EP0089885B1 - Method of surface-hardening metallic parts - Google Patents

Method of surface-hardening metallic parts Download PDF

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Publication number
EP0089885B1
EP0089885B1 EP83400541A EP83400541A EP0089885B1 EP 0089885 B1 EP0089885 B1 EP 0089885B1 EP 83400541 A EP83400541 A EP 83400541A EP 83400541 A EP83400541 A EP 83400541A EP 0089885 B1 EP0089885 B1 EP 0089885B1
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Prior art keywords
parts
gaseous mixture
percent
volume
ammonia
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EP83400541A
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German (de)
French (fr)
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EP0089885A3 (en
EP0089885A2 (en
Inventor
Michel Madsac
Thierry Hiron
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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Priority to AT83400541T priority Critical patent/ATE29154T1/en
Publication of EP0089885A2 publication Critical patent/EP0089885A2/en
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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
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
    • C23C8/30—Carbo-nitriding
    • C23C8/32—Carbo-nitriding of ferrous surfaces

Definitions

  • the invention relates to a method of surface hardening of metal parts, in particular steel parts, by nitriding or nitrocarburizing.
  • nitriding or nitrocarburizing treatments in the gas phase are intended to improve the resistance to fatigue and seizure and to increase the resistance to wear and to corrosion of steels.
  • the nitriding atmospheres used at present consist either of gaseous ammonia, or of gaseous ammonia diluted in nitrogen, or of gaseous ammonia, optionally diluted in nitrogen, in mixture with a compound capable of increasing the degree of dissociation of ammonia on contact with the part and therefore the kinetics of nitriding; this compound is, for example, an oxidizing compound such as oxygen, carbon dioxide, water vapor, or else a hydrocarbon.
  • a mixture of gaseous ammonia and a carbon-bearing gas is used, for example an endothermic gas (CO-H 2 -CO 2 - Hz-O-CH4-N2) or a hydrocarbon; there is then nitrocarburization with the formation of carbonitrides s.
  • an endothermic gas CO-H 2 -CO 2 - Hz-O-CH4-N2
  • a hydrocarbon hydrocarbon
  • FR-A-711 748 discloses a case hardening process using a gas based on ammonia and at least one of the nitrogen oxides or protoxides; the nitriding gas comprises for example 40% of ammonia and 60% of nitric oxide.
  • the subject of the invention is a method of surface hardening of metal parts, in particular of steel parts, by nitriding or nitrocarburizing which makes it possible to remedy the drawbacks mentioned above.
  • the process according to the invention consists in placing said parts in an oven and in keeping them, at a temperature between 490 ° C and 750 ° C, in an atmosphere formed by introduction into said oven of a gaseous mixture comprising in particular of ammonia and a compound accelerating the catalytic dissociation of ammonia in contact with said parts consisting of an oxidizing gas.
  • a gaseous mixture comprising in particular of ammonia and a compound accelerating the catalytic dissociation of ammonia in contact with said parts consisting of an oxidizing gas.
  • said oxidizing gas is nitrous oxide, said gaseous mixture containing approximately 0.1% to 10% by volume of said nitrous oxide.
  • said gaseous mixture can contain approximately 10% to 99% by volume of ammonia and at most 90% by volume of nitrogen.
  • said gaseous mixture also contains hydrogen in a concentration of at most 25% by volume.
  • said gaseous mixture also contains a carbon-bearing gas.
  • This carbon-bearing gas is constituted, for example, either not a hydrocarbon, such as methane or propane, in a concentration of at most 25% by volume, or by methanol in a concentration of at most 54% by volume , or by a mixture of these two compounds.
  • the treatment is carried out in an oven of the "well” type shown schematically in the attached figure.
  • This oven (1) is made of refractory material (2) coated internally with a steel jacket (3); it is fitted with heating resistors (4) and a cover (5).
  • the steel part to be treated represented in the form of a block (6), is placed inside the oven in a basket (7) which rests on a grid (8) supported by a base (9) placed on the bottom of the oven (1).
  • a turbine (10) the function of which is to continuously stir the atmosphere of the oven, is placed at a distance above the basket (7).
  • the constituents of the treatment gas mixture are fed continuously into the oven (1), either from a mixer (11), or separately, by one or more conduits (12), provided with one or more valves (13), passing through the cover (5).
  • This gaseous mixture is discharged from the oven, also continuously, through the conduit (14).
  • the treated parts are then cooled by quenching in an oil bath (not shown in the figure).
  • steel parts of grade 35CD4 are treated, and on the other hand, parts of nitriding steel of grade 40CAD612, with a gas mixture consisting of 40% NH 3 , 3% N 2 0 and 57% N 2 at a temperature of 570 ° C.
  • the oven (1) is previously heated to a temperature of 570 ° C.
  • the gaseous mixture (40% NH 3 - 3% N 2 O-57% N 2 ) from the mixer (11) is then brought into the oven (1) and it is circulated there for a certain time.
  • the part to be treated is then placed in the basket (7) which is placed in the oven (1).
  • the NH 3 - N 2 0 - N 2 gas mixture is then continuously circulated in the oven at a flow rate of 0.25 m 3 / hour.
  • the rate of renewal of the furnace atmosphere is 5 times per hour.
  • Steel parts of grade 35CD4 are treated with a gas mixture consisting of 40% NH 3 , 3% N 2 0, 10% H 2 and 37% N 2 , respectively for three hours and four hours, under conditions identical to those of example 1 (same temperature, same flow rate of the gas mixture, same rate of renewal of the furnace atmosphere).
  • steel parts of grade 35CD4 are treated, on the other hand, steel parts of grade 40CAD612, with a gas mixture consisting of 40% NH 3 , 3% N 2 0, 11% CH 3 0H and 46% N 2 , for two and a half hours, under conditions identical to those of Example 1 (flow temperature, renewal rate).
  • a first treatment was carried out for three hours with a gaseous mixture consisting of 40% NH 3 , 10% CO 2 and 50% N 2 , on the one hand on steel parts 35CD4, on the other hand on steel parts. 40CAD612 steel.
  • the thickness and hardness measurements of the nitrided layers of the parts thus treated gave the following results:
  • a third treatment was carried out for three hours with a gas mixture consisting of 40% NH 3 , 24% H 2 , 12% CO, 24% N 2 , 1.7% C0 2 and 2% H 2 0, of a part on steel parts 35CD4, on the other hand on steel parts 40CAD612.
  • the thickness and hardness measurements of the nitrided layers of the parts thus treated gave the following results:

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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)
  • Other Surface Treatments For Metallic Materials (AREA)
  • Heat Treatment Of Articles (AREA)
  • Manufacture Of Alloys Or Alloy Compounds (AREA)
  • Forging (AREA)

Abstract

In a surface hardening process for metal parts by nitriding or carbonitriding, the parts to be treated are placed in a furnace and maintained at a temperature of between 490 DEG C. and 750 DEG C., in a nitrogen bearing atmosphere, such atmosphere formed by introducing into the furnace a gaseous mixture comprising in particular ammonia and an oxidizing gas for accelerating the catalytic dissociation of ammonia in contact with said parts. The oxidizing gas is nitrous oxide and the gaseous mixture contains about 0.1% to 10% by volume of nitrous oxide. The invention is well suited for the surface hardening of steel parts.

Description

L'invention concerne un procédé de durcissement superficiel de pièces métalliques, notamment de pièces en acier, par nitruration ou nitrocarburation.The invention relates to a method of surface hardening of metal parts, in particular steel parts, by nitriding or nitrocarburizing.

Comme on le sait, les traitements de nitruration ou de nitrocarburation en phase gazeuse sont destinés à améliorer la tenue à la fatigue et au grippage et à augmenter la résistance à l'usure et à la corrosion des aciers.As is known, nitriding or nitrocarburizing treatments in the gas phase are intended to improve the resistance to fatigue and seizure and to increase the resistance to wear and to corrosion of steels.

Ces traitements consistent à maintenir les pièces en acier dans une atmosphère contenant 50% à 100% d'ammoniac gazeux à une température de 490°C à 750°C. Les pièces ainsi traitées sont ensuite refroidies par trempe en phase gazeuse ou liquide. Aux températures de traitement utilisées, l'ammoniac gazeux se décompose partiellement au contact des pièces en acier et l'azote naissant ainsi créé entre en solution solide et diffuse sur une certaine profondeur dans l'acier. Il se forme ainsi, à la surface de la pièce, une couche nitrurée qui est constituée d'une couche dite «couche de combinaison» ou «couche blanche» supportée par une couche dite «couche de diffusion». La structure et la composition de ces couches est la suivante:

  • - la couche blanche, située à l'extrême surface de la pièce, est constituée d'un mélange de nitrures s (Fe2N - Fe3N) et de nitrures y' (Fe4N).
  • - la couche de diffusion, sous-jacente à la couche blanche, est moins dure et plus épaisse que cette dernière; dans cette couche de diffusion, l'azote est en solution solide d'insertion et forme des nitrures par combinaison avec certains des éléments constituant l'acier.
These treatments consist in maintaining the steel parts in an atmosphere containing 50% to 100% of gaseous ammonia at a temperature of 490 ° C to 750 ° C. The parts thus treated are then cooled by quenching in the gas or liquid phase. At the treatment temperatures used, the gaseous ammonia partially decomposes on contact with the steel parts and the nascent nitrogen thus created enters into solid solution and diffuses over a certain depth in the steel. There is thus formed, on the surface of the part, a nitrided layer which consists of a layer called "combination layer" or "white layer" supported by a layer called "diffusion layer". The structure and composition of these layers is as follows:
  • - the white layer, located at the extreme surface of the part, consists of a mixture of nitrides s (Fe 2 N - Fe 3 N) and nitrides y '(Fe 4 N).
  • - the diffusion layer, underlying the white layer, is less hard and thicker than the latter; in this diffusion layer, the nitrogen is in solid insertion solution and forms nitrides by combination with some of the elements constituting the steel.

Les atmosphères de nitruration utilisées à l'heure actuelle sont constituées, soit d'ammoniac gazeux, soit d'ammoniac gazeux dilué dans de l'azote, soit d'ammoniac gazeux, le cas échéant dilué dans de l'azote, en mélange avec un composé susceptible d'augmenter le degré de dissociation de l'ammoniac au contact de la pièce et donc la cinétique de nitruration; ce composé est, par exemple, un composé oxydant tel que l'oxygène, le gaz carbonique, la vapeur d'eau, ou bien un hydrocarbure. Selon certains procédés, on utilise un mélange d'ammoniac gazeux et d'un gaz porteur de carbone, par exemple un gaz endothermique (CO-H2-CO2- Hz-O-CH4-N2) ou un hydrocarbure; il y a alors nitrocarburation avec formation de carbonitrures s. Dans tous les cas, selon les procédés utilisés jusqu'à présent, il est préférable d'avoir une concentration initiale d'ammoniac dans l'atmosphère utilisée d'au moins 50%.The nitriding atmospheres used at present consist either of gaseous ammonia, or of gaseous ammonia diluted in nitrogen, or of gaseous ammonia, optionally diluted in nitrogen, in mixture with a compound capable of increasing the degree of dissociation of ammonia on contact with the part and therefore the kinetics of nitriding; this compound is, for example, an oxidizing compound such as oxygen, carbon dioxide, water vapor, or else a hydrocarbon. According to certain methods, a mixture of gaseous ammonia and a carbon-bearing gas is used, for example an endothermic gas (CO-H 2 -CO 2 - Hz-O-CH4-N2) or a hydrocarbon; there is then nitrocarburization with the formation of carbonitrides s. In all cases, according to the methods used until now, it is preferable to have an initial concentration of ammonia in the atmosphere used of at least 50%.

Parmi les procédés de nitruration utilisant de l'ammoniac en mélange avec un composé oxydant, mis en oeuvre jusqu'à présent, on peut citer plus particulièrement le procédé décrit dans le brevet anglais n° 2 049 740. Ce procédé consiste à maintenir des pièces en acier, à une température de 550°C à 600°C, dans une atmosphère gazeuse constituée d'au moins 50% en volume d'ammoniac, jusqu'à 10% en volume de gaz oxydant et de gaz inerte (par exemple 70% de NH3, 7% de C02 et 23% de N2), pendant une durée de 12 à 20 heures.Among the nitriding processes using ammonia in admixture with an oxidizing compound, used up to now, there may be mentioned more particularly the process described in English Patent No. 2,049,740. This process consists in maintaining parts steel, at a temperature of 550 ° C to 600 ° C, in a gas atmosphere consisting of at least 50% by volume of ammonia, up to 10% by volume of oxidizing gas and inert gas (for example 70 % NH 3 , 7% C0 2 and 23% N 2 ), for a period of 12 to 20 hours.

Il est connu de FR-A-711 748 un procédé de cémentation avec utilisation d'un gaz à base d'ammoniac et d'au moins un des oxydes ou protoxydes de l'azote; le gaz nitrurant comprend par exemple 40% d'ammoniac et 60% d'oxyde nitrique.FR-A-711 748 discloses a case hardening process using a gas based on ammonia and at least one of the nitrogen oxides or protoxides; the nitriding gas comprises for example 40% of ammonia and 60% of nitric oxide.

Tous ces procédés permettent d'obtenir un durcissement superficiel des pièces traitées mais présentent toutefois certains inconvénients relatifs à la couche blanche. En effet:

  • - la couche blanche se développe de façon hétérogène et présente donc une épaisseur inégale sur toute la surface de la pièce.
  • - la couche blanche est fragile et peu adhérente du fait qu'elle est constituée d'un mélange intime des deux phases s (Fe2N - Fe3N) et y' (Fe4N).
  • - la couche blanche se présente sous la forme d'une zone compacte recouverte d'une zone poreuse, ce qui dans certains cas peut présenter quelques inconvénients. Or, à partir d'une certaine durée du traitement, la zone compacte atteint une épaisseur maximale limite (20 ¡.Lm) et, seule, la zone poreuse se développe.
  • - le temps d'incubation nécessaire à la nucléation des nitrures est assez long et la formation de la couche blanche exige donc un certain temps.
All these methods make it possible to obtain a surface hardening of the treated parts but nevertheless have certain drawbacks relating to the white layer. Indeed:
  • - the white layer develops heterogeneously and therefore has an uneven thickness over the entire surface of the part.
  • - The white layer is fragile and not very adherent because it consists of an intimate mixture of the two phases s (Fe 2 N - Fe 3 N) and y '(Fe 4 N).
  • - The white layer is in the form of a compact area covered with a porous area, which in some cases can have some drawbacks. However, from a certain duration of the treatment, the compact zone reaches a maximum limit thickness (20 μm) and, alone, the porous zone develops.
  • - The incubation time necessary for the nucleation of nitrides is quite long and the formation of the white layer therefore requires a certain time.

L'invention a pour objet un procédé de durcissement superficiel de pièces métalliques, notamment de pièces en acier, par nitruration ou nitrocarburation qui permet de remédier aux inconvénients rappelés ci-dessus.The subject of the invention is a method of surface hardening of metal parts, in particular of steel parts, by nitriding or nitrocarburizing which makes it possible to remedy the drawbacks mentioned above.

Le procédé conforme à l'invention consiste à placer lesdites pièces dans un four et à les maintenir, à une température comprise entre 490°C et 750°C, dans une atmosphère formée par introduction dans ledit four d'un mélange gazeux comprenant notamment de l'ammoniac et un composé accélérateur de la dissociation catalytique de l'ammoniac au contact desdits pièces constitué par un gaz oxydant. Ce procédé se caractérise en ce que ledit gaz oxydant est du protoxyde d'azote, ledit mélange gazeux contenant environ 0,1% à 10% en volume dudit protoxyde d'azote.The process according to the invention consists in placing said parts in an oven and in keeping them, at a temperature between 490 ° C and 750 ° C, in an atmosphere formed by introduction into said oven of a gaseous mixture comprising in particular of ammonia and a compound accelerating the catalytic dissociation of ammonia in contact with said parts consisting of an oxidizing gas. This process is characterized in that said oxidizing gas is nitrous oxide, said gaseous mixture containing approximately 0.1% to 10% by volume of said nitrous oxide.

Selon une caractéristique de l'invention, ledit mélange gazeux peut contenir environ 10% à 99% en volume d'ammoniac et au plus 90% en volume d'azote.According to a characteristic of the invention, said gaseous mixture can contain approximately 10% to 99% by volume of ammonia and at most 90% by volume of nitrogen.

Selon une variante de l'invention, ledit mélange gazeux contient en outre de l'hydrogène dans une concentration d'au plus 25% en volume.According to a variant of the invention, said gaseous mixture also contains hydrogen in a concentration of at most 25% by volume.

Selon une autre caractéristique de l'invention, dans le cas d'une nitrocarburation, ledit mélange gazeux contient en outre un gaz porteur de carbone. Ce gaz porteur de carbone est constitué par exemple, soit pas un hydrocarbure, tel que du méthane ou du propane, dans une concentration d'au plus 25% en volume, soit par du méthanol dans une concentration d'au plus 54% en volume, soit par un mélange de ces deux composés.According to another characteristic of the invention, in the case of nitrocarburization, said gaseous mixture also contains a carbon-bearing gas. This carbon-bearing gas is constituted, for example, either not a hydrocarbon, such as methane or propane, in a concentration of at most 25% by volume, or by methanol in a concentration of at most 54% by volume , or by a mixture of these two compounds.

A partir de 400°C, le protoxyde d'azote N20 se décompose catalytiquement facilement, libérant ainsi de l'oxygène sous forme atomique. Cet oxygène actif favorise la dissociation de l'ammoniac au contact de la pièce à traiter et permet donc une nitruration rapide et efficace de ladite pièce. Ainsi, l'utilisation d'un mélange gazeux contenant du protoxyde d'azote conforme à l'invention permet d'obtenir, du point de vue de la nitruration, des résultats nettement améliorés par rapport à l'utilisation de composés oxydants classiques tel que O2, CO2 ou H20. En effet:

  • - la couche blanche formée présente une épaisseur constante sur toute la surface de la pièce à traiter.
  • - la couche blanche n'est en général constituée que de la phase s (Fe2N - Fe3N), et est donc plus résistante. Si, parfois, pour certaines durées de traitement et teneurs en ammoniac de l'atmosphère utilisée, la phase y' (Fe,N) apparaît, cette dernière ne se mélange pas à la phase s mais se présente comme une zone mince prise en sandwich entre la zone s et la couche de diffusion; dans ces conditions, la présence de la phase y' n'affecte pas la résistance de la couche nitrurée.
  • - la zone poreuse de la couche blanche est extrêmement mince, et peut même être inexistante dans le cas de traitements de faible durée.
  • - du fait que les cinétiques de réaction sont augmentées par rapport à celles des traitements classiques, le temps d'incubation nécessaire pour la nucléation des nitrures est plus court; ceci permet, si on le désire, d'une part de pouvoir diminuer la concentration initiale du mélange gazeux en ammoniac jusqu'à 10%, d'autre part de raccourcir la durée du traitement, et également de diminuer le débit et le taux de renouvellement de l'atmosphère dans le four de traitement.
From 400 ° C, nitrous oxide N 2 0 decomposes catalytically easily, thereby releasing oxygen in atomic form. This active oxygen promotes the dissociation of ammonia on contact with the part to be treated and therefore allows rapid and effective nitriding of said part. Thus, the use of a gaseous mixture containing nitrous oxide according to the invention makes it possible to obtain, from the point of view of nitriding, clearly improved results compared to the use of conventional oxidizing compounds such as O2, CO 2 or H 2 0. Indeed:
  • - The white layer formed has a constant thickness over the entire surface of the part to be treated.
  • - The white layer is generally only made up of the s phase (Fe 2 N - Fe 3 N), and is therefore more resistant. If, sometimes, for certain treatment times and ammonia contents of the atmosphere used, the phase y '(Fe, N) appears, the latter does not mix with phase s but appears as a thin zone sandwiched between zone s and the diffusion layer; under these conditions, the presence of the phase y 'does not affect the resistance of the nitrided layer.
  • - the porous zone of the white layer is extremely thin, and may even be nonexistent in the case of treatments of short duration.
  • - the fact that the reaction kinetics are increased compared to those of conventional treatments, the incubation time necessary for the nucleation of nitrides is shorter; this allows, if desired, on the one hand to be able to decrease the initial concentration of the gaseous mixture of ammonia up to 10%, on the other hand to shorten the duration of the treatment, and also to decrease the flow rate and the rate of renewal of the atmosphere in the treatment furnace.

Les caractéristiques est avantages de l'invention apparaîtront dans la description qui suit d'exemples de réalisation, donnés à titre non limitatif, du procédé de l'invention.The characteristics and advantages of the invention will appear in the following description of exemplary embodiments, given without limitation, of the process of the invention.

Dans tous les exemples décrits ci-après, le traitement est effectué dans un four du type «puits» représenté schématiquement sur la figure jointe. Ce four (1) est en matériau réfractaire (2) revêtu intérieurement d'un chemisage en acier (3); il est muni de résistances chauffantes (4) et d'un couvercle (5). La pièce en acier à traiter, représentée sous la forme d'un bloc (6), est placée à l'intérieur du four dans un panier (7) qui repose sur une grille (8) suppportée par un socle (9) placé sur le fond du four (1). Une turbine (10), dont la fonction est de brasser en permanence l'atmosphère du four, est placée à distance au-dessus du panier (7). Les constituants du mélange gazeux de traitement sont amenés en continu dans le four (1), soit d'un mélangeur (11), soit séparément , par un ou des conduits (12), munis d'une ou de vannes (13), traversant le couvercle (5). Ce mélange gazeux est évacué du four, également en continu, par le conduit (14). Les pièces traitées sont ensuite refroidies par trempe dans un bain d'huile (non représenté sur la figure).In all the examples described below, the treatment is carried out in an oven of the "well" type shown schematically in the attached figure. This oven (1) is made of refractory material (2) coated internally with a steel jacket (3); it is fitted with heating resistors (4) and a cover (5). The steel part to be treated, represented in the form of a block (6), is placed inside the oven in a basket (7) which rests on a grid (8) supported by a base (9) placed on the bottom of the oven (1). A turbine (10), the function of which is to continuously stir the atmosphere of the oven, is placed at a distance above the basket (7). The constituents of the treatment gas mixture are fed continuously into the oven (1), either from a mixer (11), or separately, by one or more conduits (12), provided with one or more valves (13), passing through the cover (5). This gaseous mixture is discharged from the oven, also continuously, through the conduit (14). The treated parts are then cooled by quenching in an oil bath (not shown in the figure).

Le même traitement a également été effectué dans un four du type «batch» avec un bain d'huile incorporé, comportant un sas entrée-sortie d'amenée des pièces dans l'enceinte de traitement, sas muni d'un bac de trempe à l'huile.The same treatment was also carried out in an oven of the “batch” type with an incorporated oil bath, comprising an entry-exit airlock for bringing the parts into the treatment enclosure, airlock provided with a quenching tank at oil.

Bien entendu, on effectue une purge préalable du four par de l'azote gazeux avant chaque changement d'atmosphère.Of course, a preliminary purge of the oven is carried out with nitrogen gas before each change of atmosphere.

Exemple 1.Example 1.

On traite, d'une part des pièces en acier de nuance 35CD4, et d'autre part, des pièces en acier de nitruration de nuance 40CAD612, avec un mélange gazeux constitué de 40% NH3, 3% N20 et 57% N2 à une température de 570°C.On the one hand, steel parts of grade 35CD4 are treated, and on the other hand, parts of nitriding steel of grade 40CAD612, with a gas mixture consisting of 40% NH 3 , 3% N 2 0 and 57% N 2 at a temperature of 570 ° C.

De façon plus précise, on chauffe préalablement le four (1) à une température de 570°C. On amène alors le mélange gazeux (40% NH3 - 3% N2O-57% N2), provenant du mélangeur (11) dans le four (1) et on l'y fait circuler un certain temps. On place ensuite la pièce à traiter dans le panier (7) que l'on met dans le four (1). On fait alors circuler en continu le mélange gazeux NH3 - N20 - N2 dans le four à un débit de 0,25 m3/heure. Le taux de renouvellement de l'atmosphère du four est de 5 fois par heure.More specifically, the oven (1) is previously heated to a temperature of 570 ° C. The gaseous mixture (40% NH 3 - 3% N 2 O-57% N 2 ) from the mixer (11) is then brought into the oven (1) and it is circulated there for a certain time. The part to be treated is then placed in the basket (7) which is placed in the oven (1). The NH 3 - N 2 0 - N 2 gas mixture is then continuously circulated in the oven at a flow rate of 0.25 m 3 / hour. The rate of renewal of the furnace atmosphere is 5 times per hour.

On effectue ce traitement:

  • - d'une part, sur des pièces d'acier 35CD4, respectivement pendant deux heures, trois heures et quatre heures,
  • -d'autre part, sur des pièces d'acier 40CAD612, respectivement pendant deux heures, trois heures et quatre heures.
We perform this treatment:
  • - on the one hand, on pieces of steel 35CD4, respectively for two hours, three hours and four hours,
  • on the other hand, on pieces of steel 40CAD612, respectively for two hours, three hours and four hours.

Après trempe des pièces ainsi traitées dans un bain d'huile, on effectue les mesures d'épaisseur et de dureté des couches nitrurées. Les résultats obtenus sont donnés dans le tableau ci-après:

Figure imgb0001
After soaking the parts thus treated in an oil bath, the thickness and hardness measurements of the nitrided layers are carried out. The results obtained are given in the table below:
Figure imgb0001

Exemple 2.Example 2.

On traite des pièces en acier de nuance 35CD4 avec un mélange gazeux constitué de 40% NH3, 3% N20, 10% H2 et 37% N2, respectivement pendant trois heures et quatre heures, dans des conditions identiques à celles de l'exemple 1 (même température, même débit du mélange gazeux, même taux de renouvellement de l'atmosphère du four).Steel parts of grade 35CD4 are treated with a gas mixture consisting of 40% NH 3 , 3% N 2 0, 10% H 2 and 37% N 2 , respectively for three hours and four hours, under conditions identical to those of example 1 (same temperature, same flow rate of the gas mixture, same rate of renewal of the furnace atmosphere).

Après trempe des pièces ainsi traitées dans un bain d'huile, on effectue les mesures d'épaisseur et de dureté des couches nitrurées. Les résultats obtenus sont donnés dans le tableau ci-après:

Figure imgb0002
After soaking the parts thus treated in an oil bath, the thickness and hardness measurements of the nitrided layers are carried out. The results obtained are given in the table below:
Figure imgb0002

Exemple 3.Example 3.

On traite des pièces en acier de nuance 35CD4 avec un mélange gazeux constitué de 40% NH3, 3% N20, 5% C3H8 et 52% N2, pendant quatre heures, dans des conditions identiques à celles de l'exemple 1 (température, débit, taux de renouvellement).Steel parts of grade 35CD4 are treated with a gas mixture consisting of 40% NH 3 , 3% N 2 0, 5% C 3 H 8 and 52% N 2 , for four hours, under conditions identical to those of l 'example 1 (temperature, flow rate, renewal rate).

Après trempe des pièces ainsi traitées dans un bain d'huile, on effectue les mesures d'épaisseur et de dureté des couches nitrocarburées. Les résultats obtenus sont donnés dans le tableau ci-après:

Figure imgb0003
After quenching of the parts thus treated in an oil bath, the thickness and hardness measurements of the nitrocarburized layers are carried out. The results obtained are given in the table below:
Figure imgb0003

Exemple 4.Example 4.

On traite, d'une part des pièces en acier de nuance 35CD4, d'autre part, des pièces en acier de nuance 40CAD612, avec un mélange gazeux constitué de 40% NH3, 3% N20, 11% CH30H et 46% N2, pendant deux heures et demie, dans des conditions identiques à celles de l'exemple 1 (température débit, taux de renouvellement).On the one hand, steel parts of grade 35CD4 are treated, on the other hand, steel parts of grade 40CAD612, with a gas mixture consisting of 40% NH 3 , 3% N 2 0, 11% CH 3 0H and 46% N 2 , for two and a half hours, under conditions identical to those of Example 1 (flow temperature, renewal rate).

Après trempe des pièces ainsi traitées dans un bain d'huile, on effectue les mesures d'épaisseur et de dureté des couches nitrocarburées. Les résultats obtenus sont donnés dans le tableau ci-après:

Figure imgb0004
After quenching of the parts thus treated in an oil bath, the thickness and hardness measurements of the nitrocarburized layers are carried out. The results obtained are given in the table below:
Figure imgb0004

A titre de comparaison, on a effectué trois traitements de nitruration et de nitrocarburation, à l'aide d'atmosphères gazeuses classiques dans lesquelles le composé oxydant est du gaz carbonique, sur des pièces en acier identiques et dans les mêmes conditions que celles des exemples de l'invention décrits ci-dessus (température: 570°C; débit: 0,25 m3/heure; taux de renouvellement de l'atmosphère du four: 5 fois par heure).By way of comparison, three nitriding and nitrocarburizing treatments were carried out, using conventional gaseous atmospheres in which the oxidizing compound is carbon dioxide, on identical steel parts and under the same conditions as those of the examples. of the invention described above (temperature: 570 ° C; flow rate: 0.25 m 3 / hour; rate of renewal of the furnace atmosphere: 5 times per hour).

Traitement I. (comparaison nitruration)Treatment I. (nitriding comparison)

Un premier traitement a été effectué pendant trois heures avec un mélange gazeux constitué de 40% NH3, 10% CO2 et 50% N2, d'une part sur des pièces d'acier 35CD4, d'autre part sur des pièces d'acier 40CAD612. Les mesures d'épaisseur et de dureté des couches nitrurées des pièces ainsi traitées ont donné les résultats suivants:

Figure imgb0005
A first treatment was carried out for three hours with a gaseous mixture consisting of 40% NH 3 , 10% CO 2 and 50% N 2 , on the one hand on steel parts 35CD4, on the other hand on steel parts. 40CAD612 steel. The thickness and hardness measurements of the nitrided layers of the parts thus treated gave the following results:
Figure imgb0005

Traitement II. (comparaison nitrocarburation)Treatment II. (nitrocarburization comparison)

Un deuxième traitement a été effectué pendant quatre heures avec un mélange gazeux constitué de 50% NH3, 5% CO2, 15% CH4 et 30% N2 sur des pièces d'acier 35CD4. Les mesures d'épaisseur et de dureté des couches nitrurées des pièces ainsi traitées ont donné les résultats suivants:

Figure imgb0006
A second treatment was carried out for four hours with a gas mixture consisting of 50% NH 3 , 5% CO 2 , 15% CH 4 and 30% N 2 on steel parts 35CD4. The thickness and hardness measurements of the nitrided layers of the parts thus treated gave the following results:
Figure imgb0006

Traitement III. (comparaison nitrocarburation)Treatment III. (nitrocarburization comparison)

Un troisième traitement a été effectué pendant trois heures avec un mélange gazeux constitué de 40% NH3, 24% H2, 12% CO, 24% N2, 1,7% C02 et 2% H20, d'une part sur des pièces d'acier 35CD4, d'autre part sur des pièces d'acier 40CAD612. Les mesures d'épaisseur et de dureté des couches nitrurées des pièces ainsi traitées ont donné les résultats suivants:

Figure imgb0007
A third treatment was carried out for three hours with a gas mixture consisting of 40% NH 3 , 24% H 2 , 12% CO, 24% N 2 , 1.7% C0 2 and 2% H 2 0, of a part on steel parts 35CD4, on the other hand on steel parts 40CAD612. The thickness and hardness measurements of the nitrided layers of the parts thus treated gave the following results:
Figure imgb0007

Comme on le voit, grâce au procédé de l'invention, on obtient des couches nitrurées plus épaisses et une couche blanche beaucoup plus dure que par les traitements classiques.As can be seen, thanks to the process of the invention, thicker nitrided layers and a much harder white layer are obtained than by conventional treatments.

Claims (9)

1. Method of surface-hardening metal parts, especially steel parts, by nitridation according to which one places the said parts in a furnace and maintains them at a temperature of from 490°C to 750°C in an atmosphere formed by introduction of a gaseous mixture comprising especially ammo-- nia and a compound accelerating the catalytic dissociation of the ammonia in contact with the said parts and being an oxidizing gas into said furnace, characterized in that the said oxidizing gas is nitrogen monoxide, the gaseous mixture containing about 0.1 percent to 10 percent by volume of said nitrogen monoxide.
2. Method according to claim 1, characterized in that the said gaseous mixture contains about 10 percent to 99 percent by volume of ammonia.
3. Method according to one of the claims 1 or 2, characterized in that the said gaseous mixture contains at most 90 percent by volume of nitrogen.
4. Method according to one of the claims 1 to 3, characterized in that the said gaseous mixture contains at most 25 percent by volume of hydrogen.
5. Method according to one of the claims 1 to 4, characterized in that in the case of a nitrocarburation said gaseous mixture contains additionally a carbon carrier gas.
6. Method according to claim 5, characterized in that the said gaseous mixture contains at most 25 percent by volume of a hydrocarbon such as methane or propane.
7. Method according to one of the claims 5 or 6, characterized in that the said gaseous mixture contains at most 54 percent by volume of methanol.
8. Method according to one of the claims 1 to 7, characterized in that one renews the atmosphere of the furnace at least 2 to 10 times per hour.
9. Method according to one of the claims 1 to 8, characterized in that one maintains the said parts in said atmosphere during at least one hour.
EP83400541A 1982-03-23 1983-03-16 Method of surface-hardening metallic parts Expired EP0089885B1 (en)

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DE3373197D1 (en) 1987-10-01
CA1215901A (en) 1986-12-30
ATE29154T1 (en) 1987-09-15
AU1253583A (en) 1983-09-29
EP0089885A2 (en) 1983-09-28
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FR2524006A1 (en) 1983-09-30
US4531984A (en) 1985-07-30
ES520824A0 (en) 1984-01-16
ZA831884B (en) 1983-11-30

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