EP0801142A2 - Treatment method of a metallic substrate, metallic substrate thereby obtained and his applications - Google Patents

Abstract

At least the surface of a piece of metal is heated at 300-460 deg. C at a pressure of 0.5-200, preferably 0.5-20, mbars in an atmosphere containing at least one activated element which diffuses a little way into the metal, forming a surface layer comprising a solid solution free of precipitates and containing 5-50 atom% of the element. Also claimed is a piece of metal so treated to give it a hard surface layer containing at least one of carbon, boron and nitrogen in amount 5-50, preferably 10-30, atom%.

Description

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

To harden the surface of metal parts, especially steel parts, and more particularly austenitic stainless steel parts, whose resistance to wear or abrasion is poor, it is known to enrich a surface layer of the parts into an interstitial element 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) which is extremely hard and resistant to wear. This layer is a solid solution of nitrogen in the metal matrix which has the disadvantage of also comprising precipitates of nitrides which substantially degrade the corrosion resistance of the surface. This degradation in corrosion resistance makes curing by solid nitrogen solution unsuitable for use in many applications, in particular, whenever corrosion resistance is required.

In particular, in the case of martensitic stainless steel parts comprising a surface layer hardened with nitrogen, the corrosion resistance of the parts may become insufficient; in addition, the structure of the 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 less than 10 mtorr. Due to the low pressure of the gaseous atmosphere, the content of the layer interstitial surface remains low even for treatment times of several hours.

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

In the case of the surface treatment of a stainless steel part, these compounds contain chromium which is taken from 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 metal part which does not deteriorate its resistance to corrosion.

Another object of the present invention is to provide a surface treatment of a metal part which can be implemented industrially, under favorable conditions with regard to the costs and the duration of the 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 of treating the surface of a metal part, in which at least one interstitial element is introduced into a surface layer of the part close to a surface of the part by bringing into contact with the surface of the part, the interstitial element in the activated state and by diffusing the interstitial element 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 be constituted by an aliphatic or aromatic hydrocarbon or by a cyclane.

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

The gaseous compound can also be constituted by a gaseous derivative of boron, for example, a diborane.

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

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

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

The metallic part obtained, has a hardened surface layer consisting of a homogeneous solid solution of carbon or boron in the metal matrix of the part whose carbon or boron content is between 5 and 50 atom% and preferably between 10% and 30%.

The invention will now be described in a more precise but nonlimiting manner and illustrated by the examples which follow.

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

To carry out the actual treatment, the gas mixture is ionized, at least partially, by creating a cold plasma. The 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 it, and, on the other hand, creates very reactive species of carbon or boron, which are atoms in which certain electronic layers are excited. The highly reactive carbon or boron species react with the surface and penetrate inside by diffusion to form an interstitial solid solution free of precipitates.

For the diffusion of carbon or boron to take place under good conditions, the temperature of the surface must be sufficient, and preferably greater than 300 ° C. But to avoid the formation of precipitated the surface temperature must remain below 460 ° C. For the penetration of the active elements into the metal to take place satisfactorily, it is also necessary that the production of active element on the surface of the part is not too great. In fact, with carbon, for example, if the production of active carbon is too rapid, a deposit of carbon black harmful to the treatment is formed on the surface of the part. To avoid this, the active gas content of the gas mixture is limited 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 part. For example, when the active gas is methane, its content in the gas mixture is between 5 and 30% and, preferably, of the order of 10%. The gaseous atmosphere in contact with the surface of the part is maintained at a pressure which must be greater than 0.5 and which can range 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 being able itself to be the In this case, the ions are accelerated, they bombard the surface of the part which heats it up sufficiently so that there is no need to provide additional heating means.

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

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

It has been shown that by limiting the temperature of heating of the surface layer of the part during treatment, to a maximum value of 460 ° C. and by maintaining in contact with the surface of the part a gaseous atmosphere containing an element. interstitial such as carbon or boron in the activated state, at a pressure of between 0.5 and 200 mbar, a surface layer is obtained consisting of a homogeneous solid solution completely free of precipitates, of the interstitial element in the metal of the room.

The solid solution contains a high proportion of interstitial element, between 5 and 50 atom% and, generally between 10 and 30 atom%. Depending on the conditions of implementation of the treatment and in particular, depending on the duration of the 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 have, after treatment, a Vickers Hv hardness greater than 800.

This treatment, which is applicable to a very wide variety of metallic alloys and, in particular, to alloys having a cubic structure with centered face, cubic centered or tetragonal (for example, austenitic, ferritic or martensitic stainless steels), makes it possible to '' obtain a layer 1 to 60 µm thick, of a solid saturated, or even supersaturated, solution of carbon or boron, homogeneous, that is to say free of precipitates of carbides or borides, the hardness can be greater than 800 Vickers or even 1000 Vickers and which is very resistant to corrosion. Carbon can be combined with nitrogen to form the solid solution layer. For example, when the alloy is an austenitic steel, the carburetted layer cannot be attacked by chemical reagents usually 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 mechanical parts used in corrosive environments and subjected to wear or abrasion, a double surface treatment was carried out to obtain good resistance to wear and corrosion. But in a first phase, the parts are subjected 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, by example, at a temperature of the order of 360 ° C. The parts have, after treatment, a surface layer loaded with nitrogen. This layer has been found to exhibit poor corrosion resistance. In a second phase, the parts are subjected 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 that a solid solution of carbon in the steel of the part loaded with nitrogen is formed in the surface layer during the first phase. The martensitic steel parts then exhibit both high wear resistance and very good corrosion resistance.

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

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

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

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

In the case of austenitic stainless steel, it was possible to obtain a wear resistance ten times greater than the wear resistance of an untreated steel, the wear resistance being defined quantitatively at from a measurement of the weight loss of a sample subjected to wear. The treated parts also have a resistance in a salt spray greater than 1000 hours without the formation of 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 makes it possible to obtain very hard surface layers, very resistant to wear and very resistant to corrosion, can be carried out on all kinds of parts, and in particular on any mechanical part subject to wear in a corrosive environment (for example: food industry, chemical industry, nuclear industry, marine environment, biomedical applications); on any austenitic steel container which must resist scratches, for example flat austenitic stainless steel which can be coated before forming; on the blades of martensitic stainless steel cutting objects such as knives and scalpels; on orthopedic implants; on valves; on turbine or condenser parts subject to pitting corrosion. The treatment can also be carried out on a strip or on a metal blank, implemented after treatment.

The treatment applies in particular, in the case of decorative austenitic stainless steel panels, for example, to polished and / or colored panels by a process such as anodization. In this case, the hardening of the surface of the panels by forming a homogeneous solid solution of carbon in a surface layer of the panels, makes it possible to avoid risks of scratches and degradation of the aesthetic appearance of the panels.

Claims (26)

Method for treating the surface of a metal part, in which at least one interstitial element 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 in the activated state and by diffusing the interstitial element 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 a gaseous atmosphere containing the activated interstitial element is maintained 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. Method according to claim 1, characterized in that the pressure of the gaseous atmosphere is between 0.5 and 20 mbar. Process according to either of Claims 1 and 2, characterized in that the gaseous atmosphere contains at least one gaseous compound comprising the activated interstitial element, in a cold plasma. Process according to Claim 3, characterized in that the gaseous atmosphere contains a diluent for the gaseous compound consisting of at least one of the following gases: hydrogen, argon, a mixture of hydrogen and argon. Process according to either of Claims 3 and 4, characterized in that the gaseous compound consists of an aliphatic or aromatic hydrocarbon or a cyclane. Process according to either of Claims 3 and 4, characterized in that the gaseous compound consists of methane and that the gaseous atmosphere contains from 5 to 30% by volume of methane. Process according to either of Claims 3 and 4, characterized in that the gaseous compound is a gaseous derivative of boron such as a diborane. Process according to any one of Claims 3 to 7, characterized in that the gaseous atmosphere additionally contains nitrogen or a gaseous derivative of nitrogen, such as ammonia. Method according to any one of Claims 3 to 8, characterized in that the plasma is generated by an electric discharge between an anode and a cathode which may be the metallic part. Process according to any one of Claims 3 to 8, characterized 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 workpiece. Method according to any one of Claims 1 to 10, characterized in that the surface of the part is heated by radiation. Method according to any one of Claims 1 to 9, characterized in that the part is made of a metal alloy whose structure is cubic with centered face, cubic centered or tetragonal. Process according to Claim 12, characterized in that the metallic alloy is one of the following alloys: austenitic stainless steel, martensitic stainless steel, nickel-based alloy, cobalt-based alloy. Process according to Claim 12, characterized in that the metal alloy is an aluminum-based alloy or a titanium-based alloy. Method according to claim 1, in the case of a piece of martensitic stainless steel, characterized 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 in nitrogen and, - in a second phase, the part loaded with nitrogen is subjected to an atmosphere containing carbon in the activated state, at a temperature generally lower than the temperature of the treatment during the first phase and preferably between 300 and 380 ° C. Metal part comprising a surface layer hardened by an interstitial element consisting of at least one of the carbon, boron and nitrogen elements, characterized in that the surface layer consists of a homogeneous solid solution free of precipitates containing from 5 to 50% atoms of interstitial element. Metal part according to claim 16, characterized in that the surface layer contains 10 to 30 atom% of interstitial. Metal part according to either of Claims 16 and 17, characterized in that the surface layer has a thickness of between 1 and 60 µm. Metal part suivant'une any of claims 16 to 18, characterized in that the surface layer has a Vickers hardness H _v of more than 800. Use of a metal part according to any one of claims 16 to 19 as a friction part in a corrosive environment and in particular as a valve, plug, seat or cover of a tap or a valve used in a corrosive environment. Use of a metal part of austenitic stainless steel according to any one of Claims 16 to 19 as an element of a screwed assembly such as a nut, a screw or a bolt usable in a corrosive medium. Use of a metal part of austenitic stainless steel according to any one of Claims 16 to 19 as a connector for the transport of special fluids. Use of a metal part according to any one of Claims 16 to 19 as a sheet transport roller in a pickling installation. Use of a metal part according to any one of Claims 16 to 19 as a moving part of a pump used in the chemical industry or in the food industry, such as a piston, a cylinder liner, a rotor, a cage, a guide or a mixer. Use of a metal part according to any one of claims 16 to 19 as an orthopedic implant. Use of a metal part according to any one of claims 16 to 19 as a polished and / or colored decorative panel.