JPS5948703B2 - Welding method for lap joints - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welding method for lap joints made of thin plate materials, and is particularly suitable for application to Invar materials that are prone to cracking.

In recent years, LNG (liquefied natural gas) has been attracting attention as a new energy source and non-polluting energy alternative to oil, but it is important to store liquefied gases such as LNG, LPG (liquefied propane gas), or liquefied ethylene. case,
Storing at a temperature lower than room temperature allows for lower pressure and improves safety, and it is also possible to reduce the thickness of the storage tank material, making it economical and easy to construct large-capacity storage tanks. becomes.

However, since the storage tank material is always maintained at a low temperature, problems such as low-temperature brittleness occur, and it is necessary to select an appropriate low-temperature material depending on the temperature of the liquefied gas to be stored. Conventionally, aluminum alloys, austenitic stainless steels, 9% Ni steels, etc., have been used as low-temperature materials, but Invar, which is known as a constant steel, has an extremely small coefficient of thermal expansion and excellent corrosion resistance. Therefore, it is considered one of the most suitable materials for low-temperature applications.

However, this Invar is a steel that contains about 35% to 36% nickel, so it is expensive, and there are some problems with weldability, which is one of the reasons that is preventing the spread of Invar storage tanks. . In other words, when designing such a storage tank using Invar, the general method is to use Invar plate thickness of about 0.5 mm and join lap joints and helical joints by TIG (tungsten inert gas arc) welding, plasma welding, etc. However, various types of weld cracks occurred in the weld beads (mainly attached beads) of these lap joints and F-joints. For example, as shown in FIG. 1a and FIG. 1b showing the cross-sectional shape, one of them is as shown in FIG.
This crack 104 occurs in the weld bead 103 that is formed in a concave shape between the weld bead 103 and the weld bead 103. This crack 104 depends on the shape of the weld bead 103, and is caused by insufficient throat thickness to withstand the shrinkage stress peculiar to lap joints. it is conceivable that.

In addition, as shown in FIG. 2a and FIG. 2b showing the cross-sectional shape of the tack bead 204, which protrudes from the weld bead 203 formed between the upper plate 201 and the lower plate 202. The cracks 205 are caused by excessive tacking, and this is thought to be due to the portion of the tacking bead remaining during excessive tacking being overheated, resulting in cracking or high-temperature cracking due to shrinkage stress. Furthermore, when crater treatment is not performed, the first weld bead 303 formed between the upper plate 301 and the lower plate 302 is When connecting the end portion to form the second weld bead 304, a crack 306 occurs in a crater (not shown) at the end portion of the first weld bead 303 located at the seam 305. Similarly, Fig. 4a, Fig. 4b showing its planar shape, and Fig.
The crack 401 shown in FIG.
The first weld bead 4 located at the seam 406 with 05
This occurs at the end of 04, but this is considered to be an embrittlement phenomenon caused by localized heat input. The purpose of the present invention is to prevent cracks that occur when TIG welding or plasma welding is performed on lap joints formed from such thin Invar plates. The idea was to realize a low-temperature storage tank using Invar, which had been considered difficult.

The configuration of the lap joint welding method according to the present invention that achieves this object is such that when welding a lap joint consisting of an upper plate and a lower plate formed of thin plates, the main bead is wider than the width of the tack bead. The flat bead is formed into a convex shape, and the end of this bead is shifted toward the upper plate, while when connecting to the bead and forming a second bead, this bead is The second bead is superimposed on the lower plate side of the end of the first bead, so that the lower plate side of the end of the first bead is completely remelted. Features.

Hereinafter, an embodiment in which the lap joint welding method according to the present invention is applied to a thin Invar plate will be described in detail with reference to the drawings from FIG. 5 onwards.

As shown in FIG. 5a showing the external appearance of a lap joint according to this embodiment in which the final fitting work was performed after the temporary fitting work, and FIG. After temporarily attaching the joints between the upper plate 11 and the lower plate 12 made of thin plates at predetermined intervals, the actual attachment work begins.
Width of the weld bead (hereinafter referred to as the actual weld bead) 3 (length in the vertical direction in Figure 5 a)
is wider than the width of the welding bead 14 due to tack welding (hereinafter referred to as tack bead), and by completely remelting this tack bead 14 with the actual weld bead 13, cracks due to excessive tack welding can be prevented. can be completely prevented.

In addition, by making the shape of the main bead 13 flat or convex, that is, by overlaying the main bead 13, cracks depending on the shape of the main bead 13 can be prevented. The shapes of the book attaching beads 13 described below are basically all flat or convex. As shown in FIG. 6a showing the appearance of the distal end of the main bead 13, and FIGS. The end portion 15 is gradually shifted toward the upper plate 11 side, and the weld line 16 is gently curved toward the upper plate 11 side.

In this case, crater treatment is performed by gradually reducing the amount of heat input to the upper plate 11 and lower plate 12 by TIG welding, plasma welding, etc., so crater cracking can be prevented. As shown in FIG. 7a showing the external appearance of a lap joint with a second real bead formed on the distal end 15 and in FIG. 13 to further form a second bead] 7, this second bead 17 is superimposed on the lower plate 12 side of the end portion 15 of the first bead 13. In other words, the welding line 18 of the second bead 17 is set parallel to the welding line 16'' of the first bead 13, and the welding line 18 of the bead 17 is
The lower plate 12 side of the end portion 15 of No. 3 is completely melted.

This makes it possible to prevent cracks that are likely to occur in the seam 19. Although this method can be applied as is to repair work, the gist of the present invention is that the lower plate of the end portion 15 of the first mounting bead 13 is thinner than the upper plate 11 side and cannot withstand shrinkage stress. Since it is sufficient to re-melt at least the 12 side completely, the entire end portion 15 of the first attachment bead 13 located at the seam 19 may be re-melted. As described above, according to the lap joint welding method of the present invention, for materials that require delicate welding such as thin invar plates, the shape of the main bead can be changed to a flat or convex shape to increase the throat thickness. At the same time, when forming a final bead over a pre-formed bead, the original bead is completely remelted, eliminating the risk of cracking in the welded area and making it easier to weld with invar. A low temperature storage tank can be realized.

[Brief explanation of drawings]

Figures 1 to 4 are external views and cross-sectional views of lap joints showing that cracks occur when lap joints are welded using the conventional method, and Figures 5 to 7 are welding by the present invention. It is an external view of a lap joint showing a method, and its sectional view. In the drawings, 11 is the upper plate, 12 is the lower plate, 13 is the (first) final attachment bead, 14 is the temporary attachment bead, 15 is the end of the actual attachment bead, 17 is the second actual attachment bead, and 19 is the final attachment bead. This is the seam of the main bead.

Claims (1)

[Claims] 1. When welding a lap joint consisting of an upper plate and a lower plate formed of thin plates, a main bead with a width wider than the temporary bead is formed into a flat or convex shape, and the end of this main bead is While shifting it toward the upper plate side, when connecting to the main bead to form a second bead, connect the second bead to the bottom of the end of the first bead. A welding method for a lap joint, characterized in that the lower plate side of the end portion of the first permanent bead is completely remelted by overlapping it on the plate side.