JPS62252682A - Welding method for two phase stainless steel pipe - Google Patents

Abstract

PURPOSE:To achieve a high efficiency in pipe making welding and the higher stiffness of the welding metal by executing a submerged welding from the outer face, inner face or both faces of a pipe after performing a plasma welding of the seam part of a U-formed, O-formed two phase stainless steel pipe stock. CONSTITUTION:A two phase stainless steel pipe is made with a UOE method. In case of the thin thickness pipe stock subjected to a U-formed, then O-formed, a welding metal 2 is formed so as to cover a welding metal 1 by executing a submerged welding from the outer face of the pipe, after forming the welding metal 1 with the plasma welding of the Y groove of the seam part. In case of a thick pipe stock, a welding metal 4 is formed by executing the submerged welding respectively from the outer face and inner face of the pipe after forming a welding metal 3 with the plasma welding of the X groove of the seam part from the outer face. In this way the higher efficiency in the pipe making welding of the two phase stainless steel and the higher stiffness of the welding metal can simultaneously be achieved.

Description

[Detailed description of the invention] (Industrial application field) As a manufacturing method for large-diameter steel pipes, a steel plate is first formed into a U shape, and then
There is a UOE method in which a butt seam is formed at the end of a steel plate, the seam is welded, and the pipe is expanded.

The present invention relates to a seam welding method for pipe-making duplex stainless steel having a mixed structure of austenite and ferrite using the UOE method.

(Conventional technology) In the UOE pipe manufacturing method, seam welding is performed after U press and O press, but in order to match the production speed of the pipe manufacturing line, it is necessary to increase the line speed for seam welding. be. In order to improve the welding speed, it is required to increase the welding heat input, and the thicker the pipe, the greater the heat input is required. Submerged arc welding (SAW) is well known as a welding method that allows this large heat input to occur.

On the other hand, Cr stainless steel is called ferritic stainless steel because of its Mi weave, and Cr-Ni stainless steel is called austenitic stainless steel.

Ferritic stainless steel is inexpensive, has excellent corrosion resistance and weldability, but has low strength and toughness.On the other hand, austenitic stainless steel has excellent corrosion resistance, good toughness, and high strength, but is susceptible to heat effects during welding. When exposed to heat, chromium carbides are precipitated and so-called intergranular corrosion occurs, which tends to cause stress corrosion cracking and fracture under the residual tensile stress caused by welding. Duplex stainless steel is a dual phase metal of ferrite and austenite. It exhibits a structure of Cr, Nt, MO,
By controlling the component range of , N, the advantages of both the ferritic stainless steel and the austenitic stainless steel are enhanced, and the drawbacks are eliminated. Therefore, its characteristics include excellent corrosion resistance, toughness, and strength, as well as stress corrosion cracking resistance and pitting corrosion resistance even in reducing environments such as chlorine ions, making it suitable for use in the chemical industry, oil industry, or seawater. It has been widely used in heat exchangers etc. in recent years.

(Problems to be Solved by the Invention) However, when pipe-making this duplex stainless steel by the UOE method, there are the following problems in seam welding.

■ When hot cracking duplex stainless steel is welded by SAW, which is a conventional welding method, there is a strong tendency for hot cracking to occur in the welded area (which impairs weldability. This figure shows the relationship between the amount of ferrite in the weld metal and the weld metal crack length in the Palestrain test for evaluating hot cracking susceptibility when SAW welding is performed on materials with varying amounts of Ni contained. As is clear from this figure, the degree of occurrence of hot cracking is largely dependent on the amount of ferrite, and ensuring that the weld metal cracking length is 5fi or less (below line A-A' in the figure), which is acceptable for seam welding in the UOE method. In order to achieve this, the ferrite content must be in the range of 9 to 25%. However, since the ferrite content in duplex stainless steel is 40 to 60%, there is a problem that the hot cracking susceptibility of duplex stainless steel is very large. was there.

Furthermore, in welding by the SAW method, O gas, N gas, etc. in the atmosphere penetrates into the weld metal during welding and forms oxides and nitrides, so there is a problem that the toughness level is low.

■ Welding heat input limit Figure 4 shows the S for 22Cr-5,5Ni duplex stainless steel.
This figure shows the relationship between the welding heat input when performing AW welding and the weld metal crack length in a Vallethrene test.

To ensure the seam welding speed required when manufacturing duplex stainless steel pipes using the UOE method, 20 KJ/a is required.
Although a welder's thermal power of more than m is required, it can be seen from FIG. 4 that with such a large heat input, the degree of hot cracking increases rapidly.

Also, this tendency becomes more pronounced as the wall thickness of the steel pipe increases.For the above reasons, when using SAW welding as seam welding in the manufacture of duplex stainless steel pipes using the UOE method, it is important to reduce the hot cracking susceptibility to a low value. In order to suppress this, it is necessary to lower the welding heat input. As a result, the welding speed becomes slow and the wall thickness of the steel pipe that can be welded is limited.

(Means for Solving the Problems) The present invention provides a welding method for solving these problems. , is characterized in that plasma welding is first performed, and then submerged arc welding is performed from either the outer surface of the tube or the inner surface of the tube, or both.

Here, the composition of the duplex stainless steel is Cr: 20-3
0%, Ni: 4-8%, MO! It is 1 to 5%.

Further, plasma arc welding may be performed from either the outer surface of the tube or the inner surface of the tube.

(Function) Plasma arc welding (hereinafter referred to as PAW) ejects sealing gas from a specially shaped nozzle, converges and narrows the electric arc using the cooling effect of the inert shielding gas, and achieves high current density. This is a welding method that generates energy heat, and allows welding to proceed while forming a keyhole that penetrates the inner and outer surfaces of the seam of the steel plate. It has the following advantages:

■ Due to rapid heating and cooling, the solidified structure of the weld is fine, resulting in high strength and toughness levels, and good hot cracking resistance.

■ Since the welding area can be completely isolated from the atmosphere by the shielding gas, there is almost no chance of O, N, etc. entering the welding area.

Therefore, when the PAW method is applied to the production of duplex stainless steel pipes using the UOE method, it is possible to obtain good hot cracking resistance and high toughness, and furthermore, on the beat of the PAW method, dwarf heat By performing SAW welding, surface weld metal can be obtained with high efficiency, the welding speed required for UOE press pipe making, which is impossible with conventional 5A Wi welding, can be secured, and the wall thickness range that allows stable welding can be achieved. It will expand the

In addition, as an example of using the PAW method for seam welding of raw pipes using the UOE method for high-alloy steel such as duplex stainless steel, JP-A No. 6
There is a method described in Japanese Patent No. 0-154875. This method involves performing TIG welding on the inner surface of the raw pipe, and plasma welding and TAG or MIG welding on the outer surface, but this method uses TIG welding with low heat input from the inner surface, resulting in lower efficiency and poor productivity. . Furthermore, since TIG or MIG is performed together with PAW, the advantages of efficient operation of the PAW method described above are not fully utilized.

(Example) For thin-walled pipes, first check the Y of the seam as shown in
PAW weld the groove to create the weld metal in (1), and then SAW weld it from the outside of the tube to cover it (
It is preferable to make the weld metal of 2) from the viewpoint of welding heat input and welding stability. At this time, since the PAW method can stabilize welding with a relatively low heat input, SAW
Thin-walled tubes can be produced with higher tenacity of the weld metal than with other methods.

For thick-walled pipes, as shown in Figure 2, PAW welding is first performed on the
It is preferable from the viewpoint of welding heat input and welding stability to form a welded metal of 41 by welding.

At this time, the welding heat input can be dispersed between the PAW part and the SAW part, and at the same time, the PAW with low hot cracking susceptibility can be effectively utilized. It becomes possible to provide highly tough welded metal even to a pipe the size of a crane.

Figures 5(a) and 5(b) schematically show an apparatus suitable for carrying out the method of the present invention. (a) The device shown in the figure performs PAW and SAW simultaneously from the outside surface of the tube.

That is, the raw pipe (7) that has been U-pressed or O-pressed is placed on a roller truck or the like with the seam facing up, and pipe-fed in the direction of the arrow. There is a welding device fixed above the seam, and this device has a seam trough car (8) installed at the tip for aligning the seam, a plasma torch (9) for plasma welding, and then a flux side for submerged arc welding. and a welding head circle that supplies a welding wire aυ to perform submerged arc welding. Then, as the raw pipe is fed, the seam of the raw pipe is PAW-welded from the outside surface, and the welded part is then SAW-welded from the outside surface. f# junction is applied.

(b) The device shown in the figure is the device shown in (a) the PAW from the outside of the tube.
This is a device for performing SAW from the inner surface of the tube after performing SAW.

After welding the raw tube from the outside, turn it half a turn so that the seam is on the lower side, and insert the welding device into the tube so that it is positioned above the seam. The welding device consists of a fluff supply device Q for submerged arc welding fixed from the outside of the raw pipe, and a welding head ω for submerged arc welding that supplies welding wire 111). By feeding the raw pipe in this state, SAW welding is performed on the inner surface.

Duplex stainless steel material DP8 (Chemical composition C: 0
．． 02%, Si: 0.4%, Mn: 1.7%,
Cr: 22.5%, Ni: 6.5%, Mo:
3.3%) raw pipe (outer diameter 20"φ wall thickness 12.'7m)
were made by U-forming and O-forming, and the seams were welded using the method of the present invention and the conventional method for comparison. The conventional method is SA
W, the method of the present invention is PAW-3AW, and the respective conditions are shown in Table 1.

Table 1 *: J/cm In order to examine the toughness performance of the weld metals obtained by both welding construction methods, a notch Charpy impact test was conducted on the cross section of the weld using a V-notch test piece with dimensions 10x10x and depth 2x. The results are shown in Figure 7, in which the horizontal axis shows the test temperature and the vertical axis shows the absorbed energy VE, where ○ marks are for the PAW-3AW method of the present invention, and marks are for the conventional SAW method. These are the test results.

From this result, it is clear that the toughness level evaluated by the VE value of the present invention is superior to the conventional method, and that the difference in the level is especially large at low temperatures.

This is thought to be due to the use of PAW in the method of the present invention, which refines the solidified structure of the weld and reduces the amount of oxides and nitrides. This proves that the toughness level can be improved by combining them.

(Effects of the Invention) As is clear from the above explanation, by performing seam welding using the method of the present invention when manufacturing duplex stainless steel pipes using the UOE method, higher toughness can be achieved compared to when performing seam welding using the conventional SAW method. This increases the welding speed and makes it possible to manufacture thick-walled (~25.4 fi) pipes, which has an extremely large effect on improving quality and increasing productivity.

[Brief explanation of drawings]

Figure 1 is an illustration of the method of the present invention, Figure 2 is an explanatory diagram showing another embodiment, and Figure 3 is the relationship between the amount of ferrite in the weld metal and the length of palistrain weld metal cracking when stainless steel is welded. Figure 4 is a diagram showing duplex stainless steel.
Diagrams showing the relationship between welding heat input and weld metal crack length during AW welding, Figures 5(a) and 5(b) show examples of equipment used to carry out the present invention; 6 is an explanatory diagram showing an apparatus for welding the outer surface of a blank pipe, (b) is an apparatus for welding the inner surface of a blank pipe, FIG. FIG. 7 is a diagram showing the relationship between test temperature and Charpy impact value for welds obtained by the conventional method and the method of the present invention. l, 2.3.4:? 8: Coated metal, 7: Raw pipe, 8: Seam tracker, 9: Plasma torch, 10: Franks supply device, 11: Welding wire, 12: Welding head, 13: Flux. Fig. 5 @ Fig. l Ferrite t (y, ) of Azusa Kinpei Fig. 3 Fig. 2 Fig. #F heat input (Um) Fig. 4

Claims (1)

[Claims] (1) After plasma welding is first performed on the seam of the U-formed and O-formed duplex stainless steel tube,
A method for welding duplex stainless steel pipes, characterized by performing submerged arc welding from either the outer surface of the tube or the inner surface of the tube, or both.