Stainless steel welding method

Abstract

To provide a method for improving hydrogen embrittlement resistance in a weld zone after welding.SOLUTION: A stainless steel welding method of the present invention includes (i) a welding step of welding stainless steel to form a weld zone, (ii) a cold working step of cold-working the weld zone formed in the welding step (i), and (iii) an annealing step of annealing the weld zone cold-worked in the cold working step (ii) at 900°C or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for welding stainless steel. [Background technology]

[0002] It is known that hydrogen embrittlement occurs in steel materials, including stainless steel. "Hydrogen embrittlement" refers to the penetration of hydrogen into stressed areas during stress, causing embrittlement such as reduced elongation. This hydrogen embrittlement is particularly likely to occur in welded joints of steel materials, and companies are trying to prevent it by examining the process conditions and materials used during welding.

[0003] For example, Patent Document 1 describes a welding method for a welded joint in which a filler metal is added to the molten part of a base metal made of austenitic steel having a nickel (Ni) equivalent of 27.4 or more, as expressed by the following formula (1) (Ni equivalent: 12.6C + 0.35Si + 1.05Mn + Ni + 0.65Cr + 0.98Mo (1)), and the welded joint is welded. The method is characterized by using one or more filler metals selected from 316, 316L, 317, 317L, 318, and 318L as specified in JIS Z3321, and by welding the welded joint and then subjecting the weld metal portion of the welded joint to solution heat treatment.

[0004] Patent Document 2 describes a chemical composition, in mass %, of C: 0.005 to 0.1%, Si: 0.2 to 1.2%, Mn: 2.5 to 6.5%, Ni: 8 to 15%, Cr: 19 to 25%, Mo: 0.01 to 4.5%, V: 0.01 to 0.5%, Nb: 0.01 to 0.5%, Al: less than 0.05%, N: 0.15 to 0.45%, Ti: 0 to 0.5%, Cu: 0 to 3.0%, B: 0 to 0.01%, and the balance: F. and impurities, wherein the impurities are O: 0.02% or less, P: 0.05% or less, and S: 0.04% or less, respectively; preparing a second member which is an austenitic stainless steel containing C, Si, Mn, Ni, Cr, Mo, and N and has Nieq defined by formula (1) of 27% or more; and and welding the first member and the second member by gas tungsten arc welding without using any welding material, using any of a mixed gas, a mixed gas of Ar and H2, and a mixed gas of Ar, N2, and H2 as a shielding gas, wherein the heat input Q of the gas tungsten arc welding is 7 kJ / cm or less and the following formula (2) is satisfied: Nieq=Ni+Mo+Mn+0.6Cr+0.3Si+12(C+N)···(1), Q≧−0.18[H2]+4.8···(2), where the element symbols in formula (1) are substituted with the content of each element of the second member in mass %. In formula (2), the unit of Q is kJ / cm, and the mixing ratio of H2 in the shielding gas in volume % is substituted for [H2].

[0005] Patent Document 3 discloses a steel sheet having a chemical composition, in mass %, of C: 0.005 to 0.10%, Si: 1.2% or less, Mn: 4.0 to 8.0%, P: 0.03% or less, S: 0.02% or less, Ni: 14.0 to 18.0%, Cr: 20.0 to 26.0%, Mo: 1.0 to 4.0%, V: 0.50% or less, Nb: 0.50% or less, N: 0.15 to 0.45%, Cu: 0 to 3%, W: 0 to 3%, Ti: 0 to 0.5%, Al: 0 to 0.5%, and the balance: Fe and impurities, and a structure having a δ of 0.05 to 2.5% by area ratio. The document describes a welding material for austenitic stainless steel that contains ferrite and has F1, as defined by the following formula (1), of 38.0 to 48.0 (F1=10C+Si+0.5Mn-0.8Ni+1.3Cr+Cu+5Mo+2.5W+15(V+Nb+Ti)+30N+δ(1), where C, Si, Mn, Ni, Cr, Cu, Mo, W, V, Nb, Ti, and N in formula (1) are substituted with the contents of the corresponding elements in mass%, and δ in formula (1) is substituted with the amount of δ-ferrite in area%).

[0006] Patent Document 4 describes a method for manufacturing an austenitic stainless steel pipe weld joint, which includes a process of gas tungsten arc welding from the outside in a circumferential direction using a welding torch while supplying back-shielding gas to the inside of a pair of steel pipes whose ends are butted together, and changes the internal pressure of the pair of steel pipes based on the position of the welding torch.

[0007] Patent Document 5 describes a chemical composition in mass percent of C: 0.10% or less, Si: 1.0% or less, Mn: 8.0 to 11.0%, P: 0.030% or less, S: 0.0030% or less, Cr: 15.0 to 18.0%, Ni: 7.0 to 9.0%, N: 0.15 to 0.25%, B: 0.0002 to 0.01%, Mg: 0.0001 to 0.0050%, Cu: less than 1.00%, Mo: 0.5% or less, O: 0.0050% or less, Al: 0 to 0.20%, Ca: 0 to 0.01%, Nb: 0 to 0.50%, Ti: 0 to 0.50%, V: 0 to 0.50%, W: 0 to 0.50%. , Zr: 0-0.50%, Co: 0-0.50%, Ga: 0-0.010%, Hf: 0-0.10%, REM: 0-0.10%, balance: Fe and impurities, and the f-value calculated by the following formula (i) is greater than 30.0 and less than 33.5 (f-value = Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 7.55N (i), where each element symbol in the above formula (i) represents the content (% by mass) of each element contained in the steel, and if the element is not contained, it is set to zero).

[0008] Patent Document 6 describes a welding material used for gas tungsten arc welding of austenitic stainless steel, which has a chemical composition, in mass %, of C: 0.030% or less, Si: 0.10 to 0.45%, Mn: 2.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Ni: 12.0 to 14.0%, Cr: 22.0 to 26.0%, Mo: 2.0 to 3.5%, Al: less than 0.050%, N: 0.20 to 0.50%, O: 0.020% or less, Nb: 0 to 0.50%, V: 0 to 0.50%, and the balance : Fe and impurities, and satisfies the following formulas (1) to (3): austenitic stainless steel welding material (Ni + 0.7Cr + Mo + Mn - 0.3Si + 13C + 8N ≥ 35.0 (1), 0.13C + 0.05Si - 0.02Mn - 0.05Cr + 0.01Ni - 0.01Mo + 0.15N ≤ -1.00 (2), 30C + 30N + 0.5Mn + 1.8Ni - 1.5Si - 1.5Cr - Mo ≥ -10.0 (3), where the name of each element in the formula is substituted with the content of the corresponding element in mass%).

[0009] Patent Document 7 describes a clad steel plate comprising a base material and a clad material bonded to one side of the base material, wherein the base material is made of carbon steel or low-alloy steel, and the clad material is made of a corrosion-resistant alloy, and wherein at the interface between the base material and the clad material of the clad steel plate, the width in the plate thickness direction of an area having a nano-hardness of 7 GPa or more is 5 μm or less, and the ferrite phase fraction of the surface layer of the base material is more than 15% (here, the surface layer of the base material refers to a position 1 mm in the plate thickness direction from the surface of the base material). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-66586 [Patent Document 2] Japanese Patent Application Publication No. 2017-213588 [Patent Document 3] Japanese Patent Application Publication No. 2019-63868 [Patent Document 4] Japanese Patent Publication No. 2020-49526 [Patent Document 5] Japanese Patent Publication No. 2022-89303 [Patent Document 6] Japanese Patent Publication No. 2022-155987 [Patent Document 7] Japanese Patent Application Publication No. 2022-186396 Summary of the Invention [Problem to be solved by the invention]

[0011] However, as described above, in the prior art, attempts have been made to improve hydrogen embrittlement resistance by devising the structure of the base material itself to be welded, the welding method, and / or the welding method of the joint portion, which has imposed significant restrictions on the objects to be welded.

[0012] Therefore, an object of the present invention is to provide a method for improving the hydrogen embrittlement resistance of a welded portion after welding. [Means for solving the problem]

[0013] For example, the hydrogen embrittlement phenomenon in stainless steel can be explained as follows. (I) Welding transforms the austenite (fcc) structure, which is resistant to hydrogen embrittlement, into dendrite. Dendrites have a bcc crystal structure, which is unfavorable for hydrogen embrittlement resistance, so hydrogen embrittlement resistance is reduced. (II) The melting of the base material and air cooling caused by welding causes the segregation of elements such as Cr, Ni, and Mo, which are considered to be highly resistant to hydrogen embrittlement. This causes areas with low concentrations of these elements, i.e., areas that are weak in hydrogen embrittlement resistance, which promotes hydrogen embrittlement. (III) Stresses due to the above (I) and (II) and thermal loads remain in the welded parts. In a hydrogen environment, hydrogen tends to concentrate in the stressed parts, which promotes hydrogen embrittlement in these residual stress parts.

[0014] Therefore, the present inventors have investigated various means for solving the above-mentioned problems, and as a result have found that, rather than changing the base metal composition or the welding conditions themselves, applying cold working and annealing processes to the welded part after welding can return the weld structure, which is the starting point of hydrogen embrittlement, to its pre-weld state, which is resistant to hydrogen embrittlement, and thus can improve the hydrogen embrittlement resistance of the welded part, and have completed the present invention.

[0015] That is, the gist of the present invention is as follows. (1) A method for welding stainless steel, comprising: (i) a welding step of welding stainless steel to form a weld; (ii) a cold working step of cold working the weld formed in the welding step (i); and (iii) an annealing step of annealing the weld cold worked in the cold working step (ii) at 900°C or higher. The welding method according to (1), wherein the cold working is performed as in (2)(ii). (3) The welding method according to (2), wherein the cold working in (ii) is cold rolling. (4) The welding method according to (2), wherein the cold working in (ii) is cold drawing. (5) The welding method according to any one of (1) to (4), wherein the annealing step (iii) further includes a rapid cooling step for preventing sensitization. [Effects of the Invention]

[0016] The present invention provides a method for improving hydrogen embrittlement resistance in a weld after welding. [Brief explanation of the drawings]

[0017] [Figure 1] 1A and 1B are diagrams schematically illustrating a conventional welding method and an embodiment of the welding method of the present invention, respectively. [Figure 2] FIG. 2 is a diagram showing a schematic view of the appearance of a TP. [Figure 3] 1 is a graph showing stress-displacement curves of a slow strain rate tensile (SSRT) test of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0018] Preferred embodiments of the present invention will now be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity and do not accurately depict the actual dimensions and shapes. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. Note that the stainless steel welding method of the present invention is not limited to the following embodiments, and can be embodied in various forms with modifications, improvements, etc. that can be made by those skilled in the art, without departing from the spirit of the present invention.

[0019] The present invention relates to a method for welding stainless steel, which includes (i) a welding step, (ii) a cold working step of cold working the welded portion, and (iii) an annealing step of annealing the welded portion at 900°C or higher.

[0020] (i) Welding process In the welding step (i), the stainless steel is welded to form a weld.

[0021] The stainless steel is not limited, and examples of the stainless steel include austenitic stainless steel, specific examples of which include SUS301, SUS304, and SUS316.

[0022] The shape of the stainless steel is not limited. The stainless steel may be in the form of a plate or a tube (pipe). The thickness of the stainless steel is not limited.

[0023] The welding method in the welding step (i) is not limited. Examples of the welding method include fusion welding. Examples of fusion welding include arc welding, electron beam welding, gas welding, and laser welding. In one embodiment, the welding method is arc welding.

[0024] The welding step (i) forms a weld in the stainless steel.

[0025] (ii) A cold working process for cold working the welded part. In the cold working step (ii), the welded portion formed in the welding step (i) is cold worked.

[0026] Cold working is a method of working at a temperature below the recrystallization temperature of the object (weld).

[0027] Cold working is not limited to certain processes. For example, if the stainless steel is a plate-shaped stainless steel sheet, the cold working is cold rolling. "Cold rolling" is a processing method in which the material is sandwiched between rotating rolls at a temperature below the recrystallization temperature of the target (weld) and the material is thinned by pressure. For example, if the stainless steel is a tubular welded stainless steel pipe, the cold working is cold drawing. "Cold drawing" is a processing method in which the welded pipe is lubricated and then drawn using a die and plug at a temperature below the recrystallization temperature of the target (weld) to achieve the specified dimensions.

[0028] The temperature in the cold working step (ii) is not limited as long as it is below the recrystallization temperature of the welded portion.

[0029] The pressure in the cold working step (ii) is not limited.

[0030] The atmosphere in the cold working step (ii) is not limited.

[0031] The time for the cold working step (ii) is not limited.

[0032] The equipment used in the cold working step (ii) is not limited. For example, if the stainless steel is a plate-shaped stainless steel plate, a rolling mill having a reversing mechanism (a mechanism that performs rolling in the forward and reverse directions around the rolling roll) capable of multi-pass processing can be used for cold rolling. For example, if the stainless steel is a pipe-shaped stainless steel pipe, a draw bench can be used for cold drawing.

[0033] In the cold working step (ii), the welded portion is cold worked, thereby introducing strain into the welded portion.

[0034] When processing oil or the like is used in the cold working step (ii), a processing oil removal step may be further included between steps (ii) and (iii).

[0035] (iii) Annealing process in which the welded part is annealed at 900°C or higher In the annealing step (iii), the welded portion cold worked in the cold working step (ii) is annealed at 900°C or higher.

[0036] The temperature in the annealing step (iii) is not limited as long as it is 900°C or higher.

[0037] The pressure in the annealing step (iii) is not limited, and is a pressure slightly higher than atmospheric pressure so as to maintain a purged atmosphere.

[0038] The atmosphere in the annealing step (iii) is not limited, and is usually a hydrogen atmosphere or an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere.

[0039] The time for the annealing step (iii) is not limited.

[0040] The apparatus used in the annealing step (iii) is not limited, and examples of the apparatus used in the annealing step (iii) include a muffle-type annealing furnace.

[0041] The annealing step (iii) may further include a rapid cooling step to prevent sensitization.

[0042] By annealing the weld in the annealing step (iii), the strained weld recrystallizes, and the structure of the weld can be restored to the structure of the original base material.

[0043] In the present invention, step (iii) is usually carried out after step (ii).

[0044] In the present invention, the weld is subjected to strain introduction by cold working in step (ii) and annealing in step (iii) at a temperature range where recrystallization occurs at 900°C or higher in the strain-introduced portion, thereby enabling the structure of the weld to be restored to that of the original base material, thereby improving the hydrogen embrittlement resistance of the weld. Furthermore, steps (ii) and (iii) also enable the segregation of components in the weld and recovery of residual stress, resulting in further improvement in the hydrogen embrittlement resistance of the weld. Therefore, stainless steel welded by the welding method of the present invention has hydrogen embrittlement resistance equivalent to that of the base material, both at room temperature and at low temperatures.

[0045] Figure 1 shows a schematic diagram of a conventional welding method (A) and one embodiment of the welding method of the present invention (B). In the conventional welding method of Figure 1A, the weld undergoes a structural change to dendrites, non-uniform composition, and residual stress, resulting in reduced hydrogen embrittlement resistance. On the other hand, in the welding method of the present invention of Figure 1B, the weld is subjected to a cold working process and an annealing process, which recrystallizes the weld, uniformly distributes the composition, and removes residual stress, resulting in improved hydrogen embrittlement resistance. [Example]

[0046] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0047] 1. Sample Preparation (1) As an example test piece (TP), a reproduced material (welded material) was prepared that simulated a welding method including welding, cold working after welding, and annealing after cold working. Figure 2 shows the appearance of the TP. The units of values ​​in Figure 2 are mm. As can be seen from Figure 2, when conducting the tensile test described below, the welded portion of the example TP was notched so that the welded portion was the weakest point, i.e., so that a load was applied to the welded portion. (2) As a comparative example, a normal plate material (assuming a seamless pipe) having the same appearance as the TP of the example prepared in (1) was prepared. The comparative example TP was also notched at the same position as the TP of the example.

[0048] 2. Slow strain rate tensile test (SSRT) A slow strain rate tensile test was conducted to compare the welded materials of the examples with the plate materials of the comparative examples. The slow strain rate tensile test was conducted in a 70 MPa hydrogen atmosphere or air atmosphere at a temperature of −40°C or room temperature (RT, approximately 23°C) and a tension rate of 0.0048 mm / min.

[0049] 3. Test Results The test results are shown in Table 1 and Figure 3. Figure 3 shows stress-displacement curves of the slow strain rate tensile test for the examples and comparative examples.

[0050] [Table 1]

[0051] It is clear from Table 1 and Figure 3 that the elongation and strength of the welded material of the example increases under all conditions. Therefore, the occurrence of hydrogen embrittlement was not confirmed in the welded material of the example both at low temperature and at room temperature, and it was found that the welded material maintained hydrogen embrittlement resistance equal to or higher than that of the base plate material.

Claims

1. (i) a welding step of welding stainless steel to form a weld; (ii) a cold working step of cold working the welded portion formed in the welding step (i); (iii) an annealing step of annealing the welded portion cold worked in the cold working step (ii) at 900°C or higher; A method for welding stainless steel, including:

2. The welding method of claim 1 , wherein the cold working of (ii) is performed.

3. The welding method according to claim 2, wherein the cold working in (ii) is cold rolling.

4. The welding method of claim 2, wherein the cold working in (ii) is cold drawing.

5. The welding method according to any one of claims 1 to 4, wherein the annealing step (iii) further comprises a rapid cooling step to prevent sensitization.

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