JPH09308985A - Method for reducing residual stress - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely and rapidly reduce the residual stress in an arbitrary place by spraying the coolant from a heating/cooling coil to a part at which the residual stress is desired to be reduced after the desired part is heated above the prescribed temperature, and rapidly coiling it. SOLUTION: A heating/cooling coil 1 is provided with a cooling water flow passage in which the cooling water flows when heating is performed by a heating coil, and a coolant flow passage in which the coolant flows during the cooling and the coolant is sprayed to a part to be cooled for cooling, and the cooling water to constantly cool the coil during the heating flows in the cooling water flow passage. Holes are made in a part of the coolant flow passage opposite to a surface to be rapidly cooled, and the coolant is sprayed against the part for rapid cooling by flowing the coolant during the rapid cooling after the heating. Because heating and cooling can be performed by one coil 1, rapid cooling can be realized. The residual stress can surely be reduced and workability can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing / removing tensile residual stress in a welded portion of a pipe joint.

[0002]

2. Description of the Related Art In a pipe joint, tensile residual stress is generated in the vicinity of the welded portion on both the inner and outer surfaces, and stress corrosion cracking (SCC) or the like occurs. Therefore, various methods have been tried to improve the residual stress.
For example, shot peening that applies a residual compressive stress by hitting a steel ball on the surface to generate a plastic deformation region on the surface, and tensile plastic strain on the inner surface of the pipe by heating the outer surface and cooling the inner surface There have been proposed methods for reducing the above. A heating / quenching method has also been proposed in which the tensile residual stress of the pipe joint weld is removed by uniformly heating the vicinity of the pipe joint weld and then cooling the surface on the side where the residual stress is to be reduced. (JP-A-57-58
However, also in this method, a gas burner or a high-frequency heating device is usually used as a heating heat source, and a separately installed cooling jig is used as a cooling jig.

[0003]

Among the above methods, with regard to shot peening, it is difficult to apply it to the inner surface of the pipe.
Moreover, there is a problem that the application range is limited because there is a risk of contamination on the inner surface. Further, in the method of heating the outer surface and the method of cooling the inner surface, the effect of reducing the residual stress is limited to only the inner surface, and since it is necessary to heat the inner surface while cooling it, a large heating power source is required. Further, in the above heating / quenching method, when a gas burner is used as a heating power source, it is difficult to control the temperature and the workability is extremely poor. Further, heating the nozzle stub welded to the end plate causes heating from the surface, and thus it is difficult to obtain the temperature distribution necessary for reducing the residual stress. Further, when the high frequency heating device is used, a heating coil and a cooling jig are required, and there are problems that rapid cooling is difficult and man-hours are required.

The present invention intends to solve the above problems in the prior art and to provide a method capable of reliably and promptly reducing the residual stress at an arbitrary place.

[0005]

The present invention has the following constitution in order to solve the above problems. (1) In a residual stress reduction method of uniformly heating the vicinity of a welded portion of a pipe joint by high frequency heating and then cooling the surface on the side where residual stress is to be reduced, a cooling water flow in which cooling water is caused to flow when heating the high frequency heating coil. A heating / cooling coil having a passage and a cooling medium passage for injecting / cooling the cooling medium to cool / cool the cooling portion at the time of cooling, and after heating above a predetermined temperature, the cooling medium remains from the heating / cooling coil. A method for reducing residual stress in welded joints of pipe joints, characterized by spraying and quenching at locations where stress is desired to be reduced.

(2) The number of turns of the heating / cooling coil is
The residual stress reducing method according to (1) above, which is adjusted according to the heat capacity of the material to be heated on the inner and outer surfaces in the vicinity of the welded portion of the pipe joint. (3) The residual stress according to (1) or (2) above, wherein the frequency of the high frequency power source is set so that the penetration depth of the induced current flowing through the heating / cooling coil is equal to or greater than the thickness of the tube. Reduction method.

[0007]

The following effects can be expected by using the method of the present invention. The member is soaked or heated from the surface to a depth more than the depth for which the residual stress is desired to be reduced, and then the surface for which the residual stress is desired to be reduced is rapidly cooled, whereby tensile stress is generated on the surface, and tensile plastic deformation occurs. After that, when the central part of the tube thickness begins to decrease in temperature, the temperature near the surface has already decreased and it does not shrink, so the stress gradually shifts to the compression side, and finally the residual stress of compression occurs on the cooling side. Or, the residual tensile stress is relieved. Further, by making the cooling surface the inner and outer surfaces of the pipe, residual stress can be reduced on both the inner surface and the outer surface. This makes it resistant to S
CC property improves.

In the method of the present invention, the vicinity of the welded portion of the pipe joint means a region where the residual stress at the time of welding becomes tensile, and the rapid cooling is caused by the temperature difference between the inside and the outside in the thickness direction of the pipe. It means a cooling rate at which plastic strain occurs.

By using high frequency induction heating as a heating power source, by appropriately setting the frequency even for a thick tube, heat is also generated from inside the thickness of the tube. Can be heated uniformly. Further, in this method, since it is not necessary to provide a temperature gradient in the heating section, a particularly large capacity heating device is not required. The appropriate frequency referred to here is a frequency at which the penetration depth of the induced current represented by the following formula is equal to or greater than the thickness of the tube.

[Equation 1] Penetration depth = 503 × {ρ / (μ · f)} ^1/2 (m) where ρ is the resistivity (Ω · m) of the material to be heated, μ is the magnetic permeability, and f is the frequency. (Hz) is shown.

In the present invention, the heating / heating device is provided with a cooling water passage for flowing cooling water at the time of heating the heating coil and a cooling agent passage for flowing cooling agent at the time of cooling and injecting / cooling the cooling agent to the cooling portion.
A cooling coil is used, and cooling water for cooling the coil is always flowed through the cooling water flow path during heating. In the coolant channel, make a hole (or place it with a nozzle) in the part that faces the surface you want to quench, and cool the part by flowing a coolant (usually water) when quenching after heating. Spray and quench the agent. By using such a coil, heating and quenching can be performed with one coil, so that rapid quenching is possible. As a result, the residual stress can be reliably reduced and the workability is improved. In addition, the heating situation can be reproduced by controlling the position of the heating / cooling coil and the power supply to the power source, so that the temperature can be controlled accurately and the reliability of the construction method is improved.

FIG. 2 shows an example of a heating / cooling coil provided with such a cooling water flow path and a coolant flow path. The heating / cooling coil 1 of FIG. 2 has a double pipe structure in which a pipe A and a pipe B are joined by brazing and covered with a heat-resistant insulating tape 2, and one pipe A
The cooling water for cooling the coil always flows during heating,
On the other side of the pipe B, a pipe wall hole 3 is formed in a portion facing the surface to be rapidly cooled, and by flowing the coolant at the time of rapid cooling after heating, it is possible to inject and rapidly cool the coolant to that part. There is. In addition, the tube B also has a function of a heating coil,
At the time of heating, it is cooled by the cooling water of the pipe A.

For example, when it is desired to reduce the residual stress in the vicinity of the welded portion of a pipe which is through-welded to a thick plate container by the method of the present invention, heating is applied to the inner portion of the thick plate and the outer portion of the thick plate. Since there is a large difference in heat capacity between the parts, the heating method using a coil with a uniform number of turns heats the part outside the thick plate, and the temperature inside the part does not rise so much that uniform heating is desired. Absent. Therefore, uniform heating can be achieved by adjusting the number of windings of the coil, densely installing the coil in a portion having a large heat capacity, and roughly or not installing the coil in a portion having a small heat capacity. After that, rapid cooling enables uniform reduction of residual stress at each place.

Further, by setting the frequency of the high frequency power source so that the penetration depth of the induced current is equal to or greater than the thickness of the tube, uniform heating can be performed over the entire thickness of the tube.

[0014]

EXAMPLES The method of the present invention will be described more specifically with reference to the following examples. In this example, the end plate with a thickness of 160 mm is 45
The residual stress reduction method of the present invention was applied to a nozzle (outer diameter 100 mm, plate thickness 15 mm) welded at an angle of °. FIG. 1 shows a sectional view in the vicinity of the welded portion. End plate 4 is low alloy steel,
On the surface, stainless steel (clad) 6 is overlaid with a thickness of about 5 mm. Further, since the nozzle 5 is made of Inconel alloy, the Inconel alloy is welded to the welded portion in advance to form the Inconel alloy welded welded portion 7. After the build-up welding of the stainless steel 6 and the Inconel alloy, the end plate 4 is subjected to stress relief annealing at 600 ° C. The welding of the nozzle 5 was carried out after the stress relief annealing.

From the above conditions, when the residual stress reduction method of the present invention is applied to the welded portion 8, the maximum heating temperature must be lower than the stress relief annealing temperature (600 ° C. in this case), and the maximum heating temperature is 500. ℃ was made.

Since the residual stress due to welding is tensile in the range of 50 mm before and after the welded portion 8, it is necessary to heat this range to an appropriate temperature (heating target range 9 in FIG. 1). . On the other hand, in the upper part of the nozzle 5, only the nozzle 5 needs to be heated, whereas in the lower part, the nozzle 5 is heated.
Therefore, there is a large difference in heat capacity. Also, the area to be heated (heating target range 9) is 45 °
It will be distributed with a gradient.

Therefore, the heating coil 1 is a coil which is wound around the inside and outside of the nozzle 5 as shown in FIG. 1, and the shape and position of the coil on the inner surface are determined in consideration of the change in heat capacity. . That is, the inner surface coil is not installed on the upper part of the nozzle 5, which has a small heat capacity, but is concentrated on the lower part of the nozzle 5. The heating / cooling coil 1 has a double pipe structure of pipes A and B as shown in FIG. 2, and cooling water for cooling the coil is constantly supplied to one pipe A during heating. On the other side of the pipe B, holes are made in the inner surface of the nozzle stub 5 and a portion corresponding to the weld bead portion, and the coolant is injected and rapidly cooled by flowing the coolant at the time of rapid cooling after heating. I chose

The frequency of the high frequency power source for heating is 15 mm in thickness of the nozzle 5 and the material (ρ = 1 μΩ · m, μ = 1.
Considering 0), it was set to 1 kHz. The penetration depth at this frequency is about 16 mm, and an eddy current is generated in the entire thickness of the nozzle 5. Using the heating / cooling coil 1 shown in FIG.
50mm before and after the inner surface of the welded part of 350 to 500 ℃,
The weld bead could be heated to about 400-500 ° C. It was also confirmed that the temperature difference between the inner and outer surfaces of the nozzle stub 5 was within 50 ° C.

The heating / cooling coil 1 was installed at the position shown in FIG. 1 for uniform heating during heating. However, since the position where the residual stress is desired to be reduced and the position of the coil are displaced, the position of the coil is rapidly cooled. The heating / cooling coil 1 was moved up by 50 mm in order to match the position to be performed, and cooling water was flown through the pipe B to cool the inner and outer surfaces with water. The flow rate of cooling water is about 45 liters / min. 3 and 4 show residual stress distributions when the inner and outer surfaces are rapidly cooled (about 350 ° C./sec). It can be seen that the residual stress is greatly reduced on both the inner and outer surfaces. As described above, by using the method of the present invention, residual stress can be reliably and quickly reduced even in a welded portion having an uneven shape.

[0020]

The following effects are obtained by using the residual stress reducing method of the present invention. Since the residual stress on the construction surface can be reliably reduced, SCC resistance
And fatigue strength can be improved. Moreover, since the construction temperature can be a temperature that does not change the structure of the material, there is no adverse effect on any place. Since the heating coil and cooling jig are the same, the construction method has become extremely simple. Therefore, the construction time could be shortened. Since a high-frequency power source is used for heating, the heating temperature can be controlled easily. Further, since it is not necessary to cool any place at the time of heating, an expensive large capacity power source is not necessary for the heating power source.

[Brief description of drawings]

FIG. 1 is a sectional view of the vicinity of a welded portion in an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of a heating / cooling coil having a cooling water channel and a coolant channel used in the present invention.

FIG. 3 is a residual stress distribution diagram in the example of the present invention.

FIG. 4 is a residual stress distribution diagram in an example of the present invention.

Claims (3)

[Claims] 1. A residual stress reducing method for uniformly heating the vicinity of a welded portion of a pipe joint by high-frequency heating, and then cooling the surface on the side where residual stress is to be reduced. A heating / cooling coil having a cooling water flow path and a cooling agent flow path for injecting / cooling the cooling agent into the cooling part when cooling is performed, and after heating to a predetermined temperature or higher, the cooling agent from the heating / cooling coil A method for reducing residual stress in welds of pipe joints, characterized by spraying and quenching at a place where residual stress is desired to be reduced. 2. The method for reducing residual stress according to claim 1, wherein the number of turns of the heating / cooling coil is adjusted according to the heat capacity of the material to be heated on the inner and outer surfaces near the welded portion of the pipe joint. 3. The residue according to claim 1, wherein the frequency of the high frequency power source is set so that the penetration depth of the induction current flowing through the heating / cooling coil is equal to or greater than the thickness of the tube. Stress reduction method.