brazing filler metal that has excellent brazing properties and also has excellent corrosion resistance at the brazed portion.

[0016] In this specification, excellent brazability means that, after a brazing process is performed in which a steel plate coated with Ni brazing filler metal on its surface is heated at 1080°C in a nitrogen carrier gas atmosphere of 1 Torr for 10 minutes and then cooled to room temperature, the ratio of the circle-equivalent diameter of the brazing filler metal after heating to the circle-equivalent diameter of the brazing filler metal before heating (spreadability of the brazing filler metal) is 150% or more.

[0017] Furthermore, in this specification, "excellent corrosion resistance" refers to a pitting potential Vc'100 of 300 mV (vs. SCE) or more, measured in accordance with JIS G 0577:2014 by taking a 15 mm square test piece from the brazed portion (the portion to which the brazing material is attached by the brazing process) of a steel plate that has been subjected to the above-mentioned brazing process, covering the test piece with a sealing material except for an 11 mm square measurement surface, and immersing the test piece in a 3.5 mass % NaCl solution at 30°C, except for the concentration of the NaCl solution.

[0018] As mentioned above, conventional Ni brazing filler metals have the problem that their corrosion resistance decreases when used with ferritic stainless steel.

[0019] The inventors believe that the cause of this is as follows: The thermal expansion coefficient of Ni brazing filler metal is larger than that of ferritic stainless steel. Therefore, thermal stress in the tensile direction occurs in the brazing filler metal during cooling after the heat treatment in the brazing process, causing cracks in the brazed part. When condensed water penetrates into the cracks generated by the above mechanism, crevice corrosion occurs.

[0020] Based on the above mechanism, the present inventors conducted extensive research into the component elements of various brazing filler metals, their brazing properties, and the corrosion resistance of the brazed joint when brazing is performed using a Ni brazing filler metal. As a result, they discovered the following: By adding appropriate amounts of B and C to the brazing filler metal and further controlling the total content of B and C within an appropriate range, the thermal expansion coefficient of the brazing filler metal is reduced. This suppresses cracking of the brazing filler metal caused by thermal stress during brazing heat treatment, ensuring corrosion resistance while also achieving excellent brazing properties. The present invention was completed based on these findings and after further research.

[0021] That is, the gist of the present invention is as follows: [1] A Ni brazing filler metal containing, in mass%, 8.0 to 25.0% Cr, 3.0 to 20.0% Fe, 2.0 to 7.0% Mo, 1.50 to 5.00% B, and 0.10 to 1.00% C, satisfying the following formula (1), with the balance being Ni and unavoidable impurities: 2.00≦B+C≦5.50 (1) where B and C in formula (1) represent the content (mass%) of each element. [2] The Ni brazing filler metal according to [1], wherein the component composition further contains, in mass %, one or more selected from Cu: 10.0% or less, Co: 20.0% or less, Mn: 5.0% or less, W: 5.0% or less, P: 5.0% or less, and Si: 5.0% or less. [3] The Ni brazing filler metal according to [1] or [2], which is used in a heat exchanger in which at least one or more joints are assembled by brazing.

[0022] According to the present invention, it is possible to provide a Ni brazing filler metal that has excellent brazing properties and also has excellent corrosion resistance at the brazed portion.

[0023] The present invention will be specifically described below.

[0024] First, the reason for limiting the composition of the Ni brazing filler metal in the present invention will be explained. Note that the unit of the content of elements in the composition of the Ni brazing filler metal is "mass %", but hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0025] Cr: 8.0 to 25.0% Cr is an element that dissolves in the Ni alloy phase to improve corrosion resistance. If the Cr content is less than 8.0%, sufficient effects cannot be obtained. Furthermore, if the Cr content exceeds 25.0%, the melting temperature increases and brazeability deteriorates. For this reason, the Cr content is set to the range of 8.0 to 25.0%. The Cr content is preferably 10.0% or more, more preferably 12.0% or more, and even more preferably 14.0% or more. Furthermore, the Cr content is preferably 22.0% or less, more preferably 20.0% or less, and even more preferably 18.0% or less.

[0026] Fe: 3.0 to 20.0% Fe is an element that contributes to suppressing cracking in brazed joints by dissolving in the Ni alloy phase and reducing the thermal expansion coefficient. If the Fe content is less than 3.0%, sufficient effect cannot be obtained. Furthermore, if the Fe content exceeds 20.0%, the melting temperature increases and brazeability deteriorates. For this reason, the Fe content is set to the range of 3.0 to 20.0%. The Fe content is preferably 4.0% or more, more preferably 6.0% or more, and even more preferably 8.0% or more. Furthermore, the Fe content is preferably 18.0% or less, more preferably 16.0% or less, and even more preferably 14.0% or less.

[0027] Mo: 2.0 to 7.0% Like Cr, Mo is an element that dissolves in the Ni alloy phase to improve corrosion resistance. If the Mo content is less than 2.0%, sufficient effects cannot be obtained. Furthermore, if the Mo content exceeds 7.0%, the melting temperature increases and brazeability deteriorates. For this reason, the Mo content is set to the range of 2.0 to 7.0%. The Mo content is preferably 3.0% or more, more preferably 4.0% or more, and even more preferably 5.5% or more. Furthermore, the Mo content is preferably 6.0% or less.

[0028] B: 1.50 to 5.00% B is an element that forms a compound with Cr to reduce the thermal expansion coefficient and thereby suppress the occurrence of cracks in brazed joints. This effect is achieved with a B content of 1.50% or more. However, if the B content exceeds 5.00%, a hypereutectic composition results, increasing the difference between the liquidus and solidus, and reducing brazability. For this reason, the B content is set to the range of 1.50 to 5.00%. The B content is preferably 2.50% or more, more preferably 3.00% or more, and even more preferably 3.25% or more. The B content is also preferably 4.50% or less, more preferably 4.00% or less, and even more preferably 3.75% or less.

[0029] C: 0.10 to 1.00% C is an element that forms a compound with Cr to reduce the thermal expansion coefficient and thereby suppress the occurrence of cracks in brazed joints. This effect is achieved with a C content of 0.10% or more. However, if the C content exceeds 1.00%, a hypereutectic composition results, increasing the difference between the liquidus and solidus, and reducing brazability. For this reason, the C content is set to the range of 0.10 to 1.00%. The C content is preferably 0.20% or more, more preferably 0.30% or more, and even more preferably 0.40% or more. The C content is also preferably 0.80% or less, more preferably 0.70% or less, and even more preferably 0.60% or less.

[0030] 2.00≦B+C≦5.50 (1) In formula (1), B and C represent the content (mass %) of each element. In the present invention, the contents of B and C are set to predetermined values ​​to suppress cracking in the brazed portion and improve brazeability. Furthermore, the inventors have conducted extensive research and found that when B+C (the total content (mass %) of B and C) is 2.00 or more, cracking in the brazed portion can be suppressed. Although the detailed mechanism is unknown, the inventors believe as follows: During heat treatment in the brazing process, Cr—B compounds and Cr—C compounds, which have small thermal expansion coefficients, are formed, reducing the thermal expansion coefficient of the brazing filler metal to equal to or less than that of ferritic stainless steel. This suppresses the thermal stress in the tensile direction applied to the brazing filler metal during cooling after heat treatment in the brazing process, thereby suppressing cracking in the brazed portion. On the other hand, if B+C exceeds 5.50, a hypereutectic composition results, increasing the difference between the liquidus and solidus, and significantly reducing brazability. For these reasons, in the present invention, the B and C contents are set within the aforementioned ranges, and B+C is set to 2.00 or more and 5.50 or less. B+C is preferably 3.00 or more. Furthermore, B+C is preferably 4.00 or less.

[0031] The basic components (essential components) of the Ni brazing filler metal of the present invention have been described above. The remaining components of the component composition of the present invention may be Ni and unavoidable impurities. Furthermore, in the present invention, the component composition may optionally contain one or more of the following:

[0032] Cu: 10.0% or less Cu is an element that enhances corrosion resistance. To achieve this effect, when Cu is contained, the Cu content is preferably 2.0% or more. The Cu content is more preferably 4.0% or more. However, when the Cu content exceeds 10.0%, the melting temperature increases and brazing properties decrease. Therefore, when Cu is contained, the Cu content is 10.0% or less. The Cu content is preferably 8.0% or less, more preferably 6.0% or less.

[0033] Co: 20.0% or less Co is an element that enhances corrosion resistance. To achieve this effect, when Co is contained, the Co content is preferably 4.0% or more. The Co content is more preferably 8.0% or more. However, if the Co content exceeds 20.0%, the melting temperature increases and brazeability decreases. Therefore, when Co is contained, the Co content is set to 20.0% or less. The Co content is preferably 16.0% or less, more preferably 12.0% or less.

[0034] Mn: 5.0% or less Mn is an element that lowers the melting point and improves brazability. To achieve this effect, when Mn is contained, the Mn content is preferably 1.0% or more. The Mn content is more preferably 2.0% or more. However, if the Mn content exceeds 5.0%, the melting temperature increases and brazability decreases. Therefore, when Mn is contained, the Mn content is set to 5.0% or less. The Mn content is preferably 4.0% or less, more preferably 3.0% or less.

[0035] W: 5.0% or less W is an element that enhances corrosion resistance. To achieve this effect, when W is contained, the W content is preferably 1.0% or more. The W content is more preferably 2.0% or more. However, if the W content exceeds 5.0%, the melting temperature increases and brazeability decreases. Therefore, when W is contained, the W content is set to 5.0% or less. The W content is preferably 4.0% or less, more preferably 3.0% or less.

[0036] P: 5.0% or less P is an element useful for lowering the melting point of the brazing filler metal and improving brazeability. To achieve this effect, when P is contained, the P content is preferably 1.0% or more. The P content is more preferably 1.5% or more, and even more preferably 2.0% or more. However, if the P content exceeds 5.0%, a hypereutectic composition results, which reduces brazeability. Therefore, the P content is set to 5.0% or less. The P content is preferably 4.0% or less, more preferably 3.5% or less, and even more preferably 3.0% or less.

[0037] Si: 5.0% or less Si is an element useful for lowering the melting point of the brazing filler metal and improving brazeability. To achieve this effect, when Si is contained, the Si content is preferably 1.0% or more. The Si content is more preferably 1.5% or more, and even more preferably 2.0% or more. However, if the Si content exceeds 5.0%, a hypereutectic composition results, which reduces brazeability. Therefore, when Si is contained, the Si content is set to 5.0% or less. The Si content is preferably 4.0% or less, more preferably 3.5% or less, and even more preferably 3.0% or less.

[0038] Next, a preferred method for producing the Ni brazing filler metal of the present invention will be described. The method for producing the Ni brazing filler metal of the present invention is not particularly limited. For example, raw materials are weighed to have the above-mentioned composition, and the alloy obtained is completely melted in a crucible of a melting furnace. The alloy is then powdered by an atomization method or a melt-pulverization method, thereby obtaining a powdered Ni brazing filler metal. Furthermore, the alloy can be cast into a predetermined mold to obtain a rod-shaped or plate-shaped Ni brazing filler metal.

[0039] In particular, the powdered Ni brazing filler metal produced by the atomization method can be adjusted to a particle size suitable for the intended application method, and then mixed with a binder to form a paste, which can be applied to the substrate. In addition, various methods can be freely selected, such as a method of sprinkling (scattering) the binder and powdered brazing filler metal on the substrate surface, or a method of thermally spraying the powdered brazing filler metal.

[0040] The Ni brazing filler metal of the present invention as described above is suitable for use in exhaust heat recovery devices and exhaust gas recirculation devices in which at least one joint is assembled by brazing. In particular, it is suitable for use in heat exchanger components of the exhaust heat recovery devices and exhaust gas recirculation devices made of ferritic stainless steel. However, there is no problem even if it is used with other stainless steels such as austenitic stainless steel.

[0041] A Ni alloy having the composition shown in Table 1 was melted in a vacuum melting furnace to form an ingot, which was then powdered by gas atomization. This was then mixed with a binder composed of an organic substance to form a paste-like Ni brazing filler metal. The mass mixing ratio of the binder to the Ni brazing filler metal was 10% by mass. Brazing was performed on this Ni brazing filler metal as follows, and evaluations of (1) brazeability and (2) corrosion resistance were performed. The results are shown in Table 2.

[0042] (1) Evaluation of Brazeability Stainless steel test pieces measuring 50 mm wide and 50 mm long were cut and polished longitudinally with #600 emery paper to serve as substrates. As the stainless steel type, SUS444 was used for all but A23 shown in Tables 1 and 2, and SUS316L was used for A23. A Ni brazing filler metal paste was applied to the surface of the horizontally placed test piece to form a shape with a diameter of 20 mm and a thickness of 1 mm. The Ni brazing filler metal-coated test piece was then placed horizontally with the Ni brazing filler metal-coated side facing up, heated at 1080°C in a nitrogen carrier gas atmosphere of 1 Torr for 10 minutes, and then cooled to room temperature for a brazing treatment. The circle-equivalent diameter of the brazing filler metal on the surface of the test piece (the circle-equivalent diameter of the brazing filler metal after heating) was then measured. The ratio of the circle-equivalent diameter of the brazing filler metal after heating to the diameter of the brazing filler metal before heating (20 mm) (spreadability of the brazing filler metal) was then determined and evaluated according to the following criteria. The area of ​​the brazing filler metal was calculated using image analysis software (Image-J in this example), and the diameter of a circle having the same area as the calculated area was taken as the equivalent circular diameter of the brazing filler metal after heating. Spreadability of brazing filler metal after heating to before heating (%) = (equivalent circular diameter of brazing filler metal after heating / diameter of brazing filler metal before heating (20 mm)) x 100 ○ (Pass): 150% or more × (Fail): Less than 150%

[0043] (2) Evaluation of Corrosion Resistance A test piece 15 mm wide and 15 mm long was cut out from the brazed portion (the portion where the brazing material was attached by the brazing process) of the sample prepared above, and this test piece was covered with a silicone resin sealant, leaving an 11 mm square measurement surface. Next, this test piece was immersed in a 3.5 mass% NaCl solution at 30°C, and pitting potential measurement was performed in accordance with JIS G 0577:2014, except for the concentration of the NaCl solution. After holding at the natural potential for 10 minutes, the anodic current density was measured at a sweep rate of 20 mV / min to obtain a value of 1.1 mA / cm. ２ Measurements were carried out until the current density reached 100 μA / cm ２The potential at which this occurs was taken as the pitting potential Vc'100. Considering the conditions of use of the heat exchanger of the exhaust heat recovery device or EGR cooler, if the pitting potential Vc'100 is 300 mV (vs. SCE) or higher, it can be determined that the corrosion resistance is excellent. ○ (Pass): 300 mV (vs. SCE) or higher × (Fail): Less than 300 mV (vs. SCE)

[0044]

[0045]

[0046] As can be seen from Table 2, the brazeability and corrosion resistance of the brazed portion were good in all of the invention examples Nos. 1 to 23. In contrast, the comparative examples Nos. 24 to 37, which had component compositions outside the appropriate range, were unable to simultaneously satisfy the targets for brazeability and corrosion resistance after brazing.

[0047] More specifically, in Comparative Example No. 24 (symbol B1), the Cr content exceeded the upper limit of the present invention, and therefore the target brazability was not achieved. In Comparative Example No. 25 (symbol B2), the Fe content exceeded the upper limit of the present invention, and therefore the target brazability was not achieved. In Comparative Example No. 26 (symbol B3), the Mo content exceeded the upper limit of the present invention, and therefore the target brazability was not achieved. In Comparative Example No. 27 (symbol B4), the B content exceeded the upper limit of the present invention, and therefore the target brazability was not achieved. In Comparative Example No. 28 (symbol B5), the C content exceeded the upper limit of the present invention, and therefore the target brazability was not achieved.

[0048] In Comparative Example No. 29 (symbol B6), the Cr content was below the lower limit of the present invention, so the target corrosion resistance was not achieved. In Comparative Example No. 30 (symbol B7), the Fe content was below the lower limit of the present invention, so the target corrosion resistance was not achieved. In Comparative Example No. 31 (symbol B8), the Mo content was below the lower limit of the present invention, so the target corrosion resistance was not achieved. In Comparative Example No. 32 (symbol B9), the B content was below the lower limit of the present invention, so the target corrosion resistance was not achieved. In Comparative Example No. 33 (symbol B10), the C content was below the lower limit of the present invention, so the target corrosion resistance was not achieved.

[0049] In Comparative Example No. 34 (symbol B11), all components were within the specified range, but B + C exceeded 5.50, which did not satisfy formula (1), and therefore the target brazeability was not achieved. In Comparative Example No. 35 (symbol B12), all components were within the specified range, but B + C was less than 2.00, which did not satisfy formula (1), and therefore the target corrosion resistance was not achieved. Comparative Examples Nos. 36 and 37 are Ni brazing filler metals according to the JIS standard, corresponding to BNi2 and BNi5, respectively. BNi2 had low contents of Cr, Mo, and C, and therefore had insufficient corrosion resistance. BNi5 had low contents of Fe, Mo, B, and C, which resulted in insufficient brazing spread and also did not achieve the target corrosion resistance.

[0050] Furthermore, for Invention Examples Nos. 1 to 23, which had a favorable corrosion resistance evaluation, the brazed portions were observed by backscattered electron images using a Miniscope (registered trademark) TM3030plus, a scanning electron microscope (SEM) manufactured by Hitachi High-Tech Corporation. The observation conditions were an acceleration voltage of 15 kV and an observation magnification of 300x. As a result, no cracks were observed in the brazed portions. On the other hand, for Comparative Examples Nos. 29 to 33 and 35 to 37, which did not achieve the target corrosion resistance in the corrosion resistance evaluation, the brazed portions were observed in the same manner as above, and cracks were found to have occurred in the brazed portions. Therefore, in Comparative Examples Nos. 29 to 33 and 35 to 37, the cracks caused crevice corrosion, and the target corrosion resistance was not achieved.

[0051] According to the present invention, a Ni brazing filler metal can be obtained that is suitable for use in exhaust gas recirculation devices such as exhaust heat recovery devices and heat exchangers for EGR coolers that are assembled by brazing, and is therefore extremely useful industrially. The Ni brazing filler metal of the present invention is particularly suitable for use in exhaust gas recirculation devices such as exhaust heat recovery devices and heat exchangers for EGR coolers that are made of ferritic stainless steel and that are assembled by brazing.

Claims

1. A Ni brazing filler metal containing, by mass%, 8.0-25.0% Cr, 3.0-20.0% Fe, 2.0-7.0% Mo, 1.50-5.00% B, and 0.10-1.00% C, satisfying the following formula (1), with the balance being Ni and unavoidable impurities: 2.00≦B+C≦5.50 ... (1) where B and C in formula (1) represent the content (mass%) of each element.

2. The Ni brazing material according to claim 1, wherein the composition further contains, by mass%, one or more selected from the following: Cu: 10.0% or less, Co: 20.0% or less, Mn: 5.0% or less, W: 5.0% or less, P: 5.0% or less, and Si: 5.0% or less.

3. The Ni brazing filler metal according to claim 1 or 2, which is used in a heat exchanger in which at least one joint is assembled by brazing.