JPH0645866B2 - Heat treatment method for stainless steel for heater tubes - Google Patents

Description

The present invention relates to a heater tube made of stainless steel used in a high temperature and high pressure water environment, for example, a feed water heater tube for a boiling water reactor (BWR) and a wet heater tube. The present invention relates to a heat treatment method for forming a corrosion-resistant film that is effective in suppressing the generation of corrosion products in a separation / separation heater tube.

(Prior art) For heater tubes used in high-temperature and high-pressure water environments, for example, feed water heater tubes and boiling water heater tubes for boiling water reactors, austenite with excellent corrosion resistance such as SUS 304 L is used. Based stainless steel is used. However, during long-term use, it was corroded and Ni and Co in the material were eluted into the primary cooling water and brought into the reactor, where it was activated by neutron irradiation and the activated Ni and Co were activated. A clad (corrosion product) containing the like will be deposited on the pipe, the heater pipe, and the like, which poses a problem that the worker is exposed to radiation during regular inspections, which impairs safety and health.

Therefore, as one method to suppress such elution of Ni and Co and reduce the radiation dose due to these, the heater tube of the stainless steel feed water heater of the boiling water reactor with oxygen-injected high-temperature water A method and apparatus for forming an oxide film that is protective against corrosion have been proposed.
(JP-A 61-139676).

However, since this method is performed in an actual furnace, it takes a long time to form an oxide film, and it is difficult to form an optimum oxide film for suppressing metal elution due to restrictions such as processing temperature and time. is there.

(Problems to be Solved by the Invention) An object of the present invention is to provide a heater tube made of stainless steel used in a high temperature and high pressure water environment, for example, a feed water heater tube or a moisture separation heater tube of a boiling water reactor. An object of the present invention is to provide a method capable of easily forming an oxide film having an effective thickness for suppressing the elution of Ni and Co on the surface of Al in a proper thickness.

(Means for Solving the Problems) One of the inventors of the present invention has previously aimed at a Ni-based alloy heat transfer tube containing Cr or Ni, and mainly used chromium oxide on the surface to suppress the elution of Ni or Co. A heat treatment method for applying an oxide film was developed and a patent application was filed (Japanese Patent Application No. 62-212565 and Japanese Patent Application No. 62-3).
No. 16824).

After that, as a result of expanding the alloys to be studied and repeating the research, they found that the above method is also effective for a heater tube made of stainless steel, and arrived at the present invention.

Here, the gist of the present invention is "a heater tube made of stainless steel containing 12 to 20% by weight of Cr, or 12 to 20% of Cr.
A heater tube made of stainless steel containing 40% or less of Ni and Ni in an inert gas atmosphere containing 0.01 to 0.5 vol% oxygen and points A and B, B and C, and C shown in FIG. D, D and E, E and F, and F and A are heat-treated at a heating temperature and a heating time in a region surrounded by straight lines to form an oxide film mainly containing chromium oxide on the surface. Heat treatment method for stainless steel for heater tubes ”.

FIG. 1 attached shows the relationship between the heating temperature and the heating time employed in the heat treatment method according to the present invention.

That is, the heat treatment method of the present invention, a heater tube made of stainless steel in an inert gas containing a trace amount of oxygen, for example, an argon gas atmosphere containing a trace amount of oxygen, the relationship between the heating temperature and the heating time attached diagram Point A (1100 ° C, 2 minutes), B (900
℃, 2 minutes), C (800 ℃, 20 minutes), D (950 ℃, 20 minutes), E (10
(500 ℃, 5 minutes) and F (1100 ℃, 5 minutes) Heat treatment is performed at the heating temperature and heating time in the area surrounded by the straight line connecting the 6 points, and chromium oxide (Cr ₂ O ₃ ) is mainly contained on the surface. To form an oxide film.

In the present invention, the above heat treatment for forming an oxide film may be carried out separately, but by utilizing the recrystallization annealing step in the step of manufacturing the heater tube made of the stainless steel as a material. Good to do. By adjusting the conditions of recrystallization annealing to the above conditions and performing heat treatment, it is possible to perform recrystallization and oxide film formation at the same time, so it is not necessary to newly perform heat treatment for forming oxide films. .

(Operation) Hereinafter, the reason why the stainless steel to be heat treated, the heat treatment atmosphere, the heating temperature and the heating time in the present invention are limited as described above will be described in detail.

First, the object to be heat treated is a heater tube made of stainless steel containing 12 to 20% Cr, or 12 to 20% Cr and 40% Ni.
The reason for using a heater tube made of stainless steel contained below is that a heater tube made of stainless steel containing Cr or Cr and Ni in a larger amount than this does not give an oxide film by heat treatment like the method of the present invention. Both have good corrosion resistance, and the elution amount of Ni and Co is extremely small even in high-temperature water. Also, Cr
This is because stainless steel with a content of less than 12% is not used in the field intended by the present invention.

The reason for setting the heat treatment atmosphere as an inert gas atmosphere containing 0.01 to 0.5 vol% oxygen is that the oxygen potential in the atmosphere should be increased by adding a smaller amount of oxygen than in a complete inert gas atmosphere (100% inert gas). This is because an oxide film can be easily generated by increasing the temperature and performing heat treatment. But 0.01vo
When the oxygen concentration is less than 1%, it takes a long time to form an oxide film having a thickness effective for suppressing metal elution of Ni and Co. On the other hand, when the oxygen potential is increased to more than 0.5 vol%, the oxide film obtained becomes too thick, cracking and film peeling easily occur in the film, and conversely the effect of suppressing metal elution decreases. For this reason, the oxygen concentration in the inert gas atmosphere is set to 0.01 to 0.5 vol%.

The reason for limiting the heating time and the heating temperature within the range shown by the hatched lines in FIG. 1 is as follows.

When the heating time is shorter than 2 minutes indicated by the AB line and 2 to 20 minutes indicated by the BC line, the oxide film formed is thin and has a thickness effective in suppressing elution of metals such as Ni and Co in high temperature water. No oxide film was obtained, and 5 minutes indicated by the EF line
When heated for more than 5 to 20 minutes indicated by the DE line, the oxide film formed becomes too thick and cracks and film peeling are likely to occur in the film, and conversely the effect of suppressing metal elution decreases.

On the other hand, when the heating temperature is lower than 900 to 800 ° C indicated by the BC line, especially in the case of a heater tube made of austenitic stainless steel, chromium carbide is precipitated in the grain boundaries in the temperature range of 500 to 800 ° C. The reason is that a Cr-deficient layer is generated in the vicinity thereof and sensitization is likely to occur. It is possible to form an oxide film mainly composed of chromium oxide even at a low temperature of 500 ° C. or lower, but it is not practical because heating for a long time is required to obtain an oxide film having an effective thickness. Further, at a temperature higher than 1100 ° C. indicated by the AF line, the crystal grains become coarse and the mechanical properties are impaired.

If you heat-treat the heater tube made of stainless steel under the above conditions, you can effectively suppress the elution of metal on the surface, and the thickness mainly consisting of chromium oxide without cracks and 300 ~ 1500 Å An oxide film can be formed.

In the present invention, the heater tube to be heat treated is a stainless steel containing at least 12 to 20% Cr or 12 to 20% Cr and 40% or less Ni. The representative one is AISI.
Ferritic stainless steels such as Type 304L steel and 316L steel, austenitic stainless steel, 1839% Cr containing 439 steel and 13% Cr containing 410 steel.

The present invention will be further described below with reference to examples.

(Example) Using a vacuum melting furnace, alloys A and B shown in Table 1 were melted, hot forged, and hot rolled into a plate having a thickness of 7 mm.
Then, it was cold-rolled to prepare a 2.0 mm thick sample material.

After polishing this test material with emery paper (No. 800), heat treatment was performed under the heating temperature, holding time and heating atmosphere conditions shown in Table 2 to perform recrystallization annealing as well as heat treatment for forming an oxide film.

The oxide film and the metal elution amounts of Ni and Co were examined for the heat-treated test material thus obtained. The results are also shown in Table 2.

The oxide film thickness was measured by ion sputtering analysis in the plate thickness direction from the surface of the test material using IMMA (Ion Micro Mass Analyzer). The metal elution amount was measured by using the batch type autoclave (1) shown in Fig. 2. Isolate the test piece (2) in degassed pure water (3) at 215 ° C using the platinum container (4). ,Ten
I tested it for 00 hours. Then, the amount of Ni ions and the amount of Co ions eluted in pure water are measured by high frequency induction plasma emission spectroscopy (ICP).
It was measured at.

In the figure, (5) is an electric furnace heater, (6) is a thermocouple for temperature measurement, (7) is a gas inlet, (8) is a gas outlet, (9) is a pressure gauge, and (10) is Indicates a safety valve.

As is clear from Table 2, examples of the present invention which are heat-treated at the heating temperature, the heating time and the heating atmosphere specified in the present invention.
In all of the samples (Sample Nos. 1 to 6), an oxide film having an appropriate thickness is formed, and the elution amount of Ni and Co is small. In particular, with respect to Sample Nos. 2, 4, 5 and 6, the Co elution amount is 0.01 mg / or less, which is the detection limit in ICP.

On the other hand, sample Nos. 9 and 10 of the comparative example have an oxygen concentration lower than the range specified in the present invention, and sample Nos. 13 and 14 have a low heating temperature. Therefore, the oxide film obtained is too thin and the effect of suppressing metal elution is small. On the other hand, sample Nos. 7 and 8 have a high oxygen concentration, and sample Nos. 11 and 12 have a long heating time, so that the coating becomes thicker than necessary and cracks occur in places, resulting in crevice corrosion. Since the material is corroded, the effect of suppressing metal elution is small.

(Effect of the Invention) As described above, according to the method of the present invention, it is possible to relatively easily form an oxide film mainly composed of chromium oxide having an excellent metal elution suppressing effect with an appropriate thickness. Therefore, for example, when the method of the present invention is applied to a feed water heater tube or a moisture separation heater tube of a boiling water reactor, a tube having a great effect of suppressing metal elution can be produced. Further, if this heat treatment is performed in the recrystallization annealing process, it is not necessary to add a new heat treatment process.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the heating temperature and the heating time employed in the heat treatment method according to the present invention, and the range indicated by the diagonal lines is the range of the present invention. FIG. 2 is a schematic sectional view showing an autoclave for metal elution test in high temperature water used in Examples.

Claims (1)

[Claims] 1. A heater tube made of stainless steel containing 12 to 20% Cr by weight, or a stainless steel heater tube containing 12 to 20% Cr and 40% or less Ni, ~ 0.
In an inert gas atmosphere containing 5 vol% oxygen, points A and B, B and C, C and D, D and E, E and F, F shown in FIG.
And a heat treatment at a heating temperature and a heating time within a region surrounded by a straight line connecting A to form an oxide film mainly composed of chromium oxide on the surface of the stainless steel for a heater tube.