EP0885980B1 - Process for forming a superficial layer having a high hardness by plasma-free thermochemical treatment - Google Patents

Description

The present invention relates to a process for the formation, by a treatment Thermochemical without plasma, a superficial layer with hardness high and possessing tribological properties, on parts made of alloys sensitive or reactive towards nitrogen, carbon and / or oxygen.

It applies in particular but not exclusively to processing under controlled atmosphere at a high temperature, a titanium-based alloy or zirconium in an atmosphere that may include ammonia, a hydrocarbon and / or an oxidizing gas or even a gaseous composition including one or more of these compounds.

In general, we know that there are at present various techniques such thermochemical treatments.

The oldest, namely the salt bath tends to disappear because it is particularly polluting and dangerous because of the release of toxic gases and rinsing water it generates.

More recently, ion bombardment treatments involve relative vacuum heat treatment plant specially equipped with to generate on the parts to be treated a glow discharge in a treatment gas atmosphere. This technique has the disadvantage of being relatively expensive and not suitable for complex shaped parts and, in particular, of tubular shape and this, because of hollow cathode.

Furthermore, the thermochemical treatments carried out under a gaseous atmosphere atmospheric pressure, such as gaseous nitriding, consist in wearing the parts at a temperature of the order of 500 ° C to 600 ° C and to sweep them with a gas nitriding such as ammonia. It is the same with regard to treatments carried out under high pressure, "Hochdrucknitrieren von Titanwerkstoffen "HTM Harterei - Technische Mitteilungen, 46 (1991) Nov./Dez, No. 6 and in patent JP 0709541. This treatment presents the disadvantage of being long, consuming large quantities of gas from treatment and therefore to be relatively polluting.

Similarly, it has been proposed, by the patent EP-A-449793, a method consisting in inject pure nitrogen onto parts to be treated heated to a temperature of 650 ° 1000 ° C at a pressure below atmospheric pressure.

In an attempt to resolve the problems mentioned above, the plaintiff already proposed (French Patent No. 94 05062) a low nitriding process pressure carried out at a temperature of the same order as before and making operating a process gas comprising ammonia and a catalyst. This catalyst is intended to promote the dissociation of ammonia on contact parts to be treated and to oppose the recombination of the active nitrogen from of this dissociation in molecular nitrogen.

This process makes it possible to obtain excellent results for the treatment of steel and steel alloys, however, has the disadvantage of using a relatively expensive treatment gas composition and an installation Sophisticated to ensure a homogeneous sweep of the parts to be treated by the gases treatment. Another disadvantage of this method lies in the fact that in temperature and pressure conditions suitable for the treatment of steel it is inoperative on titanium or zirconium alloys. In addition, the catalyst used is incompatible with the processing of titanium and zirconium alloys because it would form a diffusion barrier to nitrogen (Ti oxide layer or Zr).

However, the aim of the invention is to provide a treatment method of this type alloys.

• un dégraissage préalable des pièces à traiter,
• l'enfournement des pièces à l'intérieur de l'enceinte,
• une première mise sous vide de l'enceinte de manière à éliminer les éléments polluants,
• une montée en température du four par convection à l'azote et/ou par rayonnement jusqu'à l'obtention d'une température de 650°C à 900°C, la pression étant maintenu à une pression inférieure ou égale à 0,1mbar,
• le maintien de cette température pendant une durée déterminée de manière à obtenir une température homogène des pièces,
• une deuxième mise sous vide éventuelle de l'enceinte pour assurer l'élimination de l'azote injecté à l'occasion de la montée en température du four,
• l'injection d'un gaz de traitement comprenant de l'ammoniaque sur les pièces à traiter portées à une température supérieur à 500°C, et le maintien de la pression à l'intérieur du four à une valeur comprise entre 100 et 600 mbars, de préférence 300 mbars, pendant une durée comprise entre quelques minutes et 24 heures ou plus en fonction de la profondeur de traitement désirée de manière à obtenir :
  • en extrême surface, une couche de combinaison de type Ti_x N_y présentant une épaisseur de quelques microns,
  • une sous-couche de diffusion de dureté plus élevée que celle du substrat,
• une troisième mise sous vide de l'enceinte pour éliminer le gaz de traitement et le refroidissement du four,
• le défoumement des pièces.

For this purpose, it proposes a method of thermochemical treatment without plasma of a superficial layer having a high hardness and having tribological properties, on parts made of alloys of sensitive or reactive metals with respect to nitrogen, such as titanium or zirconium, that it includes the following successive phases:

• a preliminary degreasing of the parts to be treated,
• the charging of parts inside the enclosure,
• a first evacuation of the chamber so as to eliminate the polluting elements,
• a temperature rise of the oven by convection with nitrogen and / or by radiation until a temperature of 650 ° C. to 900 ° C. is obtained, the pressure being maintained at a pressure less than or equal to 0.1mbar ,
• maintaining this temperature for a predetermined period so as to obtain a uniform temperature of the parts,
• a second possible evacuation of the chamber to ensure the elimination of the nitrogen injected during the rise in temperature of the oven,
• injecting a process gas comprising ammonia onto the workpieces heated to a temperature greater than 500 ° C, and maintaining the pressure inside the oven at a value between 100 and 600 mbar , preferably 300 mbar, for a period of between a few minutes and 24 hours or more depending on the desired treatment depth to obtain:
  • in extreme area Ti type of combined _x N _y layer having a thickness of a few microns,
  • a diffusion sub-layer of higher hardness than that of the substrate,
• a third evacuation of the chamber to eliminate the treatment gas and the cooling of the oven,
• the defouement of the pieces.

In the case of a nitriding treatment, this process does not use as a gas only ammonia, that is to say a relatively common gas and cheap. The treatment facility is also located greatly simplified. The substrate treated with titanium alloy or zirconium acts as a catalyst for dissociation of molecules of ammonia which adds to the thermal dissociation effect.

Moreover, at the high temperatures where the treatment is carried out, the problem previously mentioned dissociation and recombination is no longer becomes possible to obtain a good homogeneity of the treatment without having to use special provisions or catalyst gas. Only a brew of the interior atmosphere of the furnace, for example using conventional turbines, may be possibly but not necessarily carried out to increase uniformity.

This treatment makes it possible at the same time to improve the mechanical characteristics of the treated parts, in particular as regards resistance to friction and surface hardness, and to give them a pale yellow appearance that is more or less shiny and particularly aesthetic. This aspect is a function of the initial surface state and the stoichiometry of the Ti _x N _y layer.

In addition, this treatment is a diffusion treatment: it does not generate significant change in the initial roughness of the parts and it eliminates all risk detachment of the titanium nitride layer.

• le domaine biomédical : instruments et prothèses
• les sports et loisirs
• le domaine automobile et aérodynamique
• l'armement
• le domaine des arts décoratifs
• la chimie et la pétrochimie

Thanks to these features, the method according to the invention applies to the treatment of parts used in many fields of applications, namely, in particular

• the biomedical field: instruments and prostheses
• sports and recreation
• the automotive and aerodynamic field
• armament
• the field of decorative arts
• chemistry and petrochemistry

Another advantage of this process is that because it operates at low pressure (always lower than the atmospheric pressure) it only consumes very little treatment gas and is therefore not polluting with respect to the processes high pressure which require the use of specific furnaces and safety due to high pressure.

Advantageously, in the case where the treatment is a nitriding treatment, the method according to the invention may further comprise a post-processing phase intended to dehydrogenate the treated alloy.

Indeed, it is found that the atomic nitrogen supply at the surface of a room by means of the thermal and catalitic dissociation of ammonia molecules can lead, at the nitriding temperatures used, to a hydrogen enrichment and to the formation of hydrides, for example titanium hydrides of TiH type up to TiH ₂ .

Consequently, it is advisable, in some cases, to proceed with a phase additional treatment, after the low-pressure nitriding treatment of the alloy (for example titanium alloy) in order to extract this hydrogen which could be unfavorable to the fatigue strength of the part, in particular with less resistance to the propagation of cracks.

This dehydrogenation treatment can be carried out by bringing the pieces to a temperature of the order of 700 to 900 ° C for a period ranging from 1 to 5 hours, in a vacuum of 10 ^-3 to 10 ^-4 mbar.

By way of example, a TH6V-type titanium alloy, dehydrogenated at 790 ° C. for 2 hours, under a vacuum of 10 ^-4 mbar, will have its hydrogen content drop from 256 to 11 ppm.

An example of implementation of the method according to the invention will be described below in as a non-limitative example.

In this example the treatment plant used involved a furnace of vacuum heat treatment of conventional structure equipped with a turbine of flow of process gases. This oven includes a sealed enclosure containing a muffle made of a material (metal or graphite) that can not be retain pollutants (especially oxygen or water vapor) likely to affect the quality of the treatment. Inside the mitt are installed electrical heating resistors (essentially by radiation and convection) able to bring the temperature of the parts to more than 1000 ° C.

This chamber is connected on the one hand to pumping equipment suitable for realize a primary vacuum P <0.1 mbar and, on the other hand, two gas sources (one source of nitrogen and a source of ammonia) via a distribution.

After a careful degreasing, the parts to be treated (here alloy titanium TA6V) have been arranged in the muffle, on a mounting preferably in titanium alloy having previously undergone the same treatment. On this montage the parts were arranged so as to be distant from each other a few millimeters so that the diffusion is the most homogeneous possible on their surface.

Once the charge has been prepared and the enclosure is sealed, a vacuum pushed to be established (P <0.1 mbar) then maintained ten minutes of to eliminate as many pollutants as possible. The atmosphere is then a neutral atmosphere composed of nitrogen, free of oxygen and water vapor.

At the end of this preparatory phase, a temperature rise of convection oven with nitrogen (injected from the nitrogen source) and / or by radiation (resistors) until a temperature level between 500 ° C and 900 ° C, here of the order of 900 ° C.

Temperature maintenance at this level (temperature plateau) was continued for a period of time sufficient to ensure the homogeneity of the room temperature.

It was then carried out a primary vacuum of the enclosure to ensure elimination of the previously injected nitrogen and then at the start of the diffusion by an ammonia injection at a pressure P between 100 and 900 mbar, here 300 mbar, the temperature being above 500 ° C.

This diffusion phase was continued for approximately 7 hours in order to obtain a diffusion layer of approximately 0.040 mm average thickness.

At the end of the diffusion phase, a new evacuation was performed primary chamber to remove ammonia and then proceeded to a cooling the oven to room temperature. We then opened the oven and unloaded the load.

The treated parts had, at the extreme surface, a compact and homogeneous yellow layer of titanium nitride Ti _x N _y with a thickness of about 4 μm and a very high hardness (> 1000 HV), and consequently a very good resistance to friction. and excellent wear resistance. The diffusion layer (a few hundredths of a millimeter thick and with a hardness of> 400 HV) was then likely to improve the fatigue resistance (the core hardness being 339 HV).

An important advantage of this method is that it makes it possible to obtain a very good homogenous treatment even in the case of pieces of shapes and complex geometries including hollow shapes. In addition the treatment extends to the contact areas of the parts on their support.

Of course, the invention is not limited to the embodiment described above. Indeed, the treatment gas could be other than ammonia and could for example consist of a hydrocarbon-based atmosphere (C ₂ H ₂ , C ₃ H ₈ , CH ₄ ....) with a view to cementer superficially these alloys. In this case we obtain a surface layer of metallic gray color, of great hardness and having increased tribological properties.

Similarly, the treatment atmosphere could comprise an oxidizing gas such as oxygen so as to obtain a surface layer (Ti0, Ti0 ₂ , Ti ₂ O ₃ , Zr0 ₂ ..) with various colors (blue, green, violet). ) and a great hardness.

This Ti _x O _y oxide layer, of a thickness of a few microns and a hardness greater than 1000 HV is compact and homogeneous over the entire surface of the workpiece. The colors that we obtain are brilliant and very varied. It considerably improves the friction resistance of the parts. Of course the appearance of the parts is a function of the initial surface condition and the stoichiometry of the Ti _x O _y layer.

The treatment atmosphere could also consist of a combination of NH ₃ + CH ₄ so as to obtain a surface layer of carbonitrides TiC _x N _y or Zr CN pinkish color or butter.

It could also consist of a mixture of oxygen and nitrogen O ₂ + N ₂ or N ₂ 0 + N ₂ so as to obtain oxynitride layers: Ti O _x N _y ; Zr O _x N _y of various colors.

Claims (6)

1. Process for forming, by plasma-free thermochemical treatment, a superficial layer having high hardness and tribologic properties, on parts in metal alloys sensitive or reactive to nitrogen, such as titanium or zirconium, characterized in that it comprises the following successive phases:

   prior degreasing of the parts to be treated,

   furnacing the parts in the enclosure,

   producing a first vacuum in the enclosure to remove polluting elements,

   raising the furnace temperature by nitrogen convection and/or by radiation until a temperature of 650°C to 900°C is reached, the pressure being maintained at a pressure of 0.1mbar or less;

   maintaining this temperature for a determined period of time so as to obtain a homogeneous temperature of the parts,

   producing an optional second vacuum in the enclosure to ensure removal of the nitrogen injected when raising the furnace temperature,

   injecting a treatment gas containing ammonia onto the parts to be treated raised to a temperature greater than 500°C, and maintaining the pressure inside the furnace at a value of between 100 and 600 mbars, preferably 300 mbars, for a period of between a few minutes and 24 hours or more in relation to the depth of desired treatment, so as to obtain:

   on the top surface a combination layer of Ti_xN_y type having a thickness of a few microns,

   a diffusion sub-layer of greater hardness than the substrate hardness,

   producing a third vacuum in the enclosure to remove the treatment gas and cool the furnace,

   removing the parts from the furnace.
2. Process as in claim 1 characterized in that said above atmosphere is a neutral atmosphere and free of oxygen and water vapour.
3. Process as in claim 2 characterized in that said above atmosphere consists of nitrogen.
4. Process as in any of the preceding claims characterized in that comprises mixing the treatment gas.
5. Process as in claim 1, characterized in that said above treatment is a carbonitriding treatment and in that, in this case, the treatment gas is a combination of NH₃+CH₄.
6. Process as in claim 1 characterized in that said above treatment is an oxynitriding treatment and in that, in this case, the treatment gas consists of a mixture of oxygen and nitrogen.