JP2014065061A - Surface modification method and structure of nickel base alloy weld metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance health and reliability of a structure like an atomic reactor by suppressing generation and development of a stress corrosion crack of a nickel base alloy weld metal in a weld zone of the structure.SOLUTION: A nickel base alloy weld metal 4 with a columnar crystal structure in a weld zone 1 of a structure is melted, and then the melted weld metal is agitated to modify a columnar crystal structure 4a into a granular structure 5.

Description

  The present invention relates to a surface modification method and structure for a nickel-base alloy weld metal used in a structure such as a nuclear reactor.

  In a general boiling water reactor (hereinafter referred to as BWR), a reactor containment vessel is installed in the reactor building, and a reactor pressure vessel is installed on the pedestal in the reactor containment vessel. . A reactor core and cooling water are accommodated in the reactor pressure vessel, and when this cooling water flows from the lower side to the upper side of the reactor core, the temperature rises by taking away the nuclear reaction heat of the core. The heated cooling water is in a two-phase flow state of steam and water, and flows into a steam separator provided above the core. The steam is separated into water and steam by the steam separator, and the separated steam is dried by the steam dryer provided above the steam separator to become dry steam, which is connected to the reactor pressure vessel. Is provided to the turbine system via the turbine to drive the turbine system. On the other hand, the separated water flows down below the core and flows again from below to above the core, and the same cycle is repeated thereafter.

  The reactor pressure vessel is composed of a vessel main body, a lid provided so as to close the upper opening of the vessel main body, and piping for taking cooling water and leading out main steam. The pressure vessel body is composed of a cylindrical body and a lower mirror connected to the lower end of the body.

  Welding of internal structures in the reactor pressure vessel, connection pipe safe end, structures such as in-core instrumentation tubes and control rod drive mechanism guide tube penetrations, and overlay overlays on the inner surface of the reactor pressure vessel A nickel-based alloy is mainly used as the weld metal in the part (Patent Document 1).

  An example of a welded joint using such a nickel-based alloy as a weld metal is shown in FIG. The weld joint 1 is composed of base metals 2 and 3 and a weld metal 4 made of a nickel-base alloy that is butt welded to the base materials 2 and 3.

JP 2000-312969 A

  By the way, although the weld metal 4 made of a nickel-based alloy is a highly corrosion-resistant material, it is known to exhibit stress corrosion cracking (SCC) sensitivity in high-temperature pure water. In 600 alloy, the base metal has a granular structure, whereas a weld metal such as 182 alloy has a directional metal structure in which columnar crystals 4a are formed as shown in FIG. Direction from the surface to the inside of the weld metal.

  The nickel-base alloy weld metal 4 made of 182 alloy having such a metal structure has a grain boundary type SCC sensitivity along a columnar crystal in high-temperature water such as reactor cooling water, and exhibits a significant growth rate. It has been known. If a crack due to stress corrosion cracking occurs during in-service operation, the crack progresses along the columnar crystals depending on the residual stress in the weld, and the soundness of the structure may be impaired. There is.

  The present invention has been made in order to solve the above-described problems. The surface of a weld metal made of a nickel-based alloy having a columnar crystal structure in a welded joint portion such as a welded joint portion or a built-up overlay portion of a structure such as a nuclear reactor is provided. An object of the present invention is to provide a nickel-base alloy weld metal surface modification method and structure capable of improving the stress corrosion cracking resistance and improving the soundness and reliability of the structure by modification.

  In order to solve the above-mentioned problems, a surface modification method for a nickel-base alloy weld metal according to the present invention involves melting a nickel-base alloy weld metal having a columnar crystal structure in a welded portion of a structure, and then melting the weld metal. The columnar crystal structure is modified to a granular structure by stirring.

  ADVANTAGE OF THE INVENTION According to this invention, the soundness and reliability of a structure can be improved by suppressing generation | occurrence | production and progress of the stress corrosion crack of the nickel base alloy weld metal in the weld part of structures, such as a nuclear reactor.

The schematic diagram of the surface modification apparatus which concerns on 1st Embodiment. The schematic diagram which shows the action direction of Lorentz force in 1st Embodiment. The cross-sectional schematic diagram of the nickel base alloy weld metal after the surface modification which concerns on 1st Embodiment. The schematic diagram of the surface modification apparatus which concerns on 2nd Embodiment. The cross-sectional schematic diagram of the nickel base alloy weld metal after the surface modification which concerns on 3rd Embodiment. The cross-sectional schematic diagram of the weld metal of a columnar crystal structure, and a welding part.

  Hereinafter, embodiments of a surface modification method and a structure of a nickel-base alloy weld metal according to the present invention will be described with reference to the drawings.

[First Embodiment]
A surface modification method for a nickel-base alloy weld metal according to the first embodiment will be described with reference to FIGS.

(Constitution)
This embodiment demonstrates the example which applied the surface modification method of this embodiment to the welded joint part 1 as a weld part of a structural material.

  In the welded joint portion 1, for example, base materials 2 and 3 made of nickel-base alloy such as 600 alloy are butt welded with nickel-base alloy weld metal 4 made of 182 alloy or the like. The nickel-base alloy weld metal 4 has a columnar crystal structure, and in this embodiment, the surface is reformed by a surface reforming apparatus using a TIG arc as shown in FIG.

  This surface modifying apparatus includes a TIG torch 6, a magnetic material 7 disposed around the TIG torch 6, an exciting coil 8 wound around the outer periphery of the magnetic material 7, an AC rectangular wave generating power source 9, and an external power source 10. Composed.

(Function)
When the surface modification of the nickel-base alloy weld metal 4 which is a columnar crystal metal structure is performed, a TIG arc is generated by applying a voltage from the external power source 10 between the weld joint 1 and the TIG torch 6 which is an object. The molten pool 11 is formed on the surface of the nickel-base alloy weld metal 4.

  In a state where the molten pool 11 is formed, a current is supplied from the AC rectangular wave generating power source 9 to the exciting coil 8, and the magnetic material 7 around which the exciting coil 8 is wound is moved in accordance with the movement of the TIG torch 6. At this time, the AC magnetic field 12 can be formed in a direction perpendicular to the molten pool 11 by adjusting the magnetic field strength and the magnetic field frequency (direction indicated by the arrow in FIG. 1).

  As a result, as shown in FIG. 2, the action direction of the Lorentz force 14 is periodically reversed in relation to the welding current 13 flowing in the molten pool 11, and the inside of the molten pool 11 is effectively stirred. By this stirring, the growth of columnar crystals is suppressed, and as shown in FIG. 3, a granular structure is formed in the surface modified layer 5 that has been melted and solidified, and resistance to the occurrence and progress of SCC is increased.

  The thickness of the surface-modified layer 5 having the granular structure is related to the depth of the molten pool 11 formed in the nickel-base alloy weld metal 4, but the surface modification having a predetermined thickness is formed on the liquid contact side of the nickel-base alloy weld metal 4. A quality layer may be formed, and the thickness thereof is appropriately set according to the surrounding environment such as the quality of the wetted water.

(effect)
According to this embodiment, the surface layer of the nickel-base alloy weld metal 4 having a columnar crystal structure in contact with cooling water or the like is melted, stirred, and solidified. As a result, the surface-modified layer having a granular structure is formed on the nickel-base alloy weld metal 4. 5 is formed, it is possible to effectively suppress the occurrence and development of stress corrosion cracking. Thereby, the soundness and reliability of the structure can be improved.

[Second Embodiment]
The weld metal surface modification method and structure according to the second embodiment will be described with reference to FIG.
(Constitution)
The second embodiment is characterized in that surface modification is performed using laser light.

  In the present embodiment, as a surface modification device, two laser oscillators (not shown) that respectively oscillate melting laser light 15 and stirring laser light 17, and condensing optics that condenses the laser light 15. Surface using first and second laser light irradiation devices comprising a system 16, a condensing optical system 18 for condensing the laser light 17, and a shield gas nozzle 20 for supplying a shield gas 19 to the melting part Reform.

(Function)
When the surface modification of the nickel-base alloy weld metal 4 which is a columnar crystal metal structure is performed, the laser light 15 oscillated from the laser oscillator of the first laser light irradiation means is condensed by the condensing optical system 16. Then, the surface of the nickel-base alloy weld metal 4 that is a target portion for surface modification is irradiated. A shield gas 19 is supplied from the front in the traveling direction of the laser beam 15 to prevent oxidation of the weld metal. Thereby, a laser beam irradiation part melt | dissolves and the molten pool 11 is formed.

For example, a YAG laser is used as the laser beam for welding. A condensing mirror such as a parabolic mirror may be used for the condensing optical system. Further, a CO ₂ laser may be used instead of the YAG laser.

  In accordance with the melting by the first laser light irradiation means, the laser light 17 oscillated from the pulse laser oscillator of the second laser light irradiation means is condensed by the condensing optical system 18, and the molten pool 11 just before solidification is collected. Irradiate. As the laser beam 17 of the second laser beam irradiation means, a Q-switch laser or the like having a pulse width shorter than that of a normal pulse and a high peak power is used. The shock wave generated by this laser beam 17 gives a stirring action to the molten pool 11, and the growth of columnar crystals is prevented in the solidified weld metal surface layer, and as shown in FIG. A granular structure will be formed.

(effect)
According to the present embodiment, in accordance with the movement of the laser beam, the molten pool 11 is formed by the first and second laser beam irradiation means and stirred, whereby a granular structure is formed on the surface of the nickel-base alloy weld metal 4. The surface modification layer 5 is formed, and the resistance to SCC generation and progress can be improved. Thereby, the soundness and reliability of the structure can be improved.

[Third Embodiment]
A weld metal surface modification method and structure according to a third embodiment will be described with reference to FIG.
In the third embodiment, as shown in FIG. 5, after applying the surface modification method of the above embodiment, ERNiCrFe-7 having higher corrosion resistance than the material of the surface modification layer 5 finally obtained, The high corrosion resistance outermost layer 22 is formed by overlay welding of ERNiCrFe-7A or a Cr-containing Ni-based weld metal having a higher Nb concentration than that of ERNiCrFe-7A. Thereby, since corrosion resistance further improves and resistance to SCC generation and progress increases, the soundness and reliability of the structure can be further improved.

[Modification]
In the above embodiment, the example in which the surface modification method is applied to the nickel-base alloy weld metal of the weld joint portion of the structural material has been described. However, the present invention is not limited to this, and overlay welding is performed to modify the inner surface of the pressure vessel. Further, it may be applied to the nickel-base alloy weld metal 4 in the overlay portion.

  Even when a defect such as SCC already exists in the nickel-base alloy weld metal 4, the surface modification method of the present embodiment can be applied to that portion. In that case, the part including the defect is melted and solidified, and the granulation of the structure occurs together with the disappearance of the defect, and the same effect as the above-described embodiment can be obtained.

  As described above, according to the surface modification method for a nickel-base alloy weld metal according to the present embodiment, it becomes a wetted part with respect to the nickel-base alloy weld metal in a weld part of a structure such as a reactor pressure vessel. By melting and solidifying the surface of the weld metal and applying a stirring force in the process, the surface metallographic structure is granulated, thereby improving the resistance to the occurrence and progress of SCC, the soundness of the structure and Reliability can be improved. In addition, the reliability and safety of the entire nuclear reactor can be greatly improved, and the effect is great.

  As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, combinations, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Welded joint part, 2, 3 ... Base material, 4 ... Nickel base alloy weld metal, 5 ... Surface modification layer, 6 ... Tig torch, 7 ... Magnetic material, 8 ... Magnet excitation coil, 9 ... AC rectangular wave generation power supply DESCRIPTION OF SYMBOLS 10 ... External power supply, 11 ... Molten pool, 12 ... AC magnetic field, 13 ... Welding current, 14 ... Lorentz force, 15, 17 ... Laser beam, 16, 18 ... Condensing optical system, 19 ... Shield gas, 20 ... Shield Gas nozzle, 22 ... outermost layer with high corrosion resistance.

Claims (8)

  A nickel-base alloy weld metal characterized by melting a nickel-base alloy weld metal having a columnar crystal structure in a welded portion of a structure and then reforming the columnar crystal structure to a granular structure by stirring the molten weld metal. Surface modification method.   2. The surface modification method for a nickel-base alloy weld metal according to claim 1, wherein the weld metal is melted by a TIG arc, and the melted weld metal is stirred by electromagnetic induction means.   2. The surface modification method for a nickel-base alloy weld metal according to claim 1, wherein the weld metal is melted by a first laser light irradiation means, and the molten weld metal is stirred by a second laser light irradiation means. .   The high corrosion-resistant outermost layer having higher corrosion resistance than the material of the surface-modified layer is formed on the surface-modified layer modified into the granular structure. Method for surface modification of nickel-base alloy weld metal.   5. The surface modification of a nickel-base alloy weld metal according to claim 4, wherein the high corrosion-resistant outermost layer is made of ERNiCrFe-7, ERNiCrFe-7A, or a Cr-containing Ni-base weld metal having a higher Nb concentration. Method.   The method for modifying the surface of a nickel-base alloy weld metal according to any one of claims 1 to 5, wherein the welded portion of the structure is a welded joint portion or an overlay overlay portion.   A structure characterized in that the surface of the welded portion is modified into a granular structure using the surface modification method for a nickel-base alloy weld metal according to any one of claims 1 to 3 and 6.   A highly corrosion-resistant outermost layer is formed on the upper part of the surface-modified layer having a grain structure with a modified welded portion using the method for surface-modifying a nickel-base alloy weld metal according to any one of claims 4 to 6. A structure characterized by formation.

Images