Welded joint and ammonia tank provided with welded joint

Abstract

A welded joint (1) according to the present disclosure comprises a base material part (10) formed from steel and a weld metal (20) formed from steel. In a surface layer of a weld heat-affected zone (11) of the base material part (10), the absorbed energy obtained by a Charpy impact test at -33°C is 50 J or more. In a region in the vicinity of a toe zone of the weld heat-affected zone (11), the maximum Vickers hardness HHmax at a depth position of 0.5 mm in the thickness direction of the weld heat-affected zone (11) from a surface is 300 HV1 or less.

Description

Welding joint and ammonia tank including the welding joint

[0001] The present disclosure relates to a welding joint and an ammonia tank including the welding joint.

[0002] Ammonia is used as a raw material for nitrogen fertilizers such as urea, ammonium nitrate, and ammonium sulfate, or as a raw material for synthetic fibers and resins such as nylon, polyurethane, and acrylonitrile. Ammonia is further ２ attracting attention as an alternative fuel for coal-fired power generation effective for CO reduction and as a carrier of hydrogen.

[0003] Ammonia is liquefied and stored in an ammonia tank. The ammonia tank includes an outer shell, an inner shell, and a heat insulating material disposed between the outer shell and the inner shell. The inner shell includes a base material portion made of steel and a welding joint including a weld metal. The liquefied ammonia is stored at a low temperature of −33°C. Therefore, the welding joint applied to the inner shell of the ammonia tank is required to have excellent low-temperature toughness.

[0004] Further, in a liquefied ammonia environment, stress corrosion cracking (ammonia SCC) may occur. Therefore, the welding joint constituting the ammonia tank is further required to have excellent ammonia SCC resistance.

[0005] In Japanese Patent Application Laid-Open No. 2004-42133 (Patent Document 1), a welding joint having excellent ammonia SCC resistance has been proposed.

[0006] The welding joint of Patent Document 1 is a welding joint obtained by welding steel materials as base metals using a welding material. The maximum tensile residual stress on the surface near the end portion of the welded portion formed by welding is 30% or less of the yield strength of the steel material. In Patent Document 1, the ammonia SCC resistance is enhanced by adjusting the tensile residual stress of the welded portion.

[0007] Japanese Patent Application Laid-Open No. 2004-42133

[0008] However, in Patent Document 1, the compatibility between low-temperature toughness and ammonia SCC resistance has not been specifically studied.

[0009] The object of this disclosure is to provide a welded joint and an ammonia tank having excellent low-temperature toughness and excellent ammonia SCC resistance.

[0010] The welded joint of this disclosure comprises a base metal made of steel and a weld metal made of steel. The absorbed energy obtained in a Charpy impact test at -33°C in the surface layer of the heat-affected zone of the base metal is 50 J or more. In the region near the toe of the heat-affected zone, the maximum Vickers hardness H at a depth of 0.5 mm from the surface in the thickness direction of the heat-affected zone is Ｈｍａｘ The value is 300HV1 or less.

[0011] The ammonia tank of this disclosure comprises the welded joint described above.

[0012] The welded joints and ammonia tanks of this disclosure have excellent low-temperature toughness and excellent ammonia SCC resistance.

[0013] Figure 1 is a plan view showing an example of a welded joint according to this embodiment. Figure 2 is a cross-sectional view of the welded joint 1 in Figure 1 along the line II-II. Figure 3 is a schematic diagram showing the sampling locations for the V-notch test of the heat-affected zone (HAZ) of the weld. Figure 4 is an enlarged view of the region in Figure 2 that includes the weld metal and HAZ. Figure 5 is an enlarged view of the region near the toe of the HAZ in Figure 4. Figure 6 is a schematic diagram showing the sampling locations for the CTOD test specimen. Figure 7 is a schematic diagram showing the sampling locations for the four-point bending test specimen.

[0014] The inventors first investigated means to achieve both excellent low-temperature toughness and excellent ammonia SCC resistance in a welded joint comprising a base material made of steel and a weld metal made of steel. As a result, the inventors obtained the following findings.

[0015] Welded joints are manufactured by welding steel plates that make up the base material to form weld metal. At this time, the area of ​​the base material adjacent to the weld metal forms the heat-affected zone (HAZ). The HAZ is the area affected by the heat during welding. The HAZ is heated during welding and then rapidly cooled. Therefore, the hardness of the HAZ is higher than the hardness of the normal area of ​​the base material outside the HAZ.

[0016] As described above, the hardness of the heat-affected zone (HAZ) is higher than that of the normal part of the base material. Therefore, the low-temperature toughness of the HAZ is lower than that of the normal part of the base material. In welded joints, cracks tend to occur on the surface of the HAZ. Therefore, it is thought that increasing the low-temperature toughness of the surface layer of the HAZ can also increase the low-temperature toughness of the entire welded joint.

[0017] Furthermore, in the hardness distribution along the thickness of the heat absorption zone (HAZ), the hardness of the surface layer of the HAZ is significantly higher than that of the interior. This is due to the surface layer of the HAZ being cooled more rapidly than the interior. In welded joints in particular, the hardness tends to be highest in the portion of the HAZ surface adjacent to the weld metal, which is the heat source during welding, i.e., the region near the toe of the HAZ. Ammonia SCC resistance is particularly likely to occur in regions of the welded joint where the surface layer is hard. Therefore, by suppressing the maximum hardness of the surface layer near the toe of the HAZ, the ammonia SCC resistance of the welded joint can be improved.

[0018] Based on the above findings, the inventors investigated the relationship between the low-temperature toughness of the surface layer of the HAZ of a welded joint, the hardness of the surface layer in the region near the toe of the HAZ, and the low-temperature toughness and ammonia SCC resistance of the welded joint. As a result, the absorbed energy obtained in a Charpy impact test at -33°C at the surface layer of the HAZ of the base material was set to 50 J or more, and the maximum Vickers hardness H at the surface layer in the region near the toe of the HAZ was set to Ｈｍａｘ The inventors have found that by adjusting the voltage to 300HV1 or less, it is possible to achieve both excellent low-temperature toughness and excellent ammonia SCC resistance in welded joints.

[0019] The welded joint and ammonia tank of this embodiment were completed according to the above-described technical concept and have the following configuration.

[0020] The first embodiment of the welded joint comprises a base metal made of steel and a weld metal made of steel. The absorbed energy obtained in a Charpy impact test at -33°C in the surface layer of the heat-affected zone of the base metal is 50 J or more. In the region near the toe of the heat-affected zone, the maximum Vickers hardness H at a depth of 0.5 mm from the surface in the thickness direction of the heat-affected zone is Ｈｍａｘ The value is 300HV1 or less.

[0021] The welded joint of the second form is the welded joint of the first form, and further, the average Vickers hardness H at a depth of 0.5 mm in the thickness direction of the base material portion from the surface of the normal base material portion, which is the region other than the heat affected zone of welding in the base material portion Ｂａｖｅ is 240 HV1 or less. The maximum Vickers hardness H Ｈｍａｘ is 1.80 times or less of the average Vickers hardness H Ｂａｖｅ The average Vickers hardness H at a depth of 0.5 mm in the thickness direction of the weld metal from the surface of the weld metal Ｗａｖｅ is 1.70 times or less of the average Vickers hardness H Ｂａｖｅ is 1.70 times or less.

[0022] In the welded joint of the second form, the ammonia SCC resistance is further enhanced.

[0023] The welded joint of the third form is the welded joint of the first or second form, and the chemical composition of the base material portion contains, in mass%, C: 0.02 to 0.12%, and Ni: 1.00% or less. The chemical composition of the weld metal contains, in mass%, C: 0.02 to 0.20%, and Ni: 5.00% or less.

[0024] The ammonia tank of the first form includes an outer shell, an inner shell, and a heat insulating material disposed between the outer shell and the inner shell. The inner shell includes a base material portion made of steel and a weld metal made of steel. In the surface layer of the heat affected zone of welding of the base material portion, the absorbed energy obtained by the Charpy impact test at -33°C is 50 J or more. In the vicinity of the end portion of the heat affected zone of welding, the maximum Vickers hardness H at a depth of 0.5 mm in the thickness direction of the heat affected zone of welding from the surface Ｈｍａｘ is 300 HV1 or less.

[0025] The ammonia tank of the second form is the ammonia tank of the first form, and the average Vickers hardness H at a depth of 0.5 mm in the thickness direction of the base material portion from the surface of the normal base material portion, which is the region other than the heat affected zone of welding in the base material portion Ｂａｖｅ is 240 HV1 or less. Also, the maximum Vickers hardness H Ｈｍａｘ is 1.80 times or less of the average Vickers hardness H ＢａｖｅIt is less than 1.80 times. Furthermore, the average Vickers hardness H at a depth of 0.5 mm from the surface of the weld metal in the thickness direction of the weld metal is Ｗａｖｅ However, the average Vickers hardness is H Ｂａｖｅ It is less than 1.70 times that amount.

[0026] The third form of ammonia tank is the first or second form of ammonia tank, wherein the chemical composition of the base material contains, by mass%, C: 0.02 to 0.12% and Ni: 1.00% or less, and the chemical composition of the weld metal contains, by mass%, C: 0.02 to 0.20% and Ni: 5.00% or less.

[0027] The welded joint and ammonia tank of this embodiment will be described below.

[0028] [Configuration of the Welded Joint in This Embodiment] Figure 1 is a plan view showing an example of a welded joint in this embodiment. Referring to Figure 1, the welded joint 1 comprises a base material portion 10 made of steel and a weld metal 20 made of steel. The weld metal 20 is formed between a pair of base material portions 10 and is connected to each base material portion 10.

[0029] Figure 2 is a cross-sectional view of the welded joint 1 along the line II-II in Figure 1. Referring to Figures 1 and 2, in a plan view of the welded joint 1, the direction in which the weld metal 20 extends is defined as the weld metal extension direction Y. In a plan view of the welded joint 1, the direction perpendicular to the weld metal extension direction Y is defined as the width direction X. The direction perpendicular to the weld metal extension direction Y and the width direction X is defined as the thickness direction Z. In the following explanation, the width direction X will also be simply referred to as the X-axis direction. The weld metal extension direction Y will also be simply referred to as the Y-axis direction. The thickness direction Z will also be simply referred to as the Z-axis direction.

[0030] The weld metal 20 is formed by butting the ends of a pair of base metal parts 10 together and then performing welding. Well-known welding methods can be applied. Examples of welding methods include TIG welding (Gas Tungsten Arc Welding: GTAW), shielded metal arc welding (SMAW), flux-cored wire arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW).

[0031] The ends of the base material portions 10 that meet each other may or may not be beveled. If the ends of the base material portions 10 are beveled, the shape of the bevel at the end of the base material portion 10 is selected from the group consisting of, for example, a V-shaped bevel, a U-shaped bevel, and an X-shaped bevel.

[0032] As shown in Figure 2, the base metal portion 10 includes a heat-affected zone (HAZ) 11 and a normal base metal portion 12, which is the area outside the HAZ 11. The HAZ 11 is the area of ​​the base metal portion 10 adjacent to the molten line 20E of the weld metal 20, and is affected by the heat during welding. The normal base metal portion 12 is the area that is not substantially affected by the heat during welding.

[0033] [Method for distinguishing between HAZ11 and normal base material 12] HAZ11 and normal base material 12 can be distinguished by the following method. A test piece is taken that includes a cross section perpendicular to the Y-axis direction and parallel to the X-axis and Z-axis directions (i.e., the XZ plane), as shown in Figure 2. The cross section is polished to a mirror finish. Nital corrosion is performed on the cross section after mirror polishing to reveal the microstructure. HAZ11 and normal base material 12 are distinguished based on the contrast of the revealed microstructure. A person skilled in the art can easily distinguish between HAZ11 and normal base material 12 based on the contrast of the revealed microstructure.

[0034] [Characteristics of Welded Joint 1] Welded joint 1 satisfies the following characteristics: (Characteristic 1) The absorbed energy obtained in a Charpy impact test at -33°C on the surface of the HAZ 11 of the base material 10 is 50 J or more. (Characteristic 2) In the region near the toe of the HAZ 11, the maximum Vickers hardness H at a depth of 0.5 mm from the surface in the thickness direction (Z-axis direction) of the weld heat-affected zone (HAZ) 11 Ｈｍａｘ The maximum value is 300HV1 or less. The following describes each characteristic.

[0035] [(Feature 1) Low-temperature toughness of HAZ11] As described above, HAZ11 is the part of the base material 10 that is heated by the heat input of welding during the manufacturing process of the welded joint 1 and then rapidly cooled after heating. Because HAZ11 is rapidly cooled during welding, it tends to become harder compared to the normal part of the base material 12 that is not affected by heat. If the low-temperature toughness of the surface layer of HAZ11 is excessively low, cracks are likely to occur from the surface of HAZ11 of the welded joint 1. In this case, the low-temperature toughness of the entire welded joint 1 decreases.

[0036] Therefore, in the welded joint 1 of this embodiment, the absorbed energy obtained in a Charpy impact test at -33°C in the surface layer of HAZ 11 is set to 50 J or more. In this case, the low-temperature toughness of the surface layer of HAZ 11 is sufficiently high. As a result, excellent low-temperature toughness can be obtained in the welded joint 1 as a whole.

[0037] The preferred lower limit of the absorbed energy obtained in a Charpy impact test at -33°C on the surface of HAZ11 is 110 J, and more preferably 120 J. The upper limit of the absorbed energy obtained in a Charpy impact test at -33°C on the surface of HAZ11 is not particularly limited. For example, the upper limit of the absorbed energy obtained in a Charpy impact test at -33°C on HAZ11 is 350 J, for example 320 J, for example 300 J, for example 250 J.

[0038] [Method for measuring the low-temperature toughness of HAZ11] The low-temperature toughness of HAZ11 is determined by the following method. Referring to Figure 3, a full-size V-notch test specimen 40 is taken in accordance with JIS Z 2242:2023. The size of the V-notch test specimen 40 is 10 mm in width (length in the Y-axis direction) x 10 mm in thickness (length in the Z-axis direction) x 55 mm in length (length in the X-axis direction). The V-notch is positioned at the center of the X-axis direction of the V-notch test specimen 40. The V-notch extends in the Z-axis direction, and the notch depth is 2 mm. The V-notch test specimen 40 is taken so that the melting line 20E is positioned at the center of the Z-axis direction of the bottom of the V-notch.

[0039] The collected V-notch test specimen 40 is cooled to -33°C. A Charpy impact test is performed on the cooled V-notch test specimen 40 in accordance with JIS Z 2242:2023 to obtain the absorbed energy (J) at -33°C.

[0040] [(Feature 2) Maximum Vickers hardness H in the surface region near the toe of HAZ11 Ｈｍａｘ [Regarding] Figure 4 is an enlarged view of the region in Figure 2 that includes the weld metal 20 and HAZ 11. Referring to Figure 4, the surface region of HAZ 11 with a width d = 5.0 mm in the X-axis direction and away from the weld metal 20, from the toe ED which is the boundary point between the weld metal 20 and HAZ 11, is defined as the toe vicinity region 110.

[0041] As described above, ammonia SCC resistance is affected by the hardness of the surface layer of HAZ11. The surface layer of the region 110 near the toe of the HAZ11 is most affected by the heat during welding. Therefore, the hardness of the surface layer of the region 110 near the toe of the HAZ11 tends to be the highest. In this embodiment, the ammonia SCC resistance of the welded joint 1 is enhanced by suppressing the hardness of the surface layer of the region 110 near the toe, while satisfying the above feature 1.

[0042] Specifically, in the region near the tip 110, the maximum Vickers hardness H is measured at a measurement point P110 located 0.5 mm deep from the surface of HAZ11 in the thickness direction (Z-axis direction) of HAZ11. Ｈｍａｘ Set the value to 300HV1 or less. Maximum Vickers hardness H at measurement point P110. ＨｍａｘIf the hardness is 300HV1 or less, the hardness of the surface layer of HAZ11 is sufficiently suppressed. Therefore, assuming that feature 1 is satisfied, excellent ammonia SCC resistance can be obtained in the welded joint 1.

[0043] Maximum Vickers hardness H Ｈｍａｘ The preferred upper limit is 290 HV1, more preferably 280 HV1, and even more preferably 270 HV1. Maximum Vickers hardness H Ｈｍａｘ The lower limit is not particularly limited. Maximum Vickers hardness H Ｈｍａｘ The lower limit is, for example, 150HV1.

[0044] [Maximum Vickers hardness H at the surface of the region 110 near the toe end of HAZ11] Ｈｍａｘ [Measurement Method] Maximum Vickers hardness H in the surface layer of the region 110 near the toe end of the HAZ 11 of the welded joint 1. Ｈｍａｘ This can be found using the following method.

[0045] Figure 5 is an enlarged view of the region 110 near the toe of HAZ 11 in Figure 4. Referring to Figures 4 and 5, a test specimen is taken with the measurement surface being a cross section perpendicular to the weld metal extension direction Y of the weld joint 1 (i.e., the XZ plane). The test specimen is taken so that the measurement surface includes the region 110 near the toe and the surface of HAZ 11.

[0046] A line segment LS is drawn parallel to the surface of the toe region 110 of the measurement surface of the test specimen, with a depth of D1 = 0.5 mm in the thickness direction (Z-axis direction). The intersection of line segment LS and the melt line 20E is defined as point M0. Using point M0 as the base point, measurement points M1 to M10 are set at D2 = 0.5 mm intervals. A Vickers hardness test is performed on the 10 measurement points Mj (j is 1 to 10) in accordance with JIS Z 2244-1:2024. The test force is 1 kgf. The maximum value among the 10 obtained Vickers hardness values ​​is defined as the maximum Vickers hardness H. Ｈｍａｘ Let's call it (HV1).

[0047] [Effects of the Welded Joint 1 of this Embodiment] The welded joint 1 of this embodiment satisfies the above-described features 1 and 2. Therefore, the welded joint 1 provides excellent low-temperature toughness and excellent ammonia SCC resistance. These effects will be explained below.

[0048] [Regarding the low-temperature toughness of the welded joint 1] In the welded joint 1 of this embodiment, excellent low-temperature toughness can be obtained by satisfying features 1 and 2. Low-temperature toughness is evaluated by the amount of crack opening displacement (hereinafter referred to as the CTOD value δ) obtained by the CTOD test (Crack Tip Opening Displacement test) shown below.

[0049] [CTOD Test Method] The CTOD test will be conducted according to the following method: The CTOD test will be conducted in accordance with BS7448 (British Standard) Part 1 (1991) and BS7448 (British Standard) Part 2 (1997).

[0050] Specifically, as shown in Figure 6, a CTOD test specimen 50 is taken from the welded joint 1, with a Y-axis length twice the Z-axis length, an X-axis length five times the Z-axis length, and a Z-axis length equal to the total thickness of the welded joint 1. An initial crack (mechanical notch and fatigue notch) CR is introduced so that the YZ plane of the CTOD test specimen 50 becomes the fracture surface of the crack. The initial crack CR is introduced so that it extends along the entire width of the CTOD test specimen in the Z-axis direction, and the depth direction of the crack is in the Y-axis direction. Furthermore, the initial crack CR is introduced so that its end is included within the region near the toe 110.

[0051] A three-point bending test is performed on the prepared CTOD test specimen 50. At this time, a bending load is applied to the CTOD test specimen 50 so that the crack propagates in the Y-axis direction from the initial crack CR.

[0052] The test temperature is set to -10°C, and the test is conducted in air to obtain the CTOD value δ (mm). The test is performed on three CTOD test specimens 50. Of the three CTOD values ​​δ obtained, the lowest value is taken as the CTOD value δ for the welded joint 1. If the CTOD value δ at the test temperature of -10°C is 0.20 (mm) or higher, it is determined that excellent low-temperature toughness has been obtained in the welded joint 1.

[0053] [Regarding the ammonia SCC resistance of the welded joint 1] In this embodiment, the welded joint 1 is able to obtain excellent ammonia SCC resistance by satisfying features 1 and 2. Here, "obtaining excellent ammonia SCC resistance" means that no cracks are observed in the test piece in the ammonia SCC resistance evaluation method described in the next section.

[0054] [Ammonia SCC Resistance Evaluation Test] The ammonia SCC resistance of the welded joint 1 is evaluated by the following method. Figure 7 is a schematic diagram showing the sampling positions of the four-point bending test specimen. Referring to Figure 7, a four-point bending test specimen 60 having a cross section perpendicular to the weld metal extension direction Y of the welded joint 1 (i.e., the XZ plane) is taken. The four-point bending test specimen 60 has a thickness (length in the Z-axis direction) of 1.5 mm, a width (length in the Y-axis direction) of 10 mm, and a length (length in the X-axis direction) of 70 mm. The distance G between the XY plane of the four-point bending test specimen 60 and the surface of HAZ 11 is 0.5 mm. Furthermore, in the X-axis direction, the position of the toe end ED of HAZ 11 is at the center position in the longitudinal direction (X-axis direction) of the four-point bending test specimen 60 (i.e., the longitudinal length L60 and length R60 of the four-point bending test specimen 60 in Figure 7 are the same length) when the four-point bending test specimen 60 is taken.

[0055] A strain gauge is attached to the center of the surface (corresponding to the surface of the steel plate) of the collected four-point bending test specimen 60. The four-point bending test specimen with the strain gauge attached is mounted in a four-point bending jig. At this time, bending stress is applied to the four-point bending test specimen so that the amount of strain applied to the outer (tensile) surface of the four-point bending test specimen (in Figure 7, the surface of the test specimen 60 that is closer to the surface of the base material 10) corresponds to the amount of strain that corresponds to the yield stress. The four-point bending test specimen mounted in the four-point bending jig and 50 g of ammonium carbamate are placed in an autoclave. After evacuating the autoclave containing the four-point bending test specimen, the air is replaced with nitrogen. After replacing with nitrogen, 1000 g of liquid ammonia is introduced into the autoclave. The autoclave is immersed in a constant temperature bath, and while the temperature of the liquid ammonia inside the autoclave is maintained at 20°C, oxygen is introduced into the autoclave so that its partial pressure is 10% of the ammonia vapor pressure. Then, a potentiostat is used to apply a potential of +1V (vs Pt) to the four-point bending test specimen, and it is held for a predetermined time. After the predetermined time, the cross-section of the four-point bending test specimen is observed with an optical microscope to check for the presence or absence of cracks. Specifically, the four-point bending test specimen 60 after the test is cut parallel to the X-axis direction at the center of a width of 10 mm in the Y-axis direction. The observation area is defined as a 15 mm range in the X-axis direction centered on the X-axis position of the cut surface. The observation area is examined with a 500x optical microscope to check for the presence or absence of cracks. If no cracks are found, it is determined that excellent ammonia SCC resistance has been obtained.

[0056] In this embodiment, excellent ammonia SCC resistance is considered to have been achieved if, in the ammonia SCC resistance evaluation test described above, the predetermined time is set to 96 hours (4 days), and no cracks are observed in the four-point bending test specimen after 96 hours. Furthermore, if, in the ammonia SCC resistance evaluation test, the predetermined time is set to 240 hours (10 days), and no cracks are observed in the four-point bending test specimen after 240 hours, it is determined that remarkably excellent ammonia SCC resistance has been achieved.

[0057] [Preferred form of the welded joint 1 of this embodiment] Preferably, the welded joint 1 of this embodiment further satisfies the following features 3 to 5. (Feature 3) The average Vickers hardness H at a depth position P12 of 0.5 mm in the thickness direction of the base material 10 from the surface of the normal part 12 of the base material 10, which is a region of the base material 10 other than HAZ 11 Ｂａｖｅ It is 240HV1 or less. (Feature 4) Maximum Vickers hardness H Ｈｍａｘ This is an average Vickers hardness of H Ｂａｖｅ It is 1.80 times or less. (Feature 5) The average Vickers hardness H at a depth position P20 of 0.5 mm from the surface of the weld metal 20 in the thickness direction of the weld metal 20. Ｗａｖｅ However, the average Vickers hardness is H Ｂａｖｅ It is less than 1.70 times. Features 3 to 5 are explained below.

[0058] [(Feature 3) Average Vickers hardness H of the surface layer of the normal part 12 of the base material Ｂａｖｅ [Regarding this] Preferably, the average Vickers hardness H of the surface layer of the normal portion 12 of the base material. Ｂａｖｅ The value is 240HV1 or less. In this case, assuming that features 4 and 5 are satisfied, the ammonia resistance SCC of the welded joint 1 is further enhanced.

[0059] Average Vickers hardness H Ｂａｖｅ The preferred upper limit is 235 HV1, more preferably 230 HV1, even more preferably 225 HV1, and even more preferably 220 HV1. Average Vickers hardness H Ｂａｖｅ The lower limit is not particularly limited, but for example it is 130HV1.

[0060] [Average Vickers hardness H of the surface layer of the normal part 12 of the base material] Ｂａｖｅ [Measurement Method] Average Vickers hardness H of the normal portion 12 of the base material of the welded joint 1. Ｂａｖｅ This can be found using the following method.

[0061] In the normal portion 12 of the base material of the welded joint 1, a test piece is taken with a cross-section parallel to the thickness direction (Z-axis direction) of the base material portion 10 as the measuring surface. The test piece is taken so that the surface of the normal portion 12 of the base material is included in the measuring surface. The HAZ 11 and the normal portion 12 of the base material can be distinguished based on the method described in [Method for distinguishing between HAZ 11 and normal portion 12 of the base material].

[0062] On the measurement surface, ten measurement points P12 (see Figure 4) are selected at a depth of 0.5 mm from the surface of the normal portion 12 of the base material in the thickness direction of the base material portion 10. The ten measurement points P12 are arranged parallel to each other in the X-axis direction, and the distance (pitch) between adjacent measurement points P12 is 0.5 mm. A Vickers hardness test is performed on the ten selected measurement points P12 in accordance with JIS Z 2244-1:2024. The test force is 1 kgf. The arithmetic mean of the Vickers hardness of the ten obtained points is defined as the average Vickers hardness H Ｂａｖｅ (HV1) Average Vickers hardness H Ｂａｖｅ (HV1) is the integer obtained by rounding the first decimal place of the obtained arithmetic mean.

[0063] [(Feature 4) Relationship between the hardness of the surface layer of HAZ11 and the hardness of the surface layer of the normal portion 12 of the base material] Preferably, the maximum Vickers hardness H of the surface layer of the region 110 near the toe of HAZ11 Ｈｍａｘ This is the average Vickers hardness H of the surface layer of the normal part 12 of the base material. Ｂａｖｅ It is less than 1.80 times that amount.

[0064] As described above, in the welded joint 1, the maximum Vickers hardness H is the surface layer of the region 110 near the toe of the HAZ 11. Ｈｍａｘ If the hardness is 300HV1 or less, the hardness of the surface layer of HAZ11 is sufficiently suppressed. Therefore, excellent ammonia resistance SCC is obtained in the welded joint 1.

[0065] Incidentally, ammonia resistance (SCC) can be further improved not only by reducing the absolute value of the hardness of the surface layer of HAZ11, but also by suppressing variations in the hardness of the surface layer of HAZ11 in the base material portion 10 and the surface layer of the normal base material portion 12. Maximum Vickers hardness H of the surface layer near the toe region 110 ＨｍａｘHowever, the average Vickers hardness H of the surface layer of the normal part 12 of the base material Ｂａｖｅ If the ratio is 1.80 times or less, the variation in hardness on the surface of the base material 10 is sufficiently suppressed. In this case, assuming that features 3 and 5 are satisfied, the ammonia resistance SCC is further enhanced.

[0066] Preferably, a maximum Vickers hardness H Ｈｍａｘ This is an average Vickers hardness of H Ｂａｖｅ It is 1.75 times or less, more preferably 1.70 times or less, more preferably 1.65 times or less, and even more preferably 1.60 times or less.

[0067] In welded joint 1, the maximum Vickers hardness is typically H. Ｈｍａｘ This is an average Vickers hardness of H Ｂａｖｅ It becomes higher than that. On the other hand, the maximum Vickers hardness H Ｈｍａｘ average Vickers hardness H Ｂａｖｅ It is difficult to adjust the value to be close to this. Therefore, preferably, the maximum Vickers hardness H Ｈｍａｘ This is an average Vickers hardness of H Ｂａｖｅ It is 1.03 times or more, and more preferably 1.05 times or more.

[0068] [(Feature 5) Relationship between the hardness of the surface layer of the weld metal 20 and the hardness of the surface layer of the base material normal portion 12] Preferably, the average Vickers hardness H at a depth of 0.5 mm from the surface of the weld metal 20 in the thickness direction of the weld metal 20 Ｗａｖｅ However, the average Vickers hardness H of the surface layer of the normal part 12 of the base material Ｂａｖｅ It is less than 1.70 times that amount.

[0069] Feature 4 suppresses variations in hardness on the surface of the base material portion 10 (variations in hardness on the surface of HAZ 11 and the surface of the normal portion of the base material 12). Similarly, if variations in hardness on the surface of the weld metal 20 and the surface of the normal portion of the base material 12 are also suppressed, the ammonia-resistant SCC performance will be further enhanced.

[0070] The average Vickers hardness H at a depth P20 (see Figure 4) 0.5 mm from the surface of the weld metal 20 in the thickness direction of the weld metal 20 Ｗａｖｅ However, the average Vickers hardness H of the surface layer of the normal part 12 of the base material ＢａｖｅIf the hardness is 1.70 times or less, the variation in hardness of the surface layer of the weld metal 20 and the surface layer of the base material normal part 12 is sufficiently suppressed. Therefore, assuming that features 3 and 4 are satisfied, the ammonia resistance SCC is further improved.

[0071] Preferably, the average Vickers hardness H Ｗａｖｅ This is an average Vickers hardness of H Ｂａｖｅ It is 1.65 times or less, and more preferably 1.60 times or less.

[0072] In welded joint 1, the average Vickers hardness is typically H. Ｗａｖｅ This is an average Vickers hardness of H Ｂａｖｅ It will be higher than that. On the other hand, the average Vickers hardness H Ｗａｖｅ average Vickers hardness H Ｂａｖｅ It is difficult to adjust the value to be close to this. Therefore, preferably, the average Vickers hardness H Ｗａｖｅ This is an average Vickers hardness of H Ｂａｖｅ This is more than 1.03 times.

[0073] [Average Vickers hardness H of the surface layer of weld metal 20] Ｗａｖｅ [Measurement Method] Average Vickers hardness H of the surface layer of the weld metal 20 of the welded joint 1 Ｗａｖｅ This can be found using the following method.

[0074] A test specimen is taken from the center of the weld metal 20 in the width direction X, with the measurement surface being a cross-section parallel to the weld metal extension direction Y. The test specimen is taken so that the measurement surface includes the surface of the weld metal 20. On the measurement surface, 10 points are selected from the surface of the weld metal 20 at a depth of 0.5 mm in the thickness direction Z from point TP, the center of the width of the weld metal 20, and these points are used as measurement points. The 10 measurement points are arranged parallel to the Y-axis direction of the measurement surface. The distance (pitch) between adjacent measurement points P20 is 0.5 mm. A Vickers hardness test is performed on the 10 selected measurement points in accordance with JIS Z 2244-1:2024. The test force is 1 kgf. The arithmetic mean of the Vickers hardness of the 10 points obtained is taken as the average Vickers hardness H Ｗａｖｅ Let's call it (HV1).

[0075] When the welded joint 1 of this embodiment satisfies not only features 1 and 2, but also features 3 to 5, excellent low-temperature toughness is obtained, and even better ammonia SCC resistance is obtained.

[0076] Specifically, if the welded joint 1 of this embodiment satisfies not only features 1 and 2 but also features 3 to 5, even if the test time in the [Ammonia SCC Resistance Evaluation Test] is extended from 96 hours to 240 hours (10 days), no cracks are observed in the test piece after immersion in the test solution for 240 hours, and even better ammonia SCC resistance can be obtained.

[0077] [Applications of Welded Joint 1] The welded joint 1 of this embodiment can be widely applied to applications requiring low-temperature toughness and ammonia SCC resistance. The welded joint 1 of this embodiment is suitable for ammonia tank applications, for example. However, the welded joint 1 of this embodiment is not limited to the above applications.

[0078] [Chemical composition of welded joint 1] The chemical composition of the base material 10 and weld metal 20 of the welded joint 1 in this embodiment is not particularly limited and may be a well-known chemical composition. The chemical composition of the base material 10 is, for example, C: 0.02 to 0.12% and Ni: 1.00% or less by mass. The Ni content in the chemical composition of the base material 10 may be 0%. The chemical composition of the weld metal 20 is, for example, C: 0.02 to 0.20% and Ni: 5.00% or less. The Ni content in the chemical composition of the weld metal 20 may be 0%.

[0079] The preferred lower limit for the C content of the base material 10 is 0.03%, more preferably 0.04%, and even more preferably 0.05%. The preferred upper limit for the C content of the base material 10 is 0.11%, more preferably 0.10%, and even more preferably 0.09%. The preferred lower limit for the Ni content of the base material 10 is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Ni content of the base material 10 is 0.90%, more preferably 0.80%, and even more preferably 0.50%.

[0080] The preferred lower limit for the carbon content of the weld metal 20 is 0.03%, more preferably 0.05%, and still more preferably 0.08%. The preferred upper limit for the carbon content of the weld metal 20 is 0.18%, more preferably 0.16%, and still more preferably 0.14%. The preferred lower limit for the nickel content of the weld metal 20 is 0.10%, more preferably 0.50%, and still more preferably 1.00%. The preferred upper limit for the nickel content of the weld metal 20 is 4.50%, more preferably 4.00%, and still more preferably 3.50%.

[0081] [Another example of the chemical composition of the base material 10] An example of the chemical composition of the base material 10 is as follows (in mass%): C: 0.02-0.12%, Si: 0.01-0.30%, Mn: 0.50-2.00%, P: 0.025% or less, S: 0.0250% or less, Al: 0.001-0.100%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.001-0.100%, Cu: 0-1 The material contains 0.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, B: 0-0.0050%, V: 0-0.100%, Ti: 0-0.100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Sn: 0-0.1%, and Sb: 0-0.05%, with the remainder being Fe and impurities. The impurities are, for example, one or more selected from the group consisting of Sb, As, and Zn, which may be mixed in from scrap, etc. The Sb content, As content, and Zn content are, for example, 0.01% or less each. The chemical composition of the base material 10 is not limited to the above chemical composition. The above chemical composition is an example of the chemical composition of the base material 10.

[0082] [Another example of the chemical composition of weld metal 20] An example of the chemical composition of weld metal 20 is as follows, in mass%, C: 0.02-0.20%, Si: 0.01-2.00%, Mn: 0.5-3.0%, P: 0.025% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, O: 0.010% or less, Cu: 0-1.00%, Ni: 0-5.00%, Cr: 0-1.00%, Mo: 0-1.00%, Nb: 0-0.100%, V: 0-0.100%, Ti: 0-0.300%, B: 0-0.0050%, Ca: 0-0.010%, and rare earth elements: 0-0.010%, with the remainder being Fe and impurities.

[0083] [Regarding the thickness of the base material portion 10 of the welded joint 1] The thickness of the base material portion 10 of the welded joint 1 is not particularly limited. The thickness of the base material portion 10 is, for example, 6 to 60 mm. The preferred upper limit of the thickness of the base material portion 10 is 55 mm, more preferably 50 mm, and even more preferably 40 mm. The preferred lower limit of the thickness of the base material portion 10 is 10 mm, more preferably 15 mm, even more preferably 20 mm, and even more preferably more than 25 mm.

[0084] [Method for Manufacturing Welded Joint 1] An example of the method for manufacturing the welded joint 1 of this embodiment is as follows. However, the welded joint 1 of this embodiment is not limited to the method described below and may be manufactured by other methods.

[0085] The manufacturing method of the welded joint 1 of this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Welding step Each step will be described below.

[0086] [(Step 1) Material Preparation Step] In the material preparation step, a steel plate corresponding to the base material portion 10 of the welded joint 1 is prepared. Preferably, the average Vickers hardness H at a depth of 0.5 mm from the surface in the thickness direction is Ｂａｖｅ Prepare steel plates with a Vickers hardness of 240HV1 or less. However, the average Vickers hardness of the steel plates must be H Ｂａｖｅ This is not limited to the above. The chemical composition of the steel sheet is not particularly limited. The chemical composition of the steel sheet is, for example, the chemical composition of the base material 10 described above.

[0087] [(Step 2) Welding Process] Welding is performed on the prepared steel plate. A well-known welding method can be applied. Examples of welding methods include TIG welding (GTAW), shielded metal arc welding (SMAW), flux-cored wire arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW).

[0088] In welding, multi-layer welding is performed. At this time, the chemical composition of the welding material that serves as the raw material for the weld metal 20 is not particularly limited. The welding material may, for example, have the same chemical composition as the weld metal 20 described above. The chemical composition of the welding material may, for example, contain, by mass%, C: 0.02 to 0.20% and Ni: 5.00% or less.

[0089] In multilayer welding, one operation to form one layer of weld bead is called "one pass." In multilayer welding, multiple passes are performed to layer multiple weld beads and form the weld metal 20. The conditions for multilayer welding are as follows: (Condition 1) The heat input in the final pass is 3.5 to 5.0 kJ / mm. (Condition 2) The average heat input of passes other than the final pass is 1.0 to 3.0 kJ / mm. Conditions 1 and 2 will be explained below.

[0090] [Condition 1: Heat Input in the Final Pass] The heat input in the final pass affects the grain size of the HAZ 11 and the hardness of the toe-near-region 110. If the heat input in the final pass is less than 3.5 kJ / mm, the heat input to the surface of the toe-near-region 110 during welding will be insufficient. In this case, the cooling rate of the surface of the toe-near-region 110 will be excessively fast. As a result, the hardness of the toe-near-region 110 will be excessively high. On the other hand, if the heat input in the final pass exceeds 5.0 kJ / mm, the heat input to the surface of the HAZ 11 during welding will be excessive. In this case, the grain size of the HAZ 11 of the manufactured welded joint 1 will be coarse. As a result, the absorbed energy obtained in the Charpy impact test at -33°C on the surface of the HAZ 11 will be less than 50 J. Therefore, the heat input in the final pass should be set to 3.5 to 5.0 kJ / mm.

[0091] Preferably, the heat input in the final pass is 3.9 kJ / mm or more. In this case, the average Vickers hardness H at a depth of 0.5 mm in the thickness direction from the surface of the steel plate that forms the base material is Ｂａｖｅ If the value is 240HV1 or less, the manufactured welded joint will satisfy not only features 1 and 2, but also features 3 to 5.

[0092] [(Condition 2) Average heat input of passes other than the final pass] The average heat input of passes other than the final pass affects the amount of heat accumulated in the weld metal 20 and HAZ 11 before the final pass is performed. If the average heat input exceeds 3.0 kJ / mm, the amount of heat accumulated in the weld metal 20 and HAZ 11 before the final pass is performed is excessive. In this case, the crystal grains on the surface of the HAZ 11 of the manufactured welded joint 1 become coarse. As a result, the absorbed energy obtained in the Charpy impact test at -33°C on the surface of the HAZ 11 will be less than 50 J. Therefore, the average heat input of passes other than the final pass should be 3.0 kJ / mm or less. Note that if the average heat input of passes other than the final pass is too low, the weld metal 20 will not be formed at all. If the average heat input is 1.0 kJ / mm or more, the weld metal 20 will be formed.

[0093] The welded joint 1 of this embodiment is manufactured by the manufacturing method described above.

[0094] [About the ammonia tank of this embodiment] The ammonia tank of this embodiment comprises an outer shell, an inner shell, and an insulating material. The insulating material is placed between the outer shell and the inner shell. The outer shell and the insulating material have a well-known configuration. The inner shell comprises a base material made of steel and a weld metal made of steel. In other words, the inner shell includes the welded joint of this embodiment described above. Therefore, the inner shell of the ammonia tank satisfies the above-described features 1 and 2. Preferably, the inner shell further satisfies the above-described features 3 to 5. Therefore, the ammonia tank of this embodiment has excellent low-temperature toughness and excellent ammonia SCC resistance.

[0095] The effects of the welded joint of this embodiment will be specifically described below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effectiveness of the welded joint of this embodiment. Therefore, the welded joint of this embodiment is not limited to this one example of conditions.

[0096] Steel plates for each test number shown in Table 1 were prepared. The average Vickers hardness H was measured at a depth of 0.5 mm from the surface of each steel plate in the thickness direction. Ｂａｖｅ The above-mentioned average Vickers hardness H of the surface layer of the normal part 12 of the base material. Ｂａｖｅ The same method as in [Measurement Method] was used to determine the results. The obtained results are shown in Table 1. The chemical composition of the steel plates for each test number was as follows, in mass%, C: 0.02 to 0.12% and Ni: 1.00% or less.

[0097]

[0098] The ends of the steel plates for each test number were processed into a V-groove. Then, the ends of a pair of steel plates were joined together, and multi-layer welding was performed using GMAW welding. The chemical composition of the filler material (welding material) used in GMAW welding contained, by mass%, C: 0.02-0.20% and Ni: 5.00% or less. The heat input (kJ / mm) in the final pass of the multi-layer welding, and the average heat input (kJ / mm) in passes other than the final pass, are shown in Table 1. Welded joints for each test number were manufactured using the above manufacturing process.

[0099] The following tests were performed on each welded joint with the specified test number: (Test 1) Low-temperature toughness measurement test of the surface layer of the heat-affected zone (HAZ) (Test 2) Maximum Vickers hardness H in the region near the toe of the HAZ Ｈｍａｘ Measurement test (Test 3) Average Vickers hardness H of the surface layer of the normal part of the base material Ｂａｖｅ Measurement test (Test 4) Average Vickers hardness H of the surface layer of the weld metal Ｗａｖｅ Measurement tests (Test 5) CTOD test of welded joints (Test 6) Ammonia resistance SCC evaluation test of welded joints Tests 1 to 6 will be explained below.

[0100] [(Test 1) Low-temperature toughness measurement test on the surface of the HAZ] Based on the method described in [Method for measuring low-temperature toughness of HAZ11] above, the absorbed energy (J) obtained from the Charpy impact test at -33°C on the HAZ of the welded joint for each test number was determined. The results are shown in Table 1.

[0101] [(Test 2) Maximum Vickers hardness H in the region near the toe end of HAZＨｍａｘ [Measurement test of the above] Maximum Vickers hardness H at the surface of the region 110 near the toe end of HAZ11 Ｈｍａｘ Based on the method described in [Measurement Method], the maximum Vickers hardness H of the welded joint for each test number was measured. Ｈｍａｘ (HV1) was calculated. The results are shown in Table 1.

[0102] [(Test 3) Average Vickers hardness H of the surface layer of the normal part of the base material Ｂａｖｅ [Measurement test of the average Vickers hardness H of the surface layer of the normal part 12 of the base material] Ｂａｖｅ Based on the method described in [Measurement Method], the average Vickers hardness H of the welded joints for each test number was measured. Ｂａｖｅ (HV1) was calculated. The results are shown in Table 1.

[0103] [(Test 4) Average Vickers hardness H of the surface layer of the weld metal Ｗａｖｅ [Measurement test of the above] [Average Vickers hardness H of the surface layer of the weld metal 20] Ｗａｖｅ Based on the method described in [Measurement Method], the average Vickers hardness H of the welded joints for each test number was measured. Ｗａｖｅ (HV1) was calculated. The results are shown in Table 1.

[0104] [(Test 5) CTOD test of welded joints] Based on the method described in [CTOD test method] above, the CTOD value δ (mm) of each test numbered welded joint was determined. The results obtained are shown in Table 1.

[0105] [(Test 6) Ammonia SCC Resistance Evaluation Test of Welded Joints] Based on the method described in the [Ammonia SCC Resistance Evaluation Test] above, the ammonia SCC resistance of the welded joints for each test number was evaluated. For the ammonia SCC resistance evaluation test, two four-point bending test specimens were prepared for each test number. One four-point bending test specimen was immersed in the test bath for 96 hours (4 days) and then checked for cracks. The other four-point bending test specimen was immersed in the test bath for 240 hours (10 days) and then checked for cracks.

[0106] The evaluation results are shown in Table 1. In the "Ammonia SCC Resistance (4 days)" column of Table 1, "pass" indicates that, after immersion for 96 hours (4 days) in the ammonia SCC resistance evaluation test, no cracks were observed in the test specimen, indicating that excellent ammonia SCC resistance was obtained. In the "Ammonia SCC Resistance (4 days)" column, "fail" indicates that, after immersion for 96 hours (4 days) in the ammonia SCC resistance evaluation test, cracks were observed in the test specimen, indicating that sufficient ammonia SCC resistance was not obtained.

[0107] In Table 1, "pass" in the "Ammonia SCC Resistance (10 Days)" column indicates that, after immersion for 240 hours (10 days) in the ammonia SCC resistance evaluation test, no cracks were observed in the test specimen, indicating that excellent ammonia SCC resistance was achieved. In the "Ammonia SCC Resistance (10 Days)" column, "fail" indicates that, after immersion for 240 hours (10 days) in the ammonia SCC resistance evaluation test, cracks were observed in the test specimen, indicating that sufficient ammonia SCC resistance was not achieved.

[0108] [Evaluation Results] Referring to Table 1, in tests 1 to 12, the welded joints met both characteristic 1 and characteristic 2. As a result, the CTOD value δ was 0.20 mm or higher, indicating excellent low-temperature toughness in the welded joints. Furthermore, in the ammonia SCC resistance evaluation test, no cracks were observed in the test specimens after immersion in the test solution for 96 hours (4 days), indicating excellent ammonia SCC resistance in the welded joints.

[0109] In tests 1 through 6, the welded joints also met features 3 through 5. As a result, in the ammonia SCC resistance evaluation test, no cracks were observed in the test specimens after immersion in the test solution for 240 hours (10 days) in tests 1 through 6, demonstrating significantly superior ammonia SCC resistance compared to tests 7 through 12.

[0110] On the other hand, in tests 13 and 14, the heat input in the final pass of the multi-layer weld was excessive. As a result, the absorbed energy of the surface layer of the HAZ in the welded joint was less than 50 J. Consequently, the CTOD value δ was less than 0.20 mm, and sufficient low-temperature toughness could not be obtained.

[0111] In tests 15 and 16, the heat input in the final pass of the multi-layer weld was insufficient. As a result, the Vickers hardness H in the region near the toe of the weld joint was low. Ｈｍａｘ The pressure was too high. As a result, sufficient ammonia resistance (SCC) could not be achieved.

[0112] In tests 17 and 18, the average heat input was excessive in all passes except the final pass during multi-layer welding. As a result, the surface energy absorbed in the HAZ of the welded joint was less than 50 J. Consequently, the CTOD value δ was less than 0.20 mm, and sufficient low-temperature toughness was not achieved.

[0113] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

[0114] 1. Welded joint 10. Base metal portion 11. Heat-affected zone 12. Normal portion of base metal 20. Weld metal

Claims

1. The weld metal comprises a base metal made of steel and a weld metal made of steel, wherein the absorbed energy obtained in a Charpy impact test at -33°C in the surface layer of the heat-affected zone of the base metal is 50 J or more, and the maximum Vickers hardness H is measured at a depth of 0.5 mm from the surface in the thickness direction of the heat-affected zone in the region near the toe of the heat-affected zone. Ｈｍａｘ A welded joint whose voltage is 300HV1 or less.

2. The welded joint according to claim 1, further comprising: an average Vickers hardness H at a depth of 0.5 mm in the thickness direction of the base material from the surface of the normal portion of the base material, which is the region of the base material other than the heat-affected zone of the weld; Ｂａｖｅ The maximum Vickers hardness H is 240HV1 or less. Ｈｍａｘ However, the average Vickers hardness H Ｂａｖｅ The average Vickers hardness H is 1.80 times or less, and is measured at a depth of 0.5 mm from the surface of the weld metal in the thickness direction of the weld metal. Ｗａｖｅ However, the average Vickers hardness H Ｂａｖｅ A welded joint that is 1.70 times or less.

3. A welded joint according to claim 1, wherein the chemical composition of the base material contains, by mass%, C: 0.02 to 0.12% and Ni: 1.00% or less, and the chemical composition of the weld metal contains, by mass%, C: 0.02 to 0.20% and Ni: 5.00% or less.

4. The weld comprises an outer shell, an inner shell, and an insulating material disposed between the outer shell and the inner shell, wherein the inner shell comprises a base material made of steel and a weld metal made of steel, wherein the absorbed energy obtained in a Charpy impact test at -33°C on the surface layer of the heat-affected zone of the base material is 50 J or more, and the maximum Vickers hardness H is measured at a depth of 0.5 mm from the surface in the thickness direction of the heat-affected zone in the region near the toe of the heat-affected zone. Ｈｍａｘ An ammonia tank with a voltage of 300HV1 or less.

5. The ammonia tank according to claim 4, further comprising an average Vickers hardness H at a depth of 0.5 mm in the thickness direction from the surface of the normal base material portion, which is a region of the base material portion other than the weld heat affected zone. Ｂａｖｅ is 240 HV1 or less, and the maximum Vickers hardness H Ｈｍａｘ is 1.80 times or less of the average Vickers hardness H Ｂａｖｅ and the average Vickers hardness H at a depth of 0.5 mm in the thickness direction from the surface of the weld metal is Ｗａｖｅ 1.70 times or less of the average Vickers hardness H Ｂａｖｅ An ammonia tank.

6. An ammonia tank according to claim 4, wherein the chemical composition of the base material contains, by mass%, C: 0.02 to 0.12% and Ni: 1.00% or less, and the chemical composition of the weld metal contains, by mass%, C: 0.02 to 0.20% and Ni: 5.00% or less.

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