WO2014170566A1 - Thermochemical treatment method comprising a single nitriding phase before carburising - Google Patents

Thermochemical treatment method comprising a single nitriding phase before carburising Download PDF

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Publication number
WO2014170566A1
WO2014170566A1 PCT/FR2014/050581 FR2014050581W WO2014170566A1 WO 2014170566 A1 WO2014170566 A1 WO 2014170566A1 FR 2014050581 W FR2014050581 W FR 2014050581W WO 2014170566 A1 WO2014170566 A1 WO 2014170566A1
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Prior art keywords
temperature
phase
nitriding
carburizing
diffusion
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PCT/FR2014/050581
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French (fr)
Inventor
Philippe Lapierre
Jerome Lardinois
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Peugeot Citroën Automobiles SA
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Application filed by Peugeot Citroën Automobiles SA filed Critical Peugeot Citroën Automobiles SA
Priority to EP14720174.3A priority Critical patent/EP2986750A1/en
Priority to CN201480022094.0A priority patent/CN105264106A/en
Publication of WO2014170566A1 publication Critical patent/WO2014170566A1/en

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    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23CCOATING 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
    • C23C8/00Solid 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
    • C23C8/02Pretreatment of the material to be coated
    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23CCOATING 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
    • C23C8/00Solid 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
    • C23C8/06Solid 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
    • C23C8/34Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in more than one step
    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23CCOATING 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
    • C23C8/00Solid 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
    • C23C8/80After-treatment

Definitions

  • the present invention relates to a method of thermochemical treatment of steel parts to reinforce them, and a gearbox gear for a motor vehicle reinforced with such a type of treatment.
  • thermochemical treatment of this type of parts by carbonitriding.
  • These treatments comprise first a carbon and nitrogen temperature diffusion over a certain thickness of the material, before quenching the part making it possible to increase the surface hardness while keeping certain original characteristics at heart. .
  • One type of known treatment method presented in particular by the document FR-B1-284523, comprises a first stage of temperature rise to reach 930 ° C., then a second stage of equalization of this temperature, followed subsequently by a succession similar cycles comprising first a cementation phase and then a nitriding phase, and finally the thermal quenching. It remains well during all cycles of supply elements at a constant temperature.
  • a problem with this type of process is that with the carburization temperatures usually used, which are around 900 ° C, the nitriding efficiency is low. It is then necessary to have significant nitriding times to introduce enough nitrogen into the room. In addition nitriding the workpiece with a reducing gas such as ammonia causes a decarburization of the metal surface by the hydrogen released during the decomposition of this ammonia, which is reducing and consumes the carbon. This decarburization reduces the hardness, and degrades the functional performance of the room.
  • a reducing gas such as ammonia
  • the control of the nitriding at the end of the cycle is delicate because the austenitic steel grains are already saturated with carbon and nitrogen by the preceding cementation phases. Grains can no longer absorb nitrogen and release it into grain boundaries. This nitrogen combines with the additive elements such as chromium and nickel, which charges the gaskets into precipitates of nitride of chromium and silicon, as well as carbonitrides.
  • Another type of known process of treatment comprises a first stage of rise in temperature to reach 800 ° C followed by a first phase of nitriding at this temperature, then a second step of temperature rise to 980 ° C, then a succession of cycles each comprising a cementation phase followed by diffusion by maintaining this temperature, then a temperature descent step at 850 ° C followed by a second nitriding phase at this temperature. temperature, and finally the quenching phase of the metal.
  • a cementation time is obtained which comprises a succession of cementation and diffusion cycles, which is relatively long.
  • the second nitriding phase after cementation also causes a decarburization of the layer on the surface of the metal.
  • the present invention is intended to avoid these disadvantages of the prior art.
  • thermochemical treatment of steel parts to improve the mechanical characteristics, comprising a carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it first has a rise to a first temperature relatively low to begin the nitriding of the metal, then a single nitriding phase at a temperature between this first temperature and a second carburizing temperature, then a single carburizing phase, then a diffusion at this second temperature which remains constant, and finally the quenching of the metal.
  • An advantage of this treatment process is that the cementation is not followed by a nitriding phase, there is no decarburization of the surface of the workpiece, and its hardness remains at the best level.
  • the metallurgical quality is also better controlled, avoiding the formation of various nitrides in the grain boundaries that would reduce the mechanical characteristics of the part.
  • the only nitriding phase being made between the first relatively low temperature allowing this operation, and the second carburizing temperature, it is possible in certain cases to produce temperature rises simultaneously during this nitriding, which saves time on the complete cycle of treatment.
  • thermochemical treatment process according to the invention may further comprise one or more of the following characteristics, which may be combined with each other.
  • the nitriding phase is carried out at the first temperature which remains constant.
  • the nitriding phase can be followed before the carburizing phase, by a diffusion phase comprising a rise in temperature from the first temperature to the second carburizing temperature.
  • the nitriding phase can be followed immediately after the carburising phase, first comprising a temperature rise from the first temperature to the second carburizing temperature.
  • the nitriding phase comprises a rise in temperature from the first temperature to the second carburizing temperature.
  • the nitriding phase can be followed before the carburizing phase by a diffusion phase carried out at the second temperature which remains constant.
  • the first relatively low temperature for starting the nitriding of the metal is of the order of 700 ° C.
  • the second carburizing temperature is of the order of 940 ° C.
  • the invention also relates to a steel gear for a motor vehicle gearbox, reinforced by a thermochemical treatment comprising any one of the preceding features.
  • FIG. 1 is a micrographic section of an austenitic steel treated by a carbonitriding process according to the prior art
  • FIG. 2 is a micrographic section of this steel treated by a carbonitriding process according to the invention.
  • FIG. 3 is a graph showing, as a function of time, the treatment method according to the invention.
  • FIG. 4 is a graph showing the treatment method according to a first variant.
  • FIG. 5 is a graph showing the treatment method according to a second variant.
  • FIG. 1 shows the micrographic section 6 of a low carbon austenitic steel, made to a height of a few tens of micrometers below the surface 2.
  • This steel has undergone a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching.
  • a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching.
  • large precipitates of nitrides 4 forming black spots are observed, following the supply of ammonia throughout the duration of the treatment phase giving an enrichment of carbon and nitrogen. These precipitates reduce the mechanical strength of the surface layer, which can cause premature wear of highly stressed parts with stress cycles, such as gears of gearboxes for motor vehicles.
  • Figure 2 shows the micrographic section of a similar austenitic steel, having undergone a low pressure carbonitriding treatment according to the process according to the invention. For the layer a few tens of micrometers below the surface 2, there is an absence of nitride precipitate in the grain boundaries.
  • FIG. 3 shows a first carbonitriding treatment cycle according to the invention carried out in a low pressure furnace, indicating the temperature T ° as a function of time t.
  • the piece of steel to be treated is at time t0, at room temperature.
  • a first step M of progressive temperature rise of the furnace is made to reach a first temperature level T1 of nitriding, which in this example is 700 ° C.
  • the ideal nitriding temperatures are between 750 and 800 ° C. However, it can be started at 700 ° C and continued up to 900 ° C if you tolerate a little less nitrogen absorption in the room, to save processing time by reducing the rise time in temperature.
  • a third diffusion step D comprises a rise in temperature under a neutral atmosphere, to reach a second temperature plateau T2 of carburizing, which in this example is 940 ° C.
  • diffusion D of nitrogen is carried out in the metal surface layer.
  • the carburizing temperature may vary, with a treatment depth which increases as a function of this temperature.
  • a treatment depth which increases as a function of this temperature.
  • E650 the same processing depth corresponding to the standardized treatment "E650”
  • This second diffusion step D is optional, it depends on the desired carbon content at the end of treatment.
  • FIG. 4 alternatively shows a second carbonitriding treatment cycle according to the invention.
  • This variant makes it possible to optimize the treatment time, by reducing the temperature step between the nitriding phase N and the cementation phase C.
  • the nitriding being carried out with a variable temperature rising above its optimum level, the concentration Nitrogen on the part is weaker with the same nitriding time.
  • FIG. 5 alternatively presents a third carbonitriding treatment cycle according to the invention.
  • the first step M of temperature rise is identical, to reach at the first time t1 the first nitriding temperature T1.
  • a nitriding phase N comprising a temperature which initially remains at the first temperature stage T1, and which then gradually rises to reach the second temperature stage T2.
  • this process specially developed by the company carrying out the invention, makes it possible not to use particular carbonitriding processes which may be the property of suppliers of low pressure furnaces intended for this type of treatment.

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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)

Abstract

A method for thermochemically treating steel parts to improve the mechanical properties of same, comprising the diffusion of carbon by carburising and of nitrogen by nitriding in a low pressure furnace following a temperature cycle, characterised in that it first comprises an increase to a first relatively low temperature (T1) at which it is possible to start nitriding the metal, then a single nitriding phase (N) at a temperature between said first temperature (T1) and a second carburising temperature (T2), then a single carburising phase (C), followed by diffusion (D) at said second temperature (T2) which remains constant, and finally the phase of quenching (T) the metal.

Description

PROCEDE DE TRAITEMENT THERMOCHIMIQUE COMPORTANT UNE UNIQUE PHASE DE NITRURATION AVANT UNE CEMENTATION  THERMO-CHEMICAL PROCESSING METHOD COMPRISING A SINGLE NITRIDING PHASE BEFORE CEMENTATION
La présente invention concerne procédé de traitement thermochimique de pièces en acier pour les renforcer, ainsi qu'un pignon de boîte de vitesses pour un véhicule automobile renforcé avec un tel type de traitement. The present invention relates to a method of thermochemical treatment of steel parts to reinforce them, and a gearbox gear for a motor vehicle reinforced with such a type of treatment.
Pour obtenir une meilleure dureté de pièces en acier sur une certaine épaisseur à partir de la surface, en particulier pour des pignons de boîte de vitesses de véhicule automobile comprenant des dentures fortement sollicitées, il est connu de réaliser un traitement thermochimique de ce type de pièces par carbonitruration.  To obtain a better hardness of steel parts over a certain thickness from the surface, in particular for motor vehicle gearbox gears comprising highly stressed toothing, it is known to perform a thermochemical treatment of this type of parts. by carbonitriding.
Ces traitements comportent d'abord une diffusion en température de carbone et d'azote sur une certaine épaisseur de la matière, avant de réaliser une trempe de la pièce permettant d'augmenter la dureté en surface tout en gardant certaines caractéristiques d'origine à cœur.  These treatments comprise first a carbon and nitrogen temperature diffusion over a certain thickness of the material, before quenching the part making it possible to increase the surface hardness while keeping certain original characteristics at heart. .
On réalise ces types de traitement dans un four basse pression, avec des opérations comprenant suivant un profil de température défini, des phases successives de cémentation pour ajouter du carbone, de nitruration pour ajouter de l'azote, et de diffusion pour diffuser ces éléments. En fin de cycle on réalise la trempe finale.  These types of treatment are carried out in a low-pressure furnace, with operations comprising, following a defined temperature profile, successive cementation phases to add carbon, nitriding to add nitrogen, and diffusion to diffuse these elements. At the end of the cycle, the final quenching is carried out.
Un type de procédé de traitement connu, présenté notamment par le document FR-B1 -2884523, comporte une première étape de montée en température pour atteindre 930 ° C, puis une deuxième étape d'égalisation de cette température, suivie ensuite d'une succession de cycles similaires comprenant d'abord une phase de cémentation puis une phase de nitruration, et enfin la trempe thermique. On reste ainsi pendant tous les cycles d'apport des éléments, à une température constante.  One type of known treatment method, presented in particular by the document FR-B1-284523, comprises a first stage of temperature rise to reach 930 ° C., then a second stage of equalization of this temperature, followed subsequently by a succession similar cycles comprising first a cementation phase and then a nitriding phase, and finally the thermal quenching. It remains well during all cycles of supply elements at a constant temperature.
Un problème qui se pose avec ce type de procédé est qu'avec les températures de cémentation usuellement utilisées, qui sont autour de 900 ° C, le rendement de la nitruration est faible. I faut alors des temps de nitruration importants pour introduire suffisamment d'azote dans la pièce. De plus la nitruration de la pièce par un gaz réducteur comme l'ammoniac provoque une décarburation de la surface du métal par l'hydrogène libéré lors de la décomposition de cet ammoniac, qui est réducteur et consomme le carbone. Cette décarburation réduit la dureté, et dégrade la tenue fonctionnelle de la pièce. A problem with this type of process is that with the carburization temperatures usually used, which are around 900 ° C, the nitriding efficiency is low. It is then necessary to have significant nitriding times to introduce enough nitrogen into the room. In addition nitriding the workpiece with a reducing gas such as ammonia causes a decarburization of the metal surface by the hydrogen released during the decomposition of this ammonia, which is reducing and consumes the carbon. This decarburization reduces the hardness, and degrades the functional performance of the room.
Par ailleurs, la maîtrise de la nitruration en fin de cycle est délicate car les grains d'acier austénitique sont déjà saturés en carbone et en azote par les phases de cémentation précédentes. Les grains ne peuvent plus absorber d'azote et le rejettent dans les joints de grain. Cet azote s'allie avec les éléments d'addition comme le chrome et le nickel, ce qui charge les joints en précipités de nitrure de chrome et de silicium, ainsi que de carbonitrures.  Moreover, the control of the nitriding at the end of the cycle is delicate because the austenitic steel grains are already saturated with carbon and nitrogen by the preceding cementation phases. Grains can no longer absorb nitrogen and release it into grain boundaries. This nitrogen combines with the additive elements such as chromium and nickel, which charges the gaskets into precipitates of nitride of chromium and silicon, as well as carbonitrides.
Un autre type de procédé de traitement connu, présenté notamment par le document JP-B2-4655528, comporte une première étape de montée en température pour atteindre 800 °C suivie d'une premère phase de nitruration à cette température, ensuite une deuxième étape de montée en température à 980 ° C, puis une succession de cycles comprenant chacun une phase de cémentation suivie d'une diffusion en maintenant cette température, ensuite une étape de descente de température à 850 ° C suivie d'une deuxième phase de nitruration à cette température, et enfin la phase de trempe du métal.  Another type of known process of treatment, presented in particular by the document JP-B2-4655528, comprises a first stage of rise in temperature to reach 800 ° C followed by a first phase of nitriding at this temperature, then a second step of temperature rise to 980 ° C, then a succession of cycles each comprising a cementation phase followed by diffusion by maintaining this temperature, then a temperature descent step at 850 ° C followed by a second nitriding phase at this temperature. temperature, and finally the quenching phase of the metal.
On obtient pour ce procédé un temps de cémentation comprenant la succession de cycles de cémentation et de diffusion, qui est relativement long. De plus la deuxième phase de nitruration après la cémentation provoque aussi une décarburation de la couche en surface du métal.  For this process, a cementation time is obtained which comprises a succession of cementation and diffusion cycles, which is relatively long. In addition, the second nitriding phase after cementation also causes a decarburization of the layer on the surface of the metal.
La présente invention a notamment pour but d'éviter ces inconvénients de la technique antérieure.  The present invention is intended to avoid these disadvantages of the prior art.
Elle propose à cet effet un procédé de traitement thermochimique de pièces en acier pour améliorer les caractéristiques mécaniques, comprenant une diffusion de carbone par cémentation et d'azote par nitruration dans un four basse pression suivant un cycle de température, caractérisé en ce qu'il comporte d'abord une montée à une première température relativement basse permettant de commencer la nitruration du métal, puis une unique phase de nitruration à une température comprise entre cette première température et une deuxième température de cémentation, puis une unique phase de cémentation, ensuite une diffusion à cette deuxième température qui reste constante, et enfin la trempe du métal. It proposes for this purpose a method of thermochemical treatment of steel parts to improve the mechanical characteristics, comprising a carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it first has a rise to a first temperature relatively low to begin the nitriding of the metal, then a single nitriding phase at a temperature between this first temperature and a second carburizing temperature, then a single carburizing phase, then a diffusion at this second temperature which remains constant, and finally the quenching of the metal.
Un avantage de ce procédé de traitement est que la cémentation n'étant pas suivie d'une phase de nitruration, on n'a pas de décarburation de la surface de la pièce, et sa dureté reste au meilleur niveau. La qualité métallurgique est aussi mieux maîtrisée, en évitant la formation de divers nitrures dans les joints de grains qui réduiraient les caractéristiques mécaniques de la pièce.  An advantage of this treatment process is that the cementation is not followed by a nitriding phase, there is no decarburization of the surface of the workpiece, and its hardness remains at the best level. The metallurgical quality is also better controlled, avoiding the formation of various nitrides in the grain boundaries that would reduce the mechanical characteristics of the part.
De plus l'unique phase de nitruration étant faite entre la première température relativement basse permettant cette opération, et la deuxième température de cémentation, on peut dans certains cas réaliser simultanément pendant cette nitruration des montées en température, ce qui permet de gagner du temps sur le cycle complet de traitement.  In addition, the only nitriding phase being made between the first relatively low temperature allowing this operation, and the second carburizing temperature, it is possible in certain cases to produce temperature rises simultaneously during this nitriding, which saves time on the complete cycle of treatment.
Le procédé de traitement thermochimique selon l'invention peut de plus comporter une ou plusieurs des caractéristiques suivantes, qui peuvent être combinées entre elles.  The thermochemical treatment process according to the invention may further comprise one or more of the following characteristics, which may be combined with each other.
Selon un mode de réalisation, la phase de nitruration est réalisée à la première température qui reste constante.  According to one embodiment, the nitriding phase is carried out at the first temperature which remains constant.
La phase de nitruration peut être suivie avant la phase de cémentation, par une phase de diffusion comprenant une montée en température allant de la première température à la deuxième température de cémentation.  The nitriding phase can be followed before the carburizing phase, by a diffusion phase comprising a rise in temperature from the first temperature to the second carburizing temperature.
En variante la phase de nitruration peut être suivie juste après par la phase de cémentation, comprenant d'abord une montée en température allant de la première température à la deuxième température de cémentation.  Alternatively, the nitriding phase can be followed immediately after the carburising phase, first comprising a temperature rise from the first temperature to the second carburizing temperature.
Selon un autre mode de réalisation, la phase de nitruration comprend une montée en température allant de la première température à la deuxième température de cémentation. La phase de nitruration peut être suivie avant la phase de cémentation, par une phase de diffusion réalisée à la deuxième température qui reste constante. According to another embodiment, the nitriding phase comprises a rise in temperature from the first temperature to the second carburizing temperature. The nitriding phase can be followed before the carburizing phase by a diffusion phase carried out at the second temperature which remains constant.
Avantageusement, la première température relativement basse permettant de commencer la nitruration du métal, est de l'ordre de 700 °C.  Advantageously, the first relatively low temperature for starting the nitriding of the metal is of the order of 700 ° C.
Avantageusement, la deuxième température de cémentation est de l'ordre de 940 °C.  Advantageously, the second carburizing temperature is of the order of 940 ° C.
L'invention a aussi pour objet un pignon en acier pour une boîte de vitesses de véhicule automobile, renforcé par un traitement thermochimique comprenant l'une quelconque des caractéristiques précédentes.  The invention also relates to a steel gear for a motor vehicle gearbox, reinforced by a thermochemical treatment comprising any one of the preceding features.
L'invention sera mieux comprise et d'autres caractéristiques et avantages apparaîtront plus clairement à la lecture de la description ci-après, donnée à titre d'exemple et de manière non limitative en référence aux dessins annexés dans lesquels :  The invention will be better understood and other features and advantages will appear more clearly on reading the following description, given by way of example and without limitation with reference to the accompanying drawings in which:
- la figure 1 est une coupe micrographique d'un acier austénitique traité par un procédé de carbonitruration selon l'art antérieur ;  FIG. 1 is a micrographic section of an austenitic steel treated by a carbonitriding process according to the prior art;
- la figure 2 une coupe micrographique de cet acier traité par un procédé de carbonitruration selon l'invention ;  FIG. 2 is a micrographic section of this steel treated by a carbonitriding process according to the invention;
- la figure 3 est un graphique présentant en fonction du temps le procédé de traitement selon l'invention ;  FIG. 3 is a graph showing, as a function of time, the treatment method according to the invention;
- la figure 4 est un graphique présentant le procédé de traitement suivant une première variante ; et  FIG. 4 is a graph showing the treatment method according to a first variant; and
- la figure 5 est un graphique présentant le procédé de traitement suivant une deuxième variante.  FIG. 5 is a graph showing the treatment method according to a second variant.
La figure 1 présente la coupe micrographique 6 d'un acier austénitique peu chargé en carbone, réalisée sur une hauteur de quelques dizaines de micromètres en dessous de la surface 2. Cet acier a subi un traitement de carbonitruration à basse pression conforme au procédé présenté par le document FR-B1 -2884523, comprenant des phases successives de cémentation et de nitruration avant de réaliser une trempe finale. On observe à la jointure des grains des précipités importants de nitrures 4 formant des taches noires, suite à l'apport d'ammoniac pendant toute la durée de la phase de traitement donnant un enrichissement en carbone et azote. Ces précipités réduisent la résistance mécanique de la couche de surface, ce qui peut causer une usure prématurée de pièces fortement sollicitées avec des cycles de contraintes, comme les engrenages des pignons de boîtes de vitesses pour véhicules automobiles. FIG. 1 shows the micrographic section 6 of a low carbon austenitic steel, made to a height of a few tens of micrometers below the surface 2. This steel has undergone a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching. At the joint of the grains, large precipitates of nitrides 4 forming black spots are observed, following the supply of ammonia throughout the duration of the treatment phase giving an enrichment of carbon and nitrogen. These precipitates reduce the mechanical strength of the surface layer, which can cause premature wear of highly stressed parts with stress cycles, such as gears of gearboxes for motor vehicles.
La figure 2 présente la coupe micrographique d'un acier austénitique similaire, ayant subi un traitement de carbonitruration à basse pression conforme au procédé suivant l'invention. On constate pour la couche de quelques dizaines de micromètres sous la surface 2, une absence de précipité de nitrure dans les joints des grains.  Figure 2 shows the micrographic section of a similar austenitic steel, having undergone a low pressure carbonitriding treatment according to the process according to the invention. For the layer a few tens of micrometers below the surface 2, there is an absence of nitride precipitate in the grain boundaries.
La figure 3 présente un premier cycle de traitement de carbonitruration suivant l'invention réalisé dans un four basse pression, indiquant la température T° en fonction du temps t. La pièce en acier à traiter est au temps tO, à la température ambiante.  FIG. 3 shows a first carbonitriding treatment cycle according to the invention carried out in a low pressure furnace, indicating the temperature T ° as a function of time t. The piece of steel to be treated is at time t0, at room temperature.
On a d'abord jusqu'au premier temps t1 une première étape M de montée en température progressive du four, pour atteindre un premier palier de température T1 de nitruration, qui est dans cet exemple de 700 °C.  Firstly, up to the first time t1, a first step M of progressive temperature rise of the furnace is made to reach a first temperature level T1 of nitriding, which in this example is 700 ° C.
On a ensuite une deuxième étape de nitruration N, qui se poursuit jusqu'au deuxième temps t2 en restant à ce premier palier de température T1 .  Then there is a second nitriding step N, which continues until the second time t2 remaining at this first temperature level T1.
On notera que les températures idéales de nitruration se situent entre 750 et 800 °C. On peut toutefois commencer ce traitanent à 700 ° C et le poursuivre jusqu'à 900 ° C si l'on tolère un peu moins d'absorption d'azote dans la pièce, afin de gagner du temps de traitement en réduisant le temps de montée en température.  It should be noted that the ideal nitriding temperatures are between 750 and 800 ° C. However, it can be started at 700 ° C and continued up to 900 ° C if you tolerate a little less nitrogen absorption in the room, to save processing time by reducing the rise time in temperature.
On a ensuite jusqu'au troisième temps t3 une troisième étape de diffusion D comprenant une montée en température sous une atmosphère neutre, pour atteindre un deuxième palier de température T2 de cémentation, qui est dans cet exemple de 940 °C. Pendant cette toisième étape on réalise une diffusion D de l'azote dans la couche de surface du métal. Then, until the third time t3, a third diffusion step D comprises a rise in temperature under a neutral atmosphere, to reach a second temperature plateau T2 of carburizing, which in this example is 940 ° C. During this third stage, diffusion D of nitrogen is carried out in the metal surface layer.
On a ensuite une quatrième étape de cémentation C, qui se poursuit jusqu'au quatrième temps t4 en restant à ce deuxième palier de température de cémentation T2.  Then there is a fourth cementation step C, which continues until the fourth time t4 remaining at this second level of cementation temperature T2.
On notera que la température de cémentation peut varier, avec une profondeur de traitement qui augmente en fonction de cette température. Par exemple pour obtenir la même profondeur de traitement correspondant au traitement normalisé « E650 », il faut un temps de 3h à 880°C, et de 2h à 940 ° C. Il n'est pas recommandé de réaliser une cémentation au-dessus de 950 ° C car cela crée un grossissement important des grains martensitiques, et génère des pertes de tenue fonctionnelle des pièces.  It will be noted that the carburizing temperature may vary, with a treatment depth which increases as a function of this temperature. For example, to obtain the same processing depth corresponding to the standardized treatment "E650", it takes a time of 3h to 880 ° C, and 2h to 940 ° C. It is not recommended to carry out a cementation above 950 ° C because it creates a large magnification of martensitic grains, and generates losses of functional performance of the parts.
On a ensuite une cinquième étape de deuxième diffusion D dans un gaz neutre, qui se poursuit jusqu'au cinquième temps t5 en restant à ce deuxième palier de température de cémentation T2. Cette étape de deuxième diffusion D est optionnelle, elle dépend du taux de carbone souhaité en fin de traitement.  Then there is a fifth step of second diffusion D in a neutral gas, which continues until the fifth time t5 remaining at this second level of cementation temperature T2. This second diffusion step D is optional, it depends on the desired carbon content at the end of treatment.
On a enfin la dernière étape de trempe T de la pièce qui peut être faite dans un gaz, ou dans un liquide comme une huile ou un polymère.  Finally, there is the last quench step T of the part which can be made in a gas, or in a liquid such as an oil or a polymer.
On obtient ainsi à la fois un temps de traitement relativement court qui réduit les coûts de production, et un taux d'absorption de carbone et d'azote intéressant sans réaliser toutefois de précipité de nitrures au niveau des joints des grains du matériau.  This gives both a relatively short treatment time that reduces production costs, and a rate of carbon and nitrogen absorption interesting without, however, producing a nitride precipitate at the grain boundaries of the material.
La figure 4 présente en variante un deuxième cycle de traitement de carbonitruration suivant l'invention.  FIG. 4 alternatively shows a second carbonitriding treatment cycle according to the invention.
On a d'abord la première étape M de montée en température qui est identique, pour atteindre au premier temps t1 la température T1 permettant la nitruration.  First, there is the first step M of temperature rise which is identical, to reach at the first time t1 the temperature T1 allowing nitriding.
On a ensuite la deuxième étape de nitruration N qui se réalise en même temps qu'une montée en température lente et progressive, pour atteindre au deuxième temps t2 le deuxième palier de température T2 de cémentation. On a ensuite la troisième étape de diffusion D de l'azote dans la couche de surface du métal, qui se poursuit jusqu'au troisième temps t3 en restant à ce deuxième palier de température de cémentation T2. Then there is the second nitriding step N which is carried out at the same time as a slow and gradual rise in temperature, to reach the second time t2 the second temperature plateau T2 of carburizing. Then there is the third diffusion step D of the nitrogen in the metal surface layer, which continues until the third time t3 remaining at this second level of cementation temperature T2.
Les étapes suivantes identiques à celles présentées figure 3, comportent la quatrième étape de cémentation C jusqu'au quatrième temps t4 et la cinquième étape de diffusion D jusqu'au cinquième temps t5, en restant à ce deuxième palier de température de cémentation T2. On a enfin la dernière étape de trempe T.  The following steps, identical to those presented in FIG. 3, comprise the fourth cementation step C up to the fourth time t4 and the fifth diffusion step D up to the fifth time t5, remaining at this second cementation temperature step T2. Finally, we have the last stage of tempering T.
Cette variante permet d'optimiser le temps de traitement, en réduisant le palier de température entre la phase de nitruration N et la phase de cémentation C. Toutefois la nitruration étant réalisée avec une température variable montant au-dessus de son niveau optimum, la concentration en azote sur la pièce est plus faible avec un même temps de nitruration.  This variant makes it possible to optimize the treatment time, by reducing the temperature step between the nitriding phase N and the cementation phase C. However, the nitriding being carried out with a variable temperature rising above its optimum level, the concentration Nitrogen on the part is weaker with the same nitriding time.
La figure 5 présente en variante un troisième cycle de traitement de carbonitruration suivant l'invention.  FIG. 5 alternatively presents a third carbonitriding treatment cycle according to the invention.
On a d'abord la première étape M de montée en température qui est identique, pour atteindre au premier temps t1 la première température de nitruration T1 .  Firstly, the first step M of temperature rise is identical, to reach at the first time t1 the first nitriding temperature T1.
On a ensuite la deuxième étape de nitruration N qui se poursuit jusqu'au deuxième temps t2, en restant au palier de la première température de nitruration T1 comme pour le procédé présenté figure 3.  Then there is the second nitriding step N which continues until the second time t2, remaining at the level of the first nitriding temperature T1 as for the process shown in FIG.
On n'a ensuite pas d'étape de diffusion de l'azote D, mais à la place une troisième étape de début de cémentation C qui commence de suite, avec jusqu'à un troisième temps t3 une montée en température régulière pour atteindre le deuxième palier de température T2 de cémentation.  There is then no nitrogen D diffusion step, but instead a third cementation start step C which begins immediately, with up to a third time t3 a steady temperature rise to reach the second level of temperature of cementation T2.
Les étapes suivantes sont identiques à celles présentées figure 3, comportant la quatrième étape qui est la poursuite de la cémentation C jusqu'au quatrième temps t4 et la cinquième étape de diffusion D jusqu'au cinquième temps t5, en restant à ce deuxième palier de température T2. On a enfin la dernière étape de trempe T. Cette variante permet aussi d'optimiser le temps de traitement, en réduisant ou supprimant l'étape de diffusion D entre la nitruration N et la cémentation C, sans toutefois affecter le taux d'azote dans la pièce. The following steps are identical to those presented in FIG. 3, comprising the fourth step which is the continuation of carburizing C to the fourth time t4 and the fifth diffusion step D to the fifth time t5, remaining at this second level of T2 temperature. Finally, we have the last stage of tempering T. This variant also makes it possible to optimize the treatment time, by reducing or eliminating the diffusion step D between the nitriding N and the carburizing C, without, however, affecting the level of nitrogen in the part.
On réalise ainsi par ces différentes variantes une meilleure maîtrise du procédé, comprenant un enrichissement en azote uniquement avant la phase de cémentation, qui permet une bonne diffusion du carbone dans la couche traitée.  These different variants thus provide a better control of the process, comprising a nitrogen enrichment only before the carburizing phase, which allows a good diffusion of the carbon in the treated layer.
En complément on peut réaliser un mixage des différentes variantes présentées, comprenant par exemple une phase de nitruration N comportant une température qui au début reste suivant le premier palier de température T1 , et qui monte ensuite progressivement pour atteindre le deuxième palier de température T2.  In addition, it is possible to mix the different variants presented, comprising, for example, a nitriding phase N comprising a temperature which initially remains at the first temperature stage T1, and which then gradually rises to reach the second temperature stage T2.
De plus ce procédé spécialement développé par l'entreprise réalisant l'invention, permet de ne pas utiliser des procédés de carbonitruration particuliers qui peuvent être la propriété de fournisseurs de fours basse pression prévus pour ce type de traitement.  In addition, this process, specially developed by the company carrying out the invention, makes it possible not to use particular carbonitriding processes which may be the property of suppliers of low pressure furnaces intended for this type of treatment.

Claims

REVENDICATIONS
1 - Procédé de traitement thermochimique de pièces en acier pour améliorer les caractéristiques mécaniques, comprenant une diffusion de carbone par cémentation et d'azote par nitruration dans un four basse pression suivant un cycle de température, caractérisé en ce qu'il comporte d'abord une montée à une première température (T1 ) relativement basse de l'ordre de 700 °C permettant de commencer la nitrurâion du métal, puis une unique phase de nitruration (N) à une température comprise entre cette première température (T1 ) et une deuxième température de cémentation (T2), puis une unique phase de cémentation (C), ensuite une diffusion (D) à cette deuxième température (T2) qui reste constante, et enfin la phase de trempe (T) du métal. 1 - Process for the thermochemical treatment of steel parts to improve the mechanical characteristics, comprising carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it comprises first a rise to a first temperature (T1) relatively low of the order of 700 ° C to begin the nitrurâion of the metal, then a single nitriding phase (N) at a temperature between this first temperature (T1) and a second carburizing temperature (T2), then a single cementation phase (C), then a diffusion (D) at this second temperature (T2) which remains constant, and finally the quenching phase (T) of the metal.
2 - Procédé de traitement selon la revendication 1 , caractérisé en ce que la phase de nitruration (N) est réalisée à la première température (T1 ) qui reste constante.  2 - Process according to claim 1, characterized in that the nitriding phase (N) is carried out at the first temperature (T1) which remains constant.
3 - Procédé de traitement selon la revendication 2, caractérisé en ce que la phase de nitruration (N) est suivie avant la phase de cémentation (C), par une phase de diffusion (D) comprenant une montée en température allant de la première température (T1 ) à la deuxième température de cémentation (T2).  3 - Treatment process according to claim 2, characterized in that the nitriding phase (N) is followed before the carburizing phase (C) by a diffusion phase (D) comprising a rise in temperature from the first temperature (T1) at the second carburizing temperature (T2).
4 - Procédé de traitement selon la revendication 2, caractérisé en ce que la phase de nitruration (N) est suivie juste après par la phase de cémentation (C) comprenant d'abord une montée en température allant de la première température (T1 ) à la deuxième température de cémentation (T2).  4 - Process according to claim 2, characterized in that the nitriding phase (N) is followed immediately after the carburizing phase (C) comprising first a temperature rise from the first temperature (T1) to the second carburizing temperature (T2).
5 - Procédé de traitement selon la revendication 1 , caractérisé en ce que la phase de nitruration (N) comprend une montée en température allant de la première température (T1 ) à la deuxième température de cémentation (T2).  5 - Process according to claim 1, characterized in that the nitriding phase (N) comprises a temperature rise from the first temperature (T1) to the second cementation temperature (T2).
6 - Procédé de traitement selon la revendication 5, caractérisé en ce que la phase de nitruration (N) est suivie avant la phase de cémentation (C), par une phase de diffusion (D) réalisée à la deuxième température (T2) qui reste constante. 6 - Process according to claim 5, characterized in that the nitriding phase (N) is followed before the carburizing phase (C), by a diffusion phase (D) carried out at the second temperature (T2) which remains constant.
7 - Procédé de traitement selon l'une quelconque des revendications précédentes, caractérisé en ce que la deuxième température de cémentation (T2) est de l'ordre de 940 ° C.  7 - Process according to any one of the preceding claims, characterized in that the second carburizing temperature (T2) is of the order of 940 ° C.
8 - Pignon en acier pour une boîte de vitesses de véhicule automobile renforcé par un traitement thermochimique, caractérisé en ce que ce traitement est réalisé suivant l'une quelconque des revendications précédentes.  8 - Steel pinion for a motor vehicle gearbox reinforced by a thermochemical treatment, characterized in that this treatment is carried out according to any one of the preceding claims.
PCT/FR2014/050581 2013-04-18 2014-03-13 Thermochemical treatment method comprising a single nitriding phase before carburising WO2014170566A1 (en)

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