JPS6410575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a high-frequency heating coil suitable for improving stress in a welded portion in the width direction of an austenitic stainless steel product, and particularly to its shape.

[Background of the Invention] For example, austenitic stainless steel has traditionally been used for equipment and piping that constitute the primary reactor system in a BWR plant. In such welds, there is a risk that stress corrosion cracking will occur due to the combination of three factors: change in material quality, sensitization of the material, high tensile stress, and environmental conditions such as temperature cycles, leading to a decrease in plant operating efficiency. be. Stress corrosion cracking can be prevented by improving any one of the three factors mentioned above, but replacing the material is difficult in terms of process and cost.

When a pipe or pressure vessel has a joint, thermoelastic-plastic analysis using the finite element method and residual stress measurement experiments show that tensile residual stress is generated on the inner surface of the pipe or pressure vessel near the joint. It is generally known that stress corrosion cracking occurs in the heat-affected zone of the weld line, that is, within a range of about 10 mm from the weld metal weld. If the tensile residual stress caused by welding in this part is converted into compressive stress, stress corrosion cracking due to the weld line can be prevented.

One solution was proposed by Tokuko Sho.
This is the method described in Publication No. 53-38246. In this method, a coolant is present inside the piping, and the piping 2 is heated by a high-frequency heating coil 5 (Japanese Patent Laid-Open No. 1983-1995) having the shape shown in FIG.
130409). A temperature difference occurs between the inner and outer surfaces of the pipe 2 due to heating by the coil 5, and tensile and compressive yield forces are generated on the inner and outer surfaces, respectively. If heating is stopped at this point, the piping 2 is cooled by the cooling water inside the piping, and tensile residual stress is generated on the outer surface and compressive residual stress is generated on the inner surface.

The areas where the internal stress is improved by the above method are all within the width of the coil 5, and in order to prevent sensitization, uniform heating at 550°C or less is a condition.

This method uses a circumferential weld of a straight pipe-straight pipe combination,
The same applies to the nozzle stub attachment welds for main pipes of different diameters. (Unexamined Japanese Patent Publication No. 58-9786
Publication) The welding shape of piping and equipment is the circumferential joint 4 above.
There are also axial joints. Similar to the circumferential direction, there is tensile residual stress on the inner surface of the axial weld. A stress improvement method for such axial joints has not yet been developed, and it has been common practice to replace the material reluctantly.

The circumferential joint heating coil schematically shown in FIG. 15 has a shape specifically shown in FIG. 16. If we try to apply this to the weld line of an axial joint, there is a lot of waste as it heats unnecessary parts.Moreover, as shown in Fig. 17, when there is a protrusion 6 on the piping etc. The coil 5 is
Cannot be installed. Furthermore, it is difficult to process the stepped portion 7, and since there is a joint surface 8, it takes time to assemble and disassemble.

In addition, the width of the circumferential heating coil 5 is narrow compared to the long joint weld line 3 in the axial direction, and as shown in FIG. 18, the heating characteristic is narrow and simple.
It was necessary to change the position many times to heat it, and the working characteristics were very poor.

For example, pages 128-129 of ``High Frequency Heat Treatment'' published by Nikkan Kogyo Shimbun on March 15, 1950, states that in pancake-shaped coils, there is a so-called black spot in the center that is not heated. We are proposing a shape that allows current to flow in the same direction by changing the direction of rotation.

However, the cross-sectional shape is special, the shape and size of the workpiece are limited, a lot of heat is wasted, and the workability is not good. In particular, since the peripheral sides protrude toward the workpiece side more than the central part, this is not a problem if the workpiece is only a certain width.
It is quite difficult to apply to weld lines of flat workpieces, and there is no versatility.

[Object of the Invention] An object of the present invention is to provide a high-frequency heating coil with good workability, which has a small temperature drop in the center of the coil, can efficiently heat only the part along the axial joint weld line.

[Summary of the Invention] In order to achieve the above object, the present invention provides a high-frequency heating coil that has a substantially rectangular spirally wound shape and is arranged around a welding line of a tube or a container. This paper proposes a high-frequency heating coil in which the turn width of the coil is smaller than the turn width of the outer circumference.

The present invention also proposes, as a more effective means, a high-frequency heating coil in which the coil is wound vertically only in the center of the coil to further reduce the turn width.

According to these means, the temperature at the center of the coil can be made higher than before, and of course there is no so-called black spot. However, since the contact surface with the workpiece is flat, Unlike other technologies, this method does not have the disadvantage of having a special cross-sectional shape, which limits the shape and size of the workpiece. Therefore, less heat is wasted and workability is very good.

Furthermore, in order to ensure the necessary and sufficient heating range, the circumferential cross-sectional angle looking into the center of the workpiece must be adjusted.
It should be at least 120°. Further, it is desirable that the center of the coil in the circumferential direction coincides with the joint weld line, but if there is a protrusion or the like and it is necessary to avoid this, it may be shifted by ±20° in the circumferential direction.

Next, in order to heat using the above-mentioned coil, the width of the coil is about three times larger than that of the conventional one, so it is only necessary to heat the coil intermittently. Even so, it is possible to sufficiently improve stress characteristics. However, in special cases where smoother stress characteristics are desired to be improved, continuous movement may be performed while continuously energizing.

The average radius of the pipe or pressure vessel is R, and the plate thickness is t.
If the coil width is 2.7√ or more, stress can be effectively improved by high-frequency heating treatment.

However, this is only possible when the inner surface is completely cooled, that is, when the flow velocity on the inner surface is sufficiently fast and the heat transfer coefficient due to inner surface cooling is approximately 2000 Kcal/m ² h℃ or more. If this is not possible, that is, if a heat transfer coefficient of only about 100 Kcal/m ² h℃ can be obtained by natural convection, then the axial width of the shaft coupling coil can be calculated using the same symbols as above.
If it is 4√ or more, stress can be effectively improved by high frequency heat treatment.

[Embodiments of the invention]

The circumferential joint coil 5 that uniformly heats the circumferential welded part 4 of the pipe 2 was developed for nuclear reactor recirculation system piping, and the power supply capacity for heating the coil 5 was also developed for the above purpose, so the coil width was increased. There is a problem that cannot be done. Also, regarding the diameter of the pipe, it was not suitable for a vessel with a diameter larger than the reactor recirculation system pipe ^24B . Therefore, in order to efficiently heat the axial weld line of an actual machine using the current equipment, the circumferential region heated by the coil must be reduced and concentrated in the axial direction. In order to solve this problem, the coil provided by the present invention is a flat spiral coil 1 shown in FIG. The spiral coil 1 is a spirally wound coil, and is characterized by a narrower turn width at the center than the turn width at the outer circumference.

In addition, as shown in Fig. 2, the spiral coil 1 requires only 1/3 the heating area in the circumferential direction compared to the circumferential joint coil 5, so when using the current heating power source,
Can heat an area three times as wide as a circumferential joint coil. That is, the spiral coil 1 can heat the axial weld line 3 three times by heating the circumferential joint coil in one time. The spiral coil θ = 120° in Figure 2 is a value verified in prototype tests. If the angle at which the heating area in the circumferential direction is viewed is smaller than this, the stress improvement effect will also become smaller.

Next, the temperature distribution of the spiral coil will be explained. In the case of the circumferential joint coil shown in FIG. 18, the temperature near the center of the coil is highest, indicating that the magnetic field is concentrated at the center.

The difference between the circumferential joint coil 5 and the spiral coil 1 is the temperature distribution at the center of the coil, which is the temperature distribution shown in FIG. The spiral coil 1 has an M-shaped temperature distribution in which the temperature at the center of the coil is about the same as that at the ends of the coil, and the temperature near 1/4 from the ends of the coil is highest relative to the width of the coil.

This can be explained with reference to FIG. 4, which shows the direction of the current flowing through the coil in an axial cross section. The current flowing through the coil 1 is different between the right half and the left half with the center of the coil as a boundary, and the magnetic field is also generated according to the right-handed screw rule. At the center of the coil, the magnetic fields of the right half 9 and the left half 10 repel each other to form a boundary surface, and the right half and left half of the coil each constitute an independent magnetic field. Further, the magnetic flux density of the magnetic field formed independently by the left and right coils is highest at the center of the coils 9 and 10, similar to the generally known circumferential joint coil 5. As mentioned above, the spiral coil 1 has the same temperature distribution on the left and right sides of the coil center, and the temperature distribution of the spiral coil 1 is equal to that of heating the circumferential joint coil 5 of half the width twice. It has the same shape.

Therefore, as shown in Fig. 5, in order to improve the residual stress of the axial weld line 3 (length: 12 l) of the pipe 2, the conventional circumferential joint coil 5 requires 12 heat treatments. In contrast, the spiral coil 1 can complete the heat treatment in four times. Also, attachment to piping is easy because there is no joint surface 8 like the circumferential joint coil 5. Regarding the stress improvement effect, the temperature distribution is as described above for the axial weld line 3, and sufficient stress improvement can be achieved.

In FIG. 2, the spiral coil 1 is attached to the pipe 2 with respect to the axial weld line 3. A coolant is present on the inner surface of the pipe 2, and a current is applied to the spiral coil 1.
Due to the magnetic field generated by this current, an eddy current is generated in the pipe 2 in the area around which the coil 1 is wound, and the pipe itself generates heat.

The eddy current generated in the pipe has a high frequency and a high current density, and due to the skin effect of the high frequency, it concentrates near the outer surface of the pipe. Therefore, the surface of the pipe is mainly heated, and the inner surface of the pipe is cooled with water, so that a temperature difference occurs between the inner and outer surfaces of the pipe. In order to improve the internal stress of the piping from tensile stress to compressive stress, a temperature difference that causes the external surface of the piping to undergo compressive yielding and the internal surface of the piping to undergo tensile yielding is required. The spiral coil 1 is characterized by concentrating the eddy current towards the axial weld line 3 in order to provide the temperature difference. Therefore, as shown in FIG. 6, if the angle looking into the center in the circumferential direction is about 120 degrees, a sufficient stress improvement effect can be obtained.

Next, the specific shape and temperature distribution of the spiral coil will be explained with reference to FIGS. 1 and 7. The spiral coil 1 has a copper plate 11 spirally wound around the axial weld line 3. A large high-frequency current flows through the copper plate 11 and the coil itself generates heat, so a pipe 12 is attached to cool the coil. The spiral coil 1 is also constructed by attaching a cooling water hose, a cable connection terminal, a coil gap adjustment piece to the piping, a coil reinforcing plate, and the like. The difference from the known circumferential joint coil 5 is that there is no stepped portion 7 and coil joint portion 8 shown in FIG. 16.

This coil 1 was used for a prototype test to improve the stress of the axial weld line 3. Because the coil width and coil length were specified to the minimum necessary dimensions, the coil shape was not the same on the left and right sides, and the temperature The distribution is also different between the right and left halves of the coil (Figure 1).

However, the temperature range of 400°C or more necessary to improve the stress in the axial weld line 3 is the same for the right and left halves of the coil, and the shape of the piping joint is complicated, making it necessary to specify the coil width. This shows that even if this is not the case, the stress in the weld line 3 can be sufficiently improved by the spiral coil 1. Further, in the central portion of the spiral coil 1, the current directions are opposite to each other, the magnetic flux density is small, and the temperature rise is small. If the temperature in the center becomes lower, the area above 400°C where stress can be improved becomes smaller, and the performance of the spiral coil deteriorates. Note 13
represents the boundary surface of the magnetic field.

A more desirable means of the present invention for preventing and improving this is shown in FIG. 8A. This involves winding the coil vertically only in the center of the coil, inserting an insulating plate 14, and reducing the boundary surface 13 of the magnetic fields generated by opposing currents, thereby increasing the temperature in the center. In the coil winding method shown in FIG. 7, the boundary surface of the magnetic field becomes large, resulting in a temperature distribution 16 in which the temperature at the center is about 150° C. (FIG. 8B). When the vertical winding method shown in FIG. 8A is adopted, a temperature distribution 15 in which the temperature at the center of the coil rises to about 300° C. is obtained.
However, the narrower the boundary surface 13 of the magnetic field, the greater the repulsion of the magnetic field, and the greater the amount of heat generated by the central coil itself. This problem can be solved by not joining the cooling pipe 12 to the copper plate 11, but by using a hollow rectangular pipe with a cross-sectional area necessary for current flow at least in the center of the coil, as shown in FIG.

Another feature is the temperature distribution of the coil corner portion 18 shown in FIGS. 1 and 9. In this part, the current is concentrated at the corner part 18, the current flow is poor, and the eddy current generated in the pipe is also reduced. Therefore, if the corner portion 18 is formed into a curved shape as shown in FIG. 9, the concentration of current will be reduced and the eddy current generated in the pipe will also be increased.

Next, a method of improving stress along a joint using the high-frequency heating coil of the present invention will be described. FIG. 10 shows the internal stress distribution after the high-frequency heat treatment was performed only once. The stress state on the inner surface of the object to be heated when the coil position is changed and the heating treatment is performed again after high-frequency heating treatment is performed on a certain location is determined by analysis using the finite element method, as shown in FIG. 11. That is, the inner surface stress distribution shape (FIG. 10) after the first high-frequency heat treatment is shifted by the coil mounting position and superimposed, and the one with the lower stress value is selected.

By appropriately selecting the width of setting the coil mounting positions in these heat treatments at intervals, it is possible to obtain an effect better than the target value of stress improvement shown by the dashed line in Figure 12, while still being approximately one-third of the conventional stress improvement value. This means that the same or better results can be obtained with the number of times of energization.

The method of moving the coil will be explained in more detail with reference to FIGS. 13 and 14. tube 2
An axle 20 is attached to each end of the spiral coil 1 installed above the axial weld line 3,
An axle 21 is attached to the axle 20 via a bearing. One of these axles is driven by a servo motor 22. The axle 21 is housed in a rail 19 having a U-shaped cross section. This allows the spiral coil 1 to be mounted even if the axial weld line 3 is located at the bottom. When the rail 19 is installed parallel to the weld line 3, the spiral coil 1 can be moved along the weld line 3. Note that the rails 19 are supported by rail supports 23, and the rail spacing is maintained.

By heating at discrete locations in this manner, stress can be sufficiently improved. In addition, the 12th
In a special case where you want to eliminate even the small ripples in the stress improvement shown in the figure and make it extremely smooth, there is also a method of continuously supplying electricity while continuously moving, although this increases the amount of power used and is uneconomical. While cooling water is flowing inside the tube 2, electricity is applied to the spiral coil 1, and at the same time the axle 21 is turned on by the servo motor 22.
to drive the spiral coil 1 to the longitudinal joint welding part 3.
In this method, the outer surface of the tube 2 is heated while moving it at a certain speed v along the tube 2. here,
A certain speed v means that the temperature conductivity of the tube 2 is a,
When the average radius is R and the wall thickness is t, within the range of

〔Effect of the invention〕

According to the present invention, it is possible to obtain a high-frequency heating coil with good workability, which can efficiently heat only the portion along the axial joint weld line with little temperature drop in the central portion of the coil.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an embodiment of the spiral coil of the present invention together with its temperature distribution, Figure 2 is a schematic diagram showing the relationship between the coil in Figure 1 and the object to be heated, and Figure 3 is the coil in Figure 1. Fig. 4 shows the direction of the magnetic field of the coil shown in Fig. 1, and Fig. 5 shows the width relationship between the heated object and the conventional circumferential joint coil and the coil of the present invention. Figure 6 is a sectional view comparing the circumferential angles of the conventional coil and the coil of the present invention, Figure 7 is a diagram showing the arrangement of the copper plate and pipe at the center of the coil of the present invention, and Figure 8 is Fig. 7 shows an example in which the arrangement of the coils is improved; Fig. 9 shows another example in which at least the center portion is made of a square pipe; Fig. 10 shows the state of improved stress when heated only once. Figure 11 is a diagram showing the state of stress improvement when heating is performed twice by shifting the coil position.
Figure 2 is a schematic diagram showing the state of stress improvement when heating is performed multiple times while moving the coil, Figure 13 and Figure 1.
Fig. 4 is a schematic diagram showing a mechanism for moving the coil, Fig. 15 is a diagram showing the heating principle of a conventional circumferential joint coil, Fig. 16 is a diagram showing a specific example of a circumferential joint coil, and Fig. 17 is a diagram showing a protrusion. FIG. 18 is a diagram showing a pipe with an object, and FIG. 15 is a diagram showing the axial temperature distribution of the coil. 1... Spiral coil, 2... Piping (heated object),
3... Axial weld line, 4... Circumferential weld line, 5...
... Circumferential joint coil, 6 ... Protrusion, 7 ... Paralleled part, 8 ... Joint part, 9 ... Right half, 10 ... Left half, 11 ... Copper plate, 12 ... Pipe, 13 ...
Magnetic field boundary surface, 14...Insulating plate, 17... Square pipe, 18... Corner part, 19... Rail, 20
... Axle, 21 ... Wheel, 22 ... Servo motor, 23 ... Rail support.

Claims (1)

[Claims] 1. In a high-frequency heating coil that has a substantially rectangular spiral wound shape and is arranged around a weld line of a tube or container, the turn width of the coil at the center of the spiral is defined as the turn width at the outer periphery. A high-frequency heating coil characterized by being smaller than. 2. The high-frequency heating coil according to claim 1, wherein the coil is wound vertically only in the central portion of the coil to further reduce the turn width.