EP1080243B1 - Low pressure carbonitriding method for metal alloy parts - Google Patents

Abstract

The invention concerns a method for carbonitriding metal alloy parts which consists in treating said parts with a fuel mixture consisting of ethylene and hydrogen and with a nitriding gas consisting of ammonia, under pressure less than 100 hPa and at a temperature of about 750 to 1050 DEG C. The invention is particularly advantageous for treating passivable and stainless steel since it enables to attain very high surface enrichment in nitrogen.

Description

Technical area

The subject of the present invention is a carbonitriding process of alloy parts metallic.

It applies in particular to the carbonitriding of steel parts, in particular of steel rich in chromium usable in the industries of cutting edge and the automotive industry.

State of the art

Carbonitriding is a thermochemical treatment of simultaneous diffusion of carbon and nitrogen from the surface of a ferrous alloy in the solid state. It is generally carried out in a sealed oven, in which a controlled atmosphere is maintained, consisting of a support gas to which is added if necessary, to achieve the desired carbon potential, a carbon enrichment gas, and in addition, a nitrogen gas. Generally, the support gas is an endothermic generator gas comprising an alkane which is oxidized to carbon monoxide CO, since oxidation is carried out in the absence of air with respect to the stoichiometric reaction which would transform all the carbon into CO ₂ . The gases used can be nitrogen-methanol mixtures or endothermic mixtures based on hydrocarbon and ammonia, as described in “The Engineering Techniques, M1226-8 to 14, July 1994, [1].

Thus, the conventional method implements atmospheres that all contain oxygen the presence or formation of CO. Oxygen released by the decomposition of CO leads to oxidation surface of the steel which, on the one hand, brakes absorption of carbon and, on the other hand, leads to harmful structures in terms of characteristics mechanical of the treated part, contact fatigue by example. It should be noted that the carbonitrided parts that way are most often used in the state, without any mechanical touch-up of the surface.

The document FR-A-2 663 953, [2] describes a process and an installation for case hardening of low pressure metal alloy avoiding the presence oxygen. However, this low pressure technique never been considered for carrying out carbonitriding.

Statement of the invention

The subject of the present invention is a carbonitriding process which avoids the harmful presence of oxygen during treatment thermochemical diffusion of carbon and nitrogen in the metal alloy part.

According to the invention, the method of carbonitriding of metal alloy parts consists in subjecting said documents to the action of a fuel mixture consisting of ethylene and hydrogen, and the action of a nitriding gas consisting of ammonia, under a pressure of less than 100 hPa and at a temperature from about 750 to about 1050 ° C.

In this process, the supply of carbon is made by the direct dissociation of a hydrocarbon, in this case ethylene, in the enclosure of a vacuum furnace, and the supply of nitrogen comes from the dissociation. ammonia gas, depending on the thermally activated reaction: 2NH ₃ → N ₂ + 3H ₂ .

According to the invention, one uses for this carbonitriding temperatures treatment more higher than those usually used for this type of reaction, which were usually in a range from 400 to 600 ° C.

At higher temperatures used in the invention, the dissociation reaction of ammonia is thermodynamically total, but its kinetics is weak. As a result, it still exists at level of the ammonia part to be dissociated, generating active nascent nitrogen. It is for this reason that ammonia can be used for nitrogen supply.

On the other hand, the fact of working under reduced pressure, allows to benefit from a speed of passage of gas in the charge greater than the kinetics of dissociation.

The pressure used can be particularly in the range of 10 to 100 hPa.

One of the other advantages of the the invention is to be able to enrich the surface of the carbon and nitrogen part in a range of much wider temperature, from around 750 to around 1050 ° C, depending on the enrichment sequences considered. Indeed, the use of the reaction of dissociation of ethylene at low pressure allows be able to provide carbon from 750 ° C and thus lowering the temperature to benefit from a power nitriding more important ammonia because the availability of atomic nitrogen useful for diffusion, is greater. This increases the possibilities for surface carbon enrichment and in nitrogen.

Thus according to the invention, one can obtain enrichment degrees and depths in carbon and nitrogen desired by choosing the flow rates of ethylene and ammonia, the temperature and duration of treatment with the mixture fuel and nitriding gas depending on the alloy constituting said parts.

According to the invention, the carbonitriding, either by subjecting said parts to the simultaneous action of the fuel and gas mixture nitriding, either by subjecting said parts to the successive action of the fuel and gas mixture nitriding.

You can still perform the treatment by subjecting the pieces to the simultaneous action of mixing fuel and nitriding gas and then submitting them to the action of nitriding gas alone.

These steps can be repeated and combined with each other using flow rates, different temperature and duration, located in ranges given above.

Finally, the method of the invention can include an additional processing step vacuum diffusion of the parts, after they have been subjected to the action of the fuel and gas mixture nitriding. Such treatment can be carried out at a temperature from about 750 to about 1050 ° C, under pressures not exceeding 100 hPa.

Metallic alloys susceptible to be treated by the process of the invention can be of various types. We can in particular use cobalt-based steels and superalloys. Among steels, the process advantageously applies to treatment of passivable steels, containing by example 2 to 9% chromium and to the treatment of steels stainless steel containing for example 9 to 18% chromium, thanks to the low pressure technique. The treatment of such steels allows more enrich nitrogen to a high degree up to 4%.

Currently, these stainless steels are, for certain wear-related applications, used for the case hardened state. After cementation and treatment the hardened surface layer is very rich chrome carbides, which greatly degrades the corrosion resistance of these steels naturally stainless before cementation.

Being able to substitute on the surface a part of the carbon by nitrogen, allows to form precipitates of a different nature, and so consume less chromium from the matrix. Nitrogen can also partly enter into a solid solution in the matrix, its beneficial action in this form on the corrosion resistance already recognized.

By its flexibility, the method of the invention so allows on these steels to get in the layer superficial, the C / N ratio offering the best compromise for desired strength properties wear and / or corrosion resistance, by example.

In fact, the method of the invention allows, by expanding the temperature range, by possibility to link in a simple way different simultaneous or alternating enrichment sequences in carbon and / or nitrogen, to realize gradients in very varied carbon and nitrogen and this on steels very diverse, even passive.

The subject of the invention is also steel parts obtained by this process. These parts can be, for example, steel parts passivable comprising 2 to 9% chromium, which are enriched in nitrogen on their surface up to a content 2% by mass, or stainless steel parts comprising 9 to 18% chromium, which are enriched in nitrogen on their surface up to a content of 4% mass.

To implement the method of the invention, a so-called double vacuum oven can be used with hot walls or an oven with cold walls, such as the devices described in FR-A-2 663 953.

1) prévidage de la cuve du four jusqu'à une pression de 10^-1 hPa de façon à éliminer l'air,

2) remplissage de la cuve par de l'azote à la pression atmosphérique,

3) enfournement de la cuve contenant les pièces métalliques et mise sous vide de la cuve à environ 10^-2 hPa,

4) chauffage jusqu'à la température d'austénitisation avec des paliers si nécessaire, et maintien à cette température pendant 30 minutes pour l'homogénéisation des pièces,

5) introduction d'hydrogène jusqu'à 500 hPa, de préférence ou moins selon le type de four,

6) traitement de carbonitruration qui peut être effectué de différentes façons :

a) une période d'enrichissement en carbone par introduction du gaz carburant éthylène, suivie d'une période d'enrichissement en azote par introduction d'ammoniac, ou l'inverse, ou

a') période d'enrichissement en carbone et en azote par introduction simultanée d'éthylène et d'ammoniac,

7) éventuellement un traitement d'enrichissement analogue à celui de l'étape 6), ou un traitement de diffusion sous vide à une température de 750 à 1050°C, sous une pression de 10^-1 hPa, par exemple, et

8) introduction d'azote dans le four en vue du défournement.

By way of example, the method can include the following steps:

1) preheating of the oven tank to a pressure of 10 ^-1 hPa so as to eliminate the air,

2) filling the tank with nitrogen at atmospheric pressure,

3) charging the tank containing the metal parts and vacuuming the tank at around 10 ^-2 hPa,

4) heating to the austenitization temperature with stages if necessary, and maintaining at this temperature for 30 minutes for the homogenization of the parts,

5) introduction of hydrogen up to 500 hPa, preferably or less depending on the type of oven,

6) carbonitriding treatment which can be carried out in different ways:

a) a carbon enrichment period by introduction of the ethylene fuel gas, followed by a nitrogen enrichment period by introduction of ammonia, or vice versa, or

a ') carbon and nitrogen enrichment period by simultaneous introduction of ethylene and ammonia,

7) optionally an enrichment treatment similar to that of step 6), or a diffusion treatment under vacuum at a temperature of 750 to 1050 ° C., under a pressure of 10 ^-1 hPa, for example, and

8) introduction of nitrogen into the furnace with a view to charging.

Note that steps 6) and 7) can be repeated several times if necessary.

When sending ethylene gases and ammonia, the pressure in the tank is preferably maintained at approximately 25 hPa.

Detailed description of the embodiments

Other features and benefits of the invention will emerge from examples of embodiments which follow given of course by way of illustration and not limiting.

In the following examples we used the alloys listed in Table 1, the compositions (% by mass) are also given in table 1.

In these examples, we followed the mode general procedure described above, for steps 1 to 5 and 8 and steps 6 and 7 were carried out in using ethylene and ammonia flow rates and enrichment and / or dissemination sequences different.

In all the examples, a preliminary austenitization step is carried out under vacuum at 10 ^-2 hPa, at a temperature of 850 ° C., for 30 minutes. Steps 6) and 7) are then carried out under the conditions given in table 2. In this table, the reference of the alloys used, the compositions of which are given in table 1, has also been specified.

In examples 1 and 2, step 6) corresponds to carbonitriding with simultaneous shipment ethylene and ammonia and step 7) is a diffusion treatment under vacuum.

In examples 3 and 4, step 6) corresponds to carbonitriding with simultaneous shipment ethylene and ammonia (at a lower rate) and step 7) is a nitriding treatment with ammonia alone.

In examples 5 and 6, step 6) corresponds to a carburetion and step 7) to a nitriding.

In examples 7 to 9, step 6) corresponds to carbonitriding with simultaneous shipment ethylene and ammonia, but the ammonia flow is very high and step 7) is a diffusion processing under vacuum.

In examples 10 to 16, we carry out only step 6) which corresponds to a carbonitriding by simultaneous shipment of ethylene and of ammonia, for a longer period than that of previous examples.

The results obtained in each example, i.e. the surface enrichment profiles in carbon and nitrogen (in% by mass) for each of the alloys treated, are given in Tables 3 to 8.

The results obtained in examples 1 to 6 on conventional carbonitriding grades are neighbors of those that can be obtained by performing a conventional gaseous carbonitriding.

In Examples 7 to 9, good results are obtained results by processing alloys richer in chrome, therefore more passive.

In Examples 10 to 16, it is observed that we can reach very superficial values high in nitrogen on stainless steels rich in chromium where nitrogen levels reach respectively 2.86 and 4% nitrogen in Examples 14 and 15. Thus, nitrogen partially replaces surface carbon, which which allows to obtain layers with properties special.

The process of the invention is therefore very advantageous because it leads to degrees of enrichment much higher nitrogen than we can get with the conventional methods of carbonitriding where the nitrogen content at the surface are at most about 0.3%

References cited

[1]: Engineering techniques M 1226-8 to 14, July 1994

[2]: FR-A-2 663 953

Steel VS Or Cr MB V al 20 NC 6 0.17 1.60 0.85 27 CD 4 0.27 1.0 0.2 20 CD 12 0.25 3.0 0.4 32 CDV 13 0.30 3.0 1.0 0.20 40 CAD 6.12 0.40 1.8 0.25 1.0 20 DN 34.13 0.20 3.0 3.5 Z 15 CN 17.03 0.16 2.0 17.0 Z 12 CNDV 12 0.12 2.5 11.5 1.6 0.30 Ex Steel Step 6) Step 7) t
(° C) duration
(Min) C ₂ H ₄
(L / h) NH ₃
(L / h) P
(HPa) t
(° C) duration
(Min) NH ₃
(L / n) P
(HPa) 1 20NC6 850 45 50 300 25 850 45 - 0.1 2 27CD4 3 20NC6 850 45 50 100 25 850 45 100 25 4 27CD4 5 20NC6 850 45 50 - 25 850 45 100 25 6 27CD4 7 27CD4 8 32CVD13 850 45 50 600 25 850 45 - 0.1 9 Z15CN17.03 10 20NC6 11 27D4 12 20CD12 13 40CAD6.12 850 360 50 300 25 14 Z12CNDV12 15 Z15CN17.03 16 20DN34.13 Ex 1 2 Alloy 20 NC 6 27 CD 4 Depth
(Mm) VS % NOT % VS % NOT % 0.05 0.50 0.304 0.61 0.324 0.15 0.45 0.215 0.58 0.223 0.25 0.45 0.113 0.51 0.122 0.35 0.33 0,039 0.47 0.043 heart 0.19 0.0059 0.27 0.0080 Ex 3 4 Alloy 20 NC 6 27 CD 4 Depth
(Mm) VS % NOT % VS % NOT % 0.05 0.72 0.297 0.75 0.279 0.15 0.66 0.203 0.69 0.189 0.25 0.57 0.114 0.61 0,102 0.35 0.46 0.049 0.51 0,037 heart 0.19 0.0059 0.27 0.0081 Ex 5 6 Alloy 20 NC 6 27 CD 4 Depth
(Mm) VS % NOT % VS % NOT % 0.05 0.79 0,148 0.82 0,165 0.15 0.71 0.094 0.72 0.078 0.25 0.56 0,029 0.57 0,028 0.35 0.37 0.0092 0.44 0.012 heart 0.19 0.0059 0.27 0.0081 Ex 7 8 9 Alloy 27 CD4 32CDV13 Z15CN17.03 Depth
(Mm) VS % NOT % VS % NOT % VS % NOT % 0.05 0.63 0.22 0.34 0.73 0.89 2.00 0.15 0.58 0.19 0.60 0.29 0.77 0.08 0.25 0.54 0.12 0.55 0.03 0.33 0.05 0.35 0.46 0.05 0.44 0.01 0.20 0.05 Ex 10 11 12 13 Alloy 20NC6 27CD4 20CD12 40CAD6.12 Depth
(Mm) VS % NOT % VS % NOT % VS % NOT % VS % NOT % 0.05 0.98 0.52 0.93 0.44 0.60 0.89 0.81 0.98 0.15 0.86 0.51 0.86 0.44 0.54 0.80 0.77 0.84 0.25 0.81 0.45 0.79 0.41 0.54 0.55 0.89 0.48 0.35 0.73 0.31 0.77 0.31 0.73 0.12 0.80 0.04 0.45 0.65 0.20 0.66 0.24 0.57 0.04 0.66 0.01 0.55 0.56 0.09 0.51 0.15 0.46 0.02 0.57 0.01 Ex 14 15 16 Alloy Z12CNDV12 Z15CN17.03 20DN34.13 Depth
(Mm) VS % NOT % VS % NOT % VS % NOT % 0.05 0.41 2.86 0.61 4.00 0.57 0.53 0.15 2.07 0.26 2.45 0.36 0.53 0.41 0.25 1.32 0.07 1.21 0.08 0.50 0.31 0.35 1.62 0.04 0.51 0.05 0.46 0.19 0.45 0.22 0.03 0.26 0.04 0.40 0.11 0.55 0.14 0.03 0.20 0.04 0.35 0.08

Claims (12)

1. Method of carbonitriding metal alloy components, in which said components are subjected to the action of a fuel mixture made up of ethylene and hydrogen and to the action of a nitriding gas consisting of ammonia, at a pressure of less than 100 hPa and at a temperature of from about 750 to 1050°C.
2. Method according to claim 1, in which the pressure is from 10 to 100 hPa.
3. Method according to any one of claims 1 and 2, in which the metal alloy is a steel.
4. Method according to claim 3, in which the flow rates of ethylene and ammonia, the temperature and the duration of the treatment by the fuel mixture and the nitriding gas are chosen in such a way that nitrogen enrichment ranging up to 4 wt.%, of the surface of the component is obtained.
5. Method according to any one of claims 1 to 4, in which the metal alloy is a steel that can be passivated and which comprises 2 to 9 wt.% chromium.
6. Method according to any one of claims 1 to 4, in which the metal alloy is a stainless steel that comprises 9 to 18 wt.% chromium.
7. Method according to any one of claims 1 to 6, in which said components are subjected to the simultaneous action of the fuel mixture and the nitriding gas.
8. Method according to any one of claims 1 to 6, in which said components are subjected successively to the action of the fuel mixture and the nitriding gas.
9. Method according to any one of claims 1 to 6, in which the components are subjected 1) to the simultaneous action of the fuel mixture and the nitriding gas and then 2) to the action of the nitriding gas alone.
10. Method according to any one of claims 1 to 9, in which, after having subjected the components to the action of the fuel mixture and the nitriding gas, the components are subjected to a diffusion treatment under vacuum at a temperature of from about 750 to 1050°C.
11. Method according to claim 5, in which the surface of said component is enriched in nitrogen up to a content of 2 wt.%.
12. Method according to claim 6, in which the surface of said component is enriched in nitrogen up to a content of 4 wt.%.