EP1885904A1 - Low pressure carbonitriding method and device - Google Patents

Low pressure carbonitriding method and device

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Publication number
EP1885904A1
EP1885904A1 EP06743815A EP06743815A EP1885904A1 EP 1885904 A1 EP1885904 A1 EP 1885904A1 EP 06743815 A EP06743815 A EP 06743815A EP 06743815 A EP06743815 A EP 06743815A EP 1885904 A1 EP1885904 A1 EP 1885904A1
Authority
EP
European Patent Office
Prior art keywords
gas
during
temperature
carbonitriding
steps
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP06743815A
Other languages
German (de)
French (fr)
Other versions
EP1885904B2 (en
EP1885904B1 (en
Inventor
Jean Berlier
Aymeric Goldsteinas
Xavier Doussot
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ECM Technologies SAS
Original Assignee
Etudes et Constructions Mecaniques SA
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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
    • 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
    • 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
    • C23C8/34—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 more than one step

Definitions

  • the present invention relates to processes for treating steel parts, and more particularly to carbonitriding processes, that is to say introducing carbon and nitrogen at the surface of steel parts to improve its performance. hardness and fatigue resistance. Presentation of the prior art
  • a first category of carbonitriding processes corresponds to so-called high-pressure carbonitriding processes insofar as the enclosure containing the workpieces is maintained at a pressure generally close to atmospheric pressure for the duration of the treatment.
  • Such a method consists, for example, in maintaining the parts at a temperature plateau, for example at about 88O 0 C, while supplying the enclosure with a gaseous mixture of methanol and ammonia.
  • the carbonitriding step is followed a quenching step, for example a quenching with oil, and optionally a hardening step of the treated parts.
  • a second category of carbonitriding processes corresponds to so-called low pressure or reduced pressure carbonitriding processes, insofar as the enclosure containing the workpieces is maintained at a pressure generally less than a few hundred pascals (a few millibars).
  • FIG. 1 corresponds to FIG. 5 (a) of the application US 2004/0187966 and represents a temperature evolution curve 10 in the enclosure of an oven in which a carbonitriding process is carried out according to a first example of embodiment comprising seven successive steps I to VII.
  • the workpieces are heated (step I) to a temperature step 12 and held at temperature plateau 12 (step II) to achieve room temperature equalization.
  • a carburising step (step III) is carried out at the temperature step 12 by injecting into the chamber a gaseous mixture of ethylene and hydrogen and is followed by a diffusion step (step IV) carried out at temperature step 12.
  • the temperature in the chamber is then lowered (step V) to a temperature step 14 below the temperature step 12.
  • a nitriding step (step VI) is performed at temperature step 14 by injecting ammonia in the enclosure.
  • the pieces are finally quenched (stage VII), for example by quenching with oil.
  • FIG. 2 corresponds to FIG. 5 (b) of the application US 2004/0187966 and represents a curve 16 of evolution of the temperature in the enclosure of an oven in which a carbonitriding process is carried out according to a second example of realization comprising four successive steps I 'to IV.
  • Steps I 'and II' respectively correspond to steps I and II of the first exemplary embodiment.
  • Stage III ' corresponds to a carbonitriding step, carried out at a temperature step 18, during which a gaseous mixture of ethylene, hydrogen and ammonia is injected into the enclosure of the furnace.
  • Step IV corresponds to an oil quenching step.
  • a disadvantage of the first example of the carbonitriding process described in the publication US 2004/0187966 is that the nitriding step is performed after the carburizing step at a temperature step below the cementation temperature step.
  • the total treatment time can be excessively long, which makes it difficult to ⁇ used such a method in an industrial context.
  • the present invention provides a method of low-pressure carbonitriding of steel parts which allows to obtain, in a precise and reproducible manner, the desired carbon and nitrogen concentration profiles in the treated parts.
  • Another object of the present invention is to provide a carbonitriding process whose implementation is compatible with the treatment of steel parts in an industrial context.
  • the present invention also relates to a low-pressure carbonitriding furnace of steel parts to obtain, accurately and reproducibly, the desired carbon and nitrogen profiles in the treated parts.
  • Another object of the present invention is to provide a low pressure carbonitriding furnace of simple design.
  • the present invention provides a method of carbonitriding a steel part disposed in an enclosure maintained at a reduced internal pressure, the part being maintained at a temperature step.
  • the method comprises an alternation of first and second stages, a carburizing gas being injected into the chamber only during the first stages and a nitriding gas being injected into the chamber only for at least a portion of at least two seconds steps .
  • the carburising gas is propane or acetylene and the nitriding gas is ammonia.
  • a neutral gas is injected into the chamber simultaneously with the nitriding gas.
  • the nitriding gas is injected into the chamber during at least a second step for a duration less than the duration of said second step, the rest of said second step being carried out in the presence of a neutral gas.
  • the first and second steps are performed at a constant pressure of less than 1500 pascals.
  • the temperature plateau is between 800 ° C. and 1050 ° C.
  • the temperature bearing is greater than 900 ° C.
  • the present invention also provides a carbonitriding furnace for receiving a steel part, the furnace being associated with gas introduction and extraction means. controlled gases to maintain a reduced internal pressure, and comprising heating means for maintaining the workpiece at a temperature step.
  • the means of introduction include means for introducing, during an alternation of first and second steps performed at said temperature plateau, a carburization gas only during the first stages and a nitriding gas only during at least a portion of at least a second stage.
  • the intro ⁇ duction means comprise means for introducing a neutral gas.
  • FIGS. 1 and 2 illustrate conventional examples of low pressure carbonitriding process
  • Figure 3 schematically shows an embodiment of a low pressure carbonitriding furnace according to the present invention
  • FIG. 4 illustrates an example of a low pressure carbonitriding process according to the present invention
  • FIG. 5 represents an example of a nitrogen concentration profile obtained in treated steel parts according to an example of a low-pressure carbonitriding process of the invention
  • FIGS. 1 and 2 illustrate conventional examples of low pressure carbonitriding process
  • Figure 3 schematically shows an embodiment of a low pressure carbonitriding furnace according to the present invention
  • FIG. 4 illustrates an example of a low pressure carbonitriding process according to the present invention
  • FIG. 5 represents an example of a nitrogen concentration profile obtained in treated steel parts according to an example of a low-pressure carbonitriding process of the invention
  • FIGS. 1 and 2 previously described, illustrate conventional examples of low pressure carbonitriding process
  • Figure 3 schematically shows an embodiment of a low pressure carbonitriding furnace according to the present invention
  • FIGS. 9, 10 and 11 respectively illustrate another example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process.
  • the present invention consists in producing in an enclosure, containing steel parts to be treated maintained at a substantially constant temperature, an alternation of carbon enrichment stages during which a carburizing gas is injected into the chamber under a reduced pressure. and carbon diffusion steps during which the injection of the carburizing gas is interrupted.
  • the present invention consists in providing for the injection into the chamber of a nitriding gas during all or part of the carbon diffusion steps.
  • the carbon enrichment steps then correspond to nitrogen diffusion steps.
  • the nitriding gas is injected during at least a portion of at least two carbon diffusion steps, i.e. during at least a portion of a carbon diffusion step sandwiched between two enrichment stages. carbon.
  • FIG. 3 schematically shows an exemplary embodiment of a low pressure carbonitriding furnace 10 according to the present invention.
  • the furnace 10 comprises a sealed wall 12 delimiting an internal enclosure 14 in which is disposed a load to be treated 16, generally a large number of parts arranged on a suitable support.
  • a vacuum of the order of a few hundred pascals (a few millibars) can be maintained in the enclosure 14 by means of an extraction pipe 18 connected to an extractor 20.
  • An injector 22 makes it possible to introduce gases in a distributed manner into the enclosure 14. There is shown by way of example, gas inlets 22, 24, 26, 28 respectively controlled by valves 30, 32, 34, 36.
  • the temperature in the enclosure 14 can be fixed by heating means 38.
  • FIG. 4 represents a temperature evolution curve 40 in the chamber 14 of the carbonitriding furnace 10 of FIG. 3 during a carbonitriding cycle according to an exemplary carbonitriding process of the invention.
  • the method comprises an initial step H corresponding to an increase 42 of the temperature in the enclosure 14 containing the charge 16 to a temperature plateau 44 which, in the present example, is equal to 93O 0 C and which in general may correspond to temperatures between about 800 0 C and about 1050 0 C.
  • Step H is followed by a step PH equalizing the temperature of the parts constituting the load 16 at the temperature step 44.
  • Steps H and PH are carried out in the presence of a neutral gas, which is optionally added a reducing gas.
  • the neutral gas is, for example, nitrogen (N2).
  • the reducing gas for example hydrogen (H2)
  • H2 hydrogen
  • the reducing gas may be added in a proportion ranging from 1% to 5% by volume of the neutral gas.
  • Step PH is followed by an alternation of carbon enrichment steps C1 to C4, during which a cementation gas is injected into the chamber 14, and carbon diffusion steps D1 to D4 during which the The carburising gas is no longer injected into the chamber 14.
  • four enrichment steps C1 to C4 and four diffusion steps D1 to D4 are shown in FIG. 4.
  • the enrichment and diffusion steps are performed by maintaining the temperature in the chamber 14 at the temperature plateau 44.
  • an injection of a nitriding gas is carried out in the enclosure 14.
  • a quench step Q of the load 10 closes the carbonitriding cycle.
  • a vacuum is maintained in the chamber 14 at pressures of a few hundred pascals (a few millibars).
  • the injection of the carburizing gas is effected by pulses.
  • the carburising gas is for example propane (C3H8) or acetylene (C2H2). It can also be any other hydrocarbon (C ⁇ H ⁇ ) likely to dissociate at the temperatures of the enclosure to cementer the surface of the parts to be treated.
  • the nitriding gas is, for example, ammonia
  • Injection of the nitriding gas can be performed only during some of the diffusion steps.
  • the injection of the nitriding gas may be performed only during part of the step of diffu sion ⁇ .
  • a neutral gas for example nitrogen (N2) can be injected during all stages of boost and diffusion, only during the diffusion steps, or only ⁇ during part of the diffusion steps.
  • the injection of the neutral gas is regulated so as to maintain constant the pressure in the chamber 14.
  • the relative proportions of the nitriding gas and the neutral gas are determined as a function of desired nitrogen concentration profile in the treated parts.
  • the relative proportions of the nitriding gas and the neutral gas may be different for each diffusion stage during which the Nitriding gas and neutral gas are simultaneously injected into the chamber 14.
  • all the gases injected into the enclosure 14 of the furnace 10 or some of them may be mixed before the injection into the chamber 14.
  • Such a variant makes it possible, for example, during steps of raising the temperature H and equalizing the temperature PH, injecting directly into the chamber 14 a mixture of nitrogen and hydrogen of the type containing a proportion of hydrogen of less than 5% by volume, a such proportion of hydrogen excluding any risk of explosion.
  • the carbonitriding process is carried out without pressure variation and the injections, carburising gas and nitriding gas (and / or optionally neutral gas), during the steps of enrichment and diffusion, are successive and the substitution between the carburizing gas and the nitriding gas (and / or possibly the neutral gas) is likely to occur very quickly.
  • FIG. 5 shows an example of a concentration profile by weight of the nitrogen element diffused in a treated part as a function of depth, measured from the surface of the part, when the case gas is propane and gas. Nitriding is ammonia.
  • FIG. 6, 7 and 8 respectively illustrate an example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process in which the cementation gas is acetylene and gas.
  • Nitriding is ammonia.
  • the carbonitriding is carried out at a temperature level of 88O 0 C.
  • the heating steps H and temperature equalization PH last 20 minutes and are followed by an alternation of three steps of enrichment Cl, C2, C3 (respectively of 123 s, 51 s and 49 s) and three dissemination stages Dl, D2, D3 (194 s, 286 s and 2957 s, respectively).
  • FIGS. 9, 10 and 11 respectively illustrate another example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process, in which the carburizing gas is acetylene and the nitriding gas is ammonia.
  • the carbonitriding is carried out at a temperature step of 93O 0 C.
  • the heating steps H and equalization of temperature PH respectively last 29 minutes and 31 minutes and are followed by an alternation of five steps of enrichment C1 to C5 (329 s, 91 s, 80 s, 75 s and 71 s respectively) and five diffusion steps D1 to D5 (108 s, 144 s, 176 s, 208 s and 2858 s, respectively).
  • the Applicant has shown that the injection of ammonia during the diffusion steps allows an enrichment of the carburized layer of nitrogen to a depth of several hundred micrometers.
  • the nitrogen content obtained is of the order of 0.2% by weight to a few microns in depth.
  • the nitrogen content then decreases slowly from 0.2% for several hundred microns.
  • the nitrogen concentration is of the order of
  • the nitriding gas can be injected during step H of temperature rise, as soon as the temperature in the chamber 14 exceeds a given temperature, and / or during the equalizing step PH in temperature.
  • the nitriding gas is ammonia
  • the injection can be carried out as soon as the temperature in the enclosure 14 exceeds about 800 ° C. Injecting the nitriding gas only during the carbon diffusion stages allows better nitrogen and carbon enrichment of the treated parts and allows to obtain, in a precise and reproducible way, the carbon and carbon concentration profiles. desired nitrogen.
  • the nitriding gas is injected simultaneously with the carburizing gas, there is a dilution of the carburizing gas and the nitriding gas. This is not a factor favoring the reaction of carbon from the carburising gas or the reaction of the nitrogen from the nitriding gas with the parts to be treated, which slows the enrichment of the parts in nitrogen and carbon.
  • the carburizing gas and the nitriding gas are mixed, the control of the gaseous environment in the chamber 14 can hardly be carried out accurately, which makes it more difficult to obtain accurately and reproducibly, desired nitrogen and carbon component concentration profiles.
  • the diffusion of nitrogen in steel parts being, for the same processing conditions, faster than the diffusion of carbon, the injection of the nitriding gas and the cementation gas at different stages makes it possible to modify more easily the injection time of each gas while ensuring the maintenance of a constant pressure in the chamber 14.

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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)
  • Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
  • Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
  • Carbon And Carbon Compounds (AREA)

Abstract

A method for carbonitriding a steel part arranged in an enclosure maintained at a reduced internal pressure, the part being maintained at a temperature level, comprising an alternation of first and second steps, a carburizing gas being injected into the enclosure during the first steps only and a nitriding gas being injected into the enclosure only during at least part of at least two second steps.

Description

PROCEDE ET FOtIR DE CLARBONITRURATION A BASSE PRESSION PROCESS AND LOW PRESSURE CLARBONITRURATION FACTOR
Domaine de l' inventionField of the invention
La présente invention concerne les procédés de traitement de pièces en acier, et plus particulièrement les procédés de carbonitruration, c'est-à-dire d'introduction de carbone et d'azote au niveau de la surface de pièces en acier pour en améliorer la dureté et la tenue en fatigue. Exposé de l'art antérieurThe present invention relates to processes for treating steel parts, and more particularly to carbonitriding processes, that is to say introducing carbon and nitrogen at the surface of steel parts to improve its performance. hardness and fatigue resistance. Presentation of the prior art
II existe plusieurs types de procédés de carbonitru¬ ration de pièces en acier permettant l'introduction de carbone et d'azote au niveau de la surface des pièces jusqu'à des profondeurs pouvant atteindre plusieurs centaines de micro¬ mètres.There are several types of processes carbonitru ¬ ration of steel parts for the introduction of carbon and nitrogen at the surface of the parts to depths of up to several hundred micro meters ¬.
Une première catégorie de procédés de carbonitruration correspond aux procédés de carbonitruration dits à haute pression dans la mesure où l'enceinte contenant les pièces à traiter est maintenue à une pression généralement proche de la pression atmosphérique pendant toute la durée du traitement. Un tel procédé consiste, par exemple, à maintenir les pièces à un palier de température, par exemple à environ 88O0C, tout en alimentant l'enceinte avec un mélange gazeux constitué de méthanol et d'ammoniac. L'étape de carbonitruration est suivie d'une étape de trempe, par exemple une trempe à l'huile, et éventuellement d'une étape d'écrouissage des pièces traitées.A first category of carbonitriding processes corresponds to so-called high-pressure carbonitriding processes insofar as the enclosure containing the workpieces is maintained at a pressure generally close to atmospheric pressure for the duration of the treatment. Such a method consists, for example, in maintaining the parts at a temperature plateau, for example at about 88O 0 C, while supplying the enclosure with a gaseous mixture of methanol and ammonia. The carbonitriding step is followed a quenching step, for example a quenching with oil, and optionally a hardening step of the treated parts.
Une seconde catégorie de procédés de carbonitruration correspond aux procédés de carbonitruration dits à basse pression ou à pression réduite, dans la mesure où l'enceinte contenant les pièces à traiter est maintenue à une pression généralement inférieure à quelques centaines de pascals (quelques millibars) .A second category of carbonitriding processes corresponds to so-called low pressure or reduced pressure carbonitriding processes, insofar as the enclosure containing the workpieces is maintained at a pressure generally less than a few hundred pascals (a few millibars).
La publication US 2004/0187966 décrit deux exemples de procédés de carbonitruration à basse pression.Publication US 2004/0187966 describes two examples of low pressure carbonitriding processes.
La figure 1 correspond à la figure 5 (a) de la demande US 2004/0187966 et représente une courbe 10 d'évolution de la température dans l'enceinte d'un four dans lequel est réalisé un procédé de carbonitruration selon un premier exemple de réalisation comprenant sept étapes successives I à VII. Les pièces à traiter sont chauffées (étape I) jusqu'à un palier de température 12 et maintenues au palier de température 12 (étape II) pour obtenir une égalisation des températures des pièces. Une étape de cémentation (étape III) est réalisée au palier de température 12 par l'injection dans l'enceinte d'un mélange gazeux d'éthylène et d'hydrogène et est suivie d'une étape de diffusion (étape IV) réalisée au palier de température 12. La température dans l'enceinte est alors abaissée (étape V) jusqu'à un palier de température 14 inférieur au palier de température 12. Une étape de nitruration (étape VI) est réalisée au palier de température 14 en injectant de l'ammoniac dans l'enceinte. Les pièces sont finalement trempées (étape VII) , par exemple par trempe à l'huile.FIG. 1 corresponds to FIG. 5 (a) of the application US 2004/0187966 and represents a temperature evolution curve 10 in the enclosure of an oven in which a carbonitriding process is carried out according to a first example of embodiment comprising seven successive steps I to VII. The workpieces are heated (step I) to a temperature step 12 and held at temperature plateau 12 (step II) to achieve room temperature equalization. A carburising step (step III) is carried out at the temperature step 12 by injecting into the chamber a gaseous mixture of ethylene and hydrogen and is followed by a diffusion step (step IV) carried out at temperature step 12. The temperature in the chamber is then lowered (step V) to a temperature step 14 below the temperature step 12. A nitriding step (step VI) is performed at temperature step 14 by injecting ammonia in the enclosure. The pieces are finally quenched (stage VII), for example by quenching with oil.
La figure 2 correspond à la figure 5 (b) de la demande US 2004/0187966 et représente une courbe 16 d'évolution de la température dans l'enceinte d'un four dans lequel est réalisé un procédé de carbonitruration selon un second exemple de réalisa¬ tion comprenant quatre étapes successives I' à IV. Les étapes I ' et II ' correspondent respectivement aux étapes I et II du premier exemple de réalisation. L'étape III' correspond à une étape de carbonitruration, réalisée à un palier de température 18, pendant laquelle un mélange gazeux d'éthylène, d'hydrogène et d'ammoniac est injecté dans l'enceinte du four. L'étape IV correspond à une étape de trempe à l'huile. Un inconvénient du premier exemple de procédé de carbonitruration décrit dans la publication US 2004/0187966 est que l'étape de nitruration est réalisée après l'étape de cémentation, à un palier de température inférieur au palier de température de cémentation. La durée totale du traitement peut donc être excessivement longue, ce qui rend difficile l'utilisa¬ tion d'un tel procédé dans un contexte industriel.FIG. 2 corresponds to FIG. 5 (b) of the application US 2004/0187966 and represents a curve 16 of evolution of the temperature in the enclosure of an oven in which a carbonitriding process is carried out according to a second example of realization comprising four successive steps I 'to IV. Steps I 'and II' respectively correspond to steps I and II of the first exemplary embodiment. Stage III 'corresponds to a carbonitriding step, carried out at a temperature step 18, during which a gaseous mixture of ethylene, hydrogen and ammonia is injected into the enclosure of the furnace. Step IV corresponds to an oil quenching step. A disadvantage of the first example of the carbonitriding process described in the publication US 2004/0187966 is that the nitriding step is performed after the carburizing step at a temperature step below the cementation temperature step. The total treatment time can be excessively long, which makes it difficult to ¬ used such a method in an industrial context.
Un inconvénient du second exemple de procédé de carbonitruration décrit dans la publication US 2004/0187966 est dû au fait que les gaz de cémentation et de nitruration sont injectés simultanément dans l'enceinte du four. Il est alors difficile de contrôler avec précision l'ambiance gazeuse qui règne dans l'enceinte, et, par conséquent, de maîtriser, de façon précise et reproductible, les profils de concentrations d'azote et de carbone obtenus dans les pièces traitées. Résumé de l' inventionA disadvantage of the second example of the carbonitriding process described in the publication US 2004/0187966 is due to the fact that the carburizing and nitriding gases are injected simultaneously into the enclosure of the furnace. It is then difficult to precisely control the gaseous environment that prevails in the chamber, and, therefore, to control, in a precise and reproducible manner, the nitrogen and carbon concentration profiles obtained in the treated parts. Summary of the invention
La présente invention prévoit un procédé de carbonitruration à basse pression de pièces en acier qui permet d'obtenir, de façon précise et reproductible, les profils de concentrations de carbone et d'azote souhaités dans les pièces traitées.The present invention provides a method of low-pressure carbonitriding of steel parts which allows to obtain, in a precise and reproducible manner, the desired carbon and nitrogen concentration profiles in the treated parts.
Un autre objet de la présente invention est de prévoir un procédé de carbonitruration dont la mise en oeuvre est compatible avec le traitement de pièces en acier dans un contexte industriel . La présente invention vise également un four de carbonitruration à basse pression de pièces en acier permettant d'obtenir, de façon précise et reproductible, les profils de carbone et d'azote souhaités dans les pièces traitées. Un autre objet de la présente invention est de prévoir un four de carbonitruration à basse pression de conception simple.Another object of the present invention is to provide a carbonitriding process whose implementation is compatible with the treatment of steel parts in an industrial context. The present invention also relates to a low-pressure carbonitriding furnace of steel parts to obtain, accurately and reproducibly, the desired carbon and nitrogen profiles in the treated parts. Another object of the present invention is to provide a low pressure carbonitriding furnace of simple design.
Dans ce but, la présente invention prévoit un procédé de carbonitruration d'une pièce en acier disposée dans une enceinte maintenue à une pression interne réduite, la pièce étant maintenue à un palier de température. Le procédé comporte une alternance de premières et de secondes étapes, un gaz de cémentation étant injecté dans l'enceinte seulement pendant les premières étapes et un gaz de nitruration étant injecté dans l'enceinte seulement pendant au moins une partie d'au moins deux secondes étapes .For this purpose, the present invention provides a method of carbonitriding a steel part disposed in an enclosure maintained at a reduced internal pressure, the part being maintained at a temperature step. The method comprises an alternation of first and second stages, a carburizing gas being injected into the chamber only during the first stages and a nitriding gas being injected into the chamber only for at least a portion of at least two seconds steps .
Selon un mode de réalisation, le gaz de cémentation est le propane ou l'acétylène et le gaz de nitruration est l'ammoniac.According to one embodiment, the carburising gas is propane or acetylene and the nitriding gas is ammonia.
Selon un mode de réalisation, un gaz neutre est injecté dans l'enceinte simultanément au gaz de nitruration.According to one embodiment, a neutral gas is injected into the chamber simultaneously with the nitriding gas.
Selon un mode de réalisation, le gaz de nitruration est injecté dans l'enceinte au cours d'au moins une seconde étape pendant une durée inférieure à la durée de ladite seconde étape, le reste de ladite seconde étape étant réalisé en présence d'un gaz neutre.According to one embodiment, the nitriding gas is injected into the chamber during at least a second step for a duration less than the duration of said second step, the rest of said second step being carried out in the presence of a neutral gas.
Selon un mode de réalisation, les premières et secondes étapes sont réalisées à une pression constante inférieure à 1500 pascals.According to one embodiment, the first and second steps are performed at a constant pressure of less than 1500 pascals.
Selon un mode de réalisation, le palier de température est compris entre 8000C et 10500C.According to one embodiment, the temperature plateau is between 800 ° C. and 1050 ° C.
Selon un mode de réalisation, le palier de température est supérieur à 9000C. La présente invention prévoit également un four de carbonitruration destiné à recevoir une pièce en acier, le four étant associé à des moyens d'introduction de gaz et d'extraction de gaz commandés pour maintenir une pression interne réduite, et comprenant des moyens de chauffage pour maintenir la pièce à un palier de température. Les moyens d'introduction comprennent des moyens pour introduire, au cours d'une alternance de premières et de secondes étapes réalisées audit palier de température, un gaz de cémentation seulement pendant les premières étapes et un gaz de nitruration seulement pendant au moins une partie d'au moins une seconde étape .According to one embodiment, the temperature bearing is greater than 900 ° C. The present invention also provides a carbonitriding furnace for receiving a steel part, the furnace being associated with gas introduction and extraction means. controlled gases to maintain a reduced internal pressure, and comprising heating means for maintaining the workpiece at a temperature step. The means of introduction include means for introducing, during an alternation of first and second steps performed at said temperature plateau, a carburization gas only during the first stages and a nitriding gas only during at least a portion of at least a second stage.
Selon un mode de réalisation, les moyens d'intro¬ duction comprennent des moyens pour introduire un gaz neutre. Brève description des dessinsAccording to one embodiment, the intro ¬ duction means comprise means for introducing a neutral gas. Brief description of the drawings
Ces objets, caractéristiques et avantages, ainsi que d'autres de la présente invention seront exposés en détail dans la description suivante d'exemples de réalisation particuliers faite à titre non-limitatif en relation avec les figures jointes parmi lesquelles : les figures 1 et 2, précédemment décrites, illustrent des exemples classiques de procédé de carbonitruration à basse pression ; la figure 3 représente schématiquement un exemple de réalisation d'un four de carbonitruration à basse pression selon la présente invention ; la figure 4 illustre un exemple de procédé de carbonitruration à basse pression selon la présente invention ; la figure 5 représente un exemple de profil de concentration d'azote obtenu dans des pièces en acier traitées selon un exemple de procédé de carbonitruration à basse pression de l'invention ; les figures 6, 7 et 8 illustrent respectivement un autre exemple de procédé de carbonitruration selon l'invention et les profils de concentration de carbone et d'azote obtenus pour un tel procédé de carbonitruration ; et les figures 9, 10 et 11 illustrent respectivement un autre exemple de procédé de carbonitruration selon l'invention et les profils de concentration de carbone et d'azote obtenus pour un tel procédé de carbonitruration. Description détailléeThese and other objects, features and advantages of the present invention will be set forth in detail in the following description of particular embodiments given in a non-limiting manner with reference to the attached figures, in which: FIGS. 1 and 2 , previously described, illustrate conventional examples of low pressure carbonitriding process; Figure 3 schematically shows an embodiment of a low pressure carbonitriding furnace according to the present invention; FIG. 4 illustrates an example of a low pressure carbonitriding process according to the present invention; FIG. 5 represents an example of a nitrogen concentration profile obtained in treated steel parts according to an example of a low-pressure carbonitriding process of the invention; FIGS. 6, 7 and 8 respectively illustrate another example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process; and FIGS. 9, 10 and 11 respectively illustrate another example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process. detailed description
La présente invention consiste à réaliser dans une enceinte, contenant des pièces en acier à traiter maintenues à une température sensiblement constante, une alternance d'étapes d'enrichissement en carbone pendant lesquelles un gaz de cémentation est injecté dans l'enceinte sous une pression réduite et d'étapes de diffusion du carbone pendant lesquelles l'injection du gaz de cémentation est interrompue. La présente invention consiste à prévoir l'injection, dans l'enceinte, d'un gaz de nitruration pendant la totalité ou une partie des étapes de diffusion du carbone. Les étapes d'enrichissement en carbone correspondent alors à des étapes de diffusion de l'azote. Le gaz de nitruration est injecté pendant au moins une partie d'au moins deux étapes de diffusion de carbone, c'est-à-dire pendant au moins une partie d'une étape de diffusion du carbone intercalée entre deux étapes d'enrichissement en carbone. Ceci permet avantageusement de contrôler, de façon précise et reproductible, les profils de concentrations de carbone et d'azote obtenus dans les pièces traitées puisque l'injection du gaz de nitruration est réalisée séparément de l'injection du gaz de cémentation. En outre, comme l'injection du gaz de nitruration est réalisée pendant les étapes de diffusion du carbone, la durée totale du traitement de carbonitruration est sensiblement analogue à un traitement de cémentation classique. La figure 3 représente de façon schématique un exemple de réalisation d'un four de carbonitruration à basse pression 10 selon la présente invention. Le four 10 comprend une paroi étanche 12 délimitant une enceinte interne 14 dans laquelle est disposée une charge à traiter 16, généralement un grand nombre de pièces disposées sur un support approprié. Un vide de l'ordre de quelques centaines de pascals (quelques millibars) peut être maintenu dans l'enceinte 14 grâce à une canalisation d'extraction 18 reliée à un extracteur 20. Un injecteur 22 permet d'introduire des gaz de façon répartie dans l'enceinte 14. On a représenté à titre d'exemple, des entrées de gaz 22, 24, 26, 28 respectivement contrôlées par des vannes 30, 32, 34, 36. La température dans l'enceinte 14 peut être fixée par des moyens de chauffage 38.The present invention consists in producing in an enclosure, containing steel parts to be treated maintained at a substantially constant temperature, an alternation of carbon enrichment stages during which a carburizing gas is injected into the chamber under a reduced pressure. and carbon diffusion steps during which the injection of the carburizing gas is interrupted. The present invention consists in providing for the injection into the chamber of a nitriding gas during all or part of the carbon diffusion steps. The carbon enrichment steps then correspond to nitrogen diffusion steps. The nitriding gas is injected during at least a portion of at least two carbon diffusion steps, i.e. during at least a portion of a carbon diffusion step sandwiched between two enrichment stages. carbon. This advantageously makes it possible to control, in a precise and reproducible manner, the carbon and nitrogen concentration profiles obtained in the treated parts since the injection of the nitriding gas is carried out separately from the injection of the carburizing gas. In addition, since the injection of the nitriding gas is carried out during the carbon diffusion steps, the total duration of the carbonitriding treatment is substantially similar to a conventional cementation treatment. Figure 3 schematically shows an exemplary embodiment of a low pressure carbonitriding furnace 10 according to the present invention. The furnace 10 comprises a sealed wall 12 delimiting an internal enclosure 14 in which is disposed a load to be treated 16, generally a large number of parts arranged on a suitable support. A vacuum of the order of a few hundred pascals (a few millibars) can be maintained in the enclosure 14 by means of an extraction pipe 18 connected to an extractor 20. An injector 22 makes it possible to introduce gases in a distributed manner into the enclosure 14. There is shown by way of example, gas inlets 22, 24, 26, 28 respectively controlled by valves 30, 32, 34, 36. The temperature in the enclosure 14 can be fixed by heating means 38.
La figure 4 représente une courbe 40 d'évolution de la température dans l'enceinte 14 du four de carbonitruration 10 de la figure 3 au cours d'un cycle de carbonitruration selon un exemple de procédé de carbonitruration de l'invention. Le procédé comprend une étape initiale H correspondant à une élévation 42 de la température dans l'enceinte 14 contenant la charge 16 jusqu'à un palier de température 44 qui, dans le présent exemple, est égal à 93O0C et qui de façon générale peut correspondre à des températures comprises entre environ 8000C et environ 10500C. L'étape H est suivie d'une étape PH d'égalisation de la température des pièces constituant la charge 16 au palier de température 44. Les étapes H et PH sont réalisées en présence d'un gaz neutre, auquel est éventuellement ajouté un gaz réducteur. Le gaz neutre est par exemple l'azote (N2) . Le gaz réducteur, par exemple de l'hydrogène (H2) , peut être ajouté selon une proportion variant dans une plage de 1 % à 5 % en volume du gaz neutre. Pour des raisons de sécurité, il peut être souhaitable de limiter la proportion d'hydrogène à des proportions inférieures à environ 5 % pour prévenir tout risque d'explosion dans le cas où l'hydrogène viendrait accidentelle¬ ment en contact avec l'atmosphère ambiante. L'étape PH est suivie d'une alternance d'étapes d'enrichissement en carbone Cl à C4, pendant lesquelles un gaz de cémentation est injecté dans l'enceinte 14, et d'étapes de diffusion du carbone Dl à D4 pendant lesquelles le gaz de cémentation n'est plus injecté dans l'enceinte 14. A titre d'exemple, quatre étapes d'enrichissement Cl à C4 et quatre étapes de diffusion Dl à D4 sont représentées en figure 4. Les étapes d'enrichissement et de diffusion sont réalisées en maintenant la température dans l'enceinte 14 au palier de température 44. Au cours des étapes de diffusion Dl à D4, une injection d'un gaz de nitruration est réalisée dans l'enceinte 14. Une étape de trempe Q de la charge 10, par exemple une trempe au gaz, clôt le cycle de carbonitruration. Pendant les étapes H, PH, les étapes d'enrichissement Cl à C4 et les étapes de diffusion Dl à D4, un vide est maintenu dans l'enceinte 14 à des pressions de quelques centaines de pascals (quelques millibars). Selon une variante de l'invention, pendant chaque étape de cémentation, l'injection du gaz de cémentation est effectuée par impulsions.FIG. 4 represents a temperature evolution curve 40 in the chamber 14 of the carbonitriding furnace 10 of FIG. 3 during a carbonitriding cycle according to an exemplary carbonitriding process of the invention. The method comprises an initial step H corresponding to an increase 42 of the temperature in the enclosure 14 containing the charge 16 to a temperature plateau 44 which, in the present example, is equal to 93O 0 C and which in general may correspond to temperatures between about 800 0 C and about 1050 0 C. Step H is followed by a step PH equalizing the temperature of the parts constituting the load 16 at the temperature step 44. Steps H and PH are carried out in the presence of a neutral gas, which is optionally added a reducing gas. The neutral gas is, for example, nitrogen (N2). The reducing gas, for example hydrogen (H2), may be added in a proportion ranging from 1% to 5% by volume of the neutral gas. For security reasons, it may be desirable to limit the proportion of hydrogen in amounts below about 5% to prevent any risk of explosion if hydrogen accidental ¬ come in contact with the ambient atmosphere . Step PH is followed by an alternation of carbon enrichment steps C1 to C4, during which a cementation gas is injected into the chamber 14, and carbon diffusion steps D1 to D4 during which the The carburising gas is no longer injected into the chamber 14. By way of example, four enrichment steps C1 to C4 and four diffusion steps D1 to D4 are shown in FIG. 4. The enrichment and diffusion steps are performed by maintaining the temperature in the chamber 14 at the temperature plateau 44. During the diffusion steps D1 to D4, an injection of a nitriding gas is carried out in the enclosure 14. A quench step Q of the load 10, for example gas quenching, closes the carbonitriding cycle. During the steps H, PH, the enrichment steps C1 to C4 and the diffusion steps D1 to D4, a vacuum is maintained in the chamber 14 at pressures of a few hundred pascals (a few millibars). According to a variant of the invention, during each step of carburizing, the injection of the carburizing gas is effected by pulses.
Le gaz de cémentation est par exemple du propane (C3H8) ou de l'acétylène (C2H2) . Il peut aussi s'agir de tout autre hydrocarbure (CχHγ) susceptible de se dissocier aux températures de l'enceinte pour cémenter la surface des pièces à traiter. Le gaz de nitruration est par exemple de l'ammoniacThe carburising gas is for example propane (C3H8) or acetylene (C2H2). It can also be any other hydrocarbon (CχH γ ) likely to dissociate at the temperatures of the enclosure to cementer the surface of the parts to be treated. The nitriding gas is, for example, ammonia
(NH3) . En reprenant le schéma de la figure 3, on pourra faire arriver sur l'entrée 22 de la vanne 30 un hydrocarbure (Cχliγ) , sur l'entrée 24 de la vanne 32 de l'azote, sur l'entrée 36 de la vanne 34 de l'hydrogène et sur l'entrée 28 de la vanne 36 de 1 ' ammoniac .(NH3). By resuming the diagram of FIG. 3, a hydrocarbon (Cχliγ), on the inlet 24 of the valve 32 of the nitrogen, can be delivered to the inlet 22 of the valve 30 on the inlet 36 of the valve. 34 of the hydrogen and the inlet 28 of the valve 36 of ammonia.
L'injection du gaz de nitruration peut être réalisée seulement pendant certaines des étapes de diffusion. En outre, pendant une étape de diffusion au cours de laquelle du gaz de nitruration est injecté, l'injection du gaz de nitruration peut être réalisée seulement pendant une partie de l'étape de diffu¬ sion. Un gaz neutre, par exemple de l'azote (N2), peut être injecté pendant la totalité des étapes d'enrichissement et de diffusion, seulement pendant les étapes de diffusion, ou seule¬ ment pendant une partie des étapes de diffusion. L'injection du gaz neutre est régulée de façon à maintenir constante la pression dans l'enceinte 14. Lorsque du gaz de nitruration et du gaz neutre sont injectés simultanément, les proportions relatives du gaz de nitruration et du gaz neutre sont déterminées en fonction du profil de concentration d'azote souhaité dans les pièces traitées. En outre, les proportions relatives du gaz de nitruration et du gaz neutre peuvent être différentes pour chaque étape de diffusion pendant laquelle du gaz de nitruration et du gaz neutre sont injectés simultanément dans l'enceinte 14.Injection of the nitriding gas can be performed only during some of the diffusion steps. In addition, during a diffusion step in which the nitriding gas is injected, the injection of the nitriding gas may be performed only during part of the step of diffu sion ¬. A neutral gas, for example nitrogen (N2) can be injected during all stages of boost and diffusion, only during the diffusion steps, or only ¬ during part of the diffusion steps. The injection of the neutral gas is regulated so as to maintain constant the pressure in the chamber 14. When nitriding gas and neutral gas are injected simultaneously, the relative proportions of the nitriding gas and the neutral gas are determined as a function of desired nitrogen concentration profile in the treated parts. In addition, the relative proportions of the nitriding gas and the neutral gas may be different for each diffusion stage during which the Nitriding gas and neutral gas are simultaneously injected into the chamber 14.
Selon une variante de réalisation de l'invention, tous les gaz injectés dans l'enceinte 14 du four 10 ou certains d'entre eux peuvent être mélangés avant l'injection dans l'enceinte 14. Une telle variante permet par exemple, lors des étapes de montée en température H et d'égalisation de température PH, d'injecter directement dans l'enceinte 14 un mélange d'azote et d'hydrogène du type contenant une proportion d'hydrogène infé- rieure à 5 % en volume, une telle proportion d'hydrogène excluant tout risque d'explosion.According to an alternative embodiment of the invention, all the gases injected into the enclosure 14 of the furnace 10 or some of them may be mixed before the injection into the chamber 14. Such a variant makes it possible, for example, during steps of raising the temperature H and equalizing the temperature PH, injecting directly into the chamber 14 a mixture of nitrogen and hydrogen of the type containing a proportion of hydrogen of less than 5% by volume, a such proportion of hydrogen excluding any risk of explosion.
Selon le présent exemple de réalisation de 1 ' invention, le procédé de carbonitruration est mis en oeuvre sans variation de pression et les injections, du gaz de cémentation et du gaz de nitruration (et/ou éventuellement du gaz neutre), lors des étapes d'enrichissement et de diffusion, sont successives et la substitution entre le gaz de cémentation et le gaz de nitruration (et/ou éventuellement le gaz neutre) est susceptible de se produire très rapidement. La figure 5 représente un exemple de profil de concentration en poids de l'élément azote ayant diffusé dans une pièce traitée en fonction de la profondeur, mesurée à partir de la surface de la pièce, lorsque le gaz de cémentation est le propane et le gaz de nitruration est l'ammoniac. Les figures 6, 7 et 8 illustrent respectivement un exemple de procédé de carbonitruration selon l'invention et les profils de concentration de carbone et d'azote obtenus pour un tel procédé de carbonitruration dans lequel le gaz de cémentation est l'acétylène et le gaz de nitruration est l'ammoniac. Dans le présent exemple, la carbonitruration est réalisée à un palier de température de 88O0C. A titre d'exemple, les étapes de chauffage H et d'égalisation de température PH durent 20 minutes et sont suivies d'une alternance de trois étapes d'enrichissement Cl, C2, C3 (respectivement de 123 s, 51 s et 49 s) et de trois étapes de diffusion Dl, D2, D3 (respectivement de 194 s, 286 s et 2957 s) .According to the present embodiment of the invention, the carbonitriding process is carried out without pressure variation and the injections, carburising gas and nitriding gas (and / or optionally neutral gas), during the steps of enrichment and diffusion, are successive and the substitution between the carburizing gas and the nitriding gas (and / or possibly the neutral gas) is likely to occur very quickly. FIG. 5 shows an example of a concentration profile by weight of the nitrogen element diffused in a treated part as a function of depth, measured from the surface of the part, when the case gas is propane and gas. Nitriding is ammonia. FIGS. 6, 7 and 8 respectively illustrate an example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process in which the cementation gas is acetylene and gas. Nitriding is ammonia. In the present example, the carbonitriding is carried out at a temperature level of 88O 0 C. For example, the heating steps H and temperature equalization PH last 20 minutes and are followed by an alternation of three steps of enrichment Cl, C2, C3 (respectively of 123 s, 51 s and 49 s) and three dissemination stages Dl, D2, D3 (194 s, 286 s and 2957 s, respectively).
Les figures 9, 10 et 11 illustrent respectivement un autre exemple de procédé de carbonitruration selon l'invention et les profils de concentration de carbone et d'azote obtenus pour un tel procédé de carbonitruration, dans lequel le gaz de cémentation est l'acétylène et le gaz de nitruration est l'ammoniac. Dans le présent exemple, la carbonitruration est réalisée à un palier de température de 93O0C. Les étapes de chauffage H et d'égalisation de température PH durent respectivement 29 minutes et 31 minutes et sont suivies d'une alternance de cinq étapes d'enrichissement Cl à C5 (respectivement de 329 s, 91 s, 80 s, 75 s et 71 s) et de cinq étapes de diffusion Dl à D5 (respectivement de 108 s, 144 s, 176 s, 208 s et 2858 s) .FIGS. 9, 10 and 11 respectively illustrate another example of a carbonitriding process according to the invention and the carbon and nitrogen concentration profiles obtained for such a carbonitriding process, in which the carburizing gas is acetylene and the nitriding gas is ammonia. In the present example, the carbonitriding is carried out at a temperature step of 93O 0 C. The heating steps H and equalization of temperature PH respectively last 29 minutes and 31 minutes and are followed by an alternation of five steps of enrichment C1 to C5 (329 s, 91 s, 80 s, 75 s and 71 s respectively) and five diffusion steps D1 to D5 (108 s, 144 s, 176 s, 208 s and 2858 s, respectively).
La demanderesse a mis en évidence que l'injection d'ammoniac pendant les étapes de diffusion permet un enrichissement de la couche cémentée en azote sur une profondeur de plusieurs centaines de micromètres. Pour les trois exemples représentés, la teneur en azote obtenue est de l'ordre de 0,2 % en poids à quelques micromètres de profondeur. La teneur en azote décroît alors lentement à partir de 0,2 % pendant plusieurs centaines de micromètres. A titre d'exemple, pour l'exemple de réalisation précédemment décrit en relation aux figures 6, 7 et 8, la concentration en azote est de l'ordre deThe Applicant has shown that the injection of ammonia during the diffusion steps allows an enrichment of the carburized layer of nitrogen to a depth of several hundred micrometers. For the three examples shown, the nitrogen content obtained is of the order of 0.2% by weight to a few microns in depth. The nitrogen content then decreases slowly from 0.2% for several hundred microns. By way of example, for the embodiment previously described with reference to FIGS. 6, 7 and 8, the nitrogen concentration is of the order of
0,2 % à 30 μm, de 0,14 % à 60 μm, de 0,12 % à 130 μm et de 0,05 % à 200 μm.0.2% to 30 μm, 0.14% to 60 μm, 0.12% to 130 μm and 0.05% to 200 μm.
Selon une variante de l'invention, le gaz de nitruration peut être injecté pendant l'étape H de montée en température, dès que la température dans l'enceinte 14 dépasse une température donnée, et/ou pendant l'étape PH d'égalisation en température. A titre d'exemple, lorsque le gaz de nitruration est l'ammoniac, l'injection peut être réalisée dès que la température dans l'enceinte 14 dépasse environ 8000C. Le fait d'injecter le gaz de nitruration seulement pendant les étapes de diffusion du carbone permet un meilleur enrichissement en azote et en carbone des pièces traitées et permet d'obtenir, de façon précise et reproductible, les profils de concentrations de carbone et d'azote souhaités. En effet, si le gaz de nitruration est injecté simultanément au gaz de cémentation, il se produit une dilution du gaz de cémentation et du gaz de nitruration. Ceci n'est pas un facteur favorisant la réaction du carbone issu du gaz de cémentation ou la réaction de l'azote issu du gaz de nitruration avec les pièces à traiter, ce qui ralentit l'enrichissement des pièces en azote et en carbone. En outre, si le gaz de cémentation et le gaz de nitruration sont mélangés, le contrôle de l'ambiance gazeuse dans l'enceinte 14 peut difficilement être effectué avec précision, ce qui rend plus difficile l'obtention, de façon précise et reproductible, des profils de concentrations des pièces en azote et en carbone souhaités. En outre, la diffusion de l'azote dans des pièces en acier étant, pour des mêmes conditions de traitement, plus rapide que la diffusion du carbone, l'injection du gaz de nitruration et du gaz de cémentation à des étapes distinctes permet de modifier plus facilement la durée d'injection de chaque gaz tout en assurant le maintien d'une pression constante dans l'enceinte 14.According to a variant of the invention, the nitriding gas can be injected during step H of temperature rise, as soon as the temperature in the chamber 14 exceeds a given temperature, and / or during the equalizing step PH in temperature. By way of example, when the nitriding gas is ammonia, the injection can be carried out as soon as the temperature in the enclosure 14 exceeds about 800 ° C. Injecting the nitriding gas only during the carbon diffusion stages allows better nitrogen and carbon enrichment of the treated parts and allows to obtain, in a precise and reproducible way, the carbon and carbon concentration profiles. desired nitrogen. Indeed, if the nitriding gas is injected simultaneously with the carburizing gas, there is a dilution of the carburizing gas and the nitriding gas. This is not a factor favoring the reaction of carbon from the carburising gas or the reaction of the nitrogen from the nitriding gas with the parts to be treated, which slows the enrichment of the parts in nitrogen and carbon. In addition, if the carburizing gas and the nitriding gas are mixed, the control of the gaseous environment in the chamber 14 can hardly be carried out accurately, which makes it more difficult to obtain accurately and reproducibly, desired nitrogen and carbon component concentration profiles. In addition, the diffusion of nitrogen in steel parts being, for the same processing conditions, faster than the diffusion of carbon, the injection of the nitriding gas and the cementation gas at different stages makes it possible to modify more easily the injection time of each gas while ensuring the maintenance of a constant pressure in the chamber 14.
Bien entendu, la présente invention est susceptible de diverses variantes et modifications qui apparaîtront à l'homme de l'art. A titre d'exemple, l'étape de trempe au gaz précédemment décrite peut être remplacée par une étape de trempe à l'huile. Of course, the present invention is susceptible of various variations and modifications which will be apparent to those skilled in the art. For example, the previously described gas quenching step can be replaced by an oil quenching step.

Claims

REVENDICATIONS
1. Procédé de carbonitruration d'une pièce en acier disposée dans une enceinte (14) maintenue à une pression interne réduite, la pièce étant maintenue à un palier de température, caractérisé en ce qu'il comporte une alternance de premières et de secondes étapes, un gaz de cémentation étant injecté dans l'enceinte seulement pendant les premières étapes et un gaz de nitruration étant injecté dans l'enceinte seulement pendant au moins une partie d'au moins deux secondes étapes.1. A method of carbonitriding a steel part disposed in an enclosure (14) maintained at a reduced internal pressure, the part being maintained at a temperature step, characterized in that it comprises an alternation of first and second stages , a carburising gas being injected into the chamber only during the first stages and a nitriding gas being injected into the chamber only during at least a portion of at least two second stages.
2. Procédé selon la revendication 1, dans lequel le gaz de cémentation est le propane ou l'acétylène.The process of claim 1, wherein the carburising gas is propane or acetylene.
3. Procédé selon la revendication 1, dans lequel le gaz de nitruration est l'ammoniac.3. The process of claim 1, wherein the nitriding gas is ammonia.
4. Procédé selon la revendication 1, dans lequel un gaz neutre est injecté dans l'enceinte (14) simultanément au gaz de nitruration.4. The method of claim 1, wherein a neutral gas is injected into the chamber (14) simultaneously with the nitriding gas.
5. Procédé selon la revendication 1, dans lequel le gaz de nitruration est injecté dans l'enceinte (14) au cours d'au moins une seconde étape pendant une durée inférieure à la durée de ladite seconde étape, le reste de ladite seconde étape étant réalisé en présence d'un gaz neutre.5. The method of claim 1, wherein the nitriding gas is injected into the chamber (14) during at least a second step for a duration less than the duration of said second step, the rest of said second step. being performed in the presence of a neutral gas.
6. Procédé selon la revendication 1, dans lequel les premières et secondes étapes sont réalisées à une pression constante inférieure à 1500 pascals.The method of claim 1, wherein the first and second steps are performed at a constant pressure of less than 1500 pascals.
7. Procédé selon la revendication 1, dans lequel le palier de température est compris entre 8000C et 10500C.7. The method of claim 1, wherein the temperature step is between 800 0 C and 1050 0 C.
8. Procédé selon la revendication 1, dans lequel le palier de température est supérieur à 9000C.8. The method of claim 1, wherein the temperature step is greater than 900 0 C.
9. Four de carbonitruration (10) destiné à recevoir une pièce en acier, le four étant associé à des moyens d'introduction de gaz (22, 24, 26, 28) et d'extraction de gaz9. Carbonitriding furnace (10) for receiving a steel part, the furnace being associated with means for introducing gas (22, 24, 26, 28) and for extracting gas
(18, 20) commandés pour maintenir une pression interne réduite, et comprenant des moyens de chauffage (38) pour maintenir la pièce à un palier de température, caractérisé en ce que les moyens d'introduction comprennent des moyens (22, 28) pour introduire, au cours d'une alternance de premières et de secondes étapes réalisées audit palier de température, un gaz de cémentation seulement pendant les premières étapes et un gaz de nitruration seulement pendant au moins une partie d'au moins deux secondes étapes.(18, 20) controlled to maintain a reduced internal pressure, and comprising heating means (38) for maintaining the workpiece at a temperature plateau, characterized in that the insertion means comprises means (22, 28) for introducing, during an alternation of first and second steps performed at said temperature plateau, a carburization gas only during the first stages and a nitriding gas only during at least a portion of at least two second stages.
10. Four de carbonitruration (10) selon la revendication 9, dans lequel les moyens d'introduction (22, 24, 26, 28) comprennent des moyens (24) pour introduire un gaz neutre . 10. Carbonitriding furnace (10) according to claim 9, wherein the introduction means (22, 24, 26, 28) comprise means (24) for introducing a neutral gas.
EP06743815.0A 2005-04-19 2006-04-19 Low pressure carbonitriding method Active EP1885904B2 (en)

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FR0550996A FR2884523B1 (en) 2005-04-19 2005-04-19 LOW PRESSURE CARBONITRUTING PROCESS AND FURNACE
PCT/FR2006/050357 WO2006111683A1 (en) 2005-04-19 2006-04-19 Low pressure carbonitriding method and device

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DE602006010997D1 (en) 2010-01-21

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