JPH09103883A - Executing method for automatic tig welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable TIG welding to have an improved bead appearance by keeping, within a specific range of values, the weld margin groove depth and welding current of a pre-finish layer, distance between the bead end and the groove end of a finish layer, and wire deposition. SOLUTION: In the automatic TIG welding by which one side welding is performed on a carbon steel plate having a tensile strength of 780N/mm<2> grade or less, a pre-finish layer and a finish layer are formed under such conditions that the weld margin groove depth of the pre-finish layer is 2mm or less on average, that the welding current of the finish layer is 220-350A, that a distance between the bead end and the groove of the finish layer is 2-5mm, and that the wire deposition of the finish layer is 25g/min or less. The pre-finish layer is the one before the last layer in TIG(tungsten inert gas) welding, the finish layer being the last layer. In case of two sided welding, its conditions are the same as those of one side welding. Thus, a weld zone is obtained that is provided with a beautiful bead appearance, superior mechanical characteristics and bend resistant performance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic TIG welding method, and more particularly to an automatic TIG welding method for welding one side or both sides of a welded plate made of carbon steel having a tensile strength of 780 N / mm ² or less.

[0002]

2. Description of the Related Art The welded portion formed by TIG welding not only has a splendid bead appearance as compared with other arc welding such as covered arc welding and submerged arc welding, but also because the amount of oxygen in the weld metal is extremely low. , Mechanical properties, especially toughness. Also, the amount of hydrogen in the weld metal is low, and the weld crack resistance is excellent.

On the other hand, TIG welding has a drawback that the welding efficiency is low as compared with covered arc welding, but an automatic welding device capable of welding with a large welding amount has been developed, or due to a large welding amount. Even in welding, a welding material that can obtain a weld metal having good mechanical properties has been developed, and the welding performance has been improved. Therefore, TIG welding is LP
It is often used for welding important structures such as G tanks, pressure vessels, and penstocks.

[0004]

By the way, in an important welded structure, it is essential to ensure safety.
Various non-destructive inspections or destructive inspections have been performed on test welded joints that have been welded in the same manner as real welded joints or welded structures. A bending test may be adopted as one of the destructive inspections.

When the front bending test and the back bending test are performed on the TIG weld metal which is automatically welded, a large number of minute cracks may occur particularly in the weld metal near the fusion line. However, even if the surface of the test piece in which such microcracks are generated before the bending test is examined by a magnetic particle flaw detection or penetrant flaw detection test or the inside of the test piece by a radiation transmission test or the like, defects equivalent to the microcracks are not found. Often not detected.

Since the TIG weld metal has excellent weld crack resistance as described above and as described above, the microcracks are not the ones which were present in the weld metal and were not revealed by the bending test. It is thought that this is the first crack to occur due to bending deformation. Therefore, this does not immediately mean that there is a problem with the safety of the actual structure, but repair welding is necessary if the total crack length (total crack length) or the total number of cracks in the test exceeds the standard. Therefore, there is a demand for the development of a welding method that does not cause minute cracks in the weld metal even when subjected to severe bending deformation such as a bending test.

The present invention has been made in view of the above problems, and the welded portion has a splendid bead appearance, and excellent bending resistance can be obtained without impairing excellent mechanical properties. It is an object to provide an automatic TIG welding construction method.

[0008]

The automatic TIG welding execution method according to the present invention is an automatic TIG welding execution method in which a carbon steel sheet having a tensile strength of 780 N / mm ² or less is welded on one side. The average groove depth is 2 mm or less, the welding current of the finishing layer is 220 to 350 A, the distance between the bead end and the open tip of the finishing layer is 2 to 5 mm, and the welding amount of the wire of the finishing layer is 25 g / min or less. And forming a pre-finishing layer and a finishing layer.

Here, the pre-finishing layer means a layer immediately before the final layer in TIG welding, and the finishing layer means the final layer in TIG welding.

When the carbon steel sheet is welded on both sides, the front side finishing layer and the finishing layer and the back side finishing layer and the finishing layer of the carbon steel sheet are subjected to the same conditions as the above-mentioned one side welding. And TIG welding automatically.

The carbon steel having a tensile strength of 780 N / mm ² or less means JIS G3101, G3106 and G3106.
Steel plate for general structure such as 3128 or steel plate for welded structure, JI
SG3115, G3118, G3119 and G3126
Steel plates for pressure vessels, etc.

[0012]

The present inventors have developed an automatic TIG welding construction method in which the welded portion has a brilliant bead appearance, and good bending resistance can be obtained without impairing excellent mechanical properties. Various experimental studies were conducted.

First, in order to investigate the cause of the occurrence of microcracks in the welded portion, automatic TIG welding was performed on various steel sheets under various conditions using welding wires each having an appropriate composition. Then, bending test pieces were taken from these welded steel plates and subjected to a bending test.

FIG. 3 is a view showing a part of a bending test piece in which minute cracks have occurred, and FIG. 3 (a) is a top view thereof, and FIG.
(B) is an enlarged view showing an AA cross section. This figure 3
As shown in (a), in the bending test piece after the bending test, microcracks were generated in the weld metal (finishing layer) 1 formed on the upper surface of the weld metal (prefinishing layer) 3. Most of these microcracks occur in the weld metal near the fusion lines 6a and 6b, which are the boundaries between the base metal 2 and the weld metal 1 (microcracks 4), and although a small number occur, they are separated from the fusion lines 6a and 6b. It was also generated at the position where it was formed (fine crack 5). The length of the microcracks is often about 0.2 mm.

As a result of fracture surface analysis of the fracture surface of the microcracks 4 generated near the fusion line by a scanning electron microscope, it was confirmed that these microcracks were ductile fractures. That is, it is considered that these microcracks 4 are formed by the weld metal 1 in the vicinity of the fusion line exceeding the elongation at break due to the bending stress of the test and ductilely opened.

Further, it was recognized that the microcracks 4 tended to occur frequently when a low current was used during welding of the finishing layer and the bead width was widened. In the vicinity of the fusion line of the cross section of the test piece welded under such welding conditions, the penetration (b) shown in FIG.
Was shallow, and the length (a) of the region where the weld metal 1 was formed on the base material 2 in an umbrella shape was long. On the other hand, when a relatively high current is used when welding the finishing layer and the bead width is narrowly welded, the microcracks 4 hardly occur. In this case, the penetration (b) is deep and the length of the umbrella-shaped region is large. (A) was shortened.

Next, the results of measuring the hardness of the weld metal in the plate thickness direction will be described. In FIG. 4, the vertical axis represents Vickers hardness and the horizontal axis represents the distance in the thickness direction of the weld metal, and a low-temperature steel plate (SLA325B) having a plate thickness of 35 mm.
FIG. 4 is a graph showing the hardness distribution in the joint weld metal when double-sided welding is performed using a TIG welding wire for low temperature steel of 4.5% Ni system. As shown in this graph, the hardness of each finishing layer on the front side and the back side of the weld metal is higher than the hardness inside the weld metal. This is because the amount of oxygen in the TIG weld metal is extremely low (1
This is because the hardenability becomes large at about 0 ppm or less). Further, the layer inside the weld metal is tempered by the heat cycle of the subsequent welding to have an appropriate hardness (strength). Such a phenomenon that the finish layer becomes hard is particularly remarkable in the weld metal of the TIG welding material for high-strength steel having a tensile strength of 780 N / mm ² or less.

The weld metal 1 shown in FIG. 3 is a finishing layer,
As can be seen from the graph shown in FIG. 4, this portion is hard and the elongation performance is also reduced. In particular, fusion line 6
When the weld metal 1 in the vicinity of a and 6b is shallow (thin) and long, the difference in elongation performance between the base metal 2 and the weld metal imposes a load on the weld metal, so that micro-cracks 4 are formed in this portion. Is thought to occur. Therefore, in order to prevent minute cracks, it is considered effective to adjust welding conditions and positively control the bead cross-sectional shape.

On the other hand, regarding the microcracks 5 generated at positions separated from the fusion lines 6a and 6b, it was found by fracture surface analysis that the micro blowholes had their mouths widened by bending stress. It was also found that the micro-cracks caused by the blowholes occur when the amount of wire deposition during welding of the finishing layer is large. Therefore, it is considered effective to adjust the welding amount at the time of welding in order to prevent the minute cracks generated at the position separated from the fusion line. The welding method according to the present invention is based on the above measurement and consideration results.

The reasons for limiting the welding conditions in the automatic TIG welding method according to the present invention will be described below.

Depth of groove left after welding in the finishing layer: average 2 mm
Hereinafter, the groove depth of the welding residue in the pre-finishing layer refers to a distance c between the upper end surface of the weld metal after welding the pre-finishing layer 10 and the uppermost end surface of the groove, as shown in FIG. This distance is 2 on average
If it exceeds mm, an appropriate surplus may not be obtained at a welding amount (25 g / min) or less which will be described later. In addition, if the distance exceeds 2 mm on average, the bead appearance shape of the finishing layer also deteriorates. Therefore, the groove depth of the welding residue of the pre-finishing layer is 2 mm or less on average. In addition, the average value of the groove depth after welding of the pre-finishing layer is an average value obtained by dividing the groove width into four equal parts and measuring the depths at three points A, B and C as shown in FIG. is there.

Welding current of finishing layer: 220 to 350 A In general, the welding current of forming the finishing layer is preferably about 200 A, which is easy to obtain a fine bead. However, if the welding current is less than 220 A, the bead appearance is good, but the penetration between the base material and the fusion line becomes shallow, so the number of microcracks generated near the fusion line increases in the bending test. . On the other hand, if the welding current of the finishing layer exceeds 350 A, the molten metal is likely to drip when the welding is performed in the vertical position, and the appearance shape of the bead is extremely deteriorated. Therefore, the welding current of the finishing layer needs to be 220 to 350 A that can achieve both the bead appearance and the bending performance. When the welding current of the finishing layer is 220 A or more, the hardness of the finishing layer is slightly lowered due to an increase in welding heat input, and a secondary effect of improving bending resistance can be obtained.

Distance between bead end and open end of finishing layer: 2
To the distance between the bead end and the open distal end of 5mm finishing layer, as shown in FIG. 6, and the end portion of the bead of the finishing layer 11 covering the groove top surface, groove top surface of the open distal end 9a, 9b Distance from
d ₁ and Δd _{2. When} this distance exceeds 5 mm, a large number of microcracks occur near the fusion line, and when it is less than 2 mm, undercut may occur. Therefore, the distance between the bead end and the open end of the finishing layer is 2 to 5 m.
m.

Amount of wire deposited on the finishing layer: 25 g / min or less The amount of wire deposited on the finishing layer means the amount of wire deposited on the layer used to form the finishing layer. If the amount of welding exceeds 25 g / min, minute cracks due to blowholes are likely to occur in the weld metal portion away from the fusion line. Therefore, the welding amount of the wire of the finishing layer is 25 g / min or less.

In the automatic TIG welding, a wire having a diameter of 0.9 to 1.6 mm is usually used, and the automatic TIG welding method according to the present invention uses the wire within this range for welding. be able to.

Further, in the automatic TIG welding, the proper range of the arc voltage and the welding speed is narrower than that of other weldings, generally the arc voltage is about 9 to 13 V, and the welding speed at the downward welding is about 7 to 15 cm /. Min., Welding speed during vertical welding is about 4 to 7
Welding is performed at a rate of cm / min, and the appropriate ranges are not particularly limited in the present invention, but the effects of the present invention can be achieved within these ranges.

[0027]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples that fall outside the scope of the claims of the present invention.

FIG. 1 is a view showing the groove shape of the test plate used in this embodiment. This test plate has a thickness t: 35 m
m, the front side height t _{1 of} the X groove is 24 mm, the back side height t ₂ of the X groove is 9 mm, and the root height k is 2 mm. The groove angles of the front side and the back side are 50 degrees and 60 degrees, respectively. Is.

Further, as the test plates, those of the steel types shown in Table 1 below were used, and a welding wire suitable for each steel type was selected from the wires shown in Table 2 below, and automatic TIG welding was performed. The welding conditions at this time are as shown in Table 3 below. In addition, 1s shown in the column of side of the test plate in Table 3 below.
t and 2nd mean the front side and the back side of the test plate, respectively.
Further, among the welding conditions shown in Table 3 below, those that deviate from the claims of the present invention are shown by underlining their condition values.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

After performing automatic TIG welding under the welding conditions shown in Table 3 above to determine the quality of the bead appearance, the test pieces shown in FIG. 2 were cut out from each test plate. FIG. 2 is a cross-sectional view showing a sampling portion of a bending test piece in the test plate used in this example. The thickness S ₁ and S ₂ of the front bending test piece and the back bending test piece shown in FIG. 2 were both set to 10 mm.

The bending test was carried out in accordance with JIS Z3122, and the surplus of the test piece was cut to the surface of the base material so that the bending radius was twice the thickness of the test piece.

Table 4 below shows the results of the bead appearance determination, the cracks in the weld metal caused by the bending test, and the results of the comprehensive evaluation of the test pieces. In addition, the bead appearance was evaluated in four grades of excellent, good, slightly poor and bad, and each was evaluated as “◎”,
"○", "△" and "x" are shown in the column of bead appearance. Regarding this bead appearance, excellent and good cases were regarded as passing. Regarding the bending test, the number of cracks is 5 or less,
Total length of each crack is 3mm or less, maximum crack length is 1
Those having a size of mm or less were accepted. The standard of this bending test sufficiently satisfies the required value in the normal bending test. Furthermore, regarding the results of the comprehensive evaluation, those in which the results of the bead appearance and the bending test were acceptable were evaluated as “◯”, and those in which either one was unacceptable were evaluated as “X”, and the results are shown in Table 4 below.
In the pass / fail column.

[0036]

[Table 4]

As shown in Table 4 above, Example No. 1,
Regarding Nos. 5, 7, 10, 11 and 13, the bead appearance was good or better, and the cracks in the weld metal near the fusion line and at positions away from the fusion line reached the acceptance standard sufficiently. It can be seen that also has good bending resistance.

On the other hand, in Comparative Example No. 2, the welding current was smaller than the value in the predetermined range, and the distances Δd ₁ and Δd ₂ between the bead end and the open tip of the finishing layer were larger than the value in the predetermined range. Therefore, although the bead appearance was excellent, a large number of microcracks were generated near the fusion line in the bending test.

Comparative Example No. 3 was welded with the same welding current as Comparative Example No. 2, but the values of Δd ₁ and Δd ₂ were within the scope of the claims of the present invention. Therefore, although the bending resistance was improved as compared with Comparative Example No. 2, many microcracks were generated near the fusion line because the welding current of the finishing layer was low.

In Comparative Example No. 4, since the welding current of the finishing layer was larger than the value in the predetermined range, the bead appearance was slightly poor.

In Comparative Example No. 6, the groove remaining depth of the pre-finishing layer was larger than the predetermined value, and the welding amount was larger than the predetermined value because the welding was carried out aiming at an appropriate overfill shape. For this reason, although the weld metal in the vicinity of the fusion line has few microcracks, the weld metal at a position distant from the fusion line is cracked due to the large amount of welding, and the bending resistance deteriorates.

For Comparative Example _No. 8, Δd ₁ and Δd ₂
Is larger than the value in the predetermined range, so many microcracks were generated in the weld metal near the fusion line.

In Comparative Example No. 9, the welding current of the finishing layer was larger than the value in the predetermined range, and the amount of wire welding was also larger than the predetermined value. For this reason, the bead appearance was slightly poor, and in addition, cracks occurred in the weld metal at a position away from the fusion line, and the bending resistance performance deteriorated.

In Comparative Example No. 12, since Δd ₂ was larger than the value in the predetermined range, many microcracks were generated near the fusion line of Δd ₂ .

In Comparative Example No. 14, the open tip portion was melted in some places in the welded portion of the finishing layer, but Δd ₁
And Δd ₂ are smaller than the values in the predetermined range, some portions cannot be filled with the molten metal, resulting in undercut. It should be noted that the portion of the depression caused by the undercut was excluded from the bending test and no microcracks were generated in the weld metal, so the bending performance was evaluated as good.

[0046]

As described above, according to the present invention,
Automatic TIG welding of carbon steel sheets with a tensile strength of 780 N / mm ² or less under predetermined welding conditions allows the welded part to have a beautiful bead appearance, and does not impair excellent mechanical properties Bending resistance can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing a groove shape of a test plate used in Examples.

FIG. 2 is a cross-sectional view showing a sampling portion of a bending test piece in a test plate used in an example.

FIG. 3A is a top view showing a part of a bending test piece in which microcracks are generated on the surface of the weld metal, and FIG. 3B is an enlarged view showing an AA cross section of the welded portion.

FIG. 4 is a graph showing a hardness distribution in a plate thickness direction in a joint weld metal when a low temperature steel plate having a thickness of 35 mm is welded on both sides.

FIG. 5 is a cross-sectional view showing a welded portion on which a pre-finishing layer is formed.

FIG. 6 is a cross-sectional view showing a welded portion on which a finishing layer is formed.

[Explanation of symbols]

 1; Weld metal (finishing layer) 2; Base metal 3; Weld metal (pre-finishing layer) 4; Microcracks (near fusion line) 5; Microcracks (separate from fusion line) 6a, 6b; Fusion lines 9a, 9b; Open tip 10; Pre-finishing layer 11; Finishing layer

Claims (2)

[Claims] 1. In an automatic TIG welding construction method for welding one side of a carbon steel sheet having a tensile strength of 780 N / mm ² or less, an average welding groove depth of the finishing layer is 2 mm or less,
The pre-finishing layer and the finishing layer are formed by setting the welding current of the finishing layer to 220 to 350 A, the distance between the bead end and the open tip of the finishing layer to 2 to 5 mm, and the welding amount of the wire of the finishing layer to 25 g / min or less. An automatic TIG welding construction method characterized by the above. 2. In an automatic TIG welding method for welding both sides of a carbon steel sheet having a tensile strength of 780 N / mm ² or less, an average welding groove depth of the finishing layer is 2 mm or less,
The welding current of the finishing layer is 220 to 350 A, the distance between the bead end and the open tip of the finishing layer is 2 to 5 mm, and the welding amount of the wire of the finishing layer is 25 g / min or less. And a finishing layer and a finishing layer and a finishing layer on the back side are formed.