EP0801142B1 - Treatment method of a metallic substrate, metallic substrate thereby obtained and his applications - Google Patents

Description

The present invention relates to a method for processing surface of a metal part, and more particularly, a method of surface hardening by enriching carbon or boron with surface layer located near a surface of the part.

To harden the surface of metal parts, especially steel parts, and more particularly stainless steel parts austenitic, whose resistance to wear or abrasion is poor, it is known to enrich a surface layer of parts with an element interstitial such as nitrogen using a cold plasma or an ion beam. There is thus formed on the surface of the part a layer with a thickness of a few tens or a few hundred micrometers (µm) extremely hard and wear-resistant. This layer is a solution nitrogen solid in the metal matrix which has the disadvantage of also contain nitride precipitates which degrade substantially the corrosion resistance of the surface. This degradation corrosion resistance makes hardening by solid solution nitrogen, unsuitable for many applications, especially particular, whenever corrosion resistance is required.

In particular, in the case of stainless steel parts martensitic comprising a surface layer hardened with nitrogen, the corrosion resistance of the parts may become insufficient; in addition, the structure of steel can become austenitic in the surface layer, which is not favorable for ensuring good hardness.

In GB-A-226122.7, a surface treatment of a metal part has been described in which a gas atmosphere containing an element is brought into contact with a surface of the part brought to a temperature between 270 and 550 ° C. interstitial such as carbon or nitrogen, in the ionized state, at a pressure below 1.33.10 ^-2 mbar (10 mtorr). Due to the low pressure of the gaseous atmosphere, the interstitial content of the surface layer remains low even for treatment times of several hours.

In addition, in certain cases, and more particularly, for the most high processing temperatures of compounds such as carbides or nitrides are likely to form in the surface layer, in the form of precipitates.

In the case of surface treatment of a steel part stainless, these compounds contain chromium which is taken up in the surface layer of the stainless steel part. This results in a degradation of the part's resistance to corrosion.

An object of the present invention is to remedy this drawback by proposing a method for hardening the surface of a part metallic which does not deteriorate its resistance to corrosion.

Another object of the present invention is to provide a treatment of the surface of a metal part which can be used industrially, under favorable conditions with regard to costs and duration of treatment.

• on porte la pièce, au moins dans sa zone superficielle, à une température comprise entre 300 et 460°C, et
• on maintient une atmosphère gazeuse renfermant l'élément interstitiel activé au contact de la surface de la pièce, à une pression comprise entre 0,5 et 200 mbars;

de manière à obtenir, dans la couche superficielle, une solution solide exempte de précipités renfermant de 5 à 50 atomes % d'élément interstitiel.
De préférence :

• la pression de l'atmosphère gazeuse au contact de la surface de la pièce pendant le traitement est comprise entre 0,5 et 20 m bars;
• l'atmosphère gazeuse est renferme au moins un composé gazeux comportant l'élément interstitiel, dans un plasma froid.
• l'atmosphère gazeuse renferme un diluant du composé gazeux constitué par l'un au moins des gaz suivants : hydrogène, argon, mélange d'hydrogène et d'argon.

To this end, the subject of the invention is a method for treating the surface of a metal part, in which at least one interstitial element consisting of at least one of the carbon and boron elements is introduced into a surface layer of the part. adjacent to a surface of the part by bringing the interstitial element into contact with the surface of the part and causing the interstitial element to diffuse in the surface layer of the part, characterized in that:

• the part is brought, at least in its surface area, to a temperature between 300 and 460 ° C, and
• maintaining a gaseous atmosphere containing the activated interstitial element in contact with the surface of the part, at a pressure of between 0.5 and 200 mbar;

so as to obtain, in the surface layer, a solid solution free of precipitates containing from 5 to 50 atom% of interstitial element.
Preferably:

• the pressure of the gaseous atmosphere in contact with the surface of the part during the treatment is between 0.5 and 20 m bars;
• the gaseous atmosphere is contains at least one gaseous compound comprising the interstitial element, in a cold plasma.
• the gaseous atmosphere contains a diluent of the gaseous compound consisting of at least one of the following gases: hydrogen, argon, mixture of hydrogen and argon.

The gaseous compound can consist of a hydrocarbon aliphatic or aromatic or by a cyclan.

The gaseous compound can also consist of methane and in this case, the gaseous atmosphere can contain from 5 to 30% in methane volume.

The gaseous compound can also consist of a derivative boron gas, for example, a diborane.

The gaseous atmosphere may also contain nitrogen or a gaseous derivative of nitrogen such as ammonia.

The process applies, in particular, to alloy parts metallic whose structure is face centered cubic, centered cubic or tetragonal, more particularly, to stainless steel parts austenitic or martensitic, with nickel-based alloy parts and to cobalt-based alloy parts.

The process can also be applied to alloy parts metallic based on aluminum or based on titanium.

The metal part obtained, has a surface layer hardened by an interstitial element consisting of at least one of the carbon and boron elements, and optionally, additionally, with nitrogen; the surface layer consists of a homogeneous solid solution free of precipitates containing 5 and 50 atom% and preferably between 10% and 30% interstitial.

The invention will now be described in more detail, but nonlimiting and illustrated by the examples which follow.

To harden the surface of a metal part, by the process according to the invention, the part is placed in the enclosure of a treatment furnace surface by plasma. The enclosure is placed under vacuum and then introduced into the enclosure a mixture of gases, the pressure of which is lower than the pressure atmospheric can be between 0.5 and 200 mbar. The mixture of gas consists of a reactive gas on the one hand, and dilution gas on the other parts which are, for example, hydrogen and argon. The reactive gas is a gaseous compound either of carbon or of boron; for example, a aliphatic hydrocarbon, an aromatic hydrocarbon or a cyclane, and, in particular methane, or, for example, a diborane. The mixture gas may also contain a small proportion of a compound nitrogen gas.

To carry out the treatment itself, we ionize, at least partially, the gas mixture, creating a cold plasma. ionization of the gas mixture, on the one hand, creates ions which, by bombarding the surface of the part, can passivate it to make it reactive and heat, and, on the other hand, creates very reactive species of carbon or of boron, which are atoms of which certain electron layers are excited. Highly reactive carbon or boron species react with the surface and penetrate inside by diffusion to form a interstitial solid solution free of precipitates.

So that the diffusion of carbon or boron takes place in good conditions, the surface temperature must be sufficient, and preferably above 300 ° C. But to avoid the formation of precipitated the surface temperature must remain below 460 ° C. So that the penetration of the active elements in the metal takes place satisfactorily it is also necessary that the production of element active on the surface of the part is not too important. Indeed, with the carbon, for example, if the production of active carbon is too fast, it a harmful carbon black deposit forms on the surface of the part for the treatment. To avoid this, the active gas content of the product is limited. gas mixture in order to balance the kinetics of production of active element on the surface of the part, and the kinetics of penetration of the active element in the room. For example, when the active gas is methane, its content in the gas mixture, is between 5 and 30% and, from preferably around 10%. The gaseous atmosphere in contact with the surface of the part is maintained at a pressure which must be higher at 0.5 and which can go up to 200 mbar.

The plasma can be a so-called "discharge plasma", that is to say a plasma generated by an electric discharge between an anode and a cathode, the part to be treated being brought to a cathodic potential and which can itself be the cathode. In this case, the ions are accelerated, they bombard the surface of the part which heats it up enough so that there is no need to provide a means of additional heating.

The plasma can also be a plasma generated by a electromagnetic wave generator, or microwave generator, or a "post-discharge" plasma, that is to say a plasma transferred from a plasma generator to the enclosure in which it is located the room. In this case, bombardment of the workpiece surface may be insufficient to cause the necessary heating. Also, we heats the surface of the room, for example, by radiation.

The duration of the treatment depends on the thickness of the layer treated we want to obtain, this duration can vary between 1 hour and a few tens of hours.

We were able to show that by limiting the heating temperature of the surface layer of the part during treatment, at a value maximum of 460 ° C and keeping in contact with the workpiece surface a gaseous atmosphere containing an interstitial element constituted by the carbon or boron in the activated state, at a pressure between 0.5 and 200 mbar, we obtain a surface layer consisting of a homogeneous solid solution completely free of precipitates, of the element interstitial in the metal of the part.

The solid solution contains a high proportion of element interstitial, between 5 and 50 atom% and, generally between between 10 and 30 atom%. According to the conditions of implementation of the treatment and in particular, depending on the duration of treatment, it is possible to obtain a hardened surface layer with a thickness of 1 to 60 µm. The surface layer of the part may present after the treatment of a Vickers Hv hardness greater than 800.

This treatment, which is applicable to a very wide variety of metal alloys and, in particular, to alloys with a structure face centered cubic, centered cubic or tetragonal (for example, austenitic, ferritic or martensitic stainless steels), allows to obtain a layer 1 to 60 µm thick, of a solid solution saturated, or even supersaturated, with carbon or boron, homogeneous, that is to say free of carbide or boride precipitates, the hardness of which can be greater than 800 Vickers or even 1000 Vickers and that is very resistant to corrosion. Carbon can be combined with nitrogen to form the layer of solid solution. For example, when the alloy is an austenitic steel, the carburetted layer cannot be attacked by chemical reagents commonly used in metallography and has a resistance to attack by salt spray greater than 1000 hours.

In the case of martensitic stainless steel parts, for example example of mechanical parts used in corrosive environments and subjected to wear or abrasion, a double treatment of surface to obtain good resistance to wear and corrosion. But in a first phase, we submit the parts to a gaseous atmosphere containing nitrogen or a substance containing nitrogen at a temperature between 340 and 450 ° C and preferably between 350 and 380 ° C, for example, at a temperature of around 360 ° C. The pieces have, after treatment, a layer surface charged with nitrogen. This layer was found to have a poor corrosion resistance. In a second phase, we submit the parts to an atmosphere containing carbon in activated form at a temperature generally lower than the treatment temperature during the first phase and for example, between 300 and 350 ° C. The surface layer of the parts is loaded with carbon, so so that a solid solution of carbon in the steel of the nitrogen-laden part, during the first phase. The martensitic steel parts then have both a high wear resistance and very good corrosion resistance.

This treatment in two successive phases applies in particular highly stressed martensitic steel mechanical parts such as than parts used in the field of oil drilling or tools or cutting blades.

For example, we treated "quick" steel fittings 316 L stainless steel, working in a corrosive environment. The treatment has been done in an atmosphere of methane diluted in argon, at a temperature of around 400 ° C, for periods between 24 and 36 hours. The layers obtained had a thickness between 20 and 50 µm, hardness greater than 800 Vickers and resistance to the attack by salt spray greater than 1000 hours.

Also as an example, we treated steel nuts austenitic stainless steel intended for the nuclear industry, for 12 hours at a temperature of about 420 ° C. The nuts thus treated had remarkable anti-seizure characteristics due to the hardness level obtained on the threads (Hv> 800).

We were able to obtain a very strong improvement in the characteristics of many parts subject to friction and wear in the atmosphere corrosive.

In the case of austenitic stainless steel, it was possible to obtain a wear resistance ten times greater than the wear resistance of untreated steel, the wear resistance being defined quantitatively from a measure of the weight loss of a sample subjected to wear. The treated parts also hold in salt spray more than 1000 hours without pitting.

• les pièces pour robinets et vannes utilisés dans un milieu corrosif telles que les clapets, les boisseaux, les sièges et les opercules.

Among the applications to parts subjected to friction and wear in a corrosive environment, there may be mentioned:

• parts for taps and valves used in a corrosive environment such as valves, plugs, seats and lids.

• des éléments d'assemblage vissés tels que les écrous, les vis et les boulons en acier inoxydable austénitique.

The surface layer of the part in the form of a homogeneous solid solution of an interstitial such as carbon makes it possible to avoid any seizure and to obtain an improvement in the resistance to wear.

• screwed fasteners such as nuts, screws and bolts in austenitic stainless steel.

• des raccords rapides en acier inoxydable austénitique pour les fluides corrosifs.
• des rouleaux de transport de tôles dans une installation de décapage de tôles, avant revêtement électrolytique.

The hardness level obtained on the thread (Hv> 800) eliminates any risk of seizure. In addition, a salt spray test highlights a total absence of pitting corrosion after 1000 hours of testing.

• austenitic stainless steel quick couplings for corrosive fluids.
• sheet transport rollers in a sheet pickling installation, before electrolytic coating.

• des pièces mobiles de pompes utilisées dans l'industrie chimique ou l'industrie alimentaire, telles que les pistons, les chemises de cylindres, les rotors, les cages, les guides ou les mélangeurs.

The rollers must resist wear and tear on the passage of sheets and the aggressive action of acid pickling baths.

• moving parts of pumps used in the chemical or food industry, such as pistons, cylinder liners, rotors, cages, guides or mixers.

In general, the treatment according to the invention, which allows obtain very hard, wear-resistant surface layers and very resistant to corrosion, can be performed on all kinds of parts, and in particular, on any mechanical part subject to wear in corrosive environment (for example: food industry, chemical industry, nuclear industry, marine environment, biomedical applications); mostly austenitic steel container to resist scratching, for example austenitic stainless steel dishes that can be coated before forming; on the blades of sharp stainless steel objects martensitics such as knives and scalpels; on implants orthopedic; on valves; on turbine parts or condensers subject to pitting corrosion. The treatment can, also, be made on a strip or on a metal blank, set work after treatment.

The treatment applies in particular, in the case of panels in decorative austenitic stainless steel, for example, with polished panels and / or colored by a process such as anodization. In this case, the hardening of the surface of the panels by formation of a solution homogeneous solid of carbon in a surface layer of panels, avoids the risk of scratches and degradation of the aesthetic appearance of the panels.

Claims (28)

1. Process for treating the surface of a metal part, wherein at least one interstitial element consisting of at least one of the elements carbon and boron is introduced into a surface layer of the part close to a surface of the part by bringing the interstitial element in the activated state into contact with the surface of the part and by diffusing the interstitial element into the surface layer of the part, characterised in that:

   the part is heated to a temperature of between 300 and 460°C, at least in its surface zone, and

   a gaseous atmosphere containing the activated interstitial element in contact with the surface of the part is maintained at a pressure between 0.5 and 200 mbar,

   so as to obtain, in the surface layer, a solid solution free from precipitates containing 5 to 50 atoms % of interstitial element.
2. Process according to claim 1, characterised in that the pressure of the gaseous atmosphere is between 0.5 and 20 mbar.
3. Process according to one of claims 1 and 2, characterised in that the gaseous atmosphere contains at least one gaseous compound comprising the activated interstitial element, in a cold plasma.
4. Process according to claim 3, characterised in that the gaseous atmosphere contains a diluent for the gaseous compound consisting of at least one of the following gases: hydrogen, argon or a mixture of hydrogen and argon.
5. Process according to any one of claims 3 and 4, characterised in that the gaseous compound consists of an aliphatic or aromatic hydrocarbon or a cyclane.
6. Process according to any one of claims 3 and 4, characterised in that the gaseous compound consists of methane and the atmosphere contains 5 to 30% by volume of methane.
7. Process according to any one of claims 3 and 4, characterised in that the gaseous compound is a gaseous boron derivative such as a diborane.
8. Process according to any one of claims 3 to 7, characterised in that the gaseous atmosphere further contains nitrogen or a gaseous nitrogen derivative such as ammonia.
9. Process according to any one of claims 3 to 8, characterised in that the plasma is generated by an electrical discharge between an anode and a cathode, which may be the metal part.
10. Process according to any one of claims 3 to 8, characterised in that the plasma is generated by a microwave generator or an electromagnetic wave generator and optionally transferred to be brought into contact with the surface of the part.
11. Process according to any one of claims 1 to 10, characterised in that the surface of the part is heated by radiation.
12. Process according to any one of claims 1 to 9, characterised in that the part is a metal alloy the structure of which is face-centred cubic, body-centred cubic or tetragonal.
13. Process according to claim 12, characterised in that the metal alloy is one of the following alloys; austenitic stainless steel, martensitic stainless steel, nickel-based alloy or cobalt-based alloy.
14. Process according to claim 12, characterised in that the metal alloy is an aluminium-based alloy or a titanium-based alloy.
15. Process according to claim 1, in the case of a part consisting of martensitic stainless steel, characterised in that:

    in a first phase the part is subjected to an atmosphere containing nitrogen at a temperature between 340 and 450°C and preferably between 350 and 380°C, so as to charge a surface layer of the part with nitrogen, and

    in a second phase the nitrogen-charged part is subjected to an atmosphere containing carbon in the activated state, at a temperature generally below the temperature of the treatment in the first phase and preferably between 300 and 380°C.
16. Metal part comprising a surface layer hardened by an interstitial element consisting of at least one of the elements carbon and boron and optionally also nitrogen, characterised in that the surface layer is made up of a homogeneous solid solution free from precipitates containing 5 to 50 atoms % of interstitial element.
17. Metal part according to claim 16, characterised in that the surface layer contains 10 to 30 atoms % of interstitial element.
18. Metal part according to either of claims 16 and 17, characterised in that the surface layer is between 1 and 60 µm thick.
19. Metal part according to any one of claims 16 to 18, characterised in that the surface layer has a Vickers hardness H_v of more than 800.
20. Use of a metal part according to any one of claims 16 to 19 as a frictional member in a corrosive environment and particularly as a valve, bushing, seat or cap of a stopcock or of a shutoff valve used in a corrosive medium.
21. Use of a metal part made of austenitic stainless steel according to any one of claims 16 to 19 as part of a screwed assembly such as a nut, screw or bolt which may be used in a corrosive medium.
22. Use of a metal part made of austenitic stainless steel according to any one of claims 16 to 19 as a connector for transporting special fluids.
23. Use of a metal part according to any one of claims 16 to 19 as a roller for transporting sheet metal in a pickling plant.
24. Use of a metal part according to any one of claims 16 to 19 as a moving part in a pump used in the chemical industry or in the food industry, such as a piston, a cylinder sleeve, a rotor, a cage, a guide or a mixer.
25. Use of a metal part according to any one of claims 16 to 19 as an orthopaedic implant.
26. Use of a metal part according to any one of claims 16 to 19 as a polished and/or coloured decorative panel.
27. Metal part according to any one of claims 16 to 19, characterised in that it consists of a flat austenitic stainless steel product intended to undergo a shaping process.
28. Metal part according to any one of claims 16 to 19, characterised in that it consists of a cutting tool made of austenitic stainless steel, such as a knife or a scalpel.