EP2126152B1 - Processing method for cracking desensitisation using a nickel based alloy environment, mainly for a nuclear reactor fuel assembly and for a nuclear reactor, and part made of the alloy thus processed - Google Patents

Description

The invention relates to the metallurgy of nickel-based alloys, and more precisely the alloys used to manufacture structural components for nuclear reactors or for fuel assemblies inserted in said reactors.

Certain components of nuclear reactors, such as heat exchangers, cluster guide pins, piping, screws and bolts used to secure the steel components used to make the cooling circuits of light water nuclear reactors or nuclear reactors with gas heat transfer fluid or molten salt or liquid metal, are made of nickel-based alloys, for example in different types of Inconel®. These components must, at high temperature and at high pressure, have good resistance to oxidation, to corrosion, to creep and to both thermal and mechanical cyclic stresses, and this for high durations (several tens of years), and nickel-based alloys are well suited for these uses.

Light water nuclear reactor fuel assemblies can also have some of their structural components made of a nickel-based alloy, of which alloy 718 is a preferred example. This is, in particular, the case of grid springs, usually produced from strips of such alloys, and retaining springs produced either from flat semi-finished products for leaf springs, or from wires for coil springs, and fasteners, made from bars.

The general nickel-based alloys which can be used in these contexts have the following general composition, expressed in weight percentages: C ≤ 0.10%; Mn ≤ 0.5%; If ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1-15%, Nb = 0-10%, Ta ≤ 10%; the remainder being Fe, and unavoidable impurities resulting from the development. The elements for which a minimum value is not fixed can be completely absent, or present only in the trace state. We can also find, at low contents, other elements more rarely used, in order to adjust certain mechanical or chemical properties, and which will not radically modify the behavior of the alloy from the point of view of sensitivity. environment-assisted cracking, which, in an aqueous medium, results in a phenomenon of stress corrosion.

Typically, the composition of alloy 718, a particular example of such alloys, is: C ≤ 0.08%; Mn ≤ 0.35%; If ≤ 0.35%; P ≤ 0.015%; S ≤ 0.015%; Ni = 50-55%; Cr = 17-21%; Co ≤ 1%; Al = 0.2-0.8%; Mo = 2.8-3.3%; Ti = 0.65-1.15%; B ≤ 0.006%; Cu ≤ 0.3%; Nb + Ta = 4.75-5.5%; the remainder being iron and unavoidable impurities resulting from processing. It can also contain a few hundred ppm of Mg.

A problem whose importance is growing in the operation of reactors containing such components is the resistance of said components to cracking assisted by the environment. On the one hand, it is desired to lengthen the duration of the fuel assembly operating cycles as much as possible. Thus, it is desired to increase it from 12 months, which is the current usual duration, to 18 months, or even to 24 months. On the other hand, the specific conditions of the primary environment of light water reactors (LWR in English) are favorable to the development of cracking assisted by the environment. The same applies to reactors with a gas or molten salt or liquid metal heat transfer fluid, due to the very high temperatures reached which exacerbate the oxidation phenomena. Experience in a pressurized water reactor, in particular, has shown that fractures of 718 alloy grid springs could occur during use, due to an environment-assisted cracking process in this case. stress corrosion (CSC). Broken or cracked X750 Alloy Cluster Guide Pins, 600 Alloy Steam Generator Tubing, Bottom Bushings tank, welded areas, all these parts being in nickel base alloys of different shades, were also encountered.

To improve the reliability of components made of a nickel-based alloy, in particular of a 718 alloy, it is therefore necessary to find means to reduce the sensitivity of these components to cracking assisted by the environment.

So far, the solutions chosen have mainly been based on good industrial practice or palliative measures.

Thus, it has been proposed to modify the surface condition of the structural elements, mechanically (shot blasting, micro-blasting, sanding, etc.) or chemically (electro-polishing). For example, the document JP-A-2000 053 492 teaches to practice on a monocrystalline cast material in Ni-based superalloy the removal of the most superficial layer of the material, by carrying out an oxidation of said layer, then with an electrochemical polishing. After which, a heat treatment at a temperature equal to or higher than the recrystallization temperature is practiced. This removes the residual stresses on the surface of the material which make it sensitive to cracking assisted by the environment. The surface is then covered with a ceramic layer. This document teaches to apply this method to the blades of gas turbines, but the modification of the surface state of the material to remove the residual stresses was also practiced on tubes of steam generators in alloys 600 and 690.

Another method is to apply a suitable coating to the materials. Thus it is common to practice nickel plating of the grid springs of alloy 718 to reduce the number of their ruptures in service. Other types of coatings, for example surface treatments by diffusion, are also possible. So the document US-A-5 164 270 proposes to carry out an implantation of Nb and / or Zr on the surface of a ferrous alloy with 9-30% Cr and to expose it to a gas mixture O ₂ / S. This would also apply to an Ni-based alloy.

Another solution consists in carrying out global or local heat treatments at high temperature (1100 ° C.) on the structural elements, causing modifications to the microstructure of the material. Local treatments are thus carried out on the elbows of steam generators in alloy 600. In this way, we have also sought to eliminate any trace of phase δ in alloy 718 (see the document US-A-5,047,093 ).

Another solution consists in modifying the chemical composition of the material, in a more or less radical way, which can sometimes lead to the development of a new alloy shade. Alloy 600 has thus been replaced by alloy 690 for the manufacture of steam generator tubes. It is an expensive approach in terms of research and development, and it does not always lead to technically and / or economically viable results for industrial applications.

Finally, we also played not on the materials themselves, but on the design of the structures, by reducing the level of stresses to which they are subjected. Again, this approach is costly in development time anyway, and often results in failure.

In general, these rules of good practice tend more to optimize the behavior of structures with regard to the stresses seen by them, rather than to permanently and permanently improve the properties of materials and to approach their intrinsic characteristics.

The document " The effects of reactive Element Additions, Sulfur Removal, and Specimen thickness on the Oxidation Behavior of Alumina-Forming Ni- and Fe-Basis Alloys "by S. Sarioglu et al, Materials Science Forum Vols.251-254 (1997) pp.405 -412 discloses a study of the effects of sulfur concentration, sample thickness, and additions of reactive elements during isothermal and cyclic oxidation of alumina-forming alloys which include nickel-based monocrystalline superalloys , the ferritic alloys Fe-Cr-Al and NiAl. Desulfurization by annealing in hydrogen reduces the formation of interfacial porosity and can improve the resistance to cyclic oxidation until it becomes comparable to that obtained by adding reactive elements. In addition, the article " Effects of hydrogen annealing, sulfur segregation and diffusion on the cyclic oxidation resistance of superalloys ", JL Smialek et al .; Thin solid films 253 (1994), pp. 285-292 discloses effective desulfurization performed by hydrogen annealing which results in excellent resistance to cyclic oxidation for a number of advanced superalloys. The object of the invention is to propose a means of improving the performance and reliability of the components of nuclear reactors made of a nickel-based alloy subjected to conditions liable to favor the appearance of environment-assisted cracking, regardless of their design, particularly for long operating cycles. This means should also be a means of suppressing the material's sensitivity to environmentally assisted cracking, with little or no interference with the other characteristics of the material.

To this end, the invention according to claim 1 relates to a method of thermal treatment for desensitization to cracking assisted by the environment of an alloy with Ni base composition, in weight percentages: C ≤ 0.10%; Mn ≤ 0.5%; If ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1-15%, Nb = 0-10%, Ta ≤ 10%; the remainder being Fe, and unavoidable impurities resulting from the production, characterized in that the said alloy is maintained at 950-1160 ° C, in an atmosphere containing at least 100 ppm of hydrogen mixed with a neutral gas or in pure hydrogen.

Said environment-assisted cracking desensitization treatment can be carried out between 950 and 1010 ° C.

Said environment-assisted cracking desensitization treatment can be carried out between 1010 and 1160 ° C.

Said environment-assisted cracking desensitization treatment can be carried out on a semi-finished product, intended then to undergo a treatment aimed at modifying its metallurgical structure.

Said treatment can be an annealing, recrystallization, dissolution or hardening treatment, also called aging.

Said environment-assisted cracking desensitization treatment can be carried out on a product which is then no longer subjected to a treatment intended to modify its metallurgical structure.

One can proceed to a machining and / or a polishing of the alloy after its desensitization to cracking assisted by the environment.

Said desensitization treatment can be carried out in the presence of a compound having a greater avidity for oxygen than said alloy.

Said compound is a metal such as Al, Zr, Ti, Hf, or an alloy containing at least one of these metals, or an element or a compound of elements such as Mg, Ca.

At least during the environment-assisted cracking desensitization treatment, the Ni-based alloy may be wrapped in a strip of said metal or alloy or compound having a greater affinity for oxygen than said Ni base alloy.

At least during the environment-assisted cracking desensitization treatment, said Ni-based alloy may be placed in a housing comprising one or more walls made of said metal or alloy or compound having a greater avidity for oxygen than said Ni base alloy.

At least during the environment-assisted cracking desensitization treatment, said Ni-based alloy may be placed in a powder of said metal or alloy or compound having a greater avidity for oxygen than said Ni-based alloy.

The alloy may have the composition, in weight percentages: C ≤ 0.08%; Mn ≤ 0.35%; If ≤ 0.35%; P ≤ 0.015%; S ≤ 0.015%; Ni = 50-55%; Cr = 17-21%; Co ≤ 1%; Al = 0.2-0.8%; Mo = 2.8-3.3%; Ti = 0.65-1.15%; B ≤ 0.006%; Cu ≤ 0.3%; Nb + Ta = 4.75-5.5%; the remainder being Fe, and unavoidable impurities resulting from processing.

The subject of the invention is also a method of manufacturing a part made of a nickel-based alloy of composition, in weight percentages: C ≤ 0.10%; Mn ≤ 0.5%; If ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1-15%, Nb = 0-10%, Ta ≤ 10%; the remainder being Fe, and inevitable impurities resulting from the preparation, characterized in that it comprises a heat treatment for desensitization of the alloy to cracking assisted by the environment of the preceding type.

The invention according to claim 15 also relates to a part made of a nickel-based alloy, characterized in that said alloy has undergone a heat treatment for environmental assisted cracking desensitization of the preceding type.

Said part can be a structural element for assembling a nuclear reactor fuel.

Said part can then be a grid spring or holding system, or a screw.

Said part can then be made of a nickel-based alloy of composition, in weight percentages: C ≤ 0.08%; Mn ≤ 0.35%; If ≤ 0.35%; P ≤ 0.015%; S ≤ 0.015%; Ni = 50-55%; Cr = 17-21%; Co ≤ 1%; Al = 0.2-0.8%; Mo = 2.8-3.3%; Ti = 0.65-1.15%; B ≤ 0.006%; Cu ≤ 0.3%; Nb + Ta = 4.75-5.5%; the remainder being Fe, and unavoidable impurities resulting from processing.

Said part can be an element of the cooling circuits of a nuclear reactor.

Said part can then be a pipe, or a cluster guide pin, or a spring, or a heat exchanger, or a screw, or a bolt, or any other nickel-based alloy component in contact with the heat transfer fluid.

Said part can be a semi-finished product from which parts can be produced by a shaping process, or by machining, or by cutting.

Said part can then be a sheet, or a strip, or a wire, or a bar or a blank.

As will be understood, the invention is first based on the development of a heat treatment of the material, carried out under hydrogen or under a hydrogenated atmosphere, in the latter case generally in the presence of a powerful reducing agent. This treatment leads to a lasting desensitization of the alloy with respect to cracking assisted by the environment, by a mechanism which will be explained.

This desensitization treatment does not replace any thermal treatment conventionally applied by a person skilled in the art to obtain the desired mechanical characteristics, but can be added to it.

It was found that after an isothermal maintenance treatment at 980 ° C. for 100 h in a gas mixture Ar-H ₂ (5%) of a test tube taken from a strip of alloy 718, the material thus obtained saw its sensitivity to fragile intergranular rupture by environment-assisted cracking significantly reduced, and even canceled after polishing the extreme surface of the test piece.

This finding put the inventors on the path of adapting the composition of the alloy 718 and of the neighboring materials, by reducing the carbon, oxygen and nitrogen content at least in the vicinity of the surface of the parts. They were thus able to radically reduce their sensitivity to cracking assisted by the environment and to intergranular cracking at high temperature (> 350 ° C), and thus make them very well suited to the constitution of structural elements of assemblies of fuel, or cooling circuits, intended to work under conditions where normally environmentally assisted cracking is likely to be a problem. This is particularly the case for pressurized water reactors (PWR). However, the invention is also applicable to boiling water reactors (REB), and to reactors cooled by gas or molten salt or liquid metal, as well as to other devices using structural elements of nickel base alloy operating in oxidizing conditions, at medium (200-500 ° C) and high (500-1200 ° C) temperatures in liquid or gaseous medium.

However, the desensitization treatment must, if the desensitization temperature leads to a microstructure poorly suited to the application, to be supplemented by other thermal and / or thermomechanical treatments, aiming to restore the alloy to a structure and mechanical properties. making it optimally suited to the intended uses.

• la formation de CO et de CO₂ par oxydation du carbone ;
• la formation d'un ou plusieurs oxydes fragilisants tels que Cr₂O₃ ;
• une fragilisation intrinsèque des joints de grains par l'oxygène ;
• un relâchement de soufre, très fragilisant lui aussi, par réaction de l'oxygène avec les précipités contenant du soufre, ceux-ci résultant de l'élaboration en tant qu'impuretés.

The most likely mechanism for explaining the cracking of Ni-based alloys by environmentally assisted cracking in an aqueous medium, for example the primary fluid of a light water reactor, is as follows. It is based on the intergranular diffusion of oxygen atoms resulting from the dissociation of the water constituting the primary fluid. Several mechanisms can then occur at the grain boundaries which will degrade their mechanical resistance, namely:

• the formation of CO and CO ₂ by oxidation of carbon;
• the formation of one or more embrittling oxides such as Cr ₂ O ₃ ;
• intrinsic embrittlement of grain boundaries by oxygen;
• a release of sulfur, also very embrittling, by reaction of oxygen with the sulfur-containing precipitates, the latter resulting from the preparation as impurities.

A similar mechanism exists for other heat transfer fluids. In this case, the oxygen atoms come from the impurities present in the surrounding medium or even from the material itself, the least amount of oxygen being compensated for by the higher operating temperature of the nickel-based alloy component.

Previous studies (article " Oxidation Resistance and critical sulfur content of single crystal superalloys, JL Smialek, International Gas Turbine and Aeroengine Congress & Exhibition, Birmingham, 10-13.06.1996 ) have shown that prolonged exposure (8 to 100h) at high temperature (1200-1300 ° C) in a hydrogenated atmosphere made it possible to desulfurize the surface of a monocrystalline Ni-based alloy by evaporation of H ₂ S. We thus aim to reduce the problems of chipping of the material. This process cannot, however, be transposed as such to any non-monocrystalline Ni-based superalloy. Indeed, in this case, the high temperatures would cause grain growth and changes in the crystal structure which are not necessarily desired.

• un traitement à 980°C pendant 100h ; cette température permet de limiter le grossissement des grains, mais elle provoque la précipitation d'une phase δ qui, habituellement est considérée comme indésirable lorsqu'on veut éviter la fissuration assistée par l'environnement ;
• une remise en solution de la phase δ par un séjour à 1080°C pendant 1h, qui provoque également une croissance des grains ;
• un durcissement (vieillissement) à 720°C pendant 8h ou à 620°C pendant 8h.

The inventors therefore carried out a first test treatment of a test piece from a strip of composition C = 0.016%; Ni = 53.7%; B = 0.0009%; Mn = 0.11%; Mg = 0.0087%; Mo = 2.88%; Fe = 18.03%; If = 0.12%; Al = 0.54%; Co = 0.04%; P = 0.005%; Cu = 0.03%; S = 0.00034%; Ti = 1.04%; Cr = 18.1%; Nb + Ta = 5.15% under a flow of Ar-H ₂ mixture (5%), with a NiCoCrAlYTa powder covering the sample to reduce the partial pressure of oxygen. We carried out successively:

• treatment at 980 ° C for 100h; this temperature makes it possible to limit the magnification of the grains, but it causes the precipitation of a phase δ which, usually is considered undesirable when it is desired to avoid cracking assisted by the environment;
• redissolution of phase δ by staying at 1080 ° C for 1 hour, which also causes grain growth;
• hardening (aging) at 720 ° C for 8 hours or at 620 ° C for 8 hours.

After treatment, an odor of H ₂ S was released from the oven. However, fine analyzes by luminescent discharge mass spectrometry did not show a significant reduction in the sulfur content, but, on the other hand, a very strong reduction in the contents of carbon, nitrogen and, above all, oxygen elements.

A tensile test at 650 ° C in air, with a tensile speed of 10 ^-3 s ^-1 shows a facies of rupture of the test piece with some primers of intergranular rupture, but in significantly lower quantity than on samples of reference not processed.

A polishing on 15µm of each face of a test piece identical to the previous one allows to obtain a totally ductile and transgranular fracture facies, thanks to the elimination of the surface area not completely desensitized.

Polishing is an optional operation. Its introduction into the desensitization process makes it possible to reduce the duration of the heat treatment.

On the other hand, a test piece treated under the preceding conditions, apart from the absence of H ₂ in the treatment atmosphere, then polished, still exhibited an intergranular rupture facies.

• entraîne un dégazage de l'oxygène, du carbone et de l'azote présents dans l'alliage, et plus particulièrement aux joints de grains ;
• empêche une oxydation de la surface des échantillons.

The advantages of the treatment probably come from the strongly reducing nature of the atmosphere of the heat treatment, which:

• causes degassing of the oxygen, carbon and nitrogen present in the alloy, and more particularly at the grain boundaries;
• prevents oxidation of the sample surface.

This removal of the fragility at the grain boundaries is favorable to desensitization of materials to cracking assisted by the environment.

A test program was then carried out to confirm the previous good results and to determine the range of suitable treatments.

The samples were strip 0.27 mm thick, the high sensitivity to cracking assisted by the environment was known (ruptures observed during use in the reactor).

The heat treatment temperature for desensitization was 990 ° C ± 10 ° C, to avoid a magnification of the austenitic grain and limit the precipitation of phase δ.

The treatment atmosphere was Ar-H ₂ (5%).

The samples were wrapped in a FeCrAlY alloy sheet of composition: Al = 5%; C = 0.02%; Cr = 22%; Mn = 0.2%; If = 0.3%; Y = 0.1%; Zr = 0.1%; Fe = the rest.

The duration of the desensitization treatment went up to 100 hours.

• par des essais de traction sous air à 650°C à une vitesse de l'ordre de 10^-3 s^-1, les résultats en terme de mode de rupture étant considérés comme représentatifs de ceux qui seraient obtenus dans des conditions hautes températures en milieu gaz ou sel fondu ou métal liquide;
• par des essais de traction lente (vitesse de l'ordre de 1,7.10^-8 s^-1) à 350°C en milieu primaire REP (eau pure désaérée présentant un pH à 25°C égal à 6,4, contenant 2ppm de lithium ajouté sous forme de lithine et 1200ppm de bore ajouté sous forme d'acide borique, et une pression partielle d'hydrogène réglée à 0.5bar avec des teneurs en F^-, Cl^- et SO₄ ^2- inférieures à 30ppb, effectués sur des éprouvettes en V d'une forme simulant au plus près la géométrie des pieds de ressorts de grille, qui sont les zones les plus sensibles à la fissuration assistée par l'environnement ;
• et par des essais de compression lente de ressorts de grille après désensibilisation.

The quality of environment-assisted crack desensitization has been determined:

• by tensile tests in air at 650 ° C at a speed of the order of 10 ^-3 s ^-1 , the results in terms of failure mode being considered to be representative of those which would be obtained under high temperature conditions in the medium molten gas or salt or liquid metal;
• by slow traction tests (speed of the order of 1.7.10 ^-8 s ^-1 ) at 350 ° C in a primary PWR medium (deaerated pure water having a pH at 25 ° C equal to 6.4, containing 2 ppm of lithium added in the form of lithine and 1200 ppm of boron added in the form of boric acid, and a partial pressure of hydrogen adjusted to 0.5 bar with contents of F ^- , Cl ^- and SO ₄ ^2- less than 30 ppm, carried out on V-shaped test pieces of a shape closely simulating the geometry of the grid spring feet, which are the areas most sensitive to cracking assisted by the environment;
• and by slow compression tests of grid springs after desensitization.

The test specimens of alloy 718 tested, of section 2 x 0.27mm ² or 3 x 0.27mm ² , had the composition: C = 0.016%; Ni = 53.7%; B = 0.0009%; Mn = 0.11%; Mg = 0.0087%; Mo = 2.88%; Fe = 18.03%; If = 0.12%; Al = 0.54%; Co = 0.04%; P = 0.005%; Cu = 0.03%; S = 0.00034%; Ti = 1.04%; Cr = 18.1%; Nb + Ta = 5.15%.

They underwent an environmental assisted cracking desensitization heat treatment by staying at 980 ° C under an Ar - H ₂ atmosphere (5%) for a period of 0:30 to 100 hours depending on the tests, followed by aging under the same atmosphere or under vacuum at 720 ° C for 8h, then at 620 ° C for 8h, in accordance with the aging treatments usually applied to the products concerned. For two reference tests, desensitization at 980 ° C was not carried out. For a test, the coating of the sample in a sheet of FeCrAlY was replaced by the introduction of the test piece in a case of FeCrAlY.

Following this treatment, the rupture facies was examined to determine if it was intergranular (IG), transgranular (TG) or mixed (IG + TG).

The results are summarized in Table 1. <u> Table 1: Conditions for processing the tensile test samples at 650 ° C in air and at 350 ° C in the primary PWR medium - test results. </u> Trial Treatment Atmosphere Type of failure 1 720 ° C / 8h + 620 ° C / 8h empty Mixed 2 720 ° C / 8h + 620 ° C / 8h then polishing empty Mixed 3 980 ° C / 100h + 1080 ° C / 1h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty TG 4 980 ° C / 96h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ TG 5 980 ° C / 48h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ TG 6 980 ° C / 48h + 720 ° C / 8h + 620 ° C / 8h, FeCrAlY housing Ar-H ₂ TG 7 980 ° C / 48h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty TG 8 980 ° C / 39h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty TG or mixed 9 980 ° C / 36h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ TG or mixed 10 980 ° C / 33h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 11 980 ° C / 720 ° C + 30h / 8h + 620 ° C / 8h Ar-H ₂ Mixed 12 980 ° C / 27h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ Mixed 13 990 ° C / 24h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ Mixed 14 980 ° C / 24h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ Mixed 15 980 ° C / 21h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 16 980 ° C / 18h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 17 980 ° C / 15h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 18 980 ° C / 12h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ Mixed 19 980 ° C / 9 + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 20 980 ° C / 6h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 21 980 ° C / 3h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 22 980 ° C / 1h + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed 23 980 ° C / 0:30 + 720 ° C / 8h + 620 ° C / 8h Ar-H ₂ / empty Mixed

The failure modes were identical for the two test conditions.

Test pieces 1 and 2, which have not undergone a desensitization treatment, have a facies of mixed fragile intergranular and transgranular ductile rupture.

• soit un faciès de rupture mixte intergranulaire fragile et transgranulaire ductile,
• soit un faciès de rupture purement transgranulaire ductile.

Test pieces 3 to 23 which have undergone such treatment have:

• either a brittle intergranular and transgranular ductile fracture facies,
• or a purely transgranular ductile rupture facies.

The ductile transgranular character of the facies is all the more marked when the desensitization treatment has been long. From 36h, we find purely transgranular facies, and the facies become systematically purely transgranular beyond 39 hours of treatment. For treatment durations of 36 to 39 h, we are therefore at the limit of total desensitization of the samples, and obtaining partial or total desensitization is, in this case, dependent on the variability of the treatment conditions. , such as temperature.

A desensitization treatment at 980 ° C for at least 40 hours is therefore fully effective on these strips to obtain in all cases a total desensitization of the material to cracking assisted by the environment in air at 650 ° C.

Regarding the effects of the desensitization heat treatment on the microstructure of the material, the following can be said.

When an alloy 718 is treated at 850-1010 ° C, there is precipitation of a phase δ the amount of which depends on the temperature and the treatment time. The heating rate also has an important influence on the quantity of phase δ present, particularly at high temperatures, above 950 ° C. For the lowest heating rates, phase δ can form during heating. Therefore, depending on the holding temperature, the volume fraction of phase δ tends to increase if the temperature is low, or to decrease and then stabilize if the temperature is at the top of the admissible range.

Above around 1010 ° C (solvent temperature of phase δ, which can vary by a few degrees depending on the exact composition of the alloy), the grain growth increases considerably, making the microstructure less well suited for the preferred applications of the invention.

On the other hand, between 980 and 1000 ° C., for a sufficient holding time and for all the possible compositions of an alloy 718, it is possible to eliminate the small intergranular particles of phase δ and to spheroidize the insoluble precipitates.

It was also verified that the treatment atmosphere was of crucial importance for the success of the desensitization treatment, by carrying out comparative tests on samples having undergone a 96 h treatment at 980 ° C either in an Ar-H ₂ atmosphere (5%), or under vacuum. It was clearly apparent that the samples which had been treated under vacuum ruptured with a fragile intergranular rupture facies during a tensile test, while those which had been treated under a hydrogenated atmosphere had a ductile transgranular rupture facies. The presence of a hydrogenated atmosphere, containing at least 100 ppm of H ₂ mixed with a neutral gas such as Ar, or of pure H ₂ , is therefore very essential in the context of the invention.

Concerning the aging treatment which follows desensitization in the case of certain preferred applications of the invention such as the springs of the fuel assembly grid, it is usually advisable not to carry out such treatments at a temperature greater than or equal to 760 ° C. From this temperature, the phase precipitation δ is observed in the form of films or cords at the grain boundaries and a deficit of precipitates γ 'and γ "at these same places. Consequently, during autoclave tests (350 ° C) representative of the primary conditions of a PWR, cracking of samples subjected to a stress greater than or equal to the elastic limit of the alloy is often observed. According to usual designs, the phase δ formed in excess to relatively low temperature would be more damaging for sensitivity to cracking assisted by the environment than the δ phase formed at relatively high temperature (more than 950 ° C) during the desensitization treatment.

In fact, the experiments carried out by the inventors show that when an environment-assisted cracking desensitization treatment (980 ° C., 40 h) is carried out before aging (from 740 to 780 ° C, 8 h, then cooling in the oven), the sensitivity to cracking assisted by the environment is eliminated anyway, and the aging thus carried out has, in this range of treatments, no differentiated influence on cracking assisted by the environment. They just play their part usual adjustment of the mechanical properties of the material. In the present case, they increase its elastic limit.

An essential condition for the desensitization of the alloy is that the heat treatment atmosphere is not oxidizing, and, even more, the atmosphere must allow the reduction of the oxide layer generally naturally present on the surface of the material. Unless a pure hydrogen atmosphere is used, it is very preferable to carry out the desensitization treatment in the presence of a compound which captures the oxygen present with more avidity than the part to be treated.

For this purpose, a metal or other compound very eager for oxygen, such as Al, Ti, Hf, Zr, or an alloy containing at least one such metal at a high content, or an element or a compound of elements such as Mg, Ca can be used.

It is possible to cover the surface of the part with a powder of this alloy. There is then a risk of witnessing a sintering of the powder and pollution of the surface of the part, especially during the longest treatments, which makes it difficult to recover the part. However, this process has been successfully tested as part of this development.

We may therefore prefer to use two other techniques which prove to be effective and do not carry the risks presented by the use of powder.

A first technique consists in wrapping the part in a strip having the composition of the metal or of the alloy playing the role of oxygen trap.

A second technique consists in placing the part in a box comprising one or more walls of this metal or of this alloy.

As a preferred, but not exclusive, example of such an alloy, mention may be made of the FeCrAlY alloy used during the desensitization tests described above. This material, used as a constituent of catalytic converters for the automobile industry, or as a constituent of parts for machine tools or of electrical resistances, is commonly available on the market and proves to be very effective.

Tests have also been carried out intended to test the sensitivity to cracking assisted by the environment of grid springs produced from an alloy 718 of the same composition as the aforementioned tensile test pieces. They were tested at 350 ° C in a primary REP environment with a displacement speed of 10 ^-7 s ^-1 and an imposed displacement adapted to the designs tested.

On springs which have only undergone an aging treatment, without desensitization to cracking assisted by the prior environment, a multi-fracture initiation has been found on three of the four feet of the spring, with an intergranular rupture facies .

The execution, prior to aging, of a desensitization treatment for 30 h at 990 ° C. in an Ar-H ₂ 5% atmosphere, has brought an improvement, insofar as the cracks initiating the intergranular ruptures have only been observed. on one foot and were less numerous than in the absence of treatment. But the desensitization to cracking assisted by the environment was therefore not total.

On the other hand, the springs whose desensitization treatment lasted 42 hours at 990 ° C did not show any initiations of intergranular ruptures. They were therefore completely desensitized to cracking assisted by the environment, which confirmed the previous experimental results acquired on test pieces.

Other tests were carried out on test pieces of alloy 718 of composition very close to those previously mentioned, but whose experience showed that they had a less sensitivity to cracking assisted by the environment, before desensitization, than the previous, probably due to differences between the quantity of interstitials (C, N and O) present in the various batches of strips.

In certain cases, it turned out that one could obtain a complete desensitization to cracking assisted by the environment after 15h of treatment at 990 ° C ± 10 ° C. A very significant, but not always total, desensitization could, in all cases, be obtained after 30 hours of treatment at 990 ° C ± 10 ° C. From 40 hours of treatment, desensitization total environmental assisted cracking, both in air at 650 ° C and in PWR primary environment at 350 ° C, was systematic.

Under these conditions, it is proposed as treatment conditions in accordance with the invention, to carry out environmental-assisted crack desensitization of a nickel-based alloy in general, of composition C ≤ 0.10%; Mn ≤ 0.5%; If ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1-15%, Nb = 0-10%, Ta ≤ 10%; the remainder being Fe, and inevitable impurities resulting from the production, and of which the alloy 718 is a preferred but not exclusive example, by means of the following heat treatment.

The atmosphere is made up either of pure hydrogen, or of a neutral gas, such as argon, mixed with at least 100 ppm of hydrogen, the absence of oxygen being preferably guaranteed by the presence in the environment. of the part to be treated of a compound having a greater affinity for oxygen than said Ni-based alloy
Said compound can be a metal such as Al, Zr, Ti, Hf or an alloy containing at least one of these metals, such as an FeCrAlY alloy, or an element or a compound of one or more elements such as Mg or Ca ...

At least during the environment-assisted cracking desensitization treatment, the Ni-based alloy may be wrapped in a strip of said compound having a greater affinity for oxygen, carbon and nitrogen than said alloy with Ni base.

At least during the environment-assisted cracking desensitization treatment, said Ni-based alloy may be placed in a housing comprising one or more walls made with said compound having a greater avidity for oxygen than said base-based alloy Or.

At least during the environment-assisted cracking desensitization treatment, said Ni-based alloy may be immersed in a powder of said compound having a greater affinity for oxygen than said Ni-based alloy.

The precise conditions of minimum duration and temperature of treatment depend on the geometry of the products and semi-products to be desensitized, as well as on the quality of the desensitization which is sought.

• t (en heures) = 3.4 x (F%) si la fragilité initiale F est comprise entre 0 et 10%
• t (en heures) = 0.2 x (F%) si la fragilité initiale F est comprise entre 10 et 50%

The temperature of the desensitization heat treatment can be between 950 and 1160 ° C. One will generally choose one of the two ranges 950-1010 ° C and 1010-1160 ° C
The duration of said desensitization heat treatment can be determined using empirical formulations deduced from experience. For example, for a strip 0.3 mm thick and treated at 980-1000 ° C, the following formulation makes it possible to determine the minimum duration of treatment necessary to obtain a completely desensitized product:

• t (in hours) = 3.4 x (F%) if the initial fragility F is between 0 and 10%
• t (in hours) = 0.2 x (F%) if the initial fragility F is between 10 and 50%

The brittleness F of the material is here defined as being the ratio of the cumulative length of the zones with intergranular cracking and the total length of the perimeter of the rupture facies, during a test carried out in a medium representative of the operating conditions of the component.

The choice of the treatment temperature range (range 950-1010 ° C or range 1010-1160 ° C) depends essentially on the phase of preparation of the material on which this treatment is carried out and on the requirements required on the microstructure at the end of treatment.

The treatment at higher temperature is preferably carried out at the semi-finished product stage, the subsequent treatments of the production range making it possible to regenerate the microstructure of the material if it has been adversely affected by desensitization.

The treatment at lower temperature is preferably carried out at the stage of the finished product, and therefore constitutes the last stage of production, the grain size then generally not being significantly influenced by the desensitization treatment.

This choice is not, however, limiting: the high temperature treatment can be carried out on the finished product when no microstructure requirement is imposed, such as for example on the cluster guide pins. Likewise, the treatment at lower temperature can be carried out on a semi-finished product, a treatment longer than at higher temperature being, in this case, necessary to obtain total desensitization, all other things being equal.

It may, however, be desired to reduce the duration of the heat treatment, in particular when it is carried out at the semi-finished product stage. The semi-finished product thus obtained will still be slightly sensitive to cracking assisted by the surface environment at the end of the treatment, due to the edge effects which lead to a concentration of the sensitizing elements at the metal / treatment atmosphere interface. . In this case, to obtain a completely desensitized product, the heat treatment is completed by an operation of removing the surface layer which is not completely desensitized.

The removal of the surface layer can be carried out by machining and / or chemical, electrochemical or mechanical polishing.

The environment-assisted cracking desensitization treatment of said Ni-based alloy can be followed, if necessary, by heat treatments of annealing, recrystallization, dissolution or hardening (also called aging treatments) conventionally applied. by a person skilled in the art during the preparation of semi-finished products and products made of nickel-based alloys to facilitate subsequent manufacturing operations and ultimately obtain the microstructure and the mechanical characteristics necessary for the proper behavior of the components in service. An essential condition is that these possible heat treatments are carried out in a non-oxidizing atmosphere to avoid re-sensitizing the material to cracking assisted by the environment.

The invention allows parts and semi-finished products to be obtained, a non-exhaustive list of which will be given.

A part thus produced can be a structural element for assembling a nuclear reactor fuel.

Said part can then be a grid spring or holding system, or a screw.

Said part can be an element of the cooling circuits of a nuclear reactor.

Said part can then be a pipe, a cluster guide pin, a spring, a heat exchanger, a screw or a bolt, or any other nickel-based alloy component in contact with the heat transfer fluid.

A semi-finished product can be a sheet, a strip, a wire, a bar or even a blank obtained, for example, by forging, stamping, molding or even by sintering, from which parts can be produced by various processes conventional shaping, or machining, or cutting.

Alloy 718 thus treated, in particular, finds a privileged application in the manufacture of springs of grids and components of springs of system of maintenance for fuel assemblies of nuclear reactors, but can be used to constitute other parts including usage is compatible with its mechanical properties and which would be intended to be exposed in service to an environment favorable to the development of cracking assisted by the environment.

Claims (22)

1. Thermal processing method for desensitising a nickel-based alloy with respect to environmentally-assisted cracking, the alloy having the following composition in percentages by weight: C ≤ 0.10%; Mn ≤ 0.5%; Si ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12%-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1%-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1%-15%; Nb = 0-10%; Ta ≤ 10%; the balance being Fe and inevitable impurities resulting from the production operation, characterised in that the alloy is kept at 950°C-1160°C in an atmosphere containing at least 100 ppm of hydrogen mixed with an inert gas of pure hydrogen, said desensitization being considered as obtained by the fact that, after a tensile test in air at 650°C with a speed of 10^-3 s^-1, a fracture surface having both fragile intergranular and ductile transgranular features, or a fracture surface having purely ductile transgranular features, is observed.
2. Method according to claim 1, characterised in that the treatment for desensitisation with respect to environmentally-assisted cracking is performed at a temperature of between 950°C and 1010°C.
3. Method according to claim 1, characterised in that the treatment for desensitisation with respect to environmentally-assisted cracking is performed at a temperature of between 1010°C and 1160°C.
4. Method according to any one of claims 1 to 3, characterised in that the treatment for desensitisation with respect to environmentally-assisted cracking is performed on a semi-finished product which is then intended to be subjected to a treatment intended to modify its metallurgical structure.
5. Method according to claim 4, characterised in that said treatment is a treatment for annealing, recrystallisation, dissolution or hardening, carried out in a non-oxidising atmosphere.
6. Method according to any one of claims 1 to 3, characterised in that the treatment for desensitisation with respect to environmentally-assisted cracking is performed on a product which is not subsequently subjected to a treatment intended to modify its metallurgical structure.
7. Method according to any one of claims 1 to 6, characterised in that, following desensitisation with respect to environmentally-assisted cracking, the alloy is subjected to machining and/or polishing.
8. Method according to any one of claims 1 to 7, characterised in that said desensitisation treatment is performed in the presence of a compound having greater avidity for oxygen than said alloy.
9. Method according to claim 8, characterised in that said compound is a metal such as Al, Zr, Ti, Hf, or an alloy containing at least one of those metals, or an element or a compound of elements such as Mg, Ca.
10. Method according to claim 9, characterised in that, at least during the treatment for desensitisation with respect to environmentally-assisted cracking, the Ni-based alloy is wrapped in a strip of the metal or alloy or compound having greater affinity for oxygen than the Ni-based alloy.
11. Method according to claim 9, characterised in that at least during the treatment for desensitisation with respect to environmentally-assisted cracking, the Ni-based alloy is placed in a casing having one or more walls composed of the metal or alloy or compound having greater avidity for oxygen than the Ni-based alloy.
12. Method according to claim 9, characterised in that at least during the treatment for desensitisation with respect to environmentally-assisted cracking, the Ni-based alloy is placed in a powder of the metal or alloy or compound having greater avidity for oxygen than the Ni-based alloy.
13. Method according to any one of claims 1 to 12, characterised in that the alloy has the following composition in percentages by weight: C ≤ 0.08%; Mn ≤ 0.35%; Si ≤ 0.35%; P ≤ 0.015%; S ≤ 0.015%; Ni = 50%-55%; Cr = 17%-21%; Co ≤ 1%; Al = 0.2%-0.8%; Mo = 2.8%-3.3%; Ti = 0.65%-1.15%; B ≤ 0.006%; Cu ≤ 0.3%; Nb + Ta = 4.75%-5.5%; the balance being Fe and inevitable impurities resulting from the production operation.
14. Method for producing a component from a nickel-based alloy having the following composition in percentages by weight: C ≤ 0.10%; Mn ≤ 0.5%; Si ≤ 0.5%; P ≤ 0.015%; S ≤ 0.015%; Ni ≥ 40%; Cr = 12%-40%; Co ≤ 10%; Al ≤ 5%; Mo = 0.1%-15%; Ti ≤ 5%; B ≤ 0.01%; Cu ≤ 5%; W = 0.1%-15%; Nb = 0-10%; Ta ≤ 10%; the balance being Fe and inevitable impurities resulting from the production operation, characterised in that it comprises a thermal processing operation for desensitising the alloy with respect to environmentally-assisted cracking according to any one of claims 1 to 12.
15. Component produced from a nickel-based alloy, characterised in that the alloy has been subjected to a thermal processing operation for desensitisation with respect to environmentally-assisted cracking according to any one of claims 1 to 13, said desensitization being considered as obtained by the fact that, after a tensile test in air at 650°C with a speed of 10^-3 s^-1, a fracture surface having both fragile intergranular and ductile transgranular features, or a fracture surface having purely ductile transgranular features, is observed.
16. Component according to claim 15, characterised in that the component is a structural element of a nuclear reactor fuel assembly.
17. Component according to claim 16, characterised in that the component is a grid spring or a retention system, or a screw.
18. Component according to any one of claims 15 to 17, characterised in that it is produced from a nickel-based alloy having the following composition in percentages by weight: C ≤ 0.08%; Mn ≤ 0.35%; Si ≤ 0.35%; P ≤ 0.015%; S ≤ 0.015%; Ni = 50%-55%; Cr = 17%-21%; Co ≤ 1%; Al = 0.2%-0.8%; Mo = 2.8%-3.3%; Ti = 0.65%-1.15%; B ≤ 0.006%; Cu ≤ 0.3%; Nb + Ta = 4.75%-5.5%; the balance being Fe and inevitable impurities resulting from the production operation.
19. Component according to claim 15, characterised in that the component is an element of the cooling systems of a nuclear reactor.
20. Component according to claim 19, characterised in that the component is a pipe, or a cluster guide pin, or a spring, or a heat-exchanger, or a screw, or a bolt, or any other component which is made from nickel-based alloy and which comes into contact with the coolant fluid.
21. Component according to claim 15, characterised in that it is a semi-finished product from which components can be made using a shaping, machining or cutting method.
22. Component according to claim 21, characterised in that it is a sheet, or a strip, or a wire, or a bar, or a blank.