Welding method

Abstract

To provide a welding method that can increase a weld-penetration depth easily without being affected by a material quality of a material to be welded, with a low-cost facility, and further has good workability.SOLUTION: The welding method includes: a first welding step of generating arc between a material to be welded and a consumable electrode and performing MAG welding while discharging first shield gas mainly composed of inactive gas to the material to be welded so as to surround the consumable electrode; and a second welding step of generating arc between a welded part formed in the first welding step and a non-consumable electrode, and performing TIG welding while discharging second shield gas composed of inactive gas to the material to be welded so as to surround the non-consumable electrode. In the first welding step, welding is performed using shield gas containing oxygen components. In the second welding step, welding is performed without using a filler material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a welding method by arc welding. [Background technology]

[0002] TIG (Tangsten Inert Gas) welding, a type of arc welding, is widely used for welding steel materials such as stainless steel and carbon steel, as well as non-ferrous metal structures (workpieces) because it is relatively easy to perform and produces high-quality welded metal parts. However, TIG welding has a relatively small penetration depth (amount of penetration). If the penetration depth of the weld is insufficient, it is necessary to perform back welding of the workpieces after gouging, and when the workpieces are large structures, the welding work requires excessive labor and time.

[0003] In recent years, in order to solve this problem, a technique has been proposed that increases the penetration depth and improves welding efficiency by actively supplying oxygen to the molten pool (a pool of molten metal). For example, Patent Document 1 proposes a method of applying oxide flux to the welded surface of the material to be welded and then performing TIG welding. According to this method, oxygen dissolved in the molten pool due to the decomposition of the flux acts as a surface active element, and inward convection (Marangoni convection) occurs in the molten pool. This phenomenon increases the penetration depth.

[0004] Patent Document 2 also proposes a method in which, during welding, a first shielding gas made of an inert gas is discharged toward the workpieces, and a second shielding gas containing an oxidizing gas is discharged toward the workpieces around the first electrode. According to this method, oxygen is supplied to the molten pool by the second shielding gas. As described above, this supply of oxygen generates an inward convection in the molten pool, increasing the penetration depth. The first shielding gas (inert gas) is supplied toward the workpieces in order to suppress wear of the tungsten electrode caused by oxygen.

[0005] Furthermore, Patent Document 3 proposes a method in which a welding nozzle with a double-cylinder structure, which includes an inner cylinder and an outer cylinder that is shorter than the inner cylinder, is attached to the tip of a welding torch, and inert gas is supplied to the material to be welded through the inner cylinder of the welding nozzle during welding. In a method using such a welding nozzle, a pressure drop occurs in the gap between the inner and outer cylinders of the welding nozzle due to the flow of inert gas, and oxygen in the atmosphere is sucked through the gap and supplied to the molten pool. As a result, an inward convection occurs in the molten pool, and the penetration depth increases. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2004-114089 A [Patent Document 2] JP 2004-298963 A [Patent Document 3] International Publication No. 2013 / 168513 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the method disclosed in Patent Document 1 requires laborious pre-processing work of applying flux, and also requires post-processing work such as removing the flux that has hardened without being welded. Therefore, when the material to be welded is a large structure, the workability is significantly deteriorated.

[0008] On the other hand, the methods disclosed in Patent Documents 2 and 3 are easier to work with than the method disclosed in Patent Document 1 because they do not require the application of flux or the subsequent processing. However, the welding method disclosed in Patent Document 2 requires special and expensive equipment to release two types of shielding gases to the material to be welded. In addition, when the material to be welded is carbon steel, an oxide film is likely to be formed on the surface of the molten pool in the method of blowing oxygen into the molten pool as in Patent Documents 2 and 3. When an oxide film is formed on the molten pool, the introduction of oxygen into the molten pool is suppressed, making it difficult for the convection to occur, and the increase in the penetration depth is hindered. In other words, the methods disclosed in Patent Documents 2 and 3 are almost ineffective when the material to be welded is carbon steel.

[0009] The present invention has been made in consideration of the above circumstances, and has as its object to provide a welding method which can easily increase the penetration depth using inexpensive equipment, regardless of the material of the workpiece, and which is easy to work with. [Means for solving the problem]

[0010] In order to solve the above problems, the welding method of the present invention includes a first welding step in which an arc is generated between a workpiece and a consumable electrode, and welding is performed while a first shielding gas mainly composed of an inert gas is discharged onto the workpiece so as to surround the consumable electrode, and a second welding step in which an arc is generated between a weld formed in the first welding step and a non-consumable electrode, and welding is performed while a second shielding gas composed of an inert gas is discharged onto the workpiece so as to surround the non-consumable electrode, wherein the first welding step uses a first shielding gas containing an oxygen component and / or a consumable electrode containing an oxygen component, and the second welding step is performed without using a filler metal.

[0011] In this welding method, in the first welding step, welding is performed using a first shielding gas containing an oxygen component and / or a consumable electrode containing an oxygen component, so that a weld containing oxygen is formed as a first layer weld in the first welding step. Then, in the second welding step, welding is performed while generating an arc between the weld and the non-consumable electrode, so that the weld and a part of the welded material are melted and the welded material is welded. At this time, since the weld already contains oxygen, the oxygen acts as a surface active element and generates an inward convection (Marangoni convection) in the molten pool. This phenomenon increases the penetration depth (amount of penetration).

[0012] According to this welding method, it is possible to increase the penetration depth without the need for flux application or post-treatment as in the conventional technique (Patent Document 1). In addition, since the first welding step can be performed by, for example, MAG welding, and the second welding step can be performed by TIG welding, existing equipment can be used for both welding steps, and special and expensive welding equipment as in the conventional technique (Patent Document 2) is not required. Moreover, in the second welding step, the welded portion containing oxygen, which was formed in advance in the first welding step, melts to form a molten pool, and the convection occurs naturally in the molten pool. Therefore, unlike the conventional techniques (Patent Documents 2 and 3) in which oxygen is blown from the outside onto the molten pool, no oxide film is formed in the molten pool, and the penetration depth can be effectively increased even when the welded material is made of carbon steel. Therefore, according to this welding method, it is possible to provide a welding method with good workability that can easily increase the penetration depth with inexpensive equipment and is not affected by the material of the welded material, and does not require flux application or post-treatment at all. As a result, highly efficient welding work that does not require gouging is realized.

[0013] In this welding method, in the first welding step, it is preferable to set the amount of oxygen contained in the shielding gas and / or the consumable electrode so that the oxygen concentration of the weld formed in the first welding step is 900 ppm to 1800 ppm.

[0014] According to this welding method, even if the oxygen concentration is diluted by melting the materials to be welded in the second welding step, the time during which the oxygen concentration in the molten pool can be maintained at an effective oxygen concentration for activating the above-mentioned convection (approximately 100 ppm to 350 ppm) is extended, which makes it possible to more reliably increase the penetration depth of the weld.

[0015] In the above welding method, after the first welding step, the second welding step may be performed a plurality of times.

[0016] When the thickness of the material to be welded is relatively large, the oxygen concentration in the molten pool may not be sufficiently diluted to an effective oxygen concentration (100 ppm to 350 ppm) for activating the convection. In such a case, the deep penetration effect can be improved by performing the second welding step multiple times.

[0017] The welding method is applicable to butt welding in which workpieces are butted together to form a groove and welded to the groove to join the workpieces. In this case, the groove is trapezoidal and includes root faces extending in the thickness direction of each workpiece facing each other in the butt direction, extended faces extending in parallel to the butt direction and in opposite directions from one end of the root faces in the thickness direction, and a groove face that is a face defining a groove angle and extends obliquely in the thickness direction from an end of the extended face on the side opposite to the butt face with respect to the extended face, and in the first welding step, welding may be performed from the one end side of the butt faces to the butt portions of the workpieces.

[0018] This welding method is advantageous in increasing the penetration depth of the weld compared to the case of a general V-shaped groove. That is, in the case of a V-shaped groove, the electrode (non-consumable electrode) and the groove surface are relatively easy to approach each other in the second welding process, so a relatively large amount of the heat energy generated by the arc is consumed in melting the groove surface. In contrast, according to the shape of the groove portion in which the extension surface exists between the groove surfaces, the electrode and the groove surface are less likely to approach each other compared to the V-shaped groove. Therefore, the heat energy consumed by melting the groove surface is reduced, and more heat energy is directly supplied to the molten pool. This promotes a temperature rise in the molten pool, activating inward convection, and as a result, it is possible to increase the penetration depth.

[0019] In this case, the groove portion may have a V-groove between the extended surfaces, the V-groove extending in the extending direction of the groove portion.

[0020] By providing such a V-groove, it becomes possible to efficiently deposit the metal welded by using the consumable electrode (the weld metal obtained in the first welding step).

[0021] In order to solve the above problems, another welding method of the present invention includes a first welding step in which an arc is generated between a workpiece and a consumable electrode, and welding is performed while carbon dioxide gas is discharged to the workpiece so as to surround the consumable electrode, and a second welding step in which an arc is generated between a weld formed in the first welding step and a non-consumable electrode, and welding is performed while a shielding gas consisting of an inert gas is discharged to the workpiece so as to surround the non-consumable electrode, and the second welding step is characterized in that welding is performed without using a filler metal.

[0022] In this welding method, in the first welding step, welding is performed using carbon dioxide gas as a shielding gas, and an oxygen-containing weld is formed as a first layer weld in the first welding step. Then, in the second welding step, welding is performed while generating an arc between the weld and a non-consumable electrode, and the weld and a part of the weld are melted to weld the weld. At this time, since the weld already contains oxygen, the oxygen acts as a surface active element and generates an inward convection (Marangoni convection) in the molten pool. This phenomenon increases the penetration depth (amount of penetration).

[0023] According to this welding method, it is possible to increase the penetration depth without the need for flux application or post-treatment as in the conventional technique (Patent Document 1). In addition, since the first welding step can be performed by, for example, carbon dioxide gas welding, and the second welding step can be performed by TIG welding, existing equipment can be used for both welding steps, and special and expensive welding equipment as in the conventional technique (Patent Document 2) is not required. Moreover, in the second welding step, the welded portion containing oxygen, which was formed in advance in the first welding step, melts to form a molten pool, and the convection occurs naturally in the molten pool. Therefore, unlike the conventional techniques (Patent Documents 2 and 3) in which oxygen is blown from the outside onto the molten pool, no oxide film is formed in the molten pool, and the penetration depth can be effectively increased even when the welded material is made of carbon steel. Therefore, according to this welding method, it is possible to provide a welding method with good workability that can easily increase the penetration depth with inexpensive equipment and is not affected by the material of the welded material, and does not require flux application or post-treatment at all. As a result, highly efficient welding work that does not require gouging is realized.

[0024] In this welding method, in the first welding step, the amount of carbon dioxide gas is preferably set so that the oxygen concentration of the welded portion formed in the first welding step is 900 ppm to 1800 ppm.

[0025] According to this welding method, even if the oxygen concentration is diluted by melting the materials to be welded in the second welding step, the time during which the oxygen concentration in the molten pool can be maintained at an effective oxygen concentration for activating the above-mentioned convection (approximately 100 ppm to 350 ppm) is extended, which makes it possible to more reliably increase the penetration depth of the weld.

[0026] In addition, a consumable electrode containing oxygen may be used in the first welding step, and in this case, it is preferable to set the amount of carbon dioxide gas and the amount of oxygen contained in the consumable electrode so that the oxygen concentration of the weld formed in the first welding step is 900 ppm to 1800 ppm.

[0027] This welding method also extends the time during which the oxygen concentration (approximately 100 ppm to 350 ppm) effective for activating the convection can be maintained, as in the case of the above, and therefore makes it possible to more reliably increase the penetration depth of the weld. Effect of the Invention

[0028] According to the present invention described above, it is possible to provide a welding method that can easily increase the penetration depth using inexpensive equipment and is not influenced by the material of the workpiece, and that is easy to work with. As a result, highly efficient welding work that does not require gouging can be realized. [Brief description of the drawings]

[0029] [Figure 1] FIG. 2 is a process diagram illustrating the welding method of the present invention. [Diagram 2] FIG. 4 is a schematic diagram of a welding method in a first welding step. [Diagram 3] FIG. 6 is a schematic diagram of a welding method in a second welding step. [Figure 4] FIG. 2 is a schematic cross-sectional view of the workpieces showing the state of the molten pool. [Diagram 5] FIG. 1 is an explanatory diagram of a sample used in a test. [Figure 6] FIG. 4 is a schematic cross-sectional view of the workpieces, showing the welded portion in the second welding step. [Figure 7] 1 is a schematic cross-sectional view of a workpiece to be welded showing a groove portion. [Figure 8] FIG. 1 is an explanatory diagram of a sample used in a test. [Figure 9] FIG. 2 is a side view of a sample used in the test. [Figure 10] 1 is a schematic cross-sectional view of a workpiece to be welded showing a groove portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0031] FIG. 1 is a process diagram illustrating the welding method of the present invention. The welding method of the present invention (hereinafter referred to as the present welding method) is a welding method using arc welding. As shown in FIG. 1, the present welding method includes a first welding step and a second welding step. In this example, the first welding step is a welding step using MAG (Metal Active Gas) welding, and the second welding step is a welding step using TIG (Tungsten Inert Gas) welding. The welding method of each welding step will be described in detail below.

[0032] [First welding process] FIG. 2 shows an outline of the welding method (MAG welding) of the first welding step, where (a) shows a cross-section of the workpieces during welding, and (b) shows a cross-section of the workpieces after welding has been completed.

[0033] Reference numerals 101 and 102 in Fig. 2(a) denote base materials (workpieces to be welded) made of steel-based materials such as stainless steel and carbon steel, which are butted together in the left-right direction in the figure. In this example, these base materials 101 and 102 are joined by butt welding. A groove portion 110 is formed at the butted portion of the base materials 101 and 102, and in the illustrated example, a V-shaped groove that opens upward is formed. The groove portion 110 extends in a direction intersecting the butted direction, which in the illustrated example is a direction perpendicular to the paper surface.

[0034] 2(a) denotes a welding torch 10 used for MAG welding. In the first welding step, the welding torch 10 is used to perform a first layer welding from above on base materials 101, 102. The welding torch 10 includes a cylindrical nozzle 10a, an axial contact tip 12 provided at the center of the nozzle 10a, and an electrode 14 held by the contact tip 12.

[0035] The electrode 14 is a consumable electrode made of a wire-like welding wire. The welding wire is wound in a coil shape and is fed to the tip of the welding torch 10 by a feed roller 16 driven by a motor (not shown).

[0036] A shielding gas 11 (corresponding to the first shielding gas of the present invention) is discharged from the nozzle 10a toward the base materials 101, 102. The shielding gas 11 is discharged so as to surround the electrode 14, thereby shielding the arc 15 from the atmosphere. As the shielding gas 11, a mixed gas of an inert gas and a gas containing an oxygen component can be used. For example, a mixed gas of argon (Ar) and carbon dioxide gas (CO2), or a mixed gas of argon (Ar) and oxygen (O2) can be used.

[0037] In the first welding step, the electrode 14 is used as a positive electrode and the base materials 101, 102 are used as negative electrodes, and a voltage is applied between the electrode 14 and the base materials 101, 102 to cause discharge and generate an arc 15. Then, the welding torch 10 is moved along the groove portion 110, and as shown in Figures 2(a) and (b), the electrode 14 and the base materials 101, 102 are melted by the arc 15 to form a weld (welded metal portion) 17, thereby welding the base materials 101, 102.

[0038] The welding in the first welding step is performed in only one pass along the groove portion 110. During welding, a shielding gas 11 is discharged from a nozzle 10a toward the base materials 101 and 102. In this manner, the electrode 14 is melted while the shielding gas 11 containing an oxygen component is discharged toward the base materials 101 and 102, thereby forming a welded portion 17 containing oxygen (oxygen is dissolved in solid solution).

[0039] In this case, it is desirable to set the oxygen concentration in shielding gas 11 so that the concentration of oxygen contained in welded portion 17 is 900 ppm to 1800 ppm. For example, in the case of a typical gas flow rate (20 L / min to 25 L / min) in MAG welding, by using a shielding gas containing argon (Ar) with a volume ratio of 20% to 40% oxygen as shielding gas 11, it is possible to form welded portion 17 with an oxygen concentration of 900 ppm to 1800 ppm.

[0040] Forming welded portion 17 having an oxygen concentration of 900 ppm to 1800 ppm in this manner is effective in increasing the penetration depth during welding in the second welding step, as will be described later.

[0041] [Second welding process] FIG. 3 is a schematic diagram showing the welding method (TIG welding) of the second welding step, in which (a) shows a cross section of the workpieces during welding, and (b) shows a cross section of the workpieces after welding has been completed.

[0042] Reference numeral 20 in Fig. 3(a) denotes a welding torch 20 used for TIG welding. In the second welding step, the welding torch 20 is used to perform welding from above on the base materials 101, 102 after the first welding step. The welding torch 20 includes a cylindrical nozzle 20a and an axial electrode 24 provided at the center of the nozzle 20a. The electrode 24 is a tungsten electrode (non-consumable electrode). The tip of the electrode 24 is conical, and the tip angle, i.e., the electrode angle, is, for example, 90°.

[0043] From the nozzle 20a, a shielding gas 21 (corresponding to the second shielding gas of the present invention) made of an inert gas is discharged toward the base materials 101, 102. The shielding gas 21 is discharged so as to surround the electrode 24, thereby shielding the arc 25 from the atmosphere. As the inert gas, argon (Ar) or helium (He) can be used.

[0044] In the second welding step, the electrode 24 is used as a negative electrode and the base materials 101, 102 are used as positive electrodes, and a voltage is applied between the electrode 24 and the base materials 101, 102 to cause discharge and generate an arc 25. Then, the welding torch 20 is moved along the groove portion 110, more specifically along the welded portion 17 formed in the first welding step, and the welded portion 17 formed in the first welding step and the base materials 101, 102 are melted by the arc 25 to form a molten pool 27, as shown in Fig. 3(a), and the base materials 101, 102 are welded.

[0045] As described above, the welded portion 17 formed in the first welding step contains oxygen, and therefore oxygen is naturally contained in the molten pool 27 formed by melting the welded portion 17 and the base materials 101, 102. In this case, the oxygen (900 ppm to 1800 ppm) contained in the welded portion 17 is diluted by the melting of the base materials 101, 102, and as a result, the oxygen concentration in the molten pool 27 becomes approximately 100 ppm to 350 ppm.

[0046] Oxygen is a surface active element, and in a molten pool 27 that contains oxygen, the surface tension increases with increasing temperature. In the surface of a molten pool 27 that contains about 100 ppm to 350 ppm of oxygen, the high-temperature central region has a higher surface tension than the low-temperature peripheral regions, resulting in inward convection 28 in the molten pool 27, as shown in Figure 4. The occurrence of this inward convection 28 promotes the penetration of the molten pool 27, resulting in an increase in the penetration depth.

[0047] If necessary, the welding in the second welding step can be performed multiple times (multiple passes) along the groove 110 (see FIG. 1). In this case, at least the first welding (initial pass) is non-filler welding that does not use a filler metal, and if necessary, a filler metal is used in the second and subsequent welding passes. This type of welding in the second step makes it possible to form a weld 29 with a deep penetration depth, as shown in FIG. 3(b). Note that "D" in FIG. 3(b) is the penetration depth of the weld 29, and "W" is the width of the weld.

[0048] [Comparison of this welding method with conventional TIG welding methods (1)]

[0049] [Table 1]

[0050] Table 1 shows the results of a penetration depth test carried out on the present welding method (Examples 1 and 2) and conventional welding methods (Comparative Examples 1 to 3).

[0051] 5 is an explanatory diagram of the sample (test material) 30 used in the test, where (a) is a plan view and (b) is a side view of the sample 30. The sample 30 is a rectangular plate material (width W1 × length D1 × thickness H1), and a V-groove 32 is formed in the center of the width direction to resemble the groove portion 110 (groove angle θ1). The specific dimensions are W1 = 100 mm, D1 = 200 mm, H1 = 40 mm, and θ1 = 90°, and the depth of the V-groove 32 is 2 mm.

[0052] In the test, one pass of welding was performed along the V-groove 32, and the penetration depth D and width W of each weld were measured. For Examples 1 and 2 (this welding method), one pass of welding was performed in each of the first and second welding steps.

[0053] Comparative Example 1 shows the results of general TIG welding using a sample 30 made of SM490YB (carbon steel).

[0054] Comparative Example 2 shows the results of Cap A-TIG welding using a sample 30 made of SUS304 (stainless steel material). "Cap A-TIG welding" is a welding method disclosed in Prior Art 3 explained in the Background Art. That is, it is a method of TIG welding using a welding torch with a welding nozzle with a double-cylinder structure attached to the tip, in which oxygen in the atmosphere is sucked into the welding nozzle and the oxygen is supplied to the molten pool.

[0055] Comparative Example 3 shows the result of carrying out the above-mentioned Cap A-TIG welding using Sample 30 made of SS400 (carbon steel material). Note that in all of Comparative Examples 1 to 3, a general filler metal was used.

[0056] Example 1 shows the results of carrying out this welding method using a sample 30 made of SM490YB (carbon steel), and Example 2 shows the results of carrying out this welding method using a sample 30 made of SM400A (carbon steel).

[0057] As shown in Table 1, according to this welding method (Examples 1 and 2), the penetration depth D of the weld (weld metal part) is deeper than any of the conventional TIG welding methods. It is noteworthy that, although sample 30 of the example is a carbon steel material, the penetration depth D is nearly twice as deep as that of Comparative Example 3, which also used sample 30 made of carbon steel. This is because, with this welding method, no oxide film is formed during welding in the second welding step, and ideal inward convection occurs in the molten pool.

[0058] That is, in the welding method of Comparative Example 3 (Cap A-TIG welding), oxygen is blown from the outside onto the molten pool, so that an oxide film is easily formed on the surface of the molten pool. When an oxide film is formed on the molten pool, the introduction of oxygen into the molten pool is suppressed, so that the convection is less likely to occur, and the penetration depth is less likely to increase. In contrast, in the present welding method (Examples 1 and 2), as described above, a weld containing oxygen is formed in advance by MAG welding in the first welding step. Then, in the TIG welding in the second welding step, the weld formed in the first welding step and the specimen 30 (base material) melt to form a molten pool. Since the weld formed in the first welding step already contains oxygen, the molten pool formed by melting the weld and the specimen 30 naturally contains oxygen. This oxygen acts as a surface active element, and an inward convection is generated in the molten pool, so that the penetration depth increases. Therefore, in Examples 1 and 2 according to the present welding method, the penetration depth is deeper than that of Comparative Example 3, even though the specimen is a carbon steel material.

[0059] [Comparison of this welding method with conventional TIG welding methods (2)]

[0060] [Table 2]

[0061] Table 2 shows the test results of the penetration depth for this welding method (Example) and for a case where general TIG welding was performed twice (Comparative Example).

[0062] For the test, a sample 30 made of SM400A (carbon steel material) and shown in Fig. 5 was used. The amount of oxygen (oxygen concentration) originally contained in the sample 30 was 40 ppm.

[0063] In the comparative example, one pass of TIG welding was performed using MG-W50B (Solid) (manufactured by Kobe Steel, Ltd.) as a filler metal in the first welding step, and one pass of TIG welding was performed without using a filler metal in the second welding step. Note that argon (Ar) was used as the shielding gas in the first welding step, and helium (He) was used as the shielding gas in the second welding step.

[0064] On the other hand, in the example, in the first welding step, one pass of MAG welding was performed using SE-50T (Solid) (Kobe Steel, Ltd.) as a consumable electrode, and in the second welding step, one pass of TIG welding was performed without using a filler metal. In the first welding step, a shielding gas containing argon (Ar) and 40% oxygen by volume was used as the shielding gas.

[0065] As shown in Table 2, according to this welding method (Example), the penetration depth D of the weld is deeper than when general TIG welding is performed twice (Comparative Example). This shows that a significant increase in the penetration of the weld cannot be expected by simply repeating TIG welding twice, and that performing MAG welding and TIG welding under the conditions described above, as with this welding method, is effective in increasing the penetration depth of the weld.

[0066] The increase in the penetration depth of the weld is affected by the amount of oxygen (oxygen concentration) contained in the weld formed in the first welding process. During welding in the second welding process, the base material (specimen 30) melts together with the weld formed in the first welding process, so even if oxygen is contained in the weld, it is diluted when the molten pool is formed, making it difficult to maintain a sufficient amount of oxygen to generate inward convection in the molten pool.

[0067] As shown in the table, in the comparative example, the oxygen concentration in the weld formed in the first welding process is originally low at 20 ppm, and this is further diluted in the second welding process, so that the molten pool does not contain enough oxygen to generate convection.

[0068] On the other hand, in the examples, as shown in the table, the oxygen concentration of the weld formed in the first welding step is relatively high at 940 ppm, so that the molten pool contains enough oxygen (100 ppm to 350 ppm) to generate convection even after dilution in the second welding step. As a result, in the examples, inward convection occurs in the molten pool, and the penetration of the weld is significantly increased.

[0069] [Mechanical performance of welded joints] Generally, mechanical performance decreases as the amount of oxygen contained in a weld (welded metal part) increases. Therefore, in the present welding method in which a weld containing a relatively high oxygen concentration (900 ppm to 1800 ppm) is formed in the first welding step, there is concern about a decrease in mechanical performance of the welded joint obtained. However, as described above, the oxygen contained in the weld formed in the first welding step is diluted by the melting of the base metal in the second welding step. Due to this dilution, the oxygen concentration of the weld finally obtained by this welding method decreases to a level equivalent to the oxygen concentration of a weld by general TIG welding (about 10 ppm to 60 ppm). Therefore, the mechanical performance of the welded joint obtained by this welding method is comparable to that of a welded joint obtained by general TIG welding.

[0070] [Table 3]

[0071] Table 3 shows the results of a tensile test (JIS Z 2241), a bending test (JIS Z 2248), and a Charpy impact test (JIS Z 2242) conducted on a sample made of SM400A (carbon steel) for a welded joint obtained by this welding method.

[0072] As shown in this table, both the tensile test and the Charpy impact test exceeded the allowable values. The values ​​in the table are almost the same as those for welded joints obtained by general TIG welding of SM400A. In addition, no particular defects were found in the bending test.

[0073] [effect] In the above welding method, as described above, the first welding step is performed by MAG welding, and then the second welding step is performed by TIG welding. Therefore, both welding steps can be performed using existing equipment, and there is no need for special and expensive welding equipment as in the prior art (Patent Document 2 in the background art).

[0074] Furthermore, according to this welding method, even when the workpiece is made of carbon steel, the penetration depth of the weld can be increased to the same level or even higher than that of the workpiece made of stainless steel (see Table 1), and the obtained welded joint can ensure mechanical performance that is comparable to that of general TIG welding (see Table 3). Therefore, the range of application is wider than that of conventional welding methods (Patent Documents 2 and 3 in the background art). In other words, it is possible to increase the penetration depth without being influenced by the material.

[0075] Furthermore, this welding method only requires one or more TIG welding steps after MAG welding, and basically does not require pretreatment such as flux application as in the conventional welding method (Patent Document 1 in the background art) or posttreatment such as removing the hardened flux without welding. Therefore, workability is remarkably good. Therefore, according to this welding method, it is possible to increase the penetration depth even in carbon steel materials with inexpensive equipment, that is, it is possible to provide a welding method that can easily increase the penetration depth regardless of the material and has good workability. As a result, highly efficient welding work that does not require gouging is realized.

[0076] [Consideration of groove shape] 6 is a cross-sectional schematic diagram of the base materials 101, 102 (workpieces to be welded) showing the welded portion in the second welding step of the present welding method. As described above, in the second welding step of the present welding method, the base materials 101, 102 melt together with the welded portion 17 formed in the first welding step to form a molten pool 27. In the case of a V-shaped groove, the groove faces 101a, 102a (inclined faces of the base materials 101, 102 that define the groove angle) and the electrode 24 are relatively close to each other, so that the regions 103, 104 near the welded portion 17 on the groove faces 101a, 102a mainly melt. In other words, a relatively large amount of the thermal energy generated by the arc 25 is consumed in melting the base materials 101, 102.

[0077] However, to increase the penetration depth, it is desirable to efficiently supply the thermal energy generated by the arc 25 to the molten pool 27 and activate the inward convection 28 (see FIG. 4) in the molten pool 27. That is, when the molten pool 27 contains about 100 to 350 ppm of oxygen, it is desirable to increase the temperature of the central region on the surface of the molten pool 27 and increase the temperature difference between the central and peripheral regions, thereby making the surface tension of the central region greater than that of the peripheral regions.

[0078] From this viewpoint, the groove portion 110 in this welding method is preferably a trapezoidal groove as shown in Fig. 7(a) (hereinafter, sometimes referred to as a trapezoidal groove). This groove portion 110, i.e., the trapezoidal groove, is formed by root faces 101b, 102b extending in the thickness direction of the base materials 101, 102, extended faces 101c, 102c extending in the thickness direction from the upper ends of the root faces 101b, 102b in the thickness direction parallel to the butt direction and in opposite directions to each other, and the groove faces 101a, 102a which are faces defining the groove angle and extend obliquely upward in the thickness direction from the ends of the extended faces 101c, 102c on the opposite butt surface side to the extended faces 101c, 102c. 7(b), a minute V-groove may be formed at the upper end of the butted portion of the root faces 101b, 102b along the extending direction of the groove portion 110. In other words, tapered faces 101d, 102d may be formed at the end portions of the extending faces 101c, 102c (upper end portions of the root faces 101b, 102b).

[0079] [Table 4]

[0080] Table 4 shows the results of penetration depth tests conducted using this welding method for V-groove and trapezoidal groove.

[0081] For the V-groove test, a specimen having the same shape as specimen 30 shown in Fig. 5 was used. On the other hand, for the trapezoidal groove test, specimens 40A and 40B shown in Figs. 8 and 9 were used.

[0082] FIG. 8 is an explanatory diagram of the sample 40A, where (a) is a plan view and (b) is a side view of the sample 40A. The sample 40A is a rectangular plate material (width W2 × length D2 × thickness H2). In the width direction center of the sample 40A, a trapezoidal groove 42 likened to a trapezoidal groove is formed along the longitudinal direction. The trapezoidal groove 42 is formed by an inner bottom surface 42a corresponding to the extended surfaces 101c, 102c, and a pair of inclined surfaces 42b corresponding to the groove surfaces 101a, 102a. The specific dimensions are W2 = 100 mm, D2 = 500 mm, H2 = 32 mm, and θ2 = 70°, and the depth of the trapezoidal groove 42 is 10 mm.

[0083] Fig. 9 shows a side view of sample 40B. Sample 40B has minute V-grooves 43 formed in the trapezoidal groove 42 of sample 40A, which resemble the tapered surfaces 101d and 102d in Fig. 7(b), but otherwise has the same shape as sample 40A. V-groove 43 extends in the longitudinal direction at the center of the inner bottom surface 42a, and has a depth of 2 mm.

[0084] The test consisted of one pass each of the first and second welding processes, and the penetration depth D and width W of each weld were measured. The material of each of the samples 30, 40A, and 40B was SUS304 (stainless steel), and the conditions of the first and second welding processes in each test were the same.

[0085] As shown in Table 4, the penetration depth D is obviously deeper in the case of a trapezoidal groove than in the case of a V-shaped groove. This is because in the case of a trapezoidal groove, the distance between the electrode and the inclined surface 42b in the second welding process is wider than that in the case of a V-shaped groove, and more heat energy from the arc is supplied to the inner bottom surface 42a of the trapezoidal groove 42. In other words, the distance between the electrode and the inclined surface 42b reduces the heat energy consumed by melting the inclined surface 42b, and more heat energy is supplied to the molten pool. This promotes a rise in the temperature of the molten pool and activates inward convection in the molten pool, resulting in an increase in the penetration depth.

[0086] In addition, in sample 40B ("Groove Shape" in the table), in which a V-groove 43 is formed on the inner bottom surface 42a of the trapezoidal groove 42, the penetration depth D is deeper than that of sample 40A ("Surface Shape" in the table), in which the V-groove 43 is not formed, and it can be considered that the V-groove 43 contributes to the increase in the penetration depth.

[0087] In view of the above, when performing butt welding using this welding method, it can be said that it is desirable to form a groove portion 110 (trapezoidal groove) as shown in Figures 7(a) and (b) in the workpiece in order to increase the penetration depth of the weld.

[0088] The shape of the groove portion 110 shown in Fig. 7(a) and (b) is suitable for a relatively thin material to be welded. For a relatively thick material to be welded, as shown in Fig. 10, for example, a groove portion 110 having an approximately X-shape and inclined surfaces 101e and 102e extending obliquely downward in the thickness direction from the lower ends of the root faces 101b and 102b may be provided in the material to be welded. In other words, the groove portion 110 may be provided in the material to be welded, with the root faces 101b and 102b sandwiched between the upper side being a trapezoidal groove and the lower side being an inverted V-groove. In this case, a welded joint can be obtained by performing welding from the upper side (trapezoidal groove side) of the material to be welded by this welding method, and then performing, for example, general TIG welding from the lower side (inverted V-groove side) of the material to be welded. In this case, when welding from the upper side (trapezoidal groove side), it is desirable to temporarily place a backing material below the root faces 101b and 102b in order to prevent the welded portion from falling off.

[0089] The above-described welding method of the present invention is an example of a preferred embodiment, and the specific method can be appropriately changed without departing from the gist of the present invention. For example, the following method can be applied.

[0090] In the first welding step (MAG welding) of this welding method, a weld containing oxygen is formed by using a shielding gas containing an oxygen component [a mixed gas of argon (Ar) and carbon dioxide gas (CO2) / a mixed gas of argon (Ar) and oxygen (O2)]. However, a consumable electrode containing an oxygen component may be used instead of or together with the shielding gas containing an oxygen component to form a weld containing oxygen. In short, in the first welding step, a shielding gas containing an oxygen component and / or a consumable electrode containing an oxygen component may be used so that a weld with an oxygen concentration of 900 ppm to 1800 ppm is formed.

[0091] In the first welding step of the present welding method, a weld containing oxygen may be formed by using carbon dioxide (CO2) as the first shielding gas. In this case, the first welding step may be performed using general carbon dioxide gas welding equipment. Note that, in this case, a weld containing oxygen may be formed by using a consumable electrode containing an oxygen component together with the shielding gas.

[0092] Furthermore, the electrode angle of the electrode (tungsten electrode) 24 used in the second welding process (TIG welding) is 90°, but the angle may be an angle other than 90°. In other words, the plasma shape during discharge by the tungsten electrode tends to be relatively narrower when the electrode angle is relatively obtuse. Therefore, in order to increase the penetration depth of the weld, that is, to efficiently supply the thermal energy generated by the arc to the molten pool, it is desirable for the electrode angle of the electrode 24 to be 90° to 180°. [Explanation of symbols]

[0093] 10. Welding Torch 11 Shielding gas (first shielding gas) 14 electrodes (consumable electrodes) 15. Arc 17 Welding 20 Welding Torch 21 Shielding gas (secondary shielding gas) 24 electrodes (non-consumable electrodes) 25 Arc 27 Molten Pool 101, 102 Base material (material to be welded)

Claims

1. a first welding step in which an arc is generated between a workpiece and a consumable electrode, and welding is performed while a first shielding gas containing an inert gas as a main component is discharged to the workpiece so as to surround the consumable electrode; a second welding step of generating an arc between the welded portion formed in the first welding step and a non-consumable electrode, and welding the workpiece while discharging a second shielding gas made of an inert gas to surround the non-consumable electrode, In the first welding step, a first shielding gas containing an oxygen component and / or a consumable electrode containing an oxygen component is used, The welding method according to claim 1, wherein the second welding step is performed without using a filler metal.

2. The welding method according to claim 1, a first welding step for controlling the amount of oxygen contained in the shielding gas and / or the consumable electrode so that the oxygen concentration of the weld formed in the first welding step is 900 ppm to 1800 ppm;

3. The welding method according to claim 1 or 2, A welding method comprising: performing the second welding step a plurality of times after the first welding step.

4. The welding method according to claim 1 or 2, The welding method is a butt welding method in which workpieces are butted together to form a groove and then welded to the groove to join the workpieces, The groove portion is trapezoidal and includes root surfaces extending in the thickness direction of each workpiece material while facing each other in the butt direction, extended surfaces extending in parallel to the butt direction and in opposite directions from one end of the root surfaces in the thickness direction, and a groove surface that is a surface that defines a groove angle and extends obliquely in the thickness direction from an end of the extended surface on the opposite butt surface side to the extended surface, A welding method characterized in that in the first welding step, welding is performed on the butted portions of the workpieces from the one end side of the butted surfaces.

5. The welding method according to claim 4, The welding method, characterized in that the groove portion has a V-groove extending in an extension direction of the groove portion between the extended surfaces.

6. a first welding step in which an arc is generated between a workpiece and a consumable electrode, and carbon dioxide gas is discharged to the workpiece so as to surround the consumable electrode while welding; a second welding step of generating an arc between the welded portion formed in the first welding step and a non-consumable electrode, and welding the workpiece while discharging a shielding gas made of an inert gas to surround the non-consumable electrode, The welding method according to claim 1, wherein the second welding step is performed without using a filler metal.

7. The welding method according to claim 6, A welding method comprising the steps of: setting an amount of carbon dioxide gas in the first welding step so that the oxygen concentration in the weld formed in the first welding step is 900 ppm to 1800 ppm.

8. The welding method according to claim 6, a consumable electrode containing an oxygen component is used in the first welding step, and an amount of carbon dioxide gas and an amount of the oxygen component contained in the consumable electrode are set so that the oxygen concentration of the weld formed in the first welding step is 900 ppm to 1800 ppm.

Images