JP2008019499A - High-frequency induction heating stress relaxation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-frequency induction heating stress relaxation method, capable of relaxing residual stress at a welded joint part of a small-diameter thin-walled pipe. <P>SOLUTION: In the high-frequency induction heating stress relaxation method for relaxing residual stress at an inner surface of the welded joint part 4, the inner surface of the welded joint part 4 of the small-diameter thin-walled pipes 2, 3 is cooled with water while being heated by a high-frequency induction coil 6 from an outer surface of the pipe. When performing high-frequency induction heating, the heating depth S of the high-frequency induction heating is adjusted to a range of t/6 to t/2, and the distance D between the weld center and the end of the high-frequency induction coil is adjusted to equal to or larger than a value defined by the formula: 1.35×(R×t)<SP>1/2</SP>, where R is the neutral radius and t is the sheet thickness of the pipes 2, 3. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-frequency induction heating stress relaxation method for preventing stress corrosion cracking.

  Conventionally, as part of preventive maintenance of intergranular stress corrosion cracking (IGSCC), stress relaxation method (Induction Heating Stress Improvement, (Hereinafter referred to as IHSI) is performed on a welded joint of a boiling water reactor (BWR) (see, for example, Patent Document 1).

JP 2005-226112 A

  By the way, the welded joint between the water level instrumentation nozzle (nickel-chromium iron alloy, P-43) of the boiling water reactor (BWR) and the safe end (austenitic stainless steel, P-8) Compared to (welded joints of molybdenum steel and austenitic stainless steel, etc.), the effect of IHSI has not been confirmed due to the small diameter and thin wall thickness, and the combination of the base metal and the conventional one. .

  That is, the conventional IHSI has been implemented for large-diameter and thick pipe welds, but in conventional heating conditions and heating devices, a small water level measurement nozzle (P-43 + P-8 weld joint) is used. The problem is that the stress improvement effect on the pipe diameter and the thin-walled pipe shape is low, and the stress improvement effect and soundness after construction for the water level measurement nozzle (P-43 + P-8 welded joint) have not been confirmed. .

  Accordingly, an object of the present invention is to provide a high-frequency induction heating stress relaxation method that can solve the above-described problems and can relieve residual stress in a welded joint portion of a thin pipe having a small diameter.

  In order to achieve the above object, the present invention provides a method for heating a pipe inner surface of a welded joint portion of a small-diameter and thin-walled tube with a high-frequency induction coil from the outer surface of the tube while water-cooling the residual stress on the inner surface of the welded joint portion. In the high frequency induction heating stress relaxation method for relaxing the above, when the neutral radius of the tube is R and the plate thickness is t, the heating depth S of the high frequency induction heating is t / 6 or more and t / 2 or less, and The distance D between the welding center and the end of the high-frequency induction coil is expressed as follows:

The high frequency induction heating is set as described above.

  Preferably, the neutral radius R of the pipe is 33 mm or less, and the plate thickness t is 17 mm or less.

  Preferably, the maximum heating temperature of the high frequency induction heating is determined based on the material of the tube.

  Preferably, the welded pipe is made of different materials, and the high frequency induction heating is performed at a lower maximum heating temperature among the maximum heating temperatures determined based on the materials. is there.

  The pipe is composed of a nickel chromium iron alloy steel pipe having a niobium content of 1% to 3% and an austenitic stainless steel pipe having a carbon content of 0.02% or less. The temperature may be set to 650 ° C. or lower.

  The pipe is composed of a nickel chrome iron alloy steel pipe having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe having a carbon content of 0.02% or less. The heating temperature may be set to 600 ° C. or lower.

  The pipe is composed of a nickel chromium iron alloy steel pipe having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe having a carbon content of more than 0.02%. The heating temperature may be set to 550 ° C. or lower.

  Preferably, Equation 2 which is a relational expression of the temperature difference ΔT between the inner and outer surfaces required to relieve the residual stress is obtained in advance.

When the high frequency induction heating, by measuring the evaluation point temperature of tube outer surface, to calculate the inner surface temperature T _i at the number 3 and number 4,

It is confirmed that the difference ΔT between the lowest temperature of the measured evaluation points and the calculated tube inner surface temperature T _i satisfies the following equation (2).

  According to this invention, the outstanding effect that the residual stress of the weld joint part of a thin pipe | tube with a small diameter can be relieved is exhibited.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  The high-frequency induction heating stress relaxation method (hereinafter referred to as IHSI) of the present embodiment targets a welded joint portion of a small-diameter and thin-walled tube, and is bonded to, for example, a water level instrumentation nozzle of a reactor pressure vessel and the nozzle. For welded joints with safe ends.

  First, a welded joint part targeted by IHSI will be described with reference to FIGS. 1 and 2.

  As shown in FIG. 1, a water level instrumentation nozzle 2 for connecting a water level sensor (not shown) for measuring the level of cooling water in the pressure vessel 1 is attached to the reactor pressure vessel 1.

  As shown in FIG. 2, the water level instrumentation nozzle 2 has a tube shape extending outward from the pressure vessel 1, and a tube called a safe end 3 is attached to the tip thereof.

  More specifically, the water level instrumentation nozzle 2 is made of a nickel chrome iron alloy (for example, Inconel (registered trademark)), while the safe end 3 is made of austenitic stainless steel, and the mother of these nickel chrome iron alloys. A material (hereinafter referred to as P-43) and an austenitic stainless steel base material (hereinafter referred to as P-8) are joined by welding. In the present embodiment, welding is performed by adding a nickel-chromium-iron alloy welding material.

  Thus, the welded joint part 4 is composed of the base material part 2 of P-43, the base material part 3 of P-8, and the welded part 5 of nickel chrome iron alloy positioned between the base materials. The

  Next, IHSI of this embodiment will be described based on FIG.

  As shown in FIG. 3, the IHSI arranges a high frequency induction coil (hereinafter referred to as a heating coil) 6 so as to surround the outer periphery of the welded joint portion 4, and supplies a high frequency current from the high frequency heating power source 7 to the heating coil 6. The weld joint 4 is heated from the pipe outer surface side, and a liquid (specifically, water) is allowed to flow through the pipes 2 and 3 to cool the pipe inner surface of the weld joint 4 with water.

  As a result, the outer surface side of the welded joint portion 4 is heated and tends to expand. On the other hand, the inner surface side is water-cooled and does not deform, so that the inner surface is compressed and the residual stress (tensile stress) is changed to compressive stress. Migrated.

In the present embodiment, the small diameter compared to the conventional, since the target welded joint portion 4 of the thin-walled tube 2, the maximum heating temperature T _O (° C.) which is a conventional IHSI heating conditions (essential variable), The essentials of the temperature difference ΔT (° C.) between the inner and outer surfaces of the tube, the heating time τ (sec), the coil width L (mm), and the distance D (mm) between the weld line position (weld center) and the coil end were reviewed. Here, the small-diameter and thin-walled pipes 2 and 3 in this embodiment refer to those having a neutral radius R of 33 mm or less and a plate thickness t of 17 mm or less.

  As a result of reviewing the IHSI heating conditions, the heating conditions shown in Table 1 were obtained. Table 1 shows a comparison between the conventional IHSI heating conditions and the IHSI heating conditions of the present embodiment.

  As shown in Table 1, in this embodiment, since the stress relaxation target part is thin, the heating depth S (mm) was added as a heating condition as an influence factor on heating. Moreover, the distance D (mm) between the welding center and the coil end was newly set so that the heating range would not be insufficient.

  That is, in this embodiment, when the neutral radius of the pipes 2 and 3 (the radius of curvature at the center of the nominal thickness of the pipe) is R (mm) and the plate thickness is t (mm), the heating depth of the high frequency induction heating is described above. S is t / 6 or more and t / 2 or less, and the distance D between the welding center and the end of the high-frequency induction coil is

High frequency induction heating is set as described above (see FIG. 3).

  First, the heating depth S will be described with reference to FIG.

  In general, induction heating is a method in which a magnetic flux is generated in an object to be heated by passing an induction current through the heating coil 6 to induce a very high density current (eddy current), thereby heating the object to be heated. .

  The eddy current is stronger as it gets closer to the surface of the object to be heated, and becomes weaker exponentially as it goes inside. This is called the skin effect. FIG. 4 shows this skin effect.

  FIG. 4 shows the relationship between the current penetration depth S (σ in FIG. 4) and the eddy current density distribution.

  From FIG. 4, the depth to the point where the eddy current is reduced to 0.368 times the strength at the surface is defined as the heating depth S. The expression of the heating depth S is shown in Equation 6.

  In the present embodiment, the heating depth S defined as described above is set to t / 6 or more and t / 2 or less.

  This is because the application range of the plate thickness t is 8.5 to 17 mm (the plate thickness range of the actual water level instrumentation nozzle 2) and the range of the operating frequency is set to about 20 ± 10 kHz. This is because it is considered realistic from the viewpoint of heat transfer performance and performance of the heating device (high-frequency heating power source).

  That is, when the heating depth S exceeds t / 2 and the heating depth becomes deep, the temperature distribution in the tube sheet thickness direction does not have an appropriate gradient, so that a sufficient stress improvement effect cannot be obtained. On the other hand, if the heating depth S is less than t / 6, heating is performed on the surface, and for the same reason as described above, the stress improvement effect cannot be obtained sufficiently.

  Here, the heating depth S is determined by the specific resistance of the material of the tubes 2 and 3 and the frequency applied to the heating coil 6.

  Based on FIGS. 5 and 6, the relationship between the frequency f applied to the heating coil 6 and the heating depth S will be described.

  In FIG. 5, the calculation result of the heating depth S by each frequency of Inconel (trademark) (P-43) and austenitic stainless steel (P-8) is shown.

  From FIG. 5, there is no difference in the heating depth S between Inconel (registered trademark) and stainless steel, and the heating depth S is about 4 mm and the frequency setting range (f = 20 ± 10 kHz), it can be said that the heating depth S is about 3 to 5.5 mm.

  FIG. 6 shows the relationship between the frequency f and the quotient t / S obtained by dividing the plate thickness t by the heating depth S based on the result of FIG.

  From FIG. 6, when the plate thickness t is 8.5 mm, t / S is less than 2.25 when the frequency f is 10 kHz, and when the outer surface heating and the inner surface cooling are performed at the time of IHSI construction, Since it is considered that a temperature gradient cannot be obtained, it is necessary to set the frequency f to 20 kHz or more.

  Further, from FIG. 6, t / S does not exceed 6 at the maximum at any plate thickness t. This indicates that it is reasonable for the water level instrumentation nozzle 2 to maximize the frequency f used at 30 kHz.

  Next, the distance D from the welding center to the coil end will be described with reference to FIG.

  In FIG. 7, the result of the internal / external surface temperature measurement of a prior confirmation test is shown. A tube having an outer diameter of 76.5 mm and a plate thickness of 13.5 t (mm) was used as the test body. The heating coil 6 has a coil width L of 100 mm, and the IHSI specified value of the water level instrumentation nozzle 2 (for example, Iida et al., “Guidelines for stress relaxation method by high frequency induction heating (SCC countermeasure method) TNS -G2804-1985 ", thermal power generation technology association of Japan, and nuclear power generation technology committee)).

  In general, the heating coil 6 has a lower energy density because the coil density decreases toward the coil end. In particular, in the case of the water level instrumentation nozzle 2, since the plate thickness t to be applied is thin, the cooling effect of the inner surface cooling water at the time of IHSI is large, and the temperature tends to drop toward the coil end.

  For the above reasons, from the viewpoint of ensuring a sufficient heating range, the distance D from the welding center to the coil end is set to 1/2 of the coil width L (see Equation 5). That is, the heating coil 6 is arranged such that the welding center of the pipes 2 and 3 and the axial center of the heating coil 6 coincide with each other.

  Next, in FIG. 8 to FIG. 10, specimens A02 (tube diameter φ59 × plate thickness 17 t (mm)) and B02 (tube diameter φ67 × plate thickness 8.5 t (mm)) constructed under the IHSI heating conditions of this embodiment are shown. ), C05 (tube diameter φ78 × plate thickness 14.4 t (mm)) in the axial and circumferential residual stress measurement results are shown.

  From FIG. 8 to FIG. 10, the stress on the inner surface of the welded joint portion 4 in any of the specimens is mostly compressive stress (that is, the residual stress is 0 or less), and the tensile strength of the welded joint portion 4 that is a factor causing IGSCC. The stress shifted to compressive stress, confirming the stress improvement effect.

  Moreover, the stress improvement effect was seen not only on the austenitic stainless steel side (safe end 3 side) but also on the welded part 5 and the Inconel base metal side (water level instrumentation nozzle 2 side).

  As described above, in this embodiment, by setting the heating depth S to t / 6 or more and t / 2 or less and the distance D from the welding center to the coil end to be several 5 or more, even with a small diameter and thin tube, residual stress can be obtained. Can be relaxed.

Next, the maximum heating temperature T _O for high frequency induction heating will be described.

Returning to Table 1, in the present embodiment, the maximum heating temperature T _O of the high frequency induction heating is determined based on the material (material) of the pipes 2 and 3, and the welded pipes 2 and 3 are different from each other. of a material, of the maximum heating temperature T _O, which are respectively determined based on these materials, at the maximum heating temperature T _O of the lower more, the high frequency induction heating is performed.

Specifically, the pipes 2 and 3 are composed of a nickel-chromium iron alloy steel pipe 2 having a niobium content of 1% to 3% and an austenitic stainless steel pipe 3 having a carbon content of 0.02% or less. In this case, the maximum heating temperature T _{O for} high frequency induction heating is set to 650 ° C. or lower.

When the pipes 2 and 3 are composed of a nickel chromium iron alloy steel pipe 2 having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe 3 having a carbon content of 0.02% or less, The maximum heating temperature T _{O for} high frequency induction heating is set to 600 ° C. or lower.

When the pipes 2 and 3 are composed of a nickel chromium iron alloy steel pipe 2 having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe 3 having a carbon content of more than 0.02%, The maximum heating temperature T _{O for} high frequency induction heating is set to 550 ° C. or lower.

Thus, by determining the maximum heating temperature T _O of the high frequency induction heating based on the material of the pipes 2 and 3, deterioration of the pipes 2 and 3 can be suppressed, and the soundness of the welded joint part 4 can be maintained. Can do.

  From the above, it was found that the IHSI of this embodiment is effective as an IGSCC preventive maintenance measure for the water level measurement nozzle 2 (P-43 + P-8 weld joint 4).

  In addition, a mechanical test was performed on the base metal part after high frequency induction heating for the purpose of soundness confirmation, and it was confirmed that the specified value was satisfied. As a result of the mechanical test, it was found that the heating conditions of this embodiment do not adversely affect the joint performance.

  Next, in the IHSI of the present embodiment, the effect of IHSI construction is confirmed by obtaining the temperature difference ΔT between the inner and outer surfaces of the pipe during high-frequency induction heating.

  That is, in advance, Equation 8 which is a relational expression of the temperature difference ΔT between the inner and outer surfaces required to relieve the residual stress is obtained,

By measuring the evaluation point temperature at the time of the high frequency induction heating, the tube inner surface temperature T _i is calculated by Equation 9 and Equation 10,

It is confirmed that the difference ΔT between the lowest temperature of the measured evaluation points and the calculated tube inner surface temperature T _i satisfies Expression 8.

Here, in order to confirm the temperature difference ΔT between the inner and outer surfaces, which is one of the IHSI heating conditions, it is necessary to calculate the tube inner surface temperature T _i , but in the case of small diameter, thin tubes 2 and 3, conventionally, in the inner surface temperature T _i calculation method, a large difference between the inner surface temperature measured value by the test. Therefore, in this embodiment, the calculation formulas for the tube inner surface temperature T _i are reviewed, and Equations 9 and 10 are obtained.

  The results of comparing the calculated values of Equations 9 and 10 with the measured values of the tube inner surface temperature will be described with reference to FIGS.

FIGS. 11 to 13 show A given by Eq. 11 calculated from the measured values of the pipe inner and outer surface temperatures T _i and T _O and the cooling water temperature T _W and the calculated values of Eqs. 9 and 10. It is a thing.

  According to FIGS. 11 to 13, when the plate thickness t is 8.5 mm or more and 14.4 mm or less, the calculated value using Equation 9 envelops the actual measurement value, and the plate thickness t is thicker than 14.4 mm and 17 mm. In the following cases, the calculated value using Equation 10 envelops the actual measurement value.

From this, it was confirmed that the calculated value of the tube inner surface temperature T _i can be appropriately evaluated by properly using Equations 9 and 10 depending on the difference in the plate thickness t.

Here, a conventional formula for calculating the tube inner surface temperature T _i is shown in Formula 12.

Since the number 10 to the number 12 is the same except for the conditions of the thickness t, in the present embodiment, adding the number 9 in the formula of the inner surface temperature T _i that were used in IHSI method from conventional (number 10) In addition, it can be said that the equations 9 and 10 are properly used depending on the thickness t.

Thus, as a calculation formula for the tube surface temperature T _i, the difference in thickness t, the number 9, by selectively using the number 10, can be appropriately evaluated a luminal surface temperature T _i.

  As a result, the stress improvement effect can be reliably confirmed after the IHSI construction.

  In addition, this invention is not limited to the above-mentioned embodiment, Various modifications and application examples can be considered.

FIG. 1 shows a reactor pressure vessel provided with a water level instrumentation nozzle targeted by a high frequency induction heating stress relaxation method according to an embodiment of the present invention. FIG. 2 shows the welded joint portion of the present embodiment. FIG. 3 shows the welded joint portion and the high-frequency induction heating coil of this embodiment. FIG. 4 is a diagram for explaining the heating depth of induction heating. FIG. 5 is a diagram for explaining the relationship between the heating depth and the frequency. FIG. 6 is a diagram for explaining the relationship between the quotient obtained by dividing the plate thickness by the heating depth and the frequency. FIG. 7 is a diagram for explaining the distribution of the outer surface temperature in the tube axis direction during high-frequency induction heating. FIG. 8 shows an example of the residual stress measurement result of the pipe after the high frequency induction heating stress relaxation method of this embodiment is applied. FIG. 9 shows an example of the residual stress measurement result of the pipe after the high frequency induction heating stress relaxation method of this embodiment is applied. FIG. 10 shows an example of the residual stress measurement result of the pipe after the high frequency induction heating stress relaxation method of this embodiment is applied. FIG. 11 is a diagram for explaining the relationship between the measurement result of the water level instrumentation nozzle temperature and the calculation result in the present embodiment. FIG. 12 is a diagram for explaining the relationship between the measurement result of the water level instrumentation nozzle temperature and the calculation result in the present embodiment. FIG. 13 is a diagram for explaining the relationship between the measurement result of the water level instrumentation nozzle temperature and the calculation result in the present embodiment.

Explanation of symbols

2, 3 Pipe 4 Welded joint 6 High frequency induction coil R Neutral radius t Thickness D Distance between the welding center and the end of the high frequency induction coil

Claims (8)

A high-frequency induction heating stress relaxation method to relieve residual stress on the inner surface of the welded joint by heating the inner surface of the welded joint of a small-diameter and thin-walled tube with a high-frequency induction coil from the outer surface of the tube while cooling with water In
When the neutral radius of the pipe is R and the thickness is t,
The heating depth S of the high frequency induction heating is set to t / 6 or more and t / 2 or less,
And the distance D between the welding center and the end of the high frequency induction coil is given by

A high-frequency induction heating stress relaxation method characterized by performing high-frequency induction heating in the above manner.   The high frequency induction heating stress relaxation method according to claim 1, wherein the tube has a neutral radius R of 33 mm or less and a plate thickness t of 17 mm or less.   The high frequency induction heating stress relaxation method according to claim 1 or 2, wherein a maximum heating temperature of the high frequency induction heating is determined based on a material of the pipe.   4. The high frequency induction heating according to claim 3, wherein the welded pipes are made of different materials, and the high frequency induction heating is performed at a lower maximum heating temperature among the maximum heating temperatures respectively determined based on the materials. High frequency induction heating stress relaxation method.   The pipe is composed of a nickel chromium iron alloy steel pipe having a niobium content of 1% to 3% and an austenitic stainless steel pipe having a carbon content of 0.02% or less. The high frequency induction heating stress relaxation method according to any one of claims 1 to 4, wherein the temperature is set to 650 ° C or lower.   The pipe is composed of a nickel chrome iron alloy steel pipe having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe having a carbon content of 0.02% or less. The high frequency induction heating stress relaxation method according to any one of claims 1 to 4, wherein the heating temperature is set to 600 ° C or lower.   The pipe is composed of a nickel chromium iron alloy steel pipe having a niobium content of less than 1% or more than 3% and an austenitic stainless steel pipe having a carbon content of more than 0.02%. The high frequency induction heating stress relaxation method according to any one of claims 1 to 4, wherein the heating temperature is set to 550 ° C or lower. In advance, Equation 2 which is a relational expression of the temperature difference ΔT between the inner and outer surfaces required to relieve the residual stress is obtained,

When the high frequency induction heating, by measuring the evaluation point temperature of tube outer surface, to calculate the inner surface temperature T _i at the number 3 and number 4,

The high-frequency induction heating stress relaxation method according to any one of claims 1 to 7, wherein a difference ΔT between the lowest temperature among the measured evaluation points and the calculated tube inner surface temperature T _i satisfies Equation (2).

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