JP7845210B2 - Clad steel welding method - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Law Meeting Name: Study Group on the Renewal Project of the Tedorigawa Bridge on the Hokuriku Expressway (5th Meeting) Venue: Web Conference (Hosted by Central Nippon Expressway Co., Ltd.) Date: February 2, 2022

This invention relates to a clad steel welding method, and more particularly to a clad steel welding method that suppresses the dilution of the welding material, which is a corrosion-resistant metal, after base metal welding.

Clad steel is a composite steel material in which a cladding material (such as stainless steel) with excellent corrosion resistance and other functional properties is metallurgically bonded to the surface of a base material (carbon steel or low-alloy steel). It possesses the necessary strength for structural members, while also having excellent corrosion resistance and other functional properties. Furthermore, it is less expensive than steel manufactured using the same material for both the base and cladding components. These are its superior characteristics, and it is widely used in structures and components placed in corrosive environments.

However, because clad steel is a composite material, welding it requires different techniques compared to welding carbon steel to carbon steel. Conventional clad steel welding methods involve downward welding from the base metal side. However, the first layer is prone to welding defects such as incomplete melting, blowholes, and slag inclusion. Therefore, after welding the base metal, the piece is reversed and the back is gouged or ground to remove the welding defects. Then, TIG (Tungsten Inert Gas) welding or arc welding using flux-cored wire is performed downward from the clad material side.

In contrast, Patent Document 1 proposes a butt welding method for clad steel that allows welding from the inside of the pipe in all positions without having to chip away the back bead after base metal welding, and enables welding with a welding material that has a strength equal to or greater than that of the non-corrosion-resistant steel on the non-corrosion-resistant steel side, thereby achieving both corrosion resistance and high strength in the clad steel pipe.

In the technology described in Patent Document 1, the end of the clad steel 100 to be butt-welded is processed into a groove 120 of the shape shown in Figure 11 (the shape of the base material side groove 122 is not particularly limited), and then a first layer welding is performed on the base material side groove 122 on the non-corrosion-resistant steel (base material 102) side, forming a back bead 132 of a predetermined shape (H _bx / W _bx < 1, where H _bx : back bead height [mm], W _bx : back bead width [mm]) as shown in Figure 12 within the cladding material side groove 124 on the high-corrosion-resistant material (cladding material 104) side. Then, using a welding material with corrosion resistance equal to or greater than that of the high-corrosion-resistant material (cladding material 104) of the clad steel 100, the cladding material side groove 124 on the high-corrosion-resistant material (cladding material 104) side is welded without chipping away at the back bead 132.

Japanese Patent Publication No. 2021-137860

However, the present inventors have considered that the technology described in Patent Document 1 involves forming a back bead 132 made of non-corrosion-resistant steel within the groove 124 on the high-corrosion-resistant side (clamping material 104), and then welding the groove 124 on the high-corrosion-resistant side (clamping material 104) without chipping away at the back bead 132. However, since no measures are taken to reduce the ratio of the height H _bx to the width W _bx of the back bead 132 formed within the groove 124 (H _bx / W _bx ), there is a risk of dilution of the high-corrosion-resistant welding material in the vicinity of the back bead 132. Specifically, the present inventors have considered that the following problems (1) and (2) may occur.

(1) When a nickel alloy welding material is used to weld the groove 124 on the mating material side, localized corrosion resistance decreases as the dilution increases.

(2) When welding the groove 124 on the mating material side using 309 series welding material, if the dilution is large, the corrosion resistance will locally decrease and the weld metal will form a martensitic structure. A martensitic structure is extremely hard and has poor ductility, which can cause cracking. Furthermore, a martensitic structure can lead to delayed cracking due to diffusible hydrogen.

This invention has been made in view of the above points, and aims to provide a clad steel welding method that suppresses the dilution of the welding material, which is a corrosion-resistant metal, after base metal welding.

This invention solves the aforementioned problems and provides the following clad steel welding method.

In other words, the first embodiment of the clad steel welding method according to the present invention is a welding method for clad steel using stainless steel or nickel alloy as the cladding material and carbon steel or low-alloy steel as the base material, characterized by comprising: a beveling step of performing a cutback to remove the cladding material at the welding site of the clad steel to form an exposed surface of the base material; a butt joint step of butting the beveled clad steels together so that the root gap between the base materials is 3 mm or more; a backing material placement step of placing a backing material on the exposed surface of the base material after the butt joint step; a base material welding step of performing a butt joint bevel welding from the base material side after the backing material placement step; a backing material removal step of removing the backing material after the base material welding step; and a cladding material welding step of performing a butt joint bevel welding of the cladding material from the cladding material side after the backing material removal step.

In this application, carbon steel refers to a type of steel that is an alloy of iron and carbon, with a carbon content typically ranging from 0.02% to approximately 2%, and containing small amounts of silicon, manganese, phosphorus, sulfur, etc. Low-alloy steel refers to steel in which the total amount of specified alloying elements is 5% by mass or less.

Furthermore, in this application, "cutback" refers to removing the bonding material in a direction approximately parallel to the direction in which the bonding material expands, thereby exposing the base material.

A second embodiment of the clad steel welding method according to the present invention is an embodiment in which, in the first embodiment of the clad steel welding method, the backing material is made of ceramic.

A third aspect of the clad steel welding method according to the present invention is an aspect of the first or second aspect of the clad steel welding method in which, in the butt joint step, the beveled clad steels are butted together such that the parallel length of the groove bottom on the cladding material side, which includes the exposed surface, is 15 mm or more.

A fourth aspect of the clad steel welding method according to the present invention is a configuration in which, in any of the first to third aspects of the clad steel welding method, arc welding is performed from the clad material side using a flux-cored wire having a corrosion-resistant metal welding material in the clad material welding step.

A fifth aspect of the clad steel welding method according to the present invention is an aspect in which, in any of the first to fourth aspects of the clad steel welding method, the backing material is configured to be a backing material without a recess for forming a back bead.

A sixth aspect of the clad steel welding method according to the present invention is a configuration in which, in any of the first to fifth aspects of the clad steel welding method, the groove preparation step is configured to perform a cutback to remove the cladding material, thereby forming a groove wall with an angle of 130° or more with respect to the exposed surface.

A seventh aspect of the clad steel welding method according to the present invention is an aspect configured such that, in any of the first to sixth aspects of the clad steel welding method, the clad material welding step is performed without inverting the clad steel after the base material welding step has been performed.

An eighth aspect of the clad steel welding method according to the present invention is an aspect of any of the first to seventh aspects of the clad steel welding method in which, in the cladding welding step, a mixed gas of _CO2 and argon is used as a shielding gas, the _CO2 content of the mixed gas is set to 0 mol% or more and 95 mol% or less, and the heat input when performing the arc welding is set to 5 kJ/cm or more and 40 kJ/cm or less.

A ninth aspect of the clad steel welding method according to the present invention is a configuration in which, in any of the first to eighth aspects of the clad steel welding method, after the cladding welding step, at least one of mechanical polishing, electrolytic polishing, or passivation treatment is performed using an abrasive material that does not contain iron and has a particle size equivalent to or finer than F80 as defined in JIS R6001 (1998), to treat the surface of the area welded in the cladding welding step.

A tenth aspect of the clad steel welding method according to the present invention is a configuration in which, in the ninth aspect of the clad steel welding method, after performing the mechanical polishing on the surface of the area welded in the cladding welding step, either electrolytic polishing or passivation treatment is further performed on the said surface.

An eleventh aspect of the clad steel welding method according to the present invention is an aspect in which, in any of the first to tenth aspects of the clad steel welding method, the clad steel is configured to be a clad steel plate or a clad steel pipe.

According to the present invention, it is possible to provide a clad steel welding method that suppresses the dilution of the welding material, which is a corrosion-resistant metal, after base metal welding.

This diagram schematically shows the main parts of the clad steel 10 when a groove preparation process and a butt joint process are performed, relating to a clad steel welding method according to an embodiment of the present invention. This diagram schematically shows the main parts of the clad steel 10 when the backing material placement step is performed, relating to a clad steel welding method according to an embodiment of the present invention. This invention relates to a clad steel welding method according to an embodiment of the present invention, and schematically shows the main parts of the clad steel 10 when the base metal welding process is performed. This diagram schematically shows the main parts of the clad steel 10 when the backing material removal step is performed, relating to a clad steel welding method according to an embodiment of the present invention. This invention relates to a clad steel welding method according to an embodiment of the present invention, and provides a schematic diagram showing the main parts of the clad steel 10 when the cladding welding process is performed. Cross-sectional view of a specific example of a backing material that can be used in the clad steel welding method according to an embodiment of the present invention ((A) has a recess, (B) does not). (Figure (A) schematically shows the shape of a back bead 32 formed with a backing material, and (B) schematically shows the shape of a back bead 33 formed without a backing material.) A schematic diagram showing a first modified example of the joint groove 24, namely the joint groove 24A. A schematic diagram showing a second modified example of the jointed material side groove 24B, which is the jointed material side groove 24. This figure schematically shows another specific example of a backing material (backing material 29) that can be used in the clad steel welding method according to an embodiment of the present invention ((A) is a cross-sectional view of the backing material 29, and (B) is a schematic diagram showing the backing material 29 attached to the groove 24 on the mating material side). A diagram illustrating the technology described in Patent Document 1 (a cross-sectional view showing the groove shape of clad steel 100 used for butt welding). A diagram illustrating the technology described in Patent Document 1 (cross-sectional view of the shape of the back bead 132 formed by the first layer welding on the non-corrosion-resistant steel (base material 102) side).

The embodiments of the present invention will be described in detail below with reference to the drawings.

Figures 1 to 5 are schematic cross-sectional views illustrating typical process scenes of a clad steel welding method according to an embodiment of the present invention. Figure 1 is a schematic diagram illustrating the main parts of the clad steel 10 when the groove processing process and butt joint process are performed in a clad steel welding method according to an embodiment of the present invention. Figure 2 is a schematic diagram illustrating the main parts of the clad steel 10 when the backing material placement process is performed in a clad steel welding method according to an embodiment of the present invention. Figure 3 is a schematic diagram illustrating the main parts of the clad steel 10 when the base metal welding process is performed in a clad steel welding method according to an embodiment of the present invention. Figure 4 is a schematic diagram illustrating the main parts of the clad steel 10 when the backing material removal process is performed in a clad steel welding method according to an embodiment of the present invention. Figure 5 is a schematic diagram illustrating the main parts of the clad steel 10 when the cladding material welding process is performed in a clad steel welding method according to an embodiment of the present invention. Figure 6 is a cross-sectional view of a specific example of a backing material that can be used in the clad steel welding method according to an embodiment of the present invention ((A) shows a recessed backing material, and (B) shows a backing material without a recess). Figure 7 is a diagram for explaining the shape of the back bead ((A) schematically shows the shape of the back bead 32 formed with a backing material, and (B) schematically shows the shape of the back bead 33 formed without a backing material). Figure 8 is a schematic diagram showing a clad material side groove 24A, which is a first modification of the clad material side groove 24. Figure 9 is a schematic diagram showing a clad material side groove 24B, which is a second modification of the clad material side groove 24. Figure 10 is a schematic diagram showing another specific example of a backing material (backing material 29) that can be used in the clad steel welding method according to an embodiment of the present invention ((A) is a cross-sectional view of the backing material 29, and (B) is a schematic diagram showing the backing material 29 attached to the clad material side groove 24).

In the following description of the embodiments, the clad steel 10 to which the clad steel welding method according to this embodiment is applied is described as being plate-shaped. However, the shape of the clad steel to which the present invention can be applied is not limited to plate shape; any shape that allows the clad steel welding method according to the present invention to be implemented is acceptable. The clad steel welding method according to the present invention is widely applicable to clad steel of various shapes, such as plate-shaped or tubular clad steel, and other clad steel shapes. Furthermore, while the following description of the embodiments specifically mentions various materials, these are merely examples, and the present invention is not limited to these specific examples.

The clad steel 10 to which the clad steel welding method according to this embodiment is applied, as shown in Figure 1, comprises a base material 12 and a cladding material 14. The base material 12 is carbon steel or low-alloy steel, and the cladding material 14 is stainless steel or nickel alloy, and the cladding material 14 is a corrosion-resistant metal. As mentioned above, in this application, low-alloy steel refers to steel in which the total amount of a predetermined alloying element is 5% by mass or less.

The clad steel 10 to which the clad steel welding method according to this embodiment applies is, as mentioned above, stainless steel or nickel alloy. Specifically, for example, as stainless steel, austenitic stainless steels such as SUS304, SUS304L, SUS316, SUS316L, SUS316LN, SUS317, SUS317L, SUS312L, SUS836L, JSL310Mo, UNS S31254, UNS S32053, UNS S32050, UNS S08354, UNS N08904, etc. can be used. As nickel alloys, nickel alloys containing chromium and molybdenum such as NCF825, NCF625, NW6022, NW0276, UNS N08825, UNS N06625, UNS N06022, UNS N10276, etc. can be used.

In the clad steel welding method according to this embodiment, the clad steel 10 is welded in the following steps (Steps 1 to 6).

<Step 1 (Beveling and Butt Joining Process)>
A predetermined bevel is made at the end of the clad steel 10 that will be welded, and the ends of the clad steel 10 with the predetermined bevel are butted together to create a groove 20, a base material side groove 22, and a cladding material side groove 24, as shown in Figure 1. Here, the base material side groove 22 is a groove provided on the base material 12 side, and the cladding material side groove 24 is a groove provided on the cladding material 14 side, and the base material side groove 22 and the cladding material side groove 24 together are referred to as the groove 20.

The base material side groove 22 for welding the base material 12 is formed by butting the ends of the base material 12 together so that it is V-shaped as shown in Figure 1, and the root gap G of the base material side groove 22 is formed by butting the ends together so that it is 3 mm or more and 10 mm or less. By providing an appropriate root gap, integration (complete penetration) of the weld metal and the base material 12 becomes easier. In the clad steel welding method according to this embodiment, in order to provide a predetermined root gap, a backing material 26 (see Figure 2) is installed to prevent leakage of the weld metal, and an appropriate overlap is provided between the base material 12 and the backing material 26 to prevent leakage of the weld metal. If the root gap G of the base material side groove 22 is less than 3 mm, the weld metal will have difficulty entering the root gap G, making it difficult to perform complete penetration welding. Furthermore, if the root gap G of the base metal groove 22 is less than 3 mm, welding defects such as poor fusion, blowholes, and slag inclusion are likely to occur, even when using a backing material 26. On the other hand, if the root gap G exceeds 10 mm, the number of welding passes increases, reducing the efficiency of the welding operation. Also, the amount of deposited metal increases, leading to higher material costs.

Furthermore, the groove 24 on the cladding material side, where the cladding material 14 is welded, is formed by cutting back the cladding material 14 at the end of the clad steel 10 where the welding is to be performed, exposing the base metal 12. This groove processing creates a base metal exposed surface 12A where the base metal 12 is exposed over a certain area. In the clad steel welding method according to this embodiment, the base metal exposed surface 12A is provided as a plane that extends in approximately the same direction as the direction in which the surface of the cladding material 14 expands. The cladding material side groove 24 is a rectangular groove (see Figure 1) with the base metal exposed surface 12A as the groove bottom and the cladding material end face 14A of the cladding material 14 as the groove wall, and the cladding material end face (groove wall) 14A is approximately perpendicular to the base metal exposed surface (groove bottom) 12A.

In the groove 20, the base material side groove 22 is set so that the root spacing G is 3 mm or more and 10 mm or less, as described above. However, the length W of the parallel section at the bottom of the groove on the mating material side of the mating material side groove 24 is set to be 15 mm or more.

The reason for setting the length W of the parallel portion at the bottom of the groove on the mating material side of the mating material side groove 24 to 15 mm or more is to ensure that in the next step 2 (backing material placement step), the backing material 26 is in close contact with the exposed surface 12A of the base material by a sufficient distance, thereby preventing the weld metal from leaking between the backing material 26 and the exposed surface 12A of the base material. In order to prevent the weld metal from leaking between the backing material 26 and the exposed surface 12A of the base material, it is necessary to ensure a contact distance d (see Figure 2) between the backing material 26 and the exposed surface 12A of the base material of 6 mm or more on one side, and it is preferable to ensure a distance of 7 mm or more. For example, if the root spacing G of the base material side groove 22 is 3 mm, and a contact distance d of 6 mm is ensured on each side, the length W of the parallel section at the bottom of the groove on the mating material side of the mating material side groove 24 will be 3 mm + 6 mm + 6 mm = 15 mm. If a contact distance d of 7 mm is ensured on each side, the length W of the parallel section at the bottom of the groove on the mating material side of the mating material side groove 24 will be 3 mm + 7 mm + 7 mm = 17 mm. Therefore, it is necessary to ensure a parallel section W of 15 mm or more at the bottom of the groove on the mating material side of the mating material side groove 24, and it is preferable to ensure a length of 17 mm or more.

Furthermore, if the length W of the parallel section at the bottom of the groove on the mating material side of the groove 24 becomes too long, the number of welding passes for the corrosion-resistant metal welding material used for welding the mating material side groove 24 will increase, reducing the efficiency of the welding work. Additionally, the amount of weld metal deposited on the corrosion-resistant metal will increase, leading to higher material costs. Therefore, the groove spacing is preferably 40 mm or less, and more preferably 32 mm or less.

Therefore, the length W of the parallel portion at the bottom of the groove on the mating material side of the mating material side groove 24 is preferably 15 mm or more and 40 mm or less, and more preferably 17 mm or more and 32 mm or less.

<Step 2 (Backing material placement process)>
After forming the groove 20 shown in Figure 1 in Step 1 (grooving and butt joint process), in Step 2 (backing material placement process), as shown in Figure 2, the backing material 26 is placed in close contact with the exposed surface 12A of the base material. When placing the backing material 26 in close contact with the exposed surface 12A of the base material, for example, aluminum tape can be used. The length of the backing material 26 can be set to an appropriate length depending on the welding situation, and when using a short backing material, it is placed in a row. Short backing materials are easy to place along the shape of the object to be welded.

In the clad steel welding method according to this embodiment, the backing material 26 is removed in the later step 4 (backing material removal step), so the backing material 26 must be a backing material that can be removed after welding the base metal groove 22 with the base metal weld metal 30 in step 3 (base metal welding step). For example, a ceramic backing material can be removed after step 3 (base metal welding step), so it can be used as the backing material 26 in the clad steel welding method according to this embodiment. Also, since ceramics have a high melting point and hardly melt even when in contact with molten metal, and have little effect on the weld metal, it is preferable to use a ceramic backing material as the backing material 26. Specifically, a ceramic backing material can be made by molding a mixture of at least one of _SiO2 , _Al2O3 , _MgO , kaolin, and talc into a predetermined shape.

Furthermore, copper backing materials and flux backing materials can also be removed after step 3 (base metal welding process), and therefore can be used as backing materials 26 in the clad steel welding method according to this embodiment.

In the clad steel welding method according to this embodiment, in step 2 (backing material placement step), the backing material 26 is placed in close contact with the exposed surface 12A of the base material. Therefore, it is not necessary to perform chipping work on the back bead after the next step 3 (base material welding step).

When positioning the backing material 26, as described above, it is necessary to ensure a contact distance d between the backing material 26 and the exposed surface 12A of the base material of at least 6 mm on one side, and preferably at least 7 mm. Furthermore, when attaching the backing material 26 to the exposed surface 12A of the base material, this can be done using, for example, aluminum tape. However, care should be taken to ensure the flatness of the exposed surface 12A of the base material to more reliably prevent leakage of weld metal from between the backing material 26 and the exposed surface 12A. Additionally, a pressing material or jig may be used to press the backing material 26 more firmly against the exposed surface 12A of the base material.

Generally, full penetration butt welding uses a backing material with a recess for forming a back bead. In the clad steel welding method according to this embodiment, as shown in Figure 2, a backing material 26 (see Figure 6(A)) with a recess 26A for forming a back bead is used. The width of the recess 26A is equal to or greater than the root spacing G.

In the clad steel welding method according to this embodiment, even when a backing material 26 with a recess 26A is used when welding the base materials 12 together, the height of the back bead is limited to the height of the recess 26A, and by using the backing material 26, the root gap G can be widened to 3 mm or more. As shown in Figure 7(A), the height H _b1 of the back bead 32 is relatively low compared to the width W _b1 , and dilution of the welding material, which is a corrosion-resistant metal, is less likely to occur when welding the cladding materials 14 together in step 5 (cladding material welding process). On the other hand, when welding the base materials 12 together without using a backing material, as in the technique described in Patent Document 1, it is necessary to narrow the root gap G to prevent leakage of weld metal, and since the height of the back bead is not limited by the backing material, as shown in Figure 7(B), the height H _b2 of the back bead 33 that occurs in the joint groove 25 becomes relatively high with respect to the width W _b2 , and in the welding of the joint materials 14 together (welding of the joint groove 24) in step 5 (joint welding process), dilution of the welding material, which is a corrosion-resistant metal, is likely to occur locally in the vicinity of the back bead 33.

Furthermore, in the clad steel welding method according to this embodiment, a backing material 28 (see Figure 6(B)) can be used in which the surface that comes into contact with the welding area is formed only of a flat surface 28A and has no recesses. Generally, in full penetration butt welding, the formation of a back bead is confirmed to ensure that full penetration has been achieved. Therefore, in full penetration butt welding from one side, it is necessary to form a back bead on the surface opposite to the welding side. However, in the clad steel welding method according to this embodiment, as will be described later, after welding the base materials 12 together in step 3 (base material welding process), welding the cladding materials 14 together on the cladding material 14 side opposite to the base material 12 is performed in step 5 (cladding material welding process). Therefore, in the clad steel welding method according to this embodiment, it is not necessarily required to confirm the formation of a back bead in step 3 (base material welding process), and a backing material 28 without recesses can be used.

In the clad steel welding method according to this embodiment, when a backing material 28 without a recess is used, a back bead is not formed in the groove 24 on the cladding material side. Therefore, in the welding of the cladding materials 14 in step 5 (cladding material welding process), dilution of the welding material, which is a corrosion-resistant metal, is less likely to occur.

<Step 3 (Base Metal Welding Process)>
In step 2 (backing material placement step), as shown in Figure 2, the backing material 26 is placed in close contact with the exposed surface 12A of the base material. Then, in step 3 (base material welding step), a butt groove welding of the base material 12 is performed from the base material 12 side using a welding material suitable for the base material 12, and as shown in Figure 3, the base material side groove 22 is filled with base material weld metal 30 and the base material side groove 22 is welded.

Specifically, the welding method for the base metal 12 (carbon steel or low-alloy steel) in the groove 22 on the base metal side can be, for example, gas shielded arc welding (gas metal arc welding), submerged arc welding, shielded metal arc welding, self-shielded arc welding, or TIG welding. Among these, gas shielded arc welding and submerged arc welding are often used due to cost and ease of implementation.

The welding material used for welding the base metals 12 together in the groove 22 on the base metal side should, in principle, be a welding material with a composition similar to that of the base metal. However, the composition of the welding material should be selected considering component loss such as evaporation and transfer to slag during welding. Furthermore, it is preferable to use a welding material with strength equal to or greater than that of the base metal, i.e., equivalent to or overmatching.

<Step 4 (Removal of backing material)>
In step 3 (base metal welding process), as shown in Figure 3, the base metal groove 22 is welded with the base metal weld metal 30. Then, in step 4 (backing material removal process), the backing material 26 is removed as shown in Figure 4. As shown in Figure 4, a back bead 32 with a shape similar to the recess 26A of the backing material 26 is formed in the cladding material groove 24. However, in the clad steel welding method according to this embodiment, the backing material 26 is placed in step 2 (backing material placement process) before step 3 (base metal welding process). As mentioned above, the height H _b1 of the back bead 32 is relatively low compared to the width W _b1 (see Figure 7(A)). Therefore, in the groove welding of the cladding material 14 at the butt joint in step 5 (cladding material welding process), dilution of the welding material, which is a corrosion-resistant metal, is unlikely to occur.

Furthermore, in the clad steel welding method according to this embodiment, since the backing material 26 is placed in close contact with the exposed surface 12A of the base metal before the base metal welding process in step 3 is performed, welding defects in the first layer are less likely to occur, and there is no need to perform chipping work on the back bead after step 4 (backing material removal process).

<Step 5 (Welding Process for Joints)>
In step 4 (backing material removal step), as shown in Figure 4, the backing material 26 is removed to expose the joint groove 24 on the joint material side, making it possible to weld from the joint material 14 side. Then, in step 5 (joint welding step), a butt groove welding of the joint material 14 is performed from the joint material 14 side using a corrosion-resistant welding material used for welding the joint groove 24, and as shown in Figure 5, the joint groove 24 on the joint material side is filled with the joint material weld metal 34, which is a corrosion-resistant weld metal, and the joint groove 24 is welded.

Before step 5 (cladding welding process), a back bead 32 is formed in the groove 24 on the cladding side. However, in the clad steel welding method according to this embodiment, a backing material 26 is placed in step 2 (backing material placement process) before step 3 (base metal welding process). As mentioned above, the height H _b1 of the back bead 32 is relatively low compared to its width W _b1 (see Figure 7(A)), and dilution of the welding material, which is a corrosion-resistant metal, is unlikely to occur in the groove welding of the cladding 14 at the butt joint in step 5 (cladding welding process). If the backing material 26 is not placed before step 3 (base metal welding process), the height H _b2 of the back bead 33 becomes relatively large compared to its width W _b2 (see Figure 7(B)), and that area may be immediately melted in step 5 (cladding welding process), potentially causing localized dilution of the welding material.

Furthermore, in the clad steel welding method according to this embodiment, in step 2 (backing material placement step), the backing material 26 is placed in close contact with the exposed surface 12A of the base metal. Therefore, welding defects in the first layer of the base metal groove 22 are less likely to occur, eliminating the need to chip away at the back bead after step 3 (base metal welding step), and thus eliminating the need for gouging and the creation of deep grooves. For this reason, welding with corrosion-resistant welding material in step 5 (cladding material welding step) generally only requires one layer.

As mentioned above, the composite material 14 is stainless steel or a nickel alloy, and is a corrosion-resistant metal.

When using austenitic stainless steel with a pitting index of less than 31, such as SUS304, SUS304L, SUS316, SUS316L, SUS316LN, SUS317, or SUS317L, as the cladding material 14 for the clad steel 10 to which this embodiment applies, the metal component of the flux-cored wire (FCW) used as the welding material can be stainless steel of the 309 series, 309L series, 309J series, 309Mo series, 309LMo series, or 309LNb series. Specifically, for example, TS309-FB0, TS309-FB1, TS309-FC0, TS309-FC1, TS309-FM0, TS309-FM1, TS309-MI0, TS309-MI1, TS309L-FB0, TS309L-FB1, or TS309L-FC0. Flux-cored wires such as TS309L-FC1, TS309L-FM0, TS309L-FM1, TS309L-MI0, TS309L-MI1, TS309J-FB0, TS309J-FB1, TS309Mo-FB0, TS309Mo-FB1, TS309Mo-FC0, TS309Mo-FC1, TS309Mo-FM0, TS309Mo-FM1, TS309Mo-MI0, TS309Mo-MI1, TS309LMo-FB0, TS309LMo-FB1, TS309LMo-FC0, TS309LMo-FC1, TS309LMo-FM0, TS309LMo-FM1, TS309LMo-MI0, TS309LMo-MI1, and TS309LNb-FB0 (metal component is 309 series stainless steel) can be used. When 309 series stainless steel is used as the welding material, the weld metal near the boundary between the carbon steel and the weld metal is less likely to form a martensitic structure, thus suppressing crack formation. In this application, the pitting index is an index calculated using the Cr, Mo, and N content (mass%) contained in the stainless steel, and is calculated using the formula: Pitting Index = Cr + 3.3Mo + 16N (where each element symbol represents the content of that element) (expressed as a mass percentage).

If austenitic stainless steel with a pitting index of 31 or higher is used as the laminate 14, such as SUS312L, SUS836L, JSL310Mo, UNS S31254, UNS S32053, UNS S32050, UNS S08354, or UNS N08904, or NCF825, NCF625, NW6022, NW0276, UNS N08825, UNS N06625, UNS N06022, UNS When using a nickel alloy containing chromium and molybdenum, such as N10276, the metal components of the flux-cored wire (FCW) used as the welding material can be Ni6625 series, Ni6276 series, Ni6022 series, Ni6059 series, Ni6275 series, Ni6455 series, Ni6456 series, Ni6686 series, and Ni1013 series nickel alloys. Specifically, for example, TNi6625-PB0, TNi6625-PB1, TNi6625-BM0, TNi6625-BM1, TNi6625-PM0, TNi6625-PM1, TNi6276-PB0, TNi6276-PB1, TNi6276-BM0, TNi6276-BM1, TNi6276-PM0, TNi6276-PM1, TNi 6022-PB0, TNi6022-PB1, TNi6022-BM0, TNi6022-BM1, TNi6022-PM0, TNi6022-PM1, TNi6059-P B0, TNi6059-PB1, TNi6059-BM0, TNi6059-PM1, TNi6059-PB1, TNi6275-PB1, TNi6275-BM0, TNi Flux-cored wires such as 6275-PM1, TNi6455-PB1, TNi6455-BM0, TNi6455-PM1, TNi6456-PB1, TNi6456-BM0, TNi6456-PM1, TNi6686-PB1, TNi6686-BM0, TNi6686-PM1, TNi1013-PB1, TNi1013-BM0, TNi1013-PM1, and AWS A5.34 ENiMo13T1-1/4 (metal components are nickel alloys) can be used. When using nickel alloy welding materials containing Cr and Mo, the corrosion resistance of the welded area of the corrosion-resistant metal is less likely to decrease even if dilution occurs.

In this Step 5 (cladding welding process), the welding method can be TIG welding, stick welding, or MAG welding using flux-cored wire (FCW) (arc welding using a mixture of inert gas and carbon dioxide as shielding gas). MAG welding using flux-cored wire (FCW) has a higher welding efficiency compared to other welding methods (TIG welding, stick welding, etc.), and even in overhead or vertical welding, the welding efficiency does not deteriorate as much as other welding methods (TIG welding, stick welding, etc.). Therefore, from the viewpoint of workability and constructability, it is preferable to use MAG welding using flux-cored wire (FCW) as the welding method in this Step 5 (cladding welding process). In this Step 5 (cladding welding process), by using MAG welding using flux-cored wire (FCW), even when welding the cladding material 14 side by performing overhead or vertical welding without reversing the cladding steel 10 or changing the orientation of the cladding steel 10, it is possible to suppress a decrease in welding efficiency. Therefore, by using MAG welding with flux-cored wire (FCW) in step 5 (cladding welding process), the clad steel welding method according to this embodiment can be suitably used for welding clad steel to fixed structures such as bridges.

In step 5 (joint welding process), when performing MAG welding using flux-cored wire (FCW), a mixed gas of _CO2 and argon is used as the shielding gas. Increasing the proportion of argon improves the finish of the weld bead and reduces spatter, but since argon is more expensive than _CO2 , it is preferable to set the _CO2 content of the shielding gas to 0 mol% or more and 95 mol% or less.

Furthermore, in step 5 (joint welding process), it is preferable that the heat input during arc welding using flux-cored wire (FCW) in MAG welding be between 5 kJ/cm and 40 kJ/cm. If the welding heat input is less than 5 kJ/cm, it may not reach sufficient heat to adequately melt the toe and groove ends of the back bead formed on the joint side during the joint welding process, potentially leading to poor fusion. If the welding heat input exceeds 40 kJ/cm, the base metal 12 may melt excessively during the joint welding process, leading to increased dilution of the corrosion-resistant metal welding material and potentially reducing the corrosion resistance of the weld.

In step 5 (joint welding process), it is not strictly necessary to cover the surface of the joint material 14 with a sheet for protection. However, it is preferable to apply a spatter inhibitor to the surface of the joint material 14 using a spray or brush to prevent spatter from adhering to it during welding.

<Step 6 (Polishing Process)>
In step 5 (joint welding process), a groove weld is performed on the joint of the joint material 14 from the joint material 14 side using corrosion-resistant weld metal, and as shown in Figure 5, the joint material side groove 24 is filled with joint material weld metal 34, and the end of the joint material 14 is welded. Then, in step 6 (polishing process), the surface of the welded part of the joint material 14 is polished to finish it.

Although the corrosion resistance of the welded areas is reduced, polishing the surface of the welded joint of the composite material 14 removes the corrosion-reduced areas, preventing them from becoming the starting point of corrosion. Furthermore, polishing the surface of the welded joint of the composite material 14 reduces surface irregularities, making it less susceptible to the adhesion of iron powder and rust, thus preventing galvanic corrosion. Polishing the surface of the welded joint of the composite material 14 also improves its aesthetic appearance.

The surface of the welded joint of the composite material 14 can be polished using, for example, a grinder or belt sander. In the first half of the polishing process, to improve efficiency, a coarser abrasive than the F80 grit size specified in JIS R6001 (1998) may be used. However, in the final finishing stage, polishing should be performed using an abrasive with a grit size equivalent to or finer than the F80 grit size specified in JIS R6001 (1998). Polishing with a fine-grained abrasive in the final finishing stage reduces surface irregularities, resulting in a better appearance and reduced adhesion of iron filings. Larger surface irregularities make it easier for iron filings to adhere, leading to increased susceptibility to rust formation.

The abrasive material is preferably at least one of the following, which does not contain iron: aluminum oxide, silicon carbide, boron nitride, diamond, corundum, or garnet. If the abrasive contains iron, rust from the abrasive itself may adhere to the polished surface.

Furthermore, the surface of the welded joint of the connecting material 14 may be polished by electrolytic polishing. Portable electrolytic polishing equipment is also available, making it possible to perform electrolytic polishing on the welded joint of the connecting material 14 even on fixed structures such as bridges.

Alternatively, instead of electrolytic polishing, a solution containing at least one of sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, phosphoric acid, and hydrogen peroxide may be applied to the surface of the welded portion of the composite material 14 to remove welding discoloration and perform passivation treatment. It is preferable to perform at least one of the following treatments on the surface of the welded portion of the composite material 14: mechanical polishing using an abrasive, electrolytic polishing, and passivation treatment.

Furthermore, electrolytic polishing and passivation treatments can improve corrosion resistance because they preferentially dissolve iron in the passivation film on the surface of the stainless steel or nickel alloy used as the bonding material, thereby concentrating chromium.

Therefore, it is preferable to perform both mechanical polishing using abrasives and electrolytic polishing or passivation treatment. However, due to the difference in the range of polishing that can be performed (polishing is on the order of millimeters, while electrolytic polishing and passivation treatment are on the order of tens of nanometers), it is preferable to perform mechanical polishing using abrasives first, followed by electrolytic polishing or passivation treatment.

Furthermore, the surface of the welded joint on the base material 12 will be blast-treated and then painted. This treatment is the same as the usual surface treatment for steel structures.

<Supplement>
As mentioned above, in step 1 (grooving and butt jointing), a cutback process is performed on the cladding material 14 to form the base metal exposed surface 12A. Figure 1 shows the state after the cutback process is performed to remove the cladding material 14 by the same thickness as the cladding material 14. In this cutback process, as shown in Figure 8, the cladding material side groove 24A (first modified example of the cladding material side groove 24), the cutback process may be performed to a thickness greater than the cladding material 14, so that not only the cladding material 14 but also a small amount of the base metal 12 is cut off to form the base metal exposed surface 12B. If the cladding material 14 remains in the cladding material side groove 24, the cladding material 14 may melt into the welding material during welding on the base metal 12 side, potentially adversely affecting the base metal weld metal 30, so this is done to ensure that the cladding material 14 is reliably removed from the cladding material side groove 24.

Furthermore, in the cutback process for the cladding material 14, as shown in Figure 1, the cladding material side groove 24 is processed to be rectangular in shape, with the base material exposed surface 12A as the groove bottom and the cladding material end surface 14A of the cladding material 14 as the groove wall, and the cladding material end surface (groove wall) 14A is approximately perpendicular to the base material exposed surface (groove bottom) 12A. However, nickel alloys have a higher viscosity when molten than carbon steel and tend to become spherical due to surface tension, so nickel alloy welding material does not easily penetrate into fine gaps, and welding defects due to poor fusion may occur at the approximately 90° inside corner of the rectangular cladding material side groove 24 as shown in Figure 1. For this reason, when using nickel alloy welding material, it is preferable to have an isosceles trapezoidal cladding material side groove 24B as shown in Figure 9 rather than a rectangular cladding material side groove 24 as shown in Figure 1. Specifically, it is preferable to perform a cutback process on the mating material side groove 24B (a second modified example of the mating material side groove 24) so that the angle θ between the exposed base material surface (groove bottom) 12A and the mating material end surface (groove wall) 14B is 130° or more.

Furthermore, while Figure 6 shows specific cross-sectional shapes of backing materials (backing materials 26, 28) usable in the clad steel welding method according to this embodiment, the cross-sectional shapes of the backing materials usable in the clad steel welding method according to this embodiment are not limited to these. Any backing material that can ensure a predetermined contact length (6 mm or more is required, preferably 7 mm or more) on the exposed base material surface (groove bottom) 12A is acceptable, and backing materials with square or isosceles triangular cross-sections can also be used. Figure 10(A) shows the cross-sectional shape of backing material 29 with an isosceles triangular cross-section, and Figure 10(B) shows the backing material 29 with an isosceles triangular cross-section attached to the groove 24 on the clad material side.

10, 100... Clad steel 12, 102... Base metal 12A, 12B... Base metal exposed surface (groove bottom)
14, 104... Joint material; 14A, 14B... End face of joint material (groove wall)
20, 120... Groove 22, 122... Base material side groove 24, 24A, 24B, 25, 124... Joint material side groove 26, 28, 29... Backing material 26A... Recess 28A... Flat surface 30... Base material weld metal 32, 33, 132... Back bead 34... Joint material weld metal G... Root spacing W... Length of the parallel section at the bottom of the groove on the joint material side d... Contact distance between backing material 26 and the exposed surface 12A of the base material H _b1 , H _b2 , H _bx ... Back bead height W _b1 , W _b2 , W _bx ... Back bead width θ... Angle between the exposed surface 12A of the base material and the groove wall 14B

Claims (11)

A welding method for clad steel, in which stainless steel or nickel alloy is used as the cladding material and carbon steel or low-alloy steel is used as the base material,
A beveling process is performed in which a cutback is made at the part of the clad steel to be welded to remove the cladding material and to form a beveled surface of the base material,
A butt joint step in which the beveled clad steels are butted together such that the root gap between the base materials is 3 mm or more,
After the butt joint step, a backing material placement step is performed in which the backing material is placed on the exposed surface of the base material,
After the backing material placement step, a base material welding step is performed in which a butt joint groove welding is carried out from the base material side to the base material side,
After the base material welding process, a backing material removal process is performed to remove the backing material,
After the backing material removal step, a joint welding step is performed in which a groove welding of the joint is performed from the joint material side,
A clad steel welding method characterized by having the following features. The clad steel welding method according to claim 1, characterized in that the backing material is made of ceramic. The clad steel welding method according to claim 1, characterized in that, in the butt joint step, the beveled clad steels are butted together such that the parallel length of the groove bottom on the clading material side, which includes the exposed surface, is 15 mm or more. The clad steel welding method according to claim 1, characterized in that, in the aforementioned cladding welding step, arc welding is performed from the cladding side using a flux-cored wire having a corrosion-resistant metal welding material. The clad steel welding method according to any one of claims 1 to 4, characterized in that the backing material is a backing material without a recess for forming a back bead. The clad steel welding method according to any one of claims 1 to 4, characterized in that, in the groove preparation step, a cutback is performed to remove the cladding material, thereby forming a groove wall with an angle of 130° or more with respect to the exposed surface. A clad steel welding method according to any one of claims 1 to 4, characterized in that, after performing the base material welding step, the clad steel is welded without inverting it. The clad steel welding method according to claim 4, characterized in that, in the aforementioned cladding welding step, a mixed gas of _CO2 and argon is used as a shielding gas, the _CO2 content of the mixed gas is set to 0 mol% or more and 95 mol% or less, and the heat input when performing the arc welding is set to 5 kJ/cm or more and 40 kJ/cm or less. A clad steel welding method according to any one of claims 1 to 4, characterized in that, after the aforementioned cladding welding step, at least one of mechanical polishing, electrolytic polishing, or passivation treatment is performed using an abrasive material that does not contain iron and has a particle size equivalent to or finer than F80 as defined in JIS R6001 (1998), to treat the surface of the area welded in the aforementioned cladding welding step. The clad steel welding method according to claim 9, characterized in that, after performing the mechanical polishing on the surface of the welded portion in the aforementioned cladding welding process, either electrolytic polishing or passivation treatment is further performed on the said surface. The clad steel welding method according to any one of claims 1 to 4, characterized in that the clad steel is a clad steel plate or a clad steel pipe.