JPH0228424B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention solves an incomplete butt condition with a large misalignment by performing two welds: the first welding using a filler rod and the second welding without using a filler rod. The present invention relates to a method for welding metal tubes that can automatically weld metal tubes together circumferentially.

Conventionally, butt welding is often performed when welding metal tubes together. In particular, when butt welding thin-walled stainless steel pipes, it is necessary to perform complete Uranami welding, which requires a fairly high level of skill.
TIG circumferential automatic welding is being performed. However, T.I.G.
In circumferential automatic welding, welding conditions are set in advance and welding is performed automatically on the entire circumference according to a given program, so the quality of the welded part can be determined by the quality of the butt part, that is, the groove accuracy. A lot will depend on whether it's good or bad. In particular, Fig. 1 shows a longitudinal section of the welded part, and Fig. 2
As shown in the figure, if there is a step due to misalignment between the butts of metal tubes A and B, the metal melted by the arc heat will be moved by the force of the arc to the side far away from the electrode C (lower in Figure 2). The metal tubes A and B, which are base materials, are melted by the heat from the flowing molten metal. As a result, the center of the molten pool is formed away from the abutment D, and the bead E is formed as shown in Figs. Unwelded butt parts generated in separated areas
Da remains, which not only weakens welding strength but also causes crevice corrosion. Conventionally,
TIG to prevent the occurrence of such welding defects.
Automatic circumferential welding requires a high degree of groove precision, and the tolerance for misalignment and root gap is usually within 10 to 20% of the wall thickness. Therefore, the thinner the wall thickness is, the smaller the misalignment must be to keep it within the above-mentioned tolerance range. However, in reality, it is extremely difficult to butt the two metal tubes together within this groove precision, and especially when piping is performed on-site, there may be variations in the outer diameter of the metal tubes, deformation of the ends, and inadequate butting means. Therefore, it is extremely difficult to keep the misalignment within 10 to 20% of the wall thickness of the metal tube.

As a result of research aimed at providing a method for welding metal tubes by circumferential automatic welding that does not have such drawbacks, the inventor found that the purpose was achieved by performing welding twice with different purposes and effects. The present invention was completed by investigating this fact.

That is, the present invention, when butting metal tubes together and performing circumferential automatic welding along the butt line,
First, during the first welding using a filler rod, the level difference due to the misalignment that exists at the butt part is reduced, and then, sufficient heat input is applied without the filler rod to generate a back wave at the butt part. The present invention relates to a method of welding metal tubes, which is characterized by performing a second welding.

The method for welding metal tubes according to the present invention will be explained in detail below with reference to drawings and examples.

Fig. 3 is an explanatory diagram showing the state of welding by the method of the present invention on a step portion due to misalignment, with the welds divided into the first and second welds, and Fig. 4 shows the welds obtained as shown in Fig. 3. FIG.

First, the metal tube 1 and the metal tube 2 are butted against each other, but as shown in FIG. 3, there is a relatively large step 3 due to the misalignment.
(Up to about 90% of the wall thickness of the metal tubes 1 and 2 because a filler rod is used in the first welding) is not a problem when using the method of the present invention. Next, a circumferential automatic welding device is installed so that a torch 6 having an electrode 5 automatically moves around the abutting portion along the abutting line 4 to perform circumferential welding. First, the first circumferential automatic welding is performed. The purpose of this first circumferential automatic welding is to reduce the step 3, and the purpose is not to completely weld, so there is no need to melt all the way to the back side.
Although it is slightly weaker than the arc of conventional circumferential automatic welding, it is possible to use a filler rod to fill in the step 3 at the butt part and make the surface heights of the left and right metal tubes 1 and 2 almost the same. is necessary. In this way, the base material of the metal tube 2 closer to the electrode 5 is melted together with the filler rod and is flowed away from the electrode 5 by the arc force, so that the step on the abutment surface disappears and becomes an almost smooth slope. The first welded portion 7 is formed. Next, a second circumferential automatic welding is performed, but in this case, TIG welding without a filler rod is used to provide sufficient heat input to generate Uranami 8 over the entire circumference of the butt part to perform complete welding. be. However, due to the circumferential automatic welding using the filler rod in the first process, the step 3 near the butt part has been reduced and the surface heights of the left and right metal tubes 1 and 2 are almost the same, so the second process In circumferential automatic welding, the molten metal does not flow non-uniformly to one side on the surface of the butt part, and is melted to the back side while maintaining a substantially even surface shape on the left and right sides, forming the second welded part 9. Therefore, as shown in FIG. 3, the second welded part 9 has a substantially equal shape on the left and right sides around the abutment line 4, and the melting toward the back side proceeds without coming off the abutment line 4, resulting in complete abutment. The welded part is further welded to the back side, and the part eluted to the back side fills the step 3 on the back side and is generated as a good back wave 8. Eventually, a welded part 10 with an integrated cross section as shown in FIG. 4 is formed. In addition, for the first circumferential automatic welding, a filler rod is used to remove the step 3 near the butt part.
Since the surface heights of the left and right metal tubes 1 and 2 are almost the same, no filler rod is used for the second circumferential automatic welding. In this case, it is preferable to perform TIG circumferential automatic welding as the circumferential automatic welding because the second circumferential automatic welding can be performed immediately by simply removing the filler rod without replacing the electrode. Thus, according to the method of the present invention, automatic circumferential welding can be performed extremely well even if there is a large step 3 at the butt portion.

The base materials that can be used in the method of the present invention are all metal pipes that can be automatically welded circumferentially, such as various steel pipes and alloys.
Among these, it is useful for thin-walled metal pipes, especially thin-walled stainless steel pipes. Furthermore, in the method of the present invention, the metal pipe includes a so-called joint pipe.

Example 1 Thin-walled stainless steel pipes with an outer diameter of 300 mmψ and a wall thickness of 3.0 mm were butted together with an I-shaped bevel, a root gap of 0 to 1.3 mm, and a maximum misalignment of 2.0 mm (67% of the wall thickness), and were subjected to TIG as follows. Automatic circumferential welding was performed.

The first time welded with a current of 160 amperes and a speed of 15 cm/min while adding the filler rod, and the second time welded with a current of 160 amperes and a speed of 15 cm/min.
Welding was performed without a filler rod at 185 amps and a speed of 10 cm/min. As a result, it was possible to weld by producing a good back wave as shown in FIG. 4 over the entire circumference of the butt part.

Example 2 Thin-walled stainless steel pipes with an outer diameter of 80 mmψ and a wall thickness of 2.0 mm were butted together with an I-shaped bevel, a root gap of 0 to 0.7 mm, and a maximum misalignment of 1.8 mm (90% of the wall thickness), and the first TIG circumference was measured. Automatic welding current 80 amps, speed 10
After applying the filler rod at a rate of cm/min, the second time was performed without the filler rod at a current of 90 amperes and a speed of 10 cm/min. We were able to weld by generating a strong back wave.

As described in detail above, the method of the present invention does not require the strict bevel precision required for butt welding of metal pipes, reduces the labor required for beveling, and enables automatic circumferential welding at construction sites. It is valuable because it is easy to use and can reduce the cost of on-site work.

[Brief explanation of drawings]

Figure 1 is a longitudinal cross-sectional view of a welded part obtained by conventional TIG circumferential automatic welding, Figure 2 is an enlarged view of part A in Figure 1, and Figure 3 is a step part obtained by the method of the present invention due to misalignment. FIG. 4 is an explanatory view showing the welded portion divided into the first and second welding states, and FIG. 4 is a sectional view of the welded portion obtained as shown in FIG. 3. A...metal tube, B...metal tube, C...electrode, D
...Butt part, Da...Unwelded butt part, E...
...Welded part, Ea...Uranami, 1...Metal tube, 2...
Metal tube, 3... step, 4... butt line, 5... electrode, 6... torch, 7... first welding part, 8...
... Uranami, 9... Second welded part, 10... Combined welded part.

Claims (1)

[Scope of Claims] 1. When a metal tube is butted together and automatic circumferential welding is performed along the butt line, the first welding using a filler rod is performed at the butt portion. A method for welding metal tubes, which comprises reducing steps caused by misalignment, and then performing a second weld while applying sufficient heat input without a filler rod to generate a back wave at the abutting portion. 2. As the first and second circumferential automatic welding,
A method for welding metal tubes according to claim 1, which performs TIG welding.