JP7608278B2 - Welded structure and finishing method for welded structure - Google Patents

Description

The present invention relates to a welded structure with high corrosion resistance in which duplex stainless steel is welded, and a finishing method for the welded structure to obtain this.

Duplex stainless steel is known as a stainless steel with excellent strength, toughness, and corrosion resistance. Duplex stainless steel is a material in which the ferrite phase and the austenite phase are formed in roughly equal proportions, and has excellent resistance to pitting corrosion in the presence of moisture. For this reason, it is used as a material for tanks and piping in water treatment plants and chemical plants, as well as oil well tubular goods and oil transmission pipes in oil and natural gas mining plants.

Duplex stainless steel is often joined by welding because it has low susceptibility to hot and cold cracking and good weldability. Nitrogen is added to duplex stainless steel to improve strength and corrosion resistance, and to save on expensive Ni. Nitrogen added to the base metal precipitates chromium nitrides in the heat-affected zone formed by welding. As a result, a chromium-deficient layer is formed in the weld, which poses problems with the corrosion resistance of the weld.

It is known that an oxide scale forms on the surface of a welded joint when stainless steel is welded. Oxide scale is mainly composed of oxides of Fe, not Cr. When oxide scale forms, the surface of the weld shows what is known as weld burn, turning brown or purple and impairing the appearance, and it is also known that the corrosion resistance of the weld is reduced. Since gaps form with the base material and a chromium-deficient layer occurs, it is believed that the weld is particularly susceptible to localized corrosion such as pitting corrosion and crevice corrosion.

TIG welding is generally used as a method for welding duplex stainless steel. In TIG welding, a shielding gas such as argon gas is used to block oxygen and nitrogen. However, even when using a shielding gas, it is currently difficult to completely prevent the formation of oxide scale on the surface of the weld.

Duplex stainless steel is also used in environments with high chloride concentrations. Chlorine-based disinfectants such as hypochlorous acid are used in water treatment plants such as seawater desalination plants. Chlorine-based disinfectants have a high redox potential, which increases the potential of the welds of duplex stainless steel. This increase in potential increases the susceptibility to localized corrosion, so there is a demand for further improvements in corrosion resistance.

Patent Document 1 describes a duplex stainless steel that has excellent corrosion resistance in a fluid environment that contains oxidizing substances. In order to increase the corrosion resistance of the heat-affected zone, Patent Document 1 specifies the chemical composition of the welded material itself.

Patent Document 2 describes an acid cleaner used for pickling duplex stainless steel, which has excellent corrosion resistance. The acid cleaner in Patent Document 2 contains nitric acid, hydrofluoric acid, an inorganic acid other than hydrofluoric nitric acid, an organic acid, a magnesium compound, and a surfactant. This acid cleaner is said to be used on welds of duplex stainless steel.

Patent Document 3 does not mention duplex stainless steel, but does describe a modification method for improving the corrosion resistance of a welded portion of a general stainless steel. In Patent Document 3, a welded portion made of stainless steel is heated at 400 to 600° C. for 1 hour or more in a vacuum atmosphere of 10 ⁻⁴ to 10 ⁻² Torr to increase the Cr ratio on the surface of the welded portion.

JP 2018-168461 A JP 2007-297697 A Japanese Patent Application Publication No. 9-13124

To improve the corrosion resistance of duplex stainless steel welds, it is important to remove the oxide scale that forms in the welds and to give the welds a chemical composition that is highly corrosion resistant. However, even when treatment is performed to remove the oxide scale, it may not be completely removed, and the susceptibility to localized corrosion may not be sufficiently reduced. Furthermore, the treatment to remove the oxide scale itself may be a cause of the formation of starting points for localized corrosion in the base material.

Patent Documents 1 to 3 describe techniques for improving the corrosion resistance of stainless steel and its welds. However, Patent Documents 1 to 3 do not disclose the chemical composition of the weld metal in the welds. Patent Document 3 discloses that a chromium-deficient layer in the oxide film deteriorates corrosion resistance, so a method of polishing and pickling the welds is effective. However, it does not disclose the extent to which oxide scale should be removed, or the possibility that the process of removing oxide scale itself may be a cause of corrosion.

Therefore, the present invention aims to provide a welded structure that improves the corrosion resistance of the welded joint of duplex stainless steel and increases the reliability of the product in which the welded joint is formed, and a finishing method for the welded structure to obtain the same.

In order to solve the above-mentioned problems, the welded structure of the present invention is a welded structure in which duplex stainless steel is welded, the welded structure has a weld in which a base material formed of duplex stainless steel is joined via a weld metal, and the amount of Cr at a surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more the amount of Cr in the base material, in mass % excluding oxygen and carbon.

Furthermore, a method for finishing a welded structure according to the present invention is a method for finishing a welded structure in which duplex stainless steel is welded, and includes a step of removing oxide scale from a weld where a base material formed of duplex stainless steel is joined via a weld metal, and the step of removing the oxide scale is a step of removing oxide scale containing Fe from a surface of the weld metal such that the amount of Cr on the surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more the amount of Cr in the base metal, in mass % excluding oxygen and carbon.

The present invention provides a welded structure that improves the corrosion resistance of duplex stainless steel welds and increases the reliability of products in which the welds are formed, as well as a finishing method for the welded structure to obtain the same.

FIG. 1 is a diagram showing the configuration of an apparatus for performing electrolysis by a direct current method. FIG. 1 is a diagram showing the configuration of an apparatus for performing electrolysis by an indirect current method. 1 is an image showing the appearance of a welded material made by welding two pieces of duplex stainless steel together before descaling. 1 is an image showing the appearance of a welded material in which two pieces of duplex stainless steel are welded together after descaling. 1A to 1C are diagrams showing a method for providing a gap to a welded material. 1A to 1C are diagrams showing a method for providing a gap to a welded material. 1A to 1C are diagrams showing a method for providing a gap to a welded material. 1A to 1C are diagrams showing a method for providing a gap to a welded material.

Below, a welded structure and a method for finishing a welded structure according to one embodiment of the present invention will be described with reference to the drawings. Note that in each of the following drawings, the same reference numerals will be used for common configurations, and duplicate explanations will be omitted.

The welded structure according to this embodiment is a structure in which duplex stainless steel is welded. This welded structure is formed by welding together a base material, which is a material to be welded and is made of duplex stainless steel, and a mating material, which is a material to be welded. The welded structure has a welded portion between the base material and the mating material, in which the base material is joined to the mating material via the weld metal.

The welded parts of welded structures are composed of the weld metal and the heat-affected zone (HAZ). The weld metal is a region formed by integrating the base material, mating material, and deposited metal, and is formed when the base material, mating material, and filler metal used in welding melt and then solidify. The heat-affected zone is a region that is not melted but has been altered by heat, and is formed near the boundary between the base material and mating material adjacent to the weld metal.

As the base metal to be welded, duplex stainless steels with an appropriate pitting corrosion resistance index can be used, such as super duplex stainless steels such as SUS327, general-purpose duplex stainless steels such as SUS329, and lean duplex stainless steels such as SUS321 and SUS323. There are no particular limitations on the corrosion resistance index of duplex stainless steels or the fraction of austenite and ferrite phases.

As the mating material to be welded, any suitable material that can be welded can be used, such as duplex stainless steels such as super duplex stainless steel, general-purpose duplex stainless steel, lean duplex stainless steel, austenitic stainless steel, ferritic stainless steel, carbon steel, nickel-based alloy, etc. The preferred mating material is duplex stainless steel.

In the welded structure according to this embodiment, the amount of a specific element related to corrosion resistance is limited to a specific range on the surface of the weld metal. When the element amount and chemical composition of the surface of the weld metal are measured, the amount of the specific element on the surface of the weld metal is limited to a specific range, which ensures that the oxide scale on the surface of the weld is sufficiently removed and that the chemical composition of the surface layer of the weld is a composition with excellent corrosion resistance. This improves the corrosion resistance of the weld, particularly the crevice corrosion resistance.

The amount of elements related to corrosion resistance on the surface of the weld metal is measured after treatment is performed to remove the oxide scale that has formed on the surface of the weld. Oxide scale is a product such as an oxide film that is mainly composed of oxides of Fe rather than Cr. Elements related to corrosion resistance include chromium (Cr), nickel (Ni), and molybdenum (Mo), which contribute to improving corrosion resistance, and manganese (Mn), which is an indicator of a decrease in corrosion resistance.

The elemental amounts and chemical composition on the surface of the weld metal can be measured by X-ray photoelectron spectroscopy (XPS) analysis. There are no particular limitations on the measurement location, as long as it is on the surface of the weld metal exposed to the outside of the welded structure. For example, the measurement can be performed by irradiating the center of the surface of the weld metal with X-rays. Note that an oxide film mainly composed of Cr oxides may be formed on the surface of the weld metal.

XPS analysis provides qualitative and quantitative information up to a depth of several tens of nanometers from the measurement position. Therefore, if no oxide scale remains on the surface of the weld metal after treatment to remove the oxide scale has been performed, measurement results are obtained for the surface layer of the weld metal. On the other hand, if oxide scale remains on the surface of the weld metal, measurement results are obtained for both the oxide scale and the surface layer of the weld metal.

In this way, by measuring the amount of elements on the surface of the weld metal after the process to remove the oxide scale has been performed, the corrosion resistance of the weld can be accurately evaluated under conditions closer to those during practical use. The measurement results taken into account the extent to which the oxide scale has been removed and the effects of the process to remove the oxide scale itself. Therefore, by selecting appropriate chemical compositions for the base metal, mating material, and filler metal, and by selecting appropriate conditions for the process to remove the oxide scale, the corrosion resistance of the weld can be improved compared to conventional methods.

The amount of Cr on the surface of the weld metal, in mass % excluding oxygen and carbon, is preferably 1.5 times or more, more preferably 1.6 times or more, even more preferably 1.7 times or more, and even more preferably 1.8 times or more, of the amount of Cr in the base material made of duplex stainless steel.

This amount of Cr means that a large amount of Cr was detected by the measurement, and that no thick oxide scale was measured. In other words, the oxide scale on the surface of the weld metal has been sufficiently removed. Also, the surface layer of the weld metal is not deficient in Cr. Therefore, Cr has the effect of forming a passive film and improving corrosion resistance. Also, with the oxide scale sufficiently removed, high local corrosion resistance is ensured.

It is preferable that the amount of Mn on the surface of the weld metal, in mass percent excluding oxygen and carbon, be equal to or less than the amount of Mn in the base material made of duplex stainless steel.

Such an amount of Mn means that only a small amount of Mn was detected by the measurement, and that no thick oxide scale was measured. In other words, the oxide scale on the surface of the weld metal has been sufficiently removed. This is because Mn is concentrated in oxide scale that is mainly composed of Fe oxides, and dissolves from the surface layer of the weld metal when pickling, etc. is performed. Therefore, high local corrosion resistance is ensured by having the oxide scale sufficiently removed.

The amount of Ni on the surface of the weld metal, in mass % excluding oxygen and carbon, is preferably 0.4 times or more, and more preferably 0.5 times or more, the amount of Ni in the base metal made of duplex stainless steel. The amount of Mo on the surface of the weld metal is preferably 1.5 times or more, and more preferably 1.6 times or more, the amount of Mo in the base metal made of duplex stainless steel.

Such amounts of Ni and Mo mean that a large amount of Ni and Mo was detected by the measurement, and that no thick oxide scale was measured. In other words, the oxide scale on the surface of the weld metal has been sufficiently removed. Also, the surface layer of the weld metal is not deficient in Ni or Mo. Therefore, Ni has the effect of suppressing the progress of localized corrosion, etc., and Mo has the effect of strengthening the passive film and improving corrosion resistance, etc. Also, the state in which the oxide scale has been sufficiently removed ensures high localized corrosion resistance.

The amount of each element on the surface of the weld metal is limited by the mass fraction per 100 mass% of the total of the other elements excluding oxygen and carbon contained in the weld metal. Oxygen is mainly present in the oxide scale and in the oxide film mainly composed of Cr oxides, but it can be mixed into the surface layer of the weld metal from the oxide scale or the atmosphere. Carbon can be mixed in from the oil in the vacuum pump when the process to remove the oxide scale is performed under reduced pressure conditions.

In the welded structure according to this embodiment, it is preferable that at least the Cr amount is limited on the surface of the weld metal, more preferably the Cr amount and Mo amount are limited, even more preferably the Cr amount, Mo amount and Ni amount are limited, and even more preferably the Cr amount, Mo amount, Ni amount and Mn amount are limited. This is because Cr and Mo are related to basic corrosion resistance, whereas Ni is secondarily related to the progress of localized corrosion. This is because Mn is related as an indicator of the decrease in localized corrosion resistance due to oxide scale.

The use of the welded structure according to this embodiment is not particularly limited. The welded structure according to this embodiment can be used, for example, in water treatment systems, seawater desalination systems, seawater concentration systems, radioactively contaminated water treatment systems, radioactive waste liquid treatment systems, chemical plants, oil mining plants, natural gas mining plants, etc. It can be preferably used for pipes in these facilities, and joints of piping equipment such as pumps, valves, sensors, and tanks, especially liquid-contacting parts with high water or chloride concentrations.

In such a welded structure, the amount of elements related to corrosion resistance on the surface of the weld metal measured by XPS analysis is limited to a predetermined range, ensuring that oxide scale is sufficiently removed from the weld and that the entire weld metal, including the surface layer, has a chemical composition with excellent corrosion resistance. This reduces the susceptibility to localized corrosion such as pitting corrosion and crevice corrosion caused by oxide scale, and also improves the corrosion resistance of the welded metal itself. This improves the corrosion resistance of the welded part and increases the reliability of the product of the welded structure having the welded part.

In particular, even when a welded structure is placed in a corrosive environment such as seawater, where the chloride concentration exceeds 2×10 ⁴ ppm, the initiation of localized corrosion is unlikely to occur because the oxide scale has been properly removed. In a weld, the austenite phase is unlikely to grow and chromium nitrides are likely to precipitate, causing a chromium-deficient layer, but because the Cr content in the surface layer of the weld metal can be optimized, the occurrence and progression of localized corrosion in the weld can be suppressed for a long period of time.

Next, we will explain the finishing method for the welded structure to obtain the above welded structure.

The above-mentioned welded structure, in which the amount of elements related to corrosion resistance on the surface of the weld metal is limited to a predetermined range, can be obtained by a method including the steps of welding a base material, which is a material to be welded and made of duplex stainless steel, and a mating material, which is a material to be welded, together using a filler metal, and removing the oxide scale from the welded portion where the base material, which is made of duplex stainless steel, is joined via the weld metal.

As the welding method, any suitable welding method such as groove welding, fillet welding, seam welding, etc. can be used. As the material to be welded, any suitable material such as plate material, tube material, bar material, and formed material can be welded. As the welding method, any welding method such as TIG welding, MIG welding, MAG welding, shielded metal arc welding, and laser welding can be used. As the welding method, either single layer welding or laminate welding can be used. There are no particular limitations on the groove shape, laminate pass conditions, heat input conditions, etc.

When welding to form a welded structure, an appropriate filler metal can be used depending on the material to be welded. Depending on the welding method, the filler metal may be a flux-containing welding material or a flux-free welding material. By using a filler metal with an appropriate chemical composition, the amount of elements related to the corrosion resistance of the weld metal can be adjusted relative to the base metal.

It is preferable to use a filler metal that is richer in elements related to corrosion resistance than the base metal. In other words, it is preferable to use a filler metal with a chemical composition that results in a higher amount of Cr, Mo, Ni, etc. than the base metal when the base metal and filler metal are integrated. By using such a filler metal, it is possible to adjust the amount of elements related to corrosion resistance in the weld metal and increase the corrosion resistance of the weld.

As a filler metal, a material with a Cr content of 1.5 times or more that of the base material, in mass % excluding oxygen and carbon, is more preferable. A material with a Cr content of 1.5 times or more that of the base material and a Mo content of 1.5 times or more that of the base material is even more preferable. A material with a Cr content of 1.5 times or more that of the base material, a Mo content of 1.5 times or more that of the base material, and a Ni content of 0.4 times or more that of the base material is even more preferable.

In the process of removing the oxide scale from the welded joint, the oxide scale on the surface of the welded metal is removed so that the amount of Cr on the surface of the welded metal is at least 1.5 times that of the base material made of duplex stainless steel. By sufficiently removing the oxide scale, which is mainly composed of Fe oxides, the corrosion resistance of the welded joint can be improved, resulting in a welded structure with a welded joint that has high resistance to localized corrosion.

In the process of removing the oxide scale from the welded portion, it is preferable to remove the oxide scale from the surface of the welded metal so that the amount of Cr on the surface of the welded metal is 1.5 times or more that of the base metal, the amount of Mn on the surface of the welded metal is less than that of the base metal, the amount of Ni on the surface of the welded metal is 0.4 times or more that of the base metal, and the amount of Mo on the surface of the welded metal is 1.5 times or more that of the base metal.

Methods for removing oxide scale from the surface of the weld metal include (1) pickling the surface of the weld metal, (2) electrolytically treating the surface of the weld metal in a neutral salt solution, and (3) reducing heat treating the surface of the weld metal.

(1) The method of pickling the surface of the weld metal can be carried out by bringing the welded part into contact with acid. After the welded part of the welded structure is brought into contact with acid and pickled, it is washed with water and dried. The pickling process may be any of the following: immersing the welded part in an acid solution, spraying the acid solution onto the welded part with a spray or the like, applying the acid solution to the welded part with a brush or the like.

As the acid, inorganic acids such as nitric acid, sulfuric acid, hydrofluoric acid, etc., or mixed acids of one or more of these can be used. As the mixed acid, hydrofluoric acid, etc., a mixture of nitric acid and hydrofluoric acid, can be used. Pickling accelerators, pickling inhibitors, etc. can be added to the acid solution. It is preferable not to use hydrochloric acid at a high concentration, etc., because it easily acts on the surface layer of the weld metal even if there is oxide scale.

The concentration of nitric acid is preferably 2% or more and 50% or less from the viewpoint of ensuring an appropriate pickling speed. The concentration of sulfuric acid is preferably 2% or more and 50% or less from the viewpoint of ensuring an appropriate pickling speed. The concentrations of hydrofluoric nitric acid are preferably, for example, 2% or more and 50% or less for nitric acid and 0.2% or more and 10% or less for hydrofluoric acid.

The acid temperature is preferably 50°C or higher and 80°C or lower. If the temperature is 50°C or higher, oxide scale can be sufficiently removed even with a relatively short treatment time. If the temperature is 80°C or lower, uneven treatment and roughness of the base material due to a high pickling speed can be suppressed.

The time for pickling depends on the type of acid, the temperature of the acid solution, etc., but it is preferable that it be between 3 and 10 minutes. If the pickling time is too short, the oxide scale cannot be sufficiently removed, and unevenness in the treatment is likely to occur. On the other hand, if the pickling time is too long, the surface of the base is likely to become rough.

The amount of elements related to the corrosion resistance on the surface of the weld metal is affected by the degree of removal of oxide scale and the dissolution of elements on the surface layer of the weld metal, so it can be adjusted to an appropriate range by changing the type of acid, the acid temperature, the pickling treatment time, etc.

(2) The method of electrolytically treating the surface of the weld metal in a solution of neutral salt can be carried out by immersing the weld in an electrolyte of neutral salt and passing an electric current through the weld to cause an electrochemical reaction. The weld of the welded structure is immersed in an electrolyte of neutral salt and electrolytically polished, and then washed with water and dried.

The electrolyte can be an aqueous solution containing a neutral salt of an inorganic or organic acid. The neutral salt can be sodium sulfate, sodium nitrate, ammonium sulfate, or a mixture of one or more of these. The use of a neutral salt can suppress the elution of elements from the surface layer of the weld metal compared to electrolytic polishing using an acid solution. It can also reduce the amount of waste liquid generated. The use of sodium nitrate can efficiently form an oxide film mainly composed of Cr oxide on the surface of the weld while removing oxide scale.

The concentration of sodium sulfate is preferably 10% or more and 30% or less from the viewpoint of maintaining the current density and potential. The concentration of sodium nitrate is preferably 10% or more and 30% or less from the viewpoint of maintaining the current density and potential. The concentration of ammonium sulfate is preferably 10% or more and 30% or less from the viewpoint of maintaining the current density and potential.

The current density of the material to be treated in the electrolytic treatment is preferably 2 A/dm2 or ^more and 20 A/dm2 ^or less. If the current density is less than 2 A/ ^dm2 , it is difficult to sufficiently remove the oxide scale. If the current density exceeds 20 A/ ^dm2 , discoloration occurs in the material to be treated, and Cr, Mo, etc., which improve corrosion resistance, are excessively dissolved, resulting in a chemical composition of the surface layer of the weld metal with poor corrosion resistance. If the current density is 2 A/ ^dm2 or more and 20 A/ ^dm2 or less, the corrosion resistance of the base can be appropriately improved while removing the oxide scale.

The time for which electricity is passed during electrolytic treatment depends on factors such as the current density, but is preferably between 3 and 10 minutes. If the time for passing electricity is too short, the oxide scale cannot be sufficiently removed, and uneven treatment is likely to occur. On the other hand, if the time for passing electricity is too long, the surface of the base is likely to become rough.

The amount of elements related to the corrosion resistance on the surface of the weld metal is affected by the degree of removal of oxide scale, which is mainly composed of Fe oxides, and the dissolution of elements in the surface layer of the weld metal, so it can be adjusted by changing the current density, the time of electrolytic treatment, etc.

Fig. 1 is a diagram showing the configuration of an apparatus for performing electrolytic treatment by a direct current method, and Fig. 2 is a diagram showing the configuration of an apparatus for performing electrolytic treatment by an indirect current method.
1 and 2, reference numeral 1 denotes an electrolytic cell, reference numeral 2 denotes an electrolyte, reference numeral 3 denotes an electrode, reference numeral 4 denotes a welded structure which is a material to be treated, and reference numeral 5 denotes a DC power source.

As shown in Figures 1 and 2, the surface of the weld metal can be electrolytically treated by either the direct current method, in which a current is passed directly through the weld, or the indirect current method, in which a current is passed indirectly through the weld.

In the direct current method shown in FIG. 1, a welded structure 4, which is the material to be treated, and an electrode 3 as a counter electrode are placed in an electrolytic cell 1. The welded structure 4 is electrically connected to the positive electrode of a DC power source 5. The electrode 3 is electrically connected to the negative electrode of the DC power source 5. The electrode 3 can be made of carbon, tantalum, or the like. An electrolyte 2 containing dissolved neutral salt is placed in the electrolytic cell 1, and the welded structure 4 and the electrode 3 are immersed in the electrolyte 2.

In the direct current method, the welded structure 4 is used as the anode and the electrode 3 is used as the cathode, and a direct current is passed from a direct current power source 5. The current causes an electrochemical reaction in the oxide scale formed on the surface of the welded portion of the welded structure 4, which can remove the oxide scale.

In the indirect current method shown in FIG. 2, a welded structure 4, which is the material to be treated, and at least a pair of electrodes 3 are placed in an electrolytic cell 1. A plurality of electrodes 3 may be placed in tandem. Each electrode 3 is electrically connected to the positive or negative electrode of a DC power source 5. An electrolyte 2 containing dissolved neutral salt is placed in the electrolytic cell 1, and the welded structure 4 and electrodes 3 are immersed in the electrolyte 2.

In the indirect current method, a direct current is applied from a direct current power source 5 between electrodes 3 adjacent to the welded structure 4. The application of current causes polarization in the welded structure 4, which induces an electrochemical reaction in the oxide scale, mainly composed of Fe oxides, formed on the surface of the welded portion of the welded structure 4, thereby removing the oxide scale. Since the electrolytic treatment is performed in a neutral salt solution, the anodic reaction prevails over the cathodic reaction on the surface of the welded structure 4.

The direct current method shown in Figure 1 allows for electrolytic processing with excellent power efficiency. On the other hand, the indirect current method shown in Figure 2 is less power efficient but can reduce uneven processing. In Figure 2, the welded structure 4, which is the material to be treated, is fixed to the electrode 3, but it can also be moved relative to the electrode 3. The electrode 3 can be provided in a moving and scanning manner relative to the welded structure 4, or the welded structure 4 can be transported relative to the electrode 3.

(3) The method of reducing heat treating the surface of the weld metal can be performed by heat treating the weld at high temperature in a reduced pressure atmosphere.

The heat treatment temperature is preferably 400° C. or higher. The heat treatment temperature is preferably 600° C. or lower. The heat treatment time is preferably 1 hour or longer. The heat treatment time depends on the heat treatment temperature, pressure conditions, etc., but is preferably 3 hours or shorter. The pressure conditions for the heat treatment are preferably 10 ⁻² Torr or lower. When the heat treatment is performed for 1 hour or longer at 400° C. or higher in a reduced pressure atmosphere of 10 ⁻² Torr or lower, the oxide scale mainly composed of Fe oxides can be sufficiently removed.

The amount of elements related to the corrosion resistance on the surface of the weld metal is affected by the degree of removal of oxide scale, which is mainly composed of Fe oxides, and can be adjusted by changing the heat treatment temperature, heat treatment time, heat treatment pressure conditions, etc. Under low vacuum conditions, it is preferable to perform heat treatment at a low temperature or for a short time. On the other hand, under high vacuum conditions, it is possible to perform heat treatment at a high temperature or for a short time.

This method of finishing a welded structure makes it possible to obtain a welded structure in which the amount of elements related to corrosion resistance on the surface of the weld metal, as measured by XPS analysis, is limited to a predetermined range. The welded part of the welded structure is in a state in which oxide scale, mainly composed of oxides of Fe rather than Cr, has been sufficiently removed, and the entire welded metal, including the surface layer, has a chemical composition with excellent corrosion resistance. This reduces the susceptibility to localized corrosion, such as pitting corrosion and crevice corrosion, caused by oxide scale. In addition, the corrosion resistance of the welded metal itself is ensured. This improves the corrosion resistance of the welded part, thereby increasing the reliability of the welded structure product that includes the welded part.

The present invention will be described in detail below with reference to examples, but the technical scope of the present invention is not limited to these examples.

The welds of duplex stainless steels welded together were treated to remove the oxide scale from the weld metal surface, and the relationship between the chemical composition of the weld metal surface and crevice corrosion resistance was evaluated.

The materials to be welded were either S32750, a super duplex stainless steel, or S32205, a general-purpose duplex stainless steel. Plates made of the same type of duplex stainless steel were butt-welded together by TIG welding using a duplex stainless steel filler metal.

As the filler metal, a welding wire for duplex stainless steel was used, which is enriched in Cr, Mo, and Ni compared to the base metal formed of duplex stainless steel and contains Mn, Fe, _SiO2 -based flux, etc. As the shielding gas during welding, a mixed gas of argon gas and nitrogen gas was used.

Fig. 3 is an image showing the appearance of a welded material made by welding two duplex stainless steels together before descaling. Fig. 4 is an image showing the appearance of a welded material made by welding two duplex stainless steels together after descaling.
Figures 3 and 4 show a welded material obtained by butt-welding two plates of S32750, a super duplex stainless steel. Figure 3 shows the state before descaling treatment for removing oxide scale mainly composed of Fe oxides. Figure 4 shows the state after descaling treatment, and shows the welded material of Example 1 described later.

As shown in Figure 3, the surface of the weld metal in the welded area turned brown due to welding, and oxide scale consisting mainly of Fe oxides was confirmed. As shown in Figure 4, when an appropriate descaling process is performed, the oxide scale is removed, and the metallic luster of the weld metal surface is restored.

To evaluate the relationship between the chemical composition of the weld metal surface and crevice corrosion resistance, welded material made by butt-welding two sheets of duplex stainless steel like this was used. After descaling to remove oxide scale, the chemical composition of the weld metal surface was analyzed by XPS. In addition, after descaling to remove oxide scale, a crevice corrosion test was performed to evaluate crevice corrosion resistance.

The descaling methods used to remove oxide scale from the surface of the weld metal were (1) pickling the surface of the weld metal, (2) electrolytic treatment of the surface of the weld metal in a neutral salt solution, or (3) reduction heat treatment of the surface of the weld metal. The circled lines in Figure 3 indicate the measurement positions in the XPS analysis.

Crevice corrosion resistance was evaluated by immersing the welded material with a crevice in an adjustment liquid for three months and then observing the maximum corrosion depth. The adjustment liquid used was a 3.5% aqueous sodium chloride solution adjusted to pH 3.0 with hydrochloric acid at 45°C. If the maximum corrosion depth exceeded the critical crevice corrosion depth of 40 μm, it was evaluated as crevice corrosion having occurred ("X"). If the maximum corrosion depth did not exceed the critical crevice corrosion depth of 40 μm, it was evaluated as crevice corrosion not having occurred ("O").

Fig. 5 is a diagram showing a method of providing a gap to a welded material. Fig. 5A is a diagram showing a welded material in which materials to be welded are welded together. Fig. 5B is a diagram showing a state in which the surface of the welded metal is molded with an impression material. Fig. 5C is a diagram showing an operation of immersing the surface of the welded metal in an adjustment liquid. Fig. 5D is a diagram showing a state in which a gap is provided to the surface of the welded metal.
In FIG. 5, reference numeral 4 denotes a welding material which is a material to be treated, reference numeral 6 denotes a weld metal, reference numeral 7 denotes an impression material, reference numeral 8 denotes a cable tie, and reference numeral 9 denotes an adjusting liquid.

As shown in FIG. 5A, the weld metal 6 is exposed on both sides of the welded material 4, which is made by butt-welding two plates together. Gaps were formed on the surfaces of the weld metal 6 exposed on both sides of the welded material 4 by using impression material 7. The surface of the welded material 4 was left as received, and was only washed with acetone without being polished or passivated. A dental impression material made of vinyl silicone (manufactured by GC Corporation) was used as the impression material 7.

As shown in FIG. 5B, impression material 7 was pressed against both sides of weld metal 6 to form a mold of the surface of weld metal 6, and impression material 7 was once peeled off from weld material 4. The periphery of impression material 7 peeled off from weld material 4 was removed, and only the molded portion measuring 20 mm in length and 20 mm in width was cut out. Then, as shown in FIG. 5C, an adjustment liquid was dripped onto the surface of weld metal 6, and the molded impression material 7 was returned to its original position and molded.

Next, as shown in FIG. 5D, fluororesin cable ties 8 were wrapped around the impression material 7 on both sides of the welding material 4, and the impression material 7 was fixed to the welding material 4. The welding material 4 with the gap formed by the impression material 7 was left in contact with the adjustment liquid for three months, and the erosion depth of the surface of the weld metal 6 was then observed.

Tables 1 and 2 show the degree of remaining oxide scale confirmed by visual inspection, the chemical composition of the weld metal surface measured by XPS analysis, the method of descaling treatment applied to remove the oxide scale applied to the weld metal surface, and the evaluation results of crevice corrosion resistance. Table 1 shows the results when the welded material was S32750. Table 2 shows the results when the welded material was S32205.

In Tables 1 and 2, the left side of the element column shows the mass fraction of each element detected on the surface of the weld metal by XPS analysis. The mass fraction is a value converted to a fraction per 100 mass% of the total metal elements contained in the weld metal, excluding oxygen and carbon contained in the weld metal.

In Tables 1 and 2, the right side of the element column shows the mass ratio of each element detected on the surface of the weld metal to the base metal. The mass fraction is the amount of element detected on the surface of the weld metal [mass %] divided by the amount of element contained in the base metal [mass %]. For example, in the case of Cr in Example 1, it is 44 mass % on the surface of the weld metal and 25.73 mass % in the base metal, so the mass ratio is 44/25.73 = 1.71.

[Example 1]
In Example 1, the surface of the weld metal of the welded material using S32750 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of sodium sulfate was used as the electrolytic solution. In the electrolytic solution at 30°C, a current was passed through the welded material at a current density ^of 10 A/dm2 for 5 minutes.

In Example 1, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 44.00 mass%, which was 1.71 times that of the base material. No Mn was detected. The Ni content was 3.45 mass%, which was 0.54 times that of the base material. The Mo content was 5.63 mass%, which was 1.65 times that of the base material. Crevice corrosion resistance was good. It is believed that no Mn was detected because the oxide scale was sufficiently removed.

[Example 2]
In Example 2, the surface of the weld metal of the welded material using S32750 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of ammonium sulfate was used as the electrolytic solution. In the electrolytic solution at 30°C, a current was passed through the welded material at a current density ^of 10 A/dm2 for 5 minutes.

In Example 2, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 46.50 mass%, which was 1.81 times that of the base material. No Mn was detected. The Ni content was 3.15 mass%, which was 0.49 times that of the base material. The Mo content was 5.63 mass%, which was 1.65 times that of the base material. The crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Good results were obtained even when the electrolyte components were changed.

[Example 3]
In Example 3, the surface of the weld metal of the welded material using S32750 was subjected to pickling treatment. The acid used was a hydrofluoric acid-nitric acid solution containing 20% nitric acid and 0.5% hydrofluoric acid. The welded material was immersed in the acid solution at 50° C. for 5 minutes.

In Example 3, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 39.00 mass%, which was 1.52 times that of the base material. No Mn was detected. The Ni content was 3.25 mass%, which was 0.51 times that of the base material. The Mo content was 5.72 mass%, which was 1.67 times that of the base material. Crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed.

[Example 4]
In Example 4, the surface of the weld metal of the welded material using S32750 was subjected to pickling treatment. As the acid, a hydrofluoric acid-nitric acid solution containing 10% nitric acid and 0.3% hydrofluoric acid was used. The welded material was immersed in the acid solution at 50° C. for 10 minutes.

In Example 4, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 43.50 mass%, which was 1.69 times that of the base material. No Mn was detected. The Ni content was 3.65 mass%, which was 0.57 times that of the base material. The Mo content was 5.75 mass%, which was 1.68 times that of the base material. The crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Even if the acid concentration was reduced, good results were obtained by extending the treatment time.

[Example 5]
In Example 5, the surface of the weld metal of the weld material using S32750 was subjected to a reducing heat treatment. The weld material was heat treated at 400° C. for 2 hours under a reduced pressure condition of 10 ⁻² Torr.

In Example 5, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 39.60 mass%, which was 1.54 times that of the base material. No Mn was detected. The Ni content was 3.05 mass%, which was 0.48 times that of the base material. The Mo content was 5.33 mass%, which was 1.56 times that of the base material. Crevice corrosion resistance was good. It is believed that no Mn was detected because the oxide scale was sufficiently removed.

[Example 6]
In Example 6, the surface of the weld metal of the weld material using S32750 was subjected to a reducing heat treatment. The weld material was heat treated at 600° C. for 0.5 hour under reduced pressure conditions of 10 ⁻⁴ Torr.

In Example 6, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 42.30 mass%, which was 1.64 times that of the base material. No Mn was detected. The Ni content was 3.23 mass%, which was 0.50 times that of the base material. The Mo content was 5.42 mass%, which was 1.59 times that of the base material. The crevice corrosion resistance was good. It is believed that no Mn was detected because the oxide scale was sufficiently removed. Even if the treatment time was shortened, good corrosion resistance was obtained by increasing the degree of vacuum and temperature.

[Comparative Example 1]
In Comparative Example 1, the surface of the weld metal of the weld material using S32750 was evaluated without removing the oxide scale.

In Comparative Example 1, the surface of the weld metal was brown, and no metallic luster was visible. The Cr content was 28.41 mass%, which was 1.10 times that of the base metal. The Mn content was 41.68 mass%, which was 83.4 times that of the base metal. No Ni or Mo was detected. After immersion for three months, the critical crevice corrosion depth was exceeded, and the crevice corrosion resistance was poor. The reason why no Ni or Mo was detected is thought to be that the oxide scale was not removed and X-rays did not penetrate.

[Comparative Example 2]
In Comparative Example 2, the surface of the weld metal of the welded material using S32750 was pickled. The acid used was 10% hydrochloric acid to which hexamethylenetetramine was added so that the concentration was 1.0%. The welded material was immersed in the acid solution at 30° C. for 1 minute. Hexamethylenetetramine is a corrosion inhibitor that is added to general pickling solutions.

In Comparative Example 2, the degree of browning of the weld metal surface was reduced, but the metallic luster was not visible. The Cr content was 32.50 mass%, which was 1.26 times that of the base metal. No Mn, Ni, or Mo was detected. The crevice corrosion resistance was poor. This result indicates that when pickling is performed with hydrochloric acid, oxide scale may not be sufficiently removed.

[Comparative Example 3]
In Comparative Example 3, the surface of the weld metal of the welded material using S32750 was pickled. A hydrofluoric nitric acid solution containing 10% nitric acid and 0.5% hydrofluoric acid was used as the acid. The welded material was immersed in the acid solution at 30° C. for one minute. In Comparative Example 3, the hydrofluoric acid concentration, the temperature of the acid solution, and the pickling treatment time were milder than those in Example 4.

In Comparative Example 3, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 33.10 mass%, 1.29 times that of the base material. No Mn, Ni, or Mo was detected. Crevice corrosion resistance was poor. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Although the oxide scale was removed from the outside, corrosion exceeding the critical crevice corrosion depth occurred. Since the Cr content was small and no Ni or Mo was detected, it is assumed that the oxide scale remains microscopically.

[Comparative Example 4]
In Comparative Example 4, the surface of the weld metal of the welded material using S32750 was electrolytically treated in a neutral salt solution. The electrolyte used was a 10% aqueous solution of sodium sulfate. The welded material was treated in the electrolyte at 30° C. for 5 minutes at a current density of 0.5 A/ ^dm2 .

In Comparative Example 4, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 26.30 mass%, 1.02 times that of the base material. No Mn, Ni, or Mo was detected. Crevice corrosion resistance was poor. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Although the oxide scale was removed from the outside, corrosion occurred beyond the critical crevice corrosion depth. Since the Cr content was small and no Ni or Mo was detected, it is assumed that the oxide scale remains microscopically.

[Comparative Example 5]
In Comparative Example 5, the surface of the weld metal of the weld material using S32750 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of sodium sulfate was used as the electrolytic solution. In the electrolytic solution at 30°C, a current was passed through the weld material at a current density ^of 50 A/dm2 for 5 minutes. In Comparative Example 5, the current density was higher than that in Example 1.

In Comparative Example 5, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 19.80 mass%, 0.77 times that of the base material. No Mn was detected. The Ni content was 0.56 mass%, 0.09 times that of the base material. The Mo content was 1.56 mass%, 0.46 times that of the base material. The crevice corrosion resistance was poor. It is believed that the reason why no Mn was detected is that the oxide scale was removed. Although the oxide scale was removed from the outside, corrosion occurred beyond the critical crevice corrosion depth. It is presumed that this result was due to the current density being too high, which reduced the Cr, Ni, and Mo contents on the surface of the weld metal.

[Examples 1 to 6, Comparative Examples 1 to 5]
In Examples 1 to 6 and Comparative Examples 1 to 5, the N content was larger than that of the base material. N is an austenite stabilizing element, and brings about effects such as improving corrosion resistance. However, no difference in the N content was confirmed regardless of the degree of removal of oxide scale.

[Example 7]
In Example 7, the surface of the weld metal of the welded material using S32205 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of ammonium sulfate was used as the electrolytic solution. In the electrolytic solution at 30° C., a current was passed through the welded material at a current density of 10 A/dm ² for 5 minutes.

In Example 7, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 33.22 mass%, which was 1.88 times that of the base material. No Mn was detected. The Ni content was 5.62 mass%, which was 0.46 times that of the base material. The Mo content was 5.63 mass%, which was 1.82 times that of the base material. Crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed.

[Example 8]
In Example 8, the surface of the weld metal of the welded material using S32205 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of ammonium sulfate was used as the electrolytic solution. In the electrolytic solution at 30° C., a current was passed through the welded material at a current density of 10 A/dm ² for 5 minutes.

In Example 8, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 34.26 mass%, which was 1.94 times that of the base material. No Mn was detected. The Ni content was 5.45 mass%, which was 0.45 times that of the base material. The Mo content was 5.23 mass%, which was 1.69 times that of the base material. The crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Good results were obtained even when the electrolyte components were changed.

[Example 9]
In Example 9, the surface of the weld metal of the welded material using S32205 was subjected to pickling treatment. The acid used was a hydrofluoric-nitric acid solution containing 20% nitric acid and 0.5% hydrofluoric acid. The welded material was immersed in the acid solution at 50° C. for 5 minutes.

In Example 9, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 32.99 mass%, which was 1.86 times that of the base material. No Mn was detected. The Ni content was 5.52 mass%, which was 0.46 times that of the base material. The Mo content was 5.46 mass%, which was 1.76 times that of the base material. Crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed.

[Example 10]
In Example 10, the surface of the weld metal of the welded material using S32205 was subjected to pickling treatment. The acid used was a hydrofluoric-nitric acid solution containing 10% nitric acid and 0.3% hydrofluoric acid. The welded material was immersed in the acid solution at 50° C. for 10 minutes.

In Example 10, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 36.21 mass%, which was 2.04 times that of the base material. No Mn was detected. The Ni content was 5.72 mass%, which was 0.47 times that of the base material. The Mo content was 5.82 mass%, which was 1.88 times that of the base material. The crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Even if the acid concentration was reduced, good results were obtained by increasing the treatment time.

[Example 11]
In Example 11, the surface of the weld metal of the weld material using S32205 was subjected to a reducing heat treatment. The weld material was heat treated at 600° C. for 1 hour under reduced pressure conditions of 10 ⁻² Torr.

In Example 11, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 33.56 mass%, which was 1.90 times that of the base material. No Mn was detected. The Ni content was 5.66 mass%, which was 0.47 times that of the base material. The Mo content was 5.22 mass%, which was 1.68 times that of the base material. Crevice corrosion resistance was good. It is believed that no Mn was detected because the oxide scale was sufficiently removed.

[Example 12]
In Example 12, the surface of the weld metal of the weld material using S32205 was subjected to a reducing heat treatment. The weld material was heat treated at 400° C. for 1 hour under a reduced pressure condition of 10 ⁻⁴ Torr.

In Example 12, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 34.23 mass%, which was 1.93 times that of the base material. No Mn was detected. The Ni content was 5.55 mass%, which was 0.46 times that of the base material. The Mo content was 5.31 mass%, which was 1.71 times that of the base material. The crevice corrosion resistance was good. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Even if the treatment time was shortened, good corrosion resistance was obtained by increasing the degree of vacuum and temperature.

[Comparative Example 6]
In Comparative Example 6, the surface of the weld metal of the weld material using S32205 was evaluated without removing the oxide scale.

In Comparative Example 6, the surface of the weld metal was brown, and the metallic luster was not visible. The Cr content was 20.14 mass%, which was 1.14 times that of the base metal. The Mn content was 28.42 mass%, which was 24.3 times that of the base metal. No Ni or Mo was detected. After immersion for three months, the critical crevice corrosion depth was exceeded, and the crevice corrosion resistance was poor. The reason why no Ni or Mo was detected is thought to be that the oxide scale was not removed and X-rays did not penetrate.

[Comparative Example 7]
In Comparative Example 7, the surface of the weld metal of the welded material using S32205 was subjected to pickling treatment. The acid used was 10% hydrochloric acid to which hexamethylenetetramine was added so that the concentration was 1.0%. The welded material was immersed in the acid solution at 30° C. for 1 minute.

In Comparative Example 7, the degree of browning of the weld metal surface was reduced, but the metallic luster was not visible. The Cr content was 19.85 mass%, which was 1.12 times that of the base metal. No Mn, Ni, or Mo was detected. The crevice corrosion resistance was poor. This result indicates that when pickling is performed with hydrochloric acid, oxide scale may not be sufficiently removed.

[Comparative Example 8]
In Comparative Example 8, the surface of the weld metal of the welded material using S32205 was pickled. A hydrofluoric nitric acid solution containing 10% nitric acid and 0.5% hydrofluoric acid was used as the acid. The welded material was immersed in the acid solution at 30° C. for 1 minute. In Comparative Example 8, the hydrofluoric acid concentration, the temperature of the acid solution, and the pickling treatment time are milder than those in Example 10.

In Comparative Example 8, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 20.23 mass%, 1.14 times that of the base material. No Mn, Ni, or Mo was detected. Crevice corrosion resistance was poor. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Although the oxide scale was removed from the outside, corrosion exceeding the critical crevice corrosion depth occurred. Since the Cr content was small and no Ni or Mo was detected, it is assumed that the oxide scale remains microscopically.

[Comparative Example 9]
In Comparative Example 9, the surface of the weld metal of the welded material using S32205 was electrolytically treated in a neutral salt solution. The electrolyte used was a 10% aqueous solution of sodium sulfate. The welded material was treated in the electrolyte at 30° C. for 5 minutes at a current density of 0.5 A/ ^dm2 .

In Comparative Example 9, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 18.85 mass%, 1.07 times that of the base material. No Mn, Ni, or Mo was detected. Crevice corrosion resistance was poor. It is believed that Mn was not detected because the oxide scale was sufficiently removed. Although the oxide scale was removed from the outside, corrosion occurred beyond the critical crevice corrosion depth. Since the Cr content was small and no Ni or Mo was detected, it is assumed that the oxide scale remains microscopically.

[Comparative Example 10]
In Comparative Example 10, the surface of the weld metal of the weld material using S32205 was electrolytically treated in a neutral salt solution. A 10% aqueous solution of sodium sulfate was used as the electrolytic solution. In the electrolytic solution at 30°C, a current was passed through the weld material at a current density ^of 50 A/dm2 for 5 minutes. In Comparative Example 5, the current density was higher than that in Example 7.

In Comparative Example 10, the oxide scale was removed and the metallic luster of the base was visible. The Cr content was 14.80 mass%, 0.84 times that of the base material. No Mn was detected. The Ni content was 0.42 mass%, 0.03 times that of the base material. The Mo content was 1.88 mass%, 0.61 times that of the base material. The crevice corrosion resistance was poor. It is believed that no Mn was detected because the oxide scale was removed. Although the oxide scale was removed from the outside, corrosion occurred beyond the critical crevice corrosion depth. It is presumed that this result was due to the current density being too high, which reduced the Cr, Ni, and Mo contents on the surface of the weld metal.

[Examples 7 to 12, Comparative Examples 6 to 10]
In Examples 7 to 12 and Comparative Examples 6 to 10, the N content was larger than that of the base material. N is an austenite stabilizing element, and has the effect of improving corrosion resistance, etc. However, no difference in the N content was confirmed regardless of the degree of oxide scale removal.

1 Electrolytic cell 2 Electrolyte 3 Electrode 4 Material to be treated (welded structure, welded material)
5 DC power supply 6 Welding metal 7 Impression material 8 Cable tie 9 Adjustment liquid

Claims (15)

A welded structure in which duplex stainless steel is welded,
The welded structure has a welded portion in which a base material formed of duplex stainless steel is joined via a weld metal,
A welded structure, wherein the amount of Cr at a surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more the amount of Cr in the base metal, in mass % excluding oxygen and carbon. 2. The welded structure of claim 1,
A welded structure, wherein, in mass % excluding oxygen and carbon, an amount of Ni at a surface of the weld metal measured by X-ray photoelectron spectroscopy is 0.4 times or more of an amount of Ni in the base metal, and an amount of Mo at a surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more of an amount of Mo in the base metal. 3. The welded structure of claim 2,
A welded structure, wherein an amount of Mn at a surface of the weld metal measured by X-ray photoelectron spectroscopy is equal to or less than an amount of Mn in the base metal, in mass % excluding oxygen and carbon. 2. The welded structure of claim 1,
A welded structure in which the weld metal has had oxide scale removed from its surface. 2. The welded structure of claim 1,
The weld metal is a welded structure formed by integrating the base material, the mating material, and a filler metal. A method for finishing a welded structure in which duplex stainless steel is welded, comprising the steps of:
The process includes removing oxide scale from a welded portion where a base material formed of duplex stainless steel is joined via a weld metal,
The step of removing the oxide scale is a step of removing oxide scale containing Fe from a surface of the weld metal such that an amount of Cr on the surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more the amount of Cr in the base metal, in terms of mass % excluding oxygen and carbon. A method for finishing a welded structure according to claim 6, comprising the steps of:
The step of removing the oxide scale is a step of removing oxide scale containing Fe from a surface of the weld metal such that, in mass % excluding oxygen and carbon, the amount of Ni on the surface of the weld metal measured by X-ray photoelectron spectroscopy is 0.4 times or more the amount of Ni in the base metal, and the amount of Mo on the surface of the weld metal measured by X-ray photoelectron spectroscopy is 1.5 times or more the amount of Mo in the base metal. A method for finishing a welded structure according to claim 6, comprising the steps of:
The method for finishing a welded structure , wherein the step of removing the oxide scale is a step of removing oxide scale containing Fe from a surface of the weld metal such that an amount of Mn on the surface of the weld metal measured by X-ray photoelectron spectroscopy is equal to or less than an amount of Mn in the base metal, in mass % excluding oxygen and carbon. A method for finishing a welded structure according to claim 6, comprising the steps of:
The step of removing the oxide scale is a step of subjecting the surface of the weld metal to an acid pickling treatment with nitric acid, sulfuric acid, hydrofluoric acid, or a mixed acid of at least one of these. A method for finishing a welded structure according to claim 6, comprising the steps of:
The step of removing the oxide scale is a step of pickling the surface of the weld metal with nitric acid of 2% or more and 50% or less, sulfuric acid of 2% or more and 50% or less, or hydrofluoric acid-nitric acid mixture of nitric acid of 2% or more and 50% or less and hydrofluoric acid of 0.2% or more and 10% or less. A method for finishing a welded structure according to claim 6, comprising the steps of:
The method for finishing a welded structure, wherein the step of removing the oxide scale comprises subjecting the surface of the weld metal to an electrolytic treatment in a neutral salt solution. A method for finishing a welded structure according to claim 11, comprising the steps of:
The method for finishing a welded structure, wherein the neutral salt is sodium sulfate, sodium nitrate, or ammonium sulfate. A method for finishing a welded structure according to claim 11, comprising the steps of:
A method for finishing a welded structure, wherein a current density of the weld metal in the electrolytic treatment is 2 A/ ^dm2 or more and 20 A/dm2 or ^less . A method for finishing a welded structure according to claim 6, comprising the steps of:
The method for finishing a welded structure, wherein the step of removing the oxide scale comprises heat treating the surface of the weld metal in a reduced pressure atmosphere. A method for finishing a welded structure according to claim 6, comprising the steps of:
The step of removing the oxide scale comprises subjecting the surface of the weld metal to a heat treatment at 400° C. or higher for one hour or more in a reduced pressure atmosphere of 10 ⁻² Torr or lower.

Images