FR2719057A1 - Method of nitriding metallic surfaces - Google Patents

Abstract

A nitriding method is carried out in a furnace with a temp. of 500-600 deg C and low pressure. A gas mixt., injected onto the parts to be treated, consists of at least ammonia plus a catalyst forming the dissociation of ammonia to active nitrogen and opposing the recombination of active nitrogen to molecular nitrogen.

Description

PROCESS FOR LOW PRESSURE NITRURATION OF A PART

METALLIC.

  The present invention relates to a process for the nitriding at low pressure of a metal part, for example of steel, in order to improve its mechanical properties on the surface and, in particular, its resistance to wear. In general, it is known that at present there are at least three types of treatment making it possible to carry out this nitriding, namely: nitriding in salt baths, ionic nitriding and

gas nitriding.

  Nitriding in a salt bath is a particularly polluting and dangerous technique due to the release of toxic gases and the rinsing water it generates. In addition, it imposes difficult working conditions on the workforce. This is the reason

  which this technique tends to disappear.

  Ion nitriding involves a heat treatment installation under relative vacuum, specially equipped so as to generate a luminescent discharge on the parts to be treated in an atmosphere of nitriding gas. This technique has the drawback of being relatively expensive and of not being suitable for parts of complex shape and, in particular, of tubular shape, due to the phenomena of hollow cathode. Gaseous nitriding consists in bringing the parts to a treatment temperature of the order of 500 C to 600 C and sweeping them with a nitriding gas such as ammonia at atmospheric pressure. This treatment has the disadvantage of being relatively long and of consuming large quantities of treatment gas. For this same reason, this type of treatment is also polluting. In an attempt to reduce the quantities of treatment gas involved, it has also been proposed to carry out the treatment at low pressure, inside a vacuum heat treatment oven. However, in this case, we come up against the fact that at the treatment temperature, the ammonia undergoes dissociation and then recombination

  nitrogen atoms active in molecular nitrogen.

  It is clear that this recombination process goes against the desired goal, since only a small fraction of the injected gas composed of active nitrogen which has escaped recombination will be able to interact with the metal of the part to be treated to obtain the phenomenon nitriding. The use of means making it possible to prevent the treatment gas from reaching the treatment temperature before being in the immediate vicinity of the part is only possible in a very limited number of cases and is not suitable

  well especially in the case of tubular parts.

  The invention more particularly aims to remove

these disadvantages.

  - 3 - To this end, it proposes a treatment process according to which the nitriding process is carried out by bringing the parts to a treatment temperature of the order of 500 ° C. to 600 ° C. in a low pressure atmosphere, with injection on the parts of a process gas. According to the invention, this process is characterized in that the treatment gas is a gaseous mixture comprising at least ammonia as well as a catalyst promoting the dissociation of ammonia in contact with the parts to be treated and opposing recombination of active nitrogen

  from this dissociation into molecular nitrogen.

  Advantageously, the above-mentioned catalyst may consist of nitrous oxide (N20), carbon monoxide (CO), or even their hydrocarbon such as methane

or propane.

  Furthermore, the nitriding power of the mixture and, consequently, the over-nature type of the combination nitrided layer obtained as well as the quality of the diffusion layer can be controlled by diluting the gaseous mixture in a quantity of variable molecular nitrogen. depending on the desired result (elimination of iron carbonitrides in a network which weaken the part). This dilution makes it possible in particular to avoid or limit the formation of a so-called combination layer.

white layer.

  The nitrogen diffusion process in the surface layer of the metal constituting the parts to be treated can be further improved by preceding the actual treatment phase with a prior depassivation phase. This depassivation phase can be obtained by injecting a depassivation gas composed for example of ammonia and / or hydrogen, the temperature of the parts then being above a threshold temperature of the order of -4 - 400 C. In reality, the depassivation treatment may begin during the temperature rise phase of the parts and may continue during the phase of keeping the parts at the treatment temperature (between 500 ° C. and 600 ° C.). Of course, in the case where the oven is equipped with means making it possible to carry out a treatment by ion bombardment, this depassivation can be carried out by generating on the parts a luminescent discharge in an atmosphere of hydrogen and argon at

low pressure.

  The oven allowing the implementation of the process described above preferably consists of an oven of the type of those used for carburizing at low pressure which comprises an enclosure, for example with double walls, constantly cooled, a refractory muffle, housed at the interior of the enclosure, which delimits a laboratory inside which the parts to be treated can be placed, radiant heating means arranged inside the laboratory and treatment gas injectors passing through the enclosure and

  the muffle to lead inside the laboratory.

  Of course, means are also provided for generating in the enclosure a relative vacuum and means

  for adjusting the process gas flow.

  It turns out that contrary to the unfavorable prejudices resulting from the high cost and the risks of corrosion of the installation, this type of oven makes it possible to obtain surprising results mainly due to the fact that it makes it possible to generate inside the laboratory a continuous and homogeneous flow of treatment gas which arrives on the parts to be treated at a relatively low temperature, below the dissociation temperature. This is due to the fact that between the cooled wall of the enclosure and the muffle, the temperature remains at a relatively low level and that the intense heat exchanges which take place essentially by radiation -5- only affect the interior volume. of the laboratory, i.e. areas relatively close to the parts to be treated. Consequently, the treatment gases reach the treatment temperature only in contact with the parts to be treated. Therefore, a significant part of the active nitrogen released during this dissociation acts on the parts to be treated even before the phenomenon of recombination of active nitrogen into molecular nitrogen can occur. 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 are combined with the effects of the catalyst gas used to obtain the previously mentioned results which concern both the quality of the results obtained, the possibilities of action on the various treatment parameters to obtain exactly the desired nuances and the quantities of treatment gas used (these can be greatly reduced due to the gain in yield obtained and the

low pressure).

  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: FIG. 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 Figure 1.

  -6- In the example shown in FIG. 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 a coolant such as water. Thanks to this feature, the furnace has a low thermal inertia and therefore significantly faster cooling rates than those obtained in an oven with hot walls. This point is essential when it comes to treating steel grades susceptible to corrosion by intergranular precipitation. Inside the enclosure 1 is disposed a muffle 4 made of refractory material which delimits a volume V constituting the laboratory, inside which the parts 5 are placed on a support 6 carried by the bottom of the enclosure 1. The pieces can be arranged in bulk on several mesh elements arranged one above the other. The parts 5 are heated inside the laboratory by heating resistors 7 connected to an external supply circuit. The oven is also equipped with a gas circulation turbine which can be used in particular to accelerate the cooling of the interior volume of the enclosure 1. This cooling is usually obtained by introducing an inert gas (nitrogen or hydrogenated nitrogen) at a pressure lower than atmospheric pressure, convection of this gas being ensured by

the turbine 8.

  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: 7- - "Pirani" or "Penning" gauges for low pressures, when a purge limit vacuum is desired, - a pressure gauge diaphragm for working pressure. The admission of the treatment gases inside the furnace is ensured by a gas injection circuit 11 supplied from gas sources Si, S2, S3 via

  a mixer-flow regulator 12.

  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 means of treatment by ion bombardment, for example involving a high voltage electric generator connected to the wall of the enclosure and to the support structure of the parts to be treated. In the context of the method according to the invention, these processing means can be

  used to perform plasma assisted stripping.

  An important feature of the oven described above is that: a) The process gas injection pipe 11 successively passes through the double wall 2, 3 of the oven, - 8 - the intermediate space between the wall 3 and the muffle 4 before entering 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 is not subjected to

notorious warm-up.

  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 a recombination in

molecular nitrogen.

  However, the invention is not limited to such an arrangement: In fact, the injection could be carried out at another location, inside the laboratory, possibly inside the parts to be treated (case of tubulars). In this case, the injection pipe may pass into the support structure 6, the suction then being carried out at another location in the furnace, preferably at a location allowing an axial flow of treatment gas to be obtained. For this purpose, the injection pipe or the suction pipe may be extended by an injection nozzle or a suction nozzle of suitable shape, for example for carrying out the injection or the suction inside of a tubular. The nitriding gas mixture used may consist of a mixture consisting of ammonia (NH3), nitrous oxide (N20) and nitrogen (N2). In the case of plasma-assisted depassivation, it is also possible

  use hydrogen (H2) and argon (Ar).

  -9 - During the nitriding phase, the basic mixture consists of 95% to 97% NH3 and 5 to 3% N20,

  according to the steel grades treated.

  To avoid or limit the formation of a so-called "white layer" combination layer, the atmosphere can be diluted with nitrogen. The proportions of ammonia and nitrous oxide expressed above are then

  applied to the proportion of complement gas at 100%.

  The treatment temperature can vary between 500 and 600 C depending on the nuances treated and the specifications. FIG. 2 shows the different successive phases of 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 furnace is closed in a leaktight manner, the furnace is placed under high vacuum at a pressure of the order

  of 10-2 mbar, in order to obtain a purging of the oven.

  The parts are then heated to a pressure of 10-2 mbar (temperature rise phase) for one

period T1.

  When the parts reach a temperature of the order of 400 ° C. and in the case where it is desired to carry out a depassivation without ion bombardment, an injection of ammonia alone is carried out (point I) then heating is continued until obtaining the TT treatment temperature under a partial ammonia pressure of

in the range of 200 to 400 mbar.

  The actual depassivation phase is then carried out by keeping the parts at the temperature of

- 10 -

  treatment, under this partial pressure of ammonia

during a T2 period.

  In the case where ionic depassivation is carried out, the injection of ammonia is eliminated and the depassivation phase is carried out under a conventional depassivation atmosphere, for example 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 period T3, 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 T4).

  An important advantage of the method described above is that, thanks to the fact that the treatment is carried out at low pressure, it is possible to obtain rapid regulation of the nitriding potential: It suffices, in fact, to purge the oven and inject a different mixture (more or less rich in nitrogen) to vary this potential in a few minutes, which

  is not possible with conventional methods.

  Furthermore, the gaseous releases caused by nitriding at low pressure are very low and are easily treatable compared to the releases generated by the salt baths and the rinsing waters necessary for nitriding treatments in salt baths. In addition, the working conditions as well as the safety of the

work are of better quality.

- 1l -

  With respect to ionic nitriding, the method according to the invention uses less expensive means. It makes it possible to carry out, in particular on tubulars, treatments which cannot be carried out by ionic route because of the phenomena of hollow cathode. It also allows bulk treatments (impossible in ionic), thereby reducing the cost of preparing the charge. Compared to gas nitriding, the method according to the invention makes it possible to improve 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 in accordance with the method according to the invention will be described below: Test I In this test, it was a question of 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 included: - a depassivation phase under NH3 at 540 C for a period of 30 min, - a first nitriding phase at 540 C for one hour, under a treatment atmosphere comprising

% N2 - 46.5% NH3 - 3.5% N20,

  - a second nitriding phase at 540 C for an hour and a half, under a treatment atmosphere

  comprising 80% N2 - 18.6% NH3 - 1.4% N20.

- 12 -

  This treatment made it possible to obtain the following results: - absence of combination layer, - absence of networked carbonitrides, - conventional depth of nitriding: 50 to 80 μm (HV0.1 core + 100),

- surface hardness 2 950 HV5.

  Test II During this test, steel sprockets of grade 40 CD 4 steel underwent the following treatment cycle: - a depassivation phase under NH3 at 570 C for 30 min, - a nitriding phase at 540 C for 2 quarter hours under a treatment atmosphere including

% N2 - 60.5% NH3 - 4.5% N20.

  This treatment made it possible to obtain the following results: - 10 to 20 μm of combination layer, - 0.15 to 0.2 mm of diffusion layer (HV0.1 core + 100),

  - surface hardness greater than 650 HV1.

- 13 -

Claims (8)

Claims   1. Method for the nitriding at low pressure of a metal part according to which the nitriding process is carried out by bringing the parts to a treatment temperature of the order of 500 C to 600 C in an atmosphere at low pressure, with injection on the parts of a treatment gas, characterized in that the treatment gas is a gaseous mixture comprising at least ammonia as well as a catalyst promoting the dissociation of ammonia in contact with the parts to be treated and s' opposing the recombination of active nitrogen from this dissociation into molecular nitrogen.   2. Method according to claim 1, characterized in that the above-mentioned catalyst consists of nitrous oxide (N20), carbon monoxide (CO), or even in their hydrocarbon such as methane or propane.   3. Method according to one of claims 1 and   2, characterized in that it comprises the dilution of the treatment gas mixture in a variable quantity of molecular nitrogen to control the nitriding power   of the mixture and the nuances of the nitrided layer.   4. Method according to one of claims   previous, characterized in that it comprises a phase of prior deactivation.   5. Method according to claim 4, characterized in that the depassivation phase comprises the injection of a depassivation gas at a temperature   higher than a threshold temperature of around 400 C. - 14 -   6. Method according to claim 5, characterized in that the depassivation gas is ammonia and / or hydrogen.   7. Method according to one of claims 5 and   6, characterized in that the depassivation phase begins during the temperature rise phase of the parts and continues during the phase of keeping the parts at the processing temperature.   8. Method according to claim 4, characterized in that the depassivation phase comprises a depassivation by ion bombardment making   intervene a gas of deactivation such as argon.