TIG welding method

Abstract

Provided is a TIG welding method that provides excellent SCC resistance with respect to ammonia. A TIG welding method according to the present invention is for performing multi-layer welding on a steel material, and involves, by using a first welding wire having a specific chemical composition as a welding wire and a second welding wire having a different specific chemical composition having a lower C content than the chemical composition of the first welding wire, welding an initial layer and layers subsequent to the initial layer by using the first welding wire, and welding the final layer by using the second welding wire.

Description

TIG welding method

[0001] The present invention relates to a TIG welding method for steel materials, and more particularly to a TIG welding method for forming a welded joint that has excellent SCC resistance to ammonia.

[0002] The TIG welding method uses tungsten, a non-consumable material, as an electrode rod, and welds steel materials by melting a welding wire (i.e., filler metal) in an arc while blowing inert gases, typically argon gas or helium gas, onto the steel material to block the atmosphere.

[0003] Ammonia is relatively easy to liquefy under atmospheric pressure, and as such, it has attracted attention as a hydrogen carrier for realizing a decarbonized society. Technological development is underway to co-combine ammonia with coal in thermal power plants and as a marine fuel. Using ammonia as a fuel for thermal power plants requires larger ammonia storage tanks. Because ammonia is highly toxic, extreme care must be taken to prevent leakage from the tanks. Carbon steel used in the construction of ammonia tanks is susceptible to stress corrosion cracking (hereinafter also referred to as "SCC") in liquid ammonia, and the exact cause of the cracking mechanism has not yet been elucidated.

[0004] However, it is believed that there is a correlation between the occurrence of cracks and the surface hardness (HV10), which is the Vickers hardness of the surface layer of the steel material and the weld metal. It is known that when the average value of the surface hardness (HV10) of these and the ammonia contact surface exceeds 220, the occurrence of SCC increases significantly, and cracks are particularly common near the weld metal.

[0005] Ammonia is mainly used in the production of chemical fertilizers. The largest tank in Japan has a capacity of about 15,000 tons, and due to the required strength level, SLA325AN, a carbon steel plate for low-temperature pressure vessels that has been normalized, is often used as the material for the tank. In addition, during welding, a general 50 kgf / mm2 strength is used depending on the strength of the steel plate. 2Welding materials with a strength of 490 MPa or higher are used, and care is taken not to reduce the hardness of the weld. As a result, the weld metal tends to harden, with an average surface hardness (HV10) of around 250 to 280. For this reason, liquid ammonia tanks are subject to periodic open inspections, and cracks are detected and repaired during the inspections.

[0006] In response to this situation, for example, Patent Document 1 discloses an invention for a steel plate that aims to limit yield strength and improve low-temperature toughness by adjusting chemical components and improving manufacturing methods. However, Patent Document 1 does not consider a method for suppressing SCC in weld metal parts where SCC due to ammonia is particularly likely to occur.

[0007] Furthermore, as a TIG welding method using a low-carbon welding wire, for example, Patent Documents 2 and 3 disclose that the strength and toughness of the weld metal are improved by using a welding wire having a specific chemical composition.

[0008] Patent Document 4 discloses a method for welding austenitic stainless steel pipes, in which an austenitic stainless steel weld metal is joined to the root side of the groove and a martensitic stainless steel weld metal is joined to the remaining groove portion above the austenitic stainless steel weld metal. This joining hardens the surface layer of the weld metal, suppresses distortion on the inner surface of the pipe, and improves fatigue strength.

[0009] Japanese Patent No. 5428999 Japanese Patent Publication No. 159293 / 1982 Japanese Patent No. 6829111 Japanese Patent No. 108838 / 1983

[0010] However, in welded joints using these conventional carbon steel materials and welding wires, the problem of SCC formation remains unresolved due to the inevitable hardening of the weld metal surface layer regardless of the welding conditions. Because it is difficult to completely prevent SCC formation, tanks must be periodically inspected, and welding repairs are currently performed whenever cracks are discovered during the inspection. It is expected that ammonia will become an important fuel in the future. To ensure a stable energy supply, large amounts of ammonia will need to be stored, and larger ammonia land tanks are required. Increasing the size of tanks using conventional welding techniques will increase defects such as SCC due to ammonia, and welding repairs will require significant effort. Therefore, there is an urgent need to obtain welded joints with high SCC resistance. Multi-layer welding using welding wire with low alloying elements reduces the hardness of the weld metal surface layer, but it is necessary to limit heat input to prevent a decrease in joint strength and toughness, preventing improved construction efficiency.

[0011] An object of the present invention is to provide an economically advantageous TIG welding method that suppresses the average surface hardness (HV10) of the weld metal in multi-layer welding to 220 or less by using appropriate welding conditions, thereby making it less likely to cause ammonia-induced SCC.

[0012] The present inventors have conducted extensive research to achieve the above object and have obtained the following findings.

[0013] In general multi-layer welding, unless the steel material is a combination of multiple steel materials such as clad steel or an austenitic stainless steel pipe (see, for example, Patent Document 4 mentioned above), welding is performed from the first layer to the last layer using a single type of welding wire selected according to the strength grade of the steel material. Depending on the chemical composition of the welding wire, some welding wires improve the mechanical properties of the weld metal when reheated, while others have good mechanical properties in the as-welded state but deteriorate when reheated. Meanwhile, attention has been paid to the fact that the weld metal portion of a multi-layer welded joint has portions that are reheated and portions that are not reheated as-welded. That is, from the first layer to the layer before the last layer, the weld metal is cooled to room temperature after the weld metal from the welding wire melts and solidifies, and then reheated by subsequent welding. In contrast, for the final layer, once the weld metal is cooled to room temperature after melting and solidifying, the weld metal is used as a structure without being reheated unless post-weld heat treatment is performed.

[0014]

[0004] Because the thermal history applied to the final layer and the other layers (i.e., the layers from the initial layer to the layer before the final layer) is different in this way, when a joint is produced using one type of welding material, it will inevitably include a portion whose mechanical properties deteriorate due to the thermal history. By adding alloying elements such as Ni and Mo to the welding wire, it is possible to obtain good mechanical properties in both the as-welded portion that is not reheated and the portion that is reheated, but such alloying elements are expensive, which leads to a rise in the cost of the welding wire.

[0015] Based on the above-mentioned investigations, the present inventors have arrived at a welding method that can achieve both limited surface layer hardness and good low-temperature toughness in the weld metal zone by using different welding wires for the portion that is subjected to reheating in the welding of carbon steel and the final layer that is not subjected to reheating, that is, by using a combination of inexpensive welding wires. [1] A TIG welding method for forming a multi-layer weld metal on a steel material using a shielding gas made of an inert gas and a welding wire, wherein the welding wire includes a first welding wire and a second welding wire having different chemical compositions, and the chemical composition of the first welding wire contains, in mass%, C: 0.05 to 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, a chemical composition of the second welding wire containing, in mass %, C: 0.01 to 0.04%, Si: 0.02 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; the first welding wire is used for welding a first layer and layers thereafter of the multilayer structure; and the second welding wire is used for welding a final layer of the multilayer structure. [2] The TIG welding method according to [1], further containing, in addition to the chemical composition of the second welding wire, one or more selected from, by mass%, Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.030 to 0.190%, Zr: 0.01 to 0.15%, and Nb: 0.001 to 0.015%. [3] The TIG welding method according to [1] or [2], wherein welding of at least one layer of the multiple layers is weaving welding.[4] The TIG welding method according to any one of [1] to [3], wherein at least one layer of the multilayer structure is welded by hot wire welding. [5] The absorbed energy (vE) of the weld metal in a Charpy impact test at -45°C. -45 [6] The TIG welding method according to any one of [1] to [5], wherein the average surface hardness (HV10) of the weld metal is 220 or less.

[0016] The TIG welded joints obtained by the present invention exhibit high mechanical properties and high SCC resistance as welded joints for land-based ammonia tanks, while using inexpensive welding materials. This reduces the frequency of repairs for cracks inside the tank, providing significant industrial benefits.

[0017] Fig. 1 is a schematic cross-sectional view showing an example of a groove shape in a TIG welding method of the present invention. Fig. 2 is a schematic cross-sectional view showing an example of a test piece collection position for a Charpy impact test. Fig. 3 is a schematic cross-sectional view showing an example of a measurement position for a Vickers hardness test. Fig. 4 is a distribution diagram showing the results of a measurement example of a Vickers hardness test in this example. Fig. 5 is a schematic view showing an example of weaving welding. Fig. 6 is a schematic view showing an example of hot wire welding.

[0018] [Carbon Steel Material] In the present invention, TIG welding is preferably performed on a carbon steel material for an ammonia tank as a base material. The steel material may be a steel plate, and preferably includes carbon steel plates for low-temperature pressure vessels such as SLA325AN, SLA325ATMC, and SLA365.

[0019] The base material has a chemical composition that includes, for example, 0.030 to 0.090% C, 0.50% or less Si, 0.50 to 1.60% Mn, 0.015% or less P, and 0.010% or less S. It also contains 0.060% or less Al, 0.0010 to 0.0100% N, and 0.0100% or less O (oxygen). Here, "%" in relation to the chemical composition means "% by mass" (hereinafter the same applies unless otherwise specified).

[0020] The base metal further contains, as necessary, one or more of the following component elements. Since these can be contained as necessary, the content of the following component elements may be 0%. That is, the base metal contains, as necessary, one or more of Cu: 2.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Nb: 0.10% or less, Ti: 0.005 to 0.100%, Ca: 0.2000% or less, Mg: 0.0200% or less, and B: 0.0100% or less.

[0021] The remainder of the chemical composition of the base material other than the above-mentioned chemical composition is composed of Fe and unavoidable impurities, such as Sn, Sb, As, Pb, and Bi, and the total content of these elements is allowable up to 0.10%.

[0022] [Multi-layer weld metal] In the present invention, the weld metal formed by welding the above-mentioned steel materials needs to be multi-layered. If the weld metal is a single layer, there are no second or subsequent layers, and a second welding wire cannot be used, so the effects of the present invention cannot be achieved.

[0023] [TIG welding] As mentioned above, TIG welding is a method of welding steel materials by using a non-consumable tungsten electrode rod, blowing Ar gas or He gas onto the steel material while blocking air, and melting a filler material (welding wire) in an arc. This TIG welding method can be applied to various alloy steels and non-ferrous metals, and can also weld complex shapes, resulting in excellent weld quality.

[0024] An example of a TIG welding method according to the present invention will be described. As shown in Figure 1, steel plates or steel materials (thickness t: 3 to 38 mm) that serve as base materials 1 are butted together in accordance with JIS Z 3111, and a copper backing plate 3 is used to form a V-groove 2 with a groove angle θ of 50°. The groove angle θ is a value in the range of 10° to 60°.

[0025] A pure tungsten rod (e.g., 3.2 mm in diameter) is used as the electrode, and an arc is generated between the base metal and the electrode. The base metal is melted by the high-temperature arc, and a welding wire (e.g., 1.2 mm in diameter) is fed toward the molten pool using a wire feeder. The welding wire melts, solidifies, and becomes one with the base metal, forming a weld metal. The arc and weld metal are protected by an inert shielding gas during welding, resulting in high-quality weld metal.

[0026] Specific welding conditions include, for example, no preheating before welding, a downward position, a current of 150 to 300 A (DCEN), a voltage of 8 to 15 V, a welding speed of 5 to 15 cm / min, and a welding heat input of 0.7 to 3.5 kJ / mm. Furthermore, it is preferable to perform welding under conditions of an interpass temperature of 100 to 150°C, a shielding gas of Ar, and a gas flow rate of 10 to 25 L / min. The wire feed speed is preferably 600 to 5000 mm / min.

[0027] Multi-layer welding under these welding conditions forms multi-layer weld metal in the V-groove. The number of layers depends on conditions such as plate thickness, but is preferably up to 10 layers, and the number of passes is preferably in the range of 1 to 2 passes per layer.

[0028] [Welding Wire] The welding wire used in the TIG welding method according to the present invention includes a first welding wire having a specific chemical composition and a second welding wire having another specific chemical composition with a lower content of C and alloying elements than the first welding wire. By using the first welding wire for welding the first and subsequent layers and the second welding wire for welding the final layer, it is possible to ensure the mechanical properties of the weld metal and suppress SCC due to ammonia without using expensive alloying elements. The chemical compositions of the first and second welding wires are described below.

[0029] [First welding wire] In the present invention, the first welding wire is used for welding the first layer and layers thereafter, i.e., layers other than the final layer. First, the reasons for limiting the composition ranges of the chemical composition of the first welding wire will be described.

[0030] [C: 0.05 to 0.11%] C must be present in an amount of 0.05% or more to obtain the necessary strength for the weld metal. However, if the C content exceeds 0.11%, the hardness increases and the toughness decreases. Therefore, the C content is limited to the range of 0.05 to 0.11%. Furthermore, the C content is preferably in the range of 0.06 to 0.10%, and more preferably in the range of 0.07 to 0.09%.

[0031] [Si: 0.30 to 0.90%] Si is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and a content of 0.30% or more is necessary to achieve these effects. On the other hand, if Si is contained in an amount exceeding 0.90%, the toughness of the weld metal deteriorates. Therefore, the Si content is limited to the range of 0.30 to 0.90%. The Si content is preferably in the range of 0.40 to 0.80%, and more preferably in the range of 0.50 to 0.70%.

[0032] [Mn: 0.50 to 1.80%] Mn is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and a content of 0.50% or more is necessary to achieve these effects. On the other hand, if the Mn content exceeds 1.80%, the toughness of the weld metal will be significantly deteriorated. Therefore, the Mn content is limited to the range of 0.50 to 1.80%. The Mn content is preferably in the range of 0.70 to 1.50%, and more preferably in the range of 1.00 to 1.30%.

[0033] [P: 0.025% or less] If the P content exceeds 0.025%, the toughness of the weld metal deteriorates, so the P content is limited to 0.025% or less. The P content is preferably 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the P content is not particularly limited, and the P content may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the P content is preferably permissible even if it is 0.001% or more.

[0034] [S: 0.035% or less] If S is contained in an amount exceeding 0.035%, the toughness of the weld metal deteriorates, so the S content is limited to 0.035% or less. The S content is preferably 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited, and the S content may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the S content is preferably acceptable to be 0.001% or more.

[0035] [Al: 0.04 to 0.19%] An Al content of 0.04% or more is necessary for deoxidation of the weld metal, but an Al content exceeding 0.19% deteriorates the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04 to 0.19%. The Al content is preferably 0.05 to 0.10%, and more preferably 0.06 to 0.08%.

[0036] [Cu: 0.60% or less] Cu has the function of increasing the strength while maintaining high toughness of the weld metal, but if it is contained in an amount exceeding 0.60%, it causes hot embrittlement and deteriorates the surface properties. Therefore, the Cu content is limited to 0.60% or less. The Cu content is preferably 0.40% or less. On the other hand, in order to further exert the above-mentioned strength-improving effect, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more.

[0037] [Ni: 0.80% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining high toughness. However, if the Ni content exceeds 0.80%, the hardness increases and the toughness decreases. Therefore, the Ni content is limited to 0.80% or less. The Ni content is preferably 0.60% or less. On the other hand, in order to further exert the above-mentioned strength-improving effect, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

[0038] [Cr: 0.50% or less] Like Cu, Cr increases the strength of the weld metal while maintaining high toughness. However, if the Cr content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Cr content is limited to 0.50% or less. The Cr content is preferably 0.30% or less. On the other hand, in order to further exert the above-mentioned strength-improving effect, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more.

[0039] [Mo: 0.50% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining high toughness, but if the Mo content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Mo content is limited to 0.50% or less. The Mo content is preferably 0.20% or less. On the other hand, in order to further exert the above-mentioned strength-improving effect, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[0040] [N: 0.0100% or less] N is an element contained as an unavoidable impurity, and if the N content exceeds 0.0100%, toughness will be reduced. Therefore, from the viewpoint of suppressing the deterioration of toughness and weldability, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, excessive reduction will lead to an increase in the refining cost of the wire material, so from the viewpoint of cost, the N content is preferably acceptable to be 0.0010% or more.

[0041] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an unavoidable impurity, and has adverse effects such as forming oxides and becoming the origin of fracture. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the O content is preferably acceptable to be 0.0020% or more.

[0042] [Remainder Composition of First Welding Wire] The remaining chemical composition of the first welding wire other than the above-described chemical composition of the first welding wire consists of Fe and inevitable impurities. Examples of the inevitable impurity elements include Sn, Sb, As, Pb, and Bi, and a total content of these elements of 0.10% or less is acceptable. Furthermore, as long as the above-described chemical composition of the first welding wire is satisfied, the first welding wire may contain inevitable impurity elements other than these, and such embodiments are also included in the technical scope of the present invention.

[0043] Furthermore, as long as the above-described range of component composition is satisfied, multiple types of welding wires may be used sequentially as the first welding wire. In this case, welding of layers other than the final layer may be performed using the multiple types of welding wires.

[0044] [Second Welding Wire] In the present invention, the second welding wire is used for welding the final layer. Next, the chemical composition of the second welding wire will be described. The chemical composition of the second welding wire includes a basic composition and an optional selected composition. First, the reasons for limiting each composition range for the basic composition will be described, and then the reasons for specifying each composition range for the optional selected composition will be described.

[0045] [Basic Composition of Second Welding Wire] The reasons for limiting the ranges of the basic composition of the second welding wire are as follows.

[0046] [C: 0.01 to 0.04%] C must be present in an amount of 0.01% or more to obtain the necessary strength for the weld metal. However, in the as-welded state, if the C content exceeds 0.04%, the hardness increases and the toughness decreases. Therefore, the C content is limited to the range of 0.01 to 0.04%. It is essential that the upper limit of the C content be lower than the lower limit of the C content of the first welding wire in order to limit the surface hardness of the weld metal. The C content is preferably in the range of 0.02 to 0.04%, and more preferably in the range of 0.02 to 0.03%.

[0047] [Si: 0.02 to 0.90%] Si is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal. To achieve these effects, a Si content of 0.02% or more is required. On the other hand, if the Si content exceeds 0.90%, the toughness of the weld metal deteriorates. Therefore, the Si content is limited to the range of 0.02 to 0.90%. The Si content is preferably in the range of 0.20 to 0.80%, and more preferably in the range of 0.40 to 0.70%.

[0048] [Mn: 0.50 to 1.80%] Mn is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and to achieve these effects, a Mn content of 0.50% or more is necessary. On the other hand, if the Mn content exceeds 1.80%, the toughness of the weld metal will be significantly deteriorated. Therefore, the Mn content is limited to the range of 0.50 to 1.80%. The Mn content is preferably in the range of 0.70 to 1.50%, and more preferably in the range of 1.00 to 1.30%.

[0049] [P: 0.025% or less] Since P in excess of 0.025% deteriorates the toughness of the weld metal, the P content is limited to 0.025% or less. The P content is preferably 0.010% or less, and more preferably 0.003% or less. The lower limit of the P content is not particularly limited and may be 0.000%. However, since P is an element that is usually inevitably contained in steel materials as an impurity, industrially, the P content may be greater than 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, and therefore, from the viewpoint of cost, the P content is preferably acceptable to be 0.001% or more.

[0050] [S: 0.035% or less] If S exceeds 0.035%, the toughness of the weld metal deteriorates, so the S content is limited to 0.035% or less. The S content is preferably 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the S content is preferably permissible even if it is 0.001% or more.

[0051] [Al: 0.04 to 0.19%] An Al content of 0.04% or more is necessary for deoxidation of the weld metal, but an Al content exceeding 0.19% deteriorates the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04 to 0.19%. The Al content is preferably 0.05 to 0.10%, and more preferably 0.06 to 0.08%.

[0052] [N: 0.0100% or less] N is an element contained as an unavoidable impurity, and if the N content exceeds 0.0100%, toughness will be reduced. Therefore, from the viewpoint of suppressing the deterioration of toughness and weldability, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, excessive reduction will lead to an increase in the refining cost of the wire material, so from the viewpoint of cost, the N content is preferably acceptable to be 0.0010% or more.

[0053] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an unavoidable impurity, and has adverse effects such as forming oxides and becoming the origin of fracture. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.0000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the O content is preferably acceptable to be 0.0020% or more.

[0054] [Optional Composition of Second Welding Wire] The reasons for specifying each composition range for the optional composition of the second welding wire are as follows. The chemical composition of the second welding wire may contain, in addition to the basic composition, one or more elements selected from Cu, Ni, Cr, Mo, V, Ti, Zr, and Nb within each composition range shown below. The optional composition may be contained as needed, and therefore the content thereof may be 0%.

[0055] [Cu: 0.30% or less] Cu has the function of increasing the strength while maintaining high toughness of the weld metal, but if the Cu content exceeds 0.30%, hot embrittlement occurs and the surface properties deteriorate. Therefore, when Cu is added, the Cu content is preferably 0.30% or less. The Cu content is more preferably 0.25% or less. On the other hand, when Cu is added, in order to further exert the above-mentioned strength-improving effect, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

[0056] [Ni: 0.40% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining high toughness, but if the Ni content exceeds 0.40%, the hardness increases too much. Therefore, when Ni is added, the Ni content is preferably 0.40% or less. The Ni content is more preferably 0.30% or less. On the other hand, when Ni is added, in order to further exert the above-mentioned strength-improving effect, the Ni content is preferably 0.01% or more, more preferably 0.10% or more.

[0057] [Cr: 0.20% or less] Like Cu, Cr increases the strength of the weld metal while maintaining high toughness, but if the Cr content exceeds 0.20%, the hardness increases too much. Therefore, when Cr is added, the Cr content is preferably 0.20% or less. The Cr content is more preferably 0.15% or less. On the other hand, when Cr is added, in order to further exert the above-mentioned strength-improving effect, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

[0058] [Mo: 0.20% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining high toughness, but if the Mo content exceeds 0.20%, the hardness increases too much. Therefore, when Mo is added, the Mo content is preferably 0.20% or less. The Mo content is more preferably 0.10% or less. On the other hand, when Mo is added, in order to further exert the above-mentioned strength-improving effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.

[0059] [V: 0.40% or less] V increases the strength and toughness of the weld metal, but if added in excess, it forms carbides and increases hardness. Therefore, when V is added, the V content is preferably 0.40% or less. The V content is more preferably 0.20% or less. On the other hand, when V is added, in order to further exert the effect of improving the strength and toughness, the V content is preferably 0.01% or more, and more preferably 0.05% or more.

[0060] [Ti: 0.030 to 0.190%] Ti is an element that precipitates as TiO2 during solidification, suppresses coarsening of austenite in the weld metal, and contributes to high toughness by acting as ferrite transformation nuclei. If the Ti content is less than 0.030%, this effect is small, but if the Ti content exceeds 0.190%, the amount of solute Ti increases, degrading toughness. Therefore, when Ti is added, the Ti content is preferably 0.030 to 0.190%. The Ti content is more preferably 0.050 to 0.100%.

[0061] [Zr: 0.01 to 0.15%] Zr is an element that has the effect of suppressing grain growth and contributes to improving the toughness of the weld metal. However, if the Zr content is less than 0.01%, it is difficult to achieve such an effect. Furthermore, if the Zr content is excessively high, exceeding 0.15%, the amount of dissolved Zr increases, deteriorating toughness. Therefore, when Zr is added, the Zr content is preferably 0.01 to 0.15%. The Zr content is more preferably 0.03 to 0.10%.

[0062] [Nb: 0.001 to 0.015%] Nb is an element that contributes to improving the strength of the weld metal through precipitation hardening. However, if the Nb content is less than 0.001%, it is difficult to obtain such an effect. If the Nb content exceeds 0.015%, carbides become coarse, deteriorating the toughness of the weld metal. Therefore, when Nb is added, the Nb content is preferably 0.001 to 0.015%. The Nb content is more preferably 0.010% or less. On the other hand, when Ni is added, in order to further exert the above-mentioned strength-improving effect, the Ni content is more preferably 0.002% or more, and even more preferably 0.004% or more.

[0063] [Remainder Composition of Second Welding Wire] The remaining chemical composition of the second welding wire other than the above-described chemical composition of the second welding wire consists of Fe and inevitable impurities. Examples of the inevitable impurity elements include Sn, Sb, As, Pb, and Bi, and the total content of these elements is acceptable as long as it is 0.10% or less. Furthermore, as long as the above-described chemical composition of the second welding wire is satisfied, it is not prohibited for inevitable impurity elements other than these to be contained in the second welding wire, and such embodiments are also included in the technical scope of the present invention.

[0064] [Weaving Welding] In the TIG welding method according to the present invention, it is preferable that at least one layer of the multiple layers be weaved. Here, weaving welding refers to a welding technique in which a torch 10 used in TIG welding is moved left and right relative to the weld line (i.e., the center position in the groove width direction) while advancing in the welding direction (i.e., perpendicular to the paper surface), as shown in FIG. 5, for example. A welding wire (not shown in FIG. 5) is connected to the torch 10. The weaving width for each layer can be the root gap G for the first layer, or the gap at the intersection between the weld metal surface of the previous layer and both walls of the groove for other layers. This "weaving width" refers to the length of the groove width direction over the range of left and right movement of the torch.

[0065] This allows the groove surface of each layer to be sufficiently melted and promotes fusion with the weld metal. Furthermore, since the weld metal of each layer can be evenly distributed across the entire width of the groove, this is preferable because it can promote the suppression of welding defects and the improvement of the weld bead shape. In particular, in automated welding, in one-layer, one-pass welding using weaving, it is only necessary to align the tracing line of the welding torch with the center of the groove, which is also useful for reducing the labor required for setup.

[0066] [Hot Wire Welding] In the TIG welding method of the present invention, it is preferable that at least one layer of the multilayer structure be welded by hot wire welding. Here, hot wire welding refers to a welding method in which a welding wire 11 is electrically heated and fed into a molten pool 9 generated by an arc 8, as shown in FIG. 6 . The arc 8 is generated between the tungsten electrode 20 at the tip of the torch 10 and the base material 1 by supplying power from a welding power source 14 to the torch 10. The welding wire 11 is electrically heated by supplying power from a hot wire power source 15 through a contact tube 13 while being fed from a wire feeder 12 to the molten pool 9. This method enables high deposition and high-speed welding by adjusting the wire current and wire feed speed, which is preferable because it improves welding efficiency. The welding wire may be fed from either the front or rear of the welding direction.

[0067] [Chemical Composition of Welded Joint] The TIG welded joint obtained by the present invention is an arc-welded joint including a base metal and a weld metal. The base metal has the chemical composition described above. The weld metal contains, in the first and subsequent layers, C: 0.05 to 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities. The final layer of the weld metal contains 0.01 to 0.04% C, 0.02 to 0.90% Si, 0.50 to 1.80% Mn, 0.025% or less P, 0.035% or less S, 0.04 to 0.19% Al, 0.0100% or less N, and 0.0100% or less O. Optionally, it also contains one or more elements selected from 0.30% or less Cu, 0.40% or less Ni, 0.20% or less Cr, 0.20% or less Mo, 0.40% or less V, 0.030 to 0.190% Ti, 0.01 to 0.15% Zr, and 0.001 to 0.015% Nb. The remainder of the final layer of the weld metal consists of Fe and unavoidable impurities.

[0068] [Mechanical Properties of Welded Joint] The mechanical properties of the TIG welded joint obtained by the present invention preferably satisfy either one of the following conditions 1 and 2. More preferably, the mechanical properties satisfy both of them. (Condition 1) The absorbed energy (vE) of the weld metal in a Charpy impact test at -45°C -45 (Condition 2) The average surface hardness (HV10) of the weld metal is 220 or less, preferably 210 or less.

[0069] By satisfying the above condition 1, the low-temperature toughness of the ammonia tank can be ensured. By satisfying the above condition 2, it is expected that SCC caused by ammonia can be suppressed. In addition, in condition 1, the absorbed energy (vE -45 The reason for setting the strength to 47J or more is that this is the value required by the steel standards for welded structures.

[0070] The Charpy impact test under condition 1 above is carried out in accordance with the provisions of JIS Z 3128. The position at which the test specimens are taken is shown in Figure 2. The notch shape of the test specimen is a V-notch. The direction of the V-notch of the test specimen 5 is perpendicular to the surface of the steel material 1, and position a of the V-notch is the midpoint of the fusion line of the weld metal 4 on the center line of the test specimen 5. Three test specimens 5 are taken from a position 2 mm below the surface of the steel material 1. The Charpy impact test is carried out using three test specimens 5, and the absorbed energy (vE -45 ) are calculated for each of the weld metals, and the average value is used as the absorbed energy value of the weld metal of the welded joint.

[0071] The average value of the surface layer hardness (HV10) of the weld metal under the above condition 2 is determined using a Vickers hardness tester in the following manner. As shown in Figure 3, measurement positions b are set at 1 mm intervals on a line parallel to the surface of the steel material at a depth of 0.5 mm from the surface of the steel material in the cross section of the weld. The Vickers hardness measured at each measurement position b with a test force of 10 kgf is defined as the surface layer hardness (HV10), and these measured values ​​are averaged to define the surface layer hardness value of the weld metal of the welded joint.

[0072] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0073] In the examples, 15 mm thick steel materials (three types) shown in Table 1 were processed into a V-groove 2 (groove angle θ: 50°) as shown in Figure 1, and a root gap G of 3 to 5 mm was provided, followed by TIG welding of the steel materials. A copper plate was used for the backing plate 3. Steel Material No. a was TS440 MPa class steel, Steel Material No. b was TS490 MPa class steel, and Steel Material No. c was TS440 MPa class steel, and the tensile strength of each welded joint was required to be 440 MPa class and 490 MPa class, equivalent to that of the base metal.

[0074]

[0075] Next, four types of solid wire (diameter 1.2 mm) shown in Table 2 were used as welding wire. Welding Wire No. B is commonly used for steel materials with strengths of steel materials No. a and No. b, and corresponds to the first welding wire. Welding Wire Nos. A, C, and D correspond to the second welding wire, and have a lower C content than No. B. Welding Wire Nos. A and D have the basic composition and optional selected compositions of the second welding wire, while welding Wire No. C has only the basic composition of the second welding wire. The remaining chemical composition of each welding wire is Fe and unavoidable impurities.

[0076]

[0077] Using the above steel material and welding wire, multi-layer welding was performed from one side without preheating in a flat position under the various welding conditions shown in Tables 3-1, 3-2, and 4. Tables 3-1 and 3-2 also list the conditions for weaving welding and hot wire welding. In the case of "welding condition 1" shown in Table 3-1, the first layer was weaving welded, and the second to fifth layers were weaving welded and hot wire welded. In the case of "welding condition 2" shown in Table 3-2, the first to fifth layers were weaving welded and hot wire welded. The "welding condition" column in Table 4 lists the number of the welding condition applied. One welding pass was performed for each layer. The shielding gas used was 100% Ar gas by volume, and the gas flow rate was 10 to 25 L / min.

[0078]

[0079]

[0080]

[0081] Table 5 also shows the same welding conditions as above, as well as the evaluation results of the mechanical properties of the weld metal, which will be described below.

[0082]

[0083] The methods for the joint tensile test, Charpy impact test at the center of the weld metal, and Vickers hardness measurement at the surface layer of the weld metal were as follows:

[0084] The joint tensile test was carried out in accordance with the provisions of JIS Z 3121:2013.

[0085] The Charpy impact test was carried out in accordance with the provisions of JIS Z 3128. The position from which the test specimens (notch shape: V notch) were taken is shown in Figure 2. The direction of the V notch of the test specimen 5 was perpendicular to the surface of the steel material 1, and the position a of the V notch was set to the midpoint of the fusion line of the weld metal 4 on the center line of the test specimen 5. Three test specimens 5 were taken from a position 2 mm below the surface of the steel material 1. The Charpy impact test was carried out on three specimens taken from each weld joint, and the absorbed energy (vE -45) was calculated for each weld joint, and the average value was used as the absorbed energy value of the weld metal of each weld joint.

[0086] A Vickers hardness tester was used to test the Vickers hardness of the weld metal surface. As shown in Figure 3, measurement positions b were set at 1 mm intervals on a line parallel to the surface of the steel material in the weld metal at a depth of 0.5 mm from the surface of the steel material in the cross section of the weld. The Vickers hardness measured at each measurement position b with a test force of 10 kgf was taken as the surface hardness (HV10). An example of the surface hardness (HV10) measurement results is shown in Figure 4. Figure 4 shows the individual values ​​measured for weld joint No. 3 (an example of the present invention) and weld joint No. 2 (a comparative example). The average of these measurements was taken as the average surface hardness (HV10). For weld joint No. 3, the average surface hardness (HV10) was 209, which was below 220 and is therefore considered to be a joint less susceptible to stress corrosion cracking. For weld joint No. 2, the average surface hardness (HV10) was 209, which is lower than 220 and therefore considered to be a joint less susceptible to stress corrosion cracking. In specimen 2, the average surface hardness (HV10) was 238, exceeding 220, raising concerns about the occurrence of stress corrosion cracking. Note that measurement position b is located within the weld metal part, so the number of points at measurement position b varies depending on the width of the weld metal part.

[0087] [Evaluation Results] As shown in Table 5, the fracture position of the joint tensile test specimens was in the base metal in all welded joints, and therefore the weld metal has strength exceeding that of the base metal.

[0088] Welded joints Nos. 3 and 7 (invention examples) are joints in which the first layer to the layer just before the final layer were welded with welding wire No. B, and the final layer was welded with welding wire No. A. Welded joints Nos. 8 and 10 (invention examples) are joints in which the first layer to the layer just before the final layer were welded with welding wire No. B, and the final layer was welded with welding wire No. C. Welded joint No. 9 (invention example) is a joint in which the first layer to the layer just before the final layer were welded with welding wire No. B, and the final layer was welded with welding wire No. D. All of these joints had an average surface layer hardness (HV10) that satisfied the reference value (220 or less) and an absorbed energy (vE -45 ) satisfied the standard value (47 J or more).

[0089] On the other hand, in welded joints No. 1 and 5 (comparative examples), welding was performed from the first layer to the last layer using welding wire No. A, which has low C and alloying elements, and therefore the average surface layer hardness (HV10) was 220 or less, but the absorbed energy was significantly reduced due to embrittlement of the reheated portion. Welded joints No. 2 and 6 (comparative examples) were welded from the first layer to the last layer using welding wire No. B, and the average surface layer hardness (HV10) and absorbed energy (vE -45 ) did not satisfy the standard values. Welded joint No. 4 (comparative example) was made by using the opposite combination of welding wires to that of welded joint No. 3 (invention example), but the average surface hardness (HV10) and absorbed energy (vE -45 ) did not meet the standard values.

[0090] REFERENCE SIGNS LIST 1 Base material (steel material) 2 V-groove 3 Backing plate 4 Weld metal 5 Test piece for Charpy impact test 8 Arc 9 Molten pool 10 Torch 11 Welding wire 12 Wire feeder 13 Contact tube 14 Welding power source 15 Hot wire power source 20 Tungsten electrode a Notch position b Measurement position for Vickers hardness test t Plate thickness θ Groove angle

Claims

1. A TIG welding method for forming a multi-layer weld metal on a steel material using a shielding gas consisting of an inert gas and a welding wire, wherein the welding wire comprises a first welding wire and a second welding wire having different chemical compositions, and the chemical composition of the first welding wire contains, in mass %, C: 0.05 to 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, a chemical composition of the second welding wire containing, in mass %, C: 0.01 to 0.04%, Si: 0.02 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; the first welding wire is used for welding a first layer and layers thereafter of the multilayer structure; and the second welding wire is used for welding a final layer of the multilayer structure.

2. The TIG welding method according to claim 1, further comprising, in addition to the chemical composition of the second welding wire, one or more elements selected from, by mass%, Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.030 to 0.190%, Zr: 0.01 to 0.15%, and Nb: 0.001 to 0.015%.

3. The TIG welding method according to claim 1 or 2, wherein the welding of at least one layer of the multiple layers is weaving welding.

4. The TIG welding method according to any one of claims 1 to 3, wherein at least one layer of the multiple layers is welded by hot wire welding.

5. The absorbed energy (vE) of the weld metal in a Charpy impact test at -45°C -45 5. The TIG welding method according to claim 1, wherein the welding strength is 47 J or more.

6. A TIG welding method according to any one of claims 1 to 5, wherein the average surface hardness (HV10) of the weld metal is 220 or less.

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