JP2013158774A - Welding method, weld bonding structure and stainless steel welded structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the development of cracks on a crack developing route even when stress corrosion cracking occurs at a portion adjacent to a weld heat affected zone and cracks develop to the inside of a material.SOLUTION: In a welding method, a butting section of a stainless steel joint part is subjected to edge preparation, and then a welding metal is built up at the butting section of the edge prepared portion. After a section as the weld bonding section is subjected to edge preparation, the edge prepared butting section is welded by using a welding metal having a solidification mode of two-phase solidification microstructure (AF mode) comprising pro-eutectic austenite and δ ferrite in solidification phase, thereby forming a build-up layer having a solidification microstructure wherein the continuity of an interface between δ ferrite phase and γ austenite phase is divided.

Description

  The present invention relates to a welding method, a welded joint structure, and a stainless steel welded structure. For example, a welding method suitable for application to austenitic stainless steel pipes used in contact with high-temperature pure water of a nuclear power plant. And a welded joint structure and a stainless steel welded structure.

  In nuclear power plants, there have been reports of crack damage cases due to stress corrosion cracking in the vicinity of welded parts such as austenitic stainless steel pipes and furnace structures used in contact with high-temperature pure water.

  Stress corrosion cracking is one of aging degradation events that occur when material conditions, environmental conditions, and stress conditions are superimposed under certain conditions. Various countermeasure technologies have been developed so far, It has been applied to piping and furnace structures.

  For example, when stainless steel with a high carbon content is welded, chromium carbide is formed on the grain boundaries in the heat affected zone, so that a chromium deficient layer is formed in the vicinity, and corrosion resistance is reduced and stress corrosion cracking occurs. As a material improvement measure, low-carbon stainless steel has been developed as a measure to improve material quality by reducing the carbon content in stainless steel and suppressing thermal sensitization during welding. And applied to furnace structures.

  In general, when welding stainless steel pipes and structures, as shown in FIG. 1, the grooves machined on the end faces of the plant components to be welded are butted together, and this butt part is joined. Weld multiple layers of weld metal.

  In recent years, in a recirculation piping using low-carbon stainless steel, when a work hardened layer 4 is present on the surface of the weld heat affected zone 3, stress corrosion cracking has occurred, and this has advanced the weld heat affected zone 3. A case report of reaching weld metal 2 has been reported.

  In general, it is known that stress corrosion cracks generated from the work hardened layer 4 in the weld heat affected zone 3 progress to the weld metal 2 while being gradually inclined from the surface in the plate thickness direction due to the relationship with the welding residual stress. It has been. The Japan Society of Mechanical Engineers, in accordance with the standards for maintaining nuclear power facilities for power generation, if soundness is confirmed, continuous operation is permitted with cracks. From the viewpoint of improving the reliability of welded parts such as pipes and structures, Suppressing the development of stress corrosion cracking in heat affected zone and weld metal is considered as one of the most important measures.

  As a measure for suppressing the progress of stress corrosion cracking in the weld heat affected zone and the weld metal, Patent Document 1 proposes a welding joint method for piping with improved stress corrosion cracking resistance. In the welding joint method for piping described in Patent Document 1, the inner surface in the vicinity of the joint end of the pipe is overlaid using a material that is more excellent in stress corrosion cracking resistance than the base material of the pipe. A groove is formed at the joining end portion of the piled pipe leaving at least a part of the built-up portion, and the built-up portions are butted together and welded.

  On the other hand, Patent Document 2 describes a method for overlay welding of piping. In this patent document 2, before welding the pipe, a build-up weld layer in which a dendrite structure of the weld metal is grown in a direction intersecting the stress corrosion crack propagation direction is formed in the grooved portion of the pipe. The welded layers are welded together. As described above, it is possible to grow a dendritic structure of the weld metal in the direction intersecting with the stress corrosion cracking propagation direction by building up from the outer surface side to the inner surface side of the pipe in the groove processed portion of the pipe. it can.

  In this welded joint structure of pipes, stress corrosion cracks that occur near the weld heat affected zone on the inner surface side of the pipes are propagated into the weld metal by a dendrite structure formed in a direction that intersects the direction of stress corrosion crack propagation. Is suppressed.

  Further, Patent Document 3 discloses a weld that suppresses a crack generated by stress corrosion cracking generated on the surface of a plant component near the welding heat-affected zone on the inner surface side of the pipe from propagating into the weld metal. The joint structure is described.

  In this method, the grooved portion has a built-up portion by a plurality of layers of welding passes, and in this built-up portion, the first welding that is a lower layer and the first welding pass adjacent to the first welding pass are provided. A refined δ ferrite phase 5B is formed in the lower welding path along the boundary of the second welding path which is the upper layer applied on the upper layer.

  The δ ferrite phase 5B is a state in which the continuity in the direction of the δ ferrite phase 5A (see FIG. 2) extending in a certain direction is divided, and the length is shorter than that of the δ ferrite phase 5A. Further, the continuity of the interface between the δ ferrite phase 5B and the γ austenite phase 6 is shorter than the continuity of the interface between the δ ferrite phase 5A and the γ austenite phase 6.

  And in patent document 3, the area | region of the (delta) ferrite phase 5B and the (gamma) austenite phase 6 in which the continuity which exists along the boundary of an adjacent welding path was divided | segmented is formed with the width | variety of 200 micrometers-1000 micrometers, Therefore, the interface between the δ ferrite phase 5B and the γ austenite phase 6, which is the path of stress corrosion cracking, can be divided. The generated crack is restrained from progressing toward the inside of the weld metal 2.

JP 2005-28405 A JP 2009-39734 A Japanese Patent Application No. 2011-206809

  Even when stress corrosion cracking occurs near the weld heat affected zone and the crack propagates into the material, if the crack growth can be suppressed along the crack propagation path, the soundness of the portion can be secured.

  The object of the present invention is to provide a welding method capable of suppressing crack propagation on the crack propagation path even when stress corrosion cracking occurs near the weld heat-affected zone and the crack propagates into the material. And providing a welded joint structure and a stainless steel welded structure.

  In order to achieve the above object, the welding construction method of the present invention performs welding on a butt portion to be a stainless steel joint, and then welds a weld metal on the butt portion of the groove processing portion. In the construction method, after performing groove processing on the portion to be the welded joint, the solidification mode of the two-phase solidified structure (AF mode) of the primary crystal austenite + δ ferrite is applied to the butt portion of the groove processing. It welds using the welding metal which has, and forms the built-up layer with the solidification structure | tissue where the continuity of the interface of (delta) ferrite phase and (gamma) austenite phase was parted.

  Further, in order to achieve the above object, the welded joint structure of the present invention is provided with a groove processing on a butt portion which becomes a stainless steel joint portion, and a weld metal is built up on the butt portion of the groove processed portion. In the welded joint structure, the solidified mode of the two-phase solidified structure (AF mode) of primary austenite + δ ferrite is formed in the butt portion of the grooved work in which the grooved part is grooved A weld layer having a solidified structure in which the continuity of the interface between the δ ferrite phase and the γ austenite phase is divided is formed by welding a weld metal having A weld metal having a solidification mode of a two-phase solidified structure (FA mode) of primary crystal δ ferrite + austenite is welded and joined to the groove processed portion.

  Moreover, in order to achieve the above object, the stainless steel welded structure according to the present invention has a welded joint structure having the above-described configuration.

  According to the present invention, even when a stress corrosion crack occurs from the vicinity of the weld heat affected zone and the crack propagates into the material, it is possible to suppress the crack propagation on the crack propagation path.

It is a longitudinal cross-sectional view which shows the conventional welding joining structure of the recirculation system piping of the nuclear power plant which is a plant structural member. It is explanatory drawing which shows the weld solidification structure of the weld part shown in FIG. It is explanatory drawing which shows the build-up solidification structure | tissue of the build-up layer shown in FIG. It is Example 1 of the welding construction method of this invention, and is process drawing for demonstrating the welding method of a butt | matching part. It is Example 1 of the welding construction method of this invention, and is process drawing for demonstrating the welding method of a groove processing part.

  When stress corrosion cracking progresses in the weld metal part, the path of growth is affected not only by the continuity and directionality of the macroscopic weld metal dendrite structure, but also by the microscopic interface between δ ferrite and γ austenite. Has been confirmed.

  In austenitic stainless steel welding materials, the solidification mode of the weld metal (the type of primary crystal and phase change during the solidification process) is determined by the chemical composition of the weld metal.

  The solidification mode of stainless steel weld metal is austenite single phase solidification structure (A mode), primary austenite + δ ferrite two-phase solidification structure (AF mode), primary δ ferrite + austenite two-phase solidification structure (FA mode), It is classified into four types of ferrite single-phase solidified structure (F mode).

Such a change in the solidification mode can be arranged as follows by the ratio of Cr equivalent / Ni equivalent.
A, AF mode: Cr equivalent / Ni equivalent ≦ 1.48
FA mode: 1.48 ≦ Cr equivalent / Ni equivalent ≦ 1.95
A, AF mode: 1.95 ≦ Cr equivalent / Ni equivalent Here, the Cr equivalent and the Ni equivalent are determined as follows.
Cr equivalent = (% Cr) + (% Mo) +1.5 (% Si) +0.5 (% Nb)
Ni equivalent = (% Ni) +30 (% C) +0.5 (% Mn)
The weld solidification structures of the FA mode and AF mode weld metals used in the present invention are the same as those shown in FIGS. In the FA mode, the interface between the δ ferrite phase 5A and the γ austenite phase 6 has a continuous weld solidification structure, and in the AF mode, the continuity at the interface between the δ ferrite phase 5B and the γ austenite phase 6 is divided. It is known that it is easy to become an organization.

  In general, it is considered that austenitic stainless steel (SUS304, SUS316, etc.) is desirably welded using an FA mode weld metal in consideration of welding construction.

  In the present invention, after the groove processing is performed on the portion to be the welded joint portion, the continuity is divided by overlaying with a weld metal having an AF mode solidification mode on the butt portion of the groove processing. A built-up layer having a solidified structure of phase 5B and γ-austenite phase 6 is formed at the butt portion of the groove processing. And the progress of the crack of the stress corrosion crack which has progressed to the weld metal can be suppressed by welding the butt-welded portion that has been welded with a weld metal having a normal FA mode solidification mode.

  An embodiment of the present invention will be described below in consideration of the above examination results.

  As a first embodiment of the present invention, a method for welding stainless steel pipes will be described with reference to FIGS.

  As shown to Fig.4 (a), a groove processing is given to the butt | matching part of the piping 1 used as welding object, and the groove processing part 7 is formed. In addition, let the liquid-contact surface side (lower side of FIG. 4) of the piping 1 be a groove shape for performing more overlaying. Using the weld metal (for example, welding wire) which has solidification mode of AF mode to the formed groove processing part 7, a plurality of welding is given on the groove processing part 7, and the groove overlaying layer 8 is formed. . This 8 parts of the groove overlay layer is formed having a solidified structure in which the continuity of the δ ferrite phase 5B and the γ austenite phase 6 is divided.

  After forming the groove overlay layer 8 at the butt portion, as shown in FIG. 4B, the groove cladding layer 8 is subjected to pipe inner surface machining (DC machining) in order to align the groove machining and the inner diameter of the pipe. The groove processing part 9 and the DC processing part 10 are formed.

  After that, as shown in FIGS. 5A, 5B, and 5C, the groove processing portion 9 is abutted, and the groove processing portion 9 is directed to the groove processing portion 9 from the inner surface side to the outer surface side in the normal FA mode. The build-up welding 11 is performed using a weld metal having a solidification mode. The welding at this time has a heat input of less than 20 kj / cm and is performed by TIG welding. In addition to TIG welding, covered arc welding, submerged arc welding, or the like may be applied.

  According to such a present Example, the metal structure by which the continuity of the interface of (delta) ferrite 5B and (gamma) austenite 6 used as the propagation path of a stress corrosion crack was interrupted can be formed in the groove processing part 7, and a welding part can be formed. Even if a stress corrosion crack occurs in the vicinity of the weld heat-affected zone and the crack progresses into the material, the crack development of the stress corrosion crack that has progressed to the weld metal 2 Can be suppressed.

  Moreover, although the work hardening layer 4 is formed on the surface by the DC processing 10, since the portion becomes the groove overlay layer 8 in which the continuity of the interface between the δ ferrite 5B and the γ austenite 6 is divided, the stress corrosion cracking occurs. Can be suppressed.

  In addition, although the Example mentioned above demonstrated the stainless steel welded structure of the recirculation system piping of a nuclear power plant, it cannot be overemphasized that it is applicable to the stainless steel welded structure of welded joints, such as a shroud.

  DESCRIPTION OF SYMBOLS 1 ... Piping, 2 ... Weld metal, 3 ... Weld heat affected zone, 4 ... Surface hardened layer, 5A, 5B ... delta ferrite phase, 6 ... gamma austenite phase, 7 ... groove processing part, 8, 9 ... groove meat Build-up (AF mode), 10 ... DC machining part, 11 ... Overlay welding (FA mode).

Claims (4)

In the welding method of applying groove processing to the butt portion to be a stainless steel joint, and then overlaying a weld metal on the butt portion of the groove processing portion,
After performing groove processing on the portion to be the welded joint, a weld metal having a solidification mode of a two-phase solidification structure (AF mode) of primary austenite + δ ferrite is applied to the butt portion of the groove processing. And welding to form a built-up layer having a solidified structure in which the continuity of the interface between the δ ferrite phase and the γ austenite phase is divided. In the welding construction method according to claim 1,
After forming a built-up layer having a solidified structure in which the continuity of the interface between the δ ferrite phase and the γ austenite phase is divided at the butt portion of the groove processing, the groove layer is subjected to groove processing, In addition, the groove processed portion is abutted, and overlay welding is performed on the groove processed portion using a weld metal having a solidification mode of a primary crystal δ ferrite + austenite two-phase solidified structure (FA mode). Welding construction method. In the welded joint structure in which the groove processing is performed on the butt portion that becomes the stainless steel joint portion, and the weld metal is built up on the butt portion of the groove processing portion,
A weld metal having a solidification mode of a two-phase solidification structure (AF mode) of primary austenite + δ ferrite is welded to a butt portion of the groove machining in which a groove machining is performed on a portion to be the weld joint. Then, a build-up layer having a solidified structure in which the continuity of the interface between the δ ferrite phase and the γ-austenite phase is divided is formed, A welded joint structure in which weld metal having a solidification mode of a primary phase δ ferrite + austenite (FA mode) is welded and welded.   A stainless steel welded structure having the welded joint structure according to claim 3.

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