EP0010484A1 - Improvement in the chromising of steel in the gaseous phase - Google Patents

Improvement in the chromising of steel in the gaseous phase Download PDF

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Publication number
EP0010484A1
EP0010484A1 EP79400724A EP79400724A EP0010484A1 EP 0010484 A1 EP0010484 A1 EP 0010484A1 EP 79400724 A EP79400724 A EP 79400724A EP 79400724 A EP79400724 A EP 79400724A EP 0010484 A1 EP0010484 A1 EP 0010484A1
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chromium
steels
treatments
nitrogen
chromizing
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EP0010484B1 (en
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Robert Leveque
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Creusot Loire SA
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Creusot Loire SA
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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
    • C23C12/00—Solid state diffusion of at least one non-metal element other than silicon and at least one metal element or silicon into metallic material surfaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering

Definitions

  • the present invention relates to an improvement in chromium-plating by gas of steels with more than 0.2% carbon, more especially structural steels and tool steels; this improvement makes it possible to significantly increase the thickness of the chromized layer, as well as its toughness.
  • chromization of steels by gas is well known.
  • the formation of chromium-based diffusion alloys on the surface of steels has already been described in numerous invention patents.
  • the transport of chromium to the surface of the material to be treated is carried out by means of halides, which are the only chromium compounds in the vapor state at diffusion temperatures.
  • halides which are the only chromium compounds in the vapor state at diffusion temperatures.
  • the passage of chromium in solid solution in the metal takes place by exchange between the chromium halide and the iron according to a reaction which, in the case of chlorides, can be written:
  • the reaction must be carried out at high temperature and in the austenitic range, that is to say above 850 ° C. for the usual steels.
  • the surface reaction of carbon and chromium causes on the one hand the formation of a film of chromium carbides, on the other hand a diffusion of carbon towards the area.
  • the surface film consists of two types of carbides, M 23 C 6 , richer in chromium towards the surface and M 7 C 3 , poorer in chromium, towards the metal substrate.
  • M indicates a metal such as iron (Fe), chromium (Cr), nickel (Ni), etc ...
  • the surface film has a thickness between 12 and 18 microns, and a hardness level between 1200 and 1800 in the Vickers hardness scale. Chromium thus diffuses into steel over a depth generally close to 15 microns. In known methods, this pro. The chromization factor never exceeds 20 microns.
  • a first solution for increasing the thickness and the toughness of the carbide layers can be constituted by an initial treatment phase consisting of a rapid rise in temperature, in particular in the range 600-900 ° C where usually precipitate M 7 C 3 carbides in the ferritic structure.
  • an initial treatment phase consisting of a rapid rise in temperature, in particular in the range 600-900 ° C where usually precipitate M 7 C 3 carbides in the ferritic structure.
  • such a solution would present risks with regard to the initiation of cracks and pitting during heating, in particular for parts made of relatively alloyed steels having geometries which are ill-suited to the high thermal stresses which would result from such a heating cycle.
  • the present invention while applying the principle of chromizing the part in the austenitic phase, avoids the risks mentioned above by producing beforehand a nitrided layer, but without a combination layer, that is to say in such a way that it there is in no way the formation of a surface layer of iron and chromium nitrides. Indeed, a the surface layer of iron nitrides would remain relatively stable, even at high temperatures, and would constitute a real barrier opposing the diffusion of chromium, this barrier being reinforced by the formation of chromium nitrides linked to an additional supply of nitrogen.
  • the subject of the present invention is an improvement to the chromisation processes, constituted by a process for chromisation of steels over a depth e greater than 40 microns, usable for steels having a carbon content at least equal to 0.2 %, in particular for structural steels and for tool steels, characterized by the combination of three successive treatments, the first of these three treatments consisting of ionic nitriding of a surface layer with a thickness between 100 and 350 microns, this ionic nitriding being carried out in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature between 450 ° C and 650 ° C, for a period between 5 and 40 hours, so as to obtain between 1, 5% and 2.5% nitrogen in the nitrided layer, the second of these treatments consisting of chromium-forming chromium carbide gas formation, lasting between 5 and 30 hours, and reac linked to temperatures between 850 ° C.
  • the third of these three treatments being a heat treatment comprising quenching with oil of the chromized part followed by tempering at a temperature between 600 ° C. and 650 ° C, lasting between 30 minutes and 10 hours depending on the size of the part treated.
  • ionic nitriding forming the first of the three treatments and effecaée under nitrogen and hydrogen atmosphere is carried out under a partial nitrogen pressure at most equal to 1.5 millibar, and under a pressure total gaseous between 2 and 10 millibars.
  • the chromization forming the second of the three treatments uses a pulverulent mixture based on ferro-chromium and amcnonium chloride, the latter representing by weight only 0.4% to 1 1 or powdered mixture, the ferro-ehrome powder preferably having a chromium content of between 50% and 75% and a particle size of between 0.5 millimeter and 4 millimeters, without aluminous binder ml magnesium.
  • thermochemical treatments of metallic surfaces by ion bombardments and more particularly nitriding are based on properties of the electric discharge in rarefied gases, in the event of mixtures of nitrogen and hydrogen with possibly hydrocarbons.
  • the reactive gas atmosphere can be chosen independently of the need for its pyrolitic cracking since its activation is obtained by ionization. It is therefore possible to adjust the partial pressure of nitrogen so that the phase or phases provided for by the binary iron-nitrogen balance diagram are superficially formed.
  • one of the main advantages of the invention consists, thanks to ionic nitriding, in obtaining a nitrided layer without a combination layer, that is to say without iron and chromium nitrides, in a reliable and repetitive manner by adjusting the partial pressure of nitrogen as a function of the treatment temperature and the chemical composition of the steel . So, without risk of spalling, the surface steel can quickly pass into the austenitic phase at moderate temperature, because of a nitrogen content of the order of 1.5% to 2.5%.
  • gas chromization can be carried out. at a greater depth, for example up to 50 microns and even more, and with the formation on the surface of a single type of carbonitrides, in Cr 2 (C, N), which causes a significant increase in the toughness of the coating.
  • a chromium-molybdenum-vanadium steel of the type is treated 35CDV 12, therefore at 0.35% carbon, in order to obtain a chromization depth of 50 microns.
  • the ionic nitriding which constitutes the first of the three successive treatments according to the invention is carried out here in a metal enclosure provided with heat shields and cooled by circulation of water, which enclosure constitutes the anode connected to earth.
  • the electrical parameters are chosen in such a way that the current increases with the direct voltage produced by the generator and that the sample to be nitrided which constitutes the cathode is covered by the corona corresponding to the abnormal discharge regime.
  • the gaseous ions are formed and accelerated towards the sample and cause it to heat up, which is continued until the temperature chosen to carry out the rmochemical treatment.
  • the temperature regulation is obtained using a thermocouple protected by an alumina sheath and placed in the sample in. conditions which avoid arcing.
  • the pressure at which the thermochemical treatment is carried out is generally between 2.5 and 8.0 millibar; a primary pump is sufficient to create the initial vacuum and then to allow the renewal of the nitriding gas near the sample.
  • the nitriding gas mixture is composed of nitrogen and hydrogen.
  • the partial pressures of nitrogen P N for which a solid solution of nitrogen is obtained in the ferrite network are between 0.1 and 0.5 millibar.
  • the temperature is set on average at 520 ° C and does not deviate from the range between 510 and 530 ° C. Leaving aside the rise in temperature and the setting the atmosphere to low pressure, the duration of ion nitriding at the right pressure and at the right temperature is 25 hours.
  • the metal piece of CDV 12 steel 35 thus nitrided is then extracted from the ion nitriding furnace and introduced into a case-hardening box which will carry out the second treatment according to the invention, which is gas chromization.
  • the cementing agent used is a powder made up of 99.5% ferro-chromium with 60/70% chromium and for 0.5% ammonium chloride, without alumina or magnesia. This powder has a particle size between 0.5 and 4 mm, with an average size close to 2.7 mm. This powder is placed in the bottom of the case-hardening box, which has the shape of a vertical cylinder, and it is covered by a partition on which the piece of steel to be chromized is placed. At the top of the case-hardening box there is a ferro-chromium reserve in a basket used for the direct regeneration of the active vapor of chromium chloride CrCl 2 . Introduced hydrogen creates a reducing atmosphere.
  • the enclosure is brought to an average temperature of 950 ° C, not deviating from the range 920 ° C-980 ° C, for a period of 20 hours.
  • the ferrous chloride vapors from reaction (1) react on the chromium reserve placed at the top of the body, which regenerates gaseous chromium chloride CrCl 2 which participates in the chromization according to (1).
  • the chromized part undergoes the third treatment according to the invention, that is to say that it is extracted from the case hardening box, it is immediately soaked in oil , then it is introduced into a tempering oven maintained at a temperature of the order of 625 ° C., for 2 hours.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Treatment Of Steel In Its Molten State (AREA)

Abstract

The object of the present invention is an improvement in chromizing methods, constituted by a method of chromizing steels to a depth e greater than 30 microns, usable for steels with a carbon content of at least 0.2%, especially for steels for construction work and steels for tools, characterized by the combination of three successive treatments, the first of these three treatments consisting of an ionic nitriding of a surface layer between 100 and 350 microns thick, this ionic nitriding being realized in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature of between 450 DEG C. and 650 DEG C., for between 5 and 40 hours, so as to obtain between 1.5% and 2.5% nitrogen in the nitrided layer, the second of these treatments consisting of a chromizing by gaseous method forming chromium carbides, lasting between 5 and 30 hours, and realized at temperatures of between 850 DEG C. and 1,100 DEG C., the third of these three treatments being a thermal treatment comprising a quenching in oil of the chromized piece followed by a tempering at a temperature of between 600 DEG C. and 650 DEG C., lasting between 30 minutes and 10 hours, depending on the size of the piece treated.

Description

La présente invention concerne un perfectionnement à la chromisation par voie gazeuse des aciers à plus de 0,2 % de carbone, plus spécialement les aciers de construction et les aciers à outils ; ce perfectionnement permet d'accroître très notablement l'épaisseur de la couche chromisée, ainsi que sa ténacité.The present invention relates to an improvement in chromium-plating by gas of steels with more than 0.2% carbon, more especially structural steels and tool steels; this improvement makes it possible to significantly increase the thickness of the chromized layer, as well as its toughness.

La chromisation des aciers par voie gazeuse est bien connue. La formation d'alliages de diffusion à base de chrome sur la surface des aciers a déjà été décrite dans de nombreux brevets d'invention. Le transport de chrome jusqu'à la surface du matériau à traiter se fait au moyen d'halogénures, qui sont les seuls composés au chrome se trouvant à l'état de vapeur aux températures de diffusion. Le passage du chrome en solution solide dans le métal se fait par échange entre l'halogénure de chrome et le fer suivant une réaction qui, dans le cas des chlorures, peut s'écrire:

Figure imgb0001
The chromization of steels by gas is well known. The formation of chromium-based diffusion alloys on the surface of steels has already been described in numerous invention patents. The transport of chromium to the surface of the material to be treated is carried out by means of halides, which are the only chromium compounds in the vapor state at diffusion temperatures. The passage of chromium in solid solution in the metal takes place by exchange between the chromium halide and the iron according to a reaction which, in the case of chlorides, can be written:
Figure imgb0001

Pour obtenir une diffusion suffisante, la réaction doit s'effectuer à haute température et dans le domaine austénitique, c'est à dire au-delà de 850°C pour les aciers usuels. Dans les aciers dont la teneur en carbone est supérieure à 0,2 %, la réaction superficielle du carbone et du chrome entraine d'une part la formation d'une pellicule de carbures de chrome, d'autre part une diffusion du carbone vers la surface. La pellicule superficielle est constituée de deux types de carbures, M23C6, plus riche en chrome vers la surface et M7C3, plus pauvre en chrome, vers le substrat métallique.To obtain sufficient diffusion, the reaction must be carried out at high temperature and in the austenitic range, that is to say above 850 ° C. for the usual steels. In steels whose carbon content is greater than 0.2%, the surface reaction of carbon and chromium causes on the one hand the formation of a film of chromium carbides, on the other hand a diffusion of carbon towards the area. The surface film consists of two types of carbides, M 23 C 6 , richer in chromium towards the surface and M 7 C 3 , poorer in chromium, towards the metal substrate.

Dans ce qui précède et dans ce qui suit, M désigne un métal tel que le fer (Fe), le chrome (Cr), le nickel (Ni), etc ...In what precedes and in what follows, M indicates a metal such as iron (Fe), chromium (Cr), nickel (Ni), etc ...

La pellicule superficielle a une épaisseur comprise entre 12 et 18 microns, et un niveau de dureté compris entre 1200 et 1800 dans l'échelle de dureté Vickers. Le chrome diffuse ainsi dans l'acier sur une profondeur en général voisine de 15 Microns. Dans les procédés connus, cette pro. fpndeurde chromisation ne dépasse jamais 20 microns.The surface film has a thickness between 12 and 18 microns, and a hardness level between 1200 and 1800 in the Vickers hardness scale. Chromium thus diffuses into steel over a depth generally close to 15 microns. In known methods, this pro. The chromization factor never exceeds 20 microns.

L'affinité du chrome pour le carbone est telle qu'il se forme très rapidement, au cours de la montée en température de traitement, une pellicule de carbures du type M7C3 sur la surface des pièces. Cette pelli- cale gane pénécration du chrome à l'intérieur de l'acier par diffusion; il en résuite :

  • 1) la formation du deuxième type de carbures M23C6,
  • 2) l'obtention de couches superficielles de carbures relativement minces.
The affinity of chromium for carbon is such that it forms very quickly, during the rise in processing temperature, a film of carbides of the M 7 C 3 type on the surface of the parts. This film wedge gane penetration of chromium inside the steel by diffusion; it follows:
  • 1) the formation of the second type of carbides M 23 C 6 ,
  • 2) obtaining relatively thin surface layers of carbides.

Ces couches minces et biphasées présentent l'inconvénient d'être relativement fragiles, en raison de l'état des contraintes qui se trouvent dans les phases carbures après traitement thermique. Le carbure M7C3, de structure colonnaire, se trouve notamment en état de contraintes d'extension, ce qui entraine la formation de fissures qui sont souvent à l'origine des écaillages observés.These thin, two-phase layers have the drawback of being relatively fragile, due to the state of the stresses which are found in the carbide phases after heat treatment. The carbide M 7 C 3 , of columnar structure, is in particular in a state of extension stresses, which causes the formation of cracks which are often at the origin of the spallings observed.

Le but principal de la présente invention est de trouver un moyen permettant d'obtenir en surface un seul type de carbures, et ceci dans une couche de plus grande épaisseur. Un passage rapide de la pièce à traiter en phase austénitique pourrait constituer une solution pour deux raisons :

  • 1) La diffusion du carbone vers la surface est ralentie ; en effet, le . coefficient de diffusion du carbone en volume dans la phase austénitique du fer est de l'ordre de 10 -8 cm2/sec à 900°C, alors qu'il est voisin de 2.10-6 cm2/sec, à la même température dans la phase ferritique. Il en résulte donc une réduction de la vitesse de formation des carbures en sur- face et une accentuation de la diffusion du chrome en profondeur.
  • 2) Le carburé M23C6, de structure cubique à faces centrées a une maille a de'10,6 Aô, pratiquement trois fois plus grande que celle de l'austénite (a = 3,6 A ô). Ce carbure précipite donc beaucoup plus facilement dans la structure austénitique que le carbure M7C3 de structure hexagonale.
The main object of the present invention is to find a means for obtaining on the surface only one type of carbides, and this in a layer of greater thickness. A rapid passage of the part to be treated in the austenitic phase could constitute a solution for two reasons:
  • 1) The diffusion of carbon towards the surface is slowed down; indeed, the. coefficient of diffusion of carbon by volume in the austenitic phase of iron is of the order of 10 -8 cm2 / sec at 900 ° C, while it is close to 2.10 -6 cm 2 / sec, at the same temperature in the ferritic phase. This therefore results in a reduction in the rate of formation of carbides on the surface and an increase in the diffusion of chromium at depth.
  • 2) the carbide M 23 C 6, of face-centered cubic structure has a mesh has de'10,6 A O, almost three times greater than that of austenite (a = 3.6 A O). This carbide therefore precipitates much more easily in the austenitic structure than the carbide M 7 C 3 of hexagonal structure.

Donc, une première solution pour l'augmentation de l'épaisseur et de la ténacité das couches de carbures peut être constituée par une phase initiale de traitement consistant en une montée rapide en température, notamment dans le domaine 600-900°C où précipitent habituellement les carbures M7C3 dans la structure ferritique. Toutefois, une telle solution présenterait des risques au niveau des amorçages de fissurationset de tapures au cours du chauffage, notamment pour dès pièces en aciers relativement alliés présentant des géomètries peu adaptées aux fortes contraintes d'origine thermique qui résulteraient d'un tel cycle de chauffage.Therefore, a first solution for increasing the thickness and the toughness of the carbide layers can be constituted by an initial treatment phase consisting of a rapid rise in temperature, in particular in the range 600-900 ° C where usually precipitate M 7 C 3 carbides in the ferritic structure. However, such a solution would present risks with regard to the initiation of cracks and pitting during heating, in particular for parts made of relatively alloyed steels having geometries which are ill-suited to the high thermal stresses which would result from such a heating cycle. .

La présente invention, tout en appliquant le principe de chromi- ser la pièce en phase austénitique, évite les risques mentionnés ci-dessus en réalisant au préalable une couche nitrurée, mais sans couche de combinaison, ce est à dire de manière telle qu'il n'y ait en aucune façon formation d'une couche superficielle de nitrures de fer et de chrome. En effet, une couche superficielle de nitrures de fer resterait relativement stable, même à des températures élevées, et constituerait une véritable barrière s'opposant à la diffusion du chrome, cette barrière étant renforcée par la formation des nitrures de chrome liée à un apport supplémentaire d'azote.The present invention, while applying the principle of chromizing the part in the austenitic phase, avoids the risks mentioned above by producing beforehand a nitrided layer, but without a combination layer, that is to say in such a way that it there is in no way the formation of a surface layer of iron and chromium nitrides. Indeed, a the surface layer of iron nitrides would remain relatively stable, even at high temperatures, and would constitute a real barrier opposing the diffusion of chromium, this barrier being reinforced by the formation of chromium nitrides linked to an additional supply of nitrogen.

Ainsi, l'absence de couche de combinaison sur la surface des pièces à traiter ensuite par chromisation est une condition nécessaire à une bonne diffusion du chrome en profondeur, et cette condition est réalisée dans la présente invention.Thus, the absence of a combination layer on the surface of the parts to be subsequently treated by chromization is a necessary condition for good diffusion of the chromium in depth, and this condition is achieved in the present invention.

A cet effet, la présente invention a pour objet un perfectionnement aux procédés de chromisation, constitué par un procédé de chromisation des aciers sur une profondeur e supérieure à 40 microns, utilisable pour des aciers ayant une teneur en carbone au moins égale à 0,2 %, notamment pour des aciers de construction et pour des aciers à outils, caractérisé par la combinaison de trois traitements successifs, le premier de ces trois traitements consistant en une nitruration ionique d'une couche superficielle d'épaisseur comprise entre 100 et 350 microns, cette nitruration ionique étant réalisée dans une atmosphère constituée par un mélange d'azote et d'hydrogène, à une température comprise entre 450°C et 650°C, pendant une durée comprise entre 5 et 40 heures, de façon à obtenir entre 1,5 % et 2,5 % d'azote dans la couche nitrurée, le second de ces traitements consistant en une chromisation par voie gazeuse formatrice de carbures de chrome, d'une durée comprise entre 5 et 30 heures, et réalisée à des tea- pératures comprises entre 850°C et 1.1000C, le troisième de ces trois traitements étant un traitement thermique comprenant une trempe à l'huila de la pièce chromisée suivie d'un revenu à une température comprise entre 600°C et 650°C, d'une durée comprise entre 30 minutes et 10 heures selon la dimension de la pièce traitée.To this end, the subject of the present invention is an improvement to the chromisation processes, constituted by a process for chromisation of steels over a depth e greater than 40 microns, usable for steels having a carbon content at least equal to 0.2 %, in particular for structural steels and for tool steels, characterized by the combination of three successive treatments, the first of these three treatments consisting of ionic nitriding of a surface layer with a thickness between 100 and 350 microns, this ionic nitriding being carried out in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature between 450 ° C and 650 ° C, for a period between 5 and 40 hours, so as to obtain between 1, 5% and 2.5% nitrogen in the nitrided layer, the second of these treatments consisting of chromium-forming chromium carbide gas formation, lasting between 5 and 30 hours, and reac linked to temperatures between 850 ° C. and 1,100 ° C., the third of these three treatments being a heat treatment comprising quenching with oil of the chromized part followed by tempering at a temperature between 600 ° C. and 650 ° C, lasting between 30 minutes and 10 hours depending on the size of the part treated.

Suivant une caractéristique particulière de la présente invention, nitruration ionique formant le premier des trois traitements et effecaée sous atmosphère d'azote et d'hydrogène est réalisée sous une pression partielle d'azote au plus égale à 1,5 millibar, et sous une pression gazeuse totale comprise entre 2 et 10 millibars.According to a particular characteristic of the present invention, ionic nitriding forming the first of the three treatments and effecaée under nitrogen and hydrogen atmosphere is carried out under a partial nitrogen pressure at most equal to 1.5 millibar, and under a pressure total gaseous between 2 and 10 millibars.

Suivant une autre caractéristique particulière de la présente invea- tion, la chromisation formant le second-des trois traitements, réalisée par la technique connue des céments sous atmosphère réductrice à base d'hydrogène, utilise un mélange pulvérulent à base de ferro-chrome et de chlorure d'amcnonium, ce dernier ne représentant en poids que 0,4 % à 1 1 ou yélange pulvérulent, la poudre de ferro-ehrome présentant de préférence one teneur en chrome comprise entre 50 % et 75 % et une granulométrie comprise entre 0,5 millimètre et 4 millimètres, sans liant alumineux ml magnèsien.According to another particular characteristic of the present invention, the chromization forming the second of the three treatments, carried out by the known technique of cements under a reducing atmosphere based on hydrogen, uses a pulverulent mixture based on ferro-chromium and amcnonium chloride, the latter representing by weight only 0.4% to 1 1 or powdered mixture, the ferro-ehrome powder preferably having a chromium content of between 50% and 75% and a particle size of between 0.5 millimeter and 4 millimeters, without aluminous binder ml magnesium.

Dans tout ce qui précède et dans tout ce qui suit, il faut attendre par initruration ionique" un traitement thermo-chimique d'une surface métallique par bombardement ionique en gaz raréfié, réalisant une nitruration superficielle de la pièce métallique placée en cathode sousatmosphè- re d'azote et d'hydrogène à une température comprise entre 450°C et 650°C.In all of the foregoing and in all that follows, it is necessary to wait by ionic initruration "for a thermochemical treatment of a metallic surface by ion bombardment with rarefied gas, carrying out a surface nitriding of the metallic part placed in an under-cathode cathode. nitrogen and hydrogen at a temperature between 450 ° C and 650 ° C.

L'intérêt principal de la nitruration ionique est qu'elle rend possible l'exploitation de toutes les éventualités offertes par les diagrammes d'équilibre entre les éléments constituant l'acier traité et l'azote. En effet, les traitements thermochimiques de surfaces métalliques par bombardements ioniques et plus particulièrement la nitruration, sont basés sur des propriétés de la décharge électrique dans les gaz raréfiés, en l'oc- curence des mélanges d'azote et d'hydrogène avec éventuellement des hydrocarbures. L'atmosphère gazeuse réactive peut être choisie indépendamment de la nécessité de son crackage pyrolitique puisque son activation est obtenue par ionisation. Il est donc possible de régler la pression partielle d'azote de telle sorte que l'on forme superficiellement la ou les phases prévues par le diagramme d'équilibre binaire fer-azote. C'est ainsi que pour les pressions d'azote les plus faibles, on forme uniquement une couche de diffusion, solution solide d'azote dans le fer α à des températures généralement comprises entre 450° et 570°C. Dans ce même domaine de tem pérature, une augmentation de la pression partielle d'azote conduit tout d'abord à la formation d'une couche de combinaison de nitrures (Fe,N), puis de nitrures α et £ (Fe2,3N).The main interest of ionic nitriding is that it makes possible the exploitation of all the possibilities offered by the balance diagrams between the elements constituting the treated steel and the nitrogen. Indeed, the thermochemical treatments of metallic surfaces by ion bombardments and more particularly nitriding, are based on properties of the electric discharge in rarefied gases, in the event of mixtures of nitrogen and hydrogen with possibly hydrocarbons. The reactive gas atmosphere can be chosen independently of the need for its pyrolitic cracking since its activation is obtained by ionization. It is therefore possible to adjust the partial pressure of nitrogen so that the phase or phases provided for by the binary iron-nitrogen balance diagram are superficially formed. Thus, for the lowest nitrogen pressures, only a diffusion layer is formed, a solid solution of nitrogen in the α iron at temperatures generally between 450 ° and 570 ° C. In this same temperature range, an increase in the partial pressure of nitrogen leads first of all to the formation of a combination layer of nitrides (Fe, N), then of nitrides α and £ (Fe 2 , 3 NOT).

Il est possible d'obtenir dans le cas des aciers à plus de 0,2 % de carbone, et plus spécialement les aciers de construction et les aciers à ouells, des couches de diffusion d'azote d'épaisseurs comprises entre 100 et 350 microns, cette nitruration ionique étant réalisée dans une atmosphère constituée par un mélange d'azote et d'hydrogène, à une température comprise entre 450 et 570°C, pendant une durée comprise entre 5 et 40 heures de façon à obtenir par exemple sur des profondeurs de 50 à 200 microns à partir de la surface des teneurs en azote en solution solide com- rises entre 1,5 et 2,5 %.It is possible to obtain, in the case of steels with more than 0.2% carbon, and more especially structural steels and steels with ouells, nitrogen diffusion layers of thicknesses between 100 and 350 microns , this ionic nitriding being carried out in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature of between 450 and 570 ° C., for a period of between 5 and 40 hours so as to obtain, for example, at depths from 50 to 200 microns from the surface of the nitrogen contents in solid solution between 1.5 and 2.5%.

Comme on le comprend, l'un des principaux avantages de l'invention consiste, grâce à la nitruration ionique, à obtenir une couche nitrurée sans couche de combinaison, c'est à dire sans nitrures de fer et de chrome, d'une manière fiable et répétitive par ajustement de la pression partielle d'azote en fonction de la température de traitement et de la composition chimique de l'acier. Alors, sans risque de tapures, l'acier en surface peut passer rapidement en phase austénitique à température modérée, à cause d'une teneur en azote de l'ordre de 1, 5 % à 2,5 %.As will be understood, one of the main advantages of the invention consists, thanks to ionic nitriding, in obtaining a nitrided layer without a combination layer, that is to say without iron and chromium nitrides, in a reliable and repetitive manner by adjusting the partial pressure of nitrogen as a function of the treatment temperature and the chemical composition of the steel . So, without risk of spalling, the surface steel can quickly pass into the austenitic phase at moderate temperature, because of a nitrogen content of the order of 1.5% to 2.5%.

Après quoi la chromisation par voie gazeuse peut s'effectuer. à plus grande profondeur,par exemple jusqu'à 50 microns et même davantage, et avec formation en surface d'un seul type de carbonitrures, en Cr2(C,N), ce qui entraine une augmentation sensible de la ténacité du revêtement.After which gas chromization can be carried out. at a greater depth, for example up to 50 microns and even more, and with the formation on the surface of a single type of carbonitrides, in Cr 2 (C, N), which causes a significant increase in the toughness of the coating.

Afin de bien faire comprendre l'invention, on va décrire ci-après, à titre d'exemple non limitatif, un mode de réalisation du perfectionnement selon l'invention, dans lequel on traite un acier au chrome-molybdène-vanadium, de type 35CDV 12, donc à 0,35 % de carbone, en vue d'obtenir une profondeur de chromisation de 50 microns.In order to clearly understand the invention, an embodiment of the improvement according to the invention will be described below, by way of non-limiting example, in which a chromium-molybdenum-vanadium steel of the type is treated 35CDV 12, therefore at 0.35% carbon, in order to obtain a chromization depth of 50 microns.

La nitruration ionique qui constitue le premier des trois traitements successifs selon l'invention est effectuée ici dans une enceinte métallique muniede boucliers thermiques et refroidie par circulation d'eau, laquelle enceinte constitue l'anode reliée à la terre. Les paramètres électriques sont choisis de telle manière que le courant augmente avec la tension continue produite par le générateur et que l'échantillon à nitrurer qui constitue la cathode soit recouvert par l'effluve correspondant au régime de décharge anormale. A proximité de la surface cathodique, les ions gazeux sont formés et accélérés vers l'échantillon et provoquent son échauffement, que l'on poursuit jusqu'à la température choisie pour réaliser le traitement the rmochimique. La régulation de température est obtenue à l'aide d'un thermocouple protégé par une gaine en alumine et placé dans l'échantillon dans des. conditions qui permettent d'éviter l'amorçage d'arcs.The ionic nitriding which constitutes the first of the three successive treatments according to the invention is carried out here in a metal enclosure provided with heat shields and cooled by circulation of water, which enclosure constitutes the anode connected to earth. The electrical parameters are chosen in such a way that the current increases with the direct voltage produced by the generator and that the sample to be nitrided which constitutes the cathode is covered by the corona corresponding to the abnormal discharge regime. Near the cathode surface, the gaseous ions are formed and accelerated towards the sample and cause it to heat up, which is continued until the temperature chosen to carry out the rmochemical treatment. The temperature regulation is obtained using a thermocouple protected by an alumina sheath and placed in the sample in. conditions which avoid arcing.

La pression à laquelle.le traitement thermo-chimique est réalisé est généralement comprise entre 2,5 et 8,0 millibar ; une pompe primaire est suffisante pour faire le vide initial puis pour permettre le renouvellement du gaz nitrurant à proximité de l'échantillon. Le mélange gazeux nitrurant est composé d'azote et d'hydrogène. Les pressions partielles d'azote PN pour lesquelles on obtient une, solution solide d'azote dans le réseau de la ferrite sont comprises entre 0,1 et 0,5 millibar. La température est réglée en moyenne à 520°C et ne s'écarte pas du domaine compris entre 510 et 530°C. En laissant de côté la montée en température et la mise en basse pression de l'atmosphère, la durée de la nitruration ionique à bonne pression et à bonne température est de 25 heures. Par ce premier traitement, la teneur moyenne en azote de l'acier entre 50 eL 200 microns de profondeur atteint 2,1 %, et la couche nitrurée ne contient pas de nitrures de fer, ni de nitrures de chrome.The pressure at which the thermochemical treatment is carried out is generally between 2.5 and 8.0 millibar; a primary pump is sufficient to create the initial vacuum and then to allow the renewal of the nitriding gas near the sample. The nitriding gas mixture is composed of nitrogen and hydrogen. The partial pressures of nitrogen P N for which a solid solution of nitrogen is obtained in the ferrite network are between 0.1 and 0.5 millibar. The temperature is set on average at 520 ° C and does not deviate from the range between 510 and 530 ° C. Leaving aside the rise in temperature and the setting the atmosphere to low pressure, the duration of ion nitriding at the right pressure and at the right temperature is 25 hours. By this first treatment, the average nitrogen content of the steel between 50 and 200 microns deep reaches 2.1%, and the nitrided layer does not contain iron nitrides or chromium nitrides.

La pièce métallique en acier 35 CDV 12 ainsi nitrurée est alors extraite du four de nitruration ionique et introduite dans une caisse de cémentation qui va effectuer le deuxième traitement selon l'invention, qui est une chromisation par voie gazeuse.The metal piece of CDV 12 steel 35 thus nitrided is then extracted from the ion nitriding furnace and introduced into a case-hardening box which will carry out the second treatment according to the invention, which is gas chromization.

L'agent de cémentation utilisé est une poudre constituée poux 99,5 % de ferro-chrome à 60/70 % de chrome et pour 0,5 % de chlorure d'ammonium, sans alumine ni magnésie. Cette poudre présente une granulométrie comprise entre 0,5 et 4 mm, avec une dimension moyenne voisine de 2,7 mm. Cette poudre est disposée dans le fond de la caisse de cémentation, qui a la forme d'un cylindre vertical, et elle se trouve recouver- te par un cloisonnement sur lequel est placée la pièce d'acier à chromiser. A la partie supérieure de la caisse de cémentation se trouve dans un panier une réserve de ferro-chrome servant à la régénération directe de la vapeur active de chlorure de chrome CrCl2. De l'hydrogène introduit crée une atmosphère réductrice.The cementing agent used is a powder made up of 99.5% ferro-chromium with 60/70% chromium and for 0.5% ammonium chloride, without alumina or magnesia. This powder has a particle size between 0.5 and 4 mm, with an average size close to 2.7 mm. This powder is placed in the bottom of the case-hardening box, which has the shape of a vertical cylinder, and it is covered by a partition on which the piece of steel to be chromized is placed. At the top of the case-hardening box there is a ferro-chromium reserve in a basket used for the direct regeneration of the active vapor of chromium chloride CrCl 2 . Introduced hydrogen creates a reducing atmosphere.

L'enceinte est portée à une température moyenne de 950°C, ne s'écartant pas du domaine 920°C-980°C, pendant une durée de 20 heures.The enclosure is brought to an average temperature of 950 ° C, not deviating from the range 920 ° C-980 ° C, for a period of 20 hours.

Dans la caisse de cémentation se produisent les phénomènes suivants :

  • Au chauffage, le chlorure d'ammonium se dissocie. L'ion chlore ainsi libéré agit sur le chrome du ferro-chrome pour former du chlorure de chrome CrCl2 à l'état de vapeur, qui produit la chromisation superficielle selon la réaction (1) mentionnée ci-dessus,
The following phenomena occur in the cementation box:
  • When heated, the ammonium chloride dissociates. The chlorine ion thus released acts on the chromium of ferro-chromium to form chromium chloride CrCl 2 in the vapor state, which produces the surface chromization according to the reaction (1) mentioned above,

Les vapeurs de chlorure ferreux issus de la réaction (1) réagissent sur la réserve de chrome placée à la partie supérieure de la caisse, ce qui régénère du chlorure de chrome CrCl2 gazeux qui participe à la chromisation selon (1).The ferrous chloride vapors from reaction (1) react on the chromium reserve placed at the top of the body, which regenerates gaseous chromium chloride CrCl 2 which participates in the chromization according to (1).

Après 20 heures de maintien à 920°C-980°C, la pièce chromisée subit le troisième traitement selon l'invention, c'est à dire qu'elle est extraite de la caisse de cémentation, elle est immédiatement trempée à l'huile, puis elle est introduite dans un four de revenu maintenu à une température de l'ordre de 625°C, pendant 2 heures.After 20 hours of holding at 920 ° C-980 ° C, the chromized part undergoes the third treatment according to the invention, that is to say that it is extracted from the case hardening box, it is immediately soaked in oil , then it is introduced into a tempering oven maintained at a temperature of the order of 625 ° C., for 2 hours.

Après revenu, on observe :

  • - que la couche superficielle contenant les carbonitrures de chrome a une épaisseur voisine de 50 microns,
  • - que les carbonitrures de chrome de cette couche superficielle sont à peu près exclusivement du type Cr2(C,N),
  • - que la dureté de cette couche est comprise entre 1800 et 2000 dans l'échelle de Vickers,
  • - qu'elle se fissure sous une charge de 1 kilogramme-force.
After income, we observe:
  • - that the surface layer containing the chromium carbonitrides has a thickness close to 50 microns,
  • - that the chromium carbonitrides of this surface layer are almost exclusively of the Cr 2 (C, N) type,
  • - that the hardness of this layer is between 1800 and 2000 in the Vickers scale,
  • - that it cracks under a load of 1 kilogram-force.

Le revêtement chromisé ainsi obtenu dans le présent exemple selon l'invention est à comparer avec celui d'une chromisation de type connu, non précédée d'une nitruration ionique. Dans ce cas de type connu :

  • - la couche superficielle contenant les carbures de chrome a une épaisseur voisine de 15 microns,
  • - on y observe deux phases de carbures de chrome, l'une en M23C6, surtout en surface, l'autre en M7C3, vers le substrat métallique,
  • - la dureté de la couche superficielle est comprise entre 1200 et 1800 dans l'échelle de Vickers, avec des hétérogénéités liées à des porosités superficielles,
  • - la charge à partir de laquelle apparaissent les fissures aux angles des empreintes Vickers est de 300 grammes-force.
The chromized coating thus obtained in the present example according to the invention is to be compared with that of a chromization of known type, not preceded by ionic nitriding. In this case of known type:
  • - the surface layer containing the chromium carbides has a thickness close to 15 microns,
  • - two phases of chromium carbides are observed, one in M 23 C 6 , especially at the surface, the other in M 7 C 3 , towards the metal substrate,
  • the hardness of the surface layer is between 1200 and 1800 in the Vickers scale, with heterogeneities linked to surface porosities,
  • - the load from which the cracks appear at the corners of the Vickers footprints is 300 grams-force.

Il est bien entendu que l'on peut, sans sortir du cadre de l'invention, imaginer des variantes et perfectionnements de détails, de même qu'envisager l'emploi de moyens équivalents.It is understood that it is possible, without departing from the scope of the invention, to imagine variants and refinements of details, as well as to envisage the use of equivalent means.

Claims (3)

1.- Perfectionnement aux procédés de chromisation des aciers, constitué par un procédé de chromisation des aciers sur une profondeur e supérieure à 40 microns, utilisable pour des aciers ayant une teneur en carbone au moins égale à 0,2 %, caractérisé par la combinaison de trois traitements successifs, le premier de ces trois traitements consistant en une nitruration ionique d'une couche superficielle d'épaisseur comprise entre 100 et 350 microns, cette nitruration ionique étant réalisée dans une atmosphère constituée par un mélange d'azote et d'hydrogène, à une température comprise entre 450°C et 650°C, pendant une durée comprise entre 5 et 40 heures, de façon à obtenir entre 1,5 % et 2,5 % d'azote dans la couche nitrurée, le second de ces trois traitements consistant en une chromisation par voie gazeuse formatrice de carbures de chrome, d'une durée comprise entre 5 et 30 heures, et réalisée à des températures comprises entre 850°C et 1.100°C, le troisième de ces trois traitements étant un traitement thermique comprenant une trempe à l'huile de la pièce chromisée suivie d'un revenu à une température comprise entre 600°C et 650°C, d'une durée comprise entre 30 minutes et 10 heures selon la dimension de la pièce traitée.1.- Improvement to the chromizing processes of steels, constituted by a chromizing process of steels over a depth e greater than 40 microns, usable for steels having a carbon content at least equal to 0.2%, characterized by the combination of three successive treatments, the first of these three treatments consisting of ionic nitriding of a surface layer of thickness between 100 and 350 microns, this ionic nitriding being carried out in an atmosphere constituted by a mixture of nitrogen and hydrogen , at a temperature between 450 ° C and 650 ° C, for a period between 5 and 40 hours, so as to obtain between 1.5% and 2.5% of nitrogen in the nitrided layer, the second of these three treatments consisting of chromium-gas chromization forming chromium carbides, lasting between 5 and 30 hours, and carried out at temperatures between 850 ° C and 1,100 ° C, the third of these three treatments t a heat treatment comprising oil quenching of the chromized part followed by tempering at a temperature between 600 ° C and 650 ° C, lasting between 30 minutes and 10 hours depending on the size of the part processed. 2.- Procédé de chromisation des aciers selon la revendication 1, caractérisé pn ce que la nitruration ionique formant le premier des trois traitementset effectuée sous atmosphère d'azote et d'hydrogène est réalisée sous une pression partielle d'azote au plus égale à 1,5 millibar, et sous une pression gazeuse totale comprise entre 2 et 10 millibars.2. A process for chromizing steels according to claim 1, characterized in that the ionic nitriding forming the first of the three treatments and carried out under a nitrogen and hydrogen atmosphere is carried out under a partial nitrogen pressure at most equal to 1 , 5 millibar, and under a total gas pressure between 2 and 10 millibars. 3.- Procédé de chromisation des aciers selon l'une quelconque des revendications 1 et 2, dans lequel la chromisation formant le second des trois traitements est réalisée par la technique connue des céments, cement sous atmosphère réductriceà base d'hydrogène, et utilise comme/un mélange pulvérulent à base de ferro-chrome et de chlorure d'ammonium, caractérisé en ce que le mélange pulvérulent contient entre 0,4 % et 1 % de chlorure d'ammonium, et en ce que la poudre de ferro-chrome présente une teneur en chrome comprise entre 50 % et 75 % et une granulométrie comprise entre 0,5 millimètre et 4 millimètres, sans liant alumineux ni magnésien.3. A method of chromizing steels according to any one of claims 1 and 2, wherein the chromizing forming the second of the three treatments is carried out by the known technique of cements, cement under a reducing atmosphere based on hydrogen, and uses as / a powder mixture based on ferro-chromium and ammonium chloride, characterized in that the pulverulent mixture contains between 0.4% and 1% of ammonium chloride, and in that the ferro-chromium powder has a chromium content of between 50% and 75% and a particle size of between 0.5 millimeter and 4 millimeters, without aluminous or magnesium binder.
EP79400724A 1978-10-25 1979-10-08 Improvement in the chromising of steel in the gaseous phase Expired EP0010484B1 (en)

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AT79400724T ATE1529T1 (en) 1978-10-25 1979-10-08 IMPROVEMENT IN CHROMING STEEL IN THE GAS PHASE.

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FR7830308A FR2439824A1 (en) 1978-10-25 1978-10-25 IMPROVEMENT IN CHROMIZING STEELS BY GASEOUS WAY

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EP0043742A1 (en) * 1980-05-29 1982-01-13 Creusot-Loire Method of gas-chromizing steels

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EP0010484B1 (en) 1982-09-08
JPS6035989B2 (en) 1985-08-17
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JPS5558366A (en) 1980-05-01
ATE1529T1 (en) 1982-09-15

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