阅读说明：本技术 用于航空领域的表面硬化钢部件 (Case hardened steel component for aeronautical applications ) 是由 西里尔·罗杰·沃纳尔特 布鲁诺·彼得罗西 于 2020-04-30 设计创作，主要内容包括：本发明涉及用于航空领域的钢部件,该钢部件包括基体,基体至少包括碳、钴、铝以及镍,并且基体具有如下的平均原子分数：碳的平均原子分数介于0.09％至0.17％之间,钴的平均原子分数介于15.5％至18.5％之间,铝的平均原子分数小于0.1％,镍的平均原子分数介于7.2％至9.8％之间,部件被表面硬化并且还包括氮化层,氮化层至少部分地覆盖基体,并且氮化层的厚度介于5μm至180μm之间,优选地介于50μm至150μm之间。(The invention relates to a steel component for the aeronautical field, comprising a matrix comprising at least carbon, cobalt, aluminium and nickel, and having an average atomic fraction of: the average atomic fraction of carbon is between 0.09% and 0.17%, the average atomic fraction of cobalt is between 15.5% and 18.5%, the average atomic fraction of aluminum is less than 0.1%, the average atomic fraction of nickel is between 7.2% and 9.8%, the component is case hardened and further comprises a nitride layer, the nitride layer at least partially covers the substrate, and the nitride layer has a thickness between 5 μm and 180 μm, preferably between 50 μm and 150 μm.)

1. Method for manufacturing a component (1) for the aeronautical field, comprising the following successive steps:

a) providing (101) a component (1) comprising a matrix (2), the matrix (2) comprising at least carbon, cobalt, aluminum and nickel and having an average mass fraction as follows:

-the average mass fraction of carbon is between 0.09% and 0.17%,

-the average mass fraction of cobalt is between 15.5% and 18.5%,

-the average mass fraction of aluminium is less than 0.1%, and

-the average mass fraction of nickel is between 7.2% and 9.8%

b) Surface hardening (102) the component in a controlled atmosphere at a temperature between 900 ℃ and 1100 ℃,

c) quenching (103) and cryogenic treatment (104) of the component in a controlled atmosphere at a temperature between-50 ℃ and-100 ℃, and

d) ageing (105) the component in a controlled atmosphere at a temperature between 450 ℃ and 550 ℃,

characterized in that the aging treatment (105) comprises: nitriding the component in an atmosphere comprising ammonia such that the component comprises a nitrided surface, the nitriding of the component being performed simultaneously with the aging treatment.

2. Method according to claim 1, wherein the surface hardening step b) is carried out so that the average mass fraction of carbon of at least a part of the surface of the component (1) is comprised between 0.4% and 0.6%.

3. The method for manufacturing a component (1) according to claim 1 or claim 2, wherein the surface hardening step b) comprises injecting a carburizing gas into a carburizing chamber containing the component, the carburizing gas being selected from at least propane and acetylene.

4. Steel component (1) for the aeronautical field, obtainable by a process according to any one of claims 1 to 3, comprising a matrix (2), the matrix (2) comprising at least carbon, cobalt, aluminum and nickel and having an average mass fraction of:

-the average mass fraction of carbon is between 0.09% and 0.17%,

-the average mass fraction of cobalt is between 15.5% and 18.5%,

-an average mass fraction of aluminium of less than 0.1%,

-the average mass fraction of nickel is between 7.2% and 9.8%,

characterized in that the component (1) is case hardened and nitrided such that it comprises a nitride layer (3), the nitride layer (3) at least partially covering the substrate (2).

5. The component (1) according to claim 4, wherein the thickness of the nitride layer (3) is between 5 and 180 μm, preferably between 50 and 150 μm, the average mass fraction of carbon of the nitride layer is between 0.4 and 0.6% and the average mass fraction of nitrogen of the nitride layer is between 0.6 and 3%.

6. The component (1) according to claim 4 or claim 5, wherein the matrix (2) has an average mass fraction of:

-the average mass fraction of carbon is between 0.13% and 0.17%,

-the average mass fraction of cobalt is between 17.5% and 18.5%,

-the sum of the average mass fraction of molybdenum and the average mass fraction of tungsten is between 0.9% and 1.3%,

-the average mass fraction of nickel is between 9.2% and 9.8%.

7. The component (1) according to claim 4 or claim 5, wherein the matrix (2) has an average mass fraction of:

-the average mass fraction of carbon is between 0.09% and 0.13%,

-the average mass fraction of cobalt is between 15.5% and 17%,

-an average mass fraction of aluminium of less than 0.1%,

-the average mass fraction of nickel is between 7.2% and 7.8%.

8. Component according to any one of claims 4 to 7, wherein the nitride layer (3) comprises at least one element selected from nitride precipitates and carbonitride precipitates.

9. Component according to claim 8, wherein the nitride layer (3) has a predominantly martensitic phase.

10. A power transmission gear (4) comprising a component (1) according to any one of claims 5 to 9.

Technical Field

The present invention relates to steel components for the aeronautical field, preferably to power transmission gears for the aeronautical field. The invention also relates to a method for manufacturing such a component.

Background

Components used in the aerospace field, particularly power transmission gears, are typically made of steel. The steel used is generally selected to maximize heat resistance, hertzian fatigue strength, low cycle fatigue strength, and resistance to foreign body fouling. The term "steel" in the present invention refers to a metal alloy comprising mainly iron and comprising carbon, wherein the mass fraction of carbon is between 0.008% and 2%.

Two types of steel can be used in a known manner to increase the heat resistance: developed by QuesTek corporationC61^TMAndC64^TM。C61^TMis a steel having the following average mass fraction: the average mass fraction of carbon is between 0.13% and 0.17%, the average mass fraction of cobalt is between 17.5% and 18.5%, the sum of the average mass fraction of molybdenum and the average mass fraction of tungsten is between 0.9% and 1.3%, and the average mass fraction of nickel is between 9.2% and 9.8%.C64^TMAlso developed by the company QuesTek, having the following average mass fractions: the average mass fraction of carbon is between 0.09% and 0.13%, the average mass fraction of cobalt is between 15.5% and 17%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 7.8%. However,C61^TMandC64^TMthe composition of (a) does not have sufficient fatigue strength for certain applications, such as power transmission in the field of aeronautics.

For this purpose, can be toC61^TMAndC64^TMand (6) performing surface hardening. The term "case hardening" or "carburization" refers to a thermochemical treatment that enables carbon to penetrate to the surface of a steel part. Thus, the case hardening, which is carried out by enrichment in the form of a negative carbon gradient in the direction of the core of the component, makes it possible to produce a harder, stronger surface martensite layer than the core of the component. The literature of Kern et al (Kern, C.P., Wright, J.A., Sebastian, J.T., Grabowski, J.L., Jordan, D.F.,&jones, T.M., 2011, havingManufacture and processing of a new vacuum carburized gear steel with very high hardenability, AGMA technical paper 11FTM27) describes the manufacture of a steel for gears with very high hardenabilityC61^TMAndC64^TMthe process of (1), the process comprising the steps of:

-case hardening at a temperature of 1000 ℃ in the austenite range of the steel,

-a gas quenching step of quenching the gas,

-a cryogenic treatment at a temperature between-50 ℃ and-100 ℃, and

-ageing the component at a temperature between 400 ℃ and 500 ℃ in an inert gas atmosphere.

However, although the case-hardened steel thus obtained has an increased fatigue strength compared to steel not subjected to case hardening, such strength is still insufficient for using the obtained steel in certain industrial applications.

For this purpose, it is known to harden the surfaceC61^TMOrC64^TMAnd (4) carrying out shot blasting forming. Shot peening compresses a surface layer of a steel component by projecting microbeads onto the surface of the component. Thus, shot peening improves the fatigue strength of the part by introducing residual compressive stress. However, the shot peening step complicates the production of the part and increases the roughness of the part. Shot peening may not be effective when the part is used at high temperatures (e.g., at temperatures above 400 ℃). In fact, at temperatures higher than 500 ℃, the compressive stresses obtained by shot-peening can be reduced by attenuation (relaxation) of the residual stresses in the part.

Disclosure of Invention

One purpose of the present invention is to propose a solution for increasing the hardness of a part for the aeronautical field, while avoiding the drawbacks associated with shot-peening the part, which is hardened from the surfaceC61^TMOrC64^TMAnd (4) preparing.

In the context of the present invention, this object is achieved by a method for manufacturing a component for the aeronautical field, comprising the following successive steps:

a) providing a component comprising a matrix comprising at least carbon, cobalt, aluminum, and nickel, and having an average mass fraction of:

-the average mass fraction of carbon is between 0.09% and 0.17%,

-the average mass fraction of cobalt is between 15.5% and 18.5%,

-the average mass fraction of aluminium is less than 0.1%, and

-the average mass fraction of nickel is between 7.2% and 9.8%

b) Surface hardening the component at a temperature between 900 ℃ and 1100 ℃ in a controlled atmosphere,

c) quenching and cryogenic treatment of the component in a controlled atmosphere at a temperature between-50 ℃ and-100 ℃, and

d) ageing the component at a temperature between 450 ℃ and 550 ℃ in a controlled atmosphere,

the method is characterized in that the aging treatment comprises the following steps: the component is nitrided in an atmosphere comprising ammonia such that the component comprises a nitrided surface and the nitriding of the component is performed simultaneously with the aging treatment.

Advantageously, the component is case hardened so that the average mass fraction of carbon of at least a portion of the surface of the component is between 0.4% and 0.6%.

Advantageously, the component comprises a surface having a carbon surface content, and the surface hardening step of the method comprises repeating carbon enrichment cycles of the component, each carbon enrichment cycle comprising the following sub-steps:

b1) injecting a carburizing gas into a carburizing chamber containing the component to enrich the surface of the component with carbon and increase the surface carbon content of the component to a predetermined maximum surface carbon content, at a time t between 60s and 300s₁Maintaining a temperature in the chamber between 900 ℃ and 1100 ℃, and

b2) at a time t between 15s and 2000s₂An inert gas is injected into the carburizing chamber to diffuse carbon from the surface of the component into the interior of the component and reduce the surface carbon content to a predetermined minimum surface carbon content.

Advantageously, the case hardening step b) comprises injecting a carburizing gas into the carburizing chamber containing the component, the carburizing gas being selected from at least propane and acetylene.

Another subject of the invention is a steel component for the aeronautical field, which can be obtained by the method of the invention, comprising a matrix, and the matrix comprises at least carbon, cobalt, aluminum and nickel, and has the following average mass fractions:

-the average mass fraction of carbon is between 0.09% and 0.17%,

-the average mass fraction of cobalt is between 15.5% and 18.5%,

-an average mass fraction of aluminium of less than 0.1%,

-the average mass fraction of nickel is between 7.2% and 9.8%,

the component is characterized in that the component is case hardened and nitrided such that the component comprises a nitrided layer at least partially covering the substrate.

Advantageously, the component may comprise the following features taken alone or in any technically feasible combination thereof:

-the nitrided layer has a thickness comprised between 5 and 180 μm, preferably between 50 and 150 μm, an average mass fraction of carbon of the nitrided layer comprised between 0.4 and 0.6%, and an average mass fraction of nitrogen of the nitrided layer comprised between 0.6 and 3%, preferably between 0.3 and 1%.

-the matrix has an average mass fraction as follows: an average mass fraction of carbon between 0.13% and 0.17%, an average mass fraction of cobalt between 17.5% and 18.5%, the sum of the average mass fraction of molybdenum and the average mass fraction of tungsten between 0.9% and 1.3%, and an average mass fraction of nickel between 9.2% and 9.8%,

-the matrix has the following average mass fraction: the average mass fraction of carbon is between 0.09% and 0.13%, the average mass fraction of cobalt is between 15.5% and 17%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 7.8%.

-the nitride layer comprises at least one element selected from the group consisting of nitride precipitates and carbonitride precipitates.

The nitrided layer has a predominantly martensitic phase.

Another subject of the invention is a power transmission gear comprising a component according to the invention.

Drawings

Other features, objects, and advantages of the present invention will become apparent from the following description, which is to be taken in conjunction with the accompanying drawings, wherein:

fig. 1 schematically illustrates a power transmission gear according to a specific embodiment of the present invention.

[ FIG. 2 ]]FIG. 2 is a photomicrograph of a cross-section of a component according to an embodiment of the invention, wherein the component includesC61^TMA substrate.

[ FIG. 3 ]]FIG. 3 is a photomicrograph of a cross-section of a component according to an embodiment of the invention, wherein the component includesC64^TMA substrate.

Fig. 4 is a graph showing the vickers hardness distribution (profile) of two components according to two specific embodiments of the present invention and the vickers hardness distribution of two known components.

Fig. 5 illustrates a method for manufacturing a component according to a particular embodiment of the invention.

Fig. 6 schematically shows the temperature of a chamber containing a component and a controlled nitrogen potential in the chamber during a failure process of the component according to an embodiment of the invention.

Detailed Description

General structure of component 1

Referring to fig. 1, a component 1 is a steel component for the aeronautical field, comprising a substrate 2, a surface-hardened layer 3b at least partially covering the substrate 2, and a nitrided layer 3a at least partially covering the substrate 2. The component 1 has an outer surface 5. The nitrided layer 3a may be at least partially or completely merged with the surface hardened layer 3 b. In fact, during the case hardening and nitriding, the characteristic penetration lengths of the carbon and nitrogen atoms result in the formation of a case hardened layer 3b and a nitrided layer 3a, respectively, both formed from the surface 5 of the component 1 to a depth that may vary according to the layer in question.

Preferably, the power transmission means 4 comprise the component 1. The power transmission gear 4 may be used to transmit power from the engine to a rotor, such as the main rotor of a helicopter.

The matrix 2 comprises at least carbon, cobalt, aluminium and nickel, and has an average mass fraction as follows: the average mass fraction of carbon is between 0.09% and 0.17%, the average mass fraction of cobalt is between 15.5% and 18.5%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 9.8%. Thus, the substrate 2 is one whose mechanical properties are close to or equal to those of the substrateC61^TMAnd/orC64^TMSteel with mechanical properties.

Preferably, the average mass fraction of carbon of the nitrided layer 3a is between 0.4% and 0.6%. In fact, this mass fraction of carbon is lower than that of the prior art case hardened parts, enabling nitriding of the parts to increase the hardness of the parts, while avoiding the drawbacks associated with the formation of undesirable phases (e.g. retained austenite and/or intergranular precipitated networks).

Preferably, the matrix 2 has an average mass fraction as follows: the average mass fraction of carbon is between 0.13% and 0.17%, the average mass fraction of cobalt is between 17.5% and 18.5%, the sum of the average mass fraction of molybdenum and the average mass fraction of tungsten is between 0.9% and 1.3%, and the average mass fraction of nickel is between 9.2% and 9.8%. Thus, the substrate 2 hasC61^TMMechanical properties of (2).

Preferably and/or alternatively, the matrix 2 has an average mass fraction as follows: the average mass fraction of carbon is between 0.09% and 0.13%, the average mass fraction of cobalt is between 15.5% and 17%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 7.8%. Thus, the substrate 2 hasC64^TMMechanical properties of (2).

Referring to fig. 2 and 3, the thickness of the nitride layer 3a is between 5 μm and 180 μm, preferably between 50 μm and 150 μm, and the average mass fraction of nitrogen in the nitride layer 3a is between 0.2% and 3%, preferably between 0.3% and 1%. The hardness of the component is therefore higher than that of a component hardened from a surfaceC61^TMOr surface hardenedC64^TMThe hardness of the manufactured part is maintained at the same timeC61^TMOrC64^TMAnd avoids the disadvantages associated with shot peening the component.

Referring to fig. 2, at a sufficiently large distance from the surface 5 (i.e. a distance of more than 250 μm, preferably more than 500 μm from the surface 5), the composition of the matrix 2 may beC61^TMThe composition of (1). The nitride layer 3a covers the substrate 2 and comprises a surface 5 on the side of the nitride layer 3a opposite to the substrate 2. The nitrided layer 3a has, from the surface 5:

preferably, the surface bonding layer has a thickness starting from the surface, for example comprised between 0 μm and 30 μm, and at least predominantly comprises nitride, and preferably exclusively nitride, and

a diffusion layer, for example between 50 μm and 150 μm thick, located below the bonding layer.

The diffusion layer includes nitrogen, and an average mass concentration of the nitrogen in the diffusion layer is strictly lower than a mass concentration of the nitrogen in the bonding layer. In this layer, nitrogen can diffuse to the case hardened in the form of nitride precipitates and/or carbonitride precipitatesC61^TMOrC64^TMIn (1).

Referring to fig. 3, at a sufficiently large distance from the surface 5 (i.e. a distance of more than 250 μm from the surface 5,preferably a distance greater than 500 μm), the composition of the matrix 2 may beC64^TMThe composition of (1). The nitride layer 3a covers the substrate 2 and comprises a surface 5 on the side of the nitride layer 3a opposite to the substrate 2. The substrate 2 may include a diffusion layer in contact with the nitride layer 3 a. The diffusion layer comprises nitrogen, and the average mass concentration of nitrogen in the diffusion layer is strictly lower than the mass concentration of nitrogen in the nitride layer 3 a.

With reference to FIG. 4, with known case hardeningC61^TMParts and/or known surface hardeningC64^TMThe nitride layer 3a of the component 1 according to the embodiment of the present invention significantly increases the hardness of the component 1 compared to the component. Curve (a) shows the hardness profile of a component 1 according to an embodiment of the invention, the component 1 comprisingC64^TMA substrate. The hardness measured is the vickers Hardness (HV). Curve (b) shows the hardness distribution of a component 1 according to an embodiment of the invention, the component 1 comprisingC61^TMA substrate. Curve (c) shows the hardness profile of a known hard-faced component comprisingC64^TMA substrate. Curve (d) shows the hardness profile of a known hard-faced component comprisingC61^TMA substrate. On the surface of the substrate,the hardness of the components according to embodiments of the invention may be harder than known case hardeningC61^TMComponent orC64^TMThe hardness of the part is 200 HV.

Preferably, the nitride layer 3a includes at least one element selected from the group consisting of nitride precipitates and carbonitride precipitates. Therefore, the hardness of the member 1 can be improved.

Method for producing a component 1

Fig. 5 schematically shows a method for manufacturing a component 1 according to an embodiment of the invention. The method comprises the steps of 101: a component is provided that includes at least carbon, cobalt, aluminum, and nickel. The part has the following average mass fraction: the average mass fraction of carbon is between 0.09% and 0.17%, the average mass fraction of cobalt is between 15.5% and 18.5%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 9.8%. Preferably, the component may be made ofC61^TMMade, i.e. the part has the following average mass fraction: the average mass fraction of carbon is between 0.13% and 0.17%, the average mass fraction of cobalt is between 17.5% and 18.5%, the sum of the average mass fraction of molybdenum and the average mass fraction of tungsten is between 0.9% and 1.3%, and the average mass fraction of nickel is between 7.2% and 9.8%. Preferably, the component may also consist ofC64^TMMade, i.e. the part has the following average mass fraction: the average mass fraction of carbon is between 0.09% and 0.13%, the average mass fraction of cobalt is between 15.5% and 17%, the average mass fraction of aluminum is less than 0.1%, and the average mass fraction of nickel is between 7.2% and 7.8%.

The method comprises the steps of 102: the component is case hardened in a chamber whose atmosphere is controlled at a temperature between 900 ℃ and 1100 ℃. Preferably, the case hardening step 102 may include a plurality of carbon enrichment cycles 106 of the surface. The enrichment cycle 106 includes a sub-step of injecting a carburizing gas 107 and a sub-step of injecting an inert gas 108.

During the sub-step of injecting the carburizing gas 107, the carburizing gas is injected into a carburizing chamber containing the component to enrich the surface of the component with carbon and increase the surface carbon content of the component to a predetermined maximum surface carbon content. At time t₁(e.g., between 60s and 300 s), the temperature in the chamber is maintained between 900 ℃ and 1100 ℃. The carburizing gas may be selected from at least propane and acetylene. The dilution ratio of the carburizing gas injected may be between 5% and 75%, preferably between 10% and 25%, and the carburizing gas may be injected at a pressure of between 0.1 bar and 3 bar (preferably equal to 230 ± 50 mbar). The term "dilution" refers to the dilution of the carburizing gas in an inert gas (e.g., argon or nitrogen).

In the sub-step of injecting the inert gas 108, at a time t between 20 minutes and 3 hours₂An inert gas is injected into the carburizing chamber to diffuse carbon from the surface of the component into the interior of the component and reduce the surface carbon content to a predetermined minimum surface carbon content. The inert gas may be selected from argon and nitrogen. The successive enrichment cycles 106 enable the component to be enriched in carbon while preventing the mass fraction of carbon on the surface of the component from exceeding a predetermined higher surface content.

After the case hardening 102 is a quenching 103. The quenching 103 includes injecting a gas (e.g., an inert gas) into the carburizing chamber at room temperature (i.e., a temperature between 0 ℃ and 50 ℃) to stop the carburizing reaction and cause a martensitic transformation. The quenching step 103 enables the transformation of the austenite phase mainly into the martensite phase.

Quenching 103 is followed by cryogenic treatment 104. The cryogenic treatment comprises controlling the temperature of the chamber containing the component at a temperature between-100 ℃ and-50 ℃. In addition to the quenching 103, the cryogenic treatment step 104 enables the remaining portion of the austenitic phase of the component to be transformed into the martensitic phase. After the cryogenic treatment 104, the component has at least predominantly a martensitic phase, in particular 90% by volume, even more preferably only a martensitic phase.

Cryogenic treatment 104 is followed by aging treatment 105 of the component. The aging treatment 105 of the component is performed by controlling the temperature of the chamber of the component to a temperature between 450 ℃ and 550 ℃, preferably between 480 ℃ and 500 ℃. Even more preferably, when the basic body 2 of the component hasC61^TMWhen the components (A) are used, the aging treatment temperature is between 480 ℃ and 485 ℃. Thus, the ageing treatment of the component can be optimised without causing a transformation of the martensitic phase. Even more preferably, when the basic body 2 of the component hasC64^TMThe aging treatment temperature is 490-500 deg.C. Thus, the ageing treatment of the component can be optimized without causing a phase transformation in the martensite phase.

The inventors have found that it is possible to combine the ageing treatment of the component with the nitriding of the component. In other words, in the method according to an embodiment of the invention, the nitriding of the component is performed simultaneously with the ageing treatment of the component. Simultaneous implementation of the ageing treatment and nitriding is achieved by injecting ammonia (preferably free ammonia) into the chamber during ageing treatment of the component. By applying a nitrogen potential (K) in the chamber_n) To control the formation of the nitride layer 3 a. For example, a K equal to 3 is applied within 1 hour_nThen applying a K equal to 0.6 within 11 hours_nThe nitrided layer 3a can be made to meet the desire to increase the hardness level of the surface layer while avoiding the formation of undesirable metallurgical phases (e.g., an intergranular network of carbonitrides).

In fact, for most steel alloys, it is not possible to combine ageing and nitriding. Nitriding must be carried out at a temperature below the tempering temperature of the steel alloy to avoid transformation of the martensitic phase of the part by formation of carbide precipitates, resulting in metallurgical softening. However, for most steel alloys in case hardened condition for pinion applications, the tempering temperature is much lower than the minimum temperature required for nitriding. For example, the tempering temperature in the case hardened condition is less than or equal to 190 ℃ for a steel alloy having an average mass fraction of: the average mass fraction of carbon is equal to 0.16%, the average mass fraction of nickel is equal to 3.20%, the average mass fraction of chromium is equal to 1.00%, and the average mass fraction of molybdenum is equal to 0.16%. Which is too low to allow nitriding of the component while tempering the component made of such a steel alloy. In practice, nitriding requires higher temperatures, preferably temperatures higher than 450 ℃. Therefore, in the method according to the embodiment of the present invention, performing the aging treatment and the nitriding simultaneously enables the manufacturing method of the component 1 to be simplified.

Referring to fig. 6, curve (e) shows the controlled temperature in the chamber containing the component with respect to the processing time of the component during a process according to an embodiment of the present invention. Curve (f) shows a controlled nitrogen potential K in the chamber with respect to the processing time of the component during a process according to an embodiment of the invention_n. The term "nitrogen potential" refers to the ratio P_NH3/P^{3 /2} _H2Wherein P is_NH3Is the partial pressure of ammonia which dissociates at the surface 5 of the component to produce monatomic nitrogen which will diffuse into the ferritic matrix, and P^3/2 _H2Is 3/2 times the partial pressure of the dihydrogen at the surface 5.

During the aging treatment 105, in a first stage, free ammonia may be injected at a flow rate such that the nitrogen potential is between 1 and 5, preferably between 2.5 and 3.5. The duration of the first phase is between 20min and 2h, preferably between 50min and 70 min. During the second stage, the free ammonia may be injected at a flow rate such that the nitrogen potential is between 0.1 and 1, preferably between 0.3 and 0.8. The duration of the second stage is greater than 5 hours, preferably between 10 and 12 hours.