EP0707661B1 - Method of low pressure nitriding a metal workpiece and oven for carrying out said method - Google Patents

Description

The present invention relates to a method for nitriding at low pressure of a metal part, by steel example, to improve its properties mechanical on the surface and, in particular, its resistance to wear.

Generally, we know that it exists on time current at least three types of treatment allowing to carry out this nitriding, namely: nitriding in salt baths, ionic nitriding and gas nitriding.

Nitriding in a salt bath is a technique particularly polluting and dangerous due to toxic gas discharges and rinsing water it generates. In addition, it imposes on the workforce difficult working conditions. This is the reason which this technique tends to disappear.

Ion nitriding involves an installation relative vacuum heat treatment, specially equipped so as to generate on the parts to be treated a glow discharge in a gas atmosphere nitriding. This technique has the disadvantage of being relatively expensive and not suitable for parts of complex shape and, in particular, of shape tubular, due to cathode phenomena dig.

Gas nitriding involves bringing the parts to a treatment temperature of the order of 500 ° C to 600 ° C, and sweep them with nitriding gas, such as ammonia, associated with a compound accelerating the catalytic dissociation of ammonia, at pressure atmospheric. Such treatment, which is described in European patent application No. 0 089 885, presents the disadvantage of being relatively long and consuming large amounts of process gas. For this same reason, this type of treatment is also polluting.

To try to reduce the amount of process gas involved, it was also proposed to carry out the low pressure treatment, inside a vacuum heat treatment. However, in this case, we come up against the fact that at the processing temperature, ammonia dissociates, then recombines nitrogen atoms active in molecular nitrogen.

It is clear that this recombination process goes to against the desired goal, since only a small fraction of the injected gas composed of active nitrogen which has escaped recombination can interact with metal of the part to be treated to obtain the phenomenon of nitriding.

The use of means such as an injection rod constantly cooled which prevent gas treatment does not reach treatment temperature before being in the immediate vicinity of the room is not possible only in a very limited number of cases and not particularly suitable in the case of parts tubular. This is the case in the process described in the French patent No. 2,674,618.

The invention more particularly aims to eliminate these drawbacks.

To this end, it proposes a processing method according to which the process of nitriding is carried out in an oven by bringing the parts to a temperature of treatment of the order of 500 ° C to 600 ° C, in a low atmosphere pressure, with injection on the parts of a treatment gas.

• une phase de purgeage à une pression égale ou inférieure à 10^-2 mbars,
• une phase de dépassivation,
• une phase de traitement comprenant l'injection dans le four d'un gaz de traitement consistant en un mélange gazeux comprenant au moins de l'ammoniac, ainsi qu'un catalyseur favorisant la dissociation de l'ammoniac au contact des pièces à traiter, et s'opposant à la recombinaison de l'azote actif provenant de cette dissociation en azote moléculaire, cette phase de traitement s'effectuant à la température de traitement, à une pression régulée à une valeur comprise entre 200 et 400 mbars, déterminée en fonction de la nature du traitement, avec un taux de renouvellement constant de l'atmosphère gazeuse, le gaz de traitement injecté étant dilué dans une quantité d'azote moléculaire variable afin de contrôler le pouvoir nitrurant du mélange et les nuances de la couche nitrurée.

According to the invention, this process is characterized in that it comprises the following successive phases:

• a purging phase at a pressure equal to or less than 10 ^-2 mbar,
• a depassivation phase,
• a treatment phase comprising the injection into the oven of a treatment gas consisting of a gas mixture comprising at least ammonia, as well as a catalyst promoting the dissociation of ammonia on contact with the parts to be treated, and opposing the recombination of active nitrogen originating from this dissociation into molecular nitrogen, this treatment phase being carried out at the treatment temperature, at a pressure regulated to a value between 200 and 400 mbar, determined as a function of the nature of the treatment, with a constant rate of renewal of the gaseous atmosphere, the injected treatment gas being diluted in a variable quantity of molecular nitrogen in order to control the nitriding power of the mixture and the nuances of the nitrided layer.

Advantageously, the above-mentioned catalyst may consist of nitrous oxide (N ₂ O), carbon monoxide (CO), or even their hydrocarbon such as methane or propane.

As previously mentioned, the nitriding power of mixture and, therefore, the type or nature of the nitrided combination layer obtained, as well as the quality of the diffusion layer can be checked by diluting the gas mixture with an amount of nitrogen molecular variable depending on the desired result (elimination of iron carbonitrides in the network which weaken the room). This dilution allows in particular avoid or limit the formation of a layer of so-called white layer combination.

The process of diffusion of nitrogen in the layer surface of the metal constituting the parts to be treated is further improved by the depassivation phase prior. This depassivation phase can be obtained by injecting a passivation gas composed for example of ammonia and / or hydrogen, the room temperature then being greater than one threshold temperature of the order of 400 ° C. In reality, the depassivation treatment may start during the temperature rise phase of parts and may continue during the keeping parts at processing temperature (between 500 ° C and 600 ° C). Of course, if the oven is equipped with means for carrying out treatment with ion bombardment, this depassivation could be performed by generating a discharge on the parts luminescent in an atmosphere of hydrogen and argon at low pressure.

The oven allowing the implementation of the process previously described preferably consists of an oven of the type used for low carburization pressure which includes an enclosure, for example double walls, constantly cooled, a refractory flask, housed inside the enclosure, which delimits a laboratory inside which the parts to be treated can be placed, heating means by radiation arranged inside the laboratory and process gas injectors passing through the enclosure and the muffle to lead inside the laboratory. Of course, means are also provided for generate in the enclosure a relative vacuum and means for adjusting the process gas flow.

It turns out that contrary to unfavorable prejudices resulting from the high cost and the risk of corrosion of installation, this type of oven provides surprising results mainly due to the fact that it allows to generate a flow inside the laboratory continuous and homogeneous process gas arriving on the parts to be treated at a relatively temperature low, below the dissociation temperature. This is due to the fact that between the cooled wall of the enclosure and the muffle, the temperature stays at a level relatively low and that intense heat exchange which are mainly effected by radiation affect only the interior volume of the laboratory, i.e. areas relatively close to the parts to be treat.

As a result, the process gases do not reach the processing temperature when in contact with the parts to treat. Therefore, a significant part of the nitrogen active released during this dissociation acts on parts to be treated even before the phenomenon of recombination of active nitrogen into molecular nitrogen could happen. The gases resulting from this dissociation are then sucked up by the means used to generate the relative vacuum inside the oven.

The effects of this process combine with the effects of the catalyst gas used to obtain the results previously mentioned which concern quality as well of the results obtained, the possibilities for action on different processing parameters to get exactly the nuances desired and the quantities of gas of processing implemented (these can be very reduced due to the gain in yield obtained and the low pressure).

La figure 1 est une représentation schématique de l'installation ;

La figure 2 est un diagramme temporel illustrant un cycle de traitement de nitruration réalisable à l'aide de l'installation représentée figure 1.

An installation for implementing the method according to the invention will be described below, by way of nonlimiting example, with reference to the appended drawings in which:

Figure 1 is a schematic representation of the installation;

FIG. 2 is a time diagram illustrating a nitriding treatment cycle that can be carried out using the installation shown in FIG. 1.

In the example shown in Figure 1, the oven used is of the "cold wall" type, that is to say that it comprises a sealed enclosure 1 with double walls 2, 3 between which circulates a cooling fluid such as water. Thanks to this feature, the oven has low thermal inertia and therefore cooling rates significantly higher than those obtained in an oven with hot walls. This point is essential when dealing with nuances of steel susceptible to precipitation corrosion intergranular. Inside enclosure 1 is disposed a muffle 4 of refractory material which delimits a volume V constituting the laboratory, inside from which the parts 5 are placed on a support 6 carried through the bottom of the enclosure 1. The rooms can be arranged in bulk on several mesh elements arranged one above the other. The heating of 5 pieces inside the laboratory is provided by heating resistors 7 connected to a circuit external power supply. The oven is also equipped a gas circulation turbine which can be used in particular to accelerate the cooling of the interior volume of enclosure 1. This cooling is usually obtained by introduction of an inert gas (nitrogen or nitrogen hydrogenated) at a pressure below the pressure atmospheric, the convection of this gas being ensured by the turbine 8.

• des jauges "Pirani" ou "Penning" pour les faibles pressions, lorsqu'on veut obtenir un vide limite de purge,
• un manomètre à membrane pour la pression de travail.

The installation also involves pumping means 9 making it possible to establish, inside the enclosure, a limit vacuum of at least 10 ^-2 mbar in order to ensure a sufficient level of purge. These pumping means 9 are controlled by a regulation system 10 designed so as to maintain the most constant pressure possible in the treatment enclosure 1 during the nitriding cycle. Measuring the pressure inside the enclosure requires two types of sensors:

• "Pirani" or "Penning" gauges for low pressures, when we want to obtain a purge limit vacuum,
• a diaphragm pressure gauge for working pressure.

The admission of the treatment gases inside the oven is ensured by a gas injection circuit 11 supplied from gas sources S ₁ , S ₂ , S ₃ by means of a mixer-regulator of flow 12.

• la pression de 200 mbar correspondant à un minimum de consommation, et
• la pression de 400 mbar correspondant à un maximum de pénétration dans les alésages, les cavités ou les formes complexes.

The working pressure must make it possible to ensure correct renewal of the atmosphere with good penetration of the atmosphere into complex forms (blind holes, etc.) and to limit gas consumption and therefore releases as much as possible. In this spirit, the pressure has been fixed in a range of 200 to 400 mbar with a constant renewal rate:

• the pressure of 200 mbar corresponding to a minimum consumption, and
• the pressure of 400 mbar corresponding to a maximum penetration in bores, cavities or complex shapes.

The oven can also be equipped with treatment means by ion bombardment involving for example a high voltage electric generator connected to the enclosure wall and the support structure of the parts to be treated. As part of the process according to the invention, these processing means can be used to perform plasma assisted stripping.

a) Le conduit d'injection de gaz de traitement 11 traverse successivement la double paroi 2, 3 du four, l'espace intercalaire entre la paroi 3 et le moufle 4 avant de déboucher dans le laboratoire à proximité des pièces à traiter 5 et, de préférence, à distance des résistances électriques 7. De ce fait, avant de pénétrer dans le laboratoire, le gaz ne subit pas d'échauffement notoire.

b) Le conduit d'aspiration 13 passe par la structure de support 6 des pièces à traiter 5 et débouche donc au niveau des pièces à traiter. De ce fait, le gaz de traitement se trouve aspiré vers les pièces. Il effectue donc un trajet axial de durée minimum avant de venir au contact desdites pièces. En conséquence, ce n'est qu'au contact des pièces qu'il se dissocie pour libérer de l'azote actif. Par contre, cet azote actif n'a pas le temps de subir une recombinaison en azote moléculaire.

An important feature of the oven described above is that:

a) The treatment gas injection pipe 11 successively crosses the double wall 2, 3 of the furnace, the intermediate space between the wall 3 and the muffle 4 before opening into the laboratory near the parts to be treated 5 and, preferably at a distance from the electrical resistances 7. Therefore, before entering the laboratory, the gas does not undergo any noticeable heating.

b) The suction duct 13 passes through the support structure 6 of the parts to be treated 5 and therefore opens at the level of the parts to be treated. As a result, the process gas is sucked into the rooms. It therefore performs an axial path of minimum duration before coming into contact with said parts. Consequently, it is only in contact with the parts that it dissociates to release active nitrogen. On the other hand, this active nitrogen does not have time to undergo recombination into molecular nitrogen.

However, the invention is not limited to such a provision: Indeed, the injection could be performed in another location, inside the laboratory, possibly inside the rooms to treat (tubular). In this case, the conduit injection can pass into the support structure 6, aspiration then takes place at another location from the oven, preferably in a location where to obtain an axial flow of process gas. In this effect, the injection pipe or the suction pipe may be extended by an injection nozzle or a suction nozzle of suitable shape for example for inject or aspirate inside a tubular.

The nitriding gas mixture used may consist of a mixture consisting of ammonia (NH ₃ ), nitrous oxide (N ₂ O) and nitrogen (N ₂ ). In the case of plasma-assisted depassivation, hydrogen (H2) and argon (Ar) can also be used.

During the nitriding phase, the base mixture consists of 95% to 97% NH ₃ and 5 to 3% N ₂ O, depending on the grades of steel treated.

To avoid or limit the formation of a layer of so-called "white layer" combination, we can dilute the atmosphere with nitrogen. The proportions of ammonia and nitrous oxide expressed above are then applied to the proportion of complement gas at 100%.

The processing temperature can vary between 500 and 600 ° C depending on the nuances treated and the specifications charges.

Figure 2 shows the different successive phases a nitriding treatment at low pressure, according to the method according to the invention.

Once the parts have been placed on the support structure and the oven is closed in a sealed manner, a high vacuum is applied to the oven at a pressure of the order of 10 ^-2 mbar, in order to obtain purging of the oven.

The parts are then heated to a pressure of 10 ^-2 mbar (temperature rise phase) for a period T ₁ .

When the pieces reach a temperature of the order of 400 ° C and in the case where one wishes to carry out a depassivation without ion bombardment, we proceed to a injection of ammonia alone (point I) then we continue the heating until the temperature of TT treatment under partial ammonia pressure of in the range of 200 to 400 mbar.

The actual depassivation phase is then carried out by maintaining the parts at the treatment temperature, under this partial pressure of ammonia for a period T ₂ .

In the case where an ion depassivation is carried out, the ammonia injection is eliminated and the depassivation takes place under an atmosphere of conventional depassivation, for example of hydrogen and argon.

Once the depassivation phase is completed, the nitriding phase itself is started by injecting the treatment gas onto the parts to be treated. During this nitriding phase which continues during the period T ₃ , the temperature and pressure conditions are maintained.

The nitriding cycle ends with a rapid cooling phase, thanks to an injection of inert cooling gas (nitrogen or hydrogenated nitrogen), the circulation of this gas being ensured by the turbine (period T ₄ ).

An important advantage of the process described above is that, thanks to the fact that the treatment is performed at low pressure, it is possible to obtain a rapid regulation of the nitriding potential: effect, to purge the oven and inject a different mixture (more or less rich in nitrogen) for vary this potential in a few minutes, which is not possible with conventional methods.

Furthermore, the gaseous releases caused by the nitriding at low pressure are very low and are easily treatable compared to releases generated by salt baths and rinsing water necessary for nitriding treatments in salt baths. Moreover, the working conditions as well as job security work are of better quality.

With respect to ionic nitriding, the process according to the invention uses less expensive means. he allows, in particular on tubulars, treatments that cannot be carried out ionically due to hollow cathode phenomena. It also allows the carrying out bulk treatments (impossible in ionic) thereby reducing the cost of preparing the charge.

Compared to gas nitriding, the process according to the invention improves the treatment of very long tubulars by injecting the mixture gas directly into the tubulars.

In addition, it generates gas consumption and therefore lower gas emissions (3 to 5 times less).

Treatment tests according to the method according to the invention will be described below:

Trial I

• une phase de dépassivation sous NH₃ à 540°C pendant une durée de 30 mn,
• une première phase de nitruration à 540°C pendant une heure, sous une atmosphère de traitement comprenant 50 % N₂- 46,5 % NH₃- 3,5 % N₂O,
• une deuxième phase de nitruration à 540°C pendant une heure et demi, sous une atmosphère de traitement comprenant 80 % N₂ - 18,6 % NH₃ - 1,4 % N₂O.

In this test, it involved carrying out on rollers of 6 and 9.5 mm in diameter, in steel of grade Z85 WDCV 6 5 4 2, a bulk nitriding treatment. The treatment cycle more specifically understood:

• a depassivation phase under NH ₃ at 540 ° C for a period of 30 min,
• a first nitriding phase at 540 ° C. for one hour, under a treatment atmosphere comprising 50% N ₂ - 46.5% NH ₃ - 3.5% N ₂ O,
• a second nitriding phase at 540 ° C for an hour and a half, under a treatment atmosphere comprising 80% N ₂ - 18.6% NH ₃ - 1.4% N ₂ O.

• absence de couche de combinaison,
• absence de carbonitrures en réseau,
• profondeur conventionnelle de nitruration : 50 à 80 µm (HV0.1 coeur + 100),
• dureté superficielle ≥ 950 HV5.

This treatment produced the following results:

• absence of combination layer,
• absence of carbonitrides in the network,
• conventional nitriding depth: 50 to 80 µm (HV0.1 core + 100),
• surface hardness ≥ 950 HV5.

Trial II

• une phase de dépassivation sous NH₃ à 570°C pendant 30 mn,
• une phase de nitruration à 540°C pendant 2 heures et quart sous une atmosphère de traitement comprenant 35 % N₂ - 60,5 % NH₃ - 4,5 % N₂O.

During this test, steel sprockets of grade 40 CD 4 underwent the following treatment cycle:

• a depassivation phase under NH ₃ at 570 ° C for 30 min,
• a nitriding phase at 540 ° C for 2.5 hours under a treatment atmosphere comprising 35% N ₂ - 60.5% NH ₃ - 4.5% N ₂ O.

• 10 à 20 µm de couche de combinaison,
• 0,15 à 0,2 mm de couche de diffusion (HV0.1 coeur + 100),
• dureté superficielle supérieure à 650 HV1.

This treatment produced the following results:

• 10 to 20 µm combination layer,
• 0.15 to 0.2 mm of diffusion layer (HV0.1 core + 100),
• surface hardness greater than 650 HV1.

Claims (10)

1. Method for the low-pressure nitriding of a metal part, whereby the nitriding is carried out in a furnace by heating the parts to a treatment temperature of the order of 500 to 600°C, in a low-pressure atmosphere, with injection of a treatment gas onto the parts, characterised in that it comprises the following successive phases:

   a purging phase at a pressure equal to or less than 10^-2 mbars,

   a depassivation phase,

   a treatment phase comprising injection into the furnace of a treatment gas consisting of a gas compound comprising at least ammoniac, as well as a catalyst stimulating the dissociation of ammoniac upon contact with the parts to be treated and opposing recombination of the active nitrogen, produced by the dissociation, into molecular nitrogen, said treatment phase taking place at the treatment temperature, at a pressure set to a value included between 200 and 400 mbars, determined as a function of the type of treatment, with a constant renewal rate of the gas atmosphere, the treatment gas injected being diluted in a variable quantity of molecular nitrogen in order to control the nitriding capacity of the compound and the grades of the nitrided layer.
2. A method as claimed in claim 1, characterised in that said catalyst consists of nitrogen monoxide (N₂O), carbon monoxide (CO), or even hydrocarbons thereof such as methane or propane.
3. A method as claimed in claim 1, characterised in that the depassivation phase comprises the injection of a depassivation gas at a temperature above a threshold temperature of approximately 400°C.
4. A method as claimed in claim 3, characterised in that the depassivation gas is ammoniac and/or hydrogen.
5. A method as claimed in claims 3 and 4, characterised in that the depassivation phase commences during the temperature build-up phase and continues throughout the phase during which the parts are maintained at the treatment temperature.
6. A method as claimed in claim 1, characterised in that the depassivation phase comprises a depassivation by ion bombardment using a depassivation gas such as argon.
7. Utilisation of a furnace to implement the method as claimed in any of the previous claims, wherein said furnace comprises a permanently cooled enclosure (1), e.g. with double walls (2, 3), a refractory muffle (4) housed inside the enclosure (1), delimiting a heating chamber in which the parts to be treated (5) can be placed, a means (7) of heating by radiation positioned inside the heating chamber, and treatment gas injectors passing through the enclosure (1) and muffle (4) to open out inside the heating chamber, a means (9) for generating a relative vacuum inside the enclosure, and a means (12) for adjusting the flow of treatment gas,
   characterised in that, inside the heating chamber, the parts (5) are borne by a stand structure (6) which is passed through by an aspiration pipe connected to the means (9) enabling the relative vacuum to be generated, said pipe opening out at the level of said parts.
8. Utilisation of a furnace as claimed in claim 7, characterised in that the orifices of said aspiration (13) and/or gas treatment injection (11) pipe are positioned so as to obtain an axial flow of the treatment gas inside the heating chamber.
9. Utilisation of a furnace as claimed in either claim 7 or claim 8, characterised in that the stand comprises plural grid elements arranged on top of one another and on which the parts are arranged loosely.
10. Utilisation of a furnace as claimed in either claim 7 or claim 8, characterised in that said injection pipe and/or aspiration pipe are extended by an injection nozzle and/or an aspiration nozzle of suitable shape, notably for the performance of injection and/or aspiration inside the tubular part.

Images