Stainless steel welded joint, stainless steel welding method and automatic welding equipment

Abstract

To provide a stainless-steel weld joint having a weld zone with excellent quality.SOLUTION: A weld joint includes a base material comprising a stainless steel which contains Cr of 18 mass% or more, and a weld metal part. A relationship between N content [N]W of the weld metal part and N content [N]B of the base material satisfies [N]W / [N]B≥1.00, an entrainment number of dissimilar metals into the weld metal part is 1 or less per a length of 1000 mm in a weld direction of the weld metal part.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a stainless steel welded joint, a stainless steel welding method, and an automatic welding apparatus. [Background technology]

[0002] Duplex stainless steels, such as SUS329J3L, have excellent strength and corrosion resistance and are made of a two-phase structure of ferrite and austenite, with the main elements Cr, Ni, Mo, and N. For this reason, they are used in a variety of fields, including river infrastructure facilities, chemical plants, food manufacturing plants, water storage tanks, and seawater desalination plants.

[0003] In addition, austenitic stainless steels such as SUS312L and SUS836L have high corrosion resistance due to their high content of Cr, Ni, Mo, N, etc. For this reason, austenitic stainless steels are used in severe corrosive environments with high chloride ion concentrations, such as marine structures, chemical plants, and food tanks.

[0004] Non-consumable electrode welding methods such as TIG welding and plasma welding are used for the construction of stainless steel welded structures. Although non-consumable electrode welding methods are inferior to consumable electrode welding in terms of welding efficiency, they use pure Ar gas as the shielding gas, resulting in an extremely low amount of oxygen in the weld metal, excellent toughness, and the formation of high-quality weld metal with few inclusions or bubble defects, making them suitable for the construction of welded structures with strict quality requirements.

[0005] Many of the aforementioned duplex stainless steels and austenitic stainless steels contain N as a major element, but when they are non-consumable electrode welded using pure Ar shielding gas, N vaporizes during melting, causing the N content of the weld metal to be lower than that of the base metal, which can lead to reduced mechanical properties and corrosion resistance. To prevent this, there is a method for non-consumable electrode welding using a shielding gas mixed with nitrogen.

[0006] As an example of a method for welding duplex stainless steel, the following Patent Document 1 describes a welding method in which, when welding duplex stainless steel by a non-consumable electrode welding method, gases containing 60 to 100% nitrogen are used as a torch shielding gas, a back shielding gas, and an after-shielding gas. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-74738 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when non-consumable electrode gas-shielded arc welding is performed using a torch shielding gas mainly composed of nitrogen, there is a problem that the electrode melts and is rapidly consumed. In addition, if the melted and consumed tungsten electrode gets mixed into the weld metal, a welding defect called dissimilar metal inclusion occurs, and the quality of the welded joint is reduced.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a stainless steel welded joint having excellent weld quality. Another object of the present invention is to provide a stainless steel welding method and automatic welding device which suppress melting and consumption of the electrodes and do not deteriorate the quality of the weld. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention employs the following configuration. [1] A steel plate having a base material made of stainless steel containing 18% or more by mass of Cr and a weld metal part, The relationship between the N content [N]W of the weld metal portion and the N content [N]B of the base metal satisfies [N]W / [N]B≧1.00, The number of dissimilar metals in the welded metal part is 1 or less per 1000 mm of the length of the welded metal part in the welding direction. the law of nature, The dissimilar metal includes tungsten. , Stainless steel welded joints 。 [2] A method for welding a stainless steel containing 18% or more by mass of Cr as a base material by a non-consumable electrode gas-shielded arc welding method using a welding torch that can hold an electrode and eject a torch shielding gas without using a filler metal, The torch shield gas is a gas containing 65% by volume or more and 100% by volume or less of nitrogen, The electrode is a tungsten electrode, The welding torch is oriented in a direction opposite to the welding direction during welding (reverse welding), A method for welding stainless steel, the electrode having a retraction angle of 5° or more and 60° or less. [ 3 The electrode retreat angle is set to 5° or more and 30° or less. 2

[0023] A method for welding stainless steel as described in the above. [ 4 ] A pure tungsten electrode or an oxide-containing tungsten electrode is used as the electrode. 2 ]or[ 3

[0023] A method for welding stainless steel as described in the above. [ 5 ] The atmosphere at the welded part after welding is an after-shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen, [ 2 ]~[ 4 ] The method for welding stainless steel according to any one of the above items. [ 6 ] The atmosphere on the back side of the weld during and after welding is a back shield gas atmosphere containing 65 volume % to 100 volume % nitrogen. [ 2 ]~[ 5 ] The method for welding stainless steel according to any one of the above items. [ 7 An automatic welding apparatus for welding a base metal made of stainless steel containing 18% by mass or more of Cr by a non-consumable electrode gas-shielded arc welding method without using a filler metal, comprising: a welding torch that holds a tungsten electrode as an electrode and is capable of ejecting a torch shield gas; a shielding gas supply unit that supplies a gas containing 65 volume % or more and 100 volume % or less of nitrogen to the welding torch as the torch shielding gas, The automatic welding device has a welding torch oriented in a direction opposite to a welding progress direction, and a retraction angle of the electrode set in a range of 5° to 60°. [ 8 The electrode has a retreat angle in the range of 5° to 30°. 7

[0023] An automatic welding apparatus as described in the above. [ 9 The electrode is a pure tungsten electrode or an oxide-containing tungsten electrode. 7 ]or[ 8

[0023] An automatic welding apparatus as described in the above. [ 10 ] A first atmosphere adjusting unit is further provided for adjusting the atmosphere of the welded portion after welding to an after-shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen, [ 7 ]~[ 9 ] The automatic welding apparatus described in any one of the above. [ 11 ] A second atmosphere adjustment unit is further provided for adjusting the atmosphere on the back side of the weld during and after welding to a back shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen, [ 7 ]~[ 10 ] The automatic welding apparatus described in any one of the above. Effect of the Invention

[0011] According to the present invention, it is possible to provide a stainless steel welded joint having excellent quality of the weld. Furthermore, according to the present invention, it is possible to provide a method and an automatic welding apparatus for welding stainless steel, which suppress melting and consumption of the electrode and do not deteriorate the quality of the welded portion. [Brief description of the drawings]

[0012] [Figure 1]1 is a schematic diagram showing an example of an automatic welding device according to an embodiment of the present invention; [Diagram 2] FIG. 4 is a schematic diagram showing another example of an automatic welding device according to an embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram illustrating a method for welding stainless steel according to an embodiment of the present invention. [Figure 4] FIG. 2 is a schematic diagram illustrating a method for welding stainless steel according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present inventors have pursued the cause of the rapid melting and erosion of the electrode when non-consumable electrode gas-shielded arc welding is performed using a torch shielding gas mainly composed of nitrogen. It is known that nitrogen has a higher specific heat than argon, which has been used as a shielding gas in TIG welding, which is a type of non-consumable electrode gas-shielded arc welding. Due to this difference in specific heat, in non-consumable electrode gas-shielded arc welding using a gas mainly composed of nitrogen, the arc is constricted and the energy density is increased compared to when argon is used in the past. In addition, the flow rate of the plasma airflow generated around the arc is also increased. For this reason, it was found that spatter is easily scattered from the molten pool and a large amount of metal vapor tends to be generated. Here, in conventional non-consumable electrode gas-shielded arc welding, welding is performed with the welding torch standing vertically, and the electrode is located directly above the molten pool. For this reason, it has been found that in non-consumable electrode gas-shielded arc welding using a gas whose main component is nitrogen as the shielding gas, metal from spatter and metal vapor adheres to the electrode surface during welding, reducing the electrode's electron emission efficiency, resulting in an increase in electrode temperature and the electrode melting and being consumed.

[0014] As a result of extensive research, the inventors have discovered that by using reverse welding, in which the welding torch is oriented in the opposite direction to the welding progress direction, and by setting the reverse angle of the electrode to between 5° and 60°, adhesion of metal caused by spatter and metal vapor to the electrode surface can be prevented, melting and consumption of the electrode can be reduced, and inclusion of dissimilar metals in the weld metal can be reduced, thereby improving welding quality.

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A stainless steel welded joint, a stainless steel welding method, and an automatic welding apparatus according to embodiments of the present invention will be described below. Fig. 1 shows an automatic welding apparatus 1 according to the present embodiment. The automatic welding apparatus 1 shown in Fig. 1 includes a welding torch 2, a shielding gas supply unit 3 that supplies a torch shielding gas to the welding torch 2, a power source (not shown), and a control unit (not shown).

[0016] The automatic welding device 1 shown in Fig. 1 is provided with a conveying means 4 for conveying a base material 11 made of stainless steel in the opposite direction to the welding direction (the direction of arrow A in Fig. 1). The conveying means 4 may be a conveying roller as shown in Fig. 1, or a conveying stage capable of moving the base material 11 on which it is placed. Instead of the conveying means 4 for the base material 11, a driving means for moving a welding torch 2 in the welding direction (the direction of arrow B in Fig. 1) may be provided. The operation of the conveying means 4 or the driving means is controlled by a control unit.

[0017] The welding torch 2 is equipped with an electrode 5 and a gas nozzle 6 capable of ejecting torch shielding gas. The welding torch 2 is equipped with a hollow cylindrical torch housing 21 with one end 2a open, and the electrode 5 is held in a hollow portion 22 of the torch housing 21. The hollow portion 22 of the torch housing 21 also serves as a gas nozzle 6. When torch shielding gas is supplied from the shielding gas supply unit 3, the torch shielding gas can be ejected from the tip of the welding torch 2.

[0018] A power supply (not shown) is connected to the electrode 5. The power supply is connected to a control unit, and the operation of the power supply is controlled by the control unit. The electrode 5 generates an arc from its tip by the power supplied from the power supply.

[0019] The electrode 5 is a tungsten electrode. The tungsten electrode may be a pure tungsten electrode or a tungsten electrode containing an oxide, and is preferably a tungsten electrode containing an oxide because it has excellent electron emission ability. Examples of oxides include lanthana (lanthanum oxide), ceria (cerium oxide), and thorium oxide. The content of these oxides is in the range of 0.1 to 5 mass%.

[0020] In automatic welding equipment 1, the tip of welding torch 2 is oriented in the opposite direction to the welding progress direction (the direction of arrow B in FIG. 1). The retraction angle of electrode 5 is set in the range of 5° to 60°. The attitude of welding torch 2 and the retraction angle of electrode 5 will be described later.

[0021] The shielding gas supply unit 3 supplies torch shielding gas to the welding torch 2. As the torch shielding gas, a gas containing 65 volume % or more and 100 volume % or less of nitrogen is preferable, a mixed gas containing 65 volume % or more and less than 100 volume % of nitrogen with the balance being argon is more preferable, a mixed gas containing 95 volume % or more and less than 100 volume % of nitrogen with the balance being argon is even more preferable, and nitrogen gas with a nitrogen content of 100 volume % is more preferable. The shielding gas supply unit 3 is connected to the control unit, and its operation is controlled by the control unit.

[0022] The control unit controls the shielding gas supply unit 3, the power source, the conveying means 4, or the driving means to perform automatic welding on the base material 11 by the non-consumable electrode gas-shielded arc welding method.

[0023] Furthermore, the automatic welding apparatus 1 of this embodiment may be provided with a first atmosphere adjustment unit 41 and a second atmosphere adjustment unit 51, as shown in Fig. 2. Note that, although both the first atmosphere adjustment unit 41 and the second atmosphere adjustment unit 51 are illustrated in Fig. 2, either the first atmosphere adjustment unit 41 or the second atmosphere adjustment unit 51 may be provided. The first atmosphere adjustment unit 41 and the second atmosphere adjustment unit 51 are connected to a control unit (not shown), and their operations are controlled by the control unit.

[0024] The first atmosphere adjustment unit 41 is composed of a first gas box 42 and a first supply unit 43 that supplies an after-shield gas to the first gas box 42. The first gas box 42 is disposed at a position that covers the welded portion from the surface 11a side of the base material 11 after welding. The surface 11a of the base material 11 refers to the surface facing the welding torch 2. The first gas box 42 is formed of a hollow box-shaped body, and the base material 11 side is opened. By supplying the after-shield gas to the inside of the first gas box 42, it is possible to make the atmosphere on the surface 11a side of the welded portion after welding an after-shield gas atmosphere. The after-shield gas is a gas containing 65 volume % or more and 100 volume % or less of nitrogen. As a result, the welded portion immediately after welding is made into an after-shield gas atmosphere, contact of the outside air with the welded portion is avoided, and oxidation of the welded portion is reduced.

[0025] The second atmosphere adjustment unit 51 is composed of a second gas box 52 and a second supply unit 53 that supplies a back shield gas to the second gas box 52. The second gas box 53 is disposed at a position that covers the welded portion from the back surface 11b side of the base material 11 during and after welding. The back surface 11b of the base material 11 is the opposite surface to the front surface 11a. The second gas box 52 is made of a hollow box-shaped body, and the base material 11 side is opened. By supplying the back shield gas to the inside of the second gas box 52, it is possible to make the atmosphere on the back surface 11b side of the welded portion during and after welding a back shield gas atmosphere. The back shield gas is a gas containing 65 volume % or more and 100 volume % or less of nitrogen. In place of the second gas box 52, a gas nozzle that sprays the back shield gas toward the back surface 11b side of the welded portion during and after welding may be used.

[0026] The after-shield gas and the back-shield gas may be a mixed gas containing 65 volume % or more and less than 100 volume % nitrogen with the balance being argon, or a mixed gas containing 95 volume % or more and less than 100 volume % nitrogen with the balance being argon, or may be nitrogen gas with a nitrogen content of 100 volume %.

[0027] The automatic welding device 1 of this embodiment is capable of automatic welding in which the necessary welding parameters are mechanically or electrically controlled. The automatic welding device 1 of this embodiment is also capable of fully automatic welding in which the welding work is performed without the intervention of a welding operator. Furthermore, the automatic welding device 1 of this embodiment is not limited to the form shown in Fig. 1 or Fig. 2, and may be one in which welding is performed while controlling the welding torch 2 by a robot device.

[0028] Next, the welding method for stainless steel according to this embodiment will be described with reference to Figures 1 to 4. The welding method according to this embodiment can be carried out by, for example, an automatic welding device 1 shown in Figure 1 or Figure 2.

[0029] The stainless steel welding method of this embodiment is a method in which stainless steel containing 18 mass % or more of Cr is used as base metal 11, and a welding torch 2 that holds electrode 5 and can eject torch shielding gas is used to weld base metal 11 by non-consumable electrode gas-shielded arc welding without using a filler metal. Specifically, TIG welding can be applied as the non-consumable electrode gas-shielded arc welding method.

[0030] The base material 11 is stainless steel containing 18 mass % or more of Cr. Among stainless steels, stainless steels containing an austenite phase in the metal structure are particularly preferable, and more specifically, austenitic stainless steels or ferritic-austenitic duplex stainless steels are preferable.

[0031] In this embodiment, the shape of the base material 11 may be a plate material, a tube material, a bar material, a wire material, etc., and is not particularly limited. However, since the welding method of this embodiment is performed without using a filler metal, it is preferable that the plate material has a small thickness, the tube material has a small wall thickness, and the bar material and wire material have a small diameter. For example, in the case of a plate material, a steel plate with a thickness of 6 mm or less is preferable. In the case of a tube material, a steel pipe with a wall thickness of 3 mm or less is preferable. In the case of a bar material or a wire material, a diameter of 6 mm or less is preferable.

[0032] The welding method of the present embodiment may be applied to, for example, butt welding or fillet welding. In the case of butt welding, examples of the butt welding include butt welding of steel plates and butt welding of ends of steel pipes.

[0033] In the welding method of this embodiment, the groove shape does not need to be particularly limited.

[0034] In the welding method of this embodiment, the electrode 5 is a tungsten electrode. The tungsten electrode may be a pure tungsten electrode or a tungsten electrode containing oxide. By using a tungsten electrode with excellent electron emission efficiency as the electrode, an arc can be generated stably.

[0035] In the welding method of this embodiment, the torch shielding gas is a gas containing 65 volume % or more and 100 volume % or less of nitrogen. Preferably, the torch shielding gas is a mixed gas consisting of 65 volume % or more and less than 100 volume % of nitrogen and the balance argon, more preferably, a mixed gas consisting of 95 volume % or more and less than 100 volume % of nitrogen and the balance argon, and even more preferably, a nitrogen gas with a nitrogen content of 100 volume %. These gases may contain impurities. A more preferred gas is pure nitrogen gas (purity 99.995% or more (JIS K 1107:2005)), which is easy to obtain.

[0036] In the welding method of this embodiment, as shown in Fig. 3 and Fig. 4, backward welding is performed in which the tip of the welding torch 2 during welding is directed in the opposite direction to the welding advance direction C, and the backward angle θ of the electrode 5 is set to 5° to 60°, more preferably 5° to 30°. As shown in Fig. 4, a molten pool P is formed by an arc A emitted from the electrode 5, but by performing the welding by the welding torch 2 as backward welding, the electrode 5 moves away from the molten pool P as the welding advances. In addition, by tilting the electrode 5 by 5° or more toward the welding advance direction, the electrode 5 is positioned diagonally above the molten pool P. This reduces the risk of spatter S and metal vapor V generated from the molten pool P adhering to the surface of the electrode 5.

[0037] If the retraction angle θ of the electrode 5 is less than 5°, adhesion of spatters S and metal vapor V to the surface of the electrode 5 is unavoidable even when reverse welding is performed. Furthermore, if the retraction angle θ of the electrode 5 is more than 60°, the reach of the arc A from the electrode 5 to the surface 11a of the base metal 11 becomes long, and sufficient heat is not transferred to the base metal 11, making welding difficult. Therefore, the retraction angle θ of the electrode 5 is preferably in the range of 5 to 60°, and more preferably in the range of 5 to 30°.

[0038] In this embodiment, in order to further suppress the decrease in the nitrogen content in the weld metal, the atmosphere at the welded portion after welding is preferably an after-shield gas atmosphere containing 65% to 100% by volume of nitrogen. Furthermore, the atmosphere on the back side of the welded portion during and after welding may be a back-shield gas atmosphere containing 65% to 100% by volume of nitrogen. The range in which the back-shield gas atmosphere or the after-shield gas atmosphere is used is preferably a region including the weld metal whose temperature is 1000°C or higher. This brings the release of nitrogen from the weld metal and the absorption of nitrogen from the atmosphere into equilibrium, suppressing the decrease in the nitrogen content in the weld metal.

[0039] As described above, in the welding method of the present embodiment, the torch shield gas is a gas containing 65 volume % or more and 100 volume % or less of nitrogen, so that N 2 The monoatomic nitrogen (N) formed by the dissociation of N is absorbed into the molten metal, increasing the N content of the weld metal. 2 The increased partial pressure suppresses the release of N from the outermost surface of the weld metal by diffusion within the solid phase during the cooling process after solidification. 2 The higher the partial pressure, the more the release of N from the outermost surface of the weld metal is suppressed. This makes it possible to suppress the release of nitrogen from the weld metal during welding, and to prevent a decrease in the corrosion resistance of the weld metal. 2 In a gas mixture with Ar as the balance, N 2 Because the concentration is low, the N content of the weld metal cannot be increased sufficiently.

[0040] On the other hand, in non-consumable electrode gas-shielded arc welding using a gas containing 65% or more by volume of nitrogen as the torch shield gas, the arc is constricted and the energy density increases. In addition, the flow rate of the plasma airflow generated around the arc also increases. For this reason, spatter is likely to fly from the molten pool, and a large amount of metal vapor tends to be generated. If metal derived from spatter or metal vapor adheres to the surface of the electrode 5, the electron emission efficiency of the electrode 5 decreases, the electrode temperature increases, and the electrode 5 may melt and wear out. Therefore, in this embodiment, backward welding is performed while the backward angle θ of the electrode 5 is set to 5° or more and 60° or less. As a result, there is less risk that the spatter S and metal vapor V generated from the molten pool P will adhere to the surface of the electrode 5, and the electrode temperature rise is suppressed without decreasing the electron emission efficiency of the electrode, and the electrode melt and wear out is suppressed.

[0041] In addition, by making the atmosphere around the weld metal during or after welding a back-shielding gas atmosphere or an after-shielding gas atmosphere, the release of nitrogen from the weld metal in a high-temperature state immediately after welding is suppressed, and nitrogen, which is an austenite-stabilizing element, is less likely to decrease, making it possible to suppress deterioration in the mechanical properties and corrosion resistance of the weld metal.

[0042] Next, a stainless steel welded joint produced by the welding method of this embodiment will be described. The welded joint of the present embodiment includes a base material made of stainless steel containing 18 mass% or more of Cr, and a weld metal part, and the N content [N] of the weld metal part is W and N content of the base material [N] B Relationship with [N] W / [N] B ≧1.00, and the number of dissimilar metals with a diameter of 0.5 mm or more enmeshed in the weld metal part is 1 or less per 1000 mm of the length of the weld metal part in the welding direction. The dissimilar metal is tungsten.

[0043] In the welding method of this embodiment, a gas containing 65% to 100% by volume of nitrogen is used as the torch shielding gas. This prevents the evaporation of N from the molten metal that occurs in non-consumable electrode welding using argon as the torch shielding gas, and instead supplies N to the molten metal from the torch shielding gas. This reduces the N content [N] of the weld metal. W and the N content of the base material [N] B Relationship with [N] W / [N] B ≧1.00, and even if there is concern about a decrease in the properties of the weld metal (e.g., a decrease in strength or corrosion resistance) due to a decrease in the N content depending on the steel type, this decrease in properties is prevented in advance. Furthermore, in the welding method of this embodiment, melting and consumption of the electrode 5 is suppressed, as described above. Therefore, in the welded joint of this embodiment, a portion of the molten electrode 5 does not mix with the welded metal. For the above reasons, the welded joint of this embodiment can significantly improve the quality of the welded metal.

[0044] The welded joint of the present embodiment may be a welded joint formed by butt welding, or may be a joint welded by lap welding, fillet welding, etc. Examples of the welded joint formed by butt welding include a welded joint formed by butt welding steel plates together, and a welded joint formed by butt welding ends of steel pipes together.

[0045] Furthermore, as described above, the base material 11 in the welding method and welded joint of this embodiment is a stainless steel containing 18 mass% or more of Cr, and is preferably a stainless steel containing an austenitic phase in the metal structure, and more specifically, is preferably an austenitic stainless steel or a ferritic-austenitic duplex stainless steel.

[0046] Among the chemical components of austenitic stainless steels and ferritic-austenitic duplex stainless steels, the chemical components other than Cr are not particularly limited.

[0047] In stainless steel, Cr has the effect of increasing the corrosion resistance of the passive film of stainless steel. In addition, the composition of the torch shielding gas in the non-consumable electrode gas-shielded arc welding method is changed to N. 2 When the Cr content is 60 to 100% by volume, the N content in the weld metal increases significantly, which makes it easy for pore defects (blowholes, porosity) to occur. Cr has the effect of increasing the amount of N dissolved in the ferrite phase, suppressing solidification microsegregation of N, and suppressing pore defects. Since these effects cannot be sufficiently obtained when the Cr content is less than 18%, the Cr content is set to 18% by mass or more, and more preferably 20% by mass or more. There is no particular upper limit for the Cr content, but it is preferably set to 28% by mass or less from the viewpoint of cost.

[0048] As mentioned above, there is no particular limitation on the alloy elements other than Cr, but the duplex stainless steel to which this embodiment can be applied has, for example, a chemical composition in mass %, such as C: 0.001-0.030%, Si: 1.5% or less, Mn: 0.1-6.0%, P: 0.04% or less, S: 0.0100% or less, Ni: 0.1-8.0%, Cr: 18-28%, Mo: 0.1-5.0%, Cu: 0.1-2.0%, N: 0.1-0.4%, with the balance being Fe and impurities. This chemical composition is merely an example, and this embodiment is not limited thereto.

[0049] Furthermore, the austenitic stainless steel to which this embodiment can be applied contains, for example, in mass%, 18-30% Cr, 6-30% Ni, 0.01-8% Mo, 0.001-0.4% N, with the balance being Fe and impurities, and may further contain, as necessary, 0.001-0.10% C, 0.1-4.0% Si, 0.1-8.0% Mn, 0.04% or less P, 0.01% or less S, and 0.01-4.00% Cu.

[0050] As described above, according to this embodiment, it is possible to provide a stainless steel welded joint having excellent quality of the weld. Furthermore, according to the present embodiment, it is possible to provide a method and an automatic welding apparatus for welding stainless steel that suppress melting and consumption of the electrodes and does not deteriorate the quality of the welded portion. EXAMPLES

[0051] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0052] Stainless steel having the chemical composition shown in Table 1 was melted in the laboratory and then hot forged, hot rolled, cold rolled, and solution heat treated to produce welding steel plates (base material) with a thickness of 2 mm, width of 50 mm, and length of 600 mm.

[0053] Bead-on-plate welding was performed at the width center of the steel plate to be welded, using the TIG welding method with a tungsten electrode without using any welding material (filler metal), so that the total weld length was 1000 mm, under the conditions shown in Table 2. The welding was performed by reverse welding, and the electrode reverse angle, welding current, and welding speed were as shown in Table 2. The tungsten electrode was a tungsten electrode containing 2% lanthana. The area to which the back shielding gas and after shielding gas were sprayed was the area including the weld metal with a temperature of at least 1000°C or higher.

[0054] The N content of the base metal and weld metal is measured in accordance with JIS G 1228:1997, Annex 4 (Regulations) Inert Gas Fusion - Thermal Conductivity Method (1), and the N content of the weld metal [N] W and N content of the base material [N] B Ratio [N] W / [N] B asked for.

[0055] [Electrode melting and wear] After TIG welding was performed under the conditions shown in Table 2 so that the total weld length was 1000 mm, the electrode tip was observed under a microscope at 50x magnification. If no melting deformation occurred at the electrode tip, it was determined that no electrode wear occurred and was marked as "Good." If melting deformation occurred, it was marked as "Poor."

[0056] [Inclusion of dissimilar metals] X-ray examination was used to analyze whether or not dissimilar metals were entrapped in the weld metal. Tests were performed on 1000mm long weld metal, and when there was one or less piece of dissimilar metal with a diameter of 0.5mm or more, it was determined that there was almost no entrapment and was marked as "Good." When there was more than one piece of dissimilar metal with a diameter of 0.5mm or more, it was marked as "Poor."

[0057] The results are shown in Table 2.

[0058] As shown in Table 2, in all of Examples 1 to 16, the torch shielding gas contained 65% to 100% nitrogen by volume, and the electrode retreat angle was in the range of 5° to 60°, so that the electrode was prevented from being consumed and melted, and the inclusion of dissimilar metals in the weld metal was suppressed. In addition, in Examples 1 to 16, the N content [N] of the weld metal was reduced by using a gas containing 65% or more nitrogen by volume for the torch shielding gas. W is the N content of the base material [N] B More than 1.00 times ([N] W / [N] B ≧1.00), and no deterioration in the properties of the weld metal was confirmed.

[0059] On the other hand, in Comparative Examples 17 to 20, the torch shield gas was 100 volume % nitrogen gas and the electrode retreat angle was 0°, so that the tungsten electrode was consumed and melted, causing inclusion of dissimilar metals, and increasing welding defects.

[0060] [Table 1]

[0061] [Table 2] [Explanation of symbols]

[0062] 1...automatic welding device, 2...welding torch, 3...shielding gas supply unit, 4...transport means, 5...electrode, 6...gas nozzle, 11...base material, 41...first atmosphere adjustment unit, 51...second atmosphere adjustment unit.

Claims

1. The steel has a base material made of stainless steel containing 18% by mass or more of Cr, and a weld metal part, The relationship between the N content [N]W of the weld metal portion and the N content [N]B of the base metal is [N] Ｗ / [N] Ｂ ≧1.00 is satisfied, The number of inclusions of dissimilar metals in the weld metal part is 1 or less per 1000 mm of length in the welding direction of the weld metal part, 1. A stainless steel welded joint, wherein the dissimilar metals include tungsten.

2. A method for welding a base material made of stainless steel containing 18% by mass or more of Cr by a non-consumable electrode gas-shielded arc welding method using a welding torch that holds an electrode and can eject a torch shielding gas without using a filler metal, comprising: The torch shield gas is a gas containing 65% by volume or more and 100% by volume or less of nitrogen, The electrode is a tungsten electrode, The welding torch is oriented in a direction opposite to the welding direction during welding (reverse welding), The method for welding stainless steel, wherein the electrode retreat angle is 5° or more and 60° or less.

3. 3. The method for welding stainless steel according to claim 2, wherein the electrode retreat angle is 5° or more and 30° or less.

4. 4. The method for welding stainless steel according to claim 2, wherein a pure tungsten electrode or an oxide-containing tungsten electrode is used as the electrode.

5. 5. The method for welding stainless steel according to claim 2, wherein the atmosphere at the welded portion after welding is an after-shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen.

6. 6. The method for welding stainless steel according to claim 2, wherein the atmosphere on the back side of the weld during and after welding is a back shielding gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen.

7. An automatic welding apparatus for welding a base metal made of stainless steel containing 18% by mass or more of Cr by a non-consumable electrode gas-shielded arc welding method without using a filler metal, comprising: a welding torch that holds a tungsten electrode as an electrode and is capable of ejecting a torch shield gas; a shielding gas supply unit that supplies a gas containing 65 volume % or more and 100 volume % or less of nitrogen to the welding torch as the torch shielding gas, The automatic welding device has a welding torch oriented in a direction opposite to a welding progress direction, and a retraction angle of the electrode set in a range of 5° to 60°.

8. 8. The automatic welding apparatus according to claim 7, wherein the electrode rearward angle is in the range of 5 degrees to 30 degrees.

9. 9. The automatic welding apparatus according to claim 7 or 8, wherein the electrode is a pure tungsten electrode or a tungsten electrode containing oxide.

10. 10. The automatic welding apparatus according to claim 7, further comprising a first atmosphere adjustment unit for adjusting the atmosphere of the welded portion after welding to an after-shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen.

11. 11. The automatic welding apparatus according to claim 7, further comprising a second atmosphere adjustment unit for adjusting the atmosphere on the back side of the weld during and after welding to a back shield gas atmosphere containing 65 volume % or more and 100 volume % or less of nitrogen.

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