EP1583849A2

EP1583849A2 - Method of producing a zirconium alloy semi-finished product for the production of an elongated product and use thereof

Info

Publication number: EP1583849A2
Application number: EP04700467A
Authority: EP
Inventors: Pierre Barberis; Noel Rizzi; Xavier Robbe
Original assignee: Compagnie Europeenne du Zirconium Cezus SA
Current assignee: Compagnie Europeenne du Zirconium Cezus SA
Priority date: 2003-01-13
Filing date: 2004-01-07
Publication date: 2005-10-12
Also published as: FR2849866A1; RU2337176C2; RU2005125716A; US20060090821A1; WO2004072317A8; US7763132B2; WO2004072317B1; KR20050089881A; CN1735706A; FR2849866B1; JP2006519310A; WO2004072317A1

Abstract

The invention relates to a method of producing a zirconium alloy semi-finished product for the production of an elongated product and to the use thereof. According to the invention, a large ingot (1) is produced by means of zirconium alloy casting and, subsequently, the ingot is forged in order to produce the semi-finished product (3) in two steps. The first ingot (1) forging step is performed at a temperature at which the zirconium alloy is in a state comprising crystalline phases A and B, for example, said forging temperature can be between 850 DEG C and 950 DEG C.

Description

The invention relates to a process for the production of a zirconium alloy semi-product intended for the production of a long product. long product used for the production of fuel assembly elements. Fuel assemblies for nuclear reactors cooled by light water, for example nuclear reactors cooled by pressurized water (PWR) and nuclear reactors cooled by boiling water (BWR) or fuel ^" of CANDU reactors include elements consisting of a zirconium alloy having the property of having a low neutron absorption in the core of the nuclear reactor.

In the case of assemblies for nuclear reactors of the PWR type, the cladding tubes of the fuel rods and the plates used for the manufacture of the spacer grids of the fuel assembly can be made of zirconium alloy, in particular in zirconium alloy containing tin, iron, chromium and possibly nickel such as the Zircaloy 2 or Zircaloy 4 alloys. The same is true for the plugs which close the cladding tubes at their two ends. Other alloys such as the alloy known under the trade name M5 essentially containing zirconium and niobium are also used for the manufacture of fuel assembly elements in the form of flat products or long, solid or tubular. In general, the zirconium alloys used for the manufacture of elements for fuel assemblies contain at least 97% of zirconium by weight, the rest of the composition which represents at most 3% by weight, with the exception of impurities due to the development of alloys, which may consist of different elements and, in particular, iron, tin or niobium. The zirconium alloys meeting these conditions relating to their composition may be, depending on the temperature and the heat treatments they have undergone, in one or other of the two allotropic forms. ques zirconium, that is to say in alpha phase which is the stable phase at low temperature of zirconium, with compact hexagonal structure or in beta phase which is the stable phase at high temperature with cubic structure. In certain temperature zones or at the end of certain treatments, zirconium alloys, such as the technical alloys used for the manufacture of fuel assembly elements defined above may have a mixed alpha + beta structure.

The production of tubular zirconium alloy products is generally carried out by extrusion of a billet which is itself obtained from an ingot by forming and possibly machining operations. The development of long solid products (bars) is generally carried out by hot rolling, then cold hammering of semi-finished products obtained from an ingot.

Usually, an ingot of large dimensions is cast, for example having a diameter between 400 mm and 700 mm and generally between 600 mm and 660 mm. The ingot then undergoes forging operations in a temperature range in which it can be in the α, β or α + β phase (EP-0.085.552 and US-5,674,330). The ingot is forged in β phase at a temperature between 1000 ° C and 1100 ° C and generally around 1050 ° C in the case of Zircaloy 4, to obtain an intermediate product such as a bar or a product with a square section. or octagonal whose diameter of the cross section (or the diameter of the circle circumscribed in the cross section) is between 250 mm and 400 mm. For example, in the case of an octagonal section, this can have a diagonal with a length of the order of 350 mm which corresponds to the diameter of the circumscribed circle.

The intermediate product is then forged, in the α phase, at a temperature between 700 ° C and 800 ° C, for example, typically at 750 ° C until a bar having a diameter of 100 mm is obtained. at 250 mm (and typically a diameter of 205 mm).

Then quenching from the β phase (typically from a temperature between 1000 ° C and 1150 ° C), either the bar as obtained in the previous forging phase, or a block consisting of a section of cut bar, or a billet formed by a block drilled in its axial direction.

Finally, to obtain a tubular product, an billet is extruded which can either be the quenched billet obtained in the previous phase, or a billet machined from a quenched bar obtained during the previous step of manufacturing process.

To obtain a long solid product, hot rolling is carried out from a hardened bar.

In all cases, prior to the extrusion operation to obtain the final tubular product, or to the hot rolling operation to obtain a small diameter bar, a semi-finished product is produced in the form of a bar, block or billet by an elaboration process comprising a first stage of forging in β phase of the starting ingot and a second stage of forging in phase α of the intermediate product obtained at the following the first forging step in β phase.

The known transformation process which has just been described comprises a first stage of forging in the β phase at a high temperature of between 1000 ° C. and 1100 ° C. After this first forging step, the intermediate product obtained is cooled at least to the forging temperature in the α phase and generally to an ambient temperature, because the second forging step in the α phase is not carried out. immediately after the first forging step in β phase.

Forging the ingot at very high temperatures is an expensive and delicate operation. Furthermore, during the heating of the ingot to bring it to a temperature of 1000 ° C. to 1100 ° C. before the first forging step, the intermediate ingot can absorb hydrogen in contact with humid air or water, hydrogen fixing in the material in the form of hydrides.

In general, the presence of hydrides in the material in the form of large precipitates is harmful with regard to the cold formability and the corrosion resistance of the products.

The object of the invention is to propose a process for manufacturing a semi-finished product made of zirconium alloy containing. weight at least 97% of zirconium, intended for the production of at least one long product, in which an ingot of large dimensions is produced, by casting the zirconium alloy, then the semi-product intended to be subjected to forming to obtain the product long, by forging the ingot of large dimensions in two stages, this process making it possible to simplify and make less expensive the manufacture of the long product and to limit to low levels the presence of hydrides in the semi-finished product and therefore in the final long product.

For this purpose, the first stage of forging ingot of large dimensions is carried out at a temperature at which the zirconium alloy is in a state comprising the crystalline phases α and β of the zirconium alloy.

According to specific terms:

- At the temperature of the first forging step, the ingot contains a volume proportion of zirconium alloy in α phase between 10% and 90%, the rest of the zirconium alloy of the ingot being in β phase.

- The first forging step is carried out at a temperature between 850 ° C and 950 ° C;

- The first forging step is carried out at a temperature of around 900 ° C;

- The first forging step is carried out at a temperature between 600 ° C and 950 ° C;

. the second forging step is carried out at a temperature at which the zirconium alloy of an intermediate product obtained by the first ingot forging step is in the α phase;

the second forging step is carried out at a temperature at which the zirconium alloy of an intermediate product obtained at the end of the first ingot forging step is in a state comprising the crystalline phases α and β of the zirconium alloy; and - the zirconium alloy contains at most 3% by weight in total of addition elements constituted by at least one of the elements: tin, iron, chromium, nickel, oxygen, niobium, vanadium and silicon, the rest of the alloy consisting of zirconium, with the exception of unavoidable impurities. According to the invention also relates to:

the use of the process for the manufacture of a semi-finished product such as a bar or a billet intended for the production of a tubular product for the production of an element for fuel assembly such as a tube cladding or a guide tube of a fuel assembly for a water-cooled nuclear reactor or a fuel assembly element for a CANDU reactor;

- or the use of the method for the manufacture of a rod intended for the manufacture of a small diameter plug bar for the production of plugs for closing the ends of cladding tubes of fuel assembly rods for reactor nuclear.

In order to clearly understand the invention, a process will be described for manufacturing a semi-finished product intended for the production of tubular products, according to the invention, in a comparative manner with the process according to the prior art.

Figure 1 is a diagram showing in a simplified manner the different stages of the manufacturing process of the semi-finished product.

In Figure 1, there is shown a cast ingot 1 which can be a large ingot whose diameter can be between 400 mm and 700 mm and the length between 2 m and 3 m, which is obtained by casting a zirconium alloy used for the manufacture of tubular products for the production of fuel assembly elements.

The zirconium alloy can for example be a Zircaloy 2 alloy. closing, by weight, from 1.2% to 1.7% tin, from 0.07% to 0.20% iron, from 0.05% to 0.15% chromium, from 0.03% with 0.08% nickel, at most 120 ppm silicon and 150 ppm carbon, the rest of the alloy consisting of zirconium with the exception of usual impurities.

The alloy for manufacturing the long product can also be a Zircaloy 4 containing by weight, from 1.2% to 1.7% tin, from 0.18% to 0.24% iron, from 0.07% at 0.13% chromium, at most 150 ppm of carbon, the rest of the alloy consisting of zirconium and impurities. The zirconium alloy used to manufacture the long product can also be an M5 type alloy essentially containing zirconium and niobium.

According to the invention, the ingot is brought to a temperature at which the zirconium alloy is in the α + β phase, to carry out the first forging step on the ingot in the α + β phase.

The forging temperature in phase α + β (first step of the process) is chosen so that the volume proportion of phase α in the ingot alloy is between 10% and 90%, the rest of the alloy being in β phase .

Generally, the first forging step is carried out at a temperature between 850 ° C and 950 ° C and, for example, typically, at 900 ° C in the case of Zircaloy 4. At this temperature, zirconium alloys such that the Zircaloys are in the α + β phase. In the case of zirconium-niobium alloys such as the M5 alloy, the domain α + β extends over a substantially wider temperature range than in the case of Zircaloy type alloys, this range from 600 ° C. to 950 ° C.

The ingot is forged, as in the case of the process according to the prior art where this forging was carried out at high temperature (for example at 1050 ° C.), until a bar or a product is obtained. with square or octagonal section inscribed in a circle with a diameter of 250 mm to 400 mm, typically, in a circle with a diameter of 350 mm.

The substitution of forging in the α + β phase for forging in the β phase at a higher temperature makes it possible to obtain an intermediate product whose characteristics are similar to those of the usual intermediate product obtained by a first forging step in the β phase.

Lowering the forging temperature, for example 150 ° C, results in substantial savings in the implementation of the manufacturing process. In addition, forging can be carried out using conventional tools, taking into account a minor adaptation of the forging process. In the case of zirconium-niobium alloys such as M5, it is possible to carry out forging at a temperature substantially below 900 ° C., the α + β range of the alloy extending from the temperature of 600 ° C. , up to 950 ° C. In a first variant of the invention, the second phase of the forging process can be carried out to obtain the semi-finished product from the intermediate product, in the same way as in the case of the process known according to the prior art, it that is to say perform a second forging in the α phase at a temperature between 700 ° C and 800 ° C, to obtain a bar having a diameter between 100 mm and 250 mm.

According to a second variant of the method, it is possible to carry out the second forging step to obtain the semi-finished product in the form of a bar, at the same temperature as the first forging step, that is to say on the product in the α + β phase. In the figure, a schematic representation has been made of the forging installation making it possible to implement the first forging step 2 on the ingot 1 at a temperature at which the ingot 1 is in the α + β phase. There is obtained at the output of the first forging step 2 an intermediate product 3 'consisting of a bar or a product with a square or octagonal section which is subjected to a second forging step 4 to obtain the semi-finished product 3 in the form of a billet or bar from which the final long product can be obtained by extrusion or hot rolling.

The tools used in the first forging step 2 in the α + β phase and in the second forging step 4 can be conventional tools used in the context of a method according to the prior art in which the first step 2 is performed on the ingot 1 in the β phase and the second step 4 on the intermediate product 3 'in the α phase.

In the case of the invention, the second forging step 4 can be carried out at the same temperature as the first forging step 2, the intermediate product 3 'being in the α + β phase.

The second step 4 can also be carried out in the α phase, as in the case of the method of the prior art. The intermediate product 3 ′ obtained after the first forging step in the α + β phase can be subjected to a cooling step of any type.

The intermediate product 3 ′ can be immediately brought to the temperature of the second forging step, that is to say to a temperature at which the product is in the α phase or in the α + β phase.

In the case where the two forging steps are carried out in the α + β phase, provision may be made for keeping the product at temperature between the two forging steps. The two-stage forging of the ingot 1 makes it possible to obtain a bar or a billet whose diameter is between 100 and 250 mm which constitutes the semi-finished product which is then subjected to the extrusion or hot rolling operation to obtain a tubular part or a small diameter bar which can be used for the manufacture of elements for fuel assemblies for a nuclear reactor.

It has been observed, by carrying out analyzes on the semi-finished product 3 or on long products obtained from the semi-finished product, that the quantity of hydrides contained in the alloy obtained by the process according to the invention is substantially less than the amount of hydrides contained in a product according to the prior art.

In addition, the semi-finished product or long final products obtained from this semi-finished product have structural and mechanical characteristics substantially similar to those of products obtained by a process according to the prior art. In particular, the corrosion resistance and formability properties of tubular products produced from the semi-finished product according to the invention are substantially superior to those of a product obtained by the process according to the prior art.

One of the advantages of the process according to the invention is to simplify the process for manufacturing the semi-finished product by limiting the forging temperature during the first forging step and possibly eliminating cooling after the first forging step. Thus a reduction of cost and time ^'in the implementation of the process. In the case of Zircaloy 2 and Zircaloy 4 alloys, or of any other zirconium alloy containing tin, the transition to the α + β phase of the alloy, to carry out the first step and possibly the second step of the process according to the invention can lead to the formation of tin segregations. However, these segregations can be erased during subsequent treatments in the context of the preparation of the final tubular product from the semi-finished product. The same is true for the oxygen and nitrogen elements.

In the case where the method of the invention is applied to niobium alloys, as indicated above, the transition between the domains α and α + β being close to 600 ° C., the forging temperature in the α + phase β can be significantly lower than 900 ° C., however taking into account the malleability properties of the alloy at the forging temperature.

The application of the process according to the invention to other zirconium alloys than Zircaloys or niobium alloys could be envisaged. These alloys generally contain at most 3% by weight of addition elements constituted by at least one of the addition elements: tin, iron, chromium, nickel, oxygen, niobium, vanadium and silicon, the rest of the alloy consisting of zirconium and unavoidable impurities.

The invention applies in particular to the manufacture of tubular products of zirconium alloy for the production of fuel assembly elements such as cladding tubes containing fuel pellets or guide tubes for fuel assemblies. combustible.

The invention also applies to the manufacture of plug bars for producing plugs for closing the ends of the cladding tubes of fuel assembly rods.

To obtain the final products from the semi-finished product, it may be necessary to carry out operations subsequent to spinning, extrusion or hot rolling of the semi-finished product, such as pilgrim step rolling, heat treatments being able to be additionally performed between forming operations.

The invention is not strictly limited to the embodiments which have been described. The temperature of forging in the α + β phase depends on the composition of the zirconium alloy. The forming operations can be carried out using the usual means for forming in the α or β phase of the process of the prior art or other means suitable for forging in the α + β phase in one or two steps, to get the semi-finished product.

The invention applies generally to any product in technical zirconium alloy defined by the composition limits given above.

Claims

CLAIMS 1.- Method of manufacturing a semi-finished product (3) of zirconium alloy containing by weight, at least 97% of zirconium, intended for the production of at least one long product, in which an ingot is produced (1) of large dimensions by casting the zirconium alloy, then the semi-finished product (3) intended to be subjected to forming to obtain the long product, by forging the ingot of large dimensions (1) in two stages, characterized in that the first forging step (2) of the ingot (1) of large dimensions is carried out at a temperature at which the zirconium alloy is in a state comprising the crystalline phases α and β of the zirconium alloy .

2.- Method according to claim 1, characterized in that at the temperature of the first forging step, the ingot contains a volume proportion of zirconium alloy in α phase between 10% and 90%, the rest of the ingot zirconium alloy being in the β phase.

3.- Method according to any one of claims 1 and 2, characterized in that the first forging step (2) is carried out at a temperature between 850 ° C and 950 ° C.

4.- Method according to claim 3, characterized in that the first forging step is carried out at a temperature of about 900 ° C.

5.- Method according to any one of claims 1 and 2, characterized in that the first forging step is carried out at a temperature between 600 ° C and 950 ° C.

6.- Method according to any one of claims 1 to 5, characterized in that the second forging step is carried out at a temperature at which the zirconium alloy of an intermediate product (3 ') obtained by the first forging step (2) of the ingot (1) is in the α phase.

7.- Method according to any one of claims 1 to 5, charac-

- terrified by the fact that the second forging step is carried out at a temperature at which the zirconium alloy of an intermediate product (3 ') obtained at the end of the first forging step (2) of the ingot (1) is in a state comprising the crystalline phases α and β of the zirconium alloy.

8.- Method according to any one of claims 1 to 7, characterized in that the zirconium alloy contains at most 3% by weight in total of addition elements constituted by at least one of the elements: tin, iron, chromium, nickel, oxygen, niobium, vanadium and silicon, the rest of the alloy consisting of zirconium, with the exception of unavoidable impurities.

9.- Use of the method according to any one of claims 1 to 8, for the manufacture of a semi-finished product such as a bar or a billet intended for the production of a tubular product for the production of a element for fuel assembly such as a cladding tube or a guide tube for a fuel assembly for a water-cooled nuclear reactor or even a fuel assembly element for a CANDU reactor.

10.- Use of the method according to any one of claims 1 to 7, for the manufacture of a bar intended for the manufacture of a bar with small diameter plugs for the production of plugs for closing the ends of tubes cladding of fuel assembly rods for nuclear reactors.