EP3218530B1 - Method and facility for carbonitriding one or more steel parts under low pressure and at a high temperature - Google Patents

Description

The invention relates to certain thermochemical treatments which are intended to reinforce steel parts, and more precisely the carbonitriding of such steel parts.

In certain fields, such as that of vehicles, possibly motor vehicles, it is essential to reinforce the resistance of certain steel parts, and more precisely at least their resistance to fatigue, so that they can withstand significant stresses and / or in order to increase their lifespan. Such reinforcement can be obtained by carbonitriding.

It is recalled that carbonitriding is a thermochemical diffusion treatment which consists in enriching the surface of a steel with carbon and nitrogen, before a quenching step, so as to obtain a martensitic structure and a reinforcement. The nitrogen enrichment, here carried out in the austenitic phase, is called nitriding in the a phase, and the carbon enrichment is called carburization. The nitriding in the α phase (or austenitic phase) is intended to improve the fatigue life and the stability of the metallurgical structure of the steel by nitrogen penetration. Case hardening consists in bringing carbon into a steel part in order to increase its ability to be hardened and therefore allow an increase in its surface hardness, and its resistance to fatigue and wear.

Quenching is a rapid cooling in a liquid or gaseous medium which causes the appearance of a martensitic structure having a very high hardness.

As those skilled in the art know, the known carbonitriding treatments are long and give non-optimal metallurgical results because they result from compromise. In fact, they use relatively low treatment temperatures (typically around 850 ° C.) in order to optimize nitrogen enrichment (and more precisely to avoid that the major part of the ammonia (NH ₃ ) nitriding in α phase is cracked before even touching the part), but to the detriment of carbon enrichment ( which requires higher temperatures) and processing time (which must be increased due to the relatively low processing temperature).

The documents WO 2014/1770566 and FR 2 884 523 describe carbonitriding processes. The document DE 10 2013 006589 describes a carbonitriding installation.

The invention therefore aims in particular to improve the situation.

• une première étape dans laquelle on chauffe chaque pièce à une première température choisie, dans un environnement contenant un gaz neutre et sous une pression choisie,
• une deuxième étape dans laquelle on enrichit en azote dans une première chambre la pièce chauffée, par nitruration en phase α sous une deuxième température choisie inférieure à la première température,
• une troisième étape dans laquelle on enrichit en carbone dans une seconde chambre la pièce enrichie en azote, par cémentation sous une troisième température choisie strictement supérieure à la deuxième température, et
• une quatrième étape dans laquelle on trempe sous pression la pièce enrichie en azote et en carbone.

To this end, it proposes in particular a process, intended to allow carbonitriding of at least one steel part, and comprising:

• a first step in which each part is heated to a first chosen temperature, in an environment containing a neutral gas and under a selected pressure,
• a second step in which the heated part is enriched with nitrogen in a first chamber, by nitriding in the α phase at a second chosen temperature lower than the first temperature,
• a third step in which the part enriched in nitrogen is enriched in carbon in a second chamber, by carburizing under a third temperature chosen strictly higher than the second temperature, and
• a fourth step in which the part enriched in nitrogen and carbon is hardened under pressure.

As the temperature of the part is warmer than that at which the nitriding is carried out in the α phase, this prevents the nitriding gas from cracking instantly on contact and therefore it is made much more available for nitrogen enrichment. In addition, this allows better diffusion of nitrogen in the room and therefore an increase in its concentration. In addition, since the carburizing is carried out at a temperature higher than that of nitriding in phase a, the carbon enrichment of the part is thus more efficient and faster. Finally, the carbon enrichment step being carried out in a different chamber from that in which the nitrogen enrichment step is carried out, this makes it possible to vary the temperature very quickly between the nitrogen enrichment steps and in carbon.

• dans la première étape le gaz neutre peut être du diazote (ou N₂) ;
• dans la première étape la pression peut être comprise entre environ 1 bar et environ 1,5 bar. Mais elle pourrait être notablement plus basse, et par exemple similaire à la basse pression utilisée dans les deuxième et troisième étapes ;
• dans la première étape la première température peut être comprise entre environ 800°C et environ 1100 ° C ;
• dans la deuxième étape la deuxième température peut être comprise entre environ 700°C et environ 880°C;
• dans la deuxième étape on peut enrichir la pièce en azote par nitruration en phase α avec de l'ammoniac ;
• dans la troisième étape la troisième température peut être comprise entre environ 900°C et environ 1100 ° C ;
• dans la troisième étape on peut enrichir la pièce en carbone par cémentation avec de l'acétylène ;
• dans la quatrième étape la pression de trempe peut être comprise entre environ 1 bar et environ 20 bars ;
• dans la quatrième étape la trempe peut être réalisée dans un environnement contenant un gaz choisi.

The method according to the invention may include other characteristics which can be taken separately or in combination, and in particular:

• in the first step the neutral gas can be dinitrogen (or N ₂ );
• in the first stage, the pressure can be between approximately 1 bar and approximately 1.5 bar. But it could be notably lower, and for example similar to the low pressure used in the second and third stages;
• in the first step the first temperature can be between about 800 ° C and about 1100 ° C;
• in the second step the second temperature can be between about 700 ° C and about 880 ° C;
• in the second step, the part can be enriched in nitrogen by nitriding in the α phase with ammonia;
• in the third step the third temperature can be between approximately 900 ° C and approximately 1100 ° C;
• in the third step, the part can be enriched in carbon by carburizing with acetylene;
• in the fourth step the quenching pressure can be between approximately 1 bar and approximately 20 bars;
• in the fourth step the quenching can be carried out in an environment containing a chosen gas.

• au moins une chambre de chauffage propre à chauffer au moins une pièce en acier à une première température choisie, dans un environnement contenant un gaz neutre et sous une pression choisie,
• au moins une première chambre d'enrichissement propre à enrichir en azote la pièce chauffée, par nitruration en phase α sous une deuxième température choisie inférieure ou égale à la première température,
• au moins une seconde chambre d'enrichissement propre à enrichir en carbone la pièce enrichie en azote, par cémentation sous une troisième température choisie strictement supérieure à la deuxième température,
• au moins une chambre de trempe propre à tremper sous pression la pièce enrichie en azote et en carbone,
• un sas de transfert communiquant de façon contrôlée avec chacune des chambres et propre à accueillir temporairement la pièce dans un environnement où règne une atmosphère contrôlée, et
• des moyens de transfert propres à transférer la pièce d'une chambre à une autre chambre via le sas de transfert.

The invention also provides an installation, dedicated to the carbonitriding of steel parts, and comprising:

• at least one heating chamber suitable for heating at least one steel part to a first chosen temperature, in an environment containing a neutral gas and at a selected pressure,
• at least a first enrichment chamber capable of enriching the heated part with nitrogen, by nitriding in the α phase at a second selected temperature lower than or equal to the first temperature,
• at least one second enrichment chamber capable of enriching the nitrogen-enriched part with carbon, by carburizing under a third temperature chosen strictly higher than the second temperature,
• at least one quenching chamber suitable for quenching under pressure the part enriched in nitrogen and carbon,
• a transfer airlock communicating in a controlled manner with each of the bedrooms and suitable for temporarily accommodating the room in an environment where a controlled atmosphere prevails, and
• transfer means suitable for transferring the room from one room to another room via the transfer airlock.

• la figure 1 illustre schématiquement et fonctionnellement un exemple de réalisation d'une installation de carbonitruration selon l'invention, et
• la figure 2 illustre schématiquement un exemple d'algorithme mettant en œuvre un procédé de carbonitruration selon l'invention.

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings, in which:

• the figure 1 illustrates schematically and functionally an embodiment of a carbonitriding installation according to the invention, and
• the figure 2 schematically illustrates an example of an algorithm implementing a carbonitriding method according to the invention.

The object of the invention is in particular to propose a method, and an associated IC installation, intended to allow carbonitriding of part (s) made of PA steel at high temperature and at low pressure.

In what follows, it is considered, by way of nonlimiting example, that the PA steel parts are intended to equip a vehicle, possibly of automobile type. For example, it could be gearbox parts, transmission parts, or various gears. However, the invention is not limited to this application. It concerns in fact any steel part intended to equip a device, an apparatus, a system (and in particular a vehicle, whatever its type), or an installation (possibly of industrial type). Thus, it also relates in particular to certain transmission elements in the aeronautical field, and in general the parts which are mechanically stressed in wear and in fatigue.

A process for carbonitriding PA steel part (s) comprises at least first, second, third and fourth steps.

Such a process can be implemented by a carbonitriding installation IC of the type which is illustrated without limitation on the figure 1 .

As illustrated in the figure 1 , a carbonitriding installation IC, according to the invention, comprises at least one CC heating chamber, at least one first enrichment chamber CE1, at least one second enrichment chamber CE2, at least one quenching chamber CT, one ST transfer lock, and MT transfer means.

The transfer airlock ST comprises an input ES with controlled access and through which each PA (steel) part to be treated is introduced, and an output SS with controlled access and by which the PA part treated is extracted. For example, the input ES and the output SS each comprise a single or double sliding door, sealed, electrically or pneumatically controlled, and ensuring the sealed interface. This transfer lock ST communicates in a controlled manner with each of the chambers CC, CE1, CE2 and CT, and is suitable for temporarily accommodating the room PA, during each of its transfers from one room to another, in an environment where there is a controlled atmosphere intended to prevent its oxidation.

This controlled atmosphere can be a selected vacuum, preferably between about 2 millibars and about 50 millibars, and it can be neutral (for example defined by a neutral gas such as dinitrogen (or N ₂ )).

It will be noted that each PA part is preferably placed on a tray which can accommodate one or more parts to be treated. It is considered in what follows, by way of illustrative example, that only one PA part is processed at a time.

The (each) heating chamber CC is arranged so as to heat a room PA to a first chosen temperature T1, in an environment which contains a neutral gas and under a selected pressure P1. It includes access control means, such as for example a single or double sliding door, sealed, electrically or pneumatically controlled, and ensuring the sealed interface with the transfer lock ST.

For example, the neutral gas can be dinitrogen (or N ₂ ).

Also for example, the pressure P1 can be substantially equal to atmospheric pressure. Thus, it can, for example, be between approximately 1 bar and approximately 1.5 bar. But in a more economical variant, this pressure P1 can be similar (or identical) to the low pressure which is used in the enrichment chambers CE1 and CE2 (typically a few millibars).

Preferably, the first temperature T1 is between approximately 800 ° C and approximately 1100 ° C. For example, it can be chosen equal to 1050 ° C.

The (each) first enrichment chamber CE1 is arranged so as to enrich with nitrogen, under a low pressure, the part PA which has been heated in the (one) heating chamber CC, by nitriding in α phase at a second temperature. T2 chosen lower or equal to the first temperature T2 (i.e. T2 ≤ T1). Preferably, this second temperature T2 is strictly lower than the first temperature T2 (ie T2 <T1). It includes access control means, such as for example a single or double sliding door, sealed, electrically or pneumatically controlled, and ensuring the sealed interface with the transfer lock ST.

Preferably, the second temperature T2 is between about 700 ° C and about 880 ° C. For example, it can be chosen equal to 830 ° C.

For example, to carry out nitrogen enrichment by nitriding in the α phase, it is possible to use gaseous ammonia (or NH ₃ ). This gas constitutes the atmosphere inside the first enrichment chamber CE1.

The (each) second enrichment chamber CE2 is arranged so as to enrich in carbon, under a low pressure, the part PA which has been enriched in nitrogen in the (a) first enrichment chamber CE1, by carburizing under a third temperature T3 chosen strictly higher than the second temperature T2 (ie T3> T2). It includes access control means, such as for example a single or double sliding door, sealed, electrically or pneumatically controlled, and ensuring the sealed interface with the transfer lock ST.

Preferably, the third temperature T3 is between approximately 900 ° C and approximately 1100 ° C. For example, it can be chosen equal to 1050 ° C.

For example, to carry out carbon enrichment by carburizing, acetylene (or C ₂ H ₂ ) gas can be used. This gas constitutes the atmosphere inside the second CE2 enrichment chamber. However, other carburizing gases can be used, and in particular propane.

The (each) quenching chamber CT is arranged so as to harden under pressure the part PA which has been enriched with nitrogen and carbon in the first (s) CE1 and second (s) enrichment chambers. This quenching is preferably carried out under a fourth temperature T4 chosen close to ambient temperature and under a pressure P2 which is greater than or equal to atmospheric pressure. It includes access control means, such as for example a single or double sliding door, sealed, electrically or pneumatically controlled, and ensuring the sealed interface with the transfer lock ST.

For example, the quenching pressure P2 can be between approximately 1 bar and approximately 20 bars. Thus, it can, for example, be chosen equal to around 15 bars for steels containing little alloy.

It will be noted that the increase in the quenching pressure makes it possible to quench the PA parts more strongly but generates more deformations. The choice of pressure is therefore a compromise between the hardenability of the steel, the deformations and the hardness that we aim to obtain.

The quenching can be carried out by immersion in an environment which contains a chosen gas, such as for example nitrogen or helium. The quenching gas then constitutes the atmosphere inside the quenching chamber CT.

Alternatively, the quenching can be carried out by immersion in an environment which contains a chosen liquid, such as for example oil or a polymer.

The transfer means MT are arranged so as to transfer the part PA from one room to another room via the transfer lock ST. They include for example a motorized trolley (preferably electrically), comprising a plate capable of supporting at least one PA part, and mounted in translation on rails which are fixedly installed in the transfer lock ST and which communicate with the outside ( via the ES input and SS output of the transfer airlock ST) and with the different CC, CE1, CE2 and CT chambers in order to allow the transfer of the PA part.

A first step, of the method according to the invention, is carried out once at least one part PA has been installed in the (one) heating chamber CC by means of the transfer means MT (arrows F1 and F2 of the figure 1 ). This installation corresponds to sub-step 10 of the example algorithm of the figure 2 .

In this first step, the part PA is heated to the first temperature T1 chosen, in an environment containing a neutral gas (such as dinitrogen, as mentioned above), and under a pressure P1 chosen (possibly substantially equal to atmospheric pressure ).

Such heating in a neutral atmosphere and under a low pressure makes it possible to have a rate of heating of the part PA substantially faster than in the case of heating under vacuum. For example, to bring the temperature of a PA room to around 1050 ° C in a neutral atmosphere and under about 1 bar, it takes about an hour, while it takes about an hour and a quarter under vacuum. This frees up the DC heating chamber more quickly.

The first step corresponds to sub-step 20 of the example algorithm of the figure 2 .

A second step, of the method according to the invention, is carried out once the part PA has been heated to the first temperature T1 in the heating chamber CC, then installed in the (one) first enrichment chamber CE1 by means of the MV transfer means (arrows F2, F3 and F4 of the figure 1 ).

In this second step, the heated PA part is enriched in nitrogen, under low pressure (typically a few millibars), by nitriding in the α phase under the second chosen temperature T2 (less than or equal to the first temperature T1, and preferably strictly lower at T1).

The temperature T1 of the part PA being preferably initially warmer than the temperature T2 at which the nitriding is carried out in phase a, it is avoided that the nitriding gas is instantly cracked at its contact and therefore we make this gas much more available for nitrogen enrichment. In addition, this allows better diffusion of nitrogen in the PA room and therefore an increase in its concentration, in accordance with Fick's law.

Note that a maximum enrichment of the PA part with nitrogen is expected between about 800 ° C and about 850 ° C when using ammonia as nitriding gas. Indeed from about 900 ° C, the ammonia cracks 99% instantly in the atmosphere and is no longer available to enrich the PA part with nitrogen.

It will also be noted that the duration of the nitriding in the α phase can be equal to approximately ten minutes. This duration is a function of the quantity of nitrogen which it is desired to introduce into the part PA.

At the end of the nitriding in the α phase, the temperature of the part PA has become slightly lower than T1 because the nitriding temperature in the α phase T2 is strictly lower than T1. For example, if T1 is equal to 1050 ° C and the nitriding temperature in the α phase is equal to 830 ° C, the temperature of the PA room enriched in nitrogen is equal to approximately 1010 ° C after ten minutes of α phase nitriding.

The second step corresponds to sub-step 30 of the example algorithm of the figure 2 .

A third step, of the method according to the invention, is carried out once the part PA has been enriched with nitrogen in the first enrichment chamber CE1, then installed in the (one) second enrichment chamber CE2 by means MT transfer (arrows F4, F5 and F6 of the figure 1 ).

In this third step, the PA part already enriched in nitrogen is enriched in carbon, under low pressure (typically a few millibars), by carburizing under the third temperature T3 chosen (strictly higher than the second temperature T2).

The higher the third carburizing temperature T3, the more efficient and rapid the enrichment of the PA part with carbon. For example, to obtain by cementation a conventional depth called 0.4 mm E650, it takes approximately 210 minutes of treatment when the third carburizing temperature T3 is equal to 900 ° C, while it takes only 15 minutes when the third carburizing temperature T3 is equal to 1050 ° C .

It should be noted, however, that it is not recommended to use a third carburizing temperature T3 greater than 1100 ° C., as this induces a strong deterioration in the metallurgy of steels by grain enlargement. Furthermore, for the third carburizing temperatures T3 greater than 950 ° C., it is preferable to initially add alloying elements (such as, for example, niobium) to the steel of the PA part in order to prevent the magnification of the grains.

It will also be noted that the duration of the third stage can be equal to approximately fifteen minutes (ten minutes for the effective carburizing under acetylene, then five minutes for the complete diffusion of carbon in the PA part under nitrogen). This duration depends on the desired treatment depth in the PA part.

At the end of the carburizing, the temperature of the part PA has become equal to T3 because the carburizing temperature T3 is strictly higher than that which it presents at the outlet of the first enrichment chamber CE1.

The third step corresponds to sub-step 40 of the example algorithm of the figure 2 .

A fourth step, of the method according to the invention, is carried out once the part PA has been enriched with nitrogen and carbon in the first CE1 and second CE2 enrichment chambers, then installed in the (one) quenching chamber CT by means of the transfer means MT (arrows F6, F7 and F8 of the figure 1 ).

In this fourth step, the PA part enriched with nitrogen and carbon is quenched (or rapidly cooled) under pressure P2.

The fourth quenching temperature T4 is for example the ambient temperature, typically equal to around 20 ° C.

The quenching pressure P2 used is preferably between about 1 bar and about 20 bars. These much larger values than that of the low pressure used in the second and third stages make it possible to increase the cooling rate. A very fast speed makes it possible to transform the austenite enriched in nitrogen and carbon in order to form martensite and to appreciably increase the hardness of the PA part.

It will be noted that the duration of the quenching can be between approximately 2 minutes and approximately 5 minutes. This duration is mainly a function of the dimensions of the PA parts to be treated and the initial chemical composition of the steel.

The fourth step corresponds to sub-step 50 of the example algorithm of the figure 2 .

At the end of the quenching, the part PA left the heating chamber CC then the transfer airlock ST (via its output SS) by the transfer means MT (arrows F8 and F9 of the figure 1 ).

It will also be noted that the carbonitriding installation IC according to the invention may possibly comprise at least one other CC heating chamber to allow almost continuous supply of the first enrichment chamber CE1 in which the treatment time is significantly shorter. as the heating time, and / or at least one other first enrichment chamber CE1 for treating several PA pieces in parallel and / or for carrying out an enrichment in additional nitrogen, and / or at least one other second enrichment chamber CE2 for treating several PA parts in parallel and / or for carrying out an additional carbon enrichment, and / or at least one other quenching chamber CT for treating several PA parts in parallel. In particular, it is possible to envisage carrying out a second nitriding in the α phase after the carburizing to obtain a high nitrogen concentration at the surface of the part PA.

• une importante réduction du temps de traitement par rapport à une carbonitruration classique,
• une réduction notable de la consommation de gaz,
• une réduction du nombre de techniciens nécessaires au contrôle de l'installation de carbonitruration,
• une possibilité de fonctionnement en flux tendu,
• une augmentation notable de la teneur en azote dans la pièce, et donc une amélioration de ses caractéristiques fonctionnelles (et principalement de sa tenue en fatigue),
• l'obtention de pièces présentant des propriétés quasi identiques,
• une réduction du cout de traitement.

The invention has several advantages, among which:

• a significant reduction in treatment time compared to conventional carbonitriding,
• a significant reduction in gas consumption,
• a reduction in the number of technicians required to control the carbonitriding installation,
• possibility of just-in-time operation,
• a significant increase in the nitrogen content in the part, and therefore an improvement in its functional characteristics (and mainly in its resistance to fatigue),
• obtaining parts having almost identical properties,
• a reduction in the cost of treatment.

Claims (9)

1. A method of carbonitriding at least one steel part (PA) comprising a first step during which said part (PA) is heated up to a first selected temperature, in an environment containing a neutral gas and under a selected pressure, a second step of nitrogen enrichment in a first enrichment chamber (CE1) of said heated part (PA), by α-phase nitriding at a second temperature, a third step of carbon enrichment in a second enrichment chamber (CE2) of said nitrogen-enriched part (PA) by carburizing at a third temperature selected to be greater than said second temperature, and a fourth step during which said nitrogen- and carbon-enriched part (PA) is quenched under pressure, characterized in that the second temperature is selected to be lower than said first temperature.
2. The method according to claim 1, characterized in that in said first step, said neutral gas is nitrogen.
3. The method according to claim 1 or 2, characterized in that in said first step, said first temperature is in the range from approximately 800°C to approximately 1100°C.
4. The method according to any of claims 1 to 3, characterized in that said second step, said second temperature is in the range from approximately 700°C to approximately 880°C.
5. The method according to any of claims 1 to 4, characterized in that in said second step, said part (PA) is nitrogen-enriched by α-phase nitriding with ammonia.
6. The method according to any of claims 1 to 5, characterized in that in said third step, said third temperature is in the range from approximately 900°C to approximately 1100°C.
7. The method according to any of claims 1 to 6, characterized in that in said third step, said part (PA) is carbon-enriched by cementation with acetylene.
8. The method according to any of claims 1 to 7, characterized in that in said fourth step, said quenching pressure is in the range from approximately 1 bar to approximately 20 bars.
9. The method according to any of claims 1 to 8, characterized in that in said fourth step, said quenching is carried out in an environment containing a selected gas.

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