JP7475644B2 - Method for joining metal components - Google Patents

Description

The present invention relates to a joining technique for joining metal members made of iron-based materials, and in particular to a joining technique for joining metal members in an airtight manner.

Structures with sealed structures, such as various containers and pipelines, including fuel tanks for automobiles and piping for chemical equipment, are used in a variety of industrial fields. Such structures have traditionally been formed by joining multiple metal members using a welding method (joining method) such as arc welding, but it has not always been easy to ensure the strength and airtightness of the welded parts in the resulting structures. As a result, a technique has been proposed to ensure the strength and airtightness of the welded parts by providing a ring projection of a predetermined shape on one of the multiple metal members that form the structure and performing resistance welding via the ring projection (ring projection welding) (see, for example, Patent Document 1).

JP 2011-140290 A

However, when joining metal members by ring projection welding as in Patent Document 1, the ring projection itself melts and flows, causing the relative positional relationship of the metal members to change before and after joining. In addition, with welding methods such as arc welding, the metal members and weld material melt and flow at the welded portion, making it impossible to stably maintain the relative positional relationship of the metal members before and after joining. Therefore, with these conventional welding methods, the shape of the structure (joint) in which the metal members are joined varies, making it difficult to obtain a joint with high shape precision.

The purpose of the present invention is to solve the problems associated with conventional joining methods and provide a technology that improves the shape precision of joined bodies formed by joining metal members made of iron-based materials.

According to a first aspect of the present invention, there is provided a method for joining two metal members made of an iron-based material , comprising the steps of: forming a mounting hole consisting of a tapered hole and a parallel hole in one of the two metal members; and forming a tapered portion at one end of the other metal member to be joined to the first metal member, the tapered portion having a maximum outer diameter larger than the maximum inner diameter of the tapered hole, a minimum outer diameter smaller than the minimum inner diameter of the tapered hole, and an inclination identical to that of the tapered hole; inserting one end of the other metal member into the mounting hole formed in the one of the metal members; bringing the outer peripheral surface of the tapered portion into surface contact at a constant width along the inclination direction around the entire circumference of the inner peripheral surface of the tapered hole; applying pressure to the contact surface between the tapered hole and the tapered portion ; and passing current between the two metal members via the contact surface; and locally heating the contact surface to a temperature equal to or lower than the lower of the melting points of the iron-based materials forming the two metal members , thereby joining the two metal members at the contact surface .

The invention described in claim 2 is characterized in that, in the invention described in claim 1, an oxide film formed on the inner surface of the tapered hole and the outer surface of the tapered portion formed in each of the two metal components is removed before joining.

The invention described in claim 3 is the invention described in claim 1 or 2, characterized in that the two metal members are formed of austenitic stainless steel, and the joining temperature of the contact surfaces is set higher than the upper limit temperature at which sensitization occurs in the austenitic stainless steel.

The invention described in claim 4 is characterized in that, in the invention described in claim 1 or 2, the two metal members are formed of austenitic stainless steel, and the joining temperature of the contact surfaces is set higher than the temperature at which solution treatment of the austenitic stainless steel begins.

The invention described in claim 5 is characterized in that, in the invention described in claim 1 or 2, the two metal members are formed of ferritic stainless steel, and the joining temperature of the contact surfaces is set higher than the upper limit temperature at which sigma embrittlement occurs in the ferritic stainless steel.

According to the joining method of claim 1, since the joining temperature of the contact surface during joining is lower than the melting point of either of the two metal members, the two metal members are prevented from melting at the contact surface and from deforming. Therefore, the positional relationship of the two metal members can be maintained in a substantially identical state before and after joining, so that the variation in the shape of the resulting joined body can be suppressed and a joined body with higher shape accuracy can be obtained.

According to the joining method of claim 2, the oxide film formed on the inner peripheral surface of the tapered hole and the outer peripheral surface of the tapered portion is removed before joining , which promotes joining of the two metal members. Therefore, deformation of the two metal members is further suppressed, and it is possible to obtain a joined body with higher shape accuracy.

According to the joining method of claim 3, it is possible to suppress the occurrence of sensitization at the joint between two metal members joined at their contact surfaces , and therefore it is possible to suppress a decrease in corrosion resistance to intergranular corrosion at the joint.

According to the joining method of claim 4, solid solution progresses at the joint between two metal members joined at their contact surfaces , and the processing-induced martensite produced when forming the tapered hole and tapered portion can be austenitized, thereby suppressing a decrease in corrosion resistance to pitting corrosion at the joint.

According to the joining method of claim 5, it is possible to suppress the occurrence of sigma embrittlement at the joint between two metal members joined at their contact surfaces , thereby suppressing a decrease in durability of the joint.

The present invention can be realized in various forms. For example, it can be realized in the form of a joining method, a manufacturing method for containers, piping, etc. using the joining method, and a joined body, container, piping, etc. manufactured by the joining method or manufacturing method.

1A to 1C are process diagrams showing a joining process for joining two metal members according to a first embodiment of the present invention. FIG. 4 is an explanatory diagram showing how members to be joined are joined by passing an electric current through the joint; 1 is a photograph of a metal structure in the vicinity of a joint of a joint body obtained as an example of the first embodiment. FIG. 11 is a process diagram showing a part of a joining process according to a second embodiment of the present invention. 13 is a photograph of the metal structure near the joint of a joint body obtained as an example of the second embodiment.

The following describes in detail an embodiment of a method for joining metal members according to the present invention and a joined body joined by the method with reference to the drawings.

First Embodiment
1 is a process diagram showing a joining process for joining two metal members according to a first embodiment of the present invention. In the joining process of the first embodiment, first, as shown in FIG 1(a), a large diameter pipe 100 and a small diameter pipe 200 having an outer diameter smaller than that of the large diameter pipe 100 are prepared as two metal members (joined members) to be joined.

The large diameter pipe 100 is a substantially cylindrical pipe material made of austenitic stainless steel (such as SUS304), and has a mounting hole 190 formed on its side for mounting the small diameter pipe 200. The mounting hole 190 is a hole that penetrates the large diameter pipe 100 from the outer periphery to the inner periphery, and is composed of a tapered hole 191 formed on the outer periphery side (hereinafter also simply referred to as the outer periphery side) of the large diameter pipe 100, and a parallel hole 192 formed on the inner periphery side (hereinafter also simply referred to as the inner periphery side) of the tapered hole 191. In the example of FIG. 1, the mounting hole 190 consisting of the tapered hole 191 and the parallel hole 192 is formed in the large diameter pipe 100, but the parallel hole may be omitted and only the tapered hole may be formed as the mounting hole in the large diameter pipe.

The tapered hole 191 has an inner peripheral surface that is approximately a truncated cone side surface (tapered shape), and the inner diameter is formed so that it decreases from the outer peripheral side to the inner peripheral side. The parallel hole 192 is formed so that its approximately constant inner diameter is the same as the smallest inner diameter of the tapered hole 191, i.e., the inner diameter of the inner peripheral end of the tapered hole 191. In addition, the tapered hole 191 is circumferentially recessed, forming a horizontal plane F. In other words, the mounting hole 190 consisting of the tapered hole 191 and the parallel hole 192 is formed by drilling and recessing the large diameter pipe 100, which is a cylindrical base pipe.

The small diameter pipe 200 is a pipe material made of austenitic stainless steel, like the large diameter pipe 100, and has a cylindrical tubular portion 210 and a tapered portion 220 extending from one end of the tubular portion 210. The tapered portion 220 has a tapered outer surface and is formed so that the outer diameter decreases in the opposite direction (tip side) of the tubular portion 210. The small diameter pipe 200 having such a tapered portion 220 is usually formed by cutting one end of a cylindrical base pipe. In the following description, the direction from the tubular portion 210 toward the tapered portion 220 of the small diameter pipe 200 is also referred to as the tip side, and the direction from the tapered portion 220 toward the tubular portion 210 is also referred to as the rear end side.

The tapered portion 220 is formed so that its maximum outer diameter (outer diameter of the cylindrical portion 210) is larger than the maximum inner diameter (inner diameter of the outer peripheral end) of the tapered hole 191 formed in the large diameter pipe 100, and its minimum outer diameter (outer diameter of the tip end) is smaller than the minimum inner diameter (inner diameter of the inner peripheral end) of the tapered hole 191. The tapered portion 220 is also formed so that its inclination is the same as the inclination of the tapered hole 191 provided in the large diameter pipe 100. Here, the inclination of the tapered portion 220 and the tapered hole 191 refers to the inclination angle of the side of the truncated cone corresponding to the tapered shape of the outer peripheral surface and the inner peripheral surface, that is, the angle between the axis of the truncated cone and the side.

1, the tapered section 220 provided in the small diameter pipe 200 is formed so that its maximum outer diameter is larger than the maximum inner diameter of the tapered hole 191, its minimum outer diameter is smaller than the minimum inner diameter of the tapered hole 191, and its inclination is the same as that of the tapered hole 191. Therefore, the tapered section 220 provided in the small diameter pipe 200 and the tapered hole 191 provided in the large diameter pipe 100 are shaped so that they can come into surface contact with each other. In the present invention and this specification, shapes that can come into surface contact with each other in this way are also referred to as complementary shapes.

As can be seen from the above explanation, both the inner peripheral surface of tapered hole 191 and the outer peripheral surface of tapered section 220 are annular. Here, a certain part being annular means that the part has a shape that surrounds a certain area located inside it. Therefore, the inner peripheral surface of tapered hole 191 and the outer peripheral surface of tapered section 220 are annular surfaces that are complementary to each other.

After preparing the large diameter pipe 100 and the small diameter pipe 200, as shown in FIG. 1(b), the large diameter pipe 100 and the small diameter pipe 200 (i.e., the members to be joined) are assembled. Specifically, the tapered portion 220 provided on the tip side of the small diameter pipe 200 is inserted into the mounting hole 190 from the outer periphery side of the large diameter pipe 100. As a result, the small diameter pipe 200 is assembled to the large diameter pipe 100 in a state where the tapered portion 220 provided on the small diameter pipe 200 and the tapered hole 191 of the mounting hole 190 formed in the large diameter pipe 100 are in surface contact, that is, the outer periphery of the tapered portion 220 and the inner periphery of the tapered hole 191 are in contact with each other. In this way, the outer periphery of the tapered portion 220 and the inner periphery of the tapered hole 191 are formed so as to come into contact (abut) with each other, so they can also be called abutting surfaces.

As described above, the taper hole 191 is countersunk around the taper hole 191 to form a horizontal surface F, so that the taper portion 220 of the small diameter pipe 200 and the taper hole 191 of the large diameter pipe 100 are in surface contact with each other at a constant width (vertical width along the inclined surface) around the entire circumference of the taper hole 191 (i.e., the entire circumference of the taper portion 220). The shapes of the taper portion 220 of the small diameter pipe 200 and the taper hole 191 of the large diameter pipe 100 are adjusted so that when they are in surface contact as described above, the width of the surface contact portion between the two (vertical width along the inclined surface) is 0.2 mm or more and 1.0 mm or less around the entire circumference of the taper hole 191 (i.e., the entire circumference of the taper portion 220).

After the members to be joined are assembled, the assembled members to be joined (large diameter tube 100 and small diameter tube 200) are connected to a power supply unit 900, which is a source of current (FIG. 1(c)). Specifically, the large diameter tube 100 is placed on the first electrode body (lower electrode body) 910 connected to the power supply unit 900, and the second electrode body (upper electrode body) 920 connected to the power supply unit 900 is placed on the rear end side of the small diameter tube 200, as shown by the arrow.

In general, the electrical resistance (contact resistance) at the contact surface between two metal members is higher than the electrical resistance of the metal members themselves. Therefore, when two electrode bodies 910, 920 are connected to a large diameter tube 100 and a small diameter tube 200, the electrical resistance between the two electrode bodies 910, 920 is mostly generated by the contact resistance between the lower electrode body 910 and the large diameter tube 100, between the large diameter tube 100 and the small diameter tube 200, and between the small diameter tube 200 and the upper electrode body 920.

If the contact resistance between the lower electrode body 910 and the large diameter tube 100, and between the small diameter tube 200 and the upper electrode body 920, becomes high, unnecessary Joule heat will be generated when electricity is passed through the electrodes, as described below, resulting in large energy losses and the risk of the electrodes 910, 920 fusing to the members to be joined 100, 200.

In the first embodiment, as shown in FIG. 1(c), a recess 919 having a shape complementary to the outer circumferential surface of the large diameter tube 100 is formed on the upper part of the lower electrode body 910 to increase the area of the contact surface between the large diameter tube 100 and the lower electrode body 910 and reduce the contact resistance at the contact surface. In addition, a flat portion 921 is formed on the side of the upper electrode body 920 that comes into contact with the small diameter tube 200 to increase the area of the contact surface between the small diameter tube 200 and the upper electrode body 920 and reduce the contact resistance at the contact surface.

Furthermore, in the first embodiment, the electrode bodies 910, 920 are formed from chromium copper, which has high electrical conductivity, thereby lowering the electrical resistance of the electrode bodies 910, 920 themselves and further reducing the contact resistance between the electrode bodies 910, 920 and the joined members 100, 200. The electrode bodies do not necessarily have to be formed from chromium copper, and can be formed from various metal materials. However, in order to reduce the electrical resistance of the electrode bodies themselves and the contact resistance between the electrode bodies and the joined members, it is preferable to form the electrode bodies from a copper alloy, such as chromium copper.

After connecting the members to be joined 100, 200 to the electrode bodies 910, 920 in this manner, the upper electrode body 920 is pressed down toward the lower electrode body 910 as shown by the arrow in FIG. 1(d), while outputting current from the power supply device 900 through the electrode bodies 910, 920, and energizing the members to be joined 100, 200.

The power supply unit 900 has the function of outputting a pulsed current to a load (i.e., the members to be joined 100, 200) connected between the two electrode bodies 910, 920 according to preset current conditions. As the current conditions, the width of the current pulse (current flow time) and the height of the current pulse (current flow) can be set.

As described above, the contact resistance between the electrode bodies 910, 920 and the members to be joined 100, 200 is reduced, so the electrical resistance between the two electrode bodies 910, 920 is mostly the contact resistance between the large diameter tube 100 and the small diameter tube 200, that is, the contact resistance at the contact surface between the tapered portion 220 and the tapered hole 191. Therefore, when electricity is applied to the members to be joined 100, 200, Joule heat is generated mainly at the contact surface between the tapered portion 220 and the tapered hole 191, so that the contact surface is locally heated.

Then, the contact surface between the tapered portion 220 and the tapered hole 191 is locally heated by passing electricity through the tapered portion 220, and the upper electrode body 920 is pressed down toward the lower electrode body 910 to apply pressure to the contact surface, thereby joining the large diameter tube 100 and the small diameter tube 200 at the contact surface. Note that the contact surface between the tapered portion 220 and the tapered hole 191 can also be called a joint, since it is the part where the large diameter tube 100 and the small diameter tube 200 are joined.

In addition, in the first embodiment, the outer peripheral surface of the tapered portion 220 and the inner peripheral surface of the tapered hole 191, i.e., the abutment surfaces formed on the tapered portion 220 and the tapered hole 191, respectively, are annular. Therefore, the joint between the tapered hole 191 and the tapered portion 220 is annular, and the large diameter pipe 100 and the small diameter pipe 200 are joined airtight. Here, airtight means that the inner area and the outer area at the annular joint are not connected.

Figure 2 is an explanatory diagram showing how a large diameter pipe 100 and a small diameter pipe 200 (joined members) are joined by passing an electric current through them. Note that Figure 2 shows an enlarged view of the large diameter pipe 100 and the small diameter pipe 200 cut along a plane passing through the central axes of both pipes.

As described above, the tapered hole 191 provided in the large diameter pipe 100 and the tapered portion 220 provided in the small diameter pipe 200 have complementary shapes. Therefore, as shown in FIG. 2(a), when the large diameter pipe 100 and the small diameter pipe 200 are assembled, the tapered hole 191 and the tapered portion 220 are in surface contact at the joint CS.

In this state, by pushing the small diameter pipe 200 down towards the large diameter pipe 100 as shown by the open arrow in FIG. 2(b), pressure is applied to the joint CS between the tapered hole 191 and the tapered portion 220 as shown by the filled arrow. Then, when the power supply unit 900 applies electricity between the large diameter pipe 100 and the small diameter pipe 200 according to the preset current conditions, the current flows through the joint CS, and the joint CS is locally heated.

During the current flow, the heat generated at the joint CS is transferred to the surrounding area of the joint CS. When heat is transferred to the surrounding area of the joint CS in this way, there is a risk that the transferred heat will affect the material properties (corrosion resistance, mechanical properties, etc.) of the surrounding area. The surrounding area (heat-affected zone) of the joint CS, which is affected by this heat, expands as the current flow time increases. Therefore, in order to suppress the transfer of heat to the surrounding area of the joint CS and suppress the expansion of the heat-affected zone, it is preferable to make the current flow time sufficiently short compared to the expansion rate of the heat-affected zone. Specifically, when the temperature of the welded part rises to 900 to 1,000°C, the current flow time (the time Tp from when the current starts to flow until it reaches the peak) is preferably 1 ms or more and less than 100 ms, more preferably 5 ms or more and less than 50 ms, and particularly preferably 10 ms or more and less than 30 ms.

On the other hand, since the Joule heat generated by the current flow is concentrated at the joint CS, even if the current flow time is short as described above, by making the current flow sufficiently large, the temperature of the joint CS can be rapidly increased and the temperature of the joint CS can reach a temperature suitable for joining (joining temperature). The current flow is appropriately set by performing simulations, etc., based on the thus set joining temperature, current flow time, area of the joint CS, material and shape of the joined members 100, 200, etc.

The joining temperature, i.e., the temperature reached by the joint CS, is generally sufficient as long as it is equal to or higher than the temperature at which solid-phase joining is possible (solid-phase joining temperature: a thermodynamic temperature that is approximately 0.5 times the melting point). On the other hand, the upper limit of the joining temperature is set to the melting point of the metal material forming the joined members 100, 200 (hereinafter also referred to as the melting point of the joined members 100, 200). Therefore, in the first embodiment in which the joined members 100, 200 are formed of austenitic stainless steel, the joining temperature is set to the melting point of austenitic stainless steel (approximately 1400°C) or lower.

In the first embodiment, the joining temperature is set to be equal to or lower than the melting point of the joined members 100, 200. However, since the joint CS is heated to a temperature equal to or higher than the solid-phase joining temperature and pressure is applied to the joint CS, the large diameter pipe 100 and the small diameter pipe 200 are joined at the joint BR as shown in FIG. 2(c), and a joined body 10 is obtained in which the large diameter pipe 11 and the small diameter pipe 12 are integrated. Therefore, according to the first embodiment, the joined members 100, 200 can be joined at the joint CS with sufficiently high strength.

In the first embodiment, the joining temperature is set to below the melting point of the members to be joined 100, 200, so that the members to be joined 100, 200 are joined to each other without either of them melting. This prevents the members to be joined 100, 200 from melting and deforming, and as shown in FIG. 2(c), the positional relationship between the large diameter tube 11 and the small diameter tube 12 in the joined body 10 obtained after joining is maintained in substantially the same state as before joining. In this way, the positional relationship of the members to be joined is maintained in substantially the same state before and after joining, so that a joined body 10 with higher shape accuracy can be obtained.

In addition, in the first embodiment, the joining temperature is set to be equal to or lower than the melting point of the joined members 100, 200, so melting and resolidification of the joined members 100, 200 do not occur at the joints CS, BR. This suppresses the coarsening of crystal grains and the segregation of added elements and impurities that would occur due to resolidification, and suppresses the reduction in ductility and toughness due to coarsening in the region near the joint BR, and the changes in corrosion resistance and mechanical properties (material properties) due to segregation.

As described above, the joining temperature may be set to be equal to or higher than the solid-phase joining temperature. However, in the first embodiment, since the members to be joined are made of austenitic stainless steel, it is preferable to set the joining temperature higher than the upper limit temperature at which sensitization occurs in austenitic stainless steel (sensitization temperature: approximately 800°C). In this way, by setting the joining temperature higher than the sensitization temperature, sensitization of the joint BR can be suppressed, and a decrease in the corrosion resistance of the joint BR to intergranular corrosion can be suppressed.

Furthermore, the joining temperature is preferably set higher than the temperature at which solid solution (also called solution treatment) begins in austenitic stainless steel (solid solution treatment temperature: approximately 1100°C). In this way, by setting the joining temperature higher than the solid solution treatment temperature, the processing-induced martensite generated by machining or the like when forming the tapered hole 191 and the tapered portion 220 can be austenitized. Therefore, processing-induced martensite remains in the joint BR, and it is possible to suppress a decrease in the corrosion resistance of the joint BR against pitting corrosion (crevice corrosion).

<Example of the first embodiment>
In order to confirm that the joining process of the first embodiment can join the members to be joined, a joined body was prepared as an example, and the state of the joint in the obtained joined body was observed. Specifically, a large diameter pipe with a tapered hole and a small diameter pipe with a tapered portion were prepared from SUS304 blank pipes of different diameters as the members to be joined, and the members to be joined were joined as described above to obtain a joined body. Then, a region including the joint was cut out from the obtained joined body, and the metal structure of the joint was observed. In the joining, the current flow time was set to 30 ms, and the current flow was set so that the joining temperature was 900°C.

To observe the metal structure, a sample was prepared by cutting the joint along a plane passing through the central axes of both the large-diameter tube and the small-diameter tube, and the cut surface of the sample was polished. After polishing the cut surface, the sample was immersed in an etching solution to reveal the metal structure. Marble's reagent (a mixture of 4 g of copper sulfate, 20 ml of hydrochloric acid, and 20 ml of water) was used as the etching solution to reveal the metal structure of SUS304, an austenitic stainless steel.

In this way, the metal structure of the joint was observed by observing the sample with an optical microscope, which revealed the metal structure near the joint. Figure 3 is a photograph of the metal structure near the joint of the joint obtained as an example of the first embodiment.

As can be seen from Figure 3, in the examples of the first embodiment, the members to be joined (large diameter pipe and small diameter pipe) melt and the resolidified portion (also called a nugget) that occurs when the melted portion resolidifies was not formed at the joint. From this, it is considered that the joining method of the first embodiment suppresses melting of the members to be joined, and does not cause a decrease in ductility or toughness or a change in material properties due to the formation of a nugget.

In addition, although there is a gap on the inner circumference side of the large diameter pipe due to processing errors in the tapered hole and the inclination of the tapered portion, it was confirmed that the large diameter pipe and the small diameter pipe are integrated on the outer circumference side. From this, it was confirmed that the joining method of the first embodiment makes it possible to join the large diameter pipe and the small diameter pipe airtightly.

On the other hand, as shown in Figure 3, on the outer periphery of the large diameter pipe, a protrusion (burr) was formed extending from the joint toward the outer periphery. In addition, fiber flows were formed along the joint over almost the entire area of the joint. Normally, such protrusions and fiber flows are formed when the joined members undergo plastic deformation. For this reason, it is believed that when the joined body of the embodiment shown in Figure 3 was created, the strength of the joined members near the joint decreased due to the temperature rise during joining, causing plastic deformation of the joint and the area nearby.

However, as shown in FIG. 3, the size of the protrusion was sufficiently small, and the formation of the grain flow line was limited to the area very close to the joint. From this, it was confirmed that the joining method of the first embodiment does not cause a significant change in the positional relationship between the large diameter tube and the small diameter tube before and after joining, and it is possible to obtain a joined body with higher shape accuracy.

Second Embodiment
Fig. 4 is a process diagram showing a part of a joining process according to a second embodiment of the present invention. The joining process of the second embodiment differs from the joining process of the first embodiment shown in Fig. 1 in that it includes a process of removing a passivation film formed on the surfaces of the members to be joined 100, 200. Since other points are the same as those of the first embodiment, in Fig. 4, the process after the connection of the power supply device (Fig. 4(d)) is omitted from the illustration.

In the second embodiment, the same members 100 and 200 to be joined as in the first embodiment are prepared as shown in FIG. 4(a). Generally, a passive film (oxide film) is naturally formed on the surface of stainless steel as the chromium in the stainless steel is oxidized by oxygen in the air. Therefore, a passive film is naturally formed on the contact surfaces (the inner surface of the tapered hole 191 and the outer surface of the tapered portion 220) of the members 100 and 200 to be joined that come into contact during joining. However, the passive film formed naturally in this way is not necessarily uniform, and there is a risk that the thickness will be thicker than necessary.

Therefore, in the second embodiment, the prepared members 100 and 200 to be joined are subjected to an acid pickling process to remove the passive film formed on the surfaces of the members 100 and 200 to be joined. Specifically, as shown in FIG. 4(b), a cloth C containing an acid (such as an acidic electrolyte) is wrapped around the electrode 810, and while a predetermined amount of electricity is applied to the electrode 810 by the electric current application device 800, the inner surface of the tapered hole 191 and the outer surface of the tapered portion 220 are lightly traced with the cloth C wrapped around the electrode 810 (i.e., the cloth C wrapped around the electrode 810 is brought into contact with those parts). When the members 100 and 200 to be joined are subjected to an acid pickling process in this way, another electrode 820 not wrapped with the cloth C is brought into contact with the members 100 and 200 to be joined. In addition, an acid electrolyte (such as Pikaso #SUS Bright manufactured by Chemical Yamahonsha) can be suitably used as the acid for removing the passivation film formed on the surface of the joined members 100 and 200. By applying such electricity, the passivation film formed on the inner surface of the tapered hole 191 and the outer surface of the tapered portion 220 is temporarily removed. As a result, a new, extremely thin (about a few nm) and uniform passivation film is formed on the inner surface of the tapered hole 191 of the joined member 100a and the outer surface of the tapered portion 220 of the joined member 200a that have been subjected to the pickling treatment.

In this way, by performing the pickling treatment, the passivation film formed on the contact surface of the inner circumferential surface of the tapered hole 191 of the joined member 100a and the outer circumferential surface of the tapered portion 220 of the joined member 200a becomes extremely thin and uniform, so that the joining is promoted at the joint where the contact surfaces come into contact. Therefore, even if the current flow time is shortened or the joining temperature is lowered, it is possible to reliably join the joined members 100a, 200a (i.e., the inner circumferential surface of the tapered hole 191 of the joined member 100a and the outer circumferential surface of the tapered portion 220 of the joined member 200a). In this way, the amount of heat input to the joint can be reduced, so that the expansion of the heat-affected zone around the joint can be suppressed, and the influence on the material properties such as corrosion resistance and mechanical properties at the joint can be suppressed.

<Example of the second embodiment>
In order to confirm that the joining process of the second embodiment can also join the members to be joined, a joined body was prepared as an example, and the state of the joint in the obtained joined body was observed. Specifically, similar to the example of the first embodiment, a large diameter pipe and a small diameter pipe were prepared, and as described above, the passive film was removed and the members to be joined were joined to obtain a joined body. Then, a sample was prepared similar to the example of the first embodiment, and the metal structure of the joint was observed. In the joining, the current flow time was 30 ms, the same as the example of the first embodiment. Meanwhile, the current flow was set so that the joining temperature was 850° C., which is lower than that of the example of the first embodiment.

Figure 5 is a photograph of the metal structure near the joint of the joint obtained as an example of the second embodiment. As can be seen from Figure 5, even in the example of the second embodiment, no nugget was formed at the joint. From this, it is believed that the joining method of the second embodiment also suppresses melting of the joined members, and does not cause a decrease in ductility or toughness or a change in material properties due to the formation of a nugget. It was also confirmed that the large diameter pipe and the small diameter pipe were integrated on the outer periphery side of the large diameter pipe. From this, it was confirmed that the joining method of the second embodiment can also hermetically join the large diameter pipe and the small diameter pipe.

Furthermore, in the examples of the second embodiment, a protrusion was formed on the outer periphery of the large diameter pipe. However, in the examples of the second embodiment, although grain flows were formed in the joint, the area in which the grain flows were formed was smaller than in the examples of the first embodiment shown in FIG. 3. From this, it was found that in the examples of the second embodiment, by lowering the joining temperature compared to the examples of the first embodiment, the area in which the strength was reduced due to the temperature rise during joining was smaller, and plastic deformation could be suppressed. In this way, according to the joining method of the second embodiment, the plastic deformation of the joined members during joining is suppressed, and the change in the positional relationship between the large diameter pipe and the small diameter pipe before and after joining is further suppressed, and it was found that it is possible to obtain a joint with even higher shape accuracy.

<Modification of the second embodiment>
In the second embodiment, the passivation film formed on the surface of the joined members is removed by pickling treatment. However, in addition to the pickling treatment, the passivation film can also be removed by heating the prepared joined members 100, 200 in a furnace in a reducing atmosphere of hydrogen gas, carbon monoxide, or the like.

In addition, in the second embodiment, the passive film is removed prior to assembling the members 100a and 200a to be joined (Fig. 4(c)) and connecting them to the power supply 900 (Fig. 4(d)) and prior to energizing the members 100a and 200a to be joined (see Fig. 1(d)). However, the passive film can also be removed by energizing the members to be joined in a reducing atmosphere.

In this way, the joint is heated, the passivation film formed on the contact surface of the joined members is removed, and the joining is performed without the passivation film formed on the joint, promoting the joining of the joined members. Therefore, even if the current flow time is further shortened or the joining temperature is lowered, the joined members can be reliably joined, further improving the shape precision of the joined body, further suppressing the expansion of the heat-affected zone around the joint, and further suppressing the effects on material properties such as corrosion resistance and mechanical properties.

<Modification>
The present invention is not limited to the above-described embodiments, and can be embodied in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

<Modification 1>
In each of the above embodiments, the large diameter pipe 100 and the small diameter pipe 200 are provided with a tapered hole 191 and a tapered portion 220, respectively, and the inner peripheral surface of the tapered hole 191 and the outer peripheral surface (abutment surface) of the tapered portion 220 are brought into contact with each other to perform joining, but the abutment surfaces formed on the two joined members may have shapes complementary to each other and may be formed in an annular shape. For example, two cylindrical pipe materials of the same diameter may be joined at their end faces, or a first metal member provided with an annular flange may be joined to a second metal member having an attachment surface formed in a shape complementary to the flange. Furthermore, the abutment surface does not necessarily have to be annular or tapered as long as it is annular, and may be elliptical truncated cone side surface or square. Furthermore, the abutment surface does not have to be formed as a clear surface like the tapered hole 191 and the tapered portion 220 in each of the above embodiments, and may be formed at a corner portion as a micro shape.

<Modification 2>
In the above embodiments, the members to be joined 100, 200 are made of austenitic stainless steel, but the members to be joined can be made of various stainless steels, such as ferritic stainless steel (SUS430, etc.) and martensitic stainless steel (SUS410, etc.). In general, the members to be joined may be made of any material as long as it is an iron-based material (iron-based alloy and pure iron). Even in this case, it is preferable to remove the oxide film formed on the contact surface of the members to be joined, as in the second embodiment or its modified example, in order to promote the joining of the members to be joined.

When the members to be joined are made of ferritic stainless steel, the joining temperature is preferably set higher than the upper limit temperature at which σ embrittlement occurs in ferritic stainless steel (σ embrittlement temperature: approximately 800°C). In this way, by setting the joining temperature at or above the σ embrittlement temperature, the occurrence of σ embrittlement at the joint is suppressed, making it possible to suppress a decrease in the durability of the joint.

In addition, when the members to be joined are made of tool steel or structural steel, the joining temperature is preferably set to be equal to or higher than the temperature at which transformation to austenite begins (Ac1 transformation point), and more preferably equal to or higher than the temperature at which transformation to austenite is completed (Ac3 transformation point). In this way, by setting the joining temperature to be equal to or higher than the austenite transformation point (Ac1 transformation point or Ac3 transformation point), the joining becomes stronger due to the change in crystal structure when passing through the austenite transformation point, making it possible to increase the strength of the joined body at the joint.

<Modification 3>
In the above embodiments, the workpieces 100, 200 (100a, 200a) made of the same type of stainless steel (austenitic stainless steel) are joined, but the two workpieces may be made of different iron-based materials. Even in this case, the joining temperature is set to the lower of the melting points of the two workpieces or lower, thereby preventing the workpieces from melting at the joining portion. Therefore, the additive elements of the workpieces are averaged at the molten portion, and the alloy composition at the joining portion is prevented from changing. In this way, the alloy composition of the workpieces is prevented from changing at the joining portion, thereby preventing the material properties of the workpieces from changing.

However, if the two joined members are made of different iron-based materials, there is a risk that thermal cycle fatigue will occur due to differences in thermal expansion coefficients, causing damage to the joint. For this reason, it is preferable that the two joined members are made of the same type of iron-based material.

On the other hand, by forming the two joined members from different iron-based materials, each part of the joined body can be made from a more appropriate material. For example, a joined body can be formed by joining a tank made from austenitic stainless steel and a nozzle made from martensitic stainless steel. In this case, by forming the tank from austenitic stainless steel, which is easy to deep draw, it becomes easy to increase the capacity of the tank, and by forming the nozzle from martensitic stainless steel, it becomes easier to form a nozzle that requires high hardness.

The joining method according to the present invention can be used not only to join two iron-based materials to be joined, but also to join nickel-based alloys such as Inconel and Hastelloy, or to join such nickel-based alloys to iron-based materials. In addition, when using the joining method according to the present invention to join nickel-based alloys to each other, or to join such nickel-based alloys to iron-based materials, it is preferable to set the proportion of nickel components in the nickel-based alloy to 8.0% by mass or more and 20% by mass or less in terms of maintaining the joining strength.

On the other hand, in the joining method according to the present invention, it is preferable to adjust the electrical resistance of the two joined members to 140 μΩ·cm or less and the thermal conductivity of the two joined members to 30 Wm·k or less, since this allows for high strength joining with less power. In addition, it is even more preferable to adjust the electrical resistance of the two joined members to 120 μΩ·cm or less and the thermal conductivity of the two joined members to 20 Wm·k or less.

<Modification 4>
In each of the above embodiments, the current flow time is sufficiently short to suppress the expansion of the heat-affected zone around the joint CS, but if the presence of the heat-affected zone in the joint is not a problem, the current flow time can be extended. For example, if the joint obtained after joining is subjected to heat treatment (solution treatment, quenching, tempering, etc.) or if the joined members are made of soft iron, etc., the current flow time can be extended because problems such as sensitization and embrittlement do not occur even if the heat-affected zone exists.

The joining method of the present invention has the excellent effects described above, and therefore can be suitably used as a joining method for airtightly joining metal members made of various iron-based materials. In addition, the joined body of the present invention has the excellent effects described above, and therefore can be suitably used as various machine parts that require airtightness.

LIST OF SYMBOLS 10 11 Large diameter pipe 12 Small diameter pipe 100, 100a Large diameter pipe 190, 190a Mounting hole 191, 191a Tapered hole 192, 192a Parallel hole 200, 200a Small diameter pipe 210, 210a Cylindrical portion 220, 220a Tapered portion 900 Power supply unit 910 Lower electrode body 919 Recess 920 Upper electrode body 921 Planar portion BR Joint portion CS Joint portion F Horizontal surface

Claims (5)

A method for joining two metal members made of an iron-based material, comprising the steps of:
One of the two metal members has a mounting hole formed therein, the mounting hole being a tapered hole and a parallel hole.
a tapered portion is formed at one end of the other metal member joined to the one metal member, the maximum outer diameter being larger than the maximum inner diameter of the tapered hole, the minimum outer diameter being smaller than the minimum inner diameter of the tapered hole, and the inclination being the same as that of the tapered hole;
One end of the other metal member is inserted into a mounting hole formed in one of the metal members, and the outer peripheral surface of the tapered portion is brought into surface contact with the inner peripheral surface of the tapered hole at a constant width along the inclination direction over the entire circumference of the inner peripheral surface of the tapered hole,
By applying pressure to the contact surface between the tapered hole and the tapered portion and passing a current between the two metal members via the contact surface, the contact surface is locally heated to a temperature equal to or lower than the lower of the melting points of the iron-based materials forming the two metal members,
The joining method comprises joining the two metal members at the contact surface . The bonding method according to claim 1,
The present invention is characterized in that an oxide film formed on an inner peripheral surface of the tapered hole and an outer peripheral surface of the tapered portion formed in each of the two metal members is removed before joining.
Joining method. The bonding method according to claim 1 or 2,
The two metal members are made of austenitic stainless steel,
The joining temperature of the contact surfaces is set higher than the upper limit temperature at which sensitization occurs in the austenitic stainless steel.
Joining method. The bonding method according to claim 1 or 2,
The two metal members are made of austenitic stainless steel,
The joining temperature of the contact surfaces is set to be higher than a temperature at which the solution treatment of the austenitic stainless steel begins.
Joining method. The bonding method according to claim 1 or 2,
The two metal members are formed of ferritic stainless steel,
The joining temperature of the contact surfaces is set higher than the upper limit temperature at which σ embrittlement occurs in the ferritic stainless steel.
Joining method.

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