Welded joint manufacturing method, welded joint manufacturing condition calculation method, and welded joint

Abstract

Even when duplex stainless steel is welded with a large heat input, the sigma phase does not precipitate, ensuring toughness and corrosion resistance. [Solution] A method for manufacturing a welded joint includes a step of welding a base metal, which is duplex stainless steel, the main phases of which are ferrite and austenite, with a weld metal. The base metal contains, by mass, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N. The amount of ferrite in the base metal is 30% to 70% by volume. The heat input in the welding step is equal to or less than the upper heat input limit Q (J / mm) expressed by the following (Equation 1): Q = 280 × h + {80 × ([Cr] + [Mo]) 2 -4870×([Cr]+[Mo])+73800} (Formula 1) (In Equation 1, [Cr] represents the Cr content (mass%), [Mo] represents the Mo content (mass%), and h represents the thickness of the base material (mm).)

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a welded joint of duplex stainless steel, a method for calculating manufacturing conditions for a welded joint, and a welded joint. [Background technology]

[0002] Duplex stainless steel, which is composed primarily of Cr, Ni, Mo, and N and whose primary phases are ferrite and austenite, has ensured strength and corrosion resistance and is therefore used in welded structures used in environments requiring seawater resistance and sea salt particle resistance, such as ships, marine structures, bridges, seawater pumps, seawater desalination plants, river infrastructure such as sluice gates, and related equipment, as well as in environments requiring chloride resistance, such as production and storage tanks in chemical plants and food plants. Furthermore, due to the recent rise in the prices of Ni and Mo, low-cost duplex stainless steels (see Patent Document 1) have been developed that contain as little Ni and Mo as possible. These stainless steels have attracted attention as stainless steels that have high strength while maintaining corrosion resistance equivalent to that of austenitic stainless steels, the mainstream of stainless steels, and that also have low alloying costs and little price fluctuation.

[0003] When these duplex stainless steels are welded to construct steel structures, they are often used in the welded state. In particular, since the weld metal is used in its solidified state, it contains a larger amount of ferrite than steels of the same composition, which is known to result in reduced corrosion resistance and toughness. To avoid this, welding materials with increased Ni content are generally used when welding duplex stainless steels, and welding materials have also been developed that refine the crystal grains of the weld metal to improve toughness, ductility, and corrosion resistance (see Patent Document 2).

[0004] On the other hand, the weld heat-affected zone (HAZ) maintains the same composition as the steel, but its microstructure changes due to the welding thermal history, resulting in reduced corrosion resistance and toughness. In particular, in the region heated to a maximum temperature of approximately 1250°C or higher during welding (hereinafter referred to as the "high-temperature HAZ"), the microstructure changes significantly upon cooling after the ferrite phase is formed. This can result in reduced toughness and corrosion resistance. The microstructure of this region, like the weld metal, undergoes precipitation of acicular austenite at the ferrite grain boundaries during the cooling process, resulting in a dual-phase structure of ferrite and austenite. However, the relatively high cooling rate during welding suppresses austenite precipitation. Therefore, the microstructures of the weld metal and the HAZ are significantly different from those of the steel. The ferrite grains coarsen and the ferrite phase becomes significantly more abundant, potentially reducing toughness and corrosion resistance. In particular, the smaller the welding heat input, the faster the cooling rate, resulting in a greater proportion of the ferrite phase.

[0005] Furthermore, while nitrogen has a high solid solubility in the austenite phase, its solid solubility in the ferrite phase is extremely low. Because the ferrite phase is abundant in weld metal and high-temperature heat-affected zones, the nitrogen that cannot be dissolved precipitates as fine chromium nitrides within the ferrite grains. The precipitation of chromium nitrides is particularly accelerated when the welding heat input is large and the cooling rate is slow. A chromium-depleted zone forms around these finely precipitated chromium nitrides, reducing corrosion resistance. One method of resolving this issue is to perform solution heat treatment in heating equipment after welding (see Patent Documents 3 and 4). However, these methods are inefficient in welding work and cost-effective for large welded structures.

[0006] On the other hand, it is well known that during the welding process or high-temperature heat treatment of duplex stainless steel, the sigma phase, a hard and brittle intermetallic compound consisting of Fe, Cr, and Mo, precipitates, causing a decrease in toughness. Therefore, it is said that if the welding heat input is large and the cooling rate is slow during the welding process, the risk of sigma phase precipitation increases.

[0007] As such, the structure and properties of duplex stainless steel welds are governed by the cooling rate. Therefore, controlling the welding heat input, which significantly affects the cooling rate, is extremely important in welding duplex stainless steels. For this reason, the recommended welding heat input ranges shown in Table 1 below (see Non-Patent Document 1) have been proposed. Because the precipitation rate of sigma phases varies depending on the type of duplex stainless steel, the higher the content of alloying elements, the lower the recommended upper heat input limit and the narrower the recommended heat input range. Additionally, the American Petroleum Institute (API) recommends a heat input of 500 J / mm or more and 2500 J / mm or less. Therefore, current duplex stainless steel welded structures are manufactured using welding in accordance with these recommended welding heat input ranges.

[0008] [Table 1] [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2002 / 027056 [Patent Document 2] Patent No. 4531118 [Patent Document 3] Japanese Patent Application Publication No. 59-229414 [Patent Document 4] Japanese Patent Application Publication No. 60-2384323 [Non-patent literature]

[0010] [Non-Patent Document 1] Chemical Mechanical Welding Research Committee, "Guidelines for Welding Procedures of Duplex Stainless Steels", Japan Welding Society (2017) [Non-patent document 2] International Molybdenum Association, "Duplex Stainless Steel Processing Manual", 2nd Edition, IMOA Publishing (2009) Summary of the Invention [Problem to be solved by the invention]

[0011] Meanwhile, in recent years, demand for these duplex stainless steels has been increasing as corrosion-resistant materials that can withstand the severe corrosive environments that require resistance to seawater and sea salt particles in ships, marine structures, bridges, seawater pumps, seawater desalination plants, river infrastructure such as water gates, and related equipment, as well as chloride resistance in the production and storage tanks of various chemical and food plants. Furthermore, the use of thick duplex stainless steels is increasing in ships, water gates, and other river infrastructure and related equipment.

[0012] In addition, in recent years, the use of thicker duplex stainless steel has been increasing, and the application of large heat input welding is extremely effective in improving the welding efficiency of thicker materials. However, since there is an upper limit to the recommended welding heat input, the application of large heat input welding is difficult, and there is a need for a welding method for duplex stainless steel that improves welding efficiency while ensuring the corrosion resistance and toughness of the weld.

[0013] In duplex stainless steel, when the heat input is small, the cooling rate increases, the amount of ferrite increases, and corrosion resistance and toughness decrease. On the other hand, when the heat input is large, the cooling rate decreases, and sigma phase and chromium nitride precipitate, which decreases toughness and corrosion resistance. For this reason, a recommended welding heat input range is set for duplex stainless steel.

[0014] An object of the present invention is to provide a method for manufacturing a welded joint, a method for calculating manufacturing conditions for a welded joint, and a welded joint that can ensure toughness and corrosion resistance without precipitating a sigma phase even when duplex stainless steel is welded with a large heat input. [Means for solving the problem]

[0015] The present invention has been made to solve the above-mentioned problems, and is summarized as a method for manufacturing a welded joint, a method for calculating manufacturing conditions for a welded joint, and a welded joint as described below.

[0016] (1) A method for manufacturing a welded joint, comprising a welding step of welding a base material that is a duplex stainless steel, the main phase of which is a ferrite phase and an austenite phase, with a weld metal, The base material contains, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, A method for manufacturing a welded joint, wherein the amount of heat input in the welding step is equal to or less than an upper limit of heat input Q (J / mm) expressed by the following (Equation 1). Q = 280 × h + {80 × ([Cr] + [Mo]) 2 -4870×([Cr]+[Mo])+73800} (Formula 1) (In (Equation 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

[0017] (2) The method for manufacturing a welded joint according to (1), wherein the thickness of the base material is 5 mm or more.

[0018] (3) A method for manufacturing a welded joint according to (1) or (2), wherein the heat input is greater than 3500 J / mm.

[0019] (4) A method for manufacturing a welded joint according to any one of (1) to (3), wherein the heat input amount is equal to the upper heat input limit value.

[0020] (5) A method for calculating conditions for welding a base material that is a duplex stainless steel, the main phase of which is a ferrite phase and an austenite phase, with a weld metal, comprising: determining a heat input for welding; A method for calculating manufacturing conditions for a welded joint, wherein in the step of determining the heat input amount, the heat input amount is determined so that the heat input amount is equal to or less than an upper heat input limit value Q (J / mm) expressed by the following (Equation 1). Q = 280 × h + {80 × ([Cr] + [Mo]) 2-4870×([Cr]+[Mo])+73800} (Formula 1) (In (Equation 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

[0021] (6) The method for calculating manufacturing conditions for a welded joint according to (5), wherein in the step of determining the heat input, the heat input is determined so that the heat input is equal to the heat input upper limit value.

[0022] (7) A welded joint comprising a base metal that is a duplex stainless steel whose main phases are ferrite and austenite, and a weld metal, The base material contains, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, the base metal includes a high-temperature heat-affected zone in contact with the weld metal, The high-temperature heat-affected zone and the weld metal contain 30% by volume or more and 70% by volume or less of a ferrite phase, and do not contain a sigma phase.

[0023] (8) The welded joint according to (7), wherein the thickness of the base material is 5 mm or more.

[0024] (9) The welded joint according to (7) or (8), wherein the width of the high-temperature heat-affected zone is 0.5 mm or more.

[0025] (10) The welded joint according to any one of (7) to (9), wherein the high-temperature heat-affected zone and the weld metal do not contain chromium nitride.

[0026] (11) A welded joint according to any one of (7) to (10), having a critical pitting temperature of 15°C or higher.

[0027] (12) The welded joint according to (11), wherein the Charpy absorbed energy of the high-temperature heat-affected zone and the weld metal is 27 J or more. [Effects of the Invention]

[0028] According to the present invention, even when duplex stainless steel is welded with a large heat input, the sigma phase does not precipitate, and the toughness and corrosion resistance of the weld can be ensured. [Brief explanation of the drawings]

[0029] [Figure 1] FIG. 1 is a diagram showing the influence of plate thickness and heat input on the precipitation of the sigma phase in a welded portion of duplex stainless steel (SUS329J3L) used as the base metal of the welded joint according to this embodiment. [Figure 2] FIG. 2 is an isothermal transformation diagram of duplex stainless steel (SUS329J3L) used as the base metal of the welded joint according to this embodiment. [Figure 3] FIG. 3 is a diagram showing the influence of plate thickness and heat input on the cooling time from the melting point of the base metal to 700° C. for duplex stainless steel used as the base metal of the welded joint according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0030] As a result of extensive research conducted by the present inventors to solve the above problems, the present inventors have made the following findings.

[0031] (a) In duplex stainless steel, whose main phases are ferrite and austenite, the sigma phase precipitates when heat treated at approximately 800°C to 1000°C for approximately 1 to 10 minutes. The time when the sigma phase begins to precipitate depends on the composition of the duplex stainless steel, and it was found that the higher the Cr and Mo content, the shorter the precipitation start time.

[0032] (b) During the cooling process during welding, it was found that if the temperature is cooled to 700°C before the sigma phase begins to precipitate, the sigma phase does not precipitate.

[0033] (c) The cooling rate during welding depends on the plate thickness. The thicker the plate, the faster the cooling rate, and the shorter the cooling time to 700°C, making it difficult for the sigma phase to precipitate. This has led to the discovery that high heat input welding can be achieved without sigma phase precipitation.

[0034] The present inventors have conducted detailed investigations and studies into the microstructure, toughness, and corrosion resistance of welds formed by welding duplex stainless steel, the two main phases of which are ferrite and austenite.

[0035] As a result, it was discovered that in manufacturing welded joints, welding can be performed even with a large heat input without reducing toughness or corrosion resistance by welding using an upper limit heat input determined by the plate thickness, Cr content, and Mo content.

[0036] The present embodiment has been made based on the above findings. The present embodiment will be described in detail below. However, the present invention is not to be construed as being limited to the present embodiment.

[0037] (welded joints) The welded joint according to this embodiment includes a base material and a weld metal. The base material refers to the metal material to be welded after welding. The base material includes a high-temperature heat-affected zone that is affected by the heat input from welding. The weld metal refers to the portion of the molten metal that solidifies to form a joint, and the weld refers to the high-temperature heat-affected zone and the weld metal.

[0038] (Base material phase) In this embodiment, the base material of the weld joint is duplex stainless steel. In this disclosure, duplex stainless steel refers to stainless steel whose main phases are ferrite and austenite. In this disclosure, "main phases are ferrite and austenite" refers to a state in which the sum of the ferrite and austenite amounts is 95% by volume or more. In this disclosure, the ferrite amount refers to the phase fraction of the ferrite phase in the microstructure at room temperature, unless otherwise specified. In this disclosure, the austenite amount refers to the phase fraction of the austenite phase in the microstructure at room temperature, unless otherwise specified.

[0039] The ferrite phase in the base metal of the welded joint according to this embodiment contributes to improving strength and corrosion resistance. Therefore, the ferrite content of the base metal is 30% by volume or more. The ferrite content of the base metal is preferably 40% by volume or more. On the other hand, if the ferrite content is excessive, toughness and corrosion resistance decrease. Therefore, the ferrite content of the base metal is less than 70% by volume. The ferrite content of the base metal is preferably less than 60% by volume. From the viewpoints explained above, the ferrite content of the base metal is most preferably 50% by mass.

[0040] The microstructure of the base metal of the welded joint according to this embodiment is preferably a fine-grained mixed structure in which island-shaped austenite phases or block-shaped austenite phases extending in approximately one direction are dispersed in the ferrite phase. This structure can suppress the generation of chromium nitrides and improve weldability during welding. Furthermore, the microstructure of the high-temperature heat-affected zone is a two-phase structure in which lath-shaped austenite phases are precipitated at the grain boundaries of coarse ferrite grains and block-shaped austenite phases are precipitated within the grains.

[0041] If the base material according to this embodiment contains a sigma phase, the toughness and corrosion resistance will be reduced, so it is preferable that the base material according to this embodiment does not contain a sigma phase.

[0042] (Chemical composition of the base material) In the chemical composition of the base metal of the welded joint according to this embodiment, the content of each element is as follows. In this specification, "%" regarding components means "mass %" unless otherwise specified, and is distinguished from "volume %" used for the phase fraction of each phase regarding the structure.

[0043] Carbon (C) has the effect of improving strength at room temperature, but excessive C content reduces ductility and corrosion resistance. Therefore, the C content is preferably 0.10% or less, and more preferably 0.08% or less. On the other hand, in order to obtain the above effects, the C content is preferably 0.005% or more, and more preferably 0.010% or more.

[0044] It is preferable to add 0.10% or more of Si (silicon) for deoxidation. However, if Si is contained in excess, sigma phase tends to precipitate and toughness decreases. Therefore, the Si content is preferably 1.5% or less, and more preferably 1.0% or less.

[0045] Manganese (Mn) is an austenite-forming element that increases the austenite phase in duplex stainless steel, suppresses the formation of strain-induced martensite, improves toughness, and increases the solid solubility of nitrogen, suppressing the precipitation of chromium nitride in welds. Therefore, it is preferable to add 0.50% or more of Mn. Furthermore, to improve corrosion resistance, the Mn content is preferably 6.0% or less. The more preferable range of Mn content is 1.0% or more and 4.0% or less.

[0046] Since P (phosphorus) is an impurity that reduces hot workability and toughness, it is preferable to reduce its content as much as possible, preferably to 0.040% or less. However, since excessive reduction of P leads to an increase in manufacturing costs, it is preferable to keep it at 0.001% or more.

[0047] S (sulfur) is contained in steel as an impurity and is an element that reduces hot workability and ductility, so it is preferable to reduce the S content as much as possible, and it is desirable to keep it at 0.004% or less. Moreover, since excessive reduction of S leads to an increase in manufacturing costs, it is preferable to keep it at 0.001% or more.

[0048] Ni (nickel) is an austenite-forming element and, as a main element of duplex stainless steel, contributes to improving toughness. Therefore, the Ni content is preferably 0.1% or more, and more preferably 0.5% or more. On the other hand, excessive Ni content increases manufacturing costs. Therefore, the Ni content is preferably 8% or less.

[0049] Cr (chromium) is a ferrite-forming element and, as a major element of duplex stainless steel, contributes to improving corrosion resistance. For this reason, the Cr content is 20% or more. On the other hand, excessive Cr content reduces mechanical properties such as toughness. For this reason, the Cr content is 28% or less.

[0050] Mo (molybdenum) is a ferrite-forming element and, as a major element of duplex stainless steel, contributes to improving corrosion resistance. For this reason, the Mo content is 0.1% or more. However, excessive Mo content increases manufacturing costs and reduces manufacturability. For this reason, the Mo content is 5.0% or less.

[0051] Nitrogen (N) is effective in improving corrosion resistance, but it is also a strong austenite-forming element, with a particularly high diffusion rate and a tendency to redistribute, promoting the precipitation of austenite. For this reason, the N content is 0.1% or more. On the other hand, excessive N content reduces ductility. For this reason, the N content is 0.4% or less.

[0052] The base material of this embodiment may contain other elements, such as at least one of Cu, W, Co, Ca, Mg, B, and REM, where REM refers to Sc, Y, and lanthanoids, a total of 17 rare earth elements.

[0053] Cu is an element that additionally enhances the acid corrosion resistance of duplex stainless steel, stabilizes the austenite phase, and improves toughness. Therefore, the Cu content is preferably 0.3% or more. On the other hand, to suppress the occurrence of embrittlement, the Cu content is preferably 1.50% or less.

[0054] Furthermore, in order to improve hot workability, corrosion resistance, workability, etc., it is also possible to add, as necessary, W: 1.0% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.10% or less, B: 0.0050% or less, etc. The REM content mentioned above means the total content of the 17 rare earth elements mentioned above.

[0055] In the chemical composition of the base material of this embodiment, the balance is Fe (iron) and inevitable impurities. Here, "unavoidable impurities" refer to components that are inevitably mixed in due to various factors in raw materials such as ores and scraps and in the manufacturing process when the base material is industrially produced, and are acceptable within a range that does not adversely affect this embodiment.

[0056] The base material of the welded joint according to this embodiment may be at least one of lean duplex stainless steel, general-purpose duplex stainless steel, and super duplex stainless steel. Even in this case, high heat input welding can be achieved by the method for manufacturing a welded joint according to this embodiment. Examples of lean duplex stainless steel include duplex stainless steels such as SUS821L1, SUS323L (S32304), and SUS329J1. Examples of general-purpose duplex stainless steel include duplex stainless steels such as SUS329J3L (S31803) and SUS329J4L (S31260). Examples of super duplex stainless steel include duplex stainless steels such as SUS327L1 (S32750).

[0057] (Weld metal phase of welded joint) In this embodiment, the weld metal of the welded joint, i.e., the weld metal after welding, is a duplex stainless steel. In other words, the weld metal of the welded joint has a dual-phase structure in which the main phases are ferrite and austenite. The phase and composition conditions of the weld metal of the welded joint are the same as the phase and composition conditions of the base metal described above.

[0058] (Method for manufacturing welded joints) Next, a method for manufacturing a welded joint according to this embodiment will be described with reference to Figures 1, 2, and 3. The method for manufacturing a welded joint according to this embodiment is not particularly limited to a specific welding method or type of shielding gas, and TIG welding, MIG welding, MAG welding, plasma welding, submerged arc welding, and the like can be applied. The method for manufacturing a welded joint according to this embodiment also allows welding of the weld metal in an air atmosphere. Even when welding in an air atmosphere, welding with the above-mentioned heat input allows for highly efficient welding while maintaining the dual-phase structure. The shape of the welded joint is also not particularly limited.

[0059] The method for manufacturing a welded joint according to this embodiment includes a welding step in which a base material that is duplex stainless steel, the two main phases of which are ferrite and austenite, is welded with a weld metal. In this embodiment, the heat input in the welding step is set to satisfy predetermined conditions. The conditions for the heat input in the welding step are described in detail below. In this disclosure, the unit of heat input is J / mm, and the heat input refers to the value defined by the following (Equation 2). Heat input (J / mm) = welding current I (A) × arc voltage V (V) / welding speed v (mm / sec) (Equation 2)

[0060] FIG. 1 shows the effects of plate thickness and heat input on sigma phase precipitation in a welded joint made of duplex stainless steel (SUS329J3L) used as the base metal for the welded joint according to this embodiment. The SUS329J3L shown in FIG. 1 is a duplex stainless steel containing 22.6% Cr, 5.8% Ni, 3.1% Mo, 0.16% N, and the remainder Fe. FIG. 1 shows the microstructures of the weld metal and high-temperature heat-affected zone (HAZ) welded at an interpass temperature of 150°C with various heat inputs and varying plate thicknesses of the duplex stainless steel, the base metal for the welded joint. The microstructures of the weld metal and HAZ were evaluated by mirror-polishing the cross section of the welded joint, electrolytically etching it in a sodium hydroxide solution, and then examining the presence or absence of sigma phase precipitation using an optical microscope. As shown in FIG. 1, the sigma phase precipitates as the heat input increases, regardless of the plate thickness. However, the thicker the plate, the higher the upper limit of the heat input at which the sigma phase does not precipitate.

[0061] Fig. 2 is an isothermal transformation diagram of duplex stainless steel (SUS329J3L) used as the base metal of the welded joint according to this embodiment. Non-Patent Document 2 is used for the isothermal transformation diagram shown in Fig. 2. The isothermal transformation diagram shown in Fig. 2 shows precipitation start curves for sigma phase, nitrides, carbides, etc., all of which start to precipitate when the heat treatment time is approximately 100 seconds or longer. Furthermore, the sigma phase precipitates at approximately 800°C to 1000°C. Therefore, it is estimated that if the weld is cooled to 700°C or below before the sigma phase precipitation starts, the sigma phase will not precipitate.

[0062] Therefore, heat conduction analysis was performed to examine the cooling time from the melting point of the base material to 700°C when welding was performed using submerged arc welding (arc thermal efficiency: 1.0) with an interpass temperature of 150°C, while varying the plate thickness and heat input. The results are shown in Figure 3. Figure 3 is a diagram showing the influence of plate thickness and heat input on the cooling time from the melting point of the base material to 700°C for duplex stainless steel, which is used as the base material for the welded joint according to this embodiment. When the plate thickness is 5 mm or more, the greater the heat input, the longer the cooling time to 700°C. On the other hand, for any heat input, the thicker the plate, the shorter the cooling time to 700°C.

[0063] As can be seen from Figure 2, in duplex stainless steel (SUS329J3L), when the base material is cooled from its melting point to 700°C in 100 seconds, the sigma phase does not precipitate. Also, as can be seen from Figure 3, the relationship between plate thickness and heat input when the cooling time to 700°C is 100 seconds is roughly consistent with the upper limit of heat input at which the sigma phase does not precipitate for each plate thickness shown in Figure 1.

[0064] Therefore, once the cooling time to 700°C at which the sigma phase does not precipitate is determined, the relationship between the plate thickness at which the sigma phase does not precipitate and the amount of heat input can be found from Figure 3.

[0065] On the other hand, in duplex stainless steel, the Cr and Mo contents vary depending on the steel type, so the sigma phase precipitation start curve may differ from that shown in Figure 2. Therefore, isothermal heat treatment was performed on duplex stainless steel as a base material with the composition according to this embodiment, and the sigma phase precipitation start time was investigated. As a result, it was found that the sigma phase precipitation start time shortens as the Cr and Mo contents of the base material increase, and that this is expressed as a quadratic function of the sum of the Cr content and Mo content of the base material ([Cr] + [Mo]).

[0066] Based on the above findings, a method for manufacturing a welded joint according to this embodiment has been found. The method for manufacturing a welded joint according to this embodiment is a method for manufacturing a welded joint that includes a welding step of welding a base metal that is a duplex stainless steel, the main phases of which are ferrite and austenite, with a weld metal. The base metal contains, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N. The amount of ferrite in the base metal is 30% to 70% by volume. The heat input in the welding step is equal to or less than the upper heat input limit Q (J / mm) expressed by the following (Equation 1): Q = 280 × h + {80 × ([Cr] + [Mo]) 2 -4870×([Cr]+[Mo])+73800} (Formula 1) (In Equation 1, [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the thickness (mm) of the base material.) This prevents sigma phase from precipitating in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance.

[0067] In the method for manufacturing a welded joint according to this embodiment, the base metal is lean duplex stainless steel (e.g., SUS821L1), and the heat input in the welding process is preferably equal to or less than the upper heat input limit Q1 (J / mm) expressed by the following (Equation 3). This makes it possible to further suppress precipitation of sigma phase in the weld metal and high-temperature heat-affected zone, ensuring higher toughness and corrosion resistance. Q1=280×h+6000 (Formula 3)

[0068] In the method for manufacturing a welded joint according to this embodiment, the base metal is SUS329J3L, and the heat input in the welding process is preferably equal to or less than the upper heat input limit Q2 (J / mm) expressed by the following (Equation 4). This makes it possible to further suppress precipitation of sigma phase in the weld metal and high-temperature heat-affected zone, ensuring higher toughness and corrosion resistance. Q2=280×h+1500 (Formula 4)

[0069] In the method for manufacturing a welded joint according to this embodiment, the base metal is SUS329J4L, and the heat input in the welding process is preferably equal to or less than the upper heat input limit Q3 (J / mm) expressed by the following (Equation 5). This makes it possible to further suppress precipitation of sigma phase in the weld metal and high-temperature heat-affected zone, ensuring higher toughness and corrosion resistance. Q3=280×h+160 (Equation 5)

[0070] In the method for manufacturing a welded joint according to this embodiment, it is preferable that the base material is a super duplex stainless steel (e.g., SUS327L1) and that the heat input is equal to or less than the upper limit Q4 (J / mm) expressed by the following (Equation 6). This makes it possible to further suppress precipitation of sigma phase in the weld metal and high-temperature heat-affected zone, ensuring higher toughness and corrosion resistance. Q4=280×h-150 (Formula 6)

[0071] In the method for manufacturing a welded joint according to this embodiment, the base material is lean duplex stainless steel (e.g., SUS821L1), and the heat input is more preferably greater than 3000 J / mm and equal to or less than the upper heat input limit Q1 (J / mm). This makes it possible to manufacture a welded joint with high efficiency while suppressing a decrease in corrosion resistance and toughness, even in welding with a high heat input that exceeds the upper limit of the recommended welding heat input range.

[0072] In the method for manufacturing a welded joint according to this embodiment, the base material is SUS329J3L, and the heat input is more preferably greater than 3000 J / mm and equal to or less than the upper heat input limit Q2 (J / mm). This makes it possible to manufacture a welded joint with high efficiency while suppressing a decrease in corrosion resistance and toughness, even with welding with a high heat input that exceeds the upper limit of the recommended welding heat input range.

[0073] In the method for manufacturing a welded joint according to this embodiment, the base material is SUS329J4L, and the heat input is more preferably greater than 2500 J / mm and equal to or less than the upper heat input limit Q3 (J / mm). This makes it possible to manufacture a welded joint with high efficiency while suppressing a decrease in corrosion resistance and toughness, even with welding with a high heat input that exceeds the upper limit of the recommended welding heat input range.

[0074] In the method for manufacturing a welded joint according to this embodiment, the base material is super duplex stainless steel (e.g., SUS327L1), and the heat input is more preferably greater than 2000 J / mm and equal to or less than the upper heat input limit Q4 (J / mm). This makes it possible to manufacture a welded joint with high efficiency while suppressing a decrease in corrosion resistance and toughness, even in welding with a high heat input that exceeds the upper limit of the recommended welding heat input range.

[0075] In the method for manufacturing a welded joint according to this embodiment, the thickness of the base metal is preferably 5 mm or more. This allows the upper heat input limit Q (J / mm) expressed by the above formula (1) to be increased, preventing sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance, and enabling the welded joint to be manufactured with high efficiency.

[0076] In the method for manufacturing a welded joint according to this embodiment, the heat input is preferably greater than 3500 J / mm, which makes it possible to manufacture a welded joint with higher efficiency while suppressing a decrease in corrosion resistance and toughness.

[0077] In the method for manufacturing a welded joint according to this embodiment, the thickness of the base metal is more preferably 13.63 mm or more. This allows the heat input in the welding process to be greater than 3500 J / mm and equal to or less than the upper heat input limit Q (J / mm), regardless of the Cr and Mo contents of the base metal, and prevents sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance, and enabling welded joints to be manufactured with high efficiency.

[0078] In the method for manufacturing a welded joint according to this embodiment, the sum of the Cr and Mo contents in the base metal is preferably 20.1% or more and less than 23.5% by mass. This allows the heat input in the welding process to be greater than 3500 J / mm and equal to or less than the upper heat input limit Q (J / mm), regardless of the thickness h of the base metal, and prevents sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance, and enabling welded joints to be manufactured with high efficiency.

[0079] In the method for manufacturing a welded joint according to this embodiment, it is more preferable that the heat input be equal to the upper heat input limit Q (J / mm). This prevents sigma phase precipitation in the weld metal and the high-temperature heat-affected zone, ensures toughness and corrosion resistance, and maximizes the heat input, allowing for even more efficient manufacturing of the welded joint.

[0080] The welding method according to this embodiment may be TIG welding, MIG welding, MAG welding, plasma welding, submerged arc welding, or the like, and there is no need to particularly limit the welding method.

[0081] Furthermore, based on the above findings, a method for calculating the manufacturing conditions for a welded joint according to this embodiment has been found. The method for calculating the manufacturing conditions for a welded joint according to this embodiment is a method for calculating the conditions for welding, with a weld metal, a base material that is duplex stainless steel, the two main phases of which are ferrite and austenite. The method for calculating the manufacturing conditions for a welded joint according to this embodiment includes a step of determining the heat input in welding. In the step of determining the heat input, the heat input is determined so that the heat input is equal to or less than the upper heat input limit Q (J / mm) expressed by the following (Equation 1): Q = 280 × h + {80 × ([Cr] + [Mo]) 2 -4870×([Cr]+[Mo])+73800} (Formula 1) (In Equation 1, [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the thickness (mm) of the base material.) This prevents sigma phase from precipitating in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance.

[0082] In the method for calculating the manufacturing conditions for a welded joint according to this embodiment, it is most preferable that in the step of determining the heat input, the heat input is determined so that the heat input is equal to the upper heat input limit value. This prevents sigma phase precipitation in the weld metal and the high-temperature heat-affected zone, ensures toughness and corrosion resistance, and maximizes the heat input, allowing the welded joint to be manufactured with even greater efficiency.

[0083] The welded joint according to this embodiment can be obtained under the manufacturing conditions for the welded joint according to this embodiment. The welded joint according to this embodiment is a welded joint including a base material that is a duplex stainless steel, the main phases of which are ferrite and austenite, and a weld metal. The base material contains, by mass, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N. The amount of ferrite in the base material is 30% to 70% by volume. The base material includes a high-temperature heat-affected zone in contact with the weld metal. The high-temperature heat-affected zone and the weld metal contain 30% to 70% by volume ferrite phase and no sigma phase. This results in a welded joint with ensured toughness and corrosion resistance.

[0084] In the welded joint according to this embodiment, the thickness of the base metal is preferably 5 mm or more. This allows the upper heat input limit Q (J / mm) expressed by the above formula (1) to be increased, preventing sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance, and enabling highly efficient production.

[0085] In the welded joint according to this embodiment, the width of the high-temperature heat-affected zone is preferably 0.5 mm or more. Even in welded joints where welding is performed with a large heat input and the width of the high-temperature heat-affected zone is large, sigma phase does not precipitate in the weld metal or the high-temperature heat-affected zone, and the welded joint can be manufactured with high efficiency while maintaining toughness and corrosion resistance. In this disclosure, the width of the high-temperature heat-affected zone refers to the average distance from the boundary between the high-temperature heat-affected zone and the weld metal to the boundary between the high-temperature heat-affected zone and the portion of the base metal other than the high-temperature heat-affected zone. The width of the high-temperature heat-affected zone can be measured by observing the cross section of the welded joint with an optical microscope or an electron microscope such as a scanning electron microscope (SEM) and identifying the boundary between the high-temperature heat-affected zone and the weld metal, and the boundary between the high-temperature heat-affected zone and the portion of the base metal other than the high-temperature heat-affected zone.

[0086] In the welded joint according to this embodiment, the thickness of the base metal is more preferably 13.63 mm or more. This allows the heat input in the welding process to be greater than 3500 J / mm and equal to or less than the upper heat input limit Q (J / mm), regardless of the Cr and Mo contents of the base metal. This prevents sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance, and enabling highly efficient production.

[0087] In the welded joint according to this embodiment, the sum of the Cr and Mo contents in the base metal is preferably 20.1% or more and less than 23.5% by mass. This allows the heat input in the welding process to be greater than 3500 J / mm and equal to or less than the upper heat input limit Q (J / mm), regardless of the base metal thickness h, and prevents sigma phase precipitation in the weld metal and high-temperature heat-affected zone, ensuring toughness and corrosion resistance and enabling highly efficient production.

[0088] In the welded joint according to this embodiment, the high-temperature heat-affected zone and the weld metal preferably do not contain chromium nitride, which can suppress a decrease in corrosion resistance due to the formation of a chromium-depleted zone.

[0089] In the welded joint according to this embodiment, the critical pitting temperature is preferably 15° C. or higher, which means that the welded joint has high corrosion resistance.

[0090] In the welded joint according to this embodiment, the Charpy absorbed energy of the high-temperature heat-affected zone and the weld metal is preferably equal to or greater than 27 J. This means that the welded joint has high toughness.

[0091] The shape of the welded joint according to this embodiment is not particularly limited and may be a butt joint, a fillet joint, or the like. [Example]

[0092] The present invention will be described below with reference to examples, but the present embodiment is not limited to these examples.

[0093] Table 2 shows the chemical composition, thickness, ferrite content, and critical pitting temperature (CPT) of the base metal. Table 3 shows the chemical composition of the welding consumables. A V-groove with a 60° groove was prepared at the butt end of the base metal shown in Table 2, and welded joints were produced by single-electrode or dual-electrode submerged arc welding using the welding consumables shown in Table 3 under the conditions shown in Table 4. Table 4 shows the welding conditions, heat input, and upper limit heat input calculated from (Equation 1), as well as the presence or absence of sigma phase precipitation in the weld metal and high-temperature heat-affected zone, as well as the toughness, corrosion resistance, and ferrite content. In the "CPT" column of Tables 2 and 4, "<5" indicates that the CPT was less than 5°C.

[0094] [Table 2]

[0095] [Table 3]

[0096] [Table 4]

[0097] The welds of the welded joints obtained in this way were evaluated for the presence of sigma phase precipitation in the weld metal and the high-temperature heat-affected zone, as well as for toughness and corrosion resistance. The presence of sigma phase was confirmed by mirror-polishing the cross section of the weld, electrolytically etching it in a sodium hydroxide solution, and then examining it under an optical microscope. Toughness was evaluated using Charpy impact tests in accordance with JIS Z 2242. 2 mm V-notch test specimens were taken from the center of the weld metal and the fusion boundary in the high-temperature heat-affected zone, and the test temperature was 0°C. Corrosion resistance was evaluated by wet-polishing the surface of test specimens taken from the surface layer of the weld with #600 emery paper and measuring the critical pitting temperature (CPT) using the electrochemical critical pitting temperature measurement method in accordance with JIS G 0590. The higher the CPT, the less likely pitting corrosion will occur and the higher the corrosion resistance. These results are also shown in Table 4.

[0098] For example, in Example No. 1, when a welded joint was produced by submerged arc welding with a heat input of 7360 J / mm using a 20 mm thick base metal A listed in Table 2 and a welding consumable listed in Table 3a, the heat input was lower than the upper limit calculated from Equation 1 and within the range of the present invention, so no sigma phase was observed in the weld metal or the high-temperature heat-affected zone. Therefore, the Charpy absorbed energy of the weld metal and the high-temperature heat-affected zone was high, at 100 J or more, and the CPT was also equivalent to that of base metal A. Furthermore, in Example No. 2, an example according to the present embodiment, the welded joint was produced by welding a 20 mm thick base metal A listed in Table 2 using a welding consumable listed in Table 3a at a heat input of 2586 J / mm, which is within the recommended welding heat input range (800 J / mm or more and 3000 J / mm or less) proposed by various organizations. However, the toughness and corrosion resistance of Example No. 1 were equivalent to those of Example No. 2.

[0099] As is clear from Table 4, in Examples No. 1 to No. 8, no sigma phase was observed in the weld metal and high-temperature heat-affected zone, and as a result, the Charpy absorbed energy of the weld metal and high-temperature heat-affected zone was high, and the CPT was also equivalent to that of the base material.

[0100] On the other hand, in the comparative examples Nos. 9 to 12, the welding was performed with a heat input greater than the upper heat input limit calculated from the above (Equation 1), and therefore sigma phases were confirmed in the weld metal and the high-temperature heat-affected zone. As a result, the Charpy absorbed energy of the weld metal and the high-temperature heat-affected zone was significantly lower than that of the examples Nos. 1 to 8, and the CPT was also lower than that of the examples Nos. 1 to 8.

[0101] Comparative Examples Nos. 13 to 15 were welded with a heat input lower than the upper limit of the heat input range defined by the present invention, so no sigma phase was observed in the weld metal or the high-temperature heat-affected zone. Therefore, the Charpy absorbed energy of the weld metal and the high-temperature heat-affected zone was high, at 100 J or more. However, the Cr content of the base metal (symbol H) of No. 13 was less than 20%, the Mo content of the base metal (symbol I) of No. 14 was less than 0.1%, and the N content of the base metal (symbol I) of No. 15 was less than 0.1%, so the CPT was less than 5°C, significantly lower than that of Examples Nos. 1 to 8.

[0102] From the above, it has been found that by applying the method for manufacturing a welded joint according to this embodiment, even when duplex stainless steel is welded with a large heat input, the sigma phase does not precipitate, and a welded joint can be obtained in which the toughness and corrosion resistance of the weld are ensured. [Industrial Applicability]

[0103] According to the present invention, even when duplex stainless steel is welded with a large heat input, sigma phase does not precipitate, and the toughness and corrosion resistance of the weld are ensured, thereby significantly improving welding efficiency. As a result, the welded joint according to the present invention can be used as a welded structure applied in fields such as ships, marine structures, bridges, seawater pumps, river infrastructure such as sluice gates and discharge pipes, and related facilities, and will make an extremely significant contribution to industry.

Claims

1. A method for manufacturing a welded joint, comprising a welding step of welding a base material that is a duplex stainless steel having two main phases, a ferrite phase and an austenite phase, with a weld metal, The base material is lean duplex stainless steel or SUS329J3L containing, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, The method for manufacturing a welded joint, wherein the heat input in the welding step is greater than 3000 J / mm and is equal to or less than an upper heat input limit Q (J / mm) represented by the following (Equation 1). Q=280×h+{80×([Cr]+[Mo])2-4870×([Cr]+[Mo])+73800}・・・・・・・・・(Formula 1) (In (Formula 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

2. A method for manufacturing a welded joint, comprising a welding step of welding a base material that is a duplex stainless steel, the main phases of which are ferrite and austenite, with a weld metal, The base material is SUS329J4L containing, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, The method for manufacturing a welded joint, wherein the heat input in the welding step is greater than 2500 J / mm and is equal to or less than an upper heat input limit Q (J / mm) represented by the following (Equation 1). Q=280×h+{80×([Cr]+[Mo])2-4870×([Cr]+[Mo])+73800}・・・・・・・・・(Formula 1) (In (Formula 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

3. A method for manufacturing a welded joint, comprising a welding step of welding a base material that is a duplex stainless steel, the main phases of which are ferrite and austenite, with a weld metal, The base material is a super duplex stainless steel containing, by mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, The method for manufacturing a welded joint, wherein the heat input in the welding step is greater than 2000 J / mm and is equal to or less than an upper heat input limit Q (J / mm) represented by the following (Equation 1). Q=280×h+{80×([Cr]+[Mo])2-4870×([Cr]+[Mo])+73800}・・・・・・・・・(Formula 1) (In (Formula 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

4. The method for manufacturing a welded joint according to claim 1 , wherein the base metal has a plate thickness of 5 mm or more.

5. The method for manufacturing a welded joint according to claim 1 , wherein the heat input is greater than 3500 J / mm.

6. The method for manufacturing a welded joint according to claim 1 , wherein the heat input amount is equal to the upper heat input limit value.

7. A method for calculating conditions for welding a base material that is a duplex stainless steel, the main phase of which is a ferrite phase and an austenite phase, with a weld metal, comprising: determining a heat input for welding; In the step of determining the heat input amount, the heat input amount is determined so that the heat input amount is equal to or less than an upper heat input limit value Q (J / mm) expressed by the following (Equation 1). Q=280×h+{80×([Cr]+[Mo])2-4870×([Cr]+[Mo])+73800}・・・・・・・・・(Formula 1) (In (Formula 1), [Cr] represents the Cr content (mass%) of the base material, [Mo] represents the Mo content (mass%) of the base material, and h represents the plate thickness (mm) of the base material.)

8. 8. The method for calculating manufacturing conditions for a welded joint according to claim 7, wherein in the step of determining the heat input amount, the heat input amount is determined so that the heat input amount is equal to the heat input upper limit value.

9. A welded joint comprising a base material that is a duplex stainless steel having two main phases, a ferrite phase and an austenite phase, and a weld metal, The base material contains, in mass%, 20% to 28% Cr, 0.1% to 5.0% Mo, and 0.1% to 0.4% N, The amount of ferrite in the base material is 30% by volume or more and 70% by volume or less, the base metal includes a high-temperature heat-affected zone in contact with the weld metal, The high-temperature heat-affected zone and the weld metal contain 30% by volume or more and 70% by volume or less of a ferrite phase and do not contain a sigma phase, A welded joint, wherein the high-temperature heat-affected zone has a width of 0.5 mm or more.

10. A welded joint as described in claim 9, having a critical pitting corrosion temperature of 15°C or higher.

11. A welded joint as described in claim 9, wherein the Charpy absorbed energy of the high-temperature heat-affected zone and the weld metal is 27 J or more.

12. The welded joint according to any one of claims 9 to 11, wherein the base material has a plate thickness of 5 mm or more.

13. 12. The weld joint of claim 9, wherein the high-temperature heat affected zone and the weld metal are free of chromium nitrides.

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