JPH0734992B2 - Welding method for metal pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for welding a metal pipe, and particularly to welding for preventing stress corrosion cracking or the like when the inner and outer surfaces of the metal pipe come into contact with a corrosive fluid. It is about the method.

“Prior Art” In general, in metal materials such as austenitic stainless steel that is often used in nuclear power plants and chemical plants, when tensile stress coexists with a corrosion factor, corrosion cracking may rapidly progress. Are known.

Conventionally, in austenitic stainless steel pipes, when a fluid such as water, which is a corrosion factor, is inserted inside the pipe, as a technique to improve the residual stress in the vicinity of the welded joint, the steel pipe is placed outside while cooling water is inserted into the pipe. Induction heating from
A stress improving method has been considered in which a temperature difference that causes a thermal stress equal to or higher than the yield point is applied to the inner and outer surfaces of the mother pipe wall and a residual compressive stress is generated on the inner surface near the welded joint.

[Problems to be Solved by the Invention] However, such a method is applicable when the austenitic stainless steel pipe has a simple shape and a corrosive fluid is inserted thereinto. As shown in FIG. 4, when the metal tube to be improved is a double tube having a complicated shape and the welded joint 3 of the thermal sleeve 11 housed in the mother tube 10,
In a case where the inner and outer surfaces of the thermal sleeve 11 are in contact with a corrosive fluid under operating conditions, even if the mother tube 10 is heated from the outside by the induction heating, the heat transfer to the thermal sleeve 11 is not performed effectively. , Often difficult to apply.

The present invention solves the problems of the prior art as described above, and relates to the formation of a welded joint when performing welding of a joint in a metal pipe for applications in which the inner and outer surfaces come into contact with a corrosive fluid. The purpose is to take countermeasures against intergranular stress cracking on the inner and outer surfaces of the pipe.

"Means for Solving Problems" In the method for welding a metal pipe according to the present invention, prior to forming a welded joint, a first buttering for forming a buttering layer on the inner surface of the pipe separated from the center of the planned welded joint. Welding is performed in advance, and after welding the first buttering welding, welding is performed at a planned weld joint portion, and after forming the weld joint, the inside of the pipe between the first buttering layers is covered so as to cover the pipe inner surface in the vicinity of the weld joint. The second buttering welding is performed on the surface.

"Operation" By performing the first buttering welding on the inner surface of the metal pipe at a position separated by a required distance from the welding joint planned position, a part of the inner surface of the metal pipe is covered with the buttering layer, and The corrosion resistance of the buttering layer protects a part of the inner surface of the metal tube. In addition to the selection of the first buttering welding material, by setting the buttering welding heat input amount to an appropriate minimum heat input amount, it is possible to suppress the occurrence of a sensitization zone in the metal pipe structure close to the buttering layer, Even if a sensitization zone occurs, it prevents the stress corrosion cracking from occurring in the pipe due to the axial and circumferential compressive stresses as described below.

When a welded joint is formed subsequent to the first buttering welding, the welded joint portion shrinks as it cools, causing a phenomenon in which the vicinity of the first buttering layer is inside the bending moment. A stress that shifts in the compression direction in both the axial direction and the circumferential direction is applied to the inner surface of the adjacent metal tubes.

Furthermore, when the second buttering welding is performed on the inner surface of the groove welded portion, the vicinity of the inner surface of the welded joint can be protected by the corrosion resistance of both buttering layers. When the second buttering layer contracts in the axial direction and the circumferential direction, the formation range and the vicinity thereof are affected, and in the vicinity of the outer surface of the welded joint, a compressive stress is applied in both the axial direction and the circumferential direction. Becomes Further, the first buttering layer described above shifts to the direction in which the compressive stress is further applied.

As described above, in the state after welding in which the second buttering layer is formed, there is a possibility that a sensitized region is formed in the exposed portion of the welded joint or in the vicinity of the buttering layer. With respect to the portion that is subjected to compression stress, it is moved in the direction in which a compressive stress is applied, and when a corrosion factor is present, it is converted into a state in which the main inner and outer surfaces of the metal pipe can be protected from phenomena such as stress corrosion cracking.

[Example] An example of the method for welding a metal pipe according to the present invention will be described below with reference to Figs. 1 to 3.

First, an embodiment of a welded joint for a metal pipe will be described with reference to FIG. In the one embodiment, a weld groove 2 is formed inward of the metal pipe 1 in the radial direction of the metal pipe 1 at a predetermined position of the welded joint in the metal pipe 1, and the weld groove 2 has an inner side of the metal pipe 1. When the welded joint 3 is formed from the first buttered layer 4 at a position axially separated from the welded joint 3.
Of the second buttering layer 5 so as to cover the top of the welded joint 3 and the space between the first buttering layer 4.
Is formed.

Hereinafter, this welding method will be described in the order of steps.

As shown in [first formation of the buttered layer] <Configuration of welding start point> 3 (a), the center P ₀ of the weld joint planned portion
From sets the welding start point P ₁ to a position spaced in the axial direction of the metal tube 1 by a required distance L _1, the inner surface of the metal tube 1 in the welding start point P _1, the direction away from the center P ₁ 1 The beads a ₁ , a _2, and a ₃ in the buttering welding are sequentially formed.

The required distance L _{1 in} this case means the welding joint 3 and the second
2 is a distance predicted to shrink the metal tube 1 due to the formation with the buttering layer 5. According to the research by the inventors, each part of the metal tube 1 is as shown in FIG. 2 as described later in detail. Since various stresses are applied, it is preferable to set the stress within the range represented by the formula (i).

0.8 (ah) ^0.5 ≤ L ₁ ≤ 2.5 (ah) ^0.5 (i) where a: average radius of metal tube 1 h: wall thickness of metal tube 1 <setting of welding conditions> metal tube 1 is austenitic Stainless steel (eg SUS3
For example, the welding conditions for forming the first buttering layer 4 are set as follows.

Weld metal: SUS308ULC material (SCC resistance material) Welding heat input: minimum amount of heat input Q _1, however, the Q ₁ = 7 to 8 kJ / cm ...... (ii) <Formation of bead> the welding conditions, a first buttered Weld. The state of the buttering layer 4 is such that the bead formation is started from the welding start point P ₁ and the beads a ₁ , a ₂ , a ₃ are sequentially formed in the direction away from the center P _{1 as} described above. In addition, the total length (forming distance) of the beads a ₁ , a _2, and a ₃ is set to be at least 10 mm or more.

When the metal tube 1 is made of austenitic stainless steel, the metal tube 1 immediately below the first buttering layer 4 is heated by the heat of forming the first buttering layer 4.
Although a part of the internal structure of (the adjacent structure) will be heated, by performing the buttering welding work with the minimum heat input and setting the bead formation direction as described above, it becomes more sensitive. It is possible to suppress the generation of areas.

Further, the first buttering layer 4 has the weld metal as an SCC resistant material (stress corrosion corrosion resistant material), and when the weld metal is cooled from the molten state, the resolidified structure contains delta ferrite, Since it is difficult to be sensitized, the range of the first buttering layer 4 can be treated as having corrosion resistance.

It should be noted that the inner surface of the metal tube 1 in the range obtained by adding the length of the first buttering layer 4 to the required distance L ₁ on one side of the center P ₀ is also covered by the second buttering layer 5 described below. .

The portion of the first buttering layer 4 is contracted when the molten metal is solidified, so that tensile stress is applied in both the axial direction and the circumferential direction, which influences the internal structure of the metal tube 1. Appears, and the compressive stress field is partially applied, but the influence is very small compared with the influence of the welded joint 3 and the second buttering layer 5 described later. For the sake of convenience, the explanation will be given while ignoring the influence.

[Formation of welding groove] Regarding the planned location of the welded joint 3 in the metal pipe 1,
The welding groove 2 for the welded joint 3 is formed by cutting as shown by a solid line to a chain line in FIG.

[Formation of Welded Joint] With the welding groove 2 formed on the metal tube 1 abutting, welding for forming a welded joint is performed. The welding conditions for the welded joint type are: weld metal: SUS308L material, welding heat input: normal heat input Q ₂ , where Q ₂ = 10 to 15 kilojoules / cm …… (iii).

In this case, since the welding heat input amount of the welded joint portion is the normal heat input amount, the structure of the metal pipe 1 located on both sides of the welding groove 2 is affected by the welding heat and is shown in FIG. As described above, it is possible that the sensitized area X is formed along both sides of the welded joint 3, and the sensitized area X is
It reaches both the inner and outer surfaces of the metal tube 1, and the second
Improved by buttering welding.

Further, since the portion of the welded joint 3 is contracted when the molten metal is solidified, the influence of the contraction of the portion of the welded joint 3 in the axial direction and the circumferential direction is exerted on each portion of the metal pipe 1 to generate stress. Let

Examining the stress applied to each part, the welded joint 3
3 (b), the outer diameter of the metal tube 1 becomes smaller as indicated by the chain line, and the moment M acts as indicated by the arrow. As the joint 3 becomes smaller in the radial direction, a compressive stress is applied in the circumferential direction. Further, on the inner surface of the welded joint 3, tensile stress is applied in both the axial direction and the circumferential direction due to the contraction of the weld metal itself.

The shrinkage of the weld metal in the welded joint 3 is considered to affect the first buttering layer 4 and its vicinity. By reducing the size of the welded joint 3 in the circumferential direction, as shown by the chain line in FIG.
When deformed to the outer surface of the metal tube 1 on the opposite surface of the first buttering layer 4, as shown in FIG. 3 (b),
A compressive stress is applied in the circumferential direction based on the fact that a slightly small bending moment m acts and the diameter is reduced in the axial direction due to a slight tensile stress and the outer diameter of the welded joint 3 is reduced. The influence appears as if moving in the direction.

On the other hand, on the inner surface of the first buttering layer 4, the bending moment m acts opposite to the outer surface on the opposite side of the first buttering layer 4, resulting in a slight compressive stress in the axial direction, which reduces the outer diameter dimension. On the basis of this, in the circumferential direction, the influence appears so as to shift to the direction in which the compressive stress is applied.

[Formation of Second Buttering Layer] <Buttering Welding Range> The inner surface of the metal pipe 1 on which the first buttering layer 4 and the welded joint 3 are formed is formed at a position apart from the welded joint 3 on both sides. Second buttering welding is performed between the first buttering layers 4 so as to cover the welded joint 3.

<Setting of Welding Conditions> The welding conditions for forming the second buttering layer 5 are almost the same as the welding conditions for forming the first buttering layer 4, and are set as follows, for example.

Welding metal: SUS308ULC material (SCC resistant material) Welding heat input: Small heat input Q ₃ However, Q ₃ = 7 to 10 kilojoules / cm (iv) <Bead formation> Second buttering welding depending on the above conditions Do. The state of formation of the second buttering layer 5 is such that the bead c of the first layer is formed on the first buttering layer 4 or in the direction from the welding start point P ₁ to the center P ₀ of the weld joint 3. ₁・ c ₂
・ C ₃ …… is sequentially formed and beaded near the center position.
By the method of stopping cn. Also, the first layer bead
Above c _{1 to} cn, as shown in FIGS. 1 and 3 (c),
The second layer of beads d ₁ · d ₂ · d ₃ · dn and the beads of the third layer or more may be mounted as necessary to form a plurality of layers.

[State After Welding] When the second buttering welding is performed, the inner surface side of the welded joint 3 is covered with both buttering layers 4 and 5, and due to the corrosion resistance of both buttering layers 4 and 5.
The vicinity of the inner surface of the welded joint 3 can be protected.

On the other hand, the outer surface of the welded joint 3 is exposed,
The sensitization zone X located on the outer surface of the metal tube 1 comes into direct contact with the corrosive fluid due to the exposure, but as described below, it should be protected by setting a compressive stress field in the vicinity thereof. You can

As described so far, the first buttering layer 4, the welded joint 3, and the second welded groove 2 are formed on the inner surface of the metal pipe 1 and the welding groove 2.
When the welding operation is completed by sequentially forming the buttering layer 5 of No. 1, the stress applied to each part of the metal pipe 1 is
The stresses applied by the respective weldings will appear in a combined state. The outer surface of the metal tube 1 is shown by the solid line in FIG. 2, and the inner surface of the metal tube 1 is shown by the broken line in FIG. The state becomes positive (positive in FIG. 2 indicates tensile stress, and negative indicates compressive stress).

Supplementing the description based on FIG. 2, the stress applied by the second buttering welding is the same as the second buttering layer 5
Contracts in the axial and circumferential directions, respectively,
The formation of the second buttering layer 5 and its vicinity affect each part in a range wider in the axial direction than in the case of welding in the welded joint part.

That is, the second buttering layer 5 contracts in the axial direction and the circumferential direction in the range indicated by L ₁ in FIG. 2, and each part of the metal tube 1 located in this range is axially and circumferentially. Is compressed to.

When the state of the outer surface of the metal pipe 1 is examined, as shown by the solid line in FIG. 2, the outer surface of the metal pipe 1 is in the vicinity of the welded joint 3 in the range of the compressive stress in both the axial direction and the circumferential direction. Sensitization zone (L ₂ ) that is easily formed on both sides of
However, a compressive stress field is generated near the exposed outer surface. At this time, in the vicinity of the first buttering layer 4 which is separated from the welded joint 3 by the distance L ₁ , a tensile stress is applied in the axial direction and a compressive stress field is applied in the circumferential direction, but no sensitization zone is formed in this vicinity. There is no.

In addition, the inner surface of the pipe in which the metal pipe 1 and the first and second buttering layers 4 and 5 are integrated has an axial direction and a circumferential direction in the vicinity of the welded joint 3 as shown by a broken line in FIG. Both are in the range of tensile stress.

Then, as the distance from the welded joint 3 increases, the axial stress in the second buttering layer 5 changes from tensile to compressive stress, and the circumferential stress also changes to compressive,
In the vicinity of the first buttering layer 4 separated by the distance L ₁ , a compressive stress field is generated in both the axial direction and the circumferential direction. In this case, although there is no sensitization region as described above in a part of the outer surface of the metal tube 1, that is, in the vicinity of the outer surface on the opposite side on which the first buttering layer 4 is formed, the sensitization region does not exist in the axial direction. Since the tensile stress is applied, the conditions expressed by the above-mentioned formulas are set so that the magnitude of the tensile stress is sufficiently controlled.

Further supplementing the description, in such a state after welding, the sensitized region X along the thickness direction (radial direction) is formed in the structure of the metal pipe 1 in the vicinity of the welded joint 3. Even if there is a concern, by placing the second buttering layer 5 on the inner surface side of the welded joint 3,
The outer surface of the welded joint 3 can be used as a compressive stress field, and the inner surface of the welded joint 3 can be covered with a buttering layer so that the corrosion factor does not directly contact the inner surface of the metal pipe 1.
Furthermore, even if the adverse conditions in which the axial sensitization zone X is formed in the internal structure of the metal tube 1 along the second buttering layer 5 are taken into consideration, it is covered with the first and second buttering layers. And in the presence of a corrosion factor, the main inner and outer surfaces of the metal pipe 1 undergo a phenomenon such as stress corrosion cracking in response to the tensile stress of the buttering layer in the axial direction and the circumferential direction in the direction in which the compressive stress is applied. It is to convert to a state that can be protected from.

[Other Embodiments] FIG. 4 shows a structural example in which the method for welding a metal tube according to the present invention is applied to the recirculation system inlet nozzle of a boiling water reactor.

A safety end 8 is attached to the tip portion of the recirculation system inlet nozzle 7 in the reactor pressure vessel 6 by a welded joint 9, and the nozzle 7 and the safety end 8 are also attached.
As the mother pipe 10, the inner surface of the mother pipe 10, the thermal sleeve 11 for reducing the stress generation due to the temperature difference of the fluid, when the structure is attached by the base welded portion 12, in the middle of the thermal sleeve 11 The joint 3 is an application target of the metal pipe welding method according to the present invention.

In this case, the reactor cooling water, which is a corrosive fluid, is inserted into the mother tube 10 and the thermal sleeve 11, and the tubular hollow portion 13 formed between the mother tube 10 and the thermal sleeve 11 is formed. Also, the reactor cooling water, that is, the corrosive fluid intervenes.

Therefore, by handling the thermal sleeve 11 with it removed from the base welded portion 12, the welding method can be applied in accordance with the embodiment described with reference to FIGS.

In particular, in the case of the safe end 8 and the thermal sleeve 11 in FIG. 4, the case where the chain end YY 'is cut off as shown by an arrow and repaired or replaced with a new one, By connecting the parts of the welded joint 3 in the figure first and forming the base welded portion 12 shown in FIG. 4, the thermal sleeve 11 is kept inside the mother pipe 10 and the second The buttering layer 5 can be formed, and in this case, the second buttering layer 5 is formed in the hollow cylindrical portion 13 of the thermal sleeve 11 in the presence of a fluid such as reactor cooling water. You can That is, by setting the welding heat input amount for forming the second buttering layer 5 in the above-described range, the heat influence on the metal pipe 1 is suppressed,
The presence of the fluid causes the outer surface of the metal tube 1 to be cooled, but essentially does not exert a great influence.

Further, the welding groove shown in FIG. 1 can be formed in advance prior to the first buttering welding.

Furthermore, the carbon content of the metal tube 1 is low, and the wall thickness is about 10 mm.
In the above case, when the first buttering layer 4 and the second buttering layer 5 are formed, substantially the same effect can be obtained by simply heating and melting with a welding torch without adding a filler rod. Needless to say.

"Effects of the Invention" As described above, in the method for welding a metal pipe according to the present invention, the first buttering welding is performed on the inner surface of the pipe separated from the center of the planned welding spot to form the welded joint at the planned welding spot. After that, since the second buttering welding is performed between the first buttering welding portions so as to cover the welded joint portion, the following excellent effects are exhibited.

The inner surface of the welded joint can be covered with a buttering layer over a wide area so as to be protected from a corrosive fluid.

Even when a sensitized region is generated during the formation of a welded joint, it is possible to prevent a phenomenon such as stress corrosion cracking from progressing by using the vicinity of the outer surface of the welded joint as a compressive stress field.

By covering with a buttering layer, a compressive stress is applied to the metal structure immediately below it, and by forming the buttering layer in the direction along the inner surface of the metal tube, even if the sensitized area is formed, It is possible to prevent the progress of stress corrosion cracking in the thickness direction of the pipe.

It can be applied to measures against stress corrosion cracking of the thermal sleeve at the nozzle of the reactor pressure vessel.

[Brief description of drawings]

FIG. 1 is a sectional view of an essential part showing an embodiment of a welded joint formed by the method for welding a metal pipe according to the present invention, and FIG. 2 is a stress distribution diagram after treatment in the vicinity of the welded joint in FIG. 1, FIG. 3 is an explanatory view of an example of steps in the method for welding a metal pipe according to the present invention, and FIG. 4 is a longitudinal sectional view showing an example of the structure of a recirculation system inlet nozzle of a boiling water reactor. 1 ... Metal tube, 2 ... Welding groove, 3 ... Weld joint, 4 ... First buttering layer, 5 ... Second buttering layer, 6 ... Reactor pressure vessel, 7 ... Nozzle, 8: Safe end, 9: Weld joint, 10: Mother tube, 11: Thermal sleeve, 12: Base weld, 13: Cylindrical hollow part.

Claims (1)

[Claims] 1. A step of performing a first buttering welding on an inner surface of a pipe distant from the center of a planned welding joint, a step of welding the planned welding joint portion, and a pipe inner surface at the planned welding joint portion. And a step of performing second buttering welding between the first buttering welded portions.