Joined body 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 through which water or heat medium flows. Further, from the viewpoint of global environmental protection, heat exchangers are applied to environmentally friendly water heaters that have been increasingly popular in recent years, and copper is also used here.

[0003] In recent years, against the backdrop of 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] Against this 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 underway.

[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, even higher corrosion resistance than normal standards may be required.

[0006] For example, in areas with low water purification capacity, 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, a reduction in life due to corrosion has become an issue.

[0007] In the corrosion of pipes and heat exchangers like this, corrosion at the joint may be a problem. For joints of heat exchangers and the like, members (joint bodies) joined mainly by brazing are used. When stainless steel is used for these members, Ni brazing materials having corrosion resistance equivalent to that of stainless steel are mainly used for the joining.

[0008] As described above, in the field of water heaters, high corrosion resistance is required for joint members such as heat exchangers depending on the use environment. Therefore, a joint body of stainless steel brazed with a Ni brazing material (a Ni brazed joint body of stainless steel) should also exhibit sufficient corrosion resistance.

[0009] For example, Patent Document 1 discloses "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 brazing layer containing boron, in a diffusion region containing a Cr-based boride adjacent to the brazing layer of the metal member, in a direction substantially parallel to the brazing layer, with a region of a predetermined width including a diffusion region adjacent to the brazing layer where the Cr-based boride closer to the brazing layer has a higher concentration as a measurement range, and L is a line segment connecting a base point and an end point in the longitudinal direction of the measurement range, and is a line segment LSj (j = 1 to m, m is an integer of 1 or more) set with uniformity in the measurement range, Li is the length on the line segment LSj of the i-th Cr-based boride 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, and a λ value which is the average value of j λj values obtained by λj = (L - ΣLi) / n satisfies λ ≧ 10 μm."

[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, a brazing material is provided between the abutting portions of these components, and the components are brazed by heating. In this method, the brazing material is an alloy containing nickel (Ni) and phosphorus (P), and the brazing temperature is higher than 950°C and less than 1050°C.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the brazed joint described in Patent Document 1, in brazing with a brazing material containing boron, the number density of Cr boride is controlled to ensure the sulfuric acid corrosion resistance of the brazed joint. Also, in the brazing method described in Patent Document 2, by using a brazing material containing Ni and P and performing brazing at a relatively low temperature, a decrease in joint corrosion resistance is prevented.

[0013] In the brazed joint disclosed in Patent Document 1 and the brazing method disclosed in Patent Document 2, there is a problem in that sufficient corrosion resistance may not be obtained 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 an object thereof is to provide a Ni-brazed joint of stainless steel having good corrosion resistance and a method for manufacturing the same.

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

Means for Solving the Problems

[0016] Brazing is a method of joining base materials by melting a brazing material and utilizing the metallic bond generated between the brazing material and the base materials. A part of the brazing material used for joining members used in a corrosive environment is exposed to the corrosive environment simultaneously with the base material part. Therefore, for members that require high corrosion resistance such as those using stainless steel, a Ni brazing material containing a large amount of Ni with high corrosion resistance is used for the joining.

[0017] However, in a severe corrosive environment, even when a Ni brazing material is used for joining, the brazing material at the brazed part may be preferentially corroded, resulting in a decrease in the corrosion resistance of the joint.

[0018] Therefore, the present inventors examined the cause of the decrease in the corrosion resistance of the brazing material at the brazed part when brazing is performed using a Ni brazing material. As a result, it was found that the occurrence of corrosion is promoted by fine cracks generated on the surface of the brazing material at the brazed part, and the corrosion resistance of the brazed part decreases.

[0019] Regarding the promotion of corrosion occurrence due to these microcracks, the present inventors consider it to be a phenomenon based on a mechanism similar to crevice corrosion with the microcracks as crevices. That is, it is considered that as the corrosion reaction progresses, the chloride ion concentration increases inside the cracks, resulting in preferential corrosion at the crack part.

[0020] Based on the above findings, further examination was carried out on a method for obtaining a Ni brazed joint (joint) of stainless steel having desired corrosion resistance. As a result, the following findings were obtained.

[0021] That is, in the brazed portion of the joined body of stainless steels obtained by Ni brazing, by suppressing cracks generated on the surface of the brazing filler metal, 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 filler metal 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 filler metal solidifies by heating, a two-phase or more structure with different Ni concentrations is formed in the brazing filler metal of the brazed portion. By refining the structure, the brazing filler metal in the brazed portion becomes higher in strength, and the generation of cracks is suppressed.

[0024] Also, by refining the two-phase or more structure with different Ni concentrations of the brazing filler metal, it becomes difficult for the same phase to be continuous in the brazing filler metal structure in the depth direction of the brazing filler metal, 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 joined body having a brazed portion in which stainless steels are brazed with a Ni brazing filler metal, In the cross section of the brazing filler metal 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 the joined body according to [1], Cooling 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 2

Figure 3

Mode for Carrying Out the Invention

[0029] Hereinafter, the present invention will be specifically described. However, the present invention is not limited to the following embodiments.

[0030] (Stainless steel) Stainless steel serves as the base material of the brazed joint, and its shape is plate-like (stainless steel plate) or tubular (stainless steel pipe). Here, the plate-like shape includes not only flat plates but also curved plates (curved plates). The thickness of the stainless steel (plate thickness or pipe thickness) is not particularly limited, but from the viewpoint of brazing properties, it is preferably 0.1 mm or more. Also, the thickness of the stainless steel is preferably 4.0 mm or less. The thickness of the stainless steel is more preferably 0.2 mm or more, and even more preferably 0.3 mm or more. Also, the thickness of the stainless steel is more preferably 2.0 mm or less, and even more preferably 1.0 mm or less.

[0031] In addition, the component composition of the stainless steel is not particularly limited, and any components common to stainless steel may be used. For example, an iron-based alloy containing 10.5 mass% or more of Cr and 50 mass% or more of Fe may be used. As an example, austenitic stainless steel plates, austenitic-ferritic stainless steel plates, ferritic stainless steel plates, martensitic stainless steel plates, precipitation-hardening stainless steel plates, and processed products thereof defined in JIS G 4305:2021 can be used. From the perspective of reducing corrosion resistance due to sensitization of the base material, ferritic stainless steel is preferred for the component composition of the stainless steel. Also, stainless steel sanitary pipes, stainless steel pipes for general piping, stainless steel pipes for piping, and stainless steel pipes for boilers and heat exchangers defined in JIS G 3447:2015, JIS G 3448:2016, JIS G 3459:2021, JIS G 3463:2019, and JIS G 3468:2021, and processed products thereof can be used. Note that for the stainless steel plate, steel plates 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 may be a Ni alloy containing 50 mass% or more of Ni. Specifically, it may be a Ni alloy for brazing with a general component composition containing 50 mass% or more of Ni and having its melting point adjusted by adding other elements. As the composition components of the Ni brazing material, it is preferable that the brazing material structure solidified after brazing has a component composition that undergoes two-phase separation. Examples of the Ni brazing material include the brazing materials described in JIS Z 3265:1998. The shape of the Ni brazing material is not particularly limited, but for example, a paste-like brazing material (a mixture of powdered brazing material and a binder composed of an organic substance) is preferred.

[0033] (The number of Ni poor phases is 10 or more per average number per 100 μm of line segment length) The structure of the brazing filler metal in the brazed portion of the joined body according to one embodiment of the present invention is defined as follows, for example. In the brazed portion of the joined body, cutting is performed so that an arbitrary cross section including the brazing filler metal and the surface of the brazing filler metal is exposed. At this time, the cutting position is adjusted so that the thickness of the brazing filler metal is 110 μm or more. The cut test piece is resin-embedded so that the cross section is exposed and mirror-polished. The obtained embedded piece is observed by SEM. Then, from the contrast observed in the backscattered electron image, the separation of each phase with different Ni concentrations and the surface of the brazing filler metal are defined. Further, the Ni concentration of each phase is measured by an energy dispersive X-ray spectrometer (EDS), and the phase with the maximum Ni concentration (Ni-enriched phase) is determined. Among the phases separated from the contrast obtained from the backscattered electron image, a phase other than the phase with the maximum Ni concentration (Ni-enriched phase) is defined as a Ni poor phase.

[0034] Fig. 1 shows an example of a SEM backscattered electron image of the cross section of the brazing filler metal in the brazed portion of the joined body according to one embodiment of the present invention (an example of Example No. 9 described later). Taking an arbitrary point on the surface of the brazing filler metal defined as above as a measurement starting point, a straight line perpendicular to the surface of the brazing filler metal is set as shown in Fig. 1. Specifically, a straight line perpendicular to the surface of the brazing filler metal is set as follows.

[0035] (1) Taking an arbitrary point on the surface of the brazing filler metal as a 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) and the surface of the brazing filler metal with a straight line. (3) A straight line passing through the measurement starting point and perpendicular to the straight line connecting the two intersection points set in (2) is defined as a straight line perpendicular to the surface of the brazing filler metal.

[0036] Note that the above vertical straight line has a length corresponding to 110 μm or more in the SEM observation image, and is set so that a range of 110 μm or more on the straight line overlaps with the brazing material cross-section. Measure the number of Ni poor phases cut by a straight line (line segment length 100 μm) in the range of 10 μm to 110 μm from the brazing material surface among the set straight lines. Here, the number of Ni poor phases to be cut was defined as (the number of intersections of the straight line and the boundary lines of the Ni poor phase and other phases) / 2. Note that the decimal part was rounded down. In the same procedure, measure the number of Ni poor phases at a line segment length of 100 μm on 5 straight lines at different measurement locations, and calculate the average number of Ni poor phases per 100 μm of line segment length according to the following formula. However, when the Ni enrichment phase is continuous by 50% or more with respect to the thickness of the brazing material in the direction from the interface between the Ni brazing material and the stainless steel to the brazing material surface among the set straight lines, it is excluded from the measurement target. Average number of Ni poor phases per 100 μm of line segment length (pieces) = [Total number of Ni poor phases measured on 5 straight lines (line segment length 100 μm) (pieces)] / 5

[0037] The joined body in which the number of Ni poor phases thus obtained is 10 or more on average per 100 μm of line segment length exhibits better corrosion resistance. This is because in the brazing material structure where the number of Ni poor phases is 10 or more on average per 100 μm of line segment length, the generation and progression of cracks are significantly suppressed. The number of Ni poor phases is preferably 20 or more on average per 100 μm of line segment length.

[0038] (Method for manufacturing a joined body) The joined body of the present invention is manufactured, for example, by brazing as shown below. A Ni brazing material is placed (applied) between the stainless steels to be joined (base materials) or at their joined ends, the Ni brazing material is heated and melted (heating step), and then cooled and solidified (cooling step) to manufacture a Ni brazed joined body of stainless steel. In the method for manufacturing a joined body according to an embodiment of the present invention, the heating during brazing (heating step) uses, for example, a vacuum furnace or an atmosphere furnace, and the degree of vacuum is 4.0×10 -3It is preferably carried out in a vacuum atmosphere of 60 Pa or a nitrogen carrier atmosphere. The heating temperature in the brazing heating process is not particularly limited, but it is preferably carried out at 900 to 1100 °C.

[0039] Subsequently, the Ni brazing material melted by heating in the heating process is cooled and solidified (cooling process). The cooling process after the brazing heating process is an important process in the present invention. By rapidly cooling after the brazing heating process, the solidification of the brazing material proceeds rapidly, and the brazing material structure becomes a fine and discontinuous structure of the same phase. By forming such a brazing material structure, the generation and progression of cracks on the surface of the brazing material can be suppressed, and a Ni-brazed joint of stainless steel having a desired corrosion resistance can be obtained. By setting the average cooling rate in the cooling process of brazing to 30 °C / min or more in the temperature range from the maximum temperature during heating in the brazing heating process to 300 °C, a fine structure sufficient to suppress crack generation 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 process according to an embodiment of the present invention, the cooling is preferably carried out by blowing the atmosphere gas during heating in the brazing heating process onto the joint. The flow rate of the blowing gas is not particularly limited. Also, the dew point of the blowing gas is not particularly limited, but it is preferably carried out at a dew point of -20 °C or lower. In addition, 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 cooling may be started after holding at the maximum temperature for a predetermined time. That is, the maximum temperature becomes the cooling start temperature of the cooling process.

[0040] The method for measuring the temperature of the joined body (soldered part) during soldering is not particularly limited. For example, a thermocouple may be connected to the joined body using a spot welder to measure the temperature, or the temperature of the joined body may be measured using a radiation thermometer. These temperatures are preferably measured at a location within 200 mm from the location where the solder material is applied on the joined body. More preferably, it is a location within 100 mm, and even more preferably, it is a location within 50 mm.

Example

[0041] A 30 mm square (stainless steel (1)) and 20 mm × 15 mm (stainless steel (2)) were cut out from each 1 mm thick stainless steel plate of SUS304, SUS316L, SUS443J1, and SUS444 specified in JIS G 4305:2021 described in Table 1. Also, as the solder material, each powdered Ni solder material of BNi-2, BNi-5, and FP-613 (Ni-29 mass% Cr-6 mass% P-4 mass% Si) manufactured by Fukuda Metal Foil Powder Co., Ltd. described in JIS Z 3265:1998 was mixed with a binder composed of an organic substance to obtain a paste-like Ni solder material. The mass mixing ratio of the binder to the Ni solder material was 10% by mass.

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

[0043] The prepared test piece of the joined body was immersed in 400 mL of a 10 mass% NaCl aqueous solution prepared in a beaker, and the top of the beaker was covered. The beaker was left standing in a constant temperature bath maintained at 80 °C for 24 hours. The test piece held until the specified time was gently taken out, dried with a blower, and then washed with 200 mL of distilled water. After washing, the test piece was dried, the surface of the brazing material at the brazed part was observed, and the corrosion resistance was judged as follows based on the average number density of the remaining corrosion products. 〇 (Good): The average number density of the corrosion products remaining on the surface of the brazing material is 10 pieces / mm 2 Below × (Bad): The average number density of the corrosion products remaining on the surface of the brazing material is 10 pieces / mm 2 Over

[0044] Note that the observation of the surface of the brazing material and the determination of the number of corrosion products were carried out as follows. Among the brazing materials at the brazed part of the test piece subjected to the corrosion test, an arbitrary observation part was placed horizontally with respect to the electron gun of Miniscope (registered trademark) TM3030plus, a scanning electron microscope (SEM) manufactured by Hitachi High-Tech Corporation. At this time, the test piece was cut so that it could be placed on the SEM observation stage. With an acceleration voltage of 5 kV and a magnification of 300 times, a backscattered electron image was taken, and for the same field of view, a compositional analysis was performed using AZtecOne, an EDS manufactured by Oxford Instruments, to identify the corrosion products. The corrosion products refer to granular substances in which either Ni, Cr, or both have an elemental concentration of 20% or more and the elemental concentration of O is 20% or more. The number of identified corrosion products was divided by the area of the observation field of view to obtain the number density of corrosion products per field of view. In any 10 fields of view, the number density of corrosion products per field of view was calculated as described above, and the average number density of corrosion products remaining on the surface of the brazing material at the brazed part was calculated according to the following formula. Average number density of corrosion products remaining on the surface of the brazing material (pieces / mm 2 ) = [Sum of the number densities of corrosion products per field of view calculated in any 10 fields of view] / 10

[0045] Furthermore, the test piece subjected to the corrosion test was cut at the location including the brazing material as shown in Fig. 2 and embedded in an epoxy resin so that the cross-section of the brazing material was exposed. The cross-section of the embedded test piece was mirror-polished, and the obtained cross-section of the test piece was photographed with a backscattered electron image using an SEM at an acceleration voltage of 15 kV and a magnification of 300 times. Also, a compositional analysis using EDS was performed on the photographed field of view to define the Ni poor phase. For the surface of the brazing material in the obtained backscattered electron image, a 120-μm straight line perpendicular to the surface was set at arbitrary 5 points. The number of Ni poor phases cut in the range of 10 μm to 110 μm from the surface of the brazing material for each set straight line was measured, and the average number per 100 μm of the line segment length of the Ni poor phase was calculated according to the following formula. However, when the Ni enriched phase was continuous by 50% or more with respect to the thickness of the brazing material in the direction from the interface between the Ni brazing material and the stainless steel to the surface of the brazing material among the set straight lines, it was excluded from the measurement target. Average number per 100 μm of line segment length of Ni pores phase (pieces) = [Total number of Ni pores phases measured on 5 straight lines (pieces)] / 5

[0046] The results are shown in Table 1.

[0047]

Table 1

[0048] From Table 1, in the invention examples, all showed good corrosion resistance. Specifically, in Invention 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 per 100 μm of line segment length of the Ni pores phase in the brazing material cross-section was 10 or more, so better corrosion resistance was obtained.

[0049] On the other hand, in Comparative Examples No. 17 to 22 where the cooling rate was outside the appropriate range, good corrosion resistance was not 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 per 100 μm of line segment length of the Ni pores phase in the brazing material cross-section was less than 10, so good corrosion resistance was not obtained. Fig. 3 shows an example of a SEM secondary electron image of the brazing material cross-section of the brazed part of the joined body which is a comparative example of the present invention (an example of Comparative Example No. 19). As shown in Fig. 3, in the joined body of the comparative example, the structure of the brazing material in the brazed part was not refined, and the average number per 100 μm of line segment length of the predetermined Ni pores phase was not obtained.

Industrial Applicability

[0050] The Ni-brazed joined body of stainless steel of the present invention shows good corrosion resistance. Therefore, it is suitable for application to various products such as heat exchangers, electric water heaters, gas water heaters, and the peripheral piping of these products.

Claims

1. A joined body having a joined portion in which stainless steel and stainless steel are brazed with a Ni brazing material, 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. A joined body.

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

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