JP3465079B2 - Joining method - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to relates to a bonding how the metal tube using a diffusion bonding method.

[0002]

2. Description of the Related Art When joining metal materials such as steel plates and steel pipes,
A diffusion bonding method is generally used as a bonding method that causes less thermal deformation and chemical composition change than a conventional welding method involving melting of metal materials to be bonded.

FIG. 2 is a schematic diagram showing the construction of a conventional joining apparatus for joining metal materials by a diffusion joining method. In the figure, reference numeral 4 denotes a chamber. In the chamber 4, clamp mechanisms 21 and 22 for sandwiching the metal pipes 11 and 12 to be joined are arranged facing each other with an appropriate distance.
1 and 22 are driven by the hydraulic device 2 to move in the contact and separation directions, respectively, and the sandwiched metal pipes 11 and 12 are abutted against each other in an axial direction to apply pressure in the axial direction to compress the joint. . A pressure sensor 50 that monitors the pressure at the time of joining is attached to the clamp mechanisms 21 and 22, and the measurement result of the pressure sensor 50 is given to a control unit 60, which feeds back the measurement result to control the hydraulic pressure. Device 2
The welding pressure output by is controlled to maintain the joining pressure at a required pressure value.

A heating coil 30 connected to a high-frequency power source 31 is arranged substantially in the center of the clamp mechanisms 21 and 22, and a temperature sensor (not shown) for monitoring the junction temperature is provided inside the heating coil 30. It is arranged. Further, the data of the temperature in the vicinity of the joint between the metal tubes 11 and 12 measured by the temperature sensor is given to the controller 60, and the controller 60 controls the high-frequency current to be passed through the heating coil 30 by using the data, and the joint is performed. Maintain the temperature at the required temperature.

When the above-mentioned joining device is used, when the metal pipes 11 and 12 are joined in the axial direction, the clamp mechanism 2 is used.
The metal pipes 1 and 12 are sandwiched between the metal pipes 1 and 12, respectively.
The joining surfaces of 1 and 12 are butted against each other with a joining material having a lower melting point than the metal pipes 11 and 12 interposed. Next, a nitrogen gas for oxidation prevention is introduced into the chamber 4 using a gas introduction pipe (not shown) to adjust the atmosphere, and the heating coil 30
A high-frequency current is passed through to rapidly inductively heat the vicinity of the joints of the metal pipes 11 and 12 to the joint temperature at which the joint material melts. Next, the hydraulic device 2 outputs a required pressing force, and the pressing force is transmitted to the clamp mechanisms 21 and 22, respectively, and the metal pipe 1
The metal pipes 11 and 12 are joined by rapidly compressing the pipes 1 and 12 in the direction of the outline arrow in the figure. The control unit 60 controls the hydraulic device 2 and the high frequency power supply 31 to start or end heating or pressurization,
Or make adjustments. Further, the hydraulic system 2 and the high frequency power source 3 are fed back so that the joining pressure of the metal pipes 11 and 12 measured by the pressure sensor 50 and the joining temperature measured by the temperature sensor are fed back so that the joining pressure and the joining temperature become appropriate values.
Control 1

FIG. 3 is an explanatory view of plastic deformation near the joint portion of the metal pipe, and FIG. 4 is an explanatory view of pipe expansion deformation near the joint portion of the metal pipe.
FIG. 5 is an explanatory diagram of plastic deformation and pipe expansion deformation in the vicinity of the joint portion of the metal pipe. Since heat and pressure are applied to the metal pipes 11 and 12, plastic deformation occurs in the vicinity of the joint portion of the metal pipes 11 and 12, and as shown in FIG. The parts 10a, 10a are produced. Further, the present inventors have found that when the wall thickness / diameter ratio of the metal pipes 11 and 12 is small and therefore the rigidity of the metal pipes 11 and 12 is low,
As shown in FIG. 4, the vicinity of the joint portion is buckled and deformed so that the ends of each of the metal pipes 11 and 12 expand to the outside to form a protrusion 10c on the outside of the joint portion and to the inside. V
It was found that the groove-shaped depression 10b is formed.

When the metal pipes 11 and 12 are buckled, in reality, a combination of plastic deformation and pipe expansion deformation occurs.
As shown in FIG. 5, the protrusion 10a and the protrusion 10c overlap each other on the outer side of the joint to generate the protrusion 1a, and the protrusion 10a and the recess 10b overlap each other on the inner side of the joint to form the protrusion 1a. A complex unevenness having 1a and a depressed portion 1b is generated. When the protrusion 10a due to plastic deformation or the protrusion 1a and the depression 1b due to plastic deformation and pipe expansion deformation occur, the stress concentration generated at the joint increases, and the joint strength of the metal pipes 11 and 12 decreases. The fatigue strength of the joint decreases. If the metal pipes 11 and 12 do not have a high processing accuracy of the joining surfaces or the joining accuracy of the joining surfaces is low, the joining surfaces do not come into close contact with each other and are securely joined. I can't. Further, if more bonding material remains between the bonding surfaces, the bonding strength will decrease. Furthermore,
There is also a problem that the bonding time becomes long. In order to bring the joining surfaces into close contact with each other, discharge more joining material than necessary, and perform reliable joining in a short time, it is necessary to rapidly apply appropriate pressure.

In Japanese Unexamined Patent Publication No. 9-262684, the axial length for heating the joint is limited, and the protrusion length of the protrusion due to plastic deformation / the radius ratio of the joined metal pipe is 1.0 to 1.
By setting the value to 1, the joining method that satisfies the joining strength and the fatigue strength of the joint is disclosed. In addition, JP-A-9-
Japanese Patent No. 267184 discloses a joining method in which the plastic deformation is reduced and the joining strength and the fatigue strength of the joined portion are satisfied by controlling the magnitude of the applied pressure and the timing of applying the applied pressure to the metal pipe. ing. Furthermore, JP-A-11
In JP-A-104854, duplex stainless steel is used,
By limiting the axial length that heats the joint,
Disclosed is a joining method that reduces plastic deformation and satisfies the joining strength and the fatigue strength of the joint.

[0009]

In the conventional joining method, since the pipe expansion deformation due to the buckling of the joint is not taken into consideration, the composite deformation of the plastic deformation and the pipe expansion deformation cannot be effectively suppressed, and the joint strength is increased. Also, there is a problem in that the fatigue strength of the joint is reduced. In addition, the protrusion 1a on the outer side of the joint is
For example, mechanical grinding using a grinder can correct the metal tubes 11 and 12 after joining, but there is also a problem that the inner depressed portion 1b cannot be repaired after joining. Further, since the conventional joining device controls the hydraulic device 2 by using the pressing force, there is also a problem that the equipment configuration such as the control unit 60 and the pressure sensor 50 is complicated. In order to solve such a problem, the inventors have used a heater whose axial length (width) is W when the wall thickness / diameter ratio of the metal tube is small and therefore the rigidity of the metal tube is low. ,
An experiment was conducted in which pressure was applied to a metal tube with a wall thickness t so that the axial deformation length would be a, and as a result, the metal tube buckled and expanded and deformed, causing depression inside the joint. Depth d
When a depressed portion occurs, d / t (%) = (W / t) ×
We have found that the relationship of (a / t) / 2 is established.

The present invention has been made on the basis of such knowledge, and when a metal pipe having a wall thickness t is joined using a heater having a length W in the axial direction, the deformation length a of the metal pipe is W and t
By providing a joining method, it is possible to suppress the occurrence of a depression in the joint, reduce the depression depth of the depression, and prevent a decrease in joining strength and fatigue strength. The purpose is to do. Another object of the present invention is to limit the expansion W of the heater by limiting the width W of the heater and the deformation length a of the metal tube to 5 times the wall thickness t or less, thereby achieving practically sufficient joint strength and fatigue. It is to provide a joining method capable of obtaining strength. Another object of the present invention is to provide a joining method capable of efficiently reducing the depression of the joining portion and further shortening the working time by limiting the time for pressing the metal pipe to 5 seconds or less. In .

[0011]

In the joining method according to the first aspect of the present invention, two metal pipes to be joined in an axial direction are sandwiched by a joining material having a melting point lower than that of the metal pipes. And the joint portion of the metal pipe is heated using a heater until the joint temperature is equal to or higher than the melting point of the joint material, and the metal pipe is axially pressed to compress the joint portion and joint the joint portion. In the method, when it is allowed that a depressed portion having a depressed depth D due to buckling is formed inside the joined portion of the joined metal pipes having a wall thickness t, a heater having an axial length of W is used. Heating the joint and then pressurizing the metal tube until the axial deformation length a determined to satisfy the formula (W / t) × (a / t) / 2 ≦ D / t. Is characterized by.

According to the first aspect of the invention, when a metal pipe having a wall thickness t is joined by using a heater having a length W of the heater in the axial direction of the metal pipe (hereinafter referred to as the width of the heater), Depression depth / thickness ratio D / t in which the depressions formed in the joined metal pipes do not significantly affect the joining strength and the fatigue strength of the joining portion depending on the material or the thickness / diameter ratio of the metal pipe. Is obtained in advance, and the deformation length a of the metal tube in the axial direction is calculated as (W / t) × (a
/ T) / 2 ≦ D / t. Then, the joint is heated using the heater, and when the joint is heated to the joining temperature, pressure is applied to the metal pipe until the deformation length a is reached, that is, the deformation length of the metal pipe is limited. By restricting the pressure applied to the metal pipe and suppressing the occurrence of buckling, it is possible to suppress the occurrence of a depression in the joint, reduce the depression depth of the depression, and reduce the joint strength and It is possible to prevent a decrease in fatigue strength. Further, since it can be easily judged with the naked eye or a gauge, whether or not the predetermined deformation length a has been achieved, a conventional welding is provided which is provided with a heater having a predetermined width W and which controls welding pressure to perform welding. The apparatus can be used to carry out the joining method of the present invention.

The joining method according to the second aspect of the invention is characterized in that the upper limits of the length W and the deformation length a are 5t. In the second invention, when the width W of the heater and the deformation length a of the metal tube are ideally less than or equal to the value of the wall pressure t, the range in which the temperature of the metal tube rises is limited, and Minimize the range in which the strength of the metal tube is reduced and buckling is likely to occur. Also, the deformation length of the metal tube is limited to limit the pressure applied to the metal tube. Therefore, when pressure is applied to the metal pipe, the portion where the strength is reduced can be reinforced by the portion where the strength is maintained, and buckling can be prevented. However, depending on the size, material, joining conditions, etc. of the metal pipe, it may be difficult to carry out or sufficient joining strength may not be obtained. Experimentally each 5
When the deformation length a is t or less, particularly when the deformation length a is 0.4 t or more and 2 t or less, the pipe expansion deformation can be suppressed and the practically sufficient joint strength and fatigue strength can be obtained. Further, by limiting the width of the heater and the pressure applied to the metal tube, it is possible to reduce the plastic deformation of the metal tube, the bonding method of the present invention, the metal tube or metal material having high rigidity and buckling does not occur. Even when it is used for, it is possible to suppress a decrease in bonding strength and fatigue strength.

The joining method according to the third aspect of the invention is characterized in that the time for pressurizing the metal tube is set to 5 seconds or less. Third
In the invention, when excessive time is spent on heating and pressurization, the range in which the metal tube is heated and its strength decreases is expanded by heat conduction, and the strength of the metal tube is further increased by pressing for a long time. The heating and pressurization are performed in a short time because the temperature decreases and the buckling easily occurs. Since the time to reach the bonding temperature depends on the heating rate of the equipment, the time to maintain the bonding temperature,
That is, when the pressurizing time is experimentally 5 seconds, preferably 3 seconds or less, the depression of the joint can be efficiently reduced, and the working time can be shortened.

[0015]

[0016]

[0017]

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the drawings showing the embodiments thereof. FIG. 1 is a schematic diagram showing the configuration of a joining device of the present invention for joining a metal tube having a wall thickness t (mm) at a recess depth / wall thickness ratio D / t (%) of the recess portion or less. Reference numeral 4 in the drawing denotes a chamber, and a gas introduction pipe (not shown) for introducing nitrogen gas is provided in the chamber 4. In the chamber 4, clamp mechanisms 21 and 22 for sandwiching the metal pipes 11 and 12 to be joined are arranged facing each other with an appropriate distance, and the clamp mechanisms 21 and 22 are located outside the chamber 4 and are located in the hydraulic device 2. It is configured to move in the contacting / separating direction by being driven by each other to bring the sandwiched metal tubes 11 and 12 into contact with each other, and to apply a pressure to compress the joint portion.

A heating coil 3 made of copper and having one turn is arranged substantially in the center of the clamp mechanisms 21 and 22 so that the axial direction of the heating coil 3 and the contacting / separating direction are aligned with each other. The width (axial length) of the heating coil 3 is W (mm) ≦ 5t, and the diameter thereof is larger than that of the metal tubes 11 and 12. Further, a temperature sensor (not shown) for monitoring the junction temperature is arranged inside the heating coil 3. The heating coil 3 is connected to a high frequency power supply 31 outside the chamber 4. The data of the temperature in the vicinity of the joint between the metal pipes 11 and 12 measured by the temperature sensor is given to the control unit 6, and the control unit 6 controls the high frequency power supply 31 using the data, and the high frequency to be supplied to the heating coil 3. Increase or decrease the current to maintain the junction temperature at the required temperature.

Clamping mechanism 21 (22) and heating coil 3
A stopper 51 (52) for preventing the clamp mechanism 21 (22) from moving is provided between and. The stopper 51 (52) is movable in the contacting / separating direction, and a recessed portion having a recessed depth D (mm) due to buckling is allowed to be formed inside the joined portion of the joined metal tubes 11 and 12. If D
A that satisfies the relationship of / t ≧ (W / t) × (a / t) / 2
(Mm) ≤ 5t, clamp mechanism 21 at the start of joining
With reference to the position (22), it is fixed inside the chamber 4 with a distance of a / 2 (mm) from the position.

When using the above-described joining device, when joining the metal pipes 11 and 12 having the wall thickness t in the axial direction, the metal pipes 11 and 12 are clamped by the clamp mechanisms 21 and 22, respectively. The respective joining surfaces of 12 are butted against each other with a joining material having a lower melting point than the metal tubes 11 and 12 interposed. Next, the inside of the chamber 4 is gas-replaced using the gas introduction pipe to adjust the atmosphere, a high-frequency current is passed through the heating coil 3, and the melting point of the bonding material is higher than the melting point of the metal tubes 11 and 12 but lower than the melting point. The heating coil 3 induction-heats the vicinity of the joints of the metal tubes 11 and 12 up to the required joint temperature. The induction heating is rapidly performed so that the temperature rising range of the metal tubes 11 and 12 does not significantly expand beyond the width W of the heating coil 3. Further, the control unit 6 controls the high-frequency power source 31 to start, end, or adjust heating, and feeds back the junction temperature measured by the temperature sensor so that the junction temperature becomes an appropriate value. To control.

When the joining portion reaches the joining temperature, the hydraulic device 2 outputs a required pressing force, and the pressing force is applied to the clamp mechanism 2.
The metal tubes 11 and 12 are respectively transmitted to 1 and 22 to compress the metal tubes 11 and 12 in the directions of the hollow arrows in the figure. The pressurization is performed rapidly so that the range does not expand significantly beyond the width W. For this reason, the clamp mechanisms 21 and 22 that have moved in the direction of the hollow arrow mark are
Within a short time (pressurizing time T (s)) within a second, a / 2 (m) respectively
m) Move and hit the stopper. At this time, metal tube 1
Each of 1 and 12 is compressed by a / 2 (mm) and deforms,
Therefore, the joined metal tubes 11 and 12 are a
It has a deformation length of (mm). When the pressurization time T has elapsed from the start of pressurization, the control unit 6 controls the hydraulic device 2 to end the pressurization.

Table 1 shows the wall thickness t of the metal tubes 11 and 12 joined by using the joining apparatus, the width W of the heating coil 3, the deformation length a of the metal tubes 11 and 12, the pressing time T, and the depression. The actual measurement value d of the part depth is shown.

[0024]

[Table 1]

For joining, the composition is 0.25% C-0.2%.
Si-0.45% Mn-1.0% Cr-0.45% Mo
-0.02% Ti-Remainder real Fe, outer diameter 60.3 mm, and quenched (950 ° C) -tempered (620 ° C) treated seamless steel pipes were used as the metal pipes 11 and 12,
As a bonding material, a commercially available amorphous foil MBF-20
(7.0% Cr-3.0% Fe-4.5% Si-3.2
% B-balance Ni, melting point 1024 ° C.) was used. The bonding temperature was 1250 ° C., the bonding temperature was heated to about 30 s (average heating rate 40 ° C./s), and then the bonding temperature was set to 1
Hold for 80s. Further, the depression depth d was obtained by measuring the joints of the joined metal pipes 11 and 12 at four locations at intervals of 90 ° in the circumferential direction and calculating the average value thereof. Allowable depth of depression /
The value of wall thickness ratio D / t is JIS G3452, G345.
4. The oil well pipe standard API 5CT, etc. of the American Petroleum Institute recognizes that a metal pipe with a wall thickness of 4 mm or more has a wall thickness tolerance of 12.5%.
The following values were judged to be good values. The experiment numbers are from 1 to 19, and the experiment numbers 7, 8, 9, 10, 18, and 19 are comparative experiments.

Table 2 shows the heating coil width / thickness ratio W / t, the deformation length / thickness ratio a / t, and the depression depth / thickness ratio d / t (%) of the metal tubes 11 and 12. The theoretical and experimental values and the bending test results are shown. The theoretical value of d / t was calculated using (W / t) × (a / t) / 2.

[0027]

[Table 2]

In the bending test, a test piece having a circumferential length of 20 mm and an axial length of 200 mm was taken from the joined metal pipes 11 and 12 so that the joint portion was located at the center of the axial direction of the test piece. 180 degree front and back bending tests were performed with a bending radius of 10 mm, and it was judged whether the joint was open or not,
When it was opened, it was indicated by x, and when it was not opened, it was indicated by ◯.

From the table, when the heating coil width / thickness ratio W / t is 5t or less and the deformation length / thickness ratio a / t is 5t or less, the depression depth / thickness ratio is less than 10%. It can be seen that it shows a good value. When W / t is constant, a /
It can be seen that the experimental value of d / t also decreases as t decreases. Experiment number 7 has a deformation length of over 5t,
Experiment number 8 has a long pressurizing time of 8 s, and experiment numbers 9 and 1
Since the heating coil width of 0 exceeds 5 t, the depression depth / wall thickness ratio is a large value of 10% or more. Also,
Experiment Nos. 18 and 19 have a deformation length of less than 0.4 t, and therefore show a good value of the depression depth / thickness ratio of less than 1%, but the joint is open in the bending test. . When the theoretical value and the experimental value of the depression depth / thickness ratio d / t are compared, the experimental numbers with a relatively large deviation are 7 to 9, and in the case of performing the bonding method of the present invention, It matches without any practical problems.

The metal pipe used in the joining method of the present invention may be made of carbon steel, stainless steel, Ni alloy or the like, and the material is not limited. The joining material is an alloy having a melting point lower than that of the metal tube, and an appropriate material is used according to the material of the metal tube. For example, an amorphous foil of an alloy obtained by adding a melting point depressing element such as C, Si, P, or B to Ni or iron, or a sheet of powder of the alloy formed with an organic binder is used as a joint surface and a joint surface. It is sandwiched between and used. Alternatively, a layer of the alloy may be formed on the joint surface by a method such as plating or thermal spraying. Further, the joining temperature is lower than the melting point of the metal tube, and can be arbitrarily selected within the range above the melting point of the joining material. For example, when joining a carbon steel pipe, a low alloy steel pipe, and a stainless steel pipe, the proper joining temperature is in the range of about 1100 ° C to 1300 ° C. If the joining temperature is not selected above the melting point of the joining material, a sufficient liquid layer is not formed between the joining surfaces, which causes joining defects.

Further, when it is necessary to prevent the oxidation of the joint, a method of shielding the periphery of the joint with an inert gas, a method of decompressing the inside of the chamber 4 to form a vacuum, or the like is used. Further, the clamp mechanism 21 is fixed and the clamp mechanism 2
Alternatively, the metal tubes 11 and 12 may be pressurized via the two. At this time, the stopper 51 is not required between the clamp mechanism 21 and the heating coil 3, and the stopper 52 is installed only between the clamp mechanism 22 and the heating coil 3 so that the metal tubes 11 and 12 are deformed at the time of pressurization. The heating coil 3 may be configured to move in the pressurizing direction as the joint moves in the pressurizing direction. At this time, the distance between the clamp mechanism 22 and the stopper 52 is equal to the deformation length a in the axial direction.

[0032]

According to the joining method of the present invention, when joining a metal pipe having a wall thickness t using a heater having a length W in the axial direction,
By determining the deformation length a of the metal tube using W and t, the wall thickness t is determined by using the heater of the length of the heater in the axial direction of the metal tube (hereinafter referred to as the width of the heater) W. In the case of joining the metal pipes, the depression depth that does not significantly affect the joint strength and fatigue strength of the joints depending on the material of the metal pipes, the wall thickness / diameter ratio, etc. Sa / wall thickness ratio D
/ T is obtained in advance, and the deformation length a in the axial direction of the metal tube is determined so as to satisfy the formula (W / t) × (a / t) / 2 ≦ D / t. Then, the joint is heated using the heater, and when the joint is heated to the joining temperature, pressure is applied to the metal pipe until the deformation length a is reached, that is, the deformation length of the metal pipe is limited. By restricting the pressure applied to the metal pipe and suppressing the occurrence of buckling, it is possible to suppress the occurrence of a depression in the joint, reduce the depression depth of the depression, and reduce the joint strength and It is possible to prevent a decrease in fatigue strength. Further, since it can be easily judged with the naked eye or a gauge, whether or not the predetermined deformation length a has been achieved, a conventional welding is provided which is provided with a heater having a predetermined width W and which controls welding pressure to perform welding. The apparatus can be used to carry out the joining method of the present invention.

Further, by limiting the width W of the heater and the deformation length a of the metal tube to 5 times or less of the wall pressure t, the width W of the heater and the deformation length a of the metal tube are ideally set. Limits the range in which the temperature of the metal tube rises when the wall pressure is equal to or less than the value of the wall pressure t, and minimizes the range in which the strength of the metal tube is lowered by heating and buckling is easily caused. Also, the deformation length of the metal tube is limited to limit the pressure applied to the metal tube. Therefore, when pressure is applied to the metal pipe, the portion where the strength is reduced can be reinforced by the portion where the strength is maintained, and buckling can be prevented. However, depending on the size, material, joining conditions, etc. of the metal pipe, it may be difficult to carry out or sufficient joining strength may not be obtained. Experimentally, when each is 5 t or less, and particularly when the deformation length a is 0.4 t or more and 2 t or less, it is possible to suppress pipe expansion deformation and obtain practically sufficient joint strength and fatigue strength. Further, by limiting the width of the heater and the pressure applied to the metal tube, it is possible to reduce the plastic deformation of the metal tube, the bonding method of the present invention, the metal tube or metal material having high rigidity and buckling does not occur. Even when it is used for, it is possible to suppress a decrease in bonding strength and fatigue strength.

Further, by limiting the time for pressurizing the metal tube to 5 seconds or less, when excessive time is spent on heating and pressurizing, the range in which the metal tube is heated and its strength is reduced is expanded by heat conduction. In addition, since the strength of the metal tube is further lowered by the pressurization for a long time and the metal tube is easily buckled, the heating and the pressurization are performed in a short time. Since the time required to reach the bonding temperature depends on the temperature rising rate of the device, when the time for maintaining the bonding temperature, that is, the pressurizing time is experimentally 5 seconds, preferably 3 seconds or less, the depression of the bonding portion occurs. Can be efficiently reduced, and the working time can be shortened.

[0035]

[0036]

[Brief description of drawings]

FIG. 1 is a schematic view showing a configuration of a joining device used for carrying out the present invention.

FIG. 2 is a schematic diagram showing a configuration of a conventional joining device.

FIG. 3 is an explanatory diagram of plastic deformation in the vicinity of a joint portion of a metal pipe.

FIG. 4 is an explanatory view of a pipe expanding deformation in the vicinity of a joint portion of a metal pipe.

FIG. 5 is an explanatory diagram of plastic deformation and pipe expansion deformation in the vicinity of a joint portion of a metal pipe.

[Explanation of symbols]

11,12 Metal tube 2 hydraulic system 21,22 Clamp mechanism 3 heating coils 51,52 stopper 6 control unit

Claims (3)

(57) [Claims] 1. A metal pipe having a melting point lower than that of the metal pipe is butted between two metal pipes to be bonded in the axial direction, and the metal pipe is heated to a bonding temperature equal to or higher than the melting point of the metal pipe. In the joining method of heating the joined portion of the metal pipe by using a heater until the temperature reaches a certain level and compressing the joined portion by axially pressing the metal pipe, joining of the joined metal pipes having a wall thickness t If it is allowed to cause a depression with a depression depth D inside the portion, the joining portion is heated using a heater having an axial length W, and then (W / t) A joining method, characterized in that the metal tube is pressed until the deformation length a in the axial direction is determined so as to satisfy the formula of x (a / t) / 2 ≦ D / t. 2. The joining method according to claim 1, wherein the upper limits of the length W and the deformation length a are 5t. 3. The joining method according to claim 1, wherein the time for pressing the metal tube is set to 5 seconds or less.