EP2773789B1 - Carbonitriding method having a final nitridation step during temperature decrease - Google Patents

Description

The present invention claims the priority of the French application 1159878 filed on October 31, 2011 whose content (text, drawings and claims) is hereby incorporated by reference.

The present invention relates to a process for carbonitriding steel parts, including, but not limited to, parts used in the manufacture of motor vehicles. In particular the invention also applies to parts used in the manufacture of agricultural machinery, machine tool, or parts in the aeronautical field.

BACKGROUND OF THE INVENTION

Document is known EP 1885904 , a carbonitriding process for steel parts comprising alternating steps of carburizing and nitriding at a constant temperature, preceded by a temperature rise step and a temperature equalization step, and followed by a quenching step . Alternatively it is proposed to inject a nitriding gas during the temperature rise step and / or during the temperature equalization step, from a temperature of 800 ° C.

OBJECT OF THE INVENTION

The object of the invention is to improve the process of the aforementioned document, that is to say to improve the quality of the parts obtained, preferably with a reduction of the treatment time.

BRIEF DESCRIPTION OF THE INVENTION

In order to achieve this object, it is proposed according to the invention a carbonitriding process of steel parts, especially parts used in the manufacture of motor vehicles, according to claim 1.

Indeed, according to a finding that is already part of the invention, it was observed that it was possible to start the quenching from a temperature lower than the carburizing temperature. A lowering of temperature during the last nitriding step therefore makes it possible to perform this under conditions more favorable to good nitriding.

According to an advantageous version of the invention, the final nitriding step comprises a temperature step. The last nitriding step is thus carried out under optimal conditions.

Preferably, during the initial nitriding phase the temperature rise is carried out with a reduced temperature gradient compared to the simple temperature rise phase. Thus, without increasing the duration of the treatment, the nitrogen enrichment which is carried out under favorable conditions for good nitriding is increased so that it is possible to shorten or eliminate one of the subsequent nitriding steps and thus reduce the total treatment time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent on reading the following description of various nonlimiting particular embodiments of the low pressure carbonitriding process according to the invention, with reference to the 3 appended figures which are diagrams. schematics illustrating the various steps of the method according to the invention according to different embodiments.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the figure 1 the method according to the invention comprises a first step of temperature rise comprising a first phase M of simple temperature rise, illustrated by a line in continuous line, from the ambient temperature to a point at a temperature of 700 ° C. , denoted Ni1 in the figure. Depending on the composition of the steel to be treated, the phase simple rise in temperature can be carried out up to a temperature of between 700 ° C. and 750 ° C., and has a duration of between 10 min and 90 min, that is to say that the simple rise in temperature is carried out with a temperature gradient of between 8 ° C / min and 75 ° C / min.

The process then comprises an initial nitriding phase Ni with continuation of the temperature rise step up to a temperature of 940 ° C. in the example illustrated. In practice the temperature of 940 ° C corresponds to a compromise between a temperature of 860 ° C which allows a treatment of better quality and a temperature of 1000 ° C which allows for faster processing.

In the exemplary embodiment of the figure 1 , corresponding to a first embodiment of the initial nitriding phase, the rise in temperature continues on a regular basis but with a temperature gradient of between 3.5 ° C./min and 16 ° C./min less than the temperature gradient during the simple temperature rise. The duration of the initial nitriding phase is between 15 minutes and 45 minutes, depending on the amount of nitrogen that it is desired to set in this initial step and the composition of the steel to be treated.

In a manner known per se, the initial nitriding phase comprises injection phases of a nitriding gas such as alternating ammonia with diffusion phases.

According to a second embodiment of the initial nitriding phase, illustrated by the figure 2 , the rise in temperature continues with the same temperature gradient as during the simple rise in temperature to a point at a temperature between 750 ° C and 850 ° C, here 800 ° C, denoted Ni2 in the figure. The temperature is then maintained according to a plateau until a moment noted Ni3 on the figure 2 from which a high temperature rise is performed to reach the carburizing temperature.

The temperature of the bearing is chosen in a manner known per se to perform the initial nitriding phase under optimal conditions given the composition of the parts to be treated. Note in this connection that because of the bearing, the final temperature rise can be performed very rapidly, for example 80 ° C / min at 100 ° C / min without subjecting the parts to unacceptable constraints.

According to a third embodiment of the initial nitriding phase, illustrated using the figure 3 the rise in temperature continues from point Ni1 with a lower temperature gradient than in the first embodiment, preferably in a range of 2 ° C / min to 8 ° C / min, up to Ni4 moment, corresponding here to a temperature of 850 ° C, from which a high temperature rise is performed to reach the carburizing temperature, according to a gradient similar to that of the second embodiment.

Whatever the embodiment used for the initial nitriding phase, the process then comprises n alternating cementation phases with nitriding phases. In a manner known per se, the carburizing and nitriding steps comprise alternating treatment gas injection phases with diffusion phases not shown in the figures. In the figure, the diagram has been interrupted between the nitriding step N1 and the last cementation step Cn. At the end of this last cementation step Cn, the process comprises a final nitriding step Nn accompanied by a descent of temperature immediately before T quenching.

According to a first embodiment of the last nitriding step Nn, illustrated by a dashed line in the figure, the temperature is lowered continuously to a temperature within the optimum temperature range for all nitriding. remaining high enough to allow efficient quenching. In the example illustrated, the final temperature before quenching is 840 ° C. In practice results satisfactory are obtained for a final temperature before quenching between 900 ° C and 800 ° C. It has been found that this limited descent of temperature decreases the stress on the parts during quenching.

The final nitriding step has a duration of preferably between 15 min and 60 min, which corresponds to a temperature gradient of between 10 ° C / min and 1 ° C / min. As for the initial nitriding phase, the final nitriding step preferably comprises alternating nitriding gas injection phases with diffusion phases.

According to a second embodiment of the last nitriding step Nn, illustrated in FIG. figure 2 the descent of temperature is first of all carried out in a strong way, with a gradient as strong as possible without generating undue stresses in the steel, up to the optimum nitriding temperature for the steel being treated, noted Nn1 in the figure, here 840 ° C, then the temperature is maintained at a plateau until the beginning of quenching.

In practice, the method according to the invention can be implemented by combining any one of the embodiments of the initial nitriding phase with any of the embodiments of the final nitriding phase.

It will be noted that because of the increased efficiency of the nitriding phases according to the invention it is possible to replace at least one nitriding step between two carburizing steps by a simple diffusion step. Such a step is shorter than a nitriding step so that the total duration of the treatment is shortened.

Naturally, the invention is not limited to the embodiments described and variations can be made without departing from the scope of the invention as defined by the claims. In particular, the initial rise in temperature can be carried out according to a constant gradient as illustrated by a dashed line in the figure.

It will be noted that because of the increased efficiency of the nitriding phases according to the invention it is possible to replace at least one nitriding step between two carburizing steps by a simple diffusion step. Such a step is shorter than a nitriding step so that the total duration of the treatment is shortened.

Naturally, the invention is not limited to the embodiments described and variations can be made without departing from the scope of the invention as defined by the claims. In particular, the initial rise in temperature can be carried out according to a constant gradient as illustrated by a dashed line in the figure.

Claims (6)

1. Method for carbonitriding steel parts, in particular parts entering into the production of motor vehicles, having alternate cementation (C1-Cn) and nitridation (N1-Nn-1) steps at a constant temperature, preceded by a temperature increasing step, and followed by a soaking step (T), characterised in that it has a final nitridation step (Nn) accompanied by a fall in temperature immediately before the soaking (T), in that the fall in temperature is achieved up to a temperature of between 900°C and 800°C, in that the fall in temperature is achieved with a temperature gradient of between 10°C/min and 1°C/min and in that the temperature increasing step comprises one single temperature increasing phase (M) followed by an initial nitridation phase (Ni) with a continued increase in temperature and the initial nitridation phase (Ni) is carried out from a temperature of between 700°C and 750°C, and up to a temperature of between 860°C and 1000°C.
2. Carbonitriding method according to claim 1, characterised in that the final nitridation step has a temperature bearing (Nn2).
3. Carbonitriding method according to claim 1, characterised in that during the initial nitridation phase (N1), the increase in temperature is achieved with a reduced temperature gradient with respect to the single temperature increasing phase (M).
4. Carbonitriding method according to claim 3, characterised in that the initial nitridation phase (N1) has a temperature bearing (Ni2-Ni3).
5. Carbonitriding method according to claim 3, characterised in that the initial nitridation phase (N1) is immediately followed by a first cementation step (C1).
6. Carbonitriding method according to claim 1, characterised in that the initial nitridation phase (N1) is carried out with a temperature gradient of between 3.5°C/min and 10°C/min.

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