Welded joint, welded structure, and welding method

Abstract

According to this welded joint, welded structure, and welding method, a welded joint formed between steel plates of which the ends are butted against each other comprises a first welding layer and a second welding layer that is stacked on the first welding layer and is adjacent to the surface of the steel plates. The first welding layer has arrestability for suppressing propagation of brittle cracks, and the second welding layer has SCC resistance for suppressing stress corrosion cracking (SCC) caused by a fluid in contact with the surface of the steel plates.

Description

Welded joint, welded structure, and welding method

[0001] The present disclosure relates to welded joints, welded structures, and welding methods.

[0002] Patent Document 1 discloses a multi-pass butt welded joint with excellent brittle crack propagation resistance. In this multi-pass butt welded joint, a weld layer whose material properties differ from those of the surrounding material and that has the function of suppressing and stopping the propagation of brittle cracks is disposed between a front weld layer and a back weld layer.

[0003] Patent No. 4995066

[0004] According to the technology described in Patent Document 1, crack propagation is suppressed by a weld layer with excellent arrestability (crack propagation stopping properties) disposed between the front weld layer and the back weld layer. However, there is a problem in that the technology does not take into consideration the possibility of stress corrosion cracking caused by fluids or the like in contact with the front weld layer or the back weld layer.

[0005] The present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a welded joint, a welded structure, and a welding method that have excellent arrestability and high resistance to stress corrosion cracking.

[0006] The welded joint according to the present disclosure is formed between steel plates whose ends are butted together, and includes a first weld layer and a second weld layer laminated on the first weld layer and adjacent to the surface of the steel plate, wherein the first weld layer has arrestability to suppress brittle crack propagation, and the second weld layer has stress corrosion cracking (SCC) resistance to suppress SCC caused by fluids in contact with the surface of the steel plate.

[0007] In the above welded joint, the arrestability of the second welded layer may be smaller than the arrestability of the first welded layer.

[0008] In the above-described welded joint, the first welded layer may be disposed in a plane strain region of the welded joint.

[0009] In the above-described welded joint, the second welded layer may be disposed in a plane stress region of the welded joint.

[0010] In the above-described welded joint, the plurality of welded layers constituting the welded joint may be formed by multi-pass welding.

[0011] The weld joint may have a thickness measured along the plate thickness direction of less than 50 mm.

[0012] In the above-mentioned welded joint, the total thickness of the welded layers constituting the welded joint that have an arrestability lower than the arrestability of the first welded layer may be ⅓ or more of the thickness of the welded joint measured along the plate thickness direction.

[0013] The welded joint may further include a third weld layer adjacent to the back surface of the steel plate and having arrestability.

[0014] In the above welded joint, the first welded layer and the second welded layer may be included in a surface welded region formed by welding from the surface of the steel plate.

[0015] In the above welded joint, the second weld layer may be included in a surface welded region formed by welding from the surface of the steel plate, and the first welded layer may be included in a back welded region formed by welding from the back surface of the steel plate.

[0016] A welded structure according to the present disclosure includes the above-described weld joint and steel plates joined to each other by the weld joint.

[0017] A welding method according to the present disclosure includes butting ends of steel plates together, forming a first weld layer using a first welding material, and forming a second weld layer laminated on the first weld layer and adjacent to the surface of the steel plates using a second welding material, wherein the first welding material has arrestability to suppress brittle crack propagation, and the second welding material has stress corrosion cracking (SCC) resistance to suppress SCC caused by fluid contacting the surface of the steel plates.

[0018] According to the present disclosure, it is possible to provide a welded joint, a welded structure, and a welding method that have excellent arrestability and high resistance to stress corrosion cracking.

[0019] Fig. 1 is a cross-sectional view of a welded joint according to a first embodiment of the present disclosure; Fig. 2 is an analysis model corresponding to the welded joint according to the first embodiment; Fig. 3 is an analysis model of a first comparative example; Fig. 4 is an analysis model of a second comparative example; Fig. 5 is an example of a change in crack growth distance over time; Fig. 6 is an example of a change in crack growth rate over time; Fig. 7 is a cross-sectional view of a welded joint according to a second embodiment of the present disclosure; Fig. 8 is a cross-sectional view of a welded joint according to a third embodiment of the present disclosure;

[0020] Hereinafter, several exemplary embodiments will be described with reference to the drawings. Note that common parts in the drawings are given the same reference numerals, and duplicated explanations will be omitted.

[0021] [Configuration of the welded joint according to the first embodiment] Fig. 1 is a cross-sectional view of a welded joint according to the first embodiment of the present disclosure. As shown in Fig. 1, the welded joint 1 is formed between steel plates ST whose ends are butted against each other. The welded joint 1 includes a first welded layer L1 and a second welded layer L2.

[0022] The welded joint 1 and the steel plate ST joined to each other by the welded joint 1 constitute a welded structure. For example, the welded structure is a storage tank capable of storing various fluids. The welded structure may be a storage tank for storing liquefied ammonia, or a storage tank for storing liquefied petroleum gas (LPG) or liquefied natural gas (LNG). In the following description, the surface of the welded joint 1 and the surface of the steel plate ST are described as being exposed to the fluid stored in the welded structure. Examples of fluids that come into contact with the surface of the steel plate ST are not limited to those given here.

[0023] 1, the steel sheet ST is indicated by two boundary lines. The upper boundary line represents the front surface of the steel sheet ST, and the lower boundary line represents the back surface of the steel sheet ST.

[0024] The second welded layer L2 is stacked on the first welded layer L1 and is adjacent to the surface of the steel plate ST. That is, the surface of the second welded layer L2 is continuous with the surface of the steel plate ST. The first welded layer L1 and the second welded layer L2 fill the groove (groove) between the two steel plates ST. Alternatively, the welded joint 1 may include a welded layer LB in addition to the first welded layer L1 and the second welded layer L2.

[0025] Here, the first weld layer L1 has arrestability (crack propagation arrestability) that suppresses the propagation of brittle cracks. Arrestability refers to the property that brittle cracks that occur in the weld layer are arrested within the weld layer and do not propagate to the steel plate. Furthermore, the second weld layer L2 has SCC resistance that suppresses SCC caused by fluids in contact with the surface of the steel plate ST. Stress corrosion cracking occurs when a material is exposed to stress in a specific environment, for example, when its surface is in contact with a specific liquid. Stress sources include external stress and welding residual stress. SCC resistance refers to resistance to such cracks. A weld layer with high SCC resistance is less susceptible to corrosion and stress from fluids in contact with the surface of the steel plate ST and can prevent crack propagation. SCC resistance is determined by factors such as the chemical composition and heat treatment of the welding material. Furthermore, galvanic corrosion is suppressed by providing the second weld layer L2 over the first weld layer L1.

[0026] For example, the multiple weld layers that make up the welded joint 1 are formed by multi-pass welding. Multi-pass welding is a process in which welding is performed in multiple passes along the point where the ends of the steel plates ST are butted together (the weld line). Multi-pass welding is used when it is difficult to weld a wide groove in one pass.

[0027] When forming the welded joint 1, the ends of the steel plates ST are butted together, and a first weld layer is formed with a first welding material, and a second weld layer is formed with a second welding material. Here, the first welding material has arrestability, and the second welding material has SCC resistance. The second welding material may have high galvanic corrosion suppression performance.

[0028] Examples of the first welding material include Inconel, Hastelloy, austenitic stainless steel, or low-temperature steel, each containing about 70% nickel. The first welding materials listed here are generally expensive, and it is desirable to reduce their usage in order to reduce costs. The first welding material is not limited to the examples listed here.

[0029] In contrast to this, for example, when the fluid that comes into contact with the surface of the steel plate ST is ammonia, the second welding material needs to be a welding material in which the nickel content is suppressed to 5% or less. Note that the second welding material is not limited to the examples given here.

[0030] The multiple weld layers that make up the weld joint 1 are formed by sequentially stacking layers using multi-pass welding. That is, the multiple weld layers that make up the weld joint 1 are formed by a single multi-pass welding operation. In other words, the weld joint 1 is not formed by repairing an existing weld line. Furthermore, the weld joint 1 is not formed by repair welding. This allows the quality of the weld joint to be maintained when it is formed. As a result, it is possible to reliably provide a weld joint that has both arrestability and SCC resistance.

[0031] Even when a welded structure including the welded joint 1 stores a fluid, the surface of the second weld layer L2 has SCC resistance, which suppresses stress corrosion cracking caused by the fluid and allows the fluid to be stored reliably. Furthermore, galvanic corrosion is suppressed, allowing the fluid to be stored reliably. Similarly, the surface of the steel plate ST may have SCC resistance. Furthermore, the arrestability of the first weld layer L1 suppresses the occurrence of brittle cracks. Furthermore, even if a brittle crack does occur, the propagation of the brittle crack is suppressed. In this way, the arrestability of the first weld layer L1 enables a double integrity design of the storage tank.

[0032] The arrestability of the second weld layer L2 may be lower than that of the first weld layer L1. This eliminates the need to achieve both SCC resistance and arrestability for the second weld layer L2. This allows for relaxation of restrictions on the welding material required to form the second weld layer L2. Furthermore, since the area in which expensive welding materials are used to impart arrestability is limited, the cost of forming the welded joint 1 can be reduced.

[0033] The first weld layer L1 may be disposed in a plane strain region of the welded joint 1. The second weld layer L2 may be disposed in a plane stress region of the welded joint 1.

[0034] Here, the "plane strain region" refers to a region in the plate thickness direction in which, when a one-sided through crack occurs in the welded joint 1, a principal stress of 95% or more of the maximum principal stress occurring at the tip of the one-sided through crack at the plate thickness center of the welded joint 1 occurs at the tip of the one-sided through crack. Also, the "plane stress region" refers to a region other than the plane strain region.

[0035] By performing a structural analysis of the welded structure in the event of a one-sided through crack occurring in the welded joint 1, it is possible to identify the "plane strain region" and the "plane stress region." Therefore, the first weld layer L1 and the second weld layer L2 may be arranged in the "plane strain region" and the "plane stress region," respectively, determined from the results of the structural analysis of the welded structure. This ensures that arrestability is imparted to the welded joint 1. Furthermore, since the region in which expensive welding materials are used to impart arrestability is limited, the cost of forming the welded joint 1 can be reduced.

[0036] Alternatively, the thickness of the welded joint 1 may be less than 50 mm. Here, the "thickness" refers to the length measured along the plate thickness direction of the welded joint 1, which in the case of Figure 1 is the length of the welded joint 1 in the vertical direction on the paper.

[0037] Furthermore, the total thickness of the weld layers LB that constitute the weld joint 1 and that have lower arrestability than the arrestability of the first weld layer L1 may be one-third or more of the thickness of the weld joint 1 measured in the plate thickness direction. This limits the area in which expensive welding materials are used to impart arrestability, thereby reducing the cost of forming the weld joint 1.

[0038] Alternatively, the welded joint 1 may further include a third weld layer L3 adjacent to the back surface of the steel plate ST. That is, the back surface of the third weld layer L3 is continuous with the back surface of the steel plate ST. The third weld layer L3 fills the gap between the two steel plates ST.

[0039] 1, the first welded layer L1 and the second welded layer L2 are included in a front-surface welded region W1 formed by welding from the front surface of the steel plate ST, and the third welded layer L3 is included in a back-surface welded region W2 formed by welding from the back surface of the steel plate ST.

[0040] [Comparison by structural analysis] In order to confirm that the welded joint 1 of the present disclosure has a weld layer that is highly resistant to stress corrosion cracking and also has excellent arrestability, a comparison was made by structural analysis (finite element method). Fig. 2 is an analytical model corresponding to the welded joint according to the first embodiment. Fig. 3 is an analytical model of a first comparative example. Fig. 4 is an analytical model of a second comparative example.

[0041] As shown in Figure 2, in the analysis model PM, the cross-sectional shapes of the multiple weld layers of the welded joint 1 are approximated by rectangular regions. The thickness of the welded joint 1 is 15 mm, the thickness of the first welded layer L1 is 2.25 mm, and the thickness of the second welded layer L2 is 0.75 mm. The widths of the welded joint 1 and the second welded layer L2 are 9.29 mm, and the width of the welded layer LB is 4.87 mm. The structural analysis method used was the one described in "Multi-fracture Model for Analyzing the Effect of Surface Layer Toughening on Brittle Crack Arrest Behavior" (Suga Arisa, Shoji Hiroto, Ohata Mitsuru, et al., Welding Structure Symposium 2017, Proceedings of the Symposium, 2017).

[0042] 3, in the analytical model P1, the materials constituting the first weld layer L1, the second weld layer L2, and the third weld layer L3 in the analytical model PM are the same as the material constituting the weld layer LB. In the analytical model P1, the weld layer LB is formed from a welding material with low arrestability and low SCC resistance.

[0043] 4, in the analytical model P2, the material constituting the first weld layer L1 in the analytical model PM was the same as the material constituting the second weld layer L2 and the third weld layer L3. That is, in the analytical model P2, a weld layer with high arrestability is provided in the outermost layer in the plate thickness direction, which is advantageous in terms of arrestability, but the analytical model P2 does not have a weld layer with high SCC resistance, and therefore no SCC countermeasures are taken.

[0044] Fig. 5 shows an example of the change in crack growth distance over time. Fig. 6 shows an example of the change in crack growth rate over time. In Fig. 5, the solid line indicates the change in crack growth distance over time for analytical model P2, and the dotted line indicates the change in crack growth distance over time for analytical model PM. In Fig. 6, the solid line indicates the change in crack growth rate over time for analytical model P2, and the dotted line indicates the change in crack growth rate over time for analytical model PM.

[0045] In order to see the effect of providing the second weld layer L2, which has high resistance to stress corrosion cracking, on the change in crack propagation distance over time and the change in crack propagation rate over time, Figures 5 and 6 only show the analysis results for analytical model P2 and analytical model PM.

[0046] In the analysis model P1, the crack did not stop and continued to propagate. In other words, the analysis model P1 did not exhibit sufficient arrestability. In the analysis model P2, the crack stopped.

[0047] In the analytical model PM, the crack stopped without penetrating, even though the opening displacement was larger than that of the analytical model P1. Furthermore, in the analytical model PM, brittle fracture did not occur in the second weld layer L2, and it was confirmed that the crack propagated ductilely. This suggests that the analytical model PM has arrestability comparable to that of the analytical model P2.

[0048] 5 and 6, it can be said that the effect of providing the second weld layer L2 on the change in crack propagation distance over time and the change in crack propagation rate over time is small. Therefore, it was confirmed that the welded joint 1 of the present disclosure has excellent arrestability while having a weld layer that is highly resistant to stress corrosion cracking.

[0049] [Configuration of a welded joint according to a second embodiment] Fig. 7 is a cross-sectional view of a welded joint according to a second embodiment of the present disclosure. Unlike the first embodiment, in Fig. 7, the front-surface welded region W1 is formed only by the first welded layer L1 and the second welded layer L2. The back-surface welded region W2 does not have the third welded layer L3.

[0050] The welding process for forming the front-side welded region W1 and the welding process for forming the back-side welded region W2 are separate processes, and welding equipment is moved between the front and back sides of the steel plate ST. In the welded joint 1 of Figure 7, the welding material can be changed between the welding process for forming the front-side welded region W1 and the welding process for forming the back-side welded region W2. As a result, the impact on workability when forming the welded joint 1 can be minimized.

[0051] Furthermore, in the welded joint 1 of FIG. 7, the welded layer LB, which is a different material portion, does not come into contact with the first welded layer L1 or the second welded layer L2, so that galvanic corrosion can also be suppressed.

[0052] [Configuration of a welded joint according to a third embodiment] Figure 8 is a cross-sectional view of a welded joint according to a third embodiment of the present disclosure. Unlike the first embodiment, in Figure 8, the front-surface welded region W1 is formed by the second welded layer L2, and the back-surface welded region W2 is formed by the first welded layer L1. That is, the second welded layer L2 is included in the front-surface welded region W1 formed by welding from the front surface of the steel plate ST. The first welded layer L1 is included in the back-surface welded region W2 formed by welding from the back surface of the steel plate ST.

[0053] The welding process for forming the front-side welded region W1 and the welding process for forming the back-side welded region W2 are separate processes, and welding equipment is moved between the front and back sides of the steel plate ST. In the welded joint 1 of Figure 8, the welding material can be changed between the welding process for forming the front-side welded region W1 and the welding process for forming the back-side welded region W2. As a result, the impact on workability when forming the welded joint 1 can be minimized.

[0054] Furthermore, in the welded joint 1 of Figure 8, the weld layer LB, which is a dissimilar material portion, is not provided, so the weld layer LB, which is a dissimilar material portion, does not come into contact with the first weld layer L1 or the second weld layer L2, which makes it possible to suppress galvanic corrosion.

[0055] Effect of the embodiment As described above in detail, a welded joint according to the present disclosure is formed between steel plates whose ends are butted together. The welded joint includes a first weld layer and a second weld layer laminated on the first weld layer and adjacent to the surface of the steel plate. The first weld layer has arrestability that suppresses the propagation of brittle cracks, and the second weld layer has stress corrosion cracking (SCC) resistance that suppresses SCC caused by fluids in contact with the surface of the steel plate.

[0056] This makes it possible to provide a welded joint that has excellent crack arrestability and high resistance to stress corrosion cracking. In particular, it is possible to achieve both crack arrestability and resistance to stress corrosion cracking. Furthermore, by providing a second weld layer on the first weld layer, it is also possible to suppress galvanic corrosion.

[0057] When the fluid in contact with the surface of the steel plate ST is ammonia, in a structure in which the second weld layer L2 is not provided, the first weld layer L1 is required to have arrestability and SCC resistance. However, while the nickel content needs to be increased to improve arrestability, the nickel content needs to be reduced to improve SCC resistance. Thus, without the second weld layer L2, it is difficult to achieve both arrestability and SCC resistance. According to the welded joint of the present disclosure, by providing the first weld layer L1 and the second weld layer L2, it is possible to achieve both arrestability and SCC resistance.

[0058] In the welded joint according to the present disclosure, the arrestability of the second weld layer may be lower than that of the first weld layer. This eliminates the need for the second weld layer to achieve both SCC resistance and arrestability. This allows for relaxation of restrictions on the welding material required to form the second weld layer. Furthermore, since the area where expensive welding materials are used to impart arrestability is limited, the cost of forming the welded joint can be reduced.

[0059] In the welded joint according to the present disclosure, the first weld layer may be disposed in a plane strain region of the welded joint. Here, the "plane strain region" refers to a region in the thickness direction in which, when a single-sided through crack occurs in the welded joint, a principal stress of 95% or more of the maximum principal stress occurring at the tip of the single-sided through crack at the thickness center of the welded joint occurs at the tip of the single-sided through crack. This ensures that the welded joint is crack arrestable. Furthermore, the region requiring expensive welding materials to impart crack arrestability is limited, thereby reducing the cost of forming the welded joint.

[0060] In the welded joint according to the present disclosure, the second weld layer may be disposed in a plane stress region of the welded joint. Here, the "plane stress region" may be a region in the thickness direction where, when a single-sided through crack occurs in the welded joint, a principal stress that is less than 95% of the maximum principal stress that occurs at the tip of the single-sided through crack at the thickness center of the welded joint occurs at the tip of the single-sided through crack. This allows a welding material having SCC resistance to be disposed in a region where arrestability is less necessary, thereby reliably imparting SCC resistance. Furthermore, since the region where expensive welding material is used to impart arrestability is limited, the cost of forming the welded joint can be reduced.

[0061] In the welded joint according to the present disclosure, the multiple weld layers constituting the welded joint may be formed by multi-pass welding. This allows different welding materials to be used for each weld layer constituting the welded joint. Furthermore, thick steel plates can be reliably joined together.

[0062] The welded joint according to the present disclosure may have a thickness measured along the thickness direction of the plate of less than 50 mm, thereby reducing the amount of expensive welding material used to provide arrestability, thereby limiting the areas where expensive welding material is used to provide arrestability.

[0063] In the welded joint according to the present disclosure, the total thickness of the welded layers constituting the welded joint that have lower arrestability than the arrestability of the first welded layer may be one-third or more of the thickness of the welded joint measured in the plate thickness direction. This limits the area where expensive welding materials are used to impart arrestability. As a result, the cost of forming the welded joint can be reduced.

[0064] The welded joint according to the present disclosure may further include a third weld layer adjacent to the back surface of the steel plate and having arrestability. This provides the welded joint with the first weld layer and the third weld layer as welded layers having arrestability, thereby more reliably improving the arrestability of the welded joint.

[0065] In the welded joint according to the present disclosure, the first weld layer and the second weld layer may be included in a surface welded region formed by welding from the front surface of the steel plate. The welding process for forming the surface welded region and the welding process for forming the back surface welded region are separate processes, and welding equipment is moved between the front surface and back surface of the steel plate. The welding material can be changed between the welding process for forming the surface welded region and the welding process for forming the back surface welded region. As a result, the impact on workability when forming the welded joint can be minimized.

[0066] In the welded joint according to the present disclosure, the second weld layer may be included in a surface-welded region formed by welding from the surface of the steel plate, and the first weld layer may be included in a back-side welded region formed by welding from the back side of the steel plate. The welding material can be changed between the welding process for forming the surface-welded region and the welding process for forming the back-side welded region. As a result, the impact on workability when forming the welded joint can be minimized.

[0067] The welded structure according to the present disclosure includes the above-described welded joint and steel plates joined together by the welded joint. This provides a welded structure with excellent crack arrestability and high resistance to stress corrosion cracking. For example, by constructing a storage tank using the welded structure according to the present disclosure, brittle cracking and stress corrosion cracking in the welded joint can be suppressed, allowing for reliable storage of fluid.

[0068] A welding method according to the present disclosure includes butting ends of steel plates together, forming a first weld layer using a first welding material, and forming a second weld layer laminated on the first weld layer and adjacent to the surface of the steel plates using a second welding material, wherein the first welding material has arrestability to suppress brittle crack propagation, and the second welding material has stress corrosion cracking (SCC) resistance to suppress SCC caused by fluid contacting the surface of the steel plates.

[0069] This makes it possible to provide a welding method that is excellent in arrestability and has high resistance to stress corrosion cracking.

[0070] Although several embodiments have been described, modifications or variations of the embodiments can be made based on the disclosure. All components of the embodiments and all features described in the claims may be individually extracted and combined unless they are mutually inconsistent.

[0071] The entire contents of Japanese Patent Application No. 2023-167804 (filing date: September 28, 2023) are incorporated herein by reference.

[0072] 1 Welded joint L1 First welded layer L2 Second welded layer L3 Third welded layer LB Welded layer P1, P2, PM Analysis model ST Steel plate W1 Front welded area W2 Back welded area

Claims

1. A welded joint formed between steel plates with their ends butted together, comprising: a first weld layer; and a second weld layer laminated on the first weld layer and adjacent to the surface of the steel plates, wherein the first weld layer has an arrestability that suppresses the propagation of brittle cracks, and the second weld layer has stress corrosion cracking (SCC) resistance that suppresses SCC caused by fluid in contact with the surface of the steel plates.

2. The welded joint according to claim 1, wherein the arrestability of the second welded layer is smaller than that of the first welded layer.

3. The weld joint of claim 1, wherein the first weld layer is disposed in a plane strain region of the weld joint.

4. The weld joint of claim 1, wherein the second weld layer is disposed in a plane stress region of the weld joint.

5. The welded joint according to claim 1, wherein the multiple welded layers constituting the welded joint are formed by multi-pass welding.

6. The welded joint according to claim 1, having a thickness measured along the thickness direction of the plate of less than 50 mm.

7. The welded joint according to claim 1, wherein the total thickness of the welded layers constituting the welded joint, which have an arrestability smaller than the arrestability possessed by the first welded layer, is 1 / 3 or more of the thickness of the welded joint measured along the plate thickness direction.

8. The welded joint according to claim 1, further comprising a third weld layer adjacent to the back surface of the steel plate and having the arrestability.

9. The welded joint according to claim 1, wherein the first weld layer and the second weld layer are included in a surface weld area formed by welding from the surface of the steel plate.

10. The welded joint according to claim 1, wherein the second weld layer is included in a surface welded area formed by welding from the front surface of the steel plate, and the first welded layer is included in a back surface welded area formed by welding from the back surface of the steel plate.

11. A welded structure comprising: a welded joint according to any one of claims 1 to 10; and the steel plates joined to each other by the welded joint.

12. A welding method comprising the steps of: butting ends of steel plates against each other; forming a first weld layer using a first welding material having arrestability for suppressing the propagation of brittle cracks; and laminating the first weld layer to form a second weld layer adjacent to the surface of the steel plates using a second welding material having stress corrosion cracking (SCC) resistance for suppressing SCC caused by fluid in contact with the surface of the steel plates.

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