JPH08261388A - Residual stress improving method - Google Patents

Abstract

PURPOSE: To provide a residual stress improving method wherein even during a use in a high temperature state, a residual stress on the tensile side in the vicinity of a weld part is relieved or removed or the factor of an inter granular stress corrosion crack is decreased or eliminated, in a structure formed of an austenite stainless steel and a nickel alloy having sensitivity to an inter granular stress corrosion crack. CONSTITUTION: When a piping 1 joined through welding is used under environment where temperature is changed, a sleeve 3 having the coefficient of thermal expansion higher than that of a material of the piping 1 is securely joined with the outer peripheral surface, containing a weld part 2, of the piping 1 and thermal elongation on the outer peripheral side during the high temperature use of the piping 1 is promoted by thermal elongation of the sleeve 3. This constitution generates a compression stress on the inner peripheral side of the piping 1, relieves or removes a residual stress on the tensile side during welding remaining on the inner surface side of the piping 1, or causes a residual stress on the tension side to form a residual stress on the compression side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a residual stress improving method applied when a structure joined by welding is used in an environment where the temperature changes, and in particular, in-core equipment and furnaces of nuclear power plants and the like. The present invention relates to a residual stress improving method suitable for preventing stress corrosion cracking in the vicinity of welded portions such as internal structures or various pipes.

[0002]

2. Description of the Related Art For example, pipes of boiling water nuclear power plants, in-core equipment and in-core structures are generally made of special metals such as austenitic stainless steel or nickel-base alloys.

[0003] Pipes and furnace equipment made of such austenitic stainless steel or nickel base alloy,
In-furnace structures may cause intergranular stress corrosion cracking (IGSCC) when used in high temperature pure water during operation.

The main factor of the grain boundary stress corrosion cracking on the material side is the formation of grain boundary carbides in the heat affected zone of the weld due to the heat cycle such as welding and the accompanying formation of a Cr-deficient layer near the grain boundary. It is considered to be formation, that is, welding sensitization.

However, in addition to the above-mentioned factors on the material side, as a necessary condition for causing intergranular stress corrosion cracking, a tensile residual stress exceeding the yield stress is present on the surface side of the structure in contact with the high temperature fluid. Existence is mentioned.

Particularly in the case of piping of a nuclear power plant which is made of austenitic stainless steel, nickel-base alloy or the like and circulates high temperature pure water, a yield stress is applied to a portion in the vicinity of a weld on the inner peripheral surface which contacts the high temperature pure water. Excessive tensile residual stress is generated, which may cause intergranular stress corrosion cracking.

[0007]

Conventionally, when a structure joined by welding is used in an environment where the temperature changes, the residual stress on the tensile side at the time of welding remaining on the surface of the structure is Intergranular stress corrosion cracking (IGSC
C) could occur.

The present invention has been made in view of the above circumstances, and an object thereof is to obtain a structure composed of austenitic stainless steel, nickel base alloy or the like having a susceptibility to intergranular stress corrosion cracking at a high temperature. To provide a residual stress improvement method that can reduce or eliminate the factor of intergranular stress corrosion cracking by reducing, eliminating or compressing the residual stress on the tensile side near the welded part even when used below. It is to provide a residual stress improving method capable of effectively preventing corrosion itself.

[0009]

A method for improving residual stress according to the invention as defined in claim 1, wherein when the structures joined by welding are used in an environment where the temperature changes, the welding of the structures is performed. An auxiliary member made of a material having a coefficient of thermal expansion larger than that of the material of the structure is joined and fixed to one surface of the auxiliary member, and the thermal expansion of the one surface side of the structure when the structure is used at high temperature. Promoted by heat expansion,
Thereby, compressive stress is generated on the other surface side of the structure to reduce or eliminate the residual stress on the tensile side at the time of welding remaining on the other surface of the structural material, or the residual stress on the tensile side. Is a residual stress on the compression side.

According to the present invention, the thermal expansion of one surface side of the structure when used at high temperature is promoted by the thermal expansion of the auxiliary member, and a large tensile stress acts on the same surface side of the structure. On the contrary, compressive stress is generated on the inner peripheral surface side, which is the liquid contact side of the structure, due to the stress balance, and the residual stress on the tensile side generated on the inner surface side of the structure during welding is reduced or eliminated. It Alternatively, when the compression action is large, the residual stress on the tensile side changes to the residual stress on the compression side.

Therefore, according to the present invention, a structure composed of austenitic stainless steel, nickel-base alloy or the like which is susceptible to intergranular stress corrosion cracking can be used in the vicinity of the weld even when used under high temperature conditions. The factor of intergranular stress corrosion cracking can be reduced or eliminated by reducing or eliminating the residual stress on the tensile side or by setting it on the compressive stress side.

According to a second aspect of the present invention, in the residual stress improving method, when the pipes joined by welding are used in an environment where the temperature changes, the outer peripheral surface including the welded portion of the pipes is better than the pipe material. By joining and fixing the sleeve having a large coefficient of thermal expansion, the thermal expansion of the outer peripheral surface side of the pipe when used at high temperature is promoted by the thermal expansion of the sleeve, thereby generating compressive stress on the inner peripheral surface side of the pipe. The residual stress on the tensile side at the time of welding remaining on the inner surface side of the pipe is reduced or removed, or the residual stress on the tensile side is made the residual stress on the compression side.

According to the present invention, it is possible to effectively prevent stress corrosion cracking in the vicinity of welded portions of various pipes such as in-core pipes of nuclear power plants. Claim 3
The described residual stress improving method is characterized in that, in the method according to claim 2, the sleeve is composed of a plurality of divided bodies and is fastened and fixed to the outer peripheral surface of the pipe while covering the entire surface of the welded portion.

According to the present invention, the sleeve can be easily attached to an existing pipe or the like, and the technique of preventing stress corrosion cracking can be applied to an actual plant or the like. A method for improving residual stress according to a fourth aspect is characterized in that, in the method according to the first to third aspects, a corrosion resistant material is applied to the auxiliary member or the sleeve to keep the welded portion in a corrosion resistant state.

According to the present invention, by applying a corrosion-resistant metal as the material of the auxiliary member or the sleeve, the entire welded portion of various structures or pipes is covered, and the portion near the welded portion is effectively prevented from corrosion itself. it can.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the residual stress improving method according to the present invention will be described below with reference to the drawings. This example is applied to improve the residual stress of a boiling water nuclear power plant,
FIG. 1 is a cross-sectional view showing a state of a pipe subjected to the method according to the present embodiment at room temperature, FIG. 2 is a cross-sectional view showing a state of the same pipe at high temperature, and FIG. 3 is a characteristic diagram showing the action.

As shown in FIG. 1, the pipe 1 of this embodiment is joined by a welded portion 2, and the material of the pipe 1 is austenitic stainless steel or nickel-base alloy. This pipe 1 has, for example, 28
A fluid near 0 ° C circulates. Then, when the operation is stopped, the temperature drops to room temperature or 50 ° C.

A material having a larger coefficient of thermal expansion than the austenitic stainless steel or nickel-based alloy, which is the material of the pipe 1, is formed on the outer peripheral surface including the welded portion 2 of the pipe 1 used in such an environment where the temperature changes. Al alloy, 13
The sleeve 3 made of% Mn steel, ferritic stainless steel or the like is joined and fixed.

The sleeve 3 is composed of, for example, a pair of monolithic cylindrical element 3a with a flange 4, which is attached to the pipe 1 at room temperature or at a temperature of 50 ° C. or less and the bolt 5 is attached thereto. Fasten with.

When the pipe 1 is heated to a high temperature of about 280 ° C. during the plant operation, the sleeve 3 bonded and fixed to the outer peripheral portion of the pipe 1 expands more than the pipe 1, so that FIG. As shown in FIG. 2, it extends from the axial length L1 at normal temperature to the axial length L2.

As a result, the thermal expansion of the outer peripheral surface side of the pipe 1 when used at high temperature is promoted by the thermal expansion of the sleeve 3, and a large tensile stress acts on the outer peripheral surface side of the pipe 1.

On the contrary, compressive stress is generated on the inner peripheral surface side of the pipe 1 which is the liquid contact side due to the balance of stress, and the residual stress on the tensile side generated on the inner surface side of the pipe 1 during welding is reduced or To be removed. Alternatively, when the compression action is large, the residual stress on the tensile side changes to the residual stress on the compression side.

FIG. 3 shows residual stress at a point B on the outer peripheral side near the welded portion of the pipe 1 in FIGS. 1 and 2 and residual stress at a point A on the inner peripheral side at low temperature (room temperature) and high temperature. This is an observation of (heating temperature during operation). As shown in the figure, at room temperature, B near the outer peripheral surface of the pipe 1
At both the point and the point A near the inner peripheral surface, the tensile stress having a large yield stress remains.

On the other hand, under a high temperature condition during operation, the sleeve 3 joined and fixed to the outer peripheral surface of the pipe 1 expands more in the axial direction than the pipe 1 due to large thermal expansion. As shown by the arrow a in FIG. 2, the tensile stress is promoted and increases more than the yield stress at the point B near the outer peripheral surface of the pipe 1, but at the point A on the inner peripheral side of the pipe 1, the stress on the outer peripheral surface side is As a result of the compressive stress acting as indicated by the arrow b in FIG. 2 due to the balance, the residual stress on the tensile side generated during welding is greatly reduced.

Therefore, according to the present embodiment, the welded portion 2 is formed even when the pipe 1 made of austenitic stainless steel, nickel base alloy or the like having a susceptibility to intergranular stress corrosion cracking is used under a high temperature condition. By reducing or eliminating the residual stress on the tensile side in the vicinity, or by making it on the compressive stress side, the factor of intergranular stress corrosion cracking can be reduced or eliminated.

At the same time, the material of the sleeve 3 is Al
Since alloys, 13% Mn steel, ferritic stainless steel and the like are corrosion resistant metals, by covering the entire welded portion 2 of the pipe 1 with this, the portion near the welded portion 2 is effectively prevented from corrosion itself.

Although the above embodiments have been applied to the piping of a boiling water nuclear power plant, they can be appropriately applied to various in-core equipments or in-reactor structures, and in that case, the same as the above. The effect of is demonstrated.

[0028]

As described in detail in the above embodiments, according to the invention as set forth in claim 1, the thermal expansion of one surface side of the structure during high temperature use is promoted by the auxiliary member thermal expansion, and the structure is improved. A large tensile stress acts on the same surface side of the object. On the contrary, compressive stress is generated on the inner peripheral surface side, which is the liquid contact side of the structure, due to the stress balance, and the residual stress on the tensile side generated on the inner surface side of the structure during welding is reduced or eliminated. It Alternatively, when the compression action is large, the residual stress on the tensile side changes to the residual stress on the compression side. Therefore, for structures composed of austenitic stainless steel and nickel-based alloys that are susceptible to intergranular stress corrosion cracking, the residual stress on the tensile side near the weld is reduced and removed even when used under high temperature conditions. Alternatively, by making the compressive stress side, the factor of intergranular stress corrosion cracking can be reduced or eliminated.

According to the second aspect of the present invention, when the pipes joined by welding are used in an environment where the temperature changes, the outer peripheral surface including the welded portion of the pipes is expanded by heat more than the material of the pipes. The sleeve having a large coefficient is joined and fixed, and the thermal expansion of the outer peripheral surface side of the pipe at the time of high temperature use is promoted by the thermal expansion of the sleeve, thereby generating compressive stress on the inner peripheral surface side of the pipe, and Reducing or eliminating the residual stress on the tensile side during welding that remains on the inner surface of the pipe, or by making the residual stress on the tensile side the residual stress on the compression side, especially in-core piping for nuclear power plants, etc. For example, it is possible to effectively prevent the vicinity of the welded portion of various pipes from stress corrosion cracking.

According to the third aspect of the present invention, the sleeve is made up of a plurality of divided bodies and is fastened and fixed to the outer peripheral surface of the pipe so as to cover the entire surface of the welded portion. It can be easily installed, and the technology for preventing stress corrosion cracking can be applied to actual plants.

According to the fourth aspect of the present invention, a corrosion-resistant material is applied to the auxiliary member or the sleeve, and the welded portion is kept in a corrosion-resistant state. By covering the entire welded portion of the object or the pipe, the portion near the welded portion can be effectively prevented from corrosion itself.

[Brief description of drawings]

FIG. 1 is a view for explaining an embodiment of a method for improving residual stress according to the present invention, which is a cross-sectional view showing a state of a pipe subjected to the method at room temperature.

FIG. 2 is a cross-sectional view showing a state of the pipe shown in FIG. 1 at a high temperature, which is a view for explaining the embodiment.

FIG. 3 is a characteristic diagram showing an operation in the embodiment.

[Explanation of symbols]

 1 Piping 2 Weld 3 Sleeve 4 Collar 5 Bolt

Claims (4)

[Claims] 1. When a structure joined by welding is used in an environment where the temperature changes, a material having a thermal expansion coefficient larger than that of the material of the structure on one surface including a welded portion of the structure. By joining and fixing the auxiliary member consisting of, the thermal expansion of one surface side of the structure during high temperature use is promoted by the thermal expansion of the auxiliary member, whereby compressive stress is applied to the other surface side of the structure. Remaining characterized in that the residual stress on the tensile side at the time of welding remaining on the other surface of the structural material is reduced or eliminated, or the residual stress on the tensile side is made the residual stress on the compression side. Stress improvement method. 2. When a pipe joined by welding is used in an environment where the temperature changes, a sleeve having a thermal expansion coefficient larger than that of the material of the pipe is joined and fixed to the outer peripheral surface including the welded portion of the pipe. The thermal expansion of the outer peripheral surface side at the time of high temperature use of the pipe is promoted by the thermal expansion of the sleeve, thereby generating compressive stress on the inner peripheral surface side of the pipe and remaining on the inner surface side of the pipe. A method for improving residual stress, characterized in that the residual stress on the tensile side during welding is reduced or eliminated, or the residual stress on the tensile side is made the residual stress on the compression side. 3. The residual stress improving method according to claim 2, wherein the sleeve is composed of a plurality of divided bodies and is fastened and fixed to the outer peripheral surface of the pipe while covering the entire surface of the welded portion. How to improve. 4. The residual stress improving method according to claim 1, wherein a corrosion resistant material is applied to the auxiliary member or the sleeve, and the welded portion is kept in a corrosion resistant state.