JP7057141B2 - Welding structure and welding method for ferritic stainless steel sheets - Google Patents

Description

The present invention relates to a welded structure of a ferritic stainless steel sheet and a welding method for the welded structure.

As a method of welding a welded structure containing a plurality of ferritic stainless steel plates, a method of butt-welding two ferritic stainless steel plates, a method of welding using a filler metal of ferritic stainless steel, etc. Has been known for a long time. Alternatively, a method of welding using a filler metal of stainless steel, which is superior in strength and corrosion resistance to ferritic stainless steel sheets, is also generally adopted.

The portion that melts when welding stainless steel plates or when welding a stainless steel plate and a stainless steel material is referred to as a weld metal portion. The portion of the base metal on which the oxide scale is formed on the surface of the steel sheet due to the thermal history during welding is called the heat-affected zone. The heat-affected zone is generated near the boundary between the base metal and the weld metal portion, and the corrosion resistance is generally deteriorated particularly in the heat-affected zone of the stainless steel material. These weld metal parts and heat-affected zones are collectively referred to as welded parts.

For example, Patent Document 1 discloses a ferrite-based stainless steel in which the oxidation loss of Cr during welding is suppressed by adding Ti and Al in a composite manner, and the deterioration of corrosion resistance of the welded portion is improved. Further, Patent Document 2 discloses a method of suppressing a decrease in corrosion resistance of a welded portion when TIG welding is performed without performing Ar back gas sealing. Specifically, it is a ferritic stainless steel containing Cr of more than 21% by mass, and when Ni and Cu are added to TIG weld the ferritic stainless steels to each other, the thermal effect on the back surface of the stainless steel. A ferritic stainless steel having improved corrosion resistance of the portion is disclosed.

Further, Patent Document 3 describes that by adding elements such as Si, Al, and Ti, a ferritic stainless steel having excellent corrosion resistance of a welded portion can be obtained even when Ar back gas sealing is not performed. There is.

Japanese Unexamined Patent Publication No. 5-70899 JP-A-2007-302995 Japanese Unexamined Patent Publication No. 2013-204128

However, in the method of butt-welding two ferritic stainless steel sheets and the method of welding using a filler metal of ferritic stainless steel, the weld metal part becomes sharpened and the corrosion resistance deteriorates. It ends up. In addition, deep corrosion holes are generated in the heat-affected zone. On the other hand, when a welding method using a welded material of stainless steel, which is superior in strength and corrosion resistance to ferritic stainless steel plates, is adopted, the excellent corrosion resistance of the weld metal part can be guaranteed, but the heat-affected zone is still deeply corroded. There was a hole.

Further, in the ferrite-based stainless steel of Patent Document 1, when TIG welding is performed without performing Ar back gas sealing, the oxidation loss of Cr cannot be sufficiently suppressed, and the corrosion resistance of the welded portion is significantly lowered. The ferritic stainless steel of Patent Document 2 has sufficient corrosion resistance improvement when a gap is formed in the welded portion or the Cu content is out of the appropriate range (0.1% by mass to 1.0% by mass). It may not be effective and deep corrosion holes may occur. Further, in the techniques disclosed in Patent Documents 1 to 3, although corrosion is a problem only in the vicinity of the welded portion, it is necessary to replace the entire welded structure with a more expensive material, which is more than necessary. It costs money.

One aspect of the present invention has been made in view of each of the above problems, and an object thereof is in a heat-affected zone of a ferrite stainless steel sheet as a base material while maintaining a certain degree of corrosion resistance of a weld metal portion. The purpose is to effectively suppress the generation and growth of corroded holes.

In order to solve the above problems, the welded structure according to one aspect of the present invention is a welded structure including at least a first ferrite-based stainless steel plate and a second ferrite-based stainless steel plate, and the first ferrite-based stainless steel plate is described above. A welded metal portion of stainless steel having an austenite single-phase structure is formed between the stainless steel plate and the second ferrite-based stainless steel plate.

According to the above configuration, the weld metal portion is made of austenite single-phase structure stainless steel. Therefore, when the welded structure according to one aspect of the present invention is used in a corroded environment, corrosion begins to occur earlier in the weld metal portion than in the first and second ferritic stainless steel plates (base material), and after the occurrence, the corrosion begins to occur. Corrosion holes grow slowly. Further, while the corroded holes are growing in the weld metal part, the corroded holes are hardly generated or grown in the heat-affected zone of the first and second ferritic stainless steel plates, and only shallow corroded holes are formed in the weld metal part. Not done.

From the above, it is possible to effectively suppress the generation and growth of corrosion holes in the heat-affected zone of the first and second ferrite stainless steel sheets while maintaining the corrosion resistance of the weld metal portion to a certain extent.

Further, the welding method according to one aspect of the present invention is a welding method of a welded structure including at least a first ferrite-based stainless steel plate and a second ferrite-based stainless steel plate using a filler metal of austenite-based stainless steel. (I) The penetration rate of the filler metal into the first ferrite stainless steel plate and the second ferrite stainless steel plate, and (ii) at least one of the component compositions of the filler metal is the first ferrite-based steel plate. It is adjusted so as to form a welded metal portion of stainless steel having an austenite single-phase structure between the stainless steel plate and the second ferrite-based stainless steel plate.

According to the above configuration, welding of a welded structure capable of effectively suppressing the generation and growth of corrosion holes in the heat-affected zone of the first and second ferritic stainless steel plates while maintaining the corrosion resistance of the weld metal portion to a certain extent. The method can be realized.

Further, in the welding method according to one aspect of the present invention, (i) the penetration rate of the filler material and (ii) at least one of the component compositions of the filler metal are Cr equivalents in the weld metal portion. It is preferable that α is 16 or more and Ni equivalent β is 12 or more, and the relationship between the Cr equivalent α and the Ni equivalent β is adjusted so that α ≧ 1.1 × β-8.2. ..

According to the above configuration, when the welded structure is used in a corroded environment, corrosion holes are preferentially generated and grown in the weld metal part, so that the heat of the first and second ferritic stainless steels is generated. The generation and growth of corroded holes in the affected area can be suppressed more effectively.

According to one aspect of the present invention, it is possible to effectively suppress the generation and growth of corrosion holes in the heat-affected zone of the first and second ferritic stainless steel sheets while maintaining the corrosion resistance of the weld metal portion to a certain extent. ..

(A) is a cross-sectional view of a conventional welded structure. (B) is a cross-sectional view of a welded structure according to an embodiment of the present invention. It is a table which shows the component composition of the molding material of the welded structure which concerns on one Embodiment of this invention. It is an anodic polarization curve when various test pieces are arranged in a simulated environment inside a corroded hole, (a) is an anodic polarization curve when a single test piece is arranged, and (b) is a case where a connection test piece is arranged. It is a graph which shows the anodic polarization curve. It is a table which shows the component composition of the molding material of the filler metal which concerns on one Embodiment of this invention, and a comparative example. It is a table which shows the component composition of the molding material of the base material which concerns on one Embodiment of this invention, and a comparative example. It is a table which shows one Embodiment of this invention, the corrosion resistance evaluation of the welded structure which concerns on a comparative example, and the like.

[Overview of welded structure]
When the base metal and the welded portion are the same stainless steel material, the degree of deterioration in corrosion resistance in the heat-affected zone is generally larger than the degree of deterioration in corrosion resistance in the weld metal portion. Therefore, large and deep corrosion holes are generated in the heat-affected zone of the base metal, which has the lowest corrosion resistance. Here, the weld metal portion is a portion formed by the molten metal such as a filler metal, and is formed at a joint portion between two adjacent base materials. The heat-affected zone is a portion of the base metal in which an oxide scale is formed on the surface of the steel sheet due to the heat history at the time of welding, and is generated at a portion near the boundary between the base metal and the weld metal portion. Especially in the heat-affected zone of stainless steel, the corrosion resistance generally deteriorates. These weld metal parts and heat-affected zones are collectively referred to as welded parts.

For example, when two ferritic stainless steel plates (first ferritic stainless steel plate and second ferritic stainless steel plate) 2 are butted and welded as shown in FIG. 1 (a), a ferritic single-phase structure is welded to the joint portion. The welded structure 100 in which the metal portion 300 is formed is formed. In the welded structure 100, large and deep corrosion holes 2a-1 are generated in the heat-affected zone 2b-1 of the two ferrite stainless steel plates 2.

In that respect, as shown in FIG. 1B, the welded structure 1 according to the present embodiment uses a filler metal (not shown) having a structure different from that of the ferritic stainless steel plate 2 which is the base material. The weld metal portion 3 is formed. Specifically, the weld metal portion 3 is formed by using a filler metal of austenitic stainless steel, and the weld metal portion 3 has an austenite single-phase structure. The welded structure 1 according to the present embodiment is adopted for the entire structure including the welded portion.

In general, austenitic stainless steel tends to generate corrosion holes earlier than ferrite stainless steel. Therefore, in the process of exposing the welded structure 1 to the corrosive environment, the weld metal portion 3 begins to generate corrosive holes earlier than the two ferritic stainless steel plates 2. Further, after the corroded hole 3a is generated in the weld metal portion 3, the corroded hole 3a grows slowly.

Further, while the corroded holes 3a are growing, the corroded holes 2a are hardly generated or grown due to the following two points. That is, (A) where the total amount of cathode current of the welded structure 1 as a whole does not change, the anode current is excessively consumed by the generation / growth of the corrosion holes 3a, and as a result, the generation / growth of the corrosion holes 2a occurs. It is suppressed. (B) Since the electrode potential of the weld metal portion 3 becomes low due to the generation of the corroded hole 3a, a potential difference is generated between the weld metal portion 3 and the heat-affected zone 2b, and the corroded hole in the heat-affected zone 2b galvanically. The generation and growth of 2a is suppressed.

After the welded structure 1 is placed in a corroded environment through the above process, a corroded hole 2a is formed in the heat-affected zone 2b and a corroded hole 3a is formed in the weld metal portion 3 as shown in FIG. 1B. It is formed. Both the corroded holes 2a and 3a have the same size and depth, and the size is significantly smaller than that of the corroded holes 2a-1 shown in FIG. 1 (a). The depth is also significantly shallower.

[Component composition of molding material for welded structure]
In the welded structure 1 according to the present embodiment, the steel plate A in the table of FIG. 2 is used as the base material (ferritic stainless steel plate 2), and the composition of steel C is assumed as the composition of the weld metal portion 3. Has been done. When a steel plate of B steel is used as the base material, the composition of D steel is assumed as the composition of the weld metal portion 3.

Here, the A steel and the B steel are ferritic stainless steels having different amounts of Cr. Further, the C steel is SUS304L equivalent steel and the D steel is SUS316L equivalent steel, and the composition of the weld metal portion is predicted. The above-mentioned base metal and the weld metal portion constituting the welded structure 1 each have the following components.

Cr is a major component of stainless steel, which is important for ensuring corrosion resistance. Increasing the proportion of Cr oxide in the oxide film is also effective in reducing the proportion of Si oxide, Ti oxide, Al oxide and the like that are difficult to reduce. The Cr content of the base metal in the present embodiment is 17.96% by mass, and the Cr content of the C steel assumed to be the weld metal portion is 18.48% by mass. The Cr content when the base material of the B steel is used is 22.09% by mass, and the Cr content when the D steel assumed to be the weld metal portion is used is 17.48% by mass. In order to secure sufficient corrosion resistance, the Cr content is preferably 15% by mass or more, more preferably 18% by mass or more.

Ni is an effective element for obtaining an austenite single-phase structure. In addition, it has the effect of slowing the active dissolution rate of metal in the corroded pores where corrosion is progressing and suppressing the growth of the corroded pores. The Ni content of the base metal in the present embodiment is 0.16% by mass, and the Ni content of the C steel assumed to be the weld metal portion is 10.48% by mass. The Ni content when the base material of the B steel is used is 0.16% by mass, and the Ni content when the D steel assumed to be the weld metal part is used is 12.85% by mass.

Si effectively acts to improve the corrosion resistance of the welded portion when, for example, Ar gas sealing is performed and TIG welding is performed. In addition, Si is added to austenitic stainless steel for deoxidizing agents and other purposes, and also contributes to hardening of ferritic stainless steel. The Si content of the base metal in the present embodiment is 0.09% by mass, and the Si content of the C steel assumed to be the weld metal portion is 0.53% by mass. The Si content when the base material of the B steel is used is 0.25% by mass, and the Si content when the D steel assumed to be the weld metal portion is used is 0.69% by mass.

Like Cr, Mo is a basic component for ensuring stable corrosion resistance, and is mainly a component for improving corrosion resistance to seawater and various media. The Mo content of the base metal in the present embodiment is 0.95% by mass, and the Mo content of the C steel assumed to be the weld metal portion is 0.44% by mass. The Mo content when the base material of the B steel is used is 1.17% by mass, and the Mo content when the D steel assumed to be the weld metal part is used is 2.74% by mass.

C is an element inevitably contained in steel. When the C content is reduced, the steel becomes soft and the workability is improved. In addition, the formation of carbides is suppressed, and the weldability and the corrosion resistance of the weld metal portion 3 are improved. Therefore, the content of C should be low. The C content of the base metal in the present embodiment is 0.006% by mass, and the C content of the C steel assumed to be the weld metal portion is 0.015% by mass. The C content when the base material of the B steel is used is 0.011% by mass, and the C content when the D steel assumed to be the weld metal part is used is 0.018% by mass.

Like C, N is an element inevitably contained in steel. When the N content is reduced, the steel becomes soft and the workability is improved. In addition, the formation of nitrides is suppressed, and the weldability and the corrosion resistance of the weld metal portion 3 are improved. Therefore, the content of N should be low. The N content of the base metal in the present embodiment is 0.013% by mass, and the N content of the C steel assumed to be the weld metal portion is 0.016% by mass. The N content when the base material of the B steel is used is 0.012% by mass, and the N content when the D steel assumed to be the weld metal part is used is 0.009% by mass.

Mn is used as a deoxidizing agent for stainless steel. However, Mn lowers the Cr concentration in the passivation film and causes a decrease in corrosion resistance. Therefore, the Mn content should be low. The Mn content of the base metal in the present embodiment is 0.26% by mass, and the Mn content of the C steel assumed to be the weld metal portion is 1.52% by mass. The Mn content when the base material of the B steel is used is 0.16% by mass, and the Mn content when the D steel assumed to be the weld metal portion is used is 1.02% by mass.

Since P impairs the toughness of the base metal and the weld metal portion 3, the content thereof should be low. The P content of the base metal in the present embodiment is 0.034% by mass, and the P content of the C steel assumed to be the weld metal portion is 0.031% by mass. The P content when the base material of the B steel is used is 0.030% by mass, and the P content when the D steel assumed to be the weld metal part is used is 0.029% by mass.

Since S forms MnS, which tends to be the starting point of pitting corrosion, and causes a factor of lowering corrosion resistance, the content thereof should be low. The S content of the base metal in the present embodiment is 0.000% by mass, and the S content of the C steel assumed to be the weld metal portion is 0.003% by mass. The S content when the base material of the B steel is used is 0.001% by mass, and the S content when the D steel assumed to be the weld metal part is used is 0.002% by mass.

Ti is an element that contributes to the corrosion resistance of the welded portion in, for example, conventional TIG welding in which Ar back gas sealing is performed. The Ti content of the base metal in the present embodiment is 0.22% by mass, and the Ti content of the C steel assumed to be the weld metal portion is 0.001% by mass. The Ti content when the base material of B steel is used is 0.19% by mass.

Al prevents the oxidation loss of Cr by forming an Al oxide film on the surface of the heat-affected zone 2b and the like by heating at the time of welding. On the other hand, if Al is added more than necessary, the surface quality is deteriorated and the weldability is deteriorated. Therefore, it is preferable that the Al content is not so high. The Al content of the base material in this embodiment is 0.043% by mass. The Al content when the base material of the B steel is used is 0.089% by mass, and the Al content when the D steel assumed to be the weld metal part is used is 0.006% by mass.

Nb has a strong affinity with C and N, and is an effective element for preventing intergranular corrosion, which is a problem in ferritic stainless steel. On the other hand, if Nb is added more than necessary, high-temperature cracking in the weld occurs and the toughness of the weld metal portion 3 also decreases. Therefore, the content thereof should not be so high. The Nb content of the base metal in the present embodiment is 0.256% by mass, and the Nb content of the C steel assumed to be the weld metal portion is 0.035% by mass. The Nb content when the base material of the B steel is used is 0.203% by mass, and the Nb content when the D steel assumed to be the weld metal part is used is 0.022% by mass.

Regarding other component elements, it is not necessary to be particular about the corrosion resistance of the heat-affected zone 2b and the weld metal part 3, and various component compositions can be adopted depending on the application.

In addition, the content of each element described above is just an example. That is, if a welded structure composed of a base material of ferritic stainless steel and a welded metal portion of stainless steel having an austenite single-phase structure can be formed, it is made of ferritic stainless steel other than A and B steels. A base material may be used. Similarly, a filler metal material that forms the structure of the weld metal portion other than the C / D steel may be used.

Even if a base material (A steel) having a component composition as in the present embodiment is used, the size and depth are substantially the same as those of the corrosion holes 2a and 3a formed when the more expensive base material (B steel) is used. Corrosion holes 2a and 3a are formed (see (b) in FIG. 1).

Therefore, by adopting the welded structure 1 according to the present embodiment, the corrosion resistance of the weld metal portion 3 is maintained to a certain extent while suppressing the manufacturing cost, and the generation and growth of the corrosion holes 2a in the heat-affected zone 2b are effective. Can be suppressed.

[Welding method for ferritic stainless steel sheets]
The welded structure 1 according to the present embodiment is completed by forming a weld metal portion 3 using a filler metal of austenitic stainless steel as described above and joining two ferritic stainless steel plates 2. Specifically, for example, the weld metal portion 3 is formed by TIG welding with Ar gas sealing, and the two ferritic stainless steel plates 2 are joined. Further, not limited to TIG welding, any welding method may be adopted as long as other components can be melted in the weld metal portion 3 at the time of welding, such as MIG welding.

The filler metal used in the above-mentioned TIG welding needs to be devised so that the weld metal portion 3 formed in the welded structure 1 has an austenite single-phase structure. In the present embodiment, the Cr equivalent α and Ni equivalent β of the weld metal portion 3 in which the weld metal portion 3 tends to have an austenite single-phase structure are determined by using a Scheffler organization chart (not shown). Then, a filler material was selected so as to form the weld metal portion 3 having the obtained Cr equivalent α and Ni equivalent β, and TIG welding was performed.

Specifically, in the weld metal portion 3, the Cr equivalent α is 16 or more and the Ni equivalent β is 12 or more, and the relationship between the Cr equivalent α and the Ni equivalent β is α ≧ 1.1 × β-8.2. The filler metal was selected so that it would be. In the selected filler metal, both (i) the penetration rate of the filler metal into the two ferrite stainless steel plates 2 and (ii) the component composition of the filler metal are both Cr equivalent α and Ni equivalent as described above. The relationship between β and Cr equivalent α and Ni equivalent β is adjusted.

Here, the calculation formulas for obtaining the Cr equivalent α and the Ni equivalent β using the Schaeffler organization chart are the following formulas 1 and 2. In formulas 1 and 2, x represents the penetration rate of the filler material.

(Equation 1)
α = xx (Cr equivalent of filler metal) + (1-x) x (Cr equivalent of base metal)
(Equation 2)
β = xx (Ni equivalent of filler metal) + (1-x) x (Ni equivalent of base metal)
Further, for each of the filler metal and the base metal, the Cr equivalent is calculated by the following formula 3, and the Ni equivalent is calculated by the following formula 4.

(Equation 3)
Cr equivalent = Cr mass% + Mo mass% + 1.5 x Si mass% + 0.5 x Nb mass%
(Equation 4)
Ni equivalent = Ni mass% + 30 x C mass% + 0.5 x Mn mass%
The above-mentioned calculation method of Cr equivalent α and Ni equivalent β is merely an example, and is not limited to this case. In other words, a calculation formula capable of calculating Cr equivalent α and Ni equivalent β using (i) the penetration rate of the filler material into the two ferrite stainless steel plates 2 and (ii) at least one of the component compositions of the filler material. If so, any calculation formula may be used.

Further, in the weld metal portion 3, the Cr equivalent α is 16 or more and the Ni equivalent β is 12 or more, and the relationship between the Cr equivalent α and the Ni equivalent β is α ≧ 1.1 × β-8.2. It is not essential to select a suitable filler material. In other words, any filler metal in which (i) the penetration rate and (ii) at least one of the above component compositions are adjusted so that the weld metal portion 3 has an austenite single-phase structure. May be selected.

[Measurement example]
<Measurement method>
In order to confirm which of the conventional welding of the same material and the welding of different filler materials according to this embodiment causes the corrosion hole first, various test pieces are placed under a simulated environment inside the corrosion hole that reproduces the environment inside the corrosion hole. The anodic polarization curve was measured by arranging and applying a potential. Specifically, a 20% NaCl solution (adjusted to pH 1 using HCl) having a temperature of 30 ° C. is placed in an electrolytic tank (not shown), and various test pieces are immersed in this solution to perform various tests. A potential was applied to the piece. This solution was degassed by Ar and the sweep rate was set to 20 mV / min.

Regarding various test pieces, (i) ferritic stainless steel single test piece, (ii) austenite stainless steel single test piece, and (iii) ferritic stainless steel test piece and austenite stainless steel test piece. Three types of connection test pieces were used. Further, the single test pieces (i) and (ii) are made by insulatingly coating a steel plate cut into a size of 15 mm × 20 mm with a silicon seal leaving an electrode area of 1 cm ² . The connection test piece of (iii) is obtained by electrically connecting the individual test pieces of (i) and (ii) by wiring.

<Measurement result>
Hereinafter, each of the single test piece of A steel, the single test piece of C steel, and the connection test piece in which the single test piece of A steel and the single test piece of C steel are connected is immersed in the solution of the electrolytic tank. A case in which a potential is applied will be described as an example.

As shown in FIG. 3A, when a potential is applied to a single test piece of steel A, the applied potential is about −0.4 Vvs. When it was Ag / AgCl or less, the current density increased sharply. On the other hand, the applied potential is about -0.4 Vvs. The current density drops sharply from around Ag / AgCl, and the applied potential is about 0.2 Vvs. It was found that the current density became the lowest value at the time of Ag / AgCl.

Further, when a potential is applied to a single test piece of C steel, the applied potential is about −0.3 Vvs. When it was Ag / AgCl or less, the current density increased sharply. On the other hand, the applied potential is about -0.3 Vvs. The current density drops sharply from around Ag / AgCl, and the applied potential is about -0.1 Vvs. It was found that the current density became the lowest value at the time of Ag / AgCl.

Next, when a potential is applied to the connection test piece in which the single test piece of A steel and the single test piece of C steel are connected, the change in the current density is as shown in FIG. 3 (b) of the C steel. It was found that the tendency was almost the same as when the potential was applied to the single test piece alone.

Here, in the graphs of FIGS. 3A and 3B, corrosion holes start to occur in various test pieces at an applied potential having a value slightly higher than the applied potential when the current density becomes the minimum value. In this example, when a potential is applied to a single test piece of steel A, the applied potential is about 0.29 Vvs. Corrosion holes are generated in Ag / AgCl. Further, when a potential was applied to a single test piece of C steel, the applied potential was about −0.055 Vvs. Corrosion holes are generated in Ag / AgCl. Further, when a potential is applied to the connection test piece in which the single test piece of A steel and the single test piece of C steel are connected, the applied potential is about -0.05 Vvs. Corrosion holes are generated in Ag / AgCl.

As described above, the structure of the single test piece of A steel is similar to the contact mode between the heat-affected zone and the weld metal portion in the conventional "welding of the same material". Further, the connection mode between the single test piece of steel A and the single test piece of steel C is similar to the contact mode between the heat-affected zone 2b and the weld metal portion 3 in "differential filler metal welding". Therefore, it is presumed that the time when the corroded hole 3a of the weld metal portion 3 of the welded structure 1 is generated is earlier than the time when the corroded hole 2a of the heat-affected zone 2b can be generated.

After the corrosion hole 3a is generated in the weld metal portion 3, the corrosion hole grows in the weld metal portion 3, while the heat-affected zone 2b suppresses the occurrence of corrosiveness. Corrosion holes 2a are generated and difficult to grow. As a result, it is presumed that only the corrosion holes 2a smaller and shallower than the corrosion holes 3a of the weld metal portion 3 are formed in the heat-affected zone 2b.

〔Example〕
<Implementation method>
Using the filler metal a to g having a diameter of 1.2 mm shown in FIG. 4, two steel plates formed of each of the E steel to the G steel shown in FIG. 5 are fixed to the jig and sealed with Ar back gas. Then, butt welding was performed by TIG welding. The conditions for TIG welding are a current of 40 A to 50 A, a welding speed of 100 mm / min, a torch Ar gas flow rate of 10 L / min, and an electrode diameter of φ1.6 mm. The penetration rate x of the filler metal was adjusted by changing the supply rate.

The austenite phase ratio of the weld metal portion was determined to be austenite single phase when the measured value of the ferrite fraction by the ferrite scope was less than 1%.

Further, as an evaluation of the corrosion resistance of the welded portion, a dipping test using a hydrochloric acid acidic 6% ferric chloride solution was conducted. The temperature was 30 ° C. and the immersion time was 2 hours. As a result of the immersion test, the corrosion hole of the heat-affected zone was shallower than the corrosion hole of the weld metal part and the maximum erosion depth of the weld metal part was 0.1 mm or less, which passed the corrosion resistance evaluation (in FIG. 6). It was judged as "○").

<Implementation result>
No. shown in FIG. In the welded structures of 8, 9, 10, 11 and 12, the weld metal portion does not have an austenite single-phase structure due to insufficient penetration of the filler metal parts a to d, and corrosion holes exceeding 0.1 mm are formed in the weld metal portion. Occurred. In addition, No. In the welded structure of No. 13, since the Ni equivalent of the filler metal e is low, the weld metal portion does not have an austenite single-phase structure even if the penetration rate x is increased, and corrosion holes exceeding 0.1 mm are formed in the weld metal portion. Occurred.

On the other hand, No. The welded structures 14 and 15 are welded using a filler material of the same material as the steel plate. No. In the welded structures 14 and 15, corrosion holes exceeding 0.1 mm were generated in the heat-affected zone as in the conventional case.

[Additional notes]
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

1 Welded structure 2 Ferritic stainless steel sheet (1st ferritic stainless steel sheet, 2nd ferritic stainless steel sheet)
3 Welded metal part α Cr equivalent of weld metal part β Ni equivalent of weld metal part

Claims (3)

A welded structure containing at least a first ferritic stainless steel sheet and a second ferritic stainless steel sheet.
Both the first ferritic stainless steel sheet and the second ferritic stainless steel sheet contain 17% by mass or more of Cr and 0.9% by mass or more of Mo.
A weld metal portion of stainless steel having an austenite single-phase structure is formed between the first ferritic stainless steel plate and the second ferritic stainless steel plate.
In the weld metal portion, the Cr equivalent α is 17 or more and the Ni equivalent β is 12 or more, and the relationship between the Cr equivalent α and the Ni equivalent β is α ≧ 1.1 × β-8.2. Ori,
An immersion test was conducted in which the weld metal part, and the heat-affected parts of the first ferrite-based stainless steel plate and the second ferrite-based stainless steel plate were immersed in a hydrochloric acid-acidic 6% ferric chloride solution at a temperature of 30 ° C. for 2 hours. In this case, the corrosion hole of each heat-affected portion is shallower than the corrosion hole of the weld metal portion, and the maximum erosion depth of the weld metal portion is 0.1 mm or less . .. A welding method for a welded structure including at least a first ferritic stainless steel plate and a second ferritic stainless steel plate using an austenitic stainless steel filler.
(I) The penetration rate of the filler metal into the first ferritic stainless steel plate and the second ferritic stainless steel plate, and (ii) at least one of the component compositions of the filler metal is the first ferritic stainless steel plate. A welding method characterized by being adjusted so as to form a welded metal portion of stainless steel having an austenite single-phase structure between and the second ferritic stainless steel plate. (I) The penetration rate of the filler material and (ii) at least one of the component compositions of the filler material have a Cr equivalent α of 17 or more and a Ni equivalent β of 12 or more in the weld metal portion. The welding method according to claim 2, wherein the relationship between the Cr equivalent α and the Ni equivalent β is adjusted so that α ≧ 1.1 × β-8.2.

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