JPH05195191A - Surface treatment method for heat transfer tubes for nuclear reactors - Google Patents

Abstract

(57) [Abstract] [Purpose] A method for producing a tube suitable for use as a heat transfer tube such as a PWR steam generator tube in a high temperature and high pressure water environment, which has low exposure to radiation, excellent corrosion resistance, little Ni elution. provide. [Constitution] By weight, a heat transfer tube made of Ni-Cr alloy containing Mn: 0.25% or less, Cr: more than 20% and 25% or less, and Ni: 30 to 50%, 0.5 g / liter of Cr ⁶⁺ ions. Concentration added above 10 ~
After dipping in a nitric acid solution at 40% and a temperature of 40 ° C or higher for 0.1 hour or longer, in a vacuum of 10 ^-2 to 10 ^-4 torr, point A shown in attached Fig. 1
And B, B and C, C and D, D and E, E and F, and F and A are heat-treated at a temperature and holding time in a region surrounded by straight lines.

Description

Detailed Description of the Invention

[0001]

This invention relates to a pressurized water type light water reactor (P
(WR) steam generator tube, heat transfer tube surface treatment method used in high temperature and high pressure water environment.

[0002]

2. Description of the Related Art Conventionally, a light water reactor (P
Alloy 600 with Ni content of 58% or more (brand name, 75% Ni-15% Cr-9% Fe alloy) and
N such as Alloy690 (same as above, 60% Ni-30% Cr-9% Fe alloy)
i-based alloys have been used.

As a nuclear reactor material, γ-ray energy emitted by each nuclide becomes a problem from the viewpoint of radiation exposure. A breakdown of the contribution of elements to the dose rate is caused by activation of Ni.
⁵⁸ Co accounts for 70%, and ⁶⁰ Co is generated when Co is activated.
Account for about 23%. Next, ⁵⁴ Mn accounts for about 10%.
⁵¹ Cr has a γ-ray energy of 0.3 Me emitted per decay.
The contribution to the dose rate is very small because it is weak at V and its incidence is low. Therefore, in order to obtain an alloy with low radiation exposure and low radiation exposure, it is desirable that the content of ⁵⁸ Co, ⁶⁰ Co, and ⁵⁴ Mn in the alloy components, that is, Ni, Co, Mn, etc., is low. The present inventor applied for an invention of an alloy in which the chemical composition is specified so as to maintain excellent corrosion resistance in consideration of the above points, as Japanese Patent Application Nos. 3-272808 and 3-272829. did.

On the other hand, in order to reduce the exposure due to activation of corrosion products in feedwater heaters and moisture separation heater tubes of boiling water nuclear reactors, nitric acid treatment and an inert gas atmosphere containing a trace amount of oxygen are used. We also proposed a method of forming an oxide film mainly composed of Cr oxide on the surface in combination with heat treatment (JP-A No. 2-80552).

[0005]

As described above, ⁵⁸ Co, ⁶⁰ Co, and ⁵⁴ Mn, which have a large contribution of the dose rate, are considered to reduce the radiation exposure.
It is desirable that the content of Ni, Co, Mn, etc. in the material that is the source of is low. That is, Co is not contained, the content of Ni and Mn is reduced, and it is produced from a material that is also excellent in corrosion resistance in high temperature and high pressure water, and has a film for suppressing the elution of the above elements. Most desirable as a nuclear reactor heat transfer tube. It is an object of the present invention to provide a method for manufacturing such a heat transfer tube.

[0006]

The gist of the present invention resides in a surface treatment method for a nuclear reactor heat transfer tube, which is characterized by the following items.

% By weight, Mn: 0.25% or less, Cr: 20%
Over 25% and Ni: 30-50% Ni-Cr alloy heat transfer tube with Cr ⁶⁺ ion 0.5g / l or more concentration 10-
After dipping in a nitric acid solution at 40% and a temperature of 40 ° C or higher for 0.1 hour or longer, in a vacuum of 10 ^-2 to 10 ^-4 torr, point A shown in attached Fig. 1
And B, B and C, C and D, D and E, E and F, and F and A are heat-treated at a temperature and holding time in a region surrounded by straight lines.

As disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-80552, the present inventor has found that a stainless steel heat transfer tube containing 12 to 20% of Cr and 40% or less of Ni was treated with a nitric acid solution to obtain a trace amount. It was found that treatment in the temperature range of 800 ℃ to 1100 ℃ in an inert gas atmosphere containing oxygen forms a chromium oxide-based oxide film on the surface and has a great effect of suppressing Ni elution.
However, as a heat transfer tube for a PWR steam generator, a material having this chemical composition does not have sufficient corrosion resistance.

In the present invention, as a material for the heat transfer tube, an alloy having a high content of Cr and Ni and excellent in corrosion resistance is used as a material for the heat transfer tube. Next, the surface treatment conditions for optimum film formation for this alloy were investigated.

In the treatment method disclosed in Japanese Patent Laid-Open No. 80552/1990, the treatment is first carried out with a 10-40% nitric acid solution. However, the alloy having the above composition in consideration of the improvement of corrosion resistance has a high Cr content, so that the corrosion reaction in the nitric acid solution is slow and it is difficult to form a sufficient film with the above solution.
Therefore, it was decided to add Cr ⁶⁺ ions to this treatment liquid and raise the solution temperature before use. After heat treatment under the conditions of dipping in this solution and undercoating, a dense film consisting mainly of chromium oxide can be formed on the surface, and a heat transfer tube with little elution of Ni, Mn, Co, etc. Can be manufactured.

[0011]

The functions (1) to (3) of the present invention will be described in detail below.

Chemical composition of the material of the heat transfer tube As described above, Mn becomes ⁵⁴ Mn when activated, which is one of the causes of radiation exposure, so the lower the Mn, the better. However, on the other hand, it is necessary to add S (sulfur), which causes hot cracking of the alloy, to some extent to fix or deoxidize it. Therefore, the upper limit of the content of Mn is set to 0.25% (hereinafter,% with respect to the content of alloy components means weight%), and the content is used within this range.

When activated, Ni becomes ⁵⁸ Co, and is an element that has the largest contribution of the dose rate to radiation exposure. Therefore, it is desirable that the content be as low as possible in order to achieve low exposure, but if the Ni content is less than 30%, the intergranular stress corrosion cracking resistance in high temperature water is poor, so a content of 30% or more is required. Is. However, the Ni content of 50% is required to secure the corrosion resistance required in the use of the material produced by the method of the present invention.
Is enough.

Cr is also an essential element for maintaining the corrosion resistance of the alloy. If it is 20% or less, the corrosion resistance required for the above applications cannot be secured. On the other hand, if it exceeds 25%, the hot workability deteriorates, so the upper limit is made 25%.

The heat transfer tube material (alloy) which is the subject of the method of the present invention may contain 0.012% or more C in addition to the above Mn, Ni and Cr in order to secure the strength of the alloy. However, C is
If it exceeds 0.070%, the stress corrosion cracking resistance deteriorates. Therefore, the C content is preferably in the range of 0.012 to 0.070. In order to improve the hot workability of the alloy, 0.01-0.5%
For improving Ti and pitting corrosion resistance, 0.1 to 5% can be contained in total of one or two of Mo, W and V.

Among the impurity elements, regulation of Co, P and S is important. From the viewpoint of reducing exposure, Co is preferably 0.03% or less. Both P and S deteriorate the corrosion resistance, so it is desirable to set each to 0.015% or less.

The heat transfer tube is produced by a conventional tube making method using the above alloy as a raw material, for example, a hot extrusion method to produce a raw tube, and then cold drawing or cold rolling to finish it into a product size. To manufacture. Before the immersion treatment described below, heat treatment such as solution treatment or carbide precipitation treatment (grain boundary strengthening heat treatment) is performed.

Nitric Acid Solution Immersion Treatment A heat transfer tube made of the above alloy was charged with Cr ⁶⁺ ions at 0.5 g /
It is immersed in a nitric acid solution containing liters or more in a concentration of 10 to 40% and having a temperature of 40 ° C. or more to preliminarily form a base film made of chromium oxide on its surface. A suitable thickness of this film is about 10 to 30 nm.

The nitric acid concentration of 10 to 40% is a highly corrosion resistant alloy containing 20% or more of Cr, so that reaction does not occur at nitric acid concentrations of less than 10%, and a chromium oxide-based undercoat film is used. Because you cannot get. On the other hand, when the nitric acid concentration exceeds 40%, the reaction is promoted, but intergranular corrosion or the like occurs, the surface irregularities become large, and the product quality is impaired.

The reason why Cr ⁶⁺ ions are added to the solution is to enhance the reactivity of the solution. As mentioned above,
Since it contains more than 20% Cr and has higher corrosion resistance than general materials such as SUS 304 steel, the reaction does not occur unless 0.5 g / liter or more of Cr ⁶⁺ ions are added, and the target thickness of about 10 to 30 nm Is difficult to form. However, if the amount of Cr ⁶⁺ added is too large, the reaction becomes too vigorous, intergranular corrosion occurs in the material, and the unevenness of the surface tends to become large, so it is desirable to keep it to about 20 g / liter or less. Note that Cr ⁶⁺ may be added as Cr ₂ O ₃ .

By using the above treatment liquid and appropriately selecting the treatment conditions, an undercoat film having a predetermined thickness can be obtained. The treatment conditions in this case include the solution temperature and the immersion time, but the solution temperature is preferably 40 ° C. or higher. The immersion time is preferably about 0.1 to 2 hours. No reaction occurs if the treatment temperature is lower than 40 ℃. Special equipment such as pressurizers can be used at boiling temperatures above about 100 ° C, but this is not necessary. If the dipping time is less than 0.1 hours, an undercoat with a sufficient thickness cannot be obtained. Further, even if it is immersed for more than 2 hours, the oxide film does not become thick, and only corrosion progresses and surface irregularities become severe, so the immersion time is preferably 2 hours or less.

Vacuum heat treatment The heat transfer tube which has been subjected to the surface treatment as described above is changed from 10 ^-2 to 10 ^-4 to
Heat treatment is performed in a vacuum of rr at the temperature and holding time shown in the hatched portion in FIG. 1 attached. The temperature and time at each point in FIG. 1 are as follows.

Point A: 850 ° C., 0.1 hours Point B: 700 ° C., 0.1 hours Point C: 550 ° C., 1 hour Point D: 550 ° C., 10 hours Point E: 750 ° C., 0.5 hours) Point F: 850 ° C., 0.5 hours If the atmosphere of this heat treatment has a vacuum degree lower than 10 ^-2 torr, the oxide film becomes too thick and peels off easily. On the other hand, 10 ^-4
If the vacuum is higher than torr, the growth of the oxide film will be slow and a dense film of sufficient thickness cannot be obtained.

By this heat treatment, an oxide film mainly composed of dense chromium oxide from which water and OH groups in the undercoating film are removed is formed. Thus, the oxide film formed by the vacuum heating treatment after the undercoat treatment inherits the crystal structure of the film formed by the undercoat treatment and becomes a chromium oxide film with extremely high density and good adhesion.

Factors that affect the effect of suppressing the elution of metal ions are the thickness and denseness of the film. The final coating thickness is preferably 50-150 nm. 700 to 850 ℃
Temperature for less than 0.1 hours, or 550 ° C to 700
At a temperature of ℃, if the holding time shown in FIG. on the other hand,
Holding at a temperature of 750 ° C to 850 ° C for more than 0.5 hours or at a temperature of 550 ° C to 750 ° C for more than the time shown in Fig. 1 causes the film thickness to become too thick, resulting in cracking and deterioration of corrosion resistance. To do. When it comes to film compactness,
The film formed in the temperature range over 850 ° C is not dense,
The effect of suppressing metal ion elution cannot be fully exerted. At a temperature below 550 ° C, a dense film having a predetermined corrosion resistance cannot be formed.

[0026]

EXAMPLE An alloy having the chemical composition shown in Table 1 was melted by a vacuum melting method, forged and hot rolled to a thickness of 7 mm, and then cold rolled to a thickness of 4.9 mm. 20 these materials at 1055 ° C
After annealing for minutes, heat treatment was carried out at 800 ° C for 1 hour, and final processing was performed by machining and polishing with emery paper No. 320 for film treatment. The treatment conditions are shown in (1) and (2) of Table 2. Cr ⁶⁺ ions in the undercoating solution were supplied as Cr ₂ O ₃ .

Using the plate after the above treatment as a test material, a test was carried out to examine the amount of Ni eluted in high temperature water and the resistance to stress corrosion cracking (SCC).

The elution amount of Ni ions in the high temperature water was determined by using a platinum container having a capacity of 500 ml in simulated water (500 ppm B ³ +1 ppm Li ⁺ , degassed at 325 ° C.) of the primary cooling water of PWR, using a stationary autoclave. It was immersed in the solution for 300 hours, and the Ni ion in the solution was measured by ICP (high frequency induction plasma emission spectroscopy). The measured elution amount is shown in (1) and (2) of Table 2.

The resistance to stress corrosion cracking was determined by bending a 2 × 10 × 70 (mm) test piece into a U-shape, fixing it with a bolt and nut to apply stress, and then degassing it in a degassed 30% NaOH aqueous solution at 325 ° C. It was immersed for 1000 hours, and the depth of the generated crack was measured by an optical microscope and evaluated. When the crack depth was 25 μm or less, the stress corrosion cracking resistance was good (marked with ○ in Table 2 (1) and (2)).

Looking at the effect of nitric acid treatment conditions from Table 2, when the nitric acid concentration is less than 10% (No. 13) or more than 40% (No. 15), or the Cr ⁶⁺ concentration is less than 0.5 / liter. In the case of No. 14 (No. 14), the elution control of Ni is not sufficient in all cases. Further, when the treatment time is too short (No. 5) or too long (No. 6), it has an elution suppressing effect as compared with the untreated material, but is slightly inferior as compared with the case where the treatment time is appropriate.

Looking at the effect of the vacuum heat treatment, even if the nitric acid treatment is performed under proper conditions, the elution suppression is not sufficient if the conditions for the vacuum film treatment are not proper. In other words, if the temperature is too high (No.16), the processing time is short (No.18,20), or the processing time is too long (No.19,21), and the degree of vacuum is not appropriate (No. No.22,23) has a large elution amount.

The material (alloy) to be treated is not appropriate (Table 1
In the cases of B and C), the amount of elution is larger than that of the steels of the present invention as in Nos. 27 and 28, even if appropriate coating treatment is performed. The stress corrosion cracking resistance was good.

[0033]

[Table 1]

[0034]

[Table 2 (1)]

[0035]

[Table 2 (2)]

[0036]

According to the method of the present invention, a heat transfer tube having excellent corrosion resistance represented by stress corrosion cracking resistance, less metal elution, and less radiation exposure can be obtained. This heat transfer tube is extremely useful as a steam generator tube for a nuclear reactor, a moisture separation heater tube, and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing appropriate conditions (hatched portion) of vacuum heat treatment which is one step of the method of the present invention.

Claims (1)

[Claims] 1. A heat transfer tube made of a Ni-Cr alloy containing, by weight, Mn: 0.25% or less, Cr: more than 20% and 25% or less, and Ni: 30 to 50%, 0.5 g of Cr ⁶⁺ ions. / Concentration of more than 10 liters added
After dipping in a nitric acid solution at 40% and a temperature of 40 ° C or more for 0.1 hour or more, in a vacuum of 10 ^-2 to 10 ^-4 torr, points A and B, B and C, C shown in the attached FIG. And D, D and E, E and F, and F and A are heat-treated at a temperature and holding time within a region surrounded by straight lines, respectively, for a surface treatment method of a heat transfer tube for a nuclear reactor.