Joint and method for manufacturing the same

Abstract

To provide a Ni brazed joined body of stainless steel having good corrosion resistance, and a method for manufacturing the same.SOLUTION: A joined body has stainless steel and a brazing part where the stainless steel is brazed with a Ni brazing material, wherein in a cross section of a brazing material of the brazing part, an average number of Ni poor phases is 10 pieces or more per a line segment length of 100 μm. A method for manufacturing a joined body cools a temperature region from a maximum temperature during brazing to 300°C at average cooling speed of 30°C / min or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a joined body and a method for manufacturing the same, and more particularly to a Ni brazed joined body of stainless steel that exhibits good corrosion resistance.

Background Art

[0002] In the field of water heaters such as gas water heaters and electric water heaters, copper pipes are widely used as pipes for flowing water and heat medium. Further, from the viewpoint of global environmental protection, heat exchangers are applied to environment-friendly water heaters that have been increasingly popular in recent years, and copper is also used here.

[0003] In recent years, due to the increasing demand for power infrastructure in emerging countries and the electrification of automobiles, the demand for copper for electric wires and conductors has increased, and the supply and demand of copper has been in a tight state. In the future, the demand for copper is expected to increase, and reducing the amount of copper used in response to the soaring copper price has become an important issue.

[0004] From such a background, in the field of water heaters, replacement from copper to other materials, particularly stainless steel that is relatively inexpensive and has excellent corrosion resistance, is being promoted.

[0005] The heat exchangers and pipes of gas water heaters and electric water heaters are used in a state where the inside is always filled with a fluid such as high-temperature water, so excellent corrosion resistance and heat resistance are required. Therefore, stainless steel with excellent these properties is applied, but in some installed areas, higher corrosion resistance than the normal standard may be required.

[0006] For example, in areas with low water purification ability, tap water contains corrosive components, and there is a risk of corrosion inside the water heater. Further, even in use in an environment where chlorides adhere, such as in coastal areas, reduction of the lifespan due to corrosion has been an issue.

[0007] In the corrosion of such pipes and heat exchangers, corrosion at the joints can be a problem. The joints of heat exchangers and similar components mainly use brazed members (joints). When stainless steel is used for these members, nickel brazing material, which has corrosion resistance equivalent to that of stainless steel, is mainly used for joining.

[0008] As mentioned above, in the field of water heaters, high corrosion resistance is required for joint components such as heat exchangers depending on the operating environment. Therefore, stainless steel joints brazed with nickel (nickel-brazed stainless steel joints) also need to exhibit sufficient corrosion resistance.

[0009] For example, Patent Document 1 describes a brazed joint in which a first metal member made of austenitic stainless steel and a second metal member made of austenitic stainless steel are joined via a boron-containing brazing layer, wherein in a diffusion region containing Cr-based boride adjacent to the brazing layer of the metal member, a region of a predetermined width is set as the measurement range so as to include a diffusion region adjacent to the brazing layer in a direction substantially parallel to the brazing layer, with the parallel direction as the longitudinal direction, in which the Cr-based boride has a higher concentration. A brazed joint is disclosed in which, when L is the length of a line segment LSj (j=1 to m, m is an integer of 1 or more) that connects the base point and the endpoint in the longitudinal direction of the measurement range and is set uniformly within the measurement range, Li is the length of the i-th Cr-based boride on the line segment LSj from the base point on the j-th line segment LSj, and n (an integer of 1 or more) is the number of Cr-based borides counted on the j-th line segment LSj, the λ value, which is the average of j λj values ​​obtained by λj=(L-ΣLi) / n, satisfies λ≧10μm.

[0010] For example, Patent Document 2 discloses a brazing method for a heat exchanger in which a plurality of stainless steel components constituting the heat exchanger are combined in a desired arrangement, and brazing is performed by providing brazing material between the contact parts of each component and heating, wherein the brazing material is an alloy having nickel (Ni) and phosphorus (P), and the brazing temperature is higher than 950°C but less than 1050°C. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2018-75580 [Patent Document 2] Japanese Patent Publication No. 2004-298934 [Overview of the project] [Problems that the invention aims to solve]

[0012] The brazed joint described in Patent Document 1 ensures sulfuric acid corrosion resistance of the brazed joint by controlling the number density of Cr boride in brazing with a boron-containing brazing material. Furthermore, the brazing method described in Patent Document 2 prevents a decrease in joint corrosion resistance by using a brazing material containing Ni and P and performing brazing at a relatively low temperature.

[0013] The brazed joint disclosed in Patent Document 1 and the brazing method disclosed in Patent Document 2 both have the problem that they may not provide sufficient corrosion resistance in terms of chloride ion corrosion resistance, which is considered a corrosion factor in the field of water heaters.

[0014] The present invention has been made in view of the above circumstances, and aims to provide a stainless steel Ni brazed joint having good corrosion resistance and a method for manufacturing the same.

[0015] Here, "good corrosion resistance" means that when a stainless steel Ni brazed joint (hereinafter simply referred to as the joint) is immersed in a 10% by mass NaCl aqueous solution at 80°C for 24 hours, no corrosion is observed in the brazing material of the brazed joint. For detailed evaluation methods, please refer to the examples described later. [Means for solving the problem]

[0016] Brazing is a technique that joins base materials by melting a brazing material and utilizing the metallic bond that forms between the brazing material and the base material. When joining components used in corrosive environments, a portion of the brazing material is exposed to the corrosive environment at the same time as the base material. For this reason, in components requiring high corrosion resistance, such as those made of stainless steel, nickel-based brazing materials containing a large amount of nickel, which has high corrosion resistance, are used for joining.

[0017] However, in severe corrosive environments, even when nickel brazing material is used for joining, the brazing material at the brazed joint may corrode preferentially, resulting in a decrease in the corrosion resistance of the joint.

[0018] Therefore, the inventors investigated the cause of the decrease in corrosion resistance of the brazing material at the brazed joint when brazing is performed using Ni brazing material. As a result, it was found that minute cracks that occur on the surface of the brazing material at the brazed joint promote corrosion, thereby reducing the corrosion resistance of the brazed joint.

[0019] The inventors believe that this acceleration of corrosion due to microcracks is a phenomenon caused by a mechanism similar to crevice corrosion, where the microcracks act as gaps. In other words, it is thought that as the corrosion reaction progresses, the chloride ion concentration inside the cracks increases, leading to preferential corrosion at the cracked areas.

[0020] Based on the above findings, we further investigated methods for obtaining stainless steel Ni brazed joints (connections) with the desired corrosion resistance. As a result, we obtained the following findings.

[0021] That is, in the brazed portion of the joint of stainless steels obtained by Ni brazing, by suppressing cracks generated on the surface of the brazing material, desired corrosion resistance is exhibited.

[0022] The inventors further studied and found that in brazing, the faster the cooling rate after heating, the finer the brazing material structure in the brazed portion, and thus the generation and progression of cracks can be suppressed.

[0023] That is, in brazing, when the molten Ni brazing material solidifies by heating, two or more phases with different Ni concentrations are formed in the brazing material of the brazed portion. By refining the structure, the brazing material in the brazed portion becomes higher in strength, and the generation of cracks is suppressed.

[0024] Also, by refining the two or more phases with different Ni concentrations in the brazing material, it becomes difficult for the same phase to be continuous in the brazing material structure in the depth direction of the brazing material, and the progression of cracks is suppressed.

[0025] The present invention was completed after further study based on the above findings.

[0026] That is, the gist configuration of the present invention is as follows. [1] A joint having a brazed portion obtained by brazing stainless steels with a Ni brazing material, In the cross section of the brazing material of the brazed portion, a joint in which the average number of Ni poor phases is 10 or more per 100 μm of line segment length. [2] A method for manufacturing the joint according to [1], A method for manufacturing a joint, which cools the temperature range from the maximum temperature during brazing to 300 ° C. at an average cooling rate of 30 ° C. / min or more.

Effect of the Invention

[0027] According to the present invention, a Ni brazed joint of stainless steel having good corrosion resistance can be obtained.

Brief Description of the Drawings

[0028] [Figure 1] Figure 1 is an SEM backscattered electron image of the brazing material cross-section of the brazed portion of the joint in the present invention, and shows an example of an arbitrary straight line perpendicular to the surface of the brazing material (an example from Example No. 9). [Figure 2] Figure 2 shows an example of a method for taking a cross-section of the brazing material from a joint (an example from Example No. 9). [Figure 3] Figure 3 shows an example of a SEM backscattered electron image of a cross-section of the brazing material at the brazed joint of a joint, which is a comparative example of the present invention (an example of Comparative Example No. 19). [Modes for carrying out the invention]

[0029] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0030] (stainless steel) Stainless steel serves as the base material for the joint, and its shape is either plate-shaped (stainless steel plate) or tubular (stainless steel pipe). Note that "plate-shaped" here includes not only flat plates but also curved plates. The thickness of the stainless steel (plate thickness or pipe thickness) is not particularly limited, but from the viewpoint of jointability, it is preferable to have a thickness of 0.1 mm or more. Furthermore, the thickness of the stainless steel is preferably 4.0 mm or less. More preferably, the thickness of the stainless steel is 0.2 mm or more, and even more preferably 0.3 mm or more. Furthermore, the thickness of the stainless steel is preferably 2.0 mm or less, and even more preferably 1.0 mm or less.

[0031] Furthermore, the composition of the stainless steel is not particularly limited and can be any composition that is common for stainless steel. For example, an iron-based alloy containing 10.5% by mass or more of Cr and 50% by mass or more of Fe is acceptable. As an example, austenitic stainless steel sheets, austenitic-ferritic stainless steel sheets, ferritic stainless steel sheets, martensitic stainless steel sheets, and precipitation-hardening stainless steel sheets, as well as processed products thereof, as specified in JIS G 4305:2021, can be used. From the viewpoint of reducing corrosion resistance due to sensitization of the base material, ferritic stainless steel is preferred as the composition of the stainless steel. Furthermore, stainless steel sanitary pipes, stainless steel pipes for general piping, stainless steel pipes for piping, and stainless steel pipes for boilers and heat exchangers, as well as processed products thereof, as specified in JIS G 3447:2015, JIS G 3448:2016, JIS G 3459:2021, JIS G 3463:2019, and JIS G 3468:2021, can be used. In addition, stainless steel sheets with various surface finishes can be used, including No. 2B finish (annealed pickled skin pass finish), No. 2D finish (annealed pickled finish), No. 4 finish (polished finish), No. 8 finish (mirror polished finish), BA finish (bright annealed finish), HL (hairline) finish, dull finish, embossed finish, and blast finish.

[0032] (Ni brazing material) The Ni brazing material can be any Ni alloy containing 50% by mass or more Ni. Specifically, it can be any Ni alloy for brazing with a general composition, containing 50% by mass or more Ni and having its melting point adjusted by adding other elements. The composition of the Ni brazing material is preferably such that the solidified brazing material structure separates into two phases after brazing. Examples of such Ni brazing materials include those described in JIS Z 3265:1998. The shape of the Ni brazing material is not particularly limited, but a paste-like brazing material (a mixture of powdered brazing material and an organic binder) is preferred.

[0033] (The Ni-poor phase has an average of 10 or more particles per 100 μm of line segment length) The structure of the brazing material in the brazed joint of a joint according to one embodiment of the present invention is defined, for example, as follows: Cutting is performed in the brazed joint of the joint so that an arbitrary cross-section including the brazing material and the brazing material surface is exposed. At this time, the cutting position is adjusted so that the thickness of the brazing material is 110 μm or more. The cut test piece is embedded in resin so that the cross-section is exposed and then mirror polished. The resulting embedded piece is observed using a scanning electron microscope (SEM). Then, the separation of each phase with different Ni concentrations and the definition of the brazing material surface are performed from the contrast observed in the backscattered electron image. The Ni concentration of each phase is measured using an energy-dispersive X-ray spectrometer (EDS) to determine the phase with the highest Ni concentration (Ni-enriched phase). Among the phases separated from the contrast obtained from the backscattered electron image, the phase other than the phase with the highest Ni concentration (Ni-enriched phase) is defined as the Ni-poor phase.

[0034] Figure 1 shows an example of a SEM backscattered electron image of a brazing material cross-section of a brazed joint of a joint according to one embodiment of the present invention (an example of Example No. 9 described later). An arbitrary point on the surface of the brazing material defined as described above is used as the measurement starting point, and a straight line perpendicular to the brazing material surface is set as shown in Figure 1. Specifically, the straight line perpendicular to the brazing material surface is set as follows.

[0035] (1) Using any point on the surface of the brazing material as the measurement starting point, draw a circle with a diameter of 10 μm centered on that point. (2) Connect the two intersection points of the circle with a diameter of 10 μm set in (1) above and the surface of the brazing material with a straight line. (3) The line passing through the measurement starting point and perpendicular to the line connecting the two intersection points set in (2) is defined as the line perpendicular to the surface of the brazing material.

[0036] The vertical line described above is defined as having a length of 110 μm or more in the SEM observation image, and is set so that the area of ​​110 μm or more on the line overlaps with the brazing material cross-section. The number of Ni-poor phases cut by a line (segment length 100 μm) in the set line at a distance of 10 μm to 110 μm from the brazing material surface is measured. The number of Ni-poor phases cut here is calculated as (number of intersections between the line and the boundary line between the Ni-poor phase and other phases) / 2. The decimal part is truncated. The number of Ni-poor phases at a line segment length of 100 μm on five lines at different measurement locations is measured using the same procedure, and the average number of Ni-poor phases per 100 μm of line segment length is calculated according to the following formula. However, if the Ni-enriched phase is continuous for 50% or more of the thickness of the brazing material from the interface between the Ni brazing material and stainless steel toward the brazing material surface within the set line, it is excluded from the measurement target. Average number of Ni-poor phase particles per 100 μm of line segment length (particles) = [Total number of Ni-poor phase particles measured on 5 straight lines (100 μm line segment length) (particles)] / 5

[0037] Joints obtained in this manner, where the number of Ni-poor phases is 10 or more on average per 100 μm of line segment length, exhibit better corrosion resistance. This is because crack initiation and propagation are significantly suppressed in brazing structures where the number of Ni-poor phases is 10 or more on average per 100 μm of line segment length. Preferably, the number of Ni-poor phases is 20 or more on average per 100 μm of line segment length.

[0038] (Method of manufacturing the joint) The joint of the present invention is manufactured, for example, by the brazing described below. A nickel brazing material is placed (applied) between the stainless steels to be joined and stainless steel (base material), or at the joint end thereof, the nickel brazing material is heated and melted (heating step), and then cooled and solidified (cooling step) to manufacture a nickel brazed joint of stainless steel. In the manufacturing method of the joint according to one embodiment of the present invention, the heating during brazing (heating step) is performed, for example, using a vacuum furnace or an atmosphere furnace with a vacuum degree of 4.0 × 10⁻⁶ -3It is preferable to carry out the process in a vacuum atmosphere of Pa to 60 Pa or a nitrogen carrier atmosphere. The heating temperature in the brazing heating process is not particularly limited, but it is preferable to carry it out at 900 to 1100°C.

[0039] Next, the Ni brazing material that was heated and melted in the heating step is cooled and solidified (cooling step). The cooling step after the heating step of brazing is an important step in the present invention. By rapidly cooling after the heating step of brazing, the solidification of the brazing material is rapidly promoted, and the brazing material structure becomes a fine structure in which the same phase is discontinuous. By forming such a brazing material structure, the occurrence and propagation of cracks on the surface of the brazing material can be suppressed, and a stainless steel Ni brazed joint with the desired corrosion resistance can be obtained. By setting the cooling in the brazing cooling step to an average cooling rate of 30°C / min or more in the temperature range from the maximum temperature during heating in the brazing heating step to 300°C, a fine structure sufficient to suppress crack occurrence on the surface of the brazing material can be obtained. The average cooling rate in the temperature range from the maximum temperature during brazing to 300°C is preferably 70°C / min or more, and more preferably 100°C / min or more. In the cooling step according to one embodiment of the present invention, it is preferable to carry out the cooling by blowing the atmospheric gas used during heating in the brazing heating step onto the joint. The flow rate of the blown gas is not particularly limited. Furthermore, the dew point of the blown gas is not particularly limited, but it is preferable to carry out the process at a dew point of -20°C or lower. The cooling rate in the temperature range of 300°C or lower is not particularly limited. In the brazing cooling process, cooling may be started immediately after the heating temperature in the heating process reaches the maximum temperature, or the maximum temperature may be maintained for a predetermined time before cooling is started. In other words, the maximum temperature becomes the cooling start temperature in the cooling process.

[0040] The method for measuring the temperature of the joint (brazed portion) during brazing is not particularly limited. For example, a thermocouple may be connected to the joint using a spot welding machine to measure the temperature, or the temperature of the joint may be measured using a radiation thermometer. These temperatures are preferably measured within 200 mm of the area where the brazing material was applied on the joint. More preferably, they are measured within 100 mm, and even more preferably, within 50 mm. [Examples]

[0041] 30mm square (stainless steel (1)) and 20mm x 15mm (stainless steel (2)) pieces were cut from 1mm thick stainless steel sheets of SUS304, SUS316L, SUS443J1, and SUS444 as specified in JIS G 4305:2021, as listed in Table 1. The brazing material was prepared by mixing powdered nickel brazing materials BNi-2, BNi-5, and FP-613 (Ni-29mass%Cr-6mass%P-4mass%Si) manufactured by Fukuda Metal Foil & Powder Co., Ltd., as specified in JIS Z 3265:1998, with an organic binder to form a paste-like nickel brazing material. The mass mixing ratio of the binder to the nickel brazing material was 10% by mass.

[0042] A paste-like brazing material was applied to the longitudinal section of the cut stainless steel (2), and it was placed in the center of the surface of stainless steel (1) to form a T-shaped test specimen. If necessary, the test specimen was secured with stainless steel wire. The test specimen was placed horizontally in a vacuum furnace, and the furnace was evacuated. Next, nitrogen gas was introduced into the vacuum furnace. Then, the vacuum furnace was heated, and the test specimen was held in a nitrogen atmosphere of 1090°C (maximum brazing temperature) and 40 Pa for 10 minutes (brazing heating process). Next, nitrogen gas was blown into the furnace, and the specimen was cooled from the maximum temperature to 300°C at the average cooling rate shown in Table 1 (brazing cooling process). After reaching 300°C, the blowing of gas was stopped, and the specimen was cooled by furnace cooling. The temperature of the test piece during brazing was measured by spot welding a thermocouple to the midpoint of one side of a 30mm square stainless steel plate (stainless steel (1)) that was parallel to the installed 20mm x 15mm stainless steel plate (stainless steel (2)).

[0043] The test specimen of the prepared joint was immersed in 400 mL of a 10% by mass NaCl aqueous solution prepared in a beaker, and the top of the beaker was sealed. The beaker was left standing in a constant temperature bath maintained at 80°C for 24 hours. After the specified time, the test specimen was carefully removed, dried with a blower, and washed with 200 mL of distilled water. After washing, the test specimen was dried, and the brazing material surface of the brazed joint was observed. The corrosion resistance was determined as follows based on the average number density of residual corrosion products. ○ (Good): The average number density of corrosion products remaining on the brazing material surface is 10 pieces / mm 2 below × (Defective): The average number density of corrosion products remaining on the brazing material surface is 10 pieces / mm 2 super

[0044] The observation of the brazing material surface and the determination of the number of corrosion products were performed as follows. For the corrosion test specimens, an arbitrary observation area of ​​the brazing material at the brazed joint was positioned horizontally to the electron gun of a Hitachi High-Tech scanning electron microscope (SEM) Miniscope™3030plus. The specimens were cut to allow for placement on the SEM observation stand. A backscattered electron image was captured using an acceleration voltage of 5kV and a magnification of 300x. Compositional analysis of the same field of view was performed using an Oxford Instruments EDS (EDS) AZtecOne to identify corrosion products. Corrosion products are defined as granular substances containing 20% ​​or more of either Ni, Cr, or both, and 20% or more of O. The number of identified corrosion products was divided by the area of ​​the observation field to obtain the number density of corrosion products per field of view. The number density of corrosion products per field of view was calculated for 10 arbitrary fields of view as described above, and the average number density of corrosion products remaining on the brazing material surface of the brazed joint was calculated according to the following formula. Average number density of corrosion products remaining on the brazing material surface (pieces / mm²) 2 ) = [Sum of the number densities of corrosion products per field of view, calculated for any 10 fields of view] / 10

[0045] Furthermore, as shown in Figure 2, the specimens subjected to the corrosion test were cut at the location containing the brazing material and embedded in epoxy resin so that the cross-section of the brazing material was exposed. The embedded specimen cross-section was mirror-polished, and a backscattered electron image was taken of the obtained specimen cross-section using a SEM with an acceleration voltage of 15 kV and a magnification of 300x. In addition, a component composition analysis was performed on the image field using EDS to define the Ni-poor phase. A 120 μm straight line perpendicular to the brazing material surface at five arbitrary points was set in the obtained backscattered electron image. The number of Ni-poor phases cut along each set straight line at a distance from the brazing material surface in the range of 10 μm to 110 μm was measured, and the average number of Ni-poor phases per 100 μm of line segment length was calculated according to the following formula. However, if the Ni-enriched phase was continuous for 50% or more of the distance from the interface between the Ni brazing material and stainless steel toward the brazing material surface in the set straight line relative to the thickness of the brazing material, it was excluded from the measurement. Average number of Ni-Pua phase particles per 100 μm of line segment length (particles) = [Total number of Ni-Pua phase particles measured along 5 straight lines (particles)] / 5

[0046] The results are shown in Table 1.

[0047] [Table 1]

[0048] Table 1 shows that all of the inventive examples exhibited good corrosion resistance. Specifically, in inventive examples No. 1 to 16, the average cooling rate from the maximum temperature during brazing to 300°C was 30°C / min or more, and the average number of Ni-poor phase particles per 100 μm of line segment length in the brazing material cross-section was 10 or more, resulting in better corrosion resistance.

[0049] In contrast, in Comparative Examples No. 17 to 22, where the cooling rate was outside the appropriate range, good corrosion resistance could not be obtained. Specifically, in Comparative Examples No. 17 to 22, the cooling rate from the maximum temperature during brazing to 300°C was less than 30°C / min, and the average number of Ni-poor phase particles per 100 μm of line segment length in the brazing material cross-section was less than 10, so good corrosion resistance could not be obtained. Figure 3 shows an example of a SEM backscattered electron image of the brazing material cross-section of a joint that is a comparative example of the present invention (an example of Comparative Example No. 19). As shown in Figure 3, in the joint of the comparative example, the structure of the brazing material in the brazing part is not refined, and the predetermined average number of Ni-poor phase particles per 100 μm of line segment length is not obtained. [Industrial applicability]

[0050] The stainless steel nickel brazed joint of the present invention exhibits excellent corrosion resistance. Therefore, it is suitable for application in various products, including heat exchangers, electric water heaters, and gas water heaters, as well as in the surrounding piping for these products.

Claims

1. A joint having a brazed portion formed by brazing stainless steel and stainless steel with Ni brazing material, A joint in which, in the brazing material cross-section of the brazed portion, the average number of Ni-poor phases is 10 or more per 100 μm of line segment length.

2. A method for manufacturing a joint according to claim 1, A method for manufacturing a joined body, comprising cooling the temperature range from the highest temperature during brazing up to 300°C at an average cooling rate of 30°C / min or more.

Images