JPS5924915B2 - How to weld steel pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of welding steel pipes, and more particularly to a method of welding a pipe end seam in the manufacturing process of large-diameter welded steel pipes.

Generally, in a welded steel pipe, the shape and appearance of the welding start and end portions of the seam are significantly inferior to those of the center of the pipe, and internal defects are often accompanied by poor external shape.

Therefore, when welding the seam of this type of steel pipe,
Either cut off the welded end and start end of the pipe after welding, or as shown in FIG. The quality of the steel pipe 1 is ensured by allowing the weld bead 5 to escape onto the tab plate 6 during installation. The tab plate 6 is cut from the tube end 1a after welding of the seam portion 2 is completed. In particular, when the quality requirements for the welded part are strict and the seam welding process is automated, such as in the case of large-diameter welded steel pipes, the latter method of attaching a tab plate is used. However, this method has various drawbacks as described below, which significantly impede efficiency improvement and cost reduction in the manufacture of large diameter steel pipes. In other words, in addition to the original weld bead at the seam, there is a bead left on the edge of the tube end when attaching the tab plate, so for product quality reasons, it is necessary to remove this portion after welding is complete, and in some cases, the tube Cutting near the edges may shorten the product length or result in rejected products.

Moreover, as shown in FIG. 1, at the pipe end 1a of the steel pipe 1,
At both the welding start end and end end, there is a lack of a place where the heat from the arc heat source 3 can be sufficiently conducted, so the welding thermal stress is accumulated at the pipe end, and the welding thermal stress is applied to the weld seam 2 as shown by arrow 4.
It acts to tear apart. In this case, even if the number of electrodes in the heat source 3 is one or two or more, or whether the submerged arc welding uses Frahenux or the gas shielded arc welding uses shielding gas, similar tearing will occur. force is generated. When such tearing force acts on the weld bead 5 of the seam portion 2, the semi-molten portion that has not fully solidified is pulled, resulting in so-called hot cracking. In order to prevent this cracking, the tab plate 6 has a sufficiently large width L in the direction perpendicular to the axis of the weld seam 2, and the rigidity of the tab plate 6 suppresses the tearing force through the tab plate attachment bead 7. There is. In addition, in submerged arc welding, flux flows to the side to prevent the weld bead from becoming defective, and in shield gas welding, to prevent gas flow from being disturbed resulting in an unhealthy bead. Therefore, the tab plate must have a certain width. Since the bead on the tab plate and the bead at seam part 2 are partially fused, the tab plate basically needs to be made of steel with the same composition as the material of the steel pipe itself, but for the reasons mentioned above, the tab plate dimensions are The cost required for the tab board, which cannot be made very small and is essentially a waste board, is not small. Furthermore, as shown in FIG. 1, the tab plate 6 is formed to have approximately the same curvature as the tube 1 since it is welded along the tube end 1a. Therefore, when attaching a tab plate by automatic welding, it is necessary to correct the curvature of the tab plate, that is, the bend of the welding trajectory, depending on the type of steel pipe size, but this is extremely difficult.
For example, in the case of semi-automatic equipment for gas-shielded arc welding, the final operation is left to manual operation by the operator.
The tab plate mounting bead must have the function of suppressing angular deformation of the bead outside the tab plate surface in the direction of the weld line, as indicated by reference numeral 8 in FIG. If we go through the process of welding from the inside of the pipe and finishing seam welding on the outside, if the above-mentioned angular deformation occurs during welding on the inside, if we weld on the outside without correcting this angular deformation after welding on the inside, The bead on the tab plate flows out as the tab plate inclines, causing various welding troubles and at the same time impairing the integrity of the weld bead at the end of the seam tube. Therefore, the tab plate mounting bead should be welded sufficiently firmly from the viewpoint of suppressing the tearing force and angular deformation mentioned above, and the mounting welding increases the cost more than the tab plate material cost, and it is difficult to improve efficiency. is interfering with In order to solve this problem, instead of fixing the tab plate to the tube end by welding in advance, we installed a tab plate near the tube end whose position can be adjusted vertically, horizontally, and in the direction of the weld line. A plate mounting table is installed, and a steel backing plate of the minimum required size is placed on this table so as to be sandwiched from both sides and from above, and the corresponding plate is pressed against the pipe end using an appropriate pressing mechanism. There is a device that presses and welds the material (Utility Application No. 55-101431).

When seam welding is performed using this device, (a) the bead of the seam can be released onto the caul plate at the start and end of the weld, and (b) the caul plate mounting table acts as a receiving tray for flux or shielding gas. Therefore, even if the caul plate itself is minimal enough to pull the bead, there are no welding problems caused by flux or shielding gas, and (c) the pressing operation of the caul plate is easy, allowing highly efficient seam welding. However, on the other hand, there are the following disadvantages. In other words, the angular deformation of the caul plate is suppressed by the clamping mechanism from above, but the restraining force based on this suppression acts on the caul plate, and the caul plate integrated with the steel pipe after welding is removed from the mounting table. Furthermore, since the backing plate exists separately from the steel pipe to be welded and is made of steel, it does not have sufficient thermal conductivity like the tab plate, and the wall of the steel pipe If the welding heat input increases in proportion to the thickness, heat accumulation at the tube ends cannot be eliminated, and the above-mentioned tearing force acts strongly, resulting in problems such as an increase in the frequency of tube end cracks. FIG. 2 is a perspective view showing an example of a possible improvement over the prior art in internal seam welding of a large-diameter steel pipe 1. FIG. steel pipe 1
A copper plate 9 is pressed onto a position corresponding to the seam portion of the tube end 1a by an appropriate pressing means (not shown). As the pressing means in this case, a pressing mechanism as described in the above-mentioned Japanese Utility Model Application No. 101431/1988 can be effectively used.
In order to improve heat conduction from the tube end to the copper plate 9, the copper plate 9 is provided with approximately the same curvature as the tube so as to closely follow the outer shape of the tube end. In addition, in this embodiment, there is a water channel 1 in the copper plate 9.
0 is formed, and cooling water is allowed to flow from the pipe 13. However, if the thickness of the copper plate 9 is sufficient to ensure heat capacity, water cooling is not necessarily necessary. copper plate 9
A groove 11 extending in the longitudinal direction of the tube 1 and having a width and depth sufficient to surround at least the weld bead is formed on the surface of the tube. While pressing such a copper plate 9 against both pipe ends of the steel pipe 1, welding is started from above the copper plate toward the seam part 2 at the welding start end, and past the welding end, the copper plate on the terminal end side is welded. Finish welding at an appropriate position on 9.

Copper has better thermal conductivity than steel, and the copper plate 9 pressed against the tube end absorbs heat near the tube end seam well.
As mentioned in connection with the figure, even if an arc heat source arrives at the tube end, welding heat will not be accumulated in that area, and the tearing force at the tube end seam mentioned above is significantly reduced, thereby improving the quality of the tube. Edge cracking is prevented. In addition, the cooling effect of the copper plate sufficiently acts on the bead portion, and heat uniformity in the bead thickness direction is rapidly achieved, thereby preventing out-of-plane angular deformation of the bead on the copper plate. Flux or shielding gas during welding is received on the copper plate or on the copper plate mounting table, so there are no problems caused by these, and the bead at the seam can also be effectively released onto the copper plate, so the bead shape at the pipe end is It becomes extremely good. However, as shown in Figure 2, the method in which the arc is directly applied to the copper plate is effective for low heat input welding such as TIG and MIG, slag current welding such as electroslag welding, or submerged arc welding with relatively low heat input. However, if the welding heat input becomes large, the copper plate may melt due to arc heat, making it impossible to achieve the intended purpose. In order to prevent this and also solve the various problems mentioned above, the present invention aims to provide a steel pipe welding method that can effectively and easily perform seam welding without using means of welding tab plates. This is the purpose. For this purpose, the welding method according to the present invention involves pressing a metal plate having high thermal conductivity, such as a copper plate, onto the seam portions of both ends of a steel pipe to be welded, and placing a steel backing plate on the metal plate. It is installed in contact with the ends of a steel pipe, and welding starts along the seam from the caul plate at one tube end, and ends on the caul plate at the other tube end.

As the metal plate, in addition to a copper plate, an aluminum plate or the like can be used. Embodiments will be described below with reference to the drawings for implementing the present invention shown in FIGS. 3 and 4.

In the welding method according to the present invention, as shown in FIG. Make sure to project an arc upwards.

The backing plate 12 may have the minimum necessary dimensions to form the bead, and a flat plate is sufficient. However, in this case, if the surface of the caul plate 12 is made to be at the same height as the welding surface of the steel pipe, the bead will become discontinuous at the boundary between the plate and the steel pipe due to the imbalance in the amount of molten metal filled, resulting in poor bead shape. This effect extends not only to the interface but also to the vicinity of the pipe end seam. In order to prevent this, the caul plate 12 must be set slightly lower than the welding surface of the steel pipe, and a caul plate that is thinner than the steel pipe wall thickness is used for welding both the inner and outer surfaces. Of course, the depth of the groove 11 is also determined in relation to the thickness of the backing plate. The temperature rise of the steel backing plate 12 is suppressed to a low level due to the cooling effect of the copper plate 9, and there is no problem of melting and damage of the copper plate due to arc dropout.
FIG. 4 is a diagram comparing the relationship between welding heat input and angular deformation in the case of welding according to the present invention and the case of welding by conventional tab plate pressing. The △ mark indicates the case where only the copper plate is used in the embodiment shown in FIG. 2, the white circle mark indicates the case where a patch plate is embedded in the groove of the copper plate as shown in Fig. 3, and the black circle mark indicates the case where the conventional tab plate is used. As is clear from this figure, in the case of the present invention, almost no out-of-plane angular deformation occurs, whereas in the case of conventional tab plate pressing, the amount of angular deformation increases with welding heat input. Angular deformation of 3 to 60 appears for welding heat input of 30 to 110 kilojoules/CTL. In view of the gist of the present invention described above, it is more effective to water-cool the metal plate. More specifically, the effects of the present invention other than those described above will be described below.
This will be explained using tack welding as an example.

After zero forming, the steel pipe generally has an open seam due to spring back.

Therefore, after O-forming, the steel pipe is closed at the seam using cage rolls, and CO2 tack welding is performed at the seam. In this process, because the seams are open, it is impossible to attach the tab plates in a static state before tacking, and it is necessary to weld and remove the tab plates while the cage roll-roasted steel pipe is being transported. , this is an extremely difficult task. According to the present invention, attachment and detachment in such a dynamic state can be carried out very effectively. In the case of conventional seam tack welding, it is necessary to stop welding at the seam tube end, and a defective bead at that point has an adverse effect on later internal and external seam welding. Furthermore, since seam CO2 welding is performed using a DC power source, magnetic blowing peculiar to a DC arc occurs at the tube end, causing bead disturbance.

This is a phenomenon similar to heat accumulation at tube end cracks, and is caused by a rapid increase in current density at the tube end. A copper plate with good conductivity is effective in sufficiently lowering the current density at the end of the tube, and has the additional effect that magnetic blowing does not occur at all. It is clear that the present invention includes both cases where it is applied to only one end of a steel pipe to be welded, and cases where it is applied to both pipe ends, the welding start end and the welding end.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a tube end welded state by attaching a conventional tab plate, FIG. 2 is a perspective view showing one embodiment of the present invention, and FIG. 3 is a perspective view showing another embodiment of the present invention. FIG. 4 is a diagram comparing the relationship between the welding heat input and the angular deformation of the pull-out bead between the present invention and the conventional method. 1... Steel pipe, 1a... Pipe end, 2...
... seam part, 5 ... bead, 9 ...
...Copper plate, 10 ... Waterway, 11 ... Groove, 12 ... Backing plate.

Claims (1)

[Scope of Claims] 1. A metal plate with high thermal conductivity is pressed against the seam portion of the end of the steel pipe to be welded, a steel patch plate is installed on the metal plate in contact with the end of the steel pipe, and the A method of welding steel pipes, characterized in that welding starts and/or ends on a caul plate. 2. The method for welding steel pipes according to claim 1, wherein welding is started or completed while the metal plate is water-cooled. 3. Welding of steel pipes according to claim 1 or 2, characterized in that the welding is started or completed by positioning the patch plate on the metal plate so as to be slightly lower than the surface of the steel pipe welding part. Method.