Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
  • C23C8/30—Carbo-nitriding
  • C23C8/32—Carbo-nitriding of ferrous surfaces

Definitions

• the present invention relates to a process for carbonitriding metal alloy parts.
• Carbonitriding is a thermochemical treatment of simultaneous diffusion of carbon and nitrogen from the surface of a ferrous alloy in the solid state. It is generally carried out in a sealed oven, in which a controlled atmosphere is maintained, consisting of a support gas to which is added if necessary, to achieve the desired carbon potential, a carbon enrichment gas, and in addition, a nitrogen gas.
• the support gas is an endothermic generator gas comprising an alkane which is oxidized to carbon monoxide CO, since oxidation is carried out in the absence of air with respect to the stoichiometric reaction which would transform all the carbon into C0 2 .
• the gases used can be nitrogen-methanol mixtures or endothermic mixtures based on hydrocarbon and ammonia, as described in "Engineering Techniques, M1226-8 to 14, July 1994, [1].
• the conventional process uses atmospheres which all contain oxygen due to the presence or the formation of CO.
• the oxygen released by the decomposition of CO leads to a surface oxidation of the steel which, on the one hand, slows down the absorption of carbon and, on the other hand, leads to structures harmful in terms of the mechanical characteristics of the treated part, contact fatigue for example.
• the parts carbonitrided in this way are most often used in the state, without any mechanical retouching of the surface.
• the present invention relates to a carbonitriding process which makes it possible to avoid the harmful presence of oxygen, during the thermochemical treatment of diffusion of carbon and nitrogen in the metal alloy part.
• the process for carbonitriding metal alloy parts consists in subjecting said parts to the action of a fuel mixture consisting of ethylene and hydrogen, and to the action of a nitriding gas consisting of ammonia, under a pressure of less than 100 hPa and at a temperature of about 750 to about 1050 ° C.
• the supply of carbon takes place by the direct dissociation of a hydrocarbon, in this case ethylene, in the enclosure of a vacuum furnace, and the supply of nitrogen comes from the dissociation ammonia gas, depending on the thermally activated reaction:
• ammonia dissociation reaction is thermodynamically total, but its kinetics are low. Therefore, there is still at the room ammonia to dissociate, generating active nascent nitrogen. It is for this reason that ammonia can be used for nitrogen supply.
• the pressure used can be in particular in the range of 10 to 100 hPa.
• One of the other advantages of the process of the invention is that it can enrich the surface of the part with carbon and nitrogen in a much wider temperature range, from around 750 to around 1050 ° C., depending on the enrichment sequences. considered.
• the use of the ethylene dissociation reaction at low pressure makes it possible to be able to supply carbon from 750 ° C. and therefore thus by reducing the temperature to benefit from a greater nitriding power of ammonia because the availability of atomic nitrogen useful for diffusion is greater. This increases the possibilities of surface enrichment of carbon and nitrogen.
• carbonitriding can be carried out either by subjecting said parts to the simultaneous action of the fuel mixture and the nitriding gas, or by subjecting said parts to the successive action of the fuel mixture and of the nitriding gas.
• the treatment can also be carried out by subjecting the parts to the simultaneous action of the fuel mixture and the nitriding gas, then by subjecting them to the action of the nitriding gas alone.
• the method of the invention can comprise a complementary step of treatment of vacuum diffusion of the parts, after they have been subjected to the action of the fuel mixture and the nitriding gas.
• Such treatment can be carried out at a temperature of about 750 to about 1050 ° C, under pressures not exceeding 100 hPa.
• the metal alloys capable of being treated by the process of the invention can be of various types. In particular, steels and superalloys based on cobalt can be used.
• the method advantageously applies to the treatment of passivable steels, containing for example 2 to 9% of chromium and to the treatment of stainless steels containing for example 9 to 18% of chromium, thanks to the low pressure technique .
• the processing of such steels further enriches them with nitrogen to a high degree of up to 4%.
• the process of the invention therefore makes it possible on these steels to obtain in the surface layer, the C / N ratio offering the best compromise for the desired properties of wear resistance and / or corrosion resistance, for example.
• the process of the invention makes it possible, by widening the temperature range, by the possibility of linking in a simple manner different simultaneous or alternating enrichment sequences in carbon and / or nitrogen, to achieve gradients in carbon and nitrogen very varied and this on very diverse steels, even passive.
• the invention also relates to steel parts obtained by this process.
• These parts can be, for example, passivable steel parts comprising 2 to 9% chromium, which are enriched in nitrogen on their surface up to a content of 2% by mass, or stainless steel parts comprising 9 to 18 % chromium, which are enriched in nitrogen on their surface up to a content of 4% by mass.
• a double-vacuum oven known as with hot walls or an oven with cold walls, such as the devices described in FR-A-2 663 953.
• the method can include the following steps:
• carbonitriding treatment which can be carried out in different ways: a) a carbon enrichment period by introduction of the ethylene fuel gas, followed by a nitrogen enrichment period by introduction of ammonia, or vice versa, or a ') carbon and nitrogen enrichment period by simultaneous introduction of ethylene and ammonia,
• step 6 optionally an enrichment treatment analogous to that of step 6), or a diffusion treatment under vacuum at a temperature of 750 to 1050 ° C., under a pressure of 10 _1 hPa, for example, and
• steps 6) and 7) can be repeated several times if necessary.
• the pressure in the tank is preferably maintained at around 25 hPa.
• steps 1 to 5 and 8 and steps 6 and 7 were carried out using flow rates of ethylene and ammonia and enrichment sequences and / or different diffusion.
• a preliminary austenitization step is carried out under vacuum at 10 ⁇ 2 hPa, at a temperature of 850 ° C., for 30 minutes.
• Steps 6) and 7) are then carried out under the conditions given in table 2.
• the reference of the alloys used whose compositions are given in table 1 has also been specified.
• step 6) corresponds to carbonitriding with simultaneous dispatch of ethylene and ammonia and step 7) is a diffusion diffusion treatment.
• step 6) corresponds to carbonitriding with simultaneous dispatch of ethylene and ammonia (at a lower flow rate) and step 7) is a nitriding treatment with ammonia alone.
• step 6) corresponds to carburetion and step 7) to nitriding.
• step 6) corresponds to carbonitriding with simultaneous dispatch of ethylene and ammonia, but the flow rate of ammonia is very high and step 7) is a vacuum diffusion treatment.
• step 6 is carried out, which corresponds to carbonitriding by simultaneous sending of ethylene and ammonia, for a duration greater than that of the preceding examples.
• the process of the invention is therefore very advantageous since it leads to much higher degrees of nitrogen enrichment than those which can be obtained with conventional carbonitriding processes where the nitrogen contents at the surface are at most about 0.3% 10

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
• Carbon And Carbon Compounds (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Inorganic Fibers (AREA)
• Ceramic Products (AREA)

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• 1998
  • 1998-04-28 FR FR9805311A patent/FR2777911B1/fr not_active Expired - Fee Related
• 1999
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  • 1999-04-27 DE DE69902169T patent/DE69902169T2/de not_active Expired - Lifetime
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