JPH07173587A - Production of zirconium alloy welded member - Google Patents

Abstract

PURPOSE:To produce a Zr allay welded member free from deterioration of the corrosion resistance of the weld zone by subjecting an (alpha+beta) type Zr alloy to welding and thereafter executing specified cold working and heat treatment. CONSTITUTION:An (alpha+beta) type Zr allay contg. about 1 to 20wt.% Nb as a stabilizing element is subjected to welding. After that, this welded member is subjected to cold working at 5 to 30% draft and is next subjected to heat treatment at 520 to 600 deg.C for about 30min to 10hr. By this treatment, residual strains are relaxed without causing defects on the weld zone, and a (beta-Zr phase formed by the welding is transformed into an alpha-Zr phase and a (beta-Nb phase good in corrosion resistance. Thus, the corrosion resistance of the weld zone is improved, by which the (alpha+beta) type Zr alloy welded member free from deterioration in the corrosion resistance of the weld zone can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a zirconium alloy welded member in which the corrosion resistance of the welded portion does not deteriorate.

[0002]

2. Description of the Related Art Core members such as combustion channels and fuel cladding used in nuclear reactors (light water reactors) have a small neutron absorption cross section, good corrosion resistance to pure water or water vapor under high temperature and high pressure, and It is made of a zirconium alloy with suitable strength and ductility. Conventionally, zirconium contains some elements (F
[alpha] -type zirconium alloys called Zircaloy (for example, Zircaloy-2 and Zircaloy-4) having improved corrosion resistance by adding e, Cr, and Ni) have been used.

In recent years, in order to reduce the nuclear power generation cost by the light water reactor, high combustion is planned, and under such conditions, the α type zirconium alloy has insufficient mechanical strength and is deformed by creep phenomenon or the like. There was a risk that the operation of the reactor would be hindered. Therefore, the core member is (α +
Attempts have been made to fabricate β) type zirconium alloys. The (α + β) type zirconium alloy has excellent corrosion resistance, a small neutron absorption cross section, and excellent workability.
Moreover, the mechanical strength is higher than that of the α-type zirconium alloy. A typical example of such an (α + β) -type zirconium alloy is a Zr-2.5% Nb alloy in which Zr contains 2.6% by weight of Nb which is a β-stabilizing element.

[0004]

[Problems to be Solved by the Invention] However, (α +
When the β) type zirconium alloy is subjected to welding, the corrosion resistance of the welded portion deteriorates, the oxidation progresses more rapidly than the matrix, and a thick oxide film is formed. As a result, the wall thickness of the welded portion is reduced and the strength is reduced. The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a (α + β) type zirconium alloy welded member in which the corrosion resistance of the welded portion does not deteriorate.

[0005]

According to the method for producing a zirconium alloy welded member of the present invention, after performing welding on an (α + β) type zirconium alloy, cold working is performed at a working rate of 5 to 30%. Then, heat treatment at 520 to 600 ° C. is performed. The (α + β) -type zirconium alloy to be manufactured according to the present invention is not limited to the Zr-2.5% Nb alloy described above, and the Nb content of which the structure at room temperature is the (α + β) phase is 1 Any zirconium alloy of up to 20 wt% can be applied.

[0006]

[Operation] At the welded part of the (α + β) type zirconium alloy,
Along with heating and cooling of welding, residual strain and β-Zr phase are generated, which deteriorates corrosion resistance. That is, (α +
When the β) type zirconium alloy is subjected to welding, the welded portion has a microstructure that is once melted and then solidified, in other words, a needle-shaped structure. The morphology of this acicular structure depends on the cooling rate after welding. When the cooling rate is fast (about 80 ° C./second or more), Nb is dissolved in a supersaturated solid solution, and the martensite structure has a very large residual strain. On the other hand, when the cooling rate is slow, the residual strain is reduced as compared with the martensite structure, but α-Zr phase in which Nb is dissolved in about 0.5% and β in which Nb is dissolved in about 20% are dissolved. -A needle-like structure is formed by mixing with the Zr phase. The cooling rate after welding is not uniquely determined, but in any case, a needle-like structure with a large residual strain is formed, and β-
The Zr phase tends to form.

According to the present invention, since cold working is performed after welding and then phase transformation heat treatment is performed, residual strain is relaxed, and β-Zr phase is transformed into α-Zr phase and β-Nb phase, that is, corrosion resistance. Since it can be transformed into a good phase,
The corrosion resistance of the welded portion can be improved. At this time, when the cold working ratio is less than 5%, the working strain is too small, the relaxation of residual strain is difficult to occur, the phase transformation is insufficient, and the recovery of corrosion resistance is small. On the other hand, since the needle-like structure generated in the welded portion has poor cold workability, if it exceeds 30%, cracks are likely to occur during processing, which makes processing difficult. If the heat treatment temperature is lower than 520 ° C, the phase transformation is insufficient and the corrosion resistance is insufficient. On the other hand, 6
If the temperature exceeds 00 ° C, the residual strain can be relaxed,
The formation of the β-Zr phase is promoted, and on the contrary, the corrosion resistance decreases. The heat treatment time varies depending on the size of the member and the degree of cold working, but is usually about 30 minutes to 10 hours.

[0008]

EXAMPLE An (α + β) type Zr-2.5 wt% Nb alloy was melted, and the obtained ingot was forged and rolled to obtain a plate thickness of 2.5.
A plate material having a width of 150 mm and a length of 200 mm was sampled from the plate. After abutting these plate pieces, electron beam welding was performed to obtain a sample material.

For this test material, the cold
Cold rolling with maximum working rate (sheet thickness reduction rate) of 40% under working conditions
Then, after holding for 5 hours at various temperatures shown in the same table, let it cool.
did. A sample was taken from the test material that had been treated in this way.
400 ℃, 105kgf / cm ² Corrosion test in water vapor of
And measure the oxide film thickness (μm) after holding for 72 hours
It was Corrosion resistance is evaluated by oxide film thickness, and oxide film thickness
If the size is less than 3 μm, the level is 1;
With Bell 1, it was determined that the corrosion resistance was good. Those conclusions
The results are shown in Tables 1 and 2 together with the presence or absence of defects during cold working.
You The number of samples is two, and the data in the table are average values.
is there.

[0010]

[Table 1]

[0011]

[Table 2]

From Tables 1 and 2, it can be seen that the samples according to the Examples have no defects during cold working and have good corrosion resistance. In particular, when the cold working ratio is 10% or more and the heat treatment temperature is 550 to 600 ° C., the oxide film thickness is less than 2 μm, and the corrosion resistance is extremely good. Further, from the same table, data at a heat treatment temperature of 550 ° C. was extracted to investigate the relationship between the cold working rate and the oxide film thickness. The result is shown in FIG. From this, the lower limit of the cold working rate of the present invention is 5%,
Corrosion resistance is greatly improved by preferably 10% or more, 40%
You can see that it doesn't change. However, if it exceeds 30%, there is a problem in workability.

Furthermore, according to the table, the cold working rate is 10%.
Data was extracted and the relationship between the heat treatment temperature and the oxide film thickness was investigated. The result is shown in FIG. From this, it is understood that an excellent effect of improving corrosion resistance can be obtained at 520 ° C., preferably 550 ° C. or more and 600 ° C. or less, which is the range of the present invention.

[0014]

As described above, according to the method for manufacturing a zirconium alloy welded member of the present invention, after welding the (α + β) type zirconium alloy, the cold working is performed at a working rate of 5 to 30%. Since it is performed and then heat treatment at 520 to 600 ° C. is performed, residual strain of the welded portion can be relaxed, and β-Zr phase can be transformed into α-Zr phase and β-Nb phase. Can be significantly improved.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the cold workability and the corrosion resistance of (α + β) type zirconium alloy welded members.

FIG. 2 is a graph showing a relationship between heat treatment temperature and corrosion resistance of a (α + β) type zirconium alloy welded member.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuhiro Abe 2222 -1 Kobe Steel Co., Ltd. Kakogawa Research Area

Claims (1)

[Claims] 1. Zirconium which is characterized by performing a welding process on an (α + β) type zirconium alloy, then performing a cold working at a working rate of 5 to 30%, and then performing a heat treatment at 520 to 600 ° C. A method for manufacturing an alloy welded member.