EP2655684B1 - Method for reinforcing an alloy by plasma-nitriding - Google Patents

Description

TECHNICAL AREA

The present invention relates to a method of manufacturing a reinforced alloy. It relates more particularly to a process for manufacturing an alloy reinforced with nanoparticles of metal nitride.

TECHNICAL BACKGROUND

The alloys reinforced with nitride particles (so-called "NDS" alloys for "Nitride Dispersion Strenghtened") have improved mechanical properties compared to master alloys, including better tensile strength, creep, compression or fatigue.

These properties can be further improved by decreasing the size of the dispersed particles.

Many studies aim to develop a process for manufacturing an NDS alloy with small particles.

Among these methods, gaseous nitriding is commonly employed. The document " Johansson et al., Nitrogen alloyed stainless steels produced by nitridation of powder, Metal Powder Report, 1991, 46 (5), p. 65-68 Discloses a process in which titanium-containing austenitic steel powder is heated around 1000 ° C under a pure nitrous (N ₂ ) atmosphere to form precipitates of an intermediate nitride, Cr ₂ N chromium nitride Under the action of a heat treatment complementary to 1200 ° C, these precipitates are then dissolved to result in an alloy reinforced by titanium nitride dispersions.

The additional heat treatment of this nitriding process nevertheless has the disadvantage of producing dispersions with an average size of up to 300 nm. This large size of the dispersions tends to degrade the mechanical properties of the reinforced alloy.

Another type of process used to make an NDS alloy involves powder metallurgy. In the document US 4,708,742 a powder of a nitrogen donor compound (such as Cr ₂ N) is co-milled with a powder for forming the metal matrix of a reinforced alloy. The resulting powder mixture is subjected to a heat treatment to decompose the nitrogen donor so that the dinitrogen thus available forms a nitride with one of the elements of the metal matrix. After consolidation of the powder mixture, an alloy reinforced by nitride dispersions is obtained.

The heat treatment for producing nitrogen by decomposition of the nitrogen donor results in this powder metallurgy process being similar to a nitriding process.

The need to have an intermediate nitride such as Cr ₂ N before forming the final metal nitride, therefore again has an adverse effect on the size of the nanoparticles dispersed which is at best of the order of a micrometer.

The processes of the state of the art cited above therefore have the disadvantage that they do not make it possible to manufacture a reinforced alloy in which the nanoparticles have essentially a reduced average size, typically less than 50 nm.

In addition, the need to pass through an intermediate nitride means that these processes are subject to parasitic reactions that make it difficult to control the composition and the amount of particles that are present in the obtained reinforced alloy.

US 2007/295427 discloses a treated austenitic steel and the associated treatment method comprises an austenitic steel and a nonmetallic chemical element incorporated in a surface of the steel. The surface has a bilayer structure with a layer of compound above and an underlying diffusion layer, which protects the surface against embrittlement by hydrogen.

SUMMARY OF THE INVENTION

One of the aims of the invention is therefore to provide a process for manufacturing an "NDS" alloy comprising nanoparticles of which at least 80% have an average size of less than 50 nm, such a method that may allow better control of the composition. and the amount of these nanoparticles within the alloy.

The present invention thus relates to a method for manufacturing a reinforced alloy comprising a metal matrix in the volume of which nanoparticles are dispersed, of which at least 80% have an average size of 1 nm to 50 nm, the nanoparticles comprising at least one nitride selected from nitrides of at least one metal element M belonging to the group consisting of Ti, Zr, Hf, and Ta.

1. a) on réalise à une température de 200°C à 700°C une nitruration par plasma d'un alliage de base afin d'y insérer de l'azote interstitiel, l'alliage de base incorporant 0,1% à 1% en poids de l'élément métallique M et étant choisi parmi un alliage austénitique, ferritique, ferritique-martensitique ou à base nickel ;
2. b) on diffuse à une température de 350°C à 650°C l'azote interstitiel au sein de l'alliage de base ; et
3. c) on précipite le nitrure à une température de 600°C à 900°C pendant une durée de 10 minutes à 10 heures, afin de former les nanoparticules dispersées dans l'alliage renforcé.

This process comprises the following successive steps:

1. a) a plasma nitriding of a base alloy is carried out at a temperature of 200 ° C. to 700 ° C. in order to insert interstitial nitrogen, the base alloy incorporating 0.1% to 1% by weight; weight of the metal element M and being chosen from an austenitic, ferritic, ferritic-martensitic or nickel-based alloy;
2. b) diffusing at 350 ° C to 650 ° C the interstitial nitrogen within the base alloy; and
3. c) the nitride is precipitated at a temperature of 600 ° C to 900 ° C for a period of 10 minutes to 10 hours, in order to to form the nanoparticles dispersed in the reinforced alloy.

Advantageously, the process of the invention does not involve the use of an intermediate nitride intended to form the metal nitride constituting all or part of the dispersed nanoparticles.

This is made possible by the manufacturing method of the invention which comprises separate steps.

Thus, during the plasma nitriding step followed by the diffusion step, the nitrogen intended to form the nitride is introduced into the base alloy in interstitial form, namely as nitrogen in solid solution. in the base alloy, and not in molecular form N ₂ .

By its preferential chemical affinity with the metallic element M, the interstitial nitrogen then combines directly with all or part of this element, under the influence of the diffusion and / or precipitation temperature (generally under the influence of a temperature of between 500 ° C and 650 ° C) to form the nitride. Where appropriate, especially for a temperature in a common range of between 600 ° C. and 650 ° C., the diffusion and precipitation step may therefore be totally or partially overlapping.

During step c), the nitride is precipitated by a germination-growth phenomenon in order to form the nanoparticles dispersed in the reinforced alloy.

In the context of the invention, the passage through an intermediate nitride is therefore not necessary, unlike the processes of the state of the art that require a complementary heat treatment generally performed at a temperature of about 1200 ° C in order to dissociate a nitride such as Cr ₂ N.

Another advantage of the manufacturing method of the invention is that the temperature applied during its different stages can be chosen with great freedom.

Thus, the plasma nitriding step a) is carried out at a temperature of 200 ° C. to 700 ° C., preferably 200 ° C. to 600 ° C., even more preferentially from 350 ° C. to 450 ° C.

Step b) of diffusion of the interstitial nitrogen is carried out at a temperature of 350 ° C to 650 ° C, preferably 350 ° C to 500 ° C. Its duration is generally from 5 hours to 500 hours, preferably from 10 hours to 200 hours. It is generally inversely proportional to the temperature of the interstitial nitrogen diffusion step.

Once the nitrogen diffuses interstitial form in the base alloy, the precipitation temperature can advantageously be chosen to control the size of the nitride of the metal element M at the expense of the precipitation of a metal element M 'such that Cr whose dissolution of the associated nitride Cr ₂ N can only occur at a temperature of 1100 ° C.

After direct combination of the interstitial nitrogen with all or part of the metal element M in order to form the nitride, the nitride precipitation step c) is carried out at a temperature which is from 600 ° C. to 900 ° C., preferentially from 600 ° C to 800 ° C, even more preferably from 600 ° C to 700 ° C. Its duration is 10 minutes to 10 hours, preferably 30 minutes to 2 hours. It is generally inversely proportional to the temperature of the nitride precipitation step.

Such a choice of temperature is not accessible to the processes of the state of the art because the reactivity of the nitriding medium imposes on them a processing temperature which is higher and / or of more limited choice.

The absence of intermediate nitride and / or the freedom of choice in the temperature of implementation of the process of According to the invention, this method makes it possible to obtain a reinforced alloy whose matrix comprises dispersed nanoparticles of smaller average size than those obtained by the processes of the state of the art mentioned above.

DETAILED DESCRIPTION OF THE INVENTION

In the present description, the verb "to understand", "to include", "to incorporate", "to include" and its conjugate forms are open terms and thus do not exclude the presence of element (s) and / or step (s). ) additional to the element (s) and / or initial stage (s) set out after these terms. However, these open terms also include a particular embodiment in which only the element (s) and / or initial stage (s), to the exclusion of all others, are targeted; in which case the term open also refers to the closed term "consist of", "constitute of" and its conjugated forms.

The use of the undefined article "a" or "an" for an element or a step does not exclude, unless otherwise stated, the presence of a plurality of elements or steps.

Unless otherwise indicated, the chemical composition of the base alloy, the reinforced alloy or the metal matrix and the nanoparticles it contains is expressed in the present description as a percentage by weight relative to the weight of the alloy considered. .

Step a) of the manufacturing method of the invention consists of a plasma nitriding as known to those skilled in the art, described for example in the document "Techniques de l'ingénieur, reference M 1227," Nitriding, nitrocarburizing and derivatives ", Chapter 4".

It mainly comprises the formation of a plasma by imposing a potential difference between an anode and a cathode in a gaseous medium comprising nitrogen, so that reactive species are produced. The reactive species can comprise neutral (atomic N) species, or even ionized or excited (such as for example N ⁺ or N ₂ vibrationnally excited), the nitriding then being called ionic in the latter case. Using appropriate heat treatments, these species diffuse interstitial form in the base alloy to then form a nitride with the constituent atoms of this alloy.

According to the invention, the plasma nitriding is performed on a base alloy incorporating 0.1% to 1% by weight of at least one metal element M selected from Ti, Zr, Hf, or Ta, preferably 0.5 % to 1% by weight of this element.

Preferably, the metal element M is titanium.

The base alloy can be in the form of powder or part.

It is selected from an austenitic, ferritic, ferritic-martensitic or nickel-based alloy.

Plasma nitriding can be carried out using a gaseous medium comprising nitrogen (in the form of molecular nitrogen (N ₂ ) and / or as a gaseous nitrogen compound such as, for example, NH ₃ and / or N ₂ H ₂ ). The nitrogen is diluted in a chemically inert gas (vis-à-vis the other constituents of the gaseous medium), such as, for example, H ₂ .

The gaseous medium may further comprise a carbon species, such as for example CH ₄ .

The gaseous medium may for example comprise 20% to 30% by volume of N ₂ and / or of gaseous nitrogen compound, optionally supplemented with 5% to 20% by volume of the carbon species (for example CH ₄ ), the rest being constituted by the chemically inert gas (for example H ₂ ).

The pressure of the gaseous medium is generally less than atmospheric pressure, for example from 1 mbar to 100 mbar, preferably from 1 mbar to 10 mbar, still more preferably from 1.5 mbar to 5 mbar.

Plasma nitriding is generally carried out for a period of 5 hours to 300 hours, preferably from 10 hours to 200 hours, even more preferably from 24 hours to 100 hours.

Preferably, after the nitrogen diffusion step, the base alloy comprises 1000 ppm to 2000 ppm by weight of nitrogen in interstitial form, which allows the preferential formation of a nitride of the metal element M at the expense of other nitrides such as Cr ₂ N.

At the end of the manufacturing process of the invention, the reinforced alloy obtained comprises a metal matrix in which nanoparticles composed entirely or partially of at least one metal nitride are dispersed.

The metal matrix of the reinforced alloy has the chemical composition of the base alloy.

The manufacturing method of the invention also makes it possible to preserve the structure of the base alloy (austenitic, ferritic or ferritic-martensitic structure) in the reinforced alloy.

The nanoparticles are dispersed in all or part of the volume of the reinforced alloy. They represent most often 0.5% to 2% (typically 1%) of the volume of the reinforced alloy.

When the base alloy is in the form of a workpiece, the nanoparticles are dispersed in the reinforced alloy to a depth that may be between 30 μm and 1 mm, preferably between 50 μm and 500 μm, more preferably between 50 μm and 100 μm.

At least 80% of the nanoparticles have an average size of 1 nm to 50 nm, preferably at least 90% a average size of 1 nm to 10 nm, even more preferably at least 95% an average size of 0.5 nm to 5 nm.

In order to obtain such a reduction in size, the average size of the nanoparticles can be modulated by varying parameters such as the plasma nitriding temperature, the diffusion temperature, and / or the pressure of the gaseous medium.

It can also be reduced by decreasing the temperature and / or the duration of the precipitation step c), which are, for example, 850 ° C. for 1 hour.

For the purposes of the invention, the term "average size" means the mean value of the diameter of the nanoparticles when they are substantially spherical, or the average value of their main dimensions when they are not substantially spherical.

The quantity of nanoparticles (at least 80%) having a determined average size can be easily counted using a technique known to those skilled in the art such as Transmission Electron Microscopy (TEM).

The nanoparticles generally have a composition such that they comprise in atomic percentage of 30% to 70% of nitrogen, combined in the form of nitride with at least one metal element M. This quantity is conditioned by the quantity of interstitial nitrogen introduced into the base alloy, knowing that generally all of the interstitial nitrogen combines with the metal element M.

When the carbon element is additionally present in the gaseous medium in the form of carbon species, all or part of this element can be combined directly with the metal element M and optionally with nitrogen during the plasma nitriding. We then obtain nanoparticles in which the nitride is wholly or partly in the form of carbonitride of the metal element M.

As is known to those skilled in the field of metallurgy, the nitride or carbonitride of the metal element M formed does not necessarily have a defined stoichiometry. These species are most often represented by the formula M (N) or M (C, N), or alternatively the formula M _x C _y N _z in which the indices "x", "y" and "z" indicate respectively the relative atomic proportion of the elements M, C and N within the nitride or carbonitride formed.

The nitride of a metal element M may, however, comprise one or more nitrides of defined stoichiometry which may optionally coexist within the nanoparticles. For example, titanium nitride may be present in a nanoparticle in the form TiN and / or Ti ₃ N ₄ .

Preferably, the nitride present in the nanoparticles thus belongs to the group consisting of TiN, Ti ₃ N ₄ , ZrN, HfN and TaN.

Of course, the nanoparticles may also include other species that were initially present in the powders or that formed during the manufacturing process of the invention.

• de 10 à 120ppm de silicium ;
• de 10 à 100ppm de soufre ;
• moins de 20ppm de chlore ;
• de 2 à 10ppm de phosphore ;
• de 0,1 à 10ppm de bore ;
• de 0,1 à 10ppm de calcium ;
• moins de 0,1ppm de chacun des éléments suivants : lithium, fluor, métaux lourds, Sn, As, Sb.

The reinforced alloy may further comprise by weight at least one of the following (sometimes as unavoidable impurity of manufacture):

• from 10 to 120 ppm of silicon;
• from 10 to 100 ppm sulfur;
• less than 20ppm of chlorine;
• from 2 to 10 ppm of phosphorus;
• 0.1 to 10 ppm boron;
• from 0.1 to 10 ppm of calcium;
• less than 0.1 ppm of each of the following: lithium, fluorine, heavy metals, Sn, As, Sb.

The manufacturing method of the invention may comprise a consolidation step by hot spinning carried out during (optionally instead) or after step c) precipitation of the nitride, preferably at a temperature of less than or equal to 850 ° C, preferably at a temperature of 600 ° C to 850 ° C. This hot-spinning step is preferably carried out when the base alloy is in the form of a powder.

Other objects, features and advantages of the invention will now be specified in the following description of a particular embodiment of the invention, given by way of non-limiting illustration, with reference to FIG. Figure 1 attached.

BRIEF DESCRIPTION OF THE FIGURES

The Figure 1 represents a MET plate of a reinforced alloy obtained by the manufacturing method of the invention.

DESCRIPTION OF A PARTICULAR EMBODIMENT

A ferritic powder composed of Fe-18Cr-1W-0.8Ti base alloy is nitrided using the manufacturing method of the invention.

This powder has a particle size such that the average size of its grains is 100 microns.

• brassage de la poudre ;
• milieu gazeux constitué en volume de 71% H₂, 23% N₂ et 6% CH₄ ;
• pression du milieu gazeux de 2,5 mbar ;
• cycle de 15 heures de nitruration plasma réalisé à 380°C, suivi par un traitement thermique de diffusion réalisé à une température de 400°C pendant 200 heures.

The conditions for implementing the method are as follows:

• brewing the powder;
• a gaseous medium consisting in volume of 71% H ₂ , 23% N ₂ and 6% CH ₄ ;
• pressure of the gaseous medium of 2.5 mbar;
• 15 hour cycle of plasma nitriding performed at 380 ° C., followed by a diffusion heat treatment carried out at a temperature of 400 ° C. for 200 hours.

TEM analysis of the powder obtained shows the absence of nitride precipitation.

Consolidation is then carried out by hot spinning at 850 ° C for 1 hour, during which the titanium nitride precipitates.

A sample taken from the core of the obtained reinforced alloy is examined by TEM. The picture obtained represented on the Figure 1 shows the presence of numerous particles comprising titanium nitride of average size between 2 nm and 8 nm.

Claims (23)

1. Manufacturing method of a strengthened alloy comprising a metal matrix in the volume of which nanoparticles are dispersed, of which at least 80% have an average size of 1 nm to 50 nm, said nanoparticles comprising at least one nitride chosen from the nitrides of at least one metal element M belonging to the group consisting of Ti, Zr, Hf and Ta,
   the method comprising the following successive steps:

   a) performing plasma nitriding of a base alloy at a temperature of 200°C to 700°C in order to insert interstitial nitrogen therein, said base alloy incorporating 0.1 % to 1% by weight of the metal element M and being chosen from an austenitic, ferritic, ferritic-martensitic or nickel-based alloy;

   b) diffusing the interstitial nitrogen in said base alloy at a temperature of 350°C to 650°C; and

   c) precipitating the nitride at a temperature of 600°C to 900°C for a period of 10 minutes to 10 hours, in order to form said nanoparticles dispersed in the strengthened alloy.
2. Manufacturing method according to claim 1, wherein plasma nitriding is performed according to step (a) at a temperature of 200°C to 600°C.
3. Manufacturing method according to claim 1 or 2, wherein the interstitial nitrogen is diffused according to step (b) at a temperature of 350°C to 500°C.
4. Manufacturing method according to any one of the preceding claims, wherein the nitride is precipitated according to step (c) at a temperature of 600°C to 800°C.
5. Manufacturing method according to any one of the preceding claims, wherein:

   - plasma nitriding is performed according to step (a) at a temperature of 200°C to 600°C;

   - the interstitial nitrogen is diffused according to step (b) at a temperature of 350°C to 500°C; and

   - the nitride is precipitated according to step (c) at a temperature of 600°C to 800°C.
6. Manufacturing method according to any one of claims 2 to 5, wherein plasma nitriding is performed according to step (a) at a temperature of 350°C to 450°C.
7. Manufacturing method according to any one of the preceding claims, wherein the interstitial nitrogen is diffused according to step (b) for a duration from 5 hours to 500 hours.
8. Manufacturing method according to any one of claims 4 to 7, wherein the nitride is precipitated according to step (c) at a temperature of 600°C to 700°C.
9. Manufacturing method according to any one of the preceding claims, wherein said base alloy incorporates 0.5% to 1% by weight of the metal element M.
10. Manufacturing method according to any one of the preceding claims, wherein the metal element M is titanium.
11. Manufacturing method according to any one of the preceding claims, wherein the plasma nitriding is performed by means of a gaseous medium comprising nitrogen in the form of molecular nitrogen (N₂) and/or as a gaseous nitrogenous compound.
12. Manufacturing method according to claim 11, wherein the gaseous nitrogenous compound is NH₃ and/or N₂H₂.
13. Manufacturing method according to claim 11 or 12, wherein the gaseous medium comprises 20% to 30% by volume of N₂ and/or of the gaseous nitrogenous compound, the remainder consisting of the chemically inert gas.
14. Manufacturing method according to any one of the claims 11 to 13, wherein the gaseous medium also comprises a carbonaceous species.
15. Manufacturing method according to claim 14, wherein the carbonaceous species is CH₄.
16. Manufacturing method according to claim 14 or 15, wherein the gaseous medium comprises 20% to 30% by volume of N₂ and/or of the gaseous nitrogenous compound, with the carbonaceous species added to the extent of 5% to 20% by volume, the remainder consisting of the chemically inert gas.
17. Manufacturing method according to any one of the preceding claims, wherein the nitride belongs to the group consisting of TiN, Ti₃N₄, ZrN, HfN and TaN.
18. Manufacturing method according to any one of the preceding claims, wherein the nitride is wholly or partly in the form of carbonitride of the metal element M.
19. Manufacturing method according to any one of the preceding claims, wherein at least 90% of said nanoparticles have an average size of 1 nm to 10 nm.
20. Manufacturing method according to any one of the preceding claims, wherein the strengthened alloy also comprises by weight at least one of the following elements:

    - from 10 to 120 ppm of silicon;

    - from 10 to 100 ppm of sulfur;

    - less than 20 ppm of chlorine;

    - from 2 to 10 ppm of phosphorus;

    - from 0.1 to 10 ppm of boron;

    - from 0.1 to 10 ppm of calcium;

    - less than 0.1 ppm of each of the following elements: lithium, fluorine, heavy metals, Sn, As, Sb.
21. Manufacturing method according to any one of the preceding claims, comprising a step of consolidation by hot extrusion performed during or after the step c) precipitating the nitride.
22. Manufacturing method according to claim 21, wherein the hot extrusion step is performed at a temperature of less than or equal to 850°C.
23. Manufacturing method according to any one of the preceding claims, wherein the nanoparticles represent 0.5% to 2% of the volume of the strengthened alloy.

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