Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon

Definitions

• the invention relates to the metallurgy of nickel-based alloys, and more specifically to the alloys used to manufacture structural components for nuclear reactors or for fuel assemblies inserted into said reactors.
• Certain components of nuclear reactors such as heat exchangers, cluster guide pins, piping, hardware and bolts used to secure the steel components used in the cooling circuits of light water nuclear reactors or nuclear reactors with heat transfer fluid gas or molten salt or liquid metal, are made of nickel-based alloys, for example different types of Inconel®. These components must, at high temperature and at high pressure, have good resistance to oxidation, corrosion, creep and cyclic stresses both thermal and mechanical, and for long periods of time (several tens of hours). years), and the nickel-based alloys are well adapted to these uses.
• Light water nuclear reactor fuel assemblies may also have some of their structural components made of a nickel base alloy, alloy 718 of which is a prime example. This is, in particular, the case of gate springs, usually made from strips of such alloys, and holding springs made either from flat half-products for the leaf springs, or from wires for helical springs, and hardware elements made from bars.
• Another method is to apply a suitable coating on the materials.
• nickel alloy grid springs 718 to reduce the number of breaks in service.
• Other types of coatings for example diffusion surface treatments, are also possible.
• document US Pat. No. 5,164,270 proposes carrying out Nb and / or Zr implantation on the surface of a ferrous alloy with 9-30% Cr and exposing it to a gas mixture O 2 / S. This would also be applicable to a Ni-based alloy.
• Another solution is to perform global or local heat treatments at high temperature (1100 0 C) on the structural elements, causing changes in the microstructure of the material. Local treatments are thus carried out on the elbows of 600 alloy steam generators. In this way, it was also sought to eliminate any trace of phase ⁇ in alloy 718 (see document US Pat. No. 5,047,093).
• Another solution is to modify the chemical composition of the material, more or less radical, which can sometimes lead to the development of a new alloy grade.
• the alloy 600 was thus replaced by the alloy 690 for the manufacture of tubes of steam generators. This is a costly approach to R & D time, and it does not always lead to technically and / or economically viable results for industrial applications.
• the object of the invention is to propose a means of improving the performance and reliability of nickel-base alloy nuclear reactor components subjected to conditions likely to promote the appearance of environmental-assisted cracking, whatever may be the case. their design, especially for long operating cycles.
• This means should also be a means of suppressing the sensitivity of the material to environmental-assisted cracking, with little or no interference with other characteristics of the material.
• 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 half-product, which is then subjected to a treatment aimed at modifying its metallurgical structure.
• Said treatment may be a treatment of annealing, recrystallization, dissolution or hardening, also called aging.
• Said desensitization treatment for environmental assisted cracking can be carried out on a product that is no longer subjected to treatment aimed at modifying its metallurgical structure.
• the alloy can be machined and / or polished subsequent to its desensitization to environmental-assisted cracking.
• 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 1 or an alloy containing at least one of these metals, or an element or a compound of elements such as Mg 1 Ca.
• the Ni-based alloy can be wrapped in a a strip of said metal or alloy or compound having a greater affinity for oxygen than said base alloy Ni.
• said Ni-based alloy may be placed in a housing having one or more walls of said metal or alloy or compound having greater avidity for oxygen than said Ni base alloy.
• said Ni-based alloy may be placed in a powder of said metal or alloy or compound having greater avidity for oxygen than said Ni base alloy.
• the alloy may have the composition, in percentages by weight: C ⁇
• the invention also relates to a part made of a nickel-based alloy, characterized in that said alloy has undergone desensitization heat treatment to the environmental-assisted cracking of the above type.
• Said part may be a nuclear reactor fuel assembly structural element.
• Said part can then be a grid spring or holding system, or a screw.
• Said part may be an element of the cooling circuits of a nuclear reactor.
• Said part may 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 coolant.
• Said part can be a semi-product from which parts can be made 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.
• the invention is based first of all 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 reducer.
• This treatment leads to a lasting desensitization of the alloy with respect to the cracking assisted by the environment, by a mechanism that will be explained.
• This desensitization treatment is not a substitute for possible thermal treatments conventionally applied by the skilled person to obtain the desired mechanical characteristics, but may be added thereto.
• the invention is also applicable to boiling water reactors (BWRs), gas-cooled reactors, molten salt reactors or liquid metal reactors, as well as to other apparatus using nickel base alloy structural elements operating in oxidizing conditions, at medium (200-500 0 C) and high (500-1200 0 C) temperatures in liquid or gaseous medium.
• BWRs boiling water reactors
• gas-cooled reactors gas-cooled reactors
• molten salt reactors or liquid metal reactors
• liquid metal reactors as well as to other apparatus using nickel base alloy structural elements operating in oxidizing conditions, at medium (200-500 0 C) and high (500-1200 0 C) temperatures in liquid or gaseous medium.
• the desensitization treatment must, if the desensitization temperature leads to a microstructure poorly adapted to the application, be supplemented by other heat and / or thermomechanical treatments, aimed at restoring to the alloy a structure and mechanical properties making it optimally suitable for the intended uses.
• the most probable mechanism for explaining the cracking of Ni-based alloys by environmental-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. It can then occur at the level of grain boundaries several mechanisms that will degrade their mechanical strength, namely: - the formation of CO and CO 2 by oxidation of carbon;
• embrittling oxides such as Cr 2 O 3 ; an intrinsic weakening of the grain boundaries by oxygen;
• the oxygen atoms come from the impurities present in the surrounding medium or even from the material itself, the lesser amount of oxygen being compensated for by the higher operating temperature of the nickel base 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) to high temperature (1200-1300 ° C) in a hydrogenated atmosphere allowed to desulphurize the surface of a monocrystalline Ni based alloy by evaporation of H2S. It is thus intended to reduce the problem of peeling of the material.
• 10 '3 s ' 1 shows a fracture facies of the specimen with some intergranular disruption primers, but in a significantly smaller amount than on untreated reference samples.
• a 15 ⁇ m polishing of each face of a specimen identical to the previous one makes it possible to obtain a totally ductile and transgranular fracture facies, thanks to the elimination of the non-totally desensitized surface area.
• Polishing is an optional operation. Its introduction into the desensitization process reduces the duration of heat treatment.
• the samples were 0.27mm thick strips with a high sensitivity to environmental cracking (known from ruptures during reactor use).
• the heat treatment temperature for the desensitization was 99O 0 C + 10 0 C, to avoid a magnification of the austenitic grain and to limit the ⁇ phase precipitation.
• the treatment atmosphere was Ar-H 2 (5%).
• the duration of the desensitization treatment was up to 100h.
• Table 1 Sample treatment conditions of tensile tests at 650 ° C in air and at 350 ° C in primary medium REP - test results. The failure modes were identical for both test conditions.
• Specimens 1 and 2 which have not undergone desensitization treatment, have a mixed fracture intergranular and fragile transgranular ductile facies.
• Specimens 3 to 23 which have undergone such treatment have:
• the ductile transgranular character of the facies is all the more marked as the desensitization treatment has been long. From 36h, we find purely transgranular facies, and the facies become 02006
• a desensitization treatment at 98O 0 C for at least 40 h is therefore fully effective on these strips to obtain in all cases a total desensitization of the material to the environmental cracking in the air at 650 ° C.
• the amount depends on the temperature and processing time.
• the heating rate also has a significant influence on the amount of ⁇ phase present, especially at high temperatures, above 950 ° C.
• the ⁇ phase can be formed during heating. Therefore, as a function of the holding temperature, the phase volume fraction ⁇ tends to increase if the temperature is low, or to decrease and then to stabilize if the temperature is in the upper end of the admissible range.
• phase ⁇ formed in excess at relatively low temperatures would be more damaging to the sensitivity to environmental-assisted cracking than the phase ⁇ formed to relate high temperature (more than 950 0 C) during the desensitization treatment.
• An essential condition for the desensitization of the alloy is that the heat treatment atmosphere is not oxidizing, and, moreover, the atmosphere must allow the reduction of the oxide layer generally naturally present on the surface of the material.
• a pure hydrogen atmosphere it is very preferable to carry out the desensitization treatment in the presence of a compound that captures the oxygen present more greedily than the part to be treated.
• a metal or other compound very hungry for oxygen such as
• Al, Ti, Hf 1 Zr, or an alloy containing at least one such high-grade metal, or an element or compound of elements such as Mg, Ca may be used.
• a first technique is to wrap the piece in a strip having the composition of the metal or alloy acting as an oxygen trap.
• a second technique is to place the piece in a housing having one or walls of this metal or alloy.
• this metal or alloy mention may be made of the FeCrAlY alloy used in the desensitization tests described above.
• This material used as a constituent of catalytic converters for the automotive industry, or as constituent of parts for machine tools or electrical resistors, is currently available on the market and is very effective. Tests have also been carried out to test the susceptibility to environmental cracking of grid springs made of an alloy 718 of the same composition as the aforementioned tensile test pieces. They were tested at 350 ° C. in a REP primary medium with a displacement speed of 10 7 sec -1 and an imposed displacement adapted to the tested designs.
• alloy 718 is a preferred but not exclusive example, by means of the following heat treatment.
• the atmosphere consists of either pure hydrogen or 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 workpiece of a compound having a greater affinity for oxygen than said Ni-based alloy
• Said compound may be a metal such as Al, Zr, Ti, Hf or an alloy containing at least one of these metals, a FeCrAIY ailerage, or an element or a compound of one or more elements such as that Mg or Ca ...
• 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 Ni-based.
• said Ni-based alloy can be placed in a housing having one or more walls made with said compound having greater avidity for oxygen than said alloy based on Or.
• said Ni-based alloy may be dipped in a powder of said compound having a greater affinity for oxygen than said Ni-based alloy.
• the precise conditions of minimum duration and treatment temperature depend on the geometry of the products and semi-products to be desensitized, as well as the quality of the desensitization that is sought.
• the temperature of the desensitization heat treatment may be between 950 and 116O 0 C.
• the duration of said desensitization heat treatment can be determined using empirical formulations deduced from the experiment. For example, for a strip thickness of 0.3 mm and treated at 980-1000 ° C, the following formulation makes it possible to determine the minimum duration of treatment necessary to obtain a totally desensitized product:
• the fragility 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 fracture facies, during a test carried out in a medium representative of the operating conditions of the component.
• the choice of the temperature range of the treatment (range 950-
• 1010 ° C or range 1010-1160 ° C depends essentially on the development phase of the material on which this treatment is performed and the requirements on the microstructure at the end of treatment.
• the higher temperature treatment is preferably carried out at the half-product stage, the subsequent treatments of the production range making it possible to regenerate the microstructure of the material if this has been adversely affected by the desensitization.
• the lower temperature treatment is preferably carried out at the finished product stage, and thus constitutes the last stage of the elaboration, the grain size then generally not being significantly influenced by the desensitization treatment.
• the high temperature treatment can be carried out on finished product when no microstructure requirement is imposed, such as on the cluster guide pins.
• the lower temperature treatment can be carried out on a semi-finished product, a longer treatment than at a higher temperature being, in this case, necessary to obtain total desensitization, all other things being equal.
• the semi-finished product thus obtained will, at the end of the treatment, be slightly sensitive to surface-assisted cracking, because of edge effects which lead to a concentration of the sensitizing elements at the metal / treatment interface. .
• the heat treatment is completed by a removal operation of the surface layer not totally desensitized.
• the removal of the surface layer can be achieved by machining and / or chemical, electrochemical or mechanical polishing.
• the environment-assisted cracking desensitization treatment of said Ni-based alloy may be followed, if necessary, by heat treatments for annealing, recrystallization, dissolution or curing (also known as aging treatments) conventionally applied by those skilled in the art during the development of half-products and nickel-based alloy products to facilitate subsequent manufacturing operations and ultimately obtain the microstructure and mechanical characteristics necessary for the good behavior in service of the components.
• heat treatments for annealing, recrystallization, dissolution or curing also known as aging treatments
• An essential condition is that these possible heat treatments are performed in a non-oxidizing atmosphere to avoid resensitizing the material to the cracking assisted by the environment.
• a part thus produced may be a nuclear reactor fuel assembly structure element.
• Said part can then be a grid spring or holding system, or a screw.
• Said part may 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 coolant.
• a semi-finished product may be a sheet, a strip, a wire, a bar or a blank obtained, for example, by forging, stamping, molding or even by sintering, from which parts may be made by various methods. conventional shaping, or machining, or cutting.
• the alloy 718 thus treated finds a favored application in the manufacture of grid springs and holding element springs for fuel assemblies of nuclear reactors, but can be used to constitute other parts of which 'use 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.

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