CA2326239C - 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

WO 99155928 rcr / FR99 / 00998 PROCESS FOR CARBONITRLJRATION AT LOW PRESSURE OF WORKPIECES
METALLIC ALLOY
DESCRIPTION
Technical area The present invention relates to a carbonitriding process of alloy parts metallic.
It applies in particular to carbonitriding of steel parts, especially steels rich in chromium usable in the industries of advanced and the automotive industry.

State of the art Carbonitriding is a treatment thermochemical of simultaneous carbon diffusion and nitrogen from the surface of a ferrous alloy to the solid state. It is usually done in a sealed oven, in which an atmosphere is maintained controlled, consisting of a carrier gas to which we add if necessary, to reach the carbon potential desired, a carbon enrichment gas, and plus, a nitrogen gas. Generally, the carrier gas is a endothermic generator gas comprising an alkane which is oxidized to carbon monoxide CO because one realizes the oxidation in air defect compared to the reaction stoichiometric that would turn all the carbon into C02. The gases used can be nitrogen mixtures methanol or endothermic mixtures based hydrocarbon and ammonia, as described in

2 The Techniques of the Engineer, M1226-8 to 14, July 1994, [1].
Thus, the conventional method implements atmospheres that all contain oxygen due the presence or formation of CO. Oxygen released by decomposition of CO leads to oxidation superficial steel which, on the one hand, carbon absorption and, on the other hand, leads to harmful structures in terms of characteristics mechanics of the treated part, contact fatigue by example. It should be noted that the carbonitride pieces this way are most often used in the state, without any mechanical touching of the surface.
Document FR-A-2,663,953, [2] describes a process and a cementation plant of parts in Low pressure metal alloy avoiding the presence oxygen. This low pressure technique does, however, never been considered for the purpose of carbonitriding.

Presentation of the invention The present invention relates to 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 submitting the said parts to the action of a fuel mixture consisting of ethylene and hydrogen, and the action of a nitriding gas consisting of ammonia,

3 under a pressure below 100 hPa and at a temperature of about 750 to about 1050 C.
In this process, carbon input is made by the direct dissociation of a hydrocarbon, in the occurrence of ethylene, in the enclosure of a furnace empty, and the nitrogen supply comes from dissociation Ammonia gas, depending on the activated reaction thermally 2NH3 N2 + 3H2.
According to the invention, it is used for this carbonitriding treatment of higher temperatures higher than those usually used for this type of reaction, which were usually in a area of 400 to 600 C.
At the higher temperatures used in the invention, the dissociation reaction of ammonia is thermodynamically total but its kinetics is low. As a result, there is still Ammonia piece level to dissociate, generating nascent active nitrogen. It is for this reason that ammonia can be used for nitrogen input.
On the other hand, working under reduced pressure, allows to benefit from a speed of passage of gas in the load greater than the kinetics dissociation.
The pressure used can be in especially in the range of 10 to 100 hPa.
One of the other advantages of the method of the invention is to be able to enrich the surface of the carbon and nitrogen part in a field of temperature much wider, since about 750 to about 1050 C, according to the enrichment sequences

4 considered. Indeed, the use of the reaction of dissociation of ethylene at low pressure allows for can bring carbon from 750 C and so in decreasing 'the temperature to benefit from a power nitrurant more important ammonia because the availability of atomic nitrogen useful for the diffusion, is larger. This increases the opportunities for surface enrichment in carbon and in nitrogen.
Thus according to the invention, it is possible to obtain degrees and depths of enrichment in carbon and nitrogen desired by choosing appropriate flow rates of ethylene and ammonia, the temperature and duration of treatment by mixing fuel and nitriding gas depending on the alloy constituting said parts.
According to the invention, the carbonitriding, either by subjecting the said parts to the simultaneous action of the fuel mixture and the gas nitriding, or by subjecting the said parts to the successive action of the fuel mixture and the gas nitriding.
We can still perform the treatment subjecting the pieces to the simultaneous action of the mixture fuel and nitriding gas and then subjecting them to the action of the nitriding gas alone.
These steps can be repeated and combined with each other using flow rates, different temperatures and durations, located in ranges given above.
Finally, the method of the invention can understand a complementary step of treatment of vacuum diffusion of the parts, after they have have been subjected to the action of the fuel and gas mixture nitriding. Such treatment can be performed at a temperature of about 750 to about 1050 C, under pressures not exceeding 100 hPa.

5 Metal alloys susceptible to be treated by the process of the invention can to be of various types. In particular, we can use steels and superalloys based on cobalt. Among steels, the process advantageously applies to treatment of passivable steels, containing example 2 to 9% chromium and the treatment of steels stainless steels containing for example 9 to 18% of chromium, thanks to the technique of low pressure. The treatment of such steels allows more than enrich in nitrogen to a high degree that can reach 4%.
Currently, these stainless steels are, for certain wear-related applications used in the cemented state. After carburizing and treatment of use, the hardened surface layer is very rich in chromium carbides, which severely degrades the corrosion resistance of these steels naturally stainless before carburizing.
The ability to substitute on the surface part of the carbon with nitrogen, allows to form precipitates of different nature, and so consume less chromium from the matrix. Nitrogen can also part in solid solution in the matrix, its beneficial action in this form on the Corrosion resistance is 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

6 compromise for the desired properties of resistance wear and / or corrosion resistance, by example.
In fact, the method of the invention makes it possible by the widening of the temperature range, by the possibility of linking in a different simple way simultaneous or alternating enrichment sequences in carbon and / or nitrogen, to achieve gradients in carbon and nitrogen very varied and this on steels very diverse, even passive.
Also, the invention also relates to steel parts obtained by this process. These parts can be, for example, steel parts passivable comprising 2 to 9% chromium, which are enriched with nitrogen on their surface up to a 2% by weight, or stainless steel parts comprising 9 to 18% of chromium, which are enriched in nitrogen on their surface up to a level of 4%
mass.
To implement the process of the invention can be used a double vacuum furnace said hot-walled or cold-walled furnace, such as the devices described in FR-A-2 663 953.
For example, the process can understand the following steps:
1) roughing the oven bowl to a pressure of 10-1 hPa to eliminate air, 2) filling the tank with nitrogen at the atmospheric pressure, 3) charging the vessel containing the parts metal and evacuation of the tank to approximately 10-2 hPa,

7 4) heating up to the temperature austenitization with bearings if necessary, and keeping at this temperature for 30 minutes to the homogenization of the pieces, 5) introduction of hydrogen up to 500 hPa, preference or less depending on the type of oven, 6) carbonitriding treatment which can be done in different ways:
a) a period of carbon enrichment by introduction of the ethylene fuel gas, followed by a period of enrichment in nitrogen by introduction of ammonia, or the opposite, or a ') period of carbon enrichment and Nitrogen by simultaneous introduction of ethylene and ammonia, 7) optionally an enrichment treatment analogous to that of step 6), or a treatment of diffusion 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 for push.
It should be noted that steps 6) and 7) can be repeated several times if necessary.
When sending ethylene 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 the examples of embodiments = CA 02326239 2000-10-25 . . = == = .. .... ..
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= = = = = = = = s = =
= = = = = ~ = = = = = = =
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which are given, of course, for illustration and non-limiting.
In the following examples we used the alloys axéreported in Table 1 whose compositiorjlso / nr also in Table 1.
In these examples, the mode was followed general procedure described above, for steps 1 at 5 and 8 and steps 6 and 7 were done in using ethylene and ammonia flow rates and enrichment and / or dissemination sequences different.
In all the examples, a pre-vacuum austenitization step at 10-2 hPa, at a temperature of 850 C, for 30 minutes. We then perform steps 6) and 7) under the conditions given in Table 2. In this table, we have also specified the reference of the alloys used whose compositions are given in Table 1.
In examples 1 and 2, step 6) corresponds to carbonitriding with simultaneous sending of ethylene and ammonia and step 7) is a vacuum diffusion treatment.
In Examples 3 and 4, step 6) corresponds to carbonitriding with simultaneous sending ethylene and ammonia (at a lower flow rate) and step 7) is a nitriding treatment by ammonia alone. =
In Examples 5 and 6, step 6) corresponds to a carburization and step 7) to a Nitriding.
In Examples 7 to 9, step 6) corresponds to carbonitriding with simultaneous sending of ethylene and ammonia, but the flow of ammonia is ~. .
MODIFIED SHEET

9 very high and step 7) is a diffusion treatment under vacuum.
In Examples 10 to 16, it is realized only step 6) which corresponds to one.
carbonitriding by simultaneous sending of ethylene and ammonia, for a longer period than previous examples.
The results obtained in each example, that is to say the profiles of superficial enrichment carbon and nitrogen (in% by mass) for each of the treated alloys, are given in Tables 3 to 8.
The results obtained in Examples 1 to 6 on conventional shades of carbonitriding are neighbors of those that can be achieved by realizing a conventional carbonitriding gas.
In Examples 7 to 9, we obtain good results by dealing with alloys richer in chrome, so more passive.
In Examples 10 to 16, it is observed that we can superficially reach very high values high in nitrogen on stainless steels rich in chromium where the nitrogen levels reach respectively 2.86 and 4 $ of nitrogen in Examples 14 and 15. Thus, the nitrogen partly replaces the superficial carbon, this which allows to obtain layers with properties special.
The method of the invention is therefore very advantageous because it leads to degrees of enrichment in nitrogen much higher than those that we can get with the classic processes of carbonitriding where surface nitrogen levels are at most about 0.3%

References cited [1]: Techniques of the Engineer M 1226-8 to 14, July 1994 [2] = FR-A-2,663,953 Table 1 Steel C Ni Cr Mo V AI
20 CN 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 Table 2 Ex Steel Step 6) Step 7) t duration C2H4 NH3 P t duration NH3 P
(C) (min) (1 / h) (1 / h) (hPa) (C) (min) (1 / n) (hPa) 1 20NC6 850 45 50 300 25 850 45 - 0.1 8 32CVD13 850 45 50 600 25 850 45 - 0.1 9 Z15CN17.03 13 40CAD6.12 850 360 50 300 25 Z15CN17.03 16 20DN34.13 Table 3 Ex 1 2 Alloy 20 NC 6 27 CD 4 Depth C% N% C% N%
(Mm) 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 Table 4 Ex 3 4 Alloy 20 NC 6 27 CD 4 Depth C% N% C% N%
(MM) 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 Table 5 Ex 5 6 Alloy 20 NC 6 27 CD 4 Depth C% N% C% N%
(Mm) 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 Table 6 Ex 7 8 9 Alloy 27 CD4 32CDV13 Z15CN17.03 Depth C% N% C% N% C% N%
mm) 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 Table 7 Ex 10 11 12 13 Alloy 20NC6 27CD4 20CD12 40CAD6.12 Depth C% N% C 9b N% C% N% C% N%
(Mm) 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 Table 8 Ex 14 15 16 Alloy Z12CNDV12 Z15CN17.03 20DN34.13 Depth C% N% C% N% C% N6 mm) 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. Parts carbonitriding process made of metal alloy, in which the said parts to the action of a fuel mixture consisting of ethylene and hydrogen and to the action of a gas nitriding consisting of ammonia, under a pressure less than 100 hPa and at a temperature of approximately 750 to 1050°C. 2. Method according to claim 1, in which the pressure is 10 to 100 hPa. 3. Process according to any of the claims 1 and 2, wherein the metal alloy is a steel. 4. Method according to claim 3, in which the ethylene and ammonia flows, the temperature and duration of the treatment by the mixture fuel and the nitriding gas are chosen so as to obtain a nitrogen enrichment of the surface of the piece up to 4% by mass. 5. Process according to any of the claims 1 to 4, wherein the metal alloy is a passivable steel comprising 2 to 9% chromium. 6. Process according to any of claims 1 to 4, wherein the metal alloy is a stainless steel comprising 9 to 18% chromium. 7. Process according to any of claims 1 to 6, wherein said subject is parts to the simultaneous action of the fuel mixture and the nitriding gas. 8. Process according to any of the claims 1 to 6, wherein said subject is parts successively to the action of the fuel mixture, then to the action of the nitriding gas. 9. Process according to any of claims 1 to 6, in which the parts are submitted 1) to the simultaneous action of the fuel mixture and the gas nitriding, then 2) the action of the nitriding gas alone. 10. Process according to any of the claims 1 to 9, wherein after submitting the parts to the action of the fuel and gas mixture nitriding, the parts are subjected to a treatment of diffusion under vacuum, at a temperature of about 750 to 1050°C. 11. Process according to claim 5, in which the surface of said part is enriched with nitrogen up to a content of 2% by mass. 12. Process according to claim 6, in which the surface of said part is enriched with nitrogen up to a content of 4% by mass.