JPS6059983B2 - Heat treatment method for zirconium alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel method for heat treatment of zirconium alloys, and particularly to a method for heat treatment of fuel cladding tubes and channel boxes made of zirconium alloys for boiling water nuclear reactors.

Zirconium alloys have excellent corrosion resistance and a small neutron absorption cross section, so they are used for fuel rod cladding tubes and fuel assembly channel boxes, which are internal structures of nuclear reactor plants. The zirconium alloy used for the above purpose is Zircaloy 2 (Sn: about 1.5 in zirconium).
(1-2)%, Cr: about 0.1 (0.05-0.2)%
, Fe: about 0.1 (0.05-0.2)%, Ni: about 0
．． 05 ((0.01-0.2)% added) and Zircaloy 4 (Sn: about 1.5 (1
~2)%, Fe: about 0.2 (0.1-0.3)%, Cr
: about 0.1 (0.05 to 0.2)%). However, even with zirconium alloys that have excellent corrosion resistance (oxidation resistance), oxidation progresses when exposed to high-temperature, high-pressure water or steam in a furnace for a long time, and the heat transfer coefficient increases due to the thick oxide film. In some cases, nuclear reactor operation may be disrupted due to a drop in nuclear power and localized overheating.

Conventionally, as a heat treatment for suppressing the progress of oxidation, a method is known in which a zirconium alloy member is radio-frequency heated to a temperature higher than that at which a β phase is generated and then rapidly cooled (hereinafter referred to as β quench). The above-mentioned zirconium alloy has a single α phase at temperatures below 830°C, and the temperature range at which the β phase forms is 830 to 980°C, where the α+β phase forms, and a single β phase forms at temperatures higher than 980°C. be done. However, if the cooling rate during β-quenching is too rapid, the zirconium alloy member will deform. To prevent such deformation, a relatively slow (
β quenching is performed under cooling conditions (20 to 200° C./s). As a result of measuring the concentration distribution of Sn, Cr, and Fe on the surface of a channel box made of Zircaloy 4 material β-quenched at a relatively slow rate using an X-ray microanalyzer, we found that the grain boundaries Relatively coarse intermetallic compounds were precipitated, and the Sn concentration near the grain boundaries was reduced.

Nitrogen gas contained in zirconium alloys has an adverse effect on corrosion resistance, but corrosion is suppressed by the addition of Sn. Therefore, as mentioned above, the decrease in Sn concentration near the grain boundaries is
It was found that this resulted in a decrease in corrosion resistance. Furthermore, it was found that sufficient corrosion resistance could not be obtained by simply performing heat treatment in the α region after β quenching.

An object of the present invention is to provide a method for heat treating a zirconium alloy member with high corrosion resistance.

In the present invention, a zirconium alloy containing tin, chromium, and iron is rapidly cooled from a temperature range that produces a β phase (β quench), and then heat treated in an α phase region while applying tensile stress.
A heat treatment method for zirconium alloys characterized by the formation of precipitates. The temperature at which the β phase of Zircaloy alloy is formed is 830'C or higher, and as shown in the above temperature, 83
A quenched β-quenched zirconium alloy member from a single phase of α + β phase at temperatures above 0°C or β phase at temperatures higher than 98 (s) is reheated to the α-phase temperature region, and supersaturated solid solution is formed in the Zr matrix. The purpose of this method is to finely precipitate Fe and Cr in intermetallic compounds and within crystal grains and grain boundaries, and to make the Sn concentration distribution more uniform, thereby restoring ductility and improving corrosion resistance. The cooling rate from the β phase region is 20
0 to 1000°C/sec is preferred. Example Fig. 2a shows the maximum heating temperature of the reactor fuel assembly channel box made of Zircaloy 4 at 960°C (holding time: 1 minute 3 (8,960'C~70
Average cooling rate between 0'C: After β-quenching under the conditions of 700°C/sec, a member with a larger coefficient of thermal expansion is fitted into the channel box, and stress is applied by the thermal expansion. Sn, Cr, and F on the surface of sample A that was reheated and heat-treated at a temperature of ℃ for a predetermined time.
The concentration distribution of e is shown.

Figure 2b shows Sn, Cr, and
The concentration distribution of Fe is shown. Figure 2b shows that in the state of β-quenching, the concentrations of Sn, Cr, and Fe are almost uniform, and in contrast to the Zr matrix in which these elements are supersaturated as solid solutions, after β-quenching. , while applying stress 6
It can be seen that by reheating to 00° C., the Sn concentration distribution is uniform, and the Fe and Cr concentrations exhibit high peak values, and the peak positions are finely distributed within the grains and at the grain boundaries. From this, it can be seen that after β-quenching, which involves cooling sufficiently rapidly to make the Sn concentration uniform, reheating in the α-phase temperature region while applying stress will cause fine precipitation of intermetallic compounds of Cr, Fe, and Zr. It can be seen that it has the effect of making the Sn concentration distribution uniform. Further, the ductility of this material was approximately the same as that of Zircaloy 4 rolled material which was not subjected to β-quenching.

In this way, when heat treating long tubes such as fuel cladding tubes for nuclear reactors or channel boxes, it is most preferable to apply stress in the direction in which the tubes are blinded.

FIG. 3 shows the sample A and the Sn, Cr shown in FIG.
, a test piece cut out from a channel box obtained by conventional heat treatment having a concentration distribution of Fe.
20°C in water vapor at 75°C and pressure 105k9f/Cl.
It shows the weight increase due to oxidation (corrosion weight increase) after holding for 5 m and 10 m.

From FIG. 3, it can be seen that the test piece subjected to the heat treatment according to the present invention exhibits clearly less corrosion weight gain (higher corrosion resistance) than the test piece obtained by the conventional heat treatment method.

Although the above example shows an example of the α+β phase region, it is clear from experiments that similar results can be obtained when cooling is similarly performed from the β phase single phase region. According to the present invention, a zirconium alloy member having high corrosion resistance can be obtained, and a fuel cladding tube for a nuclear reactor or a fuel assembly channel box for a nuclear reactor using the member has high corrosion resistance and a long life.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the concentration distribution of Sn, Cr, and Fe on the surface of a channel box β-quenched by the conventional method, and FIG. , Cr, and Fe. FIG. 2b is a diagram showing the concentration distribution of Sn, Cr, and Fe in the β-quenched state.
The figure is a diagram showing the corrosion resistance test results of channel boxes obtained by the conventional method and the heat treatment method of the present invention.

Claims (1)

[Claims] 1. A zirconium alloy containing tin, chromium and iron,
The zirconium alloy is rapidly cooled from a temperature range where the β phase of the alloy is formed, and then heated in the α phase region of the alloy while applying tensile stress to form precipitates in the alloy. Heat treatment method. 2. The method of heat treating a zirconium alloy according to claim 1, wherein the zirconium alloy constitutes a fuel cladding tube for a nuclear reactor. 3. The method of heat treating a zirconium alloy according to claim 1, wherein the zirconium alloy constitutes a nuclear reactor fuel assembly channel box.