JP2860072B2 - Automatic TIG welding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an automatic TIG welding method, an automatic TIG welding method, in particular the tensile strength is one-side welding or both welding of the welding plate of 780N / mm ² class fewer carbon steel.

[0002]

2. Description of the Related Art Compared to other arc welding such as covered arc welding and submerged arc welding, a welded portion by TIG welding not only has a brilliant bead appearance but also has a very low oxygen content in the weld metal. Excellent mechanical properties, especially toughness. In addition, the amount of hydrogen in the weld metal is low, and the weld crack resistance is excellent.

[0003] On the other hand, TIG welding has a drawback that welding efficiency is low as compared with covered arc welding and the like, but an automatic welding apparatus capable of welding with a large welding amount has been developed, For welding, welding materials have been developed that can provide a weld metal having good mechanical properties, and welding performance has been improved. For this reason, TIG welding requires LP
It is also widely used for welding important structures such as G tanks, pressure vessels and penstocks.

[0004]

However, since it is essential that safety is ensured for important welded structures,
Various non-destructive inspections, destructive inspections, and the like have been performed on test welded joints that have undergone the same welding as actual welded joints or actual welded structures. A bending test may be employed as one of the destructive inspections.

When a front bending test and a back bending test are performed on the automatically welded TIG weld metal, a large number of small cracks may occur particularly in the weld metal near the fusion line. However, even if the surface of the test piece where such micro-cracks occur before the bending test is inspected by a magnetic particle flaw detection test or a penetrant flaw detection test or the inside of the test piece by a radiographic test or the like, a defect equivalent to a micro-crack does not occur. Often not detected.

[0006] Because of these findings and the fact that the TIG weld metal is excellent in resistance to weld cracking as described above, the micro-cracks are not the defects that were present in the weld metal that were revealed by the bending test. It is considered that this is the first crack generated by undergoing bending deformation. Therefore, it does not immediately imply that there is a problem with the safety of the actual structure. However, if the total length of cracks (total crack length) or the total number of cracks in the test exceeds the standard, repair welding is necessary. Therefore, there is a demand for the development of a welding method that does not cause minute cracks in the weld metal even under severe bending deformation such as a bending test.

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

[0008]

SUMMARY OF THE INVENTION An automatic TIG welding method according to the present invention is directed to an automatic TIG welding method for one-side welding of a carbon steel sheet having a tensile strength of 780 N / mm ² class or less. 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 of the finishing layer and the open end is 2 to 5 mm, and the welding amount of the wire of the finishing layer is 25 g / min or less. Forming a pre-finishing layer and a finishing layer.

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

In the case where the carbon steel sheet is welded on both sides, under the same conditions as in the case of the above-mentioned single-side welding, the front pre-finish layer and the finish layer on the front side and the finish pre-finish layer and the finish layer on the back side of the carbon steel sheet are provided. And automatic TIG welding.

The carbon steel having a tensile strength of 780 N / mm ² or less is defined in JIS G3101, G3106 and G3106.
General structural steel plate or welded structural steel plate such as 3128, JI
SG3115, G3118, G3119 and G3126
And the like.

[0012]

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

First, in order to investigate the cause of the generation of microcracks in a welded portion, automatic TIG welding was performed on various steel sheets under various conditions using welding wires having appropriate component compositions. Then, bending test pieces were sampled from these welded steel plates and subjected to bending tests.

FIG. 3 is a view showing a part of a bending test piece in which a microcrack has occurred. 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, a minute crack was generated in the weld metal (finish layer) 1 formed on the upper surface of the weld metal (pre-finish layer) 3. Most of these minute cracks are generated 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 (small cracks 4). (Micro crack 5). In addition, the length of each micro-crack is often about 0.2 mm.

The fracture surface of the microcracks 4 generated near the fusion line was analyzed by a scanning electron microscope, and it was confirmed that these microcracks were ductile fractures. In other words, it is considered that these microcracks 4 are caused by the ductile opening of the weld metal 1 near the fusion line beyond the breaking elongation due to the bending stress of the test.

Further, it was recognized that the microcracks 4 tended to occur frequently when a low current was used at the time of welding 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.
And the length (a) of the region where the weld metal 1 was formed in an umbrella shape on the base material 2 was long. On the other hand, when a relatively high current is used to weld the finishing layer and the bead width is narrowed, little cracks 4 hardly occur. In this case, the penetration (b) is deep, and the length of the umbrella-shaped region is large. (A) was shorter.

Next, the results of measuring the hardness of the weld metal in the thickness direction will be described. FIG. 4 shows a low-temperature steel plate (SLA325B) having a thickness of 35 mm, where the vertical axis represents Vickers hardness and the horizontal axis represents the distance in the thickness direction of the weld metal.
FIG. 4 is a graph showing a hardness distribution in a joint weld metal when both surfaces are welded using a 4.5% Ni-based low temperature steel TIG welding wire. As shown in this graph, the hardness of any of the finish layers 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 oxygen content in the TIG weld metal is extremely low (1
This is because the hardenability is increased. Further, the layer inside the weld metal is tempered by a heat cycle by the subsequent welding, and has an appropriate hardness (strength). Such a phenomenon that the finish layer becomes hard is particularly remarkable in a weld metal of a TIG welding material for high-tensile 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
If the weld metal 1 near a and 6b is shallow (thin) and long, the difference in elongation performance between the base metal 2 and the weld metal places a burden on the weld metal. Is thought to occur. Therefore, in order to prevent micro cracks, it is considered effective to adjust the welding conditions and actively control the bead cross-sectional shape.

On the other hand, as for the micro-crack 5 generated at a position apart from the fusion lines 6a and 6b, it was found from the fracture surface analysis that the micro-blow hole was widened by bending stress. It has also been found that the fine cracks due to the blow holes are generated when the welding amount of the wire at the time of welding the finishing layer is large. Therefore, it is considered effective to adjust the amount of welding at the time of welding in order to prevent minute cracks generated at a position separated from the fusion line. The welding construction method according to the present invention is based on the above measurement and consideration results.

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

[0021] Groove depth before welding of the layer before finishing: 2 mm on average
Hereinafter, the groove depth of the welding residue in the pre-finish layer means a distance c between the upper end surface of the weld metal after welding the pre-finish 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 margin may not be obtained with a welding amount (25 g / min) or less described below. In addition, if the distance exceeds 2 mm on average, the bead appearance of the finished layer also deteriorates. Therefore, the welding residual groove depth of the layer before finishing is set to an average of 2 mm or less. As shown in FIG. 5, the average value of the groove depth before welding is the average value obtained by dividing the groove width into four equal parts and measuring the depth at three points A, B, and C. is there.

Welding current of the finishing layer: 220 to 350 A Generally, the welding current for forming the finishing layer is preferably about 200 A, at which a brilliant bead can be easily obtained. However, when the welding current is less than 220 A, although the bead appearance is good, the penetration of the base material and the vicinity of the fusion line becomes shallow, so that the number of micro cracks generated near the fusion line in the bending test increases. . On the other hand, when the welding current of the finishing layer exceeds 350 A, when welding is performed in a vertical position, the molten metal is easily dripped, and the external shape of the bead is extremely deteriorated. Therefore, the welding current of the finishing layer needs to be 220 to 350 A which can achieve both bead appearance and bending performance. When the welding current of the finishing layer is 220 A or more, the hardness of the finishing layer is slightly reduced due to an increase in welding heat input, and a secondary effect of improving bending resistance can be obtained.

The distance between the bead end and the open end of the finishing layer: 2
As shown in FIG. 6, the distance between the bead end of the finishing layer and the open end is 5 mm, and the end of the bead of the finishing layer 11 covering the top surface of the groove and the open ends 9a and 9b of the top surface of the groove, as shown in FIG. Distance Δ
d _1, refers to a [Delta] d _2, when the distance exceeds 5 mm, microcracking ends up number generated in the fusion line vicinity, whereas if it is less than 2 mm, there are cases where undercut occurs. Therefore, the distance between the bead end and the open end of the finishing layer is 2 to 5 m.
m.

The amount of welding of the finishing layer wire: 25 g / min or less The amount of welding of the finishing layer wire refers to the amount of welding of the wire used when forming the finishing layer. If the amount of welding exceeds 25 g / min, micro-cracks due to blow holes are likely to occur in the weld metal away from the fusion line. Therefore, the welding amount of the wire of the finishing layer is set to 25 g / min or less.

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

In the automatic TIG welding, the appropriate range of the arc voltage and the welding speed is narrower than other weldings. Generally, the arc voltage is about 9 to 13 V, and the welding speed in the downward welding is about 7 to 15 cm /. Min, the welding speed in vertical welding is about 4-7
The welding is performed at a rate of cm / min, and the appropriate range is not particularly limited in the present invention. However, if the range is within these ranges, the effects of the present invention can be obtained.

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

For the test plates, steel types shown in Table 1 below were used, and welding wires suitable for each steel type were 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, 1 s shown in the side column of the test plate in Table 3 below
t and 2nd mean the front side and the back side of the test plate, respectively.
In addition, among the welding conditions shown in Table 3 below, those that deviate from the scope of the claims of the present invention are indicated by underlining the condition values.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

Automatic TIG welding was performed according to the welding conditions shown in Table 3 above, and after judging the quality of the bead appearance, a test piece shown in FIG. 2 was cut out from each test plate. FIG. 2 is a cross-sectional view showing a sampled portion of a bending test piece in the test plate used in this example. The thicknesses S ₁ and S ₂ of the front bending test piece and the back bending test piece shown in FIG. 2 were both 10 mm.

The bending test was performed in accordance with JIS Z3122. The excess of the test piece was cut to the surface of the base material, and 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 generated by the bending test, and the results of the comprehensive evaluation of the test pieces. In addition, the bead appearance was evaluated in four stages of excellent, good, slightly poor and poor, and was evaluated as “◎”,
"○", "△" and "x" are shown in the column of bead appearance. Regarding the appearance of the bead, the case of excellent and good was judged as acceptable. In the bending test, the number of cracks was 5 or less,
Total length of each crack is 3mm or less, maximum crack length is 1
mm or less was accepted. The standard of the bending test sufficiently satisfies the required value in a normal bending test. Furthermore, regarding the results of the comprehensive evaluation, those in which the bead appearance and the results of the bending test were acceptable were evaluated as “○”, and those in which one of them was unacceptable were evaluated as “×”, and the following Table 4
In the pass / fail column.

[0036]

[Table 4]

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

On the other hand, in Comparative Example No. 2, the welding current is smaller than the value in the predetermined range, and the distances Δd ₁ and Δd ₂ between the bead end and the open end of the finishing layer are larger than the value in the predetermined range. For this reason, although the bead appearance was excellent, an extremely large number of minute cracks occurred near the fusion line in the bending test.

In Comparative Example No. 3, welding was performed 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. For this reason, although the bending resistance was improved as compared with Comparative Example No. 2, many small cracks 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 remaining depth of the groove of the pre-finish layer was larger than a predetermined value, and welding was performed with an aim of an appropriate overfill shape, so that the welding amount of the wire was larger than the predetermined value. For this reason, although the weld metal near the fusion line has few cracks, the amount of welding is large, so that the weld metal at a position away from the fusion line is cracked, and the bending resistance is reduced.

For Comparative Example _No. 8, Δd ₁ and Δd ₂
Is larger than the value in the predetermined range, a large number of minute cracks occurred in the weld metal near the fusion line.

In Comparative Example No. 9, the welding current of the finishing layer is larger than the predetermined range, and the welding amount of the wire is 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 was reduced.

In Comparative Example No. 12, since Δd ₂ was larger than the value in the predetermined range, many small cracks occurred near the fusion line of Δd ₂ .

In Comparative Example No. 14, although the open end portions of the welded portion of the pre-finish layer were molten at some points, Δd ₁
And Δd ₂ were smaller than the values in the predetermined range, there were portions that could not be filled with the molten metal, and undercuts occurred. In addition, the recessed portion caused by the undercut was excluded from the object of the bending test, and since the microcracks did not occur in the weld metal, the bending performance was evaluated as good.

[0046]

As described above, according to the present invention,
By performing automatic TIG welding on a carbon steel sheet having a tensile strength of 780 N / mm ² or less under predetermined welding conditions, the welded portion has a brilliant bead appearance and is excellent without impairing 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 an example.

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

FIG. 3 (a) is a top view showing a part of a bending test piece in which microcracks have occurred on the surface of a weld metal, and FIG. 3 (b) is an enlarged view showing an AA cross section of the welded portion.

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

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

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

[Explanation of symbols]

 Reference Signs List 1: weld metal (finish layer) 2: base metal 3: weld metal (pre-finish layer) 4: micro crack (near fusion line) 5: micro crack (separate from fusion line) 6a, 6b; fusion line 9a, 9b; Open tip 10; Finishing layer 11; Finishing layer

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-1339 (JP, A) JP-A-7-266038 (JP, A) JP-A-63-290687 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) B23K 9 / 095,9 / 167,9 / 23

Claims (2)

(57) [Claims] 1. An automatic TIG welding method for single-side welding a carbon steel sheet having a tensile strength of 780 N / mm ² or less in a single-sided welding method.
The pre-finish layer and the finish layer are formed by setting the welding current of the finish layer to 220 to 350 A, the distance between the bead end and the open end of the finish layer to 2 to 5 mm, and the welding amount of the wire of the finish layer to 25 g / min or less. An automatic TIG welding method, characterized in that: 2. An automatic TIG welding method for double-side welding a carbon steel sheet having a tensile strength of 780 N / mm ² or less in a double-sided welding method.
When the welding current of the finishing layer is 220 to 350 A, the distance between the bead end and the open end 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, the front finishing layer on the front side of the carbon steel sheet And forming a finishing layer, a pre-finishing layer on the back side, and a finishing layer.