EP0707661B1 - Method of low pressure nitriding a metal workpiece and oven for carrying out said method - Google Patents
Method of low pressure nitriding a metal workpiece and oven for carrying out said method Download PDFInfo
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- EP0707661B1 EP0707661B1 EP95918040A EP95918040A EP0707661B1 EP 0707661 B1 EP0707661 B1 EP 0707661B1 EP 95918040 A EP95918040 A EP 95918040A EP 95918040 A EP95918040 A EP 95918040A EP 0707661 B1 EP0707661 B1 EP 0707661B1
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- Prior art keywords
- treatment
- gas
- phase
- parts
- depassivation
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- 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
- C23C8/08—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 only one element being applied
- C23C8/24—Nitriding
- C23C8/26—Nitriding of ferrous surfaces
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- 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
- C23C8/36—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 using ionised gases, e.g. ionitriding
- C23C8/38—Treatment of ferrous surfaces
Definitions
- the present invention relates to a method for nitriding at low pressure of a metal part, by steel example, to improve its properties mechanical on the surface and, in particular, its resistance to wear.
- nitriding in salt baths we know that it exists on time current at least three types of treatment allowing to carry out this nitriding, namely: nitriding in salt baths, ionic nitriding and gas nitriding.
- Nitriding in a salt bath is a technique particularly polluting and dangerous due to toxic gas discharges and rinsing water it generates. In addition, it imposes on the workforce difficult working conditions. This is the reason which this technique tends to disappear.
- Ion nitriding involves an installation relative vacuum heat treatment, specially equipped so as to generate on the parts to be treated a glow discharge in a gas atmosphere nitriding.
- This technique has the disadvantage of being relatively expensive and not suitable for parts of complex shape and, in particular, of shape tubular, due to cathode phenomena dig.
- Gas nitriding involves bringing the parts to a treatment temperature of the order of 500 ° C to 600 ° C, and sweep them with nitriding gas, such as ammonia, associated with a compound accelerating the catalytic dissociation of ammonia, at pressure atmospheric.
- nitriding gas such as ammonia
- Such treatment which is described in European patent application No. 0 089 885, presents the disadvantage of being relatively long and consuming large amounts of process gas. For this same reason, this type of treatment is also polluting.
- the invention more particularly aims to eliminate these drawbacks.
- the above-mentioned catalyst may consist of nitrous oxide (N 2 O), carbon monoxide (CO), or even their hydrocarbon such as methane or propane.
- the nitriding power of mixture and, therefore, the type or nature of the nitrided combination layer obtained, as well as the quality of the diffusion layer can be checked by diluting the gas mixture with an amount of nitrogen molecular variable depending on the desired result (elimination of iron carbonitrides in the network which weaken the room). This dilution allows in particular avoid or limit the formation of a layer of so-called white layer combination.
- the process of diffusion of nitrogen in the layer surface of the metal constituting the parts to be treated is further improved by the depassivation phase prior.
- This depassivation phase can be obtained by injecting a passivation gas composed for example of ammonia and / or hydrogen, the room temperature then being greater than one threshold temperature of the order of 400 ° C.
- the depassivation treatment may start during the temperature rise phase of parts and may continue during the keeping parts at processing temperature (between 500 ° C and 600 ° C).
- this depassivation could be performed by generating a discharge on the parts luminescent in an atmosphere of hydrogen and argon at low pressure.
- the oven allowing the implementation of the process previously described preferably consists of an oven of the type used for low carburization pressure which includes an enclosure, for example double walls, constantly cooled, a refractory flask, housed inside the enclosure, which delimits a laboratory inside which the parts to be treated can be placed, heating means by radiation arranged inside the laboratory and process gas injectors passing through the enclosure and the muffle to lead inside the laboratory.
- an enclosure for example double walls, constantly cooled, a refractory flask, housed inside the enclosure, which delimits a laboratory inside which the parts to be treated can be placed
- heating means by radiation arranged inside the laboratory and process gas injectors passing through the enclosure and the muffle to lead inside the laboratory.
- means are also provided for generate in the enclosure a relative vacuum and means for adjusting the process gas flow.
- this type of oven provides surprising results mainly due to the fact that it allows to generate a flow inside the laboratory continuous and homogeneous process gas arriving on the parts to be treated at a relatively temperature low, below the dissociation temperature. This is due to the fact that between the cooled wall of the enclosure and the muffle, the temperature stays at a level relatively low and that intense heat exchange which are mainly effected by radiation affect only the interior volume of the laboratory, i.e. areas relatively close to the parts to be treat.
- the process gases do not reach the processing temperature when in contact with the parts to treat. Therefore, a significant part of the nitrogen active released during this dissociation acts on parts to be treated even before the phenomenon of recombination of active nitrogen into molecular nitrogen could happen.
- the gases resulting from this dissociation are then sucked up by the means used to generate the relative vacuum inside the oven.
- the oven used is of the "cold wall” type, that is to say that it comprises a sealed enclosure 1 with double walls 2, 3 between which circulates a cooling fluid such as water. Thanks to this feature, the oven has low thermal inertia and therefore cooling rates significantly higher than those obtained in an oven with hot walls. This point is essential when dealing with nuances of steel susceptible to precipitation corrosion intergranular.
- a muffle 4 of refractory material which delimits a volume V constituting the laboratory, inside from which the parts 5 are placed on a support 6 carried through the bottom of the enclosure 1.
- the rooms can be arranged in bulk on several mesh elements arranged one above the other.
- the heating of 5 pieces inside the laboratory is provided by heating resistors 7 connected to a circuit external power supply.
- the oven is also equipped a gas circulation turbine which can be used in particular to accelerate the cooling of the interior volume of enclosure 1. This cooling is usually obtained by introduction of an inert gas (nitrogen or nitrogen hydrogenated) at a pressure below the pressure atmospheric, the convection of this gas being ensured by the turbine 8.
- an inert gas nitrogen or nitrogen hydrogenated
- the admission of the treatment gases inside the oven is ensured by a gas injection circuit 11 supplied from gas sources S 1 , S 2 , S 3 by means of a mixer-regulator of flow 12.
- the oven can also be equipped with treatment means by ion bombardment involving for example a high voltage electric generator connected to the enclosure wall and the support structure of the parts to be treated.
- treatment means by ion bombardment involving for example a high voltage electric generator connected to the enclosure wall and the support structure of the parts to be treated.
- these processing means can be used to perform plasma assisted stripping.
- the invention is not limited to such a provision: Indeed, the injection could be performed in another location, inside the laboratory, possibly inside the rooms to treat (tubular).
- the conduit injection can pass into the support structure 6, aspiration then takes place at another location from the oven, preferably in a location where to obtain an axial flow of process gas.
- the injection pipe or the suction pipe may be extended by an injection nozzle or a suction nozzle of suitable shape for example for inject or aspirate inside a tubular.
- the nitriding gas mixture used may consist of a mixture consisting of ammonia (NH 3 ), nitrous oxide (N 2 O) and nitrogen (N 2 ).
- NH 3 ammonia
- N 2 O nitrous oxide
- N 2 nitrogen
- hydrogen (H2) and argon (Ar) can also be used.
- the base mixture consists of 95% to 97% NH 3 and 5 to 3% N 2 O, depending on the grades of steel treated.
- the processing temperature can vary between 500 and 600 ° C depending on the nuances treated and the specifications charges.
- Figure 2 shows the different successive phases a nitriding treatment at low pressure, according to the method according to the invention.
- a high vacuum is applied to the oven at a pressure of the order of 10 -2 mbar, in order to obtain purging of the oven.
- the parts are then heated to a pressure of 10 -2 mbar (temperature rise phase) for a period T 1 .
- the actual depassivation phase is then carried out by maintaining the parts at the treatment temperature, under this partial pressure of ammonia for a period T 2 .
- the nitriding phase itself is started by injecting the treatment gas onto the parts to be treated. During this nitriding phase which continues during the period T 3 , the temperature and pressure conditions are maintained.
- the nitriding cycle ends with a rapid cooling phase, thanks to an injection of inert cooling gas (nitrogen or hydrogenated nitrogen), the circulation of this gas being ensured by the turbine (period T 4 ).
- inert cooling gas nitrogen or hydrogenated nitrogen
- An important advantage of the process described above is that, thanks to the fact that the treatment is performed at low pressure, it is possible to obtain a rapid regulation of the nitriding potential: effect, to purge the oven and inject a different mixture (more or less rich in nitrogen) for vary this potential in a few minutes, which is not possible with conventional methods.
- gaseous releases caused by the nitriding at low pressure are very low and are easily treatable compared to releases generated by salt baths and rinsing water necessary for nitriding treatments in salt baths.
- working conditions as well as job security work are of better quality.
- the process according to the invention uses less expensive means. he allows, in particular on tubulars, treatments that cannot be carried out ionically due to hollow cathode phenomena. It also allows the carrying out bulk treatments (impossible in ionic) thereby reducing the cost of preparing the charge.
- the process according to the invention improves the treatment of very long tubulars by injecting the mixture gas directly into the tubulars.
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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)
- Furnace Details (AREA)
Description
La présente invention concerne un procédé pour la nitruration à basse pression d'une pièce métallique, par exemple en acier, en vue d'améliorer ses propriétés mécaniques en surface et, en particulier, sa résistance à l'usure.The present invention relates to a method for nitriding at low pressure of a metal part, by steel example, to improve its properties mechanical on the surface and, in particular, its resistance to wear.
D'une façon générale, on sait qu'il existe à l'heure actuelle au moins trois types de traitement permettant d'effectuer cette nitruration, à savoir : la nitruration en bains de sels, la nitruration ionique et la nitruration gazeuse.Generally, we know that it exists on time current at least three types of treatment allowing to carry out this nitriding, namely: nitriding in salt baths, ionic nitriding and gas nitriding.
La nitruration par bain de sel est une technique particulièrement polluante et dangereuse en raison des rejets de gaz toxiques et des eaux de rinçage qu'elle engendre. En outre, elle impose à la main-d'oeuvre des conditions de travail difficiles. C'est la raison pour laquelle cette technique tend à disparaítre.Nitriding in a salt bath is a technique particularly polluting and dangerous due to toxic gas discharges and rinsing water it generates. In addition, it imposes on the workforce difficult working conditions. This is the reason which this technique tends to disappear.
La nitruration ionique fait intervenir une installation de traitement thermique sous vide relatif, spécialement équipée de manière à engendrer sur les pièces à traiter une décharge luminescente dans une atmosphère de gaz nitrurant. Cette technique présente l'inconvénient d'être relativement coûteuse et de ne pas convenir pour des pièces de forme complexe et, en particulier, de forme tubulaire, et ce, en raison des phénomènes de cathode creuse.Ion nitriding involves an installation relative vacuum heat treatment, specially equipped so as to generate on the parts to be treated a glow discharge in a gas atmosphere nitriding. This technique has the disadvantage of being relatively expensive and not suitable for parts of complex shape and, in particular, of shape tubular, due to cathode phenomena dig.
La nitruration gazeuse consiste à porter les pièces à une température de traitement de l'ordre de 500°C à 600°C, et de les balayer avec un gaz de nitruration, tel que de l'ammoniac, associé à un composé accélérateur de la dissociation catalytique de l'ammoniac, à la pression atmosphérique. Un tel traitement, qui se trouve décrit dans la demande de brevet européen N° 0 089 885, présente l'inconvénient d'être relativement long et de consommer de grandes quantités de gaz de traitement. Pour cette même raison, ce type de traitement est également polluant.Gas nitriding involves bringing the parts to a treatment temperature of the order of 500 ° C to 600 ° C, and sweep them with nitriding gas, such as ammonia, associated with a compound accelerating the catalytic dissociation of ammonia, at pressure atmospheric. Such treatment, which is described in European patent application No. 0 089 885, presents the disadvantage of being relatively long and consuming large amounts of process gas. For this same reason, this type of treatment is also polluting.
Pour tenter de réduire les quantités de gaz de traitement mises en jeu, on a également proposé d'effectuer le traitement à basse pression, à l'intérieur d'un four de traitement thermique sous vide. Toutefois, dans ce cas, on se heurte au fait qu'à la température de traitement, l'ammoniac subit une dissociation, puis une recombinaison des atomes d'azote actifs en azote moléculaire.To try to reduce the amount of process gas involved, it was also proposed to carry out the low pressure treatment, inside a vacuum heat treatment. However, in this case, we come up against the fact that at the processing temperature, ammonia dissociates, then recombines nitrogen atoms active in molecular nitrogen.
Il est clair que ce processus de recombinaison va à l'encontre du but recherché, puisque seule une faible fraction du gaz injecté composée d'azote actif qui a échappé à la recombinaison pourra interagir avec le métal de la pièce à traiter pour obtenir le phénomène de nitruration.It is clear that this recombination process goes to against the desired goal, since only a small fraction of the injected gas composed of active nitrogen which has escaped recombination can interact with metal of the part to be treated to obtain the phenomenon of nitriding.
L'usage de moyens tels qu'une canne d'injection constamment refroidie qui permettent d'éviter que le gaz de traitement n'atteigne la température de traitement avant d'être à proximité immédaite de la pièce n'est possible que dans un nombre très limité de cas et ne convient pas bien notamment dans le cas de pièces tubulaires. Tel est le cas dans le procédé décrit dans le brevet français N° 2 674 618.The use of means such as an injection rod constantly cooled which prevent gas treatment does not reach treatment temperature before being in the immediate vicinity of the room is not possible only in a very limited number of cases and not particularly suitable in the case of parts tubular. This is the case in the process described in the French patent No. 2,674,618.
L'invention a plus particulièrement pour but de supprimer ces inconvénients.The invention more particularly aims to eliminate these drawbacks.
Elle propose à cet effet un procédé de traitement selon lequel le processus de nitruration est réalisé dans un four en portant les pièces à une température de traitement de l'ordre de 500° C à 600° C, dans une atmosphère à basse pression, avec injection sur les pièces d'un gaz de traitement.To this end, it proposes a processing method according to which the process of nitriding is carried out in an oven by bringing the parts to a temperature of treatment of the order of 500 ° C to 600 ° C, in a low atmosphere pressure, with injection on the parts of a treatment gas.
Selon l'invention ce procédé est caractérisé en ce qu'il comprend les phases successives suivantes :
- une phase de purgeage à une pression égale ou inférieure à 10-2 mbars,
- une phase de dépassivation,
- une phase de traitement comprenant l'injection dans le four d'un gaz de traitement consistant en un mélange gazeux comprenant au moins de l'ammoniac, ainsi qu'un catalyseur favorisant la dissociation de l'ammoniac au contact des pièces à traiter, et s'opposant à la recombinaison de l'azote actif provenant de cette dissociation en azote moléculaire, cette phase de traitement s'effectuant à la température de traitement, à une pression régulée à une valeur comprise entre 200 et 400 mbars, déterminée en fonction de la nature du traitement, avec un taux de renouvellement constant de l'atmosphère gazeuse, le gaz de traitement injecté étant dilué dans une quantité d'azote moléculaire variable afin de contrôler le pouvoir nitrurant du mélange et les nuances de la couche nitrurée.
- a purging phase at a pressure equal to or less than 10 -2 mbar,
- a depassivation phase,
- a treatment phase comprising the injection into the oven of a treatment gas consisting of a gas mixture comprising at least ammonia, as well as a catalyst promoting the dissociation of ammonia on contact with the parts to be treated, and opposing the recombination of active nitrogen originating from this dissociation into molecular nitrogen, this treatment phase being carried out at the treatment temperature, at a pressure regulated to a value between 200 and 400 mbar, determined as a function of the nature of the treatment, with a constant rate of renewal of the gaseous atmosphere, the injected treatment gas being diluted in a variable quantity of molecular nitrogen in order to control the nitriding power of the mixture and the nuances of the nitrided layer.
Avantageusement, le susdit catalyseur pourra consister en du protoxyde d'azote (N2O), en de l'oxyde de carbone (CO), voire même en leur hydrocarbure tel que du méthane ou du propane. Advantageously, the above-mentioned catalyst may consist of nitrous oxide (N 2 O), carbon monoxide (CO), or even their hydrocarbon such as methane or propane.
Comme précédemment mentionné, le pouvoir nitrurant du mélange et, en conséquence, le type ou la nature de la couche nitrurée de combinaison obtenue, ainsi que la qualité de la couche de diffusion pourront être contrôlés en diluant le mélange gazeux dans une quantité d'azote moléculaire variable en fonction du résultat recherché (élimination des carbonitrures de fer en réseau qui fragilisent la pièce). Cette dilution permet notamment d'éviter ou de limiter la formation d'une couche de combinaison dite couche blanche.As previously mentioned, the nitriding power of mixture and, therefore, the type or nature of the nitrided combination layer obtained, as well as the quality of the diffusion layer can be checked by diluting the gas mixture with an amount of nitrogen molecular variable depending on the desired result (elimination of iron carbonitrides in the network which weaken the room). This dilution allows in particular avoid or limit the formation of a layer of so-called white layer combination.
Le processus de diffusion de l'azote dans la couche superficielle du métal constituant les pièces à traiter est en outre amélioré grâce à la phase de dépassivation préalable. Cette phase de dépassivation peut être obtenue en effectuant une injection d'un gaz de dépassivation composé par exemple d'ammoniac et/ou de l'hydrogène, la température des pièces étant alors supérieure à une température de seuil de l'ordre de 400°C. En réalité, le traitement de dépassivation pourra débuter pendant la phase de montée en température des pièces et pourra se poursuivre pendant la phase de maintien des pièces à la température de traitement (entre 500°C et 600°C). Bien entendu, dans le cas où le four est équipé de moyens permettant d'effectuer un traitement par bombardement ionique, cette dépassivation pourra être effectuée en engendrant sur les pièces une décharge luminescente dans une atmosphère d'hydrogène et d'argon à basse pression.The process of diffusion of nitrogen in the layer surface of the metal constituting the parts to be treated is further improved by the depassivation phase prior. This depassivation phase can be obtained by injecting a passivation gas composed for example of ammonia and / or hydrogen, the room temperature then being greater than one threshold temperature of the order of 400 ° C. In reality, the depassivation treatment may start during the temperature rise phase of parts and may continue during the keeping parts at processing temperature (between 500 ° C and 600 ° C). Of course, if the oven is equipped with means for carrying out treatment with ion bombardment, this depassivation could be performed by generating a discharge on the parts luminescent in an atmosphere of hydrogen and argon at low pressure.
Le four permettant la mise en oeuvre du procédé précédemment décrit consiste, de préférence, en un four du type de ceux utilisés pour la cémentation à basse pression qui comprend une enceinte, par exemple à double parois, constamment refroidie, un moufle réfractaire, logé à l'intérieur de l'enceinte, qui délimite un laboratoire à l'intérieur duquel les pièces à traiter peuvent être placées, des moyens de chauffage par rayonnement disposés à l'intérieur du laboratoire et des injecteurs de gaz de traitement traversant l'enceinte et le moufle pour déboucher à l'intérieur du laboratoire. Bien entendu, des moyens sont également prévus pour engendrer dans l'enceinte un vide relatif et des moyens de réglage du débit du gaz de traitement.The oven allowing the implementation of the process previously described preferably consists of an oven of the type used for low carburization pressure which includes an enclosure, for example double walls, constantly cooled, a refractory flask, housed inside the enclosure, which delimits a laboratory inside which the parts to be treated can be placed, heating means by radiation arranged inside the laboratory and process gas injectors passing through the enclosure and the muffle to lead inside the laboratory. Of course, means are also provided for generate in the enclosure a relative vacuum and means for adjusting the process gas flow.
Il s'avère que contrairement aux préjugés défavorables résultant du coût élevé et des risques de corrosion de l'installation, ce type de four permet d'obtenir des résultats surprenants principalement dus au fait qu'il permet d'engendrer à l'intérieur du laboratoire un flux continu et homogène de gaz de traitement qui arrive sur les pièces à traiter à une température relativement basse, en dessous de la température de dissociation. Ceci est dû au fait qu'entre la paroi refroidie de l'enceinte et le moufle, la température demeure à un niveau relativement bas et que les échanges thermiques intenses qui s'effectuent essentiellement par rayonnement n'affectent que le volume intérieur du laboratoire, c'est-à-dire des zones relativement proches des pièces à traiter.It turns out that contrary to unfavorable prejudices resulting from the high cost and the risk of corrosion of installation, this type of oven provides surprising results mainly due to the fact that it allows to generate a flow inside the laboratory continuous and homogeneous process gas arriving on the parts to be treated at a relatively temperature low, below the dissociation temperature. This is due to the fact that between the cooled wall of the enclosure and the muffle, the temperature stays at a level relatively low and that intense heat exchange which are mainly effected by radiation affect only the interior volume of the laboratory, i.e. areas relatively close to the parts to be treat.
En conséquence, les gaz de traitement n'atteignent la température de traitement qu'au contact des pièces à traiter. De ce fait, une partie importante de l'azote actif libéré lors de cette dissociation agit sur les pièces à traiter avant même que le phénomène de recombinaison de l'azote actif en azote moléculaire puisse se produire. Les gaz résultant de cette dissociation sont ensuite aspirés par les moyens utilisés pour engendrer le vide relatif à l'intérieur du four.As a result, the process gases do not reach the processing temperature when in contact with the parts to treat. Therefore, a significant part of the nitrogen active released during this dissociation acts on parts to be treated even before the phenomenon of recombination of active nitrogen into molecular nitrogen could happen. The gases resulting from this dissociation are then sucked up by the means used to generate the relative vacuum inside the oven.
Les effets de ce processus se combinent avec les effets du gaz catalyseur utilisé pour obtenir les résultats précédemment évoqués qui concernent aussi bien la qualité des résultats obtenus, les possibilités d'action sur les différents paramètres de traitement pour obtenir exactement les nuances souhaitées et les quantités de gaz de traitement mis en oeuvre (celles-ci peuvent être très réduites du fait du gain en rendement obtenu et de la faible pression).The effects of this process combine with the effects of the catalyst gas used to obtain the results previously mentioned which concern quality as well of the results obtained, the possibilities for action on different processing parameters to get exactly the nuances desired and the quantities of gas of processing implemented (these can be very reduced due to the gain in yield obtained and the low pressure).
Une installation pour la mise en oeuvre du procédé selon
l'invention sera décrite ci-après, à titre d'exemple non
limitatif, avec référence aux dessins annexés dans
lesquels :
Dans l'exemple représenté sur la figure 1, le four
utilisé est de type à "parois froides", c'est-à-dire
qu'il comprend une enceinte étanche 1 à double parois 2,
3 entre lesquelles circule un fluide de refroidissement
tel que de l'eau. Grâce à cette particularité, le four
présente une faible inertie thermique et donc des
vitesses de refroidissement sensiblement supérieures à
celles obtenues dans un four à parois chaudes. Ce point
est primordial lorsqu'il s'agit de traiter des nuances
d'acier sensibles à la corrosion par précipitation
intergranulaire. A l'intérieur de l'enceinte 1 est
disposé un moufle 4 en matériau réfractaire qui délimite
un volume V constituant le laboratoire, à l'intérieur
duquel les pièces 5 sont placées sur un support 6 porté
par le fond de l'enceinte 1. Les pièces pourront être
disposées en vrac sur plusieurs éléments grillagés
disposés les uns au-dessus des autres. Le chauffage des
pièces 5 à l'intérieur du laboratoire est assuré par des
résistances chauffantes 7 connectées à un circuit
d'alimentation extérieur. Le four est par ailleurs équipé
d'une turbine de circulation des gaz utilisable notamment
pour accélérer le refroidissement du volume intérieur de
l'enceinte 1. Ce refroidissement est habituellement
obtenu par introduction d'un gaz inerte (azote ou azote
hydrogéné) à une pression inférieure à la pression
atmosphérique, la convection de ce gaz étant assurée par
la turbine 8.In the example shown in Figure 1, the oven
used is of the "cold wall" type, that is to say
that it comprises a sealed enclosure 1 with double walls 2,
3 between which circulates a cooling fluid
such as water. Thanks to this feature, the oven
has low thermal inertia and therefore
cooling rates significantly higher than
those obtained in an oven with hot walls. This point
is essential when dealing with nuances
of steel susceptible to precipitation corrosion
intergranular. Inside enclosure 1 is
disposed a muffle 4 of refractory material which delimits
a volume V constituting the laboratory, inside
from which the
L'installation fait en outre intervenir des moyens de pompage 9 permettant d'établir, à l'intérieur de l'enceinte, un vide limite d'au moins 10-2 mbar afin d'assurer un niveau de purge suffisant. Ces moyens de pompage 9 sont commandés par un système de régulation 10 conçu de manière à maintenir une pression la plus constante possible dans l'enceinte de traitement 1 lors du cycle de nitruration. La mesure de la pression à l'intérieur de l'enceinte nécessite deux types de capteurs :
- des jauges "Pirani" ou "Penning" pour les faibles pressions, lorsqu'on veut obtenir un vide limite de purge,
- un manomètre à membrane pour la pression de travail.
- "Pirani" or "Penning" gauges for low pressures, when we want to obtain a purge limit vacuum,
- a diaphragm pressure gauge for working pressure.
L'admission des gaz de traitement à l'intérieur du four
est assurée par un circuit d'injection de gaz 11 alimenté
à partir de sources de gaz S1, S2, S3 par l'intermédiaire
d'un mélangeur-régulateur de débit 12.The admission of the treatment gases inside the oven is ensured by a
La pression de travail doit permettre d'assurer un renouvellement correct de l'atmosphère avec une bonne pénétration de l'atmosphère dans les formes complexes (trous borgnes...) et de limiter au maximum les consommations de gaz et donc les rejets. Dans cet esprit, la pression a été fixée dans une fourchette de 200 à 400 mbar avec un taux de renouvellement constant :
- la pression de 200 mbar correspondant à un minimum de consommation, et
- la pression de 400 mbar correspondant à un maximum de pénétration dans les alésages, les cavités ou les formes complexes.
- the pressure of 200 mbar corresponding to a minimum consumption, and
- the pressure of 400 mbar corresponding to a maximum penetration in bores, cavities or complex shapes.
Le four peut être en outre équipé de moyens de traitement par bombardement ionique faisant par exemple intervenir un générateur électrique à haute tension connecté à la paroi de l'enceinte et à la structure de support des pièces à traiter. Dans le cadre du procédé selon l'invention, ces moyens de traitement peuvent être utilisés pour effectuer un décapage assisté par plasma.The oven can also be equipped with treatment means by ion bombardment involving for example a high voltage electric generator connected to the enclosure wall and the support structure of the parts to be treated. As part of the process according to the invention, these processing means can be used to perform plasma assisted stripping.
Une particularité importante du four précédemment décrit
réside dans le fait que :
Toutefois, l'invention ne se limite pas à une telle disposition : En effet, l'injection pourrait être réalisée en un autre emplacement, à l'intérieur du laboratoire, éventuellement à l'intérieur des pièces à traiter (cas des tubulaires). Dans ce cas, le conduit d'injection pourra passer dans la structure de support 6, l'aspiration s'effectuant alors en un autre emplacement du four, de préférence en un emplacement permettant d'obtenir un flux axial de gaz de traitement. A cet effet, le conduit d'injection ou le conduit d'aspiration pourront être prolongés par une buse d'injection ou une buse d'aspiration de forme appropriée par exemple pour réaliser l'injection ou l'aspiration à l'intérieur d'un tubulaire.However, the invention is not limited to such a provision: Indeed, the injection could be performed in another location, inside the laboratory, possibly inside the rooms to treat (tubular). In this case, the conduit injection can pass into the support structure 6, aspiration then takes place at another location from the oven, preferably in a location where to obtain an axial flow of process gas. In this effect, the injection pipe or the suction pipe may be extended by an injection nozzle or a suction nozzle of suitable shape for example for inject or aspirate inside a tubular.
Le mélange gazeux nitrurant utilisé pourra consister en un mélange constitué d'ammoniac (NH3), de protoxyde d'azote (N2O) et d'azote (N2). Dans le cas d'une dépassivation assistée par plasma, on peut également utiliser de l'hydrogène (H2) et de l'argon (Ar). The nitriding gas mixture used may consist of a mixture consisting of ammonia (NH 3 ), nitrous oxide (N 2 O) and nitrogen (N 2 ). In the case of plasma-assisted depassivation, hydrogen (H2) and argon (Ar) can also be used.
Lors de la phase de nitruration, le mélange de base est constitué de 95 % à 97 % de NH3 et de 5 à 3 % de N2O, selon les nuances d'acier traitées.During the nitriding phase, the base mixture consists of 95% to 97% NH 3 and 5 to 3% N 2 O, depending on the grades of steel treated.
Pour éviter ou limiter la formation d'une couche de combinaison dite "couche blanche", on peut diluer l'atmosphère avec de l'azote. Les proportions d'ammoniac et de protoxyde d'azote exprimées ci-dessus sont alors appliquées à la proportion de gaz de complément à 100 %.To avoid or limit the formation of a layer of so-called "white layer" combination, we can dilute the atmosphere with nitrogen. The proportions of ammonia and nitrous oxide expressed above are then applied to the proportion of complement gas at 100%.
La température de traitement peut varier entre 500 et 600°C selon les nuances traitées et le cahier des charges.The processing temperature can vary between 500 and 600 ° C depending on the nuances treated and the specifications charges.
La figure 2 montre les différentes phases successives d'un traitement de nitruration à basse pression, conformément au procédé selon l'invention.Figure 2 shows the different successive phases a nitriding treatment at low pressure, according to the method according to the invention.
Une fois les pièces disposées sur la structure de support et le four refermé de façon étanche, on procède à une mise sous vide poussé du four à une pression de l'ordre de 10-2 mbar, afin d'obtenir un purgeage du four.Once the parts have been placed on the support structure and the oven is closed in a sealed manner, a high vacuum is applied to the oven at a pressure of the order of 10 -2 mbar, in order to obtain purging of the oven.
On procède ensuite au chauffage des pièces à la pression de 10-2 mbar (phase de montée en température) pendant une période T1.The parts are then heated to a pressure of 10 -2 mbar (temperature rise phase) for a period T 1 .
Lorsque les pièces atteignent une température de l'ordre de 400°C et dans le cas où l'on souhaite effectuer une dépassivation sans bombardement ionique, on procède à une injection d'ammoniac seul (point I) puis on poursuit le chauffage jusqu'à l'obtention de la température de traitement TT sous une pression partielle d'ammoniac de l'ordre de 200 à 400 mbar.When the pieces reach a temperature of the order of 400 ° C and in the case where one wishes to carry out a depassivation without ion bombardment, we proceed to a injection of ammonia alone (point I) then we continue the heating until the temperature of TT treatment under partial ammonia pressure of in the range of 200 to 400 mbar.
On réalise ensuite la phase de dépassivation proprement dite en maintenant les pièces à la température de traitement, sous cette pression partielle d'ammoniac pendant une période T2.The actual depassivation phase is then carried out by maintaining the parts at the treatment temperature, under this partial pressure of ammonia for a period T 2 .
Dans le cas où l'on effectue une dépassivation ionique, l'injection d'ammoniac est supprimée et la phase de dépassivation s'effectue sous une atmosphère de dépassivation classique, par exemple d'hydrogène et d'argon.In the case where an ion depassivation is carried out, the ammonia injection is eliminated and the depassivation takes place under an atmosphere of conventional depassivation, for example of hydrogen and argon.
Une fois la phase de dépassivation achevée, on entame la phase de nitruration proprement dite en injectant le gaz de traitement sur les pièces à traiter. Pendant cette phase de nitruration qui se poursuit pendant la période T3, les conditions de température et de pression sont maintenues.Once the depassivation phase is completed, the nitriding phase itself is started by injecting the treatment gas onto the parts to be treated. During this nitriding phase which continues during the period T 3 , the temperature and pressure conditions are maintained.
Le cycle de nitruration se termine par une phase de refroidissement rapide, grâce à une injection de gaz de refroidissement inerte (azote ou azote hydrogéné), la circulation de ce gaz étant assurée par la turbine (période T4).The nitriding cycle ends with a rapid cooling phase, thanks to an injection of inert cooling gas (nitrogen or hydrogenated nitrogen), the circulation of this gas being ensured by the turbine (period T 4 ).
Un avantage important du procédé précédemment décrit consiste en ce que, grâce au fait que le traitement est effectué à basse pression, il est possible d'obtenir une régulation rapide du potentiel nitrurant : Il suffit, en effet, de réaliser une purge du four et d'injecter un mélange différent (plus ou moins riche en azote) pour faire varier ce potentiel en quelques minutes, ce qui n'est pas possible avec les procédés classiques.An important advantage of the process described above is that, thanks to the fact that the treatment is performed at low pressure, it is possible to obtain a rapid regulation of the nitriding potential: effect, to purge the oven and inject a different mixture (more or less rich in nitrogen) for vary this potential in a few minutes, which is not possible with conventional methods.
Par ailleurs, les rejets gazeux occasionnés par la nitruration à basse pression sont très faibles et sont facilement traitables comparés aux rejets engendrés par les bains de sels et les eaux de rinçage nécessaires aux traitements de nitruration en bains de sels. De plus, les conditions de travail ainsi que la sécurité du poste de travail sont de meilleure qualité. Furthermore, the gaseous releases caused by the nitriding at low pressure are very low and are easily treatable compared to releases generated by salt baths and rinsing water necessary for nitriding treatments in salt baths. Moreover, the working conditions as well as job security work are of better quality.
Vis-à-vis de la nitruration ionique, le procédé selon l'invention met en oeuvre des moyens moins onéreux. Il permet d'effectuer, notamment sur des tubulaires, des traitements non réalisables par voie ionique en raison des phénomènes de cathode creuse. Il permet en outre la réalisation de traitements en vrac (impossible en ionique) en diminuant ainsi le coût de préparation de la charge.With respect to ionic nitriding, the process according to the invention uses less expensive means. he allows, in particular on tubulars, treatments that cannot be carried out ionically due to hollow cathode phenomena. It also allows the carrying out bulk treatments (impossible in ionic) thereby reducing the cost of preparing the charge.
Par rapport à la nitruration gazeuse, le procédé selon l'invention permet d'améliorer le traitement de tubulaires de forte longueur par l'injection du mélange gazeux directement dans les tubulaires.Compared to gas nitriding, the process according to the invention improves the treatment of very long tubulars by injecting the mixture gas directly into the tubulars.
En outre, il engendre une consommation de gaz et donc des rejets gazeux moins importants (3 à 5 fois moindre).In addition, it generates gas consumption and therefore lower gas emissions (3 to 5 times less).
Des essais de traitement conformément au procédé selon l'invention seront décrits ci-après :Treatment tests according to the method according to the invention will be described below:
Dans cet essai, il s'agissait de réaliser sur des galets de 6 et de 9,5 mm de diamètre, en acier de nuance Z85 WDCV 6 5 4 2 un traitement de nitruration en vrac. Le cycle de traitement a plus précisément compris :
- une phase de dépassivation sous NH3 à 540°C pendant une durée de 30 mn,
- une première phase de nitruration à 540°C pendant une heure, sous une atmosphère de traitement comprenant 50 % N2- 46,5 % NH3- 3,5 % N2O,
- une deuxième phase de nitruration à 540°C pendant une heure et demi, sous une atmosphère de traitement comprenant 80 % N2 - 18,6 % NH3 - 1,4 % N2O.
- a depassivation phase under NH 3 at 540 ° C for a period of 30 min,
- a first nitriding phase at 540 ° C. for one hour, under a treatment atmosphere comprising 50% N 2 - 46.5% NH 3 - 3.5% N 2 O,
- a second nitriding phase at 540 ° C for an hour and a half, under a treatment atmosphere comprising 80% N 2 - 18.6% NH 3 - 1.4% N 2 O.
Ce traitement a permis d'obtenir les résultats suivants :
- absence de couche de combinaison,
- absence de carbonitrures en réseau,
- profondeur conventionnelle de nitruration : 50 à 80 µm (HV0.1 coeur + 100),
- dureté superficielle ≥ 950 HV5.
- absence of combination layer,
- absence of carbonitrides in the network,
- conventional nitriding depth: 50 to 80 µm (HV0.1 core + 100),
- surface hardness ≥ 950 HV5.
Au cours de cet essai, des pignons automobiles en acier de nuance 40 CD 4 ont subi le cycle de traitement suivant :
- une phase de dépassivation sous NH3 à 570°C pendant 30 mn,
- une phase de nitruration à 540°C pendant 2 heures et quart sous une atmosphère de traitement comprenant 35 % N2 - 60,5 % NH3 - 4,5 % N2O.
- a depassivation phase under NH 3 at 570 ° C for 30 min,
- a nitriding phase at 540 ° C for 2.5 hours under a treatment atmosphere comprising 35% N 2 - 60.5% NH 3 - 4.5% N 2 O.
Ce traitement a permis d'obtenir les résultats suivants :
- 10 à 20 µm de couche de combinaison,
- 0,15 à 0,2 mm de couche de diffusion (HV0.1 coeur + 100),
- dureté superficielle supérieure à 650 HV1.
- 10 to 20 µm combination layer,
- 0.15 to 0.2 mm of diffusion layer (HV0.1 core + 100),
- surface hardness greater than 650 HV1.
Claims (10)
- Method for the low-pressure nitriding of a metal part, whereby the nitriding is carried out in a furnace by heating the parts to a treatment temperature of the order of 500 to 600°C, in a low-pressure atmosphere, with injection of a treatment gas onto the parts, characterised in that it comprises the following successive phases:a purging phase at a pressure equal to or less than 10-2 mbars,a depassivation phase,a treatment phase comprising injection into the furnace of a treatment gas consisting of a gas compound comprising at least ammoniac, as well as a catalyst stimulating the dissociation of ammoniac upon contact with the parts to be treated and opposing recombination of the active nitrogen, produced by the dissociation, into molecular nitrogen, said treatment phase taking place at the treatment temperature, at a pressure set to a value included between 200 and 400 mbars, determined as a function of the type of treatment, with a constant renewal rate of the gas atmosphere, the treatment gas injected being diluted in a variable quantity of molecular nitrogen in order to control the nitriding capacity of the compound and the grades of the nitrided layer.
- A method as claimed in claim 1, characterised in that said catalyst consists of nitrogen monoxide (N2O), carbon monoxide (CO), or even hydrocarbons thereof such as methane or propane.
- A method as claimed in claim 1, characterised in that the depassivation phase comprises the injection of a depassivation gas at a temperature above a threshold temperature of approximately 400°C.
- A method as claimed in claim 3, characterised in that the depassivation gas is ammoniac and/or hydrogen.
- A method as claimed in claims 3 and 4, characterised in that the depassivation phase commences during the temperature build-up phase and continues throughout the phase during which the parts are maintained at the treatment temperature.
- A method as claimed in claim 1, characterised in that the depassivation phase comprises a depassivation by ion bombardment using a depassivation gas such as argon.
- Utilisation of a furnace to implement the method as claimed in any of the previous claims, wherein said furnace comprises a permanently cooled enclosure (1), e.g. with double walls (2, 3), a refractory muffle (4) housed inside the enclosure (1), delimiting a heating chamber in which the parts to be treated (5) can be placed, a means (7) of heating by radiation positioned inside the heating chamber, and treatment gas injectors passing through the enclosure (1) and muffle (4) to open out inside the heating chamber, a means (9) for generating a relative vacuum inside the enclosure, and a means (12) for adjusting the flow of treatment gas,
characterised in that, inside the heating chamber, the parts (5) are borne by a stand structure (6) which is passed through by an aspiration pipe connected to the means (9) enabling the relative vacuum to be generated, said pipe opening out at the level of said parts. - Utilisation of a furnace as claimed in claim 7, characterised in that the orifices of said aspiration (13) and/or gas treatment injection (11) pipe are positioned so as to obtain an axial flow of the treatment gas inside the heating chamber.
- Utilisation of a furnace as claimed in either claim 7 or claim 8, characterised in that the stand comprises plural grid elements arranged on top of one another and on which the parts are arranged loosely.
- Utilisation of a furnace as claimed in either claim 7 or claim 8, characterised in that said injection pipe and/or aspiration pipe are extended by an injection nozzle and/or an aspiration nozzle of suitable shape, notably for the performance of injection and/or aspiration inside the tubular part.
Applications Claiming Priority (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR9405062A FR2719057B1 (en) | 1994-04-22 | 1994-04-22 | Process for the nitriding at low pressure of a metallic part and oven for the implementation of said process. |
FR9405062 | 1994-04-22 | ||
FR9411483 | 1994-09-23 | ||
FR9411483A FR2725015B1 (en) | 1994-09-23 | 1994-09-23 | OVEN FOR USE IN LOW PRESSURE NITRURATION OF A METAL PART |
PCT/FR1995/000522 WO1995029269A1 (en) | 1994-04-22 | 1995-04-21 | Method of low pressure nitriding a metal workpiece and oven for carrying out said method |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0707661A1 EP0707661A1 (en) | 1996-04-24 |
EP0707661B1 true EP0707661B1 (en) | 2000-03-15 |
Family
ID=26231123
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP95918040A Expired - Lifetime EP0707661B1 (en) | 1994-04-22 | 1995-04-21 | Method of low pressure nitriding a metal workpiece and oven for carrying out said method |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0707661B1 (en) |
DE (2) | DE69515588T2 (en) |
TW (1) | TW279902B (en) |
WO (1) | WO1995029269A1 (en) |
Families Citing this family (6)
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DE19947381B4 (en) * | 1999-10-01 | 2011-06-22 | METAPLAS IONON Oberflächenveredelungstechnik GmbH, 51427 | Apparatus for the heat treatment of workpieces, in particular for gas nitriding, nitrocarburizing and oxidizing |
DE10118494C2 (en) * | 2001-04-04 | 2003-12-11 | Aichelin Gesmbh Moedling | Process for low pressure carbonitriding of steel parts |
DE102009002985A1 (en) * | 2009-05-11 | 2010-11-18 | Robert Bosch Gmbh | Process for carbonitriding |
GB2497354B (en) * | 2011-12-07 | 2014-09-24 | Solaris Holdings Ltd | Method of improvement of mechanical properties of products made of metals and alloys |
DE102014213510A1 (en) * | 2014-07-11 | 2016-02-18 | Robert Bosch Gmbh | Method for nitriding a component of a fuel injection system |
CN110747430B (en) * | 2019-10-25 | 2020-12-15 | 西南交通大学 | Low-pressure gas rapid nitriding method |
Family Cites Families (13)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB564841A (en) * | 1943-02-13 | 1944-10-16 | John Fallon | Improvements in furnaces heated by gaseous or liquid fuels |
DE1933439A1 (en) * | 1968-07-01 | 1970-01-15 | Gen Electric | Nitriding process for surface hardening stainless steels - without the use of activators |
FR2264891B1 (en) * | 1974-03-19 | 1977-10-07 | Michel Henri | |
SE7700229L (en) * | 1976-01-22 | 1977-07-23 | Western Electric Co | PROCEDURE FOR COATING THE SUBSTRATE BY PRECIPITATION FROM A PLASMA |
FI63783C (en) * | 1981-09-30 | 1983-08-10 | Kymin Oy Kymmene Ab | FOERFARANDE FOER NITRERING VID LAOGT TRYCK MED HJAELP AV GLIMURLADDNING |
FR2524006B1 (en) * | 1982-03-23 | 1985-10-11 | Air Liquide | PROCESS FOR THE SURFACE CURING OF METAL PARTS |
JPS60211061A (en) * | 1984-04-05 | 1985-10-23 | Toyota Central Res & Dev Lab Inc | Ion-nitrifying method of aluminum material |
DE3705710A1 (en) * | 1986-02-24 | 1987-08-27 | Ohara Co | METHOD FOR NITRATING THE SURFACE OF TITANIUM MOLDED PARTS, AND NITRATING TREATMENT DEVICE |
GB8608717D0 (en) * | 1986-04-10 | 1986-05-14 | Lucas Ind Plc | Metal components |
US4926793A (en) * | 1986-12-15 | 1990-05-22 | Shin-Etsu Handotai Co., Ltd. | Method of forming thin film and apparatus therefor |
US5039357A (en) * | 1990-06-15 | 1991-08-13 | Dynamic Metal Treating, Inc. | Method for nitriding and nitrocarburizing rifle barrels in a fluidized bed furnace |
FR2674618B1 (en) * | 1991-03-27 | 1998-03-13 | Etudes Const Mecaniques | NITRURATION PROCESS AND OVEN. |
DE4208848C2 (en) * | 1991-12-04 | 2001-08-30 | Ald Vacuum Techn Ag | Process for the thermochemical after-treatment of steels and metals |
-
1995
- 1995-04-21 WO PCT/FR1995/000522 patent/WO1995029269A1/en active IP Right Grant
- 1995-04-21 DE DE69515588T patent/DE69515588T2/en not_active Expired - Lifetime
- 1995-04-21 EP EP95918040A patent/EP0707661B1/en not_active Expired - Lifetime
- 1995-04-21 DE DE0707661T patent/DE707661T1/en active Pending
- 1995-05-23 TW TW84105155A patent/TW279902B/zh not_active IP Right Cessation
Also Published As
Publication number | Publication date |
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WO1995029269A1 (en) | 1995-11-02 |
EP0707661A1 (en) | 1996-04-24 |
DE69515588T2 (en) | 2000-09-07 |
DE707661T1 (en) | 1996-10-10 |
DE69515588D1 (en) | 2000-04-20 |
TW279902B (en) | 1996-07-01 |
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