JPS6411394B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an arc welding method for continuously welding while automatically feeding a welding wire, and particularly to a method for preventing high-temperature cracking at the start and end portions of a welded material.

Arc welding, in which a welding wire is automatically fed and moved along a welding line while continuously maintaining an arc, ie, generally referred to as automatic welding, is widely used. Typical examples are submerged arc welding, carbon dioxide shielded arc welding, and MIG.
Arc welding method, etc.

All of these welding methods are extremely highly efficient and high quality welding methods. However, due to its high efficiency, the weld metal is large, and strain is added during its composition and solidification process, resulting in cracks in particular localized areas, such as end cracks during single-layer welding on one side, or pipe end cracks in straight seam welded steel pipes. High temperature cracks may occur.

The present invention aims to prevent high-temperature cracking at the beginning and end of these welded materials and to obtain a stable, high-quality product over the entire length in the welding direction, and provides a method thereof.

As shown in FIG. 1, the strain phenomenon associated with welding is different between the welding start and end and the welding center, and is usually largest at the welding end, followed by the start and center, and smaller at the center.

In Figure 1, A is size 30″OD x 0.25″WT
pipe, first electrode 1000A×33V, second electrode
730A x 37V, 3rd electrode 600A x 39V, 3-electrode submerged arc welding, welding speed 2.1m/
It is min. B is a pipe of size 30″OD x 0.72″WT, first electrode 1300A x 35V, second electrode 930A x
Three-electrode submerged arc welding was performed at 37V and the third electrode was 800A x 39V, and the welding speed was 1.4m/min.

Furthermore, according to the research results of the present inventors, the tensile state that occurs during the solidification process of weld metal, such as the strain level, strain rate, and change in strain over time, largely depends on the welding speed if other conditions are constant. I discovered that.

Figure 2 is an explanatory diagram of these, and the size
A pipe of 30″OD
×33V, 2nd electrode 500A×37V, 3rd electrode 400A×
Welding speed V for 39V 3-electrode submerged arc welding
= 1.2m/min. As you can see from this figure, by lowering the welding speed,
Both the strain level and strain rate are reduced, and if the speed is halved, it is almost halved.

Therefore, the technical idea of the present invention is to locally reduce the welding strain only at the beginning and end where the thermal strain during welding is greatest, thereby obtaining a healthier weld metal while minimizing the drop in efficiency. .

The contents will be explained in detail below using illustrated examples.
Fig. 3 shows an example in which the arc welding method of the present invention is applied to a workpiece having a total length of 12 m, and Fig. 3a shows the moving speed of the arc (hereinafter referred to as the welding speed). In this case, the steady state speed is 2.0 m/min, and the initial speed from the starting point S is 1.5 m/min. As for the initial speed near the starting point S, it is preferable to have a lower one in the local area, but the slower it is, the greater the acceleration will be when returning to a steady state, which is a problem since the commercial value will be inversely reduced in terms of bead appearance. It is.

Therefore, the present inventors found from many experimental results that by slowing down the initial speed to 50% of the steady state, the thermal strain was significantly reduced and the bead appearance was acceptable compared to that in the steady state. It was confirmed that Practically speaking, it is recommended to choose a value that balances both bead appearance problems and thermal distortion reduction within a range of 50% or more of the steady state, or to focus on either of them. The value may be selected, preferably 50 to 80% of the steady state.

After the initial speed is set as described above and welding is started, the welding speed is gradually increased over a range of at least 2 m from the welding start point so as to return to the steady state welding speed. In this case, the acceleration is preferably constant.

The reason for limiting the 2m position is that, as shown in Figure 1, the thermal strain is large in the range from the end to around 2m, and of course the speed gradually increasing area extends beyond 2m to the middle part. I don't mind, but
There is no point in doing so as it would be disadvantageous in terms of efficiency.

The above conditions are for the welding speed on the welding start side, but in parallel with this, the welding current also needs to be changed to follow this. The conditions and reasons for limitation are the same as those for welding speed. That is, the reason why it is necessary to change the current in accordance with the change in welding speed is due to the following two reasons.

(a) If the welding speed is varied while keeping the current constant, the amount of wire melting per unit length of welding changes, making it difficult to form an appropriate bead. In other words, when the welding speed is increased, the current becomes insufficient, and when the welding speed is decreased, the current becomes excessive, and the former causes welding defects such as undercuts and the latter causes welding defects such as overlap. Therefore, it is necessary to control the ratio of current and speed to an appropriate value.

(b) Similarly, if the welding speed is varied while keeping the current constant, the penetration depth changes, making it difficult to form a joint. In other words, increasing the welding speed will result in insufficient penetration, and decreasing the welding speed will result in excessive welding, which may lead to burn-through in some cases. For the above reasons, it is necessary to change the welding current in accordance with the change in welding speed.

Further, regarding arc voltage, for example, in the case of submerged arc welding, it changes depending on the flux, etc., and in the case of gas shield welding, it changes depending on the type of gas. An appropriate voltage is selected to maintain a good bead shape, but it is necessary to find the appropriate value experimentally for each individual.

That is, it is necessary to increase or decrease the arc voltage in accordance with the increase or decrease in the current value.

As shown in Figure 3b, the steady state current is
1050A, and the initial current is 800A. In this case, the speed acceleration is balanced,
The current is gradually increased from the lowest possible value to the steady state in a range of at least 2 m from the welding start point in a range of 50% or more of the steady state current. In this case too, the initial current value is preferably 50 to 80
% range is preferable.

As described above, after the welding speed and welding current are gradually increased to reach the steady-state state, high-speed welding in the steady state is continued.

Furthermore, in a range of at least 2 m from the terminal end F, the welding speed and welding current are gradually reduced, contrary to the starting end S side. The welding speed near the terminal end F and the welding current are both selected to be as low as possible within a range of 50% or more of the steady state. In this case as well, it is preferable that the range is from 50 to 80%, and that the gradually decreasing acceleration is also preferably constant.

Examples of the present invention for welding large-diameter steel pipes will be described below.

Material: API 5LX/X-52; C: 0.11%, Mn: 1.30%, Nb: 0.02% or less Welding material: Wire Y-D; C: 0.12%, Mn: 1.93% or less Flux: NF-820 SiO ₂ :35%,
MnO: 40% Pipe _size : 30"OD Internal welding was performed.

Welding method, 2-wire tandem welding, Figure 3 a-b
The experiment was carried out under the conditions shown in .

As a result, as shown in Fig. 3c, a stable condition was obtained throughout the entire length of the welded object of 12 m, and a good bead appearance was obtained that did not lose its commercial value.

[Brief explanation of drawings]

FIG. 1 shows the distribution of thermal strain state in the length direction of the welded part obtained by welding by the conventional method. FIG. 2 shows the thermal strain state that changes depending on the difference in welding speed.
FIG. 3 shows the following situation in the embodiment of the present invention. a is a state of change in welding speed, b is a state of change in welding current, and c is a state of thermal strain distribution in the length direction of the welded object obtained as a result.

Claims (1)

[Claims] 1. In an arc welding method in which welding is performed continuously while automatically feeding a welding wire, arc movement speed and welding current can be maintained at 50% or more of the steady state for a range of at least 2 m from the welding start point. The arc is gradually increased from a low value to a steady state value, followed by steady high speed welding in the middle zone of the weld, and then arc An arc welding method characterized by changing the moving speed and welding current so that they gradually decrease to the lowest possible value within a range of 50% or more of the steady state.