JP2016078060A - Weld joint of duplex stainless steel and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a weld joint recovering the corrosion resistance of the area heated at high temperature of approx. 1,100°C or higher as the highest arrival temperature in the weld heat affected zone formed by welding a duplex stainless steel, and having excellent corrosion resistance in a corrosive environment.SOLUTION: A weld joint comprises the duplex stainless steel material which contains 20 mass% or more of Cr and 0.1 mass% or more of N in a chemical composition, comprises two phases of ferrite and austenite, and satisfies the expression: 0.0092×Q/h≤L≤0.029×Q/h-1, when defining the board thickness of the duplex stainless steel material as h (mm), and defining the distance between the melting boundaries of re-welded metal and initial weld joint after re-welding the welded metal with the heat input amount Q (J/mm) as L (mm).SELECTED DRAWING: Figure 3

Description

  The present invention relates to a welded structure used in an environment where seawater resistance and sea salt particle resistance are required, such as a ship, an offshore structure, a bridge, a seawater pump, and a seawater desalination device, and a chemical plant, food production. Duplex stainless steel used for assembling welded structures used in environments where chloride resistance is required, such as in plants, where the heat affected zone has corrosion resistance equivalent to or better than steel in a corrosive environment The present invention relates to a welded joint of steel and a method for manufacturing the same.

  The duplex stainless steel is a high-strength and highly corrosion-resistant stainless steel having Cr, Ni, Mo, and N as main elements and a two-phase structure of ferrite and austenite. In addition, as for duplex stainless steel, low-cost duplex stainless steel (for example, Patent Document 1) in which the amount of Ni and the amount of Mo is reduced as much as possible has been developed due to the recent rise in Ni and Mo. It has attracted attention as a stainless steel that has the same characteristics as a certain austenitic stainless steel, has a low alloy cost, and has little price fluctuation.

  On the other hand, in recent years, seawater resistance, sea salt particle resistance, such as ships, offshore structures, bridges, seawater pumps, seawater desalination equipment, and various chemical plants, food manufacturing plants, etc., which are demanded of chloride resistance As a corrosion resistant material that can withstand a corrosive environment, the demand for duplex stainless steel is increasing.

  By the way, when constructing a steel structure by welding the duplex stainless steel, many of them are used as they are without being subjected to post heat treatment after welding from the viewpoint of maintaining corrosion resistance. In particular, since the weld metal remains solidified, it is known that the amount of ferrite increases compared to a steel material having the same composition, and the corrosion resistance decreases. In order to avoid this, welding of duplex stainless steel generally uses a welding material with an increased amount of Ni, and further refines the crystal grain of the weld metal to improve toughness, ductility and corrosion resistance. Welding materials have also been developed (Patent Document 2). Here, the amount of ferrite indicates the phase fraction of the ferrite phase in the present specification unless otherwise specified.

  The weld heat affected zone remains the same composition as the steel material, but the structure changes due to the welding heat history, and the corrosion resistance decreases. At the time of welding, the structure of the region where the maximum reached temperature is about 1100 ° C. or less is almost the same as the structure of the steel part (base material part) that is not affected by heat, but film-like chromium nitride is present at the ferrite grain boundaries. It may precipitate and corrosion resistance may fall. In order to improve the corrosion resistance of the weld heat affected zone heated to this temperature region, it is effective to adjust the amount of alloy elements such as Ni and N to lower the nitride precipitation temperature (Patent Document 3).

  On the other hand, the region where the maximum temperature reached during welding is a high temperature of about 1100 ° C. or higher (hereinafter referred to as “high temperature heat affected zone” in this specification) is cooled after it once becomes a ferrite single phase. For this reason, acicular austenite precipitates at the ferrite grain boundary during the cooling process, resulting in a two-phase structure of ferrite and austenite. However, since the cooling rate during welding is relatively large, precipitation of austenite is suppressed. Therefore, the structure of the high-temperature heat-affected zone is greatly different from that of the base material, and the ferrite grains become coarse and the amount of ferrite becomes extremely large.

  Carbon and nitrogen have a high solid solubility in austenite, but since the solid solubility in ferrite is very small, the high temperature heat-affected zone has a large amount of ferrite, and carbon and nitrogen that cannot be completely dissolved are present. As chromium carbonitride, it precipitates finely in ferrite grains. Thus, a chromium-deficient layer is formed around the finely precipitated chromium carbonitride, and the corrosion resistance is lowered. As a method for improving this, it is disclosed that solution heat treatment is performed in a heating facility after welding (Patent Documents 4 and 5). However, in the case of a large-sized welded structure, these methods have poor welding construction efficiency and cannot be said to be effective methods from the viewpoint of cost.

  A temper bead method is known as a method for performing a heat treatment on a welding heat-affected zone without using such heating equipment (Patent Documents 6 and 7). The temper bead method is a method for recovering the toughness of the hardened part formed when repair welding carbon steel.After welding the first layer to the surface of the object using a predetermined welding wire, The remaining layers are stacked and welded, and tempered by the welding heat of this remaining layer welding to recover toughness deterioration due to hardening generated in the steel material.

International Publication No. 2002/027056 Japanese Patent No. 4531118 JP 2012-197509 A JP 59-229414 A JP 60-2384323 A JP 2000-271742 A JP 2012-115886 A

As described above, when duplex stainless steel is welded, there are two sites where corrosion resistance is a concern. It is the area | region (high temperature heat affected zone) heated to high temperature whose highest reached temperature is 1100 degreeC or more among a weld metal part and a heat affected zone.
The weld metal portion can improve the corrosion resistance of the weld metal itself by using a welding material to which a large amount of Ni is added. On the other hand, in order to improve the corrosion resistance of the high temperature heat affected zone, it is necessary to control the structure, and there is no effective means. The conventional temper bead method involves tempering a portion of the base material that has been quenched and hardened by the first layer welding with the welding heat of the remaining layer welding, and does not stand from the viewpoint of controlling the structure. Therefore, an object of the present invention is to improve the corrosion resistance by controlling the structure of the high-temperature heat-affected zone in welding of duplex stainless steel. And it aims at providing the welding method of the duplex stainless steel for obtaining the duplex stainless steel welded joint which has the outstanding corrosion resistance in a corrosive environment by solving this subject, and obtaining it.

As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.
(A) In ordinary duplex stainless steel welding, a region heated to a high temperature of about 1100 ° C. or higher among the heat affected zone (high temperature heat affected zone) is a fusion boundary line (welded metal and stainless steel). It was found to be included in a range of about 1 mm from the boundary line of the steel. In addition, the range of 1 mm from the melting boundary line means a range within 1 mm perpendicular to the melting boundary line in the cross section perpendicular to the welding line of the weld joint in the stainless steel.

(B) In the component of duplex stainless steel, if Cr: 20% or more, N: 0.1% or more (both mass%), by heat-treating the high-temperature heat-affected zone at 700 ° C to 1000 ° C, It has been found that the amount of ferrite can be suppressed, and as a result, the corrosion resistance is improved.

(C) In order to heat-treat the high-temperature heat-affected zone at 700 ° C. to 1000 ° C., it was found that this can be realized by re-welding the weld metal portion and limiting the welding heat input and re-welding position.

The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) A welded joint in which at least one is mass%, contains Cr: 20% or more, N: 0.1% or more, and is a duplex stainless steel composed of ferrite and austenite. The weld phase is re-welded at least once on the weld metal constituting the ferrite phase in the high-temperature heat-affected zone within the stainless steel and within 1 mm from the melting boundary line that is the boundary between the weld metal constituting the joint and the Slentes steel. A duplex stainless steel welded joint characterized in that the phase fraction of is less than 70% by volume.
(2) The welded joint is characterized by comprising, in mass%, Cr: 20% or more, N: 0.1% or more, and welding two-phase stainless steels whose structure is composed of ferrite and austenite. The duplex stainless steel welded joint according to (1).
(3) In the welded joint, the critical pitting corrosion occurrence temperature of the high temperature heat affected zone is not less than the critical pitting corrosion occurrence temperature of the stainless steel before welding. (1) or (2) Duplex stainless steel welded joint.
(4) At least one of the mass%, Cr: 20% or more, N: 0.1% or more, the structure is a welded joint manufacturing method that is a duplex stainless steel composed of ferrite and austenite, Reweld at least once on the weld metal constituting the joint, Q (J / mm) of the heat input of the reweld, the boundary line between the weld metal constituting the joint and the weld metal by re-welding, and the joint When the shortest distance between the weld metal to be configured and the melting boundary line that is the boundary of the Slentes steel is L (mm), and the thickness of the stainless steel is h (mm),
0.0092 * Q / h <= L <= 0.029 * Q / h-1, It is a manufacturing method of the duplex stainless steel welded joint characterized by the above-mentioned.
(5) The welded joint is characterized in that, in mass%, Cr: 20% or more, N: 0.1% or more is contained, and duplex stainless steels whose structures are composed of ferrite and austenite are welded together. The method for producing a duplex stainless steel welded joint according to (4).
(6) In the weld joint, the critical pitting corrosion temperature of the high temperature heat affected zone is equal to or higher than the critical pitting corrosion temperature of the stainless steel before welding. (4) or (5) Method for producing a duplex stainless steel welded joint.

  According to the present invention, the corrosion resistance of the welded heat affected zone formed by welding the duplex stainless steel is improved particularly in the region where the highest temperature is about 1100 ° C. or higher, and the corrosion resistance of the welded portion in a corrosive environment. Can be greatly improved.

It is a figure which shows sectional drawing of the welded joint by the butt welding which shows an example of conventional embodiment. It is a figure which shows the influence of the heat processing temperature which acts on the ferrite content of the high temperature heat affected zone in this invention. It is a figure which shows sectional drawing of the welded joint by the butt welding which shows an example of embodiment of this invention.

The present inventors investigated and examined in detail the structure and corrosion resistance of the weld heat affected zone formed by welding the duplex stainless steel.
As a result, in various duplex stainless steel welded joints, by re-welding the weld metal at a certain distance from the melting boundary line, the structure of the weld heat-affected zone near the melting boundary line changes, and the corrosion resistance can be improved. I found.
Hereinafter, the present invention will be described in detail. In this specification, “%” relating to a component means “mass%” unless otherwise specified, and is distinguished from “volume%” used in the phase fraction of each phase relating to a structure.

  First, the duplex stainless steel defined by the present invention is limited by the structure and chemical composition. The structure is a duplex stainless steel made of ferrite and austenite. In particular, in the case of stainless steel, a ferrite single phase is formed in a temperature range of 1100 ° C. or higher, but a two-phase structure of ferrite and austenite is formed at room temperature. Specifically, the ferrite content (phase fraction of the ferrite phase) at room temperature is 35% to 70% by volume, and the balance is the austenite phase.

  The component is stainless steel containing 20% or more of Cr and 0.1% or more of N by mass%. If the above structure is satisfied, alloy elements other than Cr and N are not particularly limited.

  Cr is a ferrite-forming element and contributes to the improvement of corrosion resistance as a main element of duplex stainless steel. However, if the Cr content is less than 20%, sufficient corrosion resistance cannot be obtained, so the content is limited to 20% or more.

  N is a powerful austenite-generating element as well as effective in improving corrosion resistance. In particular, N promotes the precipitation of austenite because of its high diffusion rate and easy redistribution. If the N content is less than 0.1%, sufficient corrosion resistance and austenite precipitation promoting effect cannot be obtained, so the content is limited to 0.1% or more.

  Next, the present invention will be described with reference to FIGS. 1 and 3 by taking a welded joint by butt welding as an example of an embodiment of the present invention as an example. In the present invention, the welding method is not particularly limited, and TIG welding, MIG welding, MAG welding, plasma welding, submerged welding, laser welding, and the like can be applied. Further, the shape of the joint is not particularly limited.

  When a butt-welded joint as shown in FIG. 1 is made of duplex stainless steel, a weld heat affected zone near the fusion boundary 3, particularly a region where the maximum temperature reached about 1100 ° C. or more (high temperature heat affected zone (see FIG. In the hatching region 1) 1), coarse ferrite grains are formed, and austenite precipitates from the ferrite grain boundaries. However, the amount of precipitated austenite is small, and the amount of ferrite is about 70% by volume or more. Many fine precipitates are observed in the ferrite grains. That is, the structure is completely different from that of the steel (base metal part) at a portion not affected by the welding heat. This is because when this region is heated to about 1100 ° C. or higher during welding and once cooled to a ferrite single phase, the cooling rate is relatively high, so that precipitation of austenite from the ferrite grain boundaries is suppressed. This is because. Furthermore, because the amount of ferrite increases, carbon and nitrogen that cannot be completely dissolved are finely precipitated in the ferrite grains as chromium carbonitride. In this way, in the weld heat-affected zone of the welded joint, since the maximum temperature reached 1100 ° C. or higher, the amount of ferrite is as high as about 70% by volume or more, and fine chromium carbonitride precipitates in the ferrite grains, resulting in corrosion resistance. Decreases. Such a high-temperature heat-affected zone 4 varies depending on the welding heat input, but in general welding, the high-temperature heat-affected zone 4 is a region within about 1 mm from the melting boundary 3 to the base material side. In addition, the fusion boundary line 3 is a boundary between the weld metal 2 and the Slentes steel constituting the weld joint. Moreover, the range of 1 mm from the melting boundary line means a range within 1 mm perpendicular to the melting boundary line in the cross section perpendicular to the welding line of the weld joint in the stainless steel.

  That is, the phase distribution of ferrite and austenite at the high temperature heat-affected zone 4 as it is welded is thermodynamically non-equilibrium due to the relatively high cooling rate. However, when this thermodynamically non-equilibrium structure is heat-treated at a temperature of about 700 ° C. to 1000 ° C., the phase fraction of ferrite and austenite approaches the equilibrium state, and austenite reprecipitates, and the amount of ferrite increases. Less (Figure 2). Furthermore, in the duplex stainless steel containing 0.1% or more of N, since the diffusion rate of N is large, redistribution of N easily occurs, austenite precipitates more, and the amount of ferrite is steel (base material part). It becomes 35 volume%-70 volume% equivalent. On the other hand, when the N content is less than 0.1%, the amount of ferrite remains as high as about 70% by volume even when heat treatment is performed at 700 ° C. to 1000 ° C. As described above, since the degree of decrease in the ferrite amount varies depending on the N content, as described above, the N content is limited to 0.1% or more in order to sufficiently exhibit the austenite precipitation promoting effect.

  However, even when the N content is 0.1% or more, in the case of the heat treatment at a temperature lower than 700 ° C. and the heat treatment at a temperature higher than 1000 ° C., the change in the ferrite amount is small and remains about 70% by volume or more. This is because when a temperature is lower than 700 ° C., precipitation of austenite becomes difficult, and when the temperature is higher than 1000 ° C., the ferrite single-phase region is heated again to be cooled. Further, in the case of heat treatment at about 700 ° C. to 1000 ° C., since austenite is reprecipitated, carbon and nitrogen are dissolved in austenite, and the amount of carbon and nitrogen in the ferrite grains is reduced. Precipitation of chromium carbonitride is suppressed.

  That is, when a welded joint is made of the duplex stainless steel material 1, the surface of the high temperature heat affected zone 4 within about 1 mm from the melting boundary line, that is, the region where the maximum temperature reached 1100 ° C. or more and the ferrite content is 70 By heating the weld heat affected zone that has reached volume% or more to about 700 ° C to 1000 ° C, the amount of ferrite is reduced to the same level as steel, and the precipitation of chromium carbonitride in the ferrite grains is suppressed. As a result, the corrosion resistance is improved.

  Based on the above findings, in the present invention, welding heat is applied as the heat source for reheating to about 700 ° C. to 1000 ° C.

  On the other hand, there is a temper bead method for recovering the toughness of the hardened zone of the weld heat affected zone as a method of using the welding heat for the modification of the structure of the weld heat affected zone (Patent Document). 6, 7). However, the target part effective in this temper bead method (hardening zone of the heat affected zone) and the heating temperature zone are heated to about 700 ° C. to 1000 ° C. within a range of 1 mm from the melting boundary line intended by the present invention. It is different from the condition range of That is, it is difficult to apply the temper bead method described in Patent Documents 6 and 7 to the problem to be solved by the present invention.

  That is, the conventional temper bead method involves tempering a portion where the base material has been quenched and hardened by the first layer welding with the welding heat of the remaining layer welding, and is not based on the viewpoint of controlling the structure. Therefore, in the welding of duplex stainless steel, the temper bead method cannot be immediately applied to improve the corrosion resistance by controlling the structure of the high temperature heat affected zone.

  Further, as a method of re-welding the welded portion, tanning welding or face-to-face welding of the weld toe portion by the TIG method is widely known in order to improve fatigue strength. However, these methods are intended to improve the welded portion shape, and it is difficult to improve the corrosion resistance of the weld heat affected zone of the duplex stainless steel. In other words, by re-welding the welded part of duplex stainless steel by this tanning welding or face welding, a new weld heat affected zone is formed, and the part (from the melting boundary line of tanning welding or face welding) This is because the corrosion resistance of the region within 1 mm is lowered.

  Therefore, in the present invention, as shown in FIG. 3, welding is performed again on the weld metal 6 (hereinafter referred to as re-welding), and the initial welded joint (hereinafter referred to as the first welded joint in this manner). The surface structure and corrosion resistance of the weld heat affected zone were investigated. Regarding the welding position of the re-welded metal (welded metal generated by re-welding), the molten boundary line 9 of the initial welded joint and the molten boundary line 10 of the re-welded metal 8 (the initial weld metal and the re-welded metal constituting the joint) Various studies were conducted using the shortest distance L to the parameter as a parameter. As a result, when L is small, the high-temperature heat-affected zone 7 whose corrosion resistance has deteriorated in the initial welded joint is heated again to the ferrite single phase region of 1100 ° C. or higher by re-welding, and the amount of ferrite remains large. Conversely, when L is large, the high-temperature heat-affected zone is not heated to a predetermined 700 ° C. to 1000 ° C. even by welding heat from re-welding, and is only heated to a temperature range below that, so the amount of ferrite changes. No improvement was observed in corrosion resistance. That is, in order to improve the corrosion resistance by reheating the region within 1 mm from the melting boundary line in the weld heat affected zone of the initial welded joint, that is, the surface of the high temperature heat affected zone 7 to 700 ° C. to 1000 ° C. It was revealed that there is an appropriate range for the welding position. On the other hand, since the heat history in re-welding also changes depending on the welding heat input and the plate thickness, they affect the appropriate range of the welding position for re-welding.

Therefore, the appropriate range of the distance L between the molten boundary line 9 of the initial welded joint and the molten boundary line 10 of the re-welded metal is set as the welding heat input Q and the plate thickness h of the re-welding to improve the corrosion resistance. A heat conduction analysis was conducted so that the surface of the affected part was 700 ° C to 1000 ° C. As a result, when the distance L between the melting boundary line of the initial welded joint and the melting boundary line of the re-welded metal is smaller than 0.0092 × Q / h (mm), the high temperature heat affected zone is 1100 ° C. or higher again. It was found that the amount of ferrite remains large and the corrosion resistance is not improved because it is heated to the ferrite single phase region. Similarly, if the distance L between the melting boundary line of the initial welded joint and the melting boundary line of the re-welded metal is larger than 0.029 × Q / h-1 (mm) as a result of the heat conduction analysis, the high temperature heat affected zone However, since it was not heated to 700 ° C., the amount of ferrite remained large, and it was also found that no improvement in corrosion resistance was observed. Therefore,
0.0092 × Q / h ≦ L ≦ 0.029 × Q / h−1 (Formula 1)
It has been found that satisfying this is a requirement for an appropriate welding position of the re-welded metal 8 in order to improve the corrosion resistance of the surface of the high-temperature heat-affected zone 7.
Here, the unit of the distance L between the melting boundary line of the initial welded joint and the melting boundary line of the re-welded metal is mm, the unit of the plate thickness h is mm, and the unit of the welding heat input Q is J / mm. The amount of heat input Q is defined by the following (Formula 2).
Weld heat input Q (J / mm) = Welding current I (A) × Welding voltage V (V) / Welding speed v (mm / sec) (2)

  FIG. 3 shows a case where the corrosion resistance of the weld heat affected zone on the right side of the initial welded joint is improved. When the corrosion resistance of the weld heat affected zone on the left side of the initial welded joint is improved, the initial welded joint is shown in FIG. The position of the re-welded metal is set with reference to the left melting boundary line.

  The re-welding method according to the present invention is not particularly limited, and TIG welding, MIG welding, MAG welding, plasma welding, submerged welding, laser welding, and the like can be applied. The welding material to be used is not particularly limited, but is preferably the same as the welding material used for manufacturing the joint. This is because the corrosion resistance of the weld metal part can be maintained by using the same welding material. In TIG welding or laser welding, welding may be performed without using a welding material. Further, the shape of the initial welded joint is not particularly limited, such as a fillet joint in addition to the butt joint. By re-welding to the welding position specified by (Equation 1) and (Equation 2) above, the structure of the weld heat affected zone in the initial welded joint is improved, and a welded joint of duplex stainless steel with excellent corrosion resistance is obtained. It is done.

Hereinafter, the present invention will be described with reference to examples.
Table 1 shows the critical pitting corrosion temperature (CPT) measured by a ferric chloride immersion test in accordance with the chemical composition, ferrite content, and ASTM G48 Method E regulations of various duplex stainless steel materials used as a base material. Table 2 shows the chemical composition of the welding material for duplex stainless steel. FIG. 1 shows the welding method and welding conditions shown in Table 3 using a V groove having a groove angle of 60 ° at the butt end of the duplex stainless steel material shown in Table 1 and using the welding material shown in Table 2. A welded joint was produced. The combinations are shown in the column of “Initial Welded Joint” in Table 4. Also, in the column of “Initial weld joint” in Table 4, in the weld heat affected zone of each weld joint, unlike the steel structure, coarse ferrite grains are formed, and the maximum ultimate temperature is estimated to be 1100 ° C. or higher. The width of the region to be processed and the amount of ferrite in the region are also shown. The information on the structure of the heat-affected zone was evaluated by mirror-polishing the cross section of the welded joint, performing electrolytic etching in a sodium hydroxide solution, and then performing optical microscope observation and image analysis. Next, as shown in FIG. 3, re-welding was performed on each welded joint thus produced on the weld metal. Reweld welding method, welding conditions, and welding position (distance L between the melting boundary line of the initial welded joint and the melting boundary line of the rewelded metal (same as L shown in FIG. 3)) are shown in “Rewelding”. ”Column. In addition, the welding material in re-welding was the same as the welding material of the initial welded joint.

  In the welded portion thus obtained, the ferrite content and corrosion resistance of the region (high temperature heat affected zone) in which the highest reached temperature is estimated to be 1100 ° C. or higher in the heat affected zone of the initial welded joint were evaluated. The results are also shown in Table 4. The ferrite content was measured by mirror-polishing the weld cross section, performing electrolytic etching in a sodium hydroxide solution, and then analyzing the image by observation with an optical microscope near the surface layer. The width of the high-temperature heat-affected zone, that is, the width from the melting boundary line was measured by observing a portion where the amount of ferrite is more than 70% by volume as the high-temperature heat-affected zone by optical microscope observation. In addition, the corrosion resistance was evaluated by wet-polishing the surface of the test piece taken from the surface layer of the welded portion with # 600 emery paper, in compliance with ASTM G48 Method E regulations, and by the ferric chloride immersion test (the critical pitting corrosion temperature ( CPT) was measured.

  For example, the symbol No. in Table 4 which is an example of the present invention. 1, when a welded joint was produced by MAG welding under the welding conditions in Table 3a using the welding material in Table 2a using the steel material A in Table 1 having a plate thickness of 12 mm, The width was 0.4 mm, and the ferrite content in the region was 79% by volume. Next, when this weld metal is re-welded by the TIG method, the appropriate range of the welding position for re-welding is determined based on the welding boundary of the initial welded joint and the re-welding from the welding heat input, plate thickness, and equation (1). The distance from the molten boundary line of the weld metal is 0.96 to 2.02 mm, and the distance (L) between the molten boundary line of the initial welded joint and the molten boundary line of the re-welded metal is 1.5 mm. Welding was performed at the welding position. As a result, the ferrite content in the region within 0.4 mm from the melting boundary line (that is, the high temperature heat affected zone) in the weld heat affected zone of the initial welded joint is reduced from 79% by volume to 59% by volume, and the limit of the part The pitting corrosion occurrence temperature (CPT) was 15 ° C. equivalent to the critical pitting corrosion occurrence temperature (CPT) of the steel material A shown in Table 1.

  Thus, as is apparent from Table 4, the re-welding position L in the range of the present invention was within the range of 0.0092 × Q / h to 0.029 × Q / h−1. 1-No. In the present invention example 7, the amount of ferrite in the high-temperature heat-affected zone is greatly reduced before re-welding, and the critical pitting corrosion temperature (CPT) is the critical pitting corrosion temperature of each steel material shown in Table 1 (CPT). It can be seen that the corrosion resistance of the weld heat affected zone has been recovered by the present invention.

  On the other hand, no. In the comparative examples of 9, 10 and 13, since the re-welding position L is smaller than 0.0092 × Q / h, the region where the highest temperature reached is estimated to be 1100 ° C. or higher in the heat affected zone of the initial welded joint is again By heating to a ferrite single phase region of 1100 ° C. or higher, the amount of ferrite in that region is almost the same as the amount of ferrite before re-welding and is as high as 70% by volume or more, and the critical pitting corrosion temperature (CPT) ) Is lower than the critical pitting corrosion temperature (CPT) of each steel shown in Table 1, and no recovery of the corrosion resistance of the weld heat affected zone is observed.

  No. In the comparative examples of 8, 11, 12, and 14, since the re-welding position L is larger than 0.029 × Q / h−1, the high temperature heat affected zone can be reheated to a temperature range of 700 ° C. to 1000 ° C. The amount of ferrite in that region is almost the same as the amount of ferrite before re-welding and is as high as 70% by volume or more. Further, the critical pitting corrosion temperature (CPT) is also the limit of each steel material shown in Table 1. It is lower than the pitting corrosion temperature (CPT), and no recovery of the corrosion resistance of the weld heat affected zone is observed.

  No. In Comparative Example 15, the re-welding position L is within the range of 0.0092 × Q / h to 0.029 × Q / h−1 of the present invention range, so the amount of ferrite in the high temperature heat affected zone is re-welded. However, since the Cr content of the steel material is less than 20%, the critical pitting corrosion temperature (CPT) is lower than the critical pitting corrosion temperature (CPT) of the steel materials shown in Table 1. Yes.

  No. In the comparative example of 16, the re-welding position L is in the range of 0.0092 × Q / h to 0.029 × Q / h−1 of the present invention range, but the N amount of the steel is less than 0.1%. Therefore, the precipitation of austenite due to reheating is small. Therefore, the ferrite content in the high temperature heat-affected zone is as high as about 70% by volume even after re-welding, and the critical pitting corrosion temperature (CPT) is also shown in Table 1. It is lower than the critical pitting corrosion temperature (CPT) of steel.

  From the above, by applying the duplex stainless steel welding method of the present invention, the corrosion resistance of the weld heat-affected zone heated to a high temperature of about 1100 ° C. or higher is recovered, and is equivalent to steel in a corrosive environment. It has been found that a welded joint with excellent corrosion resistance can be obtained.

  According to the present invention, among the weld heat affected zone formed by welding duplex stainless steel, the corrosion resistance of the region heated to a high temperature of about 1100 ° C. or higher is recovered, particularly in a corrosive environment. This greatly improves the corrosion resistance of the weld. As a result, the deterioration of the corrosion resistance of the weld heat-affected zone of duplex stainless steel, which has been a problem in the past, has been improved, and the seawater resistance and seawater resistance of ships, marine structures, bridges, seawater pumps, seawater desalination equipment, etc. It can be used in fields where particle properties and chloride resistance such as various chemical plants and food production plants are required.

DESCRIPTION OF SYMBOLS 1 Duplex stainless steel material 2 Weld metal 3 Melting boundary line 4 Weld heat affected zone (high temperature heat affected zone) where the highest temperature reaches 1100 ° C or more
5 Duplex stainless steel material 6 Weld metal of initial welded joint 7 Weld heat affected zone (high temperature heat affected zone) with maximum temperature of 1100 ° C or higher
8 Weld metal of re-weld 9 Melting boundary of early welded joint 10 Melting boundary of re-welded metal

Claims (6)

  At least one is a welded joint which is a duplex stainless steel containing at least one mass%, Cr: 20% or more, N: 0.1% or more, and whose structure is composed of ferrite and austenite, and constituting the joint Re-weld at least once on the metal, and the phase content of the ferrite phase in the high-temperature heat-affected zone within the stainless steel and within 1 mm from the melting boundary line that is the boundary between the weld metal and the Slentes steel constituting the joint A duplex stainless steel welded joint characterized in that the rate is less than 70% by volume.   The weld joint is characterized by comprising, in mass%, Cr: 20% or more, N: 0.1% or more, and welded two-phase stainless steels whose structure is composed of ferrite and austenite. Item 2. The duplex stainless steel welded joint according to item 1.   3. The duplex stainless steel according to claim 1, wherein, in the welded joint, a critical pitting corrosion temperature of the high temperature heat affected zone is equal to or higher than a critical pitting corrosion temperature of the stainless steel before welding. Welded joints. A method for producing a welded joint, wherein at least one of them is by mass%, Cr: 20% or more, N: 0.1% or more, and the structure is a duplex stainless steel composed of ferrite and austenite, Reweld at least once on the weld metal to be constructed, Q (J / mm) as the heat input of the re-weld, the boundary line between the weld metal constituting the joint and the weld metal by re-welding, and the weld constituting the joint When the shortest distance between the metal and the melting boundary line that is the boundary between the Slentes steel is L (mm), and the thickness of the stainless steel is h (mm),
0.0092 * Q / h <= L <= 0.029 * Q / h-1, It is a manufacturing method of the duplex stainless steel welded joint characterized by the above-mentioned.   2. The weld joint according to claim 1, wherein the welded joint contains, in mass%, Cr: 20% or more, N: 0.1% or more, and a duplex stainless steel composed of ferrite and austenite. 4. A method for producing a duplex stainless steel welded joint according to 4.   6. The duplex stainless steel according to claim 4, wherein in the welded joint, a critical pitting corrosion temperature of the high temperature heat affected zone is equal to or higher than a critical pitting corrosion temperature of the stainless steel before welding. A method for manufacturing a welded joint.