JP2003149366A - Wrapper tube with welded joint, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wrapper tube made of ferritic steel and its manufacturing method that can realize weld structure without the formation of a δ-ferrite phase at a weld part to a handling head or an entrance nozzle made of austenitic stainless steel. SOLUTION: In this wrapper tube 20 made of ferritic steel for a fast breeder reactor fuel assembly, the handling head 22 and the entrance nozzle 24 made of austenitic stainless steel are jointed to both end parts, and short tubular welded joints 28 made of austenitic stainless steel are welded to both ends of a wrapper tube body 26 and put in the eliminated state of the δ-ferrite phase of a weld part by normalizing. Such a wrapper tube with the welded joints can be manufactured by welding to joint a ferritic steel round pipe and an austenitic stainless steel round pipe, and after drawing them out in hexagonal tube shape, eliminating the δ-ferrite phase formed at the weld part by normalizing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic steel wrapper tube for a fast reactor fuel assembly having a structure in which an austenitic stainless steel handling head and an entrance nozzle are joined and welded at both ends, and a method for manufacturing the same. Is.

[0002]

2. Description of the Related Art A fuel assembly for a fast breeder reactor is shown in FIG.
As shown in FIG. 2, a fuel element 12 constituted by bundling a large number of fuel pins around which a spacer wire is wound is housed in a trumpet tube 10 which is a hexagonal tube.
The upper end port of 0 is joined with an upper end pipe plug called a handling head 14 by welding means, and the lower end port of the trumpet pipe 10 is joined with a lower end pipe plug called an entrance nozzle 16 by welding means. It is a structure. Reference numeral 18 represents a spacer pad.

In the conventional fuel assembly for a fast breeder reactor, the trumpet tube 10 and the upper and lower end tube plugs (the handling head 14 and the entrance nozzle 16) are all made of austenitic stainless steel. .

However, when austenitic stainless steel is used for the trumpet tube, there is a problem that the trumpet tube is susceptible to considerable deformation due to swelling (volume expansion) when it is irradiated with fast neutrons in the furnace. . Therefore, ferritic steel or the like has been developed as a swelling-resistant material, and it has become necessary to change the material from austenitic stainless steel to ferritic steel or the like in order to improve the swelling resistance of the trumpet tube.

On the other hand, as an in-core material for a fast breeder reactor, austenitic stainless steel, which is relatively excellent in corrosion resistance in a liquid sodium environment and resistance to neutron irradiation embrittlement, is used. The plate is also made of austenitic stainless steel. Therefore, if the entrance nozzle is made of ferritic steel, there is a concern that liquid sodium may leak from the fitting portion between the core support plate and the entrance nozzle due to the difference in thermal expansion. Therefore, the entrance nozzle is preferably made of austenitic stainless steel, like the core support plate.

For these reasons, when ferritic steel is applied to the trumpet tube material, it is necessary to perform dissimilar material welding between the ferritic steel which is the trumpet tube material and the austenitic stainless steel which is the entrance nozzle material.

[0007]

When welding a dissimilar material between a trumpet tube made of ferritic steel and an entrance nozzle and a handling head made of austenitic stainless steel, for example, when TIG welding is performed, δ ferrite showing brittleness at the welded portion. It is inevitable that phases will form. Even when electron beam welding, which has a smaller heat input than TIG welding, is selected as the welding method, the amount of δ ferrite produced can be reduced to a considerable extent as compared with TIG welding, but it cannot be eliminated. It is generally known that a material in which a δ ferrite phase is generated causes a decrease in toughness during a period of use.

Therefore, it has been attempted to reduce the produced δ ferrite phase by performing tempering after welding.
However, the heat treatment after assembling the fuel assembly requires a large-scale heat treatment furnace, and even if the heat treatment is performed in such a state, the effect of reducing the δ ferrite phase can hardly be expected.

Further, since the heat-affected zone of the ferritic steel trumpet tube is hardened by welding, post-weld heat treatment is indispensable for coping with it, but as described above, it is necessary for heat treatment after fuel assembly assembly. A large-scale heat treatment furnace is required.

Further, since bending occurs due to the difference in thermal expansion during welding of dissimilar materials, it is necessary to correct the bending, but it is extremely difficult to correct the bending after assembling the fuel assembly. Further, even if the bend can be corrected, it cannot be completely corrected, which causes a cost increase. Therefore, it has been considered unsuitable to directly weld the austenitic stainless steel entrance nozzle or the handling head to the ferritic steel wrapper pipe.

An object of the present invention is to provide a ferritic steel trumpet tube capable of realizing a welded structure in which a delta ferrite phase is not formed at a welded portion with a handling head or entrance nozzle made of austenitic stainless steel, and a method for producing the same. Is.

[0012]

SUMMARY OF THE INVENTION The present invention is a ferritic steel wrapper tube for a fast reactor fuel assembly in which an austenitic stainless steel handling head and an entrance nozzle are joined and welded at both ends. Made of austenitic stainless steel, welded to both ends of the main body of the made trumpet tube, and is a trumpet tube with welded joint in which the δ ferrite phase of the welded part disappears by normalizing. .

Such a trumpet tube with a welded joint is manufactured by first joining and welding a ferritic steel round tube and an austenitic stainless steel round tube in a state of round tubes, drawing them into a hexagonal tube, and then welding by normalizing. It can be manufactured by a procedure of eliminating the δ ferrite phase generated in the part.

Here, the chemical composition of the ferritic steel round tube is C
= 0.09 to 0.15% by weight, Si ≤ 0.10% by weight,
Mn = 0.40 to 0.80% by weight, P ≦ 0.030% by weight, S ≦ 0.030% by weight, Ni = 0.20 to 0.60
% By weight, Cr = 10.0 to 12.0% by weight, Mo = 0.
30 to 0.70% by weight, W = 1.70 to 2.30% by weight, V = 0.15 to 0.25% by weight, Nb = 0.020
˜0.080% by weight and N = 0.030 to 0.070% by weight are preferable. Normalizing is 1025 to 107
It is carried out at 5 ° C, more preferably 1045 to 1050 ° C.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic explanatory view showing an example of a trumpet tube according to the present invention and a fuel assembly for a fast breeder reactor, which is assembled using the trumpet tube.
Indicates the state after assembly. In this fuel assembly for a fast breeder reactor, a handling head 22 and an entrance nozzle 24 made of austenitic stainless steel are joined and welded to the upper and lower ends of a trumpet tube 20 made of ferritic steel (mostly made of ferritic steel). Structure. In the present invention, in the trumpet tube 20, the short tubular weld joints 28 made of austenitic stainless steel are welded to both ends of the trumpet tube body 26 made of ferritic steel, respectively, and the δ ferrite phase generated at the welded portion is smoothed. It is in a state of disappearance due to the processing, and this is a characteristic point. With the structure having such a welded joint, in the fuel assembly assembling process, as in the prior art, the austenitic stainless steels (the handling head 22 and the short tubular welded joint 28, and the short tubular welded joint 28 are joined together. And welding can be performed at the entrance nozzle 24).

For example, the chemical composition is C = 0.09 to 0.1.
5% by weight, Si ≦ 0.10% by weight, Mn = 0.40
0.80% by weight, P ≦ 0.030% by weight, S ≦ 0.03
0% by weight, Ni = 0.20 to 0.60% by weight, Cr = 1
0.0-12.0 wt%, Mo = 0.30-0.70 wt%, W = 1.70-2.30 wt%, V = 0.15
A ferritic steel round tube having 0.25% by weight, Nb = 0.020 to 0.080% by weight, and N = 0.030 to 0.070% by weight is used. The ferritic steel round tube and austenitic stainless steel round tube are joined and welded, drawn into a hexagonal tube, and then subjected to a normalizing treatment at 1025 ° C. to 1075 ° C. to eliminate the δ ferrite phase generated in the welded portion. Can be made.

An example of the manufacturing process is shown in FIG. Round pipe made of ferritic steel and austenitic stainless steel (eg S
A round tube made of US316 steel) is joined and welded in a state where the round tubes are joined together, and then softening annealing is performed. For the welding and annealing performed here, general methods that have been conventionally used may be adopted. Next, the round tube welded and welded is hexagonally drawn and subjected to normalization / tempering. After that, the dimensional adjustment is performed by hexagonal drawing at a cold working rate of 2% or less, the bend is corrected by a hydraulic press, the outer surface is ground, and the length is cut to manufacture a trumpet tube with a welded joint. Then, chamfering, degreasing and dimension inspection are performed. Since no heat treatment is performed after normalizing and tempering, the mechanical properties of the trumpet tube are maintained.

In the above manufacturing process, the normalizing process must be performed after welding the trumpet tube body and the short tube.
Welded austenitic stainless steel short pipe and ferritic steel trumpet tube main body that had been subjected to normalization treatment to ensure the prescribed mechanical properties, and then annealed it. This is because the phase does not disappear and remains as it is.

Welding is also possible between hexagonal tubes, but in the case of hexagonal tubes, it is difficult to groove the hexagonal tube butts and to weld the hexagonal R part. Since the welding is relatively easy, it is preferable to perform the welding in the state of the round tubes as described above. Further, in the case of welding round pipes together, since hexagonal drawing is performed after welding, there is also an advantage that the dimensional accuracy such as straightness and twist is excellent. By the way, in welding of hexagonal tubes, bending occurs due to heat input during welding, and there is also a problem that bending cannot be completely corrected by dimensional adjustment by drawing processing with a working rate of 2% or less and hydraulic correction.

The normalizing temperature is 1025 ° C. or higher in consideration of the temperature range in which the δ ferrite phase disappears and the temperature range in which mechanical properties such as tensile strength and impact characteristics required for ferritic steel can be secured. It is set to 1075 ° C (including both end temperatures). If higher than this temperature range, for example 11
At a temperature of 00 ° C. or higher, the δ ferrite phase is stably present, so the temperature must be 1100 ° C. or lower. In the temperature range over 1075 ° C. (1100 ° C. or less), the creep rupture strength and the tensile strength show high values, but the toughness deteriorates due to the growth of crystal grains. Such a temperature range is not suitable for a trumpet tube that emphasizes toughness. Also, 102
Below 5 ° C, the required mechanical properties cannot be satisfied.
For these reasons, the normalizing temperature is 10 as above.
The temperature is preferably 25 ° C to 1075 ° C. In tempering, the δ ferrite phase disappeared by normalization does not occur again by tempering, so that the required mechanical properties and A
Considering one transformation point, it is preferable to carry out in the range of 700 ° C to 750 ° C.

[0021]

[Example] Ferrite / martensite steel was melted in a vacuum melting furnace to obtain an ingot. After hot forging this into a billet by machining, it was hot extruded to produce a blank tube. A round tube was obtained by cold working this raw tube. On the other hand,
Austenitic stainless steel is commercially available SUS316
Was billeted and hot extruded, and then rounded into a round tube. Table 1 shows the chemical composition of the ferritic steel pipe and the SUS316 steel pipe. The amount of each component is shown in% by weight.

[0022]

[Table 1]

These round tubes were TIG-welded to each other using an Inconel type filler material, subjected to a heat treatment after welding at 800 ° C. for 30 minutes, and then drawn into hexagonal tubes. As the final heat treatment, normalize and temper, 1050 ° C x 40 minutes +720
It was carried out at 40 ° C for 40 minutes. After that, dimensional adjustment / bending correction (cold workability of 2% or less) of the trumpet tube, outer surface polishing, and cutting to a fixed length were performed to manufacture a trumpet tube provided with an austenitic stainless steel welded joint.

After the appearance inspection was performed on the manufactured trumpet tube with welded joint and it was confirmed that there was no welding defect, the squareness and the twist were measured, but there was no problem.
Table 2 shows the measurement results. It should be noted that in Table 2, planes A to F represent the six sides of the hexagonal tube.

[0025]

[Table 2]

Next, the structure of the cross section of the welded portion was observed. As a result, in the method of the present invention, welding defects and cracks did not occur at all, and a sound weld was formed. Also, FIG.
As shown in Fig. 5, δ
No formation of ferrite phase was observed. On the other hand, in the prior art, the amount of δ ferrite was 12.7% in the case of TIG welding and 0.8% in the case of EB (electron beam) welding.

[0027]

As described above, according to the present invention, short tubular welded joints made of austenitic stainless steel are pre-welded to both ends of a ferritic steel trumpet tube body, and δ-ferrite of the welded portion is obtained by normalizing. Since it is a trumpet tube with welded joints in which the phases have disappeared, a fast reactor consisting of an austenitic stainless steel handling head, a ferritic steel trumpet tube, and an austenitic stainless steel entrance nozzle using normal welding technology. The fuel assembly can be assembled. Moreover, when joining and welding the handling head and entrance nozzle made of austenitic stainless steel to both ends, it is possible to prevent deterioration of toughness.

Further, according to the present invention, a ferritic steel round tube and an austenitic stainless steel round tube are first joined and welded in a state where the round tubes are joined together, drawn into a hexagonal tube, and then formed in the welded portion by normalizing. Since it is a method for manufacturing a trumpet tube with a welded joint so as to eliminate the ferrite phase, it can be manufactured easily with small bending, with high dimensional accuracy.

According to the present invention, it becomes possible to use a ferritic steel with a small swelling as a trumpet tube for a fast reactor fuel assembly, and as a result, the fuel assembly structure can be applied to an actual machine without changing the structure. It will be possible.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a fuel assembly using a trumpet tube with a welded joint according to the present invention.

FIG. 2 is a flowchart showing an example of the manufacturing process.

FIG. 3 is an explanatory view of an observation result showing the presence or absence of a δ ferrite phase in the related art and the present invention.

FIG. 4 is an explanatory view showing an example of a fuel assembly for a fast breeder reactor.

[Explanation of symbols]

20 trumpet tube 22 Handling head 24 entrance nozzle 26 Trumpet tube body 28 Short tubular welded joint

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/00 302 C22C 38/48 38/48 G21C 21/02 N G21C 3/30 B23K 103: 02 21 / 02 G21C 3/30 GDFH // B23K 103: 02 V (72) Inventor Shigeharu Ukai 4002 Narita-cho, Oarai-cho, Higashi-Ibaraki-gun, Ibaraki-ken 4002 Nuclear Fuel Cycle Development Organization Oarai Engineering Center (72) Inventor Takanari Okuda Shimonoseki City, Yamaguchi Prefecture 13-1 Minatomachi Shinko Special Steel Tube Co., Ltd. (72) Inventor Yuyuki Fujiwara 1-5-8 Wakihama Kaigandori, Chuo-ku, Kobe-shi, Hyogo Kobelco Research Institute F Term (reference) 4E001 AA03 CA03 CC03 DG04 4K042 AA06 BA02 BA06 BA11 CA16 DA04 DC02

Claims (4)

[Claims] 1. A ferritic steel trumpet tube for a fast reactor fuel assembly, in which a handling head made of austenitic stainless steel and an entrance nozzle are joined and welded at both ends. , A short tubular weld joint made of austenitic stainless steel is welded,
A trumpet tube with a welded joint, characterized in that the δ ferrite phase in the weld has disappeared by normalizing. 2. A method for producing a trumpet tube with a welded joint according to claim 1, wherein a ferritic steel round tube and an austenitic stainless steel round tube are joined and welded, drawn into a hexagonal tube, and then normalized. A method for manufacturing a trumpet tube with a welded joint, characterized in that the δ ferrite phase generated in the welded portion is eliminated. 3. The ferritic steel round tube has a chemical composition
C = 0.09 to 0.15% by weight, Si ≦ 0.10% by weight, Mn = 0.40 to 0.80% by weight, P ≦ 0.030
% By weight, S ≦ 0.030% by weight, Ni = 0.20-0.
60% by weight, Cr = 10.0 to 12.0% by weight, Mo =
0.30 to 0.70% by weight, W = 1.70 to 2.30% by weight, V = 0.15 to 0.25% by weight, Nb = 0.02
The method for producing a trumpet tube with a welded joint according to claim 2, wherein 0 to 0.080% by weight and N = 0.030 to 0.070% by weight. 4. The method for manufacturing a trumpet tube with a welded joint according to claim 3, wherein the normalizing is performed at 1025 ° C. to 1075 ° C.