JP2000190076A - Tig welding method for pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily weld a stainless steel pipe at a degree of 250A nominal diameter with a single layer welding pass, and obtain a good welding quality while a preliminary processing or a welding operation against a pipe is simple. SOLUTION: Welding end faces of a pipe 1 are butted and temporarily tacked, and a torch 2 is opposed to an I-shaped groove 1A or a Y-shaped groove 1B formed on the welding end faces at an approximately right angle. Then, arc is discharged while a shield gas G is emitted from the torch 2, so as to weld the groove across the entire periphery of the pipe 1. The mix gas composed of argon and hydrogen, or of argon, helium and hydrogen is used as the shield gas G. Then, concentration of hydrogen in the mix gas is adjusted to be 7-20 vol.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a TIG welding method for pipes, and more particularly to a method for pre-processing and welding a pipe which is simple and capable of welding a stainless steel pipe having a nominal diameter of about 250 A with a single-layer welding pass. Further, the present invention relates to a TIG welding method for pipes capable of obtaining good welding quality.

[0002]

2. Description of the Related Art For example, in piping construction of a chemical plant or the like, a nominal diameter is 15A to 250A and a wall thickness is 2.1 to 250A.
An austenitic stainless steel pipe of about 4.0 mm is often used. As such a pipe welding method, TIG welding is generally performed in which a large V-shaped groove is formed on the outer peripheral side of the weld end face of the pipe. And this TI
In G welding, 100 is usually used as a shielding gas.
Vol% of argon is used.

In the TIG welding, when the thickness of the pipe is, for example, 3.4 mm, the inner circumference of the pipe is usually provided with
A root gap of about 2 to 3.5 mm is provided, and a 60 ° V-shaped groove is provided on the outer peripheral side. And this V
At the groove of the mold, at least two layers of welding paths are formed over the entire circumference while melting the welding rod (welding rod). that time,
Sufficient care must be taken to ensure that a good reverse ridge is formed on the inner peripheral side of the pipe and that a good surface bead is formed on the outer peripheral side. Therefore, in the conventional TIG welding, there is a problem that the operation requires skill and the operation efficiency is extremely low.

[0004] On the other hand, in automatic TIG welding, the amount of weld metal that is melted is increased by changing the shielding gas used from 100 Vol% argon to a mixed gas. That is, in the automatic TIG welding, a mixed gas of 95 vol% of argon (Ar) and 5 vol% of hydrogen (H2) is used as a shielding gas, and the working efficiency is improved by a single-layer welding pass.

[0005]

By the way, the conventional TI
In G welding, a large V-shaped groove must be machined and a welding rod must be used, so that efficient work cannot be expected. On the other hand, in automatic TIG welding, it is a fact that only a pipe having a thickness of about 3 mm can be welded by a single-layer welding pass.

The present invention has been made in view of the above circumstances, and has as its object to simplify the pre-processing and welding work on a pipe and to weld a stainless steel pipe up to a nominal diameter of about 250 A with a single-layer welding pass. It is another object of the present invention to provide a TIG welding method for pipes, which is capable of obtaining good welding quality.

[0007]

In order to achieve the above-mentioned object, a TIG welding method for a pipe according to the present invention is characterized in that the welded end faces of the pipes are butted and temporarily fixed to each other to form an I formed on the welded end face.
A TIG welding method for welding the groove over the entire circumference of the pipe by causing the torch to face the mold groove or the Y-shaped groove substantially perpendicularly and performing arc discharge while injecting a shielding gas from the torch. And a gas mixture of argon and hydrogen as the shielding gas, or
A mixed gas composed of argon, helium, and hydrogen is used, and the concentration of hydrogen in the mixed gas is 7 to 20 V
ol%.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a TIG welding method for a pipe according to the present invention will be described with reference to the drawings. FIG. 1 is an explanatory view of a pipe welding operation to which the pipe TIG welding method of the present invention is applied, and is a front view showing a welded portion of the pipe as a cross section. FIG. 2 is a photograph as a substitute for a drawing, showing the situation of the coarsening of the tissue particles at the boundary between the bead portion and the base material in Example 1 and Comparative Example 1.

In the TIG welding method for pipes according to the present invention, as shown in FIG. 1, the welded end faces of the pipes (1), (1) are butted and temporarily fixed, and an I-shaped groove formed on the welded end face is formed. By causing the torch (2) to face the (1A) or the Y-shaped groove (1B) substantially perpendicularly, and performing arc discharge (A) while injecting the shielding gas (G) from the torch (2), the pipe ( The I-shaped groove (1A) or the Y-shaped groove (1B) is welded over the entire circumference of 1) and (1).

An example of the pipe (1) is an austenitic stainless steel pipe used as a standard in pipe construction. The nominal diameter of the stainless steel pipe suitable for the present invention is about 15A to 250A, and the wall thickness is 2.1 to 4.0.
It is about 0 mm.

In the case of a stainless steel pipe having a wall thickness of 4 mm or less, the welded end faces of the pipes (1) and (1) are butt-joined while keeping the cut face, and the welded end faces are temporarily fixed by spot welding, as shown in FIG. As shown in a), pipes (1),
An I-shaped groove (1A) is formed on the welded end face of (1). On the other hand, in a stainless steel pipe having a wall thickness exceeding 4 mm,
Each of the pipes (1) and (1) has 45
By performing chamfering of about ° and butt-welding the end faces of the pipes and performing spot welding, as shown in FIG. 1B, the Y-shaped groove (1B) is formed on the welded end faces of the pipes (1) and (1). ) Is formed.

The torch (2) has a configuration similar to that of a torch used for general TIG welding, and includes a tungsten electrode (2A) for generating an arc discharge (A) between itself and a pipe (1) as a base material. ) And a gas nozzle (2B) for ejecting a shielding gas (G) around the arc discharge (A).

The welding operation using the torch (2) is performed without using a welding rod (welding rod), unlike a general TIG welding method. At that time, the torch (2) is supported substantially perpendicular to the pipe (1), and the tip of the tungsten electrode (2A) is directed to the I-shaped groove (1A) or the Y-shaped groove (1B).

In the TIG welding method for a pipe according to the present invention, a mixed gas composed of argon and hydrogen or a mixed gas composed of argon, helium and hydrogen is used as the shielding gas (G). The concentration of hydrogen in the mixed gas is adjusted to 7 to 20% by volume. That is, as the shielding gas (G), for example, hydrogen is 7 to 20 Vol% and argon is 93 to 80 Vol%.
1% of two kinds of mixed gas or hydrogen is 7-20V
% of helium, 5 to 20% by volume of helium, and 8% of argon.
Three mixed gases of 8 to 60% by volume are used.

The hydrogen concentration in the shielding gas (G) is:
This greatly affects the thermal efficiency of arc discharge (A) in TIG welding of a pipe, and when the hydrogen concentration increases, the current value required to obtain the ideal penetration amount of the deposited metal decreases.
For example, the shielding gas (G) is 100 vol% of argon.
, The necessary pulse current value is about 170 A and the base current value is about 70 A. However, if the shielding gas (G) is a mixed gas of 85 Vol% argon and 15 Vol% hydrogen, the necessary pulse current value is 40 A. The base current value drops to about 30A. In other words, when the hydrogen concentration in the shielding gas (G) increases, the thermal efficiency of the arc discharge (A) increases even at the same current value, and the penetration amount of the deposited metal increases. Therefore, the shielding gas (G)
Is preferably 7% by volume or more.

On the other hand, the hydrogen concentration in the shielding gas (G) affects the sealing performance of the arc discharge (A) in TIG welding of a pipe. If the hydrogen concentration is too high, the specific gravity of the shielding gas (G) becomes light. Therefore, it diffuses upward, and the sealing performance of the arc discharge (A) decreases. If the hydrogen concentration is too high, the shielding gas (G) burns near the side surface of the arc discharge (A), which adversely affects the welding operation. Therefore,
The hydrogen concentration in the shielding gas (G) is 20 Vol%
The following is preferred.

In the TIG welding method for pipes according to the present invention, it is not necessary to machine a large V-shaped groove on the welded end face of the pipes (1), (1). Alternatively, the I-shaped groove (1A) may be formed by simply butting the welded end faces of the pipes (1) and (1). Therefore, the pre-processing for the pipes (1) and (1) is simple.

In welding the pipes (1) and (1) with the I-shaped groove (1A) or the Y-shaped groove (1B), no welding rod (welding rod) is used. The work itself is simple. Therefore, without special skill,
The welding operation can be easily and efficiently performed in a short time.

In the TIG welding method for pipes according to the present invention, the shielding gas (G) is 7 to 20 Vo.
Since a mixed gas composed of 1% of hydrogen is used, the thermal efficiency of the arc discharge (A) is increased, and the amount of the deposited metal to be melted is increased. As a result, even with a small Y-shaped groove (1B) or an I-shaped groove (1A), a stainless steel pipe having a nominal diameter of about 250A can be easily welded with a single-layer welding pass.

Further, since the total heat input to the pipes (1) and (1) can be reduced by the single-layer welding pass, coarsening of the structure of the welded portion can be suppressed and the residual stress of the welded portion can be reduced. . Further, the bead width of the welded portion can be reduced. Moreover, in the pipe TIG welding method of the present invention, the shielding gas (G) has a hydrogen concentration of 20 V
% or less, the welding work can be performed in a good state without problems of lowering the sealing performance of the arc discharge (A) and burning of the shielding gas (G).

[0021]

[Example 1] By the TIG welding method for a pipe according to the present invention, the pipe (1),
The I-shaped groove (1A) of (1) was welded over the entire circumference by a single-layer pass. Nominal diameter 125A for pipe (1)
An austenitic stainless steel pipe (thickness: 3.4 mm) was used. Hydrogen 10V as shielding gas (G)
ol%, 90 vol% of argon mixed gas was used. The base current of the torch (2) was set at 35A, and the pulse current was set at 70A. Then, the cross-sectional width of the bead portion was measured, and the appearance of the bead portion was observed. In addition, a cross-sectional enlarged photograph of the boundary between the bead portion and the base material was taken to observe the state of coarsening of the tissue particles.

As a result, the maximum width of the bead portion was 3 mm, and it was confirmed that the width was sufficiently small. In addition, it was confirmed that the front bead portion had a good appearance with little swelling, and the appearance of the rim portion was also good. Further, as shown in FIG. 2A, the size of the tissue particles at the boundary is substantially the same as the size of the tissue particles at the base material portion, and it is clear that the coarsening of the tissue particles hardly occurs. found.

[Comparative Example 1] The same pipe as in Example 1 was prepared, and the pipe V was welded by a conventional pipe TIG welding method.
The mold groove was welded over the entire circumference by a two-layer pass. The V-shaped groove was set at 60 °, and a root gap of about 3 mm was formed. As a shielding gas, 100 Vol% argon was used. The base current of the torch was set at 70A, and the pulse current was set at 170A. Then, as in Example 1, the cross-sectional width of the bead portion was measured, and the appearance of the bead portion was observed. Further, the state of coarsening of the tissue particles at the boundary between the bead portion and the base material was observed.

As a result, the width of the bead portion was a maximum of 6 mm, and the front bead portion had an appearance with a large rise. Furthermore, as shown in FIG. 2B, the size of the tissue particles at the boundary was found to be larger than that of the base material.

[0025]

As described above, according to the pipe TIG welding method of the present invention, it is not necessary to machine a large V-shaped groove on the welded end face of the pipe, and it is possible to machine a small Y-shaped groove or a pipe. Can be welded by an I-shaped groove which is merely abutted with the welded end faces, and the pre-processing on the pipe is simple. In addition, the welding operation is simple because no welding rod (welding rod) is used in the welding operation. Therefore, the welding operation can be performed easily and efficiently in a short time without special skill.

In particular, according to the pipe TIG welding method of the present invention, since a mixed gas composed of 7 to 20% by volume of hydrogen is used as a shielding gas, the thermal efficiency of arc discharge is increased, and the amount of the deposited metal to be deposited is reduced. Can be increased. As a result, even with a small Y-shaped groove or an I-shaped groove, a stainless steel pipe having a nominal diameter of about 250 A can be easily welded with a single-layer welding pass. Further, since the total amount of heat input to the pipe can be reduced, coarsening of the structure of the welded portion can be suppressed, the residual stress in the welded portion can be reduced, and the bead width of the welded portion can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a pipe welding operation to which a pipe TIG welding method according to the present invention is applied, and is a front view showing a welded portion of the pipe as a cross section.

FIG. 2 is a photograph substituted for a drawing, showing a state of coarsening of tissue particles at a boundary portion between a bead portion and a base material in Example 1 and Comparative Example 1.

[Explanation of symbols]

 1: Pipe 1A: I-shaped groove 1B: Y-shaped groove 2: Torch 2A: Tungsten electrode 2B: Gas nozzle A: Arc discharge G: Shield gas

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4E001 AA03 BB06 BB07 CA03 CC03 DA01 DD02 DD03 DD06 DF03 DF05 DF06 EA08 4E081 AA14 BA03 BA19 BA27 BB15 CA07 CA11 DA02 DA05 DA11 DA18 DA39 FA01

Claims (1)

[Claims] Claims: 1. A pipe is welded and end-fixed temporarily by welding.
The torch is substantially perpendicularly opposed to the I-shaped groove or the Y-shaped groove formed on the welded end face, and arc discharge is performed while injecting a shielding gas from the torch, thereby forming the groove over the entire circumference of the pipe. A TIG welding method for welding
A mixed gas composed of argon and hydrogen or a mixed gas composed of argon, helium and hydrogen is used as the shielding gas, and the concentration of hydrogen in the mixed gas is adjusted to 7 to 20% by volume. TIG welding method for pipes.