JP2017179427A - Welded joint of duplex stainless steel, welding method of duplex stainless steel and manufacturing method of welded joint of duplex stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded joint having excellent corrosion resistance under corrosion environment by improving corrosion resistance of a duplex stainless steel weld metal.SOLUTION: A welding method of a duplex stainless steel is provided in which the duplex stainless steel containing, by mass%, Cr:18% or more, Ni:0.1% or more, N:0.1% or more, and having a duplex structure of ferrite and austenite, the ferrite amount being 30 to 70 vol.%, is welded by non-consumable electrode type welding with a heat input Q (J/mm) and using a gaseous mixture of an Ar gas and a nitrogen gas as a shielding gas, the nitrogen gas amount being represented by the following formula. When Q≤1000 J/mm, nitrogen gas amount (vol.%)≥3.6×10Q-6.4×10Q+3. When Q≥1000 J/mm, nitrogen gas amount (mass%)≥3.3×10Q+3×10Q-0.133.SELECTED DRAWING: Figure 4

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 marine structure and a seawater desalination apparatus, or a chemical plant, a food production plant, a storage tank, etc. When welding duplex stainless steel used for assembly of welded structures used in various corrosive environments, such as TIG welding, it is possible to secure weld metal with corrosion resistance equivalent to that of steel in corrosive environments. The present invention relates to a welded joint of duplex stainless steel, a welding method of duplex stainless steel, and a manufacturing method of a welded joint of duplex stainless steel.

  Duplex stainless steel represented by SUS329J3L has excellent corrosion resistance, seawater resistance such as marine structures and seawater desalination equipment, sea salt particle resistance, various chemical plants, food manufacturing plants, storage As a corrosion-resistant material that can withstand severe corrosive environments that require chloride resistance such as tanks, it is applied to a wide range of fields. The duplex stainless steel is a stainless steel which has Cr, Ni, Mo, N as main elements, and has been adjusted so that the phase ratio of ferrite and austenite is about 50% by volume to ensure strength and corrosion resistance. Due to the recent rise in Ni and Mo, low-cost duplex stainless steels (for example, Patent Document 1) in which the amount of Ni and the amount of Mo are reduced as much as possible have been developed, and are equivalent to the austenitic stainless steel that is the mainstream of stainless steel. It has attracted attention as a stainless steel having corrosion resistance, low alloy cost, and low price fluctuation.

  The welding methods for constructing these duplex stainless steel welded structures include TIG welding, plasma welding, laser welding and other non-consumable electrode welding, MIG welding, and gas shielded arc welding using a flux-cored wire. In addition, consumable electrode welding such as coated arc welding and submerged arc welding is applied. Among these, non-consumable electrode type welding is inferior to consumable electrode type welding in terms of welding efficiency, but because pure Ar gas is used for the shielding gas, the amount of oxygen in the weld metal is extremely low and toughness is excellent. Suitable for the construction of welded structures with strict quality requirements.

  On the other hand, in welding construction of duplex stainless steel, post-heat treatment is not performed after welding from the viewpoint of maintaining corrosion resistance, and the steel is used as it is. In particular, since the weld metal is used as it is solidified, the amount of ferrite is increased and the corrosion resistance is reduced as compared with a steel material having the same composition. In particular, when the amount of ferrite in the weld metal exceeds 65% by volume, the corrosion resistance of the weld metal is drastically reduced (for example, Non-Patent Document 1 and Non-Patent Document 2). The reason for this is that in the component system of duplex stainless steel, it solidifies in a ferrite single phase, and when it becomes below about 1100 ° C. in the subsequent cooling process, acicular austenite precipitates at the ferrite grain boundary, and ferrite and austenite Although it becomes a two-phase structure, since the cooling rate at the time of welding is large, precipitation of austenite is suppressed, and the amount of ferrite is extremely large compared to steel materials. Further, in the duplex stainless steel, N is one of the main elements, and this N acts as an austenite stable element to increase the amount of austenite and is added as an element improving the corrosion resistance. However, in a high temperature state during welding, N easily evaporates, and N in the weld metal is less than in steel, so the amount of ferrite is greater than in steel. C and N have a high solid solubility in austenite, but the solid solubility of C and N in ferrite is extremely small. Therefore, in a weld metal with a large amount of ferrite, C and N cannot be completely dissolved. N is finely precipitated in the ferrite grains as chromium carbonitride. Thus, a chromium-deficient layer is formed around the finely precipitated chromium carbonitride, and the corrosion resistance is lowered.

In order to avoid this, in the welding of duplex stainless steel, a welding material in which the amount of Ni is increased from the steel material is generally used so that the amount of ferrite is 65 volume% or less even in a weld metal that is solidified. The In addition, a welding material in which the toughness, ductility and corrosion resistance are improved by refining the crystal grains of the weld metal has been developed (for example, Patent Document 2). However, when the supply amount of welding material is not sufficient due to defects in welding conditions or when the welding heat input is out of the range of 500 to 3500 J / mm, the amount of ferrite remains large and the corrosion resistance is reduced. To do. Furthermore, as described above, the precipitation of austenite in the weld metal depends on the cooling rate of about 1100 ° C. or less, and when the heat input is small and the cooling rate is extremely high, the welding material with increased Ni is used. However, the precipitation of austenite is suppressed and the amount of ferrite increases. In addition, in non-consumable electrode type welding such as TIG welding and plasma welding, welding is often performed without using a welding material. In that case, the amount of ferrite is surely increased, and the corrosion resistance is lower than that of steel. .
Thus, in the welding of duplex stainless steel, it is difficult to adjust the ferrite content, and a welding method is desired in which the corrosion resistance of the weld metal is equivalent to that of the base material.

International Publication No. 2002/027056 Japanese Patent No. 4531118

Ogawa, Ozeki: Journal of the Japan Welding Society, 57 (1988), p92 Miura, Takaso, Kudo, Tsuge: Proceedings of the Japan Welding Society, 7 (1989), p94

  The present invention relates to a non-consumable electrode type welding method for duplex stainless steel in view of the above-described conventional state of the art, regardless of the presence or absence of the use of welding material, the amount of supply, the amount of heat input, the size of the cooling rate, etc. , A method for welding a duplex stainless steel, which can stably adjust the ferrite content in the weld metal and ensure the corrosion resistance of the weld metal as well as a steel material, a method for manufacturing a welded joint of a duplex stainless steel, and a duplex stainless steel An object is to provide a welded joint of steel.

The present inventors investigated and examined in detail the microstructure and corrosion resistance of a weld metal formed by welding duplex stainless steel.
As a result, it is clear that the weld metal microstructure can be changed and the corrosion resistance can be improved by adding a proper amount of nitrogen gas to the shield gas and performing non-consumable electrode welding in various duplex stainless steel welded joints. I made it.

The present invention solves the above-mentioned problems, and the gist thereof is as follows.
(1) By mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the ferrite content is 30-70 A welded joint made of a duplex stainless steel base material having a volume% and a weld metal part, wherein the ferrite content of the weld metal part is 65% by volume or less.
(2) By mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the ferrite content is 30-70 It is a method of welding a duplex stainless steel of volume% by non-consumable electrode type welding with a heat input Q (J / mm) and using a mixed gas of Ar gas and nitrogen gas as a shielding gas. A method for welding duplex stainless steel, wherein the amount of nitrogen gas is represented by the following formula:
When Q ≦ 1000 J / mm Nitrogen gas amount (volume%) ≧ 3.6 × 10 ⁻⁶ Q ² −6.4 × 10 ⁻³ Q + 3
When Q ≧ 1000 J / mm Nitrogen gas amount (volume%) ≧ 3.3 × 10 ⁻⁸ Q ² + 3 × 10 ⁻⁴ Q−0.133
However, heat input Q (J / mm) = welding current I (A) × welding voltage V (V) / welding speed v (mm / second)
(3) By mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the ferrite content is 30-70 A weld joint comprising a duplex stainless steel base material having a volume% and a weld metal part having a ferrite content of 65 volume% or less is produced by the duplex stainless steel welding method described in (2) above. To manufacture welded joints of duplex stainless steel.

  ADVANTAGE OF THE INVENTION According to this invention, the corrosion resistance of the weld metal formed by welding duplex stainless steel can be improved, and the corrosion resistance of a weld metal part can be improved significantly in a corrosive environment.

It is the figure which showed the influence of the heat input amount which has on the ferrite amount of a weld metal. It is the figure which showed the influence of the heat input which has on the amount of nitrogen in a weld metal. It is the figure which showed the relationship between the nitrogen content in shield gas, and the nitrogen content in a weld metal. It is the figure which showed the influence of the amount of heat inputs and the amount of nitrogen in shield gas which acts on the ferrite content of a weld metal.

  Hereinafter, the present invention will be described in detail. In the following description, “%” means “mass%” unless otherwise specified, and represents “volume%” used for the phase fraction of each phase and “volume” representing the ratio of the mixed gas used for the shielding gas. % ".

  The duplex stainless steel defined by the present invention is limited by the microstructure and chemical composition. That is, it is structurally a two-phase structure of ferrite + austenite at room temperature, and more specifically, the ferrite content of the room temperature structure is 30% by volume to 70% by volume, and the component is mass%. , Cr is 18% or more, Ni is 0.1% or more and N is 0.1% or more, and the other alloy elements are not particularly limited as long as the above microstructure is satisfied.

  Further, the shape of the duplex stainless steel is not particularly limited, such as a plate material, a pipe material, a wire material, and the like.

  If the amount of ferrite in the duplex stainless steel is less than 30% by volume and more than 70% by volume, the corrosion resistance of the duplex stainless steel decreases, so the amount of ferrite is limited to 30 to 70% by volume.

  The reason for limiting the Cr content is that Cr is a ferrite-forming element and contributes to the improvement of the corrosion resistance as a main element of the duplex stainless steel. However, if the content is less than 18%, sufficient corrosion resistance cannot be obtained. , Limited to 18% or more. The content of Cr is preferably 20% or more, and more preferably 23% or more. The upper limit of the Cr content is not particularly specified, but is preferably 27% or less from the viewpoint of cost.

  The reason for limiting the Ni content is that Ni is an austenite-forming element and stabilizes the austenite phase to improve ductility and toughness. However, if the content is less than 0.1%, the austenite phase is not stable. Since it is sufficient and the toughness deteriorates, it is limited to 0.1% or more. The Ni content is preferably 0.5% or more, and more preferably 1.0% or more. The upper limit of the Ni content is not particularly specified, but is preferably 7.0% or less from the viewpoint of cost.

  As a reason for limiting the content of N, N is effective in improving corrosion resistance and at the same time is a strong austenite-forming element, and particularly has a high diffusion rate and is likely to cause redistribution, thereby promoting precipitation of austenite. If the 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. The upper limit of the N content does not need to be specified in particular, but when the base material is a duplex stainless steel having a relatively low N of 0.30% or less, the amount of ferrite in the weld metal part tends to increase. The effect of the present invention is remarkably exhibited. The upper limit of the N content is preferably 0.25% or less, and more preferably 0.20% or less.

  As described above, alloy elements other than Cr, Ni, and N are not particularly limited. Examples of the duplex stainless steel to which the present invention can be applied include C: 0.025% or less, Si: 1.0% or less, Mn: 0.5 to 6.0%, Ni: 0.1 to 7.0%, Cr: 18 to 27%, P: 0.040% or less, S: 0.0100% or less, Mo: 0.05 -4.0%, Cu: 0.10-10.50%, N: 0.1-0.30%, remainder Fe and an unavoidable impurity composition are mentioned. This composition is merely an example, and the present invention is not limited to this composition.

  In order to ensure the corrosion resistance of the duplex stainless steel, C is preferably made a content of 0.025% or less. If it is 0.025% or less, the production | generation of Cr carbide | carbonized_material will be suppressed and corrosion resistance will improve. On the other hand, since extremely reducing the C content significantly increases the cost, the lower limit of the C content is preferably set to 0.001%.

  Si is added in an amount of 0.10% or more for deoxidation. By setting the Si content to 1.0% or less, toughness can be improved. A preferable range of the Si content is 0.20 to 0.60%.

  Mn increases the austenite phase in duplex stainless steel, suppresses the formation of work-induced martensite, improves toughness, increases the solid solubility of nitrogen, and suppresses the precipitation of nitride in the weld. To 0.50% or more. By setting the Mn content to 6.0% or less, the corrosion resistance can be improved. A preferable range of the Mn content is 1.50 to 4.00%, and a more preferable range is more than 2.00% and less than 3.00%.

  P and S are inevitably mixed elements, but are elements that lower the corrosion resistance of stainless steel, so it is preferable to reduce them as much as possible. P is 0.040% or less, and S is 0.0100% or less. It is desirable. Moreover, since excessive reduction leads to an increase in cost, P is preferably 0.001% or more and S is preferably 0.0001% or more.

  Since Mo is a very effective element that improves the corrosion resistance of the duplex stainless steel, 0.05% or more is contained. Toughness can be improved by making Mo content 4.0% or less. A preferable range of the Mo content is 2.4 to 3.2%, and a more preferable range is 2.5 to 2.9%.

  Cu is an element that additionally increases the corrosion resistance of the duplex stainless steel to acids, and has the effect of stabilizing the austenite phase and improving toughness. The occurrence of embrittlement can be suppressed by setting the Cu content to 1.50% or less. A preferable range of the Cu content is 0.3 to 1.5%.

  Further, in order to improve hot workability, corrosion resistance, workability, etc., Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.10% or less, B: 0.0. 0050% or less, Sn: 1.0% or less, Sb: 1.0%, Co: 0.50% or less, V: 0.50% or less, W: 1.0% or less can also be added.

  In addition, about 0.005% or more about Ca and Mg, since the stable effect is acquired, a preferable range is 0.0005 to 0.0050%. If REM is made 0.005% or more, a stable effect can be obtained, so a preferred range is 0.005 to 0.10%. If B is 0.0003% or more, a stable effect can be obtained, so a preferable range is 0.0003 to 0.0050%.

  Sn is a selective element that additionally improves the corrosion resistance. The content of Sn that stably exhibits this effect is 0.050% or more. By making the Sn content 1.0% or less, the hot workability can be improved.

  Sb, like Sn, is an element useful for improving corrosion resistance when contained in a trace amount, and is contained in a range that does not impair the low cost. Corrosion resistance can be improved by setting the Sb content to 0.001% or more. By making the Sb content 1.0% or less, workability can be improved. A preferable range of the Sb content is 0.01 to 0.30%.

  Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is selectively added. The Co content that exhibits this effect is 0.02% or more. By making the Co content 0.50% or less, an increase in cost due to excessive addition can be prevented. A preferable range of the Co content is 0.04% or more and less than 0.30%.

  V is an element effective for additionally improving the corrosion resistance of the duplex stainless steel. V needs to be added in an amount of 0.05% or more in order to obtain the above effect. By setting the V content to 0.50% or less, good hot workability can be ensured. A preferable range of the V content is 0.06% to 0.20%.

  W is an element that is selectively added to additionally enhance the corrosion resistance of the duplex stainless steel. The content of W that stably exhibits this effect is 0.05% or more. By making the content of W 1.0% or less, an increase in cost due to excessive addition can be prevented. A preferable range of the W content is 0.10% to 0.50%.

  Next, the inventor of the present invention has a 22.5% Cr-5.7% Ni-3% Mo-0.158% N component in the above-mentioned limited range and a two-phase ferrite content of 51% by volume. The relationship between the amount of heat input and the amount of ferrite in the weld metal when stainless steel was TIG welded without using a welding wire with 100 volume% Ar gas as a shielding gas was investigated.

  The amount of ferrite in the weld metal was measured by cutting the weld metal, mirror polishing, performing electrolytic etching in a sodium hydroxide solution, and then performing image analysis with an optical microscope. The result is shown in FIG. In any amount of heat input, the amount of ferrite is greater than 51% by volume of the steel material, and when the amount of heat input is 1000 J / mm or less, the amount of ferrite increases rapidly as the amount of heat input decreases. This is because the smaller the heat input, the higher the cooling rate of about 1100 ° C. or less, which is the temperature range in which austenite precipitates, so that austenite precipitation is suppressed. In addition, when the heat input is 1000 J / mm or more, the amount of ferrite increases as the heat input increases. This is because N, which is an austenite-generating element, evaporates during high temperatures during welding.

  In FIG. 2, a duplex stainless steel having a component of 22.5% Cr-5.7% Ni-3% Mo-0.158% N and a ferrite content of 51% by volume is added to 100% by volume Ar. The relationship between the amount of heat input and the amount of N in the weld metal when TIG welding is performed without using a welding wire with a gas as a shielding gas is shown. In any amount of heat input, the N amount of the weld metal is smaller than the N amount of the steel material, and the N amount in the weld metal decreases as the heat input increases. As described above, this is because N evaporated to the outside during the high temperature during welding. That is, as the heat input decreases, the cooling rate increases and the amount of ferrite increases, and as the heat input increases, the evaporation of the N amount increases and the amount of ferrite increases. Therefore, the ferrite content in the weld metal is determined by the superposition of the cooling rate depending on the heat input and the evaporation amount of N.

  As a method for suppressing such an increase in the amount of ferrite in the weld metal, it is conceivable to add N, which is a strong austenite-forming element, to the weld metal. Therefore, welding was performed by using a mixed gas of Ar gas and nitrogen gas as the shielding gas and changing the mixing ratio of the nitrogen gas. 22.5% Cr-5.7% Ni-3% Mo-0.158% N component and ferrite volume 51 volume% duplex stainless steel, Ar gas and nitrogen gas mixed gas shielding gas The relationship between the amount of nitrogen gas in the shielding gas and the amount of N in the weld metal when TIG welding is performed without using a welding wire at a heat input of 1000 J / mm is shown in FIG. By increasing the nitrogen gas in the shielding gas, the amount of N in the weld metal also increases and is larger than the amount of N in the steel material.

  From this result, FIG. 4 shows the relationship between the amount of nitrogen gas in the shield gas and the amount of ferrite in the weld metal when the heat input is changed. In FIG. 4, the case where the ferrite content is 65% by volume or less is indicated by ◯, and the case where the ferrite content exceeds 65% by volume is indicated by x. Here, the reason for distinguishing the amount of ferrite by 65% by volume is that in the above-mentioned Non-Patent Document 1 and Non-Patent Document 2, when the ferrite content of the weld metal is 65% by volume or less, fine chromium carbonitride in the ferrite grains. This is because the precipitation of the product is suppressed and good corrosion resistance can be secured. Thus, in order to ensure the corrosion resistance of the weld metal, the ferrite content in the weld metal needs to be 65% by volume or less. For that purpose, nitrogen gas is mixed in the shielding gas, and N in the weld metal is mixed. It is effective to increase the amount. However, the amount of nitrogen gas to be mixed in the shielding gas necessary to reduce the ferrite content in the weld metal to 65% by volume or less varies depending on the welding heat input as shown in FIG. This is because the cooling rate and the N evaporation amount change depending on the welding heat input. Therefore, the minimum limit value of the amount of nitrogen gas to be mixed in the shielding gas necessary to reduce the ferrite content in the weld metal to 65% by volume or less is determined by the welding heat input. That is, the amount of heat input is about 1000 J / mm, which is the smallest, and the amount of nitrogen gas to be mixed with the shield gas increases as the amount of heat input is smaller or larger. This is because, as described above, the amount of ferrite in the weld metal is determined by the superposition of the cooling rate depending on the heat input and the evaporation amount of N.

From the above knowledge, in the present invention, in order to ensure the corrosion resistance of the weld metal using the heat input Q (J / mm), the amount of ferrite in the weld metal is reduced to 65% by volume or less. The amount of nitrogen gas to be mixed is limited as follows.
When Q ≦ 1000 J / mm Nitrogen gas amount (% by volume) ≧ 3.6 × 10 ⁻⁶ Q ² −6.4 × 10 ⁻³ Q + 3 (Formula 1)
When Q ≧ 1000 J / mm Nitrogen gas amount (volume%) ≧ 3.3 × 10 ⁻⁸ Q ² + 3 × 10 ⁻⁴ Q−0.133 (Equation 2)
It has been found that satisfying the requirement is a shielding gas requirement for reducing the amount of ferrite in the weld metal to 65% by volume or less and improving the corrosion resistance of the weld metal.
Here, the unit of the heat input Q is J / mm, and the heat input Q is defined by the following (Formula 3).
Heat input Q (J / mm) = welding current I (A) × welding voltage V (V) / welding speed v (mm / second) (Equation 3)

  The amount of ferrite in the weld metal can be measured by mirror-polishing the weld metal, performing electrolytic etching in a sodium hydroxide solution, and then performing image analysis with an optical microscope.

  The duplex stainless steel welding method of the present invention is a non-consumable electrode welding method such as TIG welding, plasma welding, laser welding, etc., and the welding method is not particularly limited, and the welding material to be used is also particularly limited. There is no. Furthermore, even in welding with heat input outside the range of 500 to 3500 J / mm, where it has been impossible to sufficiently reduce the ferrite content even when using a welding material, the ferrite content of the weld metal part is 65 volume% or less. It is possible to Moreover, you may weld without using a welding material. In particular, when a duplex stainless steel having a relatively small N content of 0.30% or less is welded without using a welding material, a weld joint having a weld metal portion with a ferrite content of 65% by volume or less can be provided. . Furthermore, the shape of the welded joint is not particularly limited, such as a fillet joint in addition to the butt joint. A welded joint of duplex stainless steel that improves the microstructure of the weld metal and has excellent corrosion resistance by welding using the mixed gas of the nitrogen gas amount specified in (Formula 1) and (Formula 2) above. can get.

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 the base material.
Table 2 shows the chemical composition of the welding material for duplex stainless steel.
When welding is performed using the welding materials shown in Table 2 by providing a V groove with a groove angle of 60 ° at the butt end of the duplex stainless steel material shown in Table 1 under the welding method and welding conditions shown in Table 3. In addition, two types of welded joints were evaluated when welding was performed without using a groove in the duplex stainless steel material of Table 1 and without using a welding material. The shielding gas is a mixed gas of Ar gas and nitrogen gas, and the amount of nitrogen gas in the used mixed gas is shown in Table 3. Also, when the welding heat input is 1000 J / mm or less, the value of the limit nitrogen gas amount calculated by (Equation 1) above, and when the welding heat input is 1000 J / mm or more, it is calculated by (Equation 2) above. Table 3 also shows the value of the limit nitrogen gas amount.
In the welded joint thus obtained, the ferrite content and corrosion resistance of the weld metal were evaluated. The results are also shown in Table 3.

The amount of ferrite was measured by mirror-polishing the weld metal cross section, performing electrolytic etching in a sodium hydroxide solution, and then performing image analysis by observation with an optical microscope. Corrosion resistance was evaluated by wet-polishing the surface of a test piece taken from a weld metal with # 600 emery paper, in accordance with ASTM G48 Method E regulations, and the critical pitting corrosion temperature (CPT) by ferric chloride immersion test. Was measured.
For example, the symbol No. in Table 3 which is an example of the present invention. 2, the steel material B in Table 1 was used as the shielding gas with a welding heat input of 1250 J / mm using the welding material a in Table 2 and a 99.6 vol% Ar gas + 0.4 vol% N ₂ mixed gas as the shielding gas. This is a case where a welded joint is produced by TIG welding, and since the welding heat input exceeds 1000 J / mm, the limit nitrogen gas amount calculated by (Equation 2) is 0.29% by volume, which is within the scope of the present invention. In. As a result, the ferrite content of the weld metal is 59% by volume, and the critical pitting corrosion temperature (CPT) of the weld metal is 15 ° C., which is equivalent to the critical pitting corrosion temperature (CPT) of the steel material B shown in Table 1. became.

Thus, as is apparent from Table 3, when the shielding gas within the scope of the present invention, that is, when the heat input is 1000 J / mm or less, the nitrogen gas amount is 3.6 × 10 ⁻⁶ Q ² -6.4 × 10. ⁻³ Q + 3 (volume%) or more, and when the heat input is 1000 J / mm or more, the amount of nitrogen gas is 3.3 × 10 ⁻⁸ Q ² + 3 × 10 ⁻⁴ Q−0.133 (volume%) or more. No. 1 welded with Ar gas and nitrogen mixed gas as shield gas 1-No. In the present invention example 9, the ferrite content of the weld metal is 65 volume% or less, and the critical pitting corrosion temperature (CPT) is equivalent to the critical pitting corrosion temperature (CPT) of each steel shown in Table 1. In addition, according to the present invention, it can be seen that the corrosion resistance of the weld metal is equivalent to that of the steel material.

  On the other hand, no. In Comparative Examples 10, 12, and 15, the welding heat input is 1000 J / mm or less, and a mixed gas having a nitrogen gas amount smaller than the limit nitrogen gas amount calculated by using the above (Equation 1) from the heat input amount is used. Therefore, the ferrite content of the weld metal is more than 65% by volume, and the critical pitting corrosion temperature (CPT) is lower than the critical pitting corrosion temperature (CPT) of each steel shown in Table 1.

  No. In the comparative examples of 11, 13, 14, 16 and 17, the welding heat input is 1000 J / mm or more, and the amount of nitrogen gas is less than the limit nitrogen gas calculated from the heat input using the above (Equation 2). Since a mixed gas was used, the ferrite content of the weld metal was more than 65% by volume, and the critical pitting corrosion temperature (CPT) was also lower than the critical pitting corrosion temperature (CPT) of each steel shown in Table 1. Yes.

  No. In the comparative examples 18 and 19, the welding heat input is 1000 J / mm or more, and a mixed gas having a nitrogen gas amount larger than the limit nitrogen gas amount calculated by using the above (Equation 2) from the heat input amount is used. Since the weld metal has a ferrite content of less than 65% by volume, the Cr content of the steel material is less than 18%, or the N content of the steel material is less than 0.1%. ) Is lower than the critical pitting corrosion temperature (CPT) of the steel shown in Table 1.

  From the above, it was found that by applying the duplex stainless steel welding method of the present invention, the corrosion resistance of the weld metal is improved, and a welded joint having excellent corrosion resistance equivalent to that of a steel material in a corrosive environment can be obtained.

  According to the present invention, the corrosion resistance of the weld metal formed by welding the duplex stainless steel is improved, and the corrosion resistance of the weld metal portion is greatly improved in a corrosive environment. As a result, the deterioration of the corrosion resistance of weld metal parts of duplex stainless steel, which has been a problem in the past, has been improved, seawater resistance such as marine structures and seawater desalination devices, seawater particle resistance, and various chemicals. The application as a welded structure used in fields where chloride resistance is required, such as plants, food production plants, storage tanks, etc., has been expanded, and the industrial contribution is extremely large.

Claims (3)

In mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the amount of ferrite is 30 to 70% by volume It is a welded joint composed of a certain duplex stainless steel base material and a weld metal part,
A welded joint of duplex stainless steel, wherein the weld metal portion has a ferrite content of 65% by volume or less. In mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the amount of ferrite is 30-70% by volume A non-consumable electrode type welding with a heat input Q (J / mm), and a welding gas using a mixed gas of Ar gas and nitrogen gas as a shielding gas,
A method for welding duplex stainless steel, characterized in that the amount of nitrogen gas is expressed by the following equation.
When Q ≦ 1000 J / mm Nitrogen gas amount (volume%) ≧ 3.6 × 10 ⁻⁶ Q ² −6.4 × 10 ⁻³ Q + 3
When Q ≧ 1000 J / mm Nitrogen gas amount (volume%) ≧ 3.3 × 10 ⁻⁸ Q ² + 3 × 10 ⁻⁴ Q−0.133
However, heat input Q (J / mm) = welding current I (A) × welding voltage V (V) / welding speed v (mm / second)   In mass%, Cr: 18% or more, Ni: 0.1% or more, N: 0.1% or more, and a two-phase structure of ferrite and austenite, the amount of ferrite is 30 to 70% by volume A two-phase stainless steel base material and a welded joint comprising a weld metal part having a ferrite content of 65% by volume or less are manufactured by the duplex stainless steel welding method according to claim 2. Manufacturing method of stainless steel welded joints.

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