EP2773788B1 - Method for low-pressure carbonitriding having an extended temperature range in an initial nitridation phase - Google Patents

Description

The present invention claims the priority of the French application 1159875 filed on October 31, 2011 whose content (text, drawings and claims) is hereby incorporated by reference.

The present invention relates to a low pressure carbonitriding process for 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 low pressure carbonitriding process of 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 step quenching. 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 and 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 view of the achievement of this object, it is proposed according to the invention a low pressure carbonitriding process of steel parts, in particular parts used in the manufacture of motor vehicles, according to claim 1.

Thus, without increasing the duration of the temperature rise stage, the nitrogen enrichment which is carried out under conditions favorable to good nitriding is increased so that it is possible to shorten or to eliminate one of the stages of subsequent nitriding and thus reduce the total treatment time.

According to an advantageous version of the invention, the initial nitriding phase is immediately followed by a first cementation step. Thus, the total elimination of the temperature equalization phase makes it possible to lengthen the initial nitriding phase in an optimum temperature range for nitriding.

According to another advantageous aspect of the invention, during the initial nitriding phase the temperature rise is carried out with a reduced temperature gradient compared to the simple temperature rise phase. Thus the treatment time is further increased in an optimum temperature range for nitriding.

According to yet another advantageous aspect of the invention, the process comprises a final nitriding step accompanied by a lowering of temperature immediately before the quenching. Thus the final nitriding step is also carried out in an optimum temperature range, so that the quality of the treatment is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear on reading the following description of a particular embodiment of the low pressure carbonitriding process according to the invention, with reference to the figure 1 annexed illustrating the different steps of the method according to the invention. The figures 2 and 3 illustrate a low pressure carbonitriding process.

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 simple temperature rise phase 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 temperature rise 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 figure 2 the temperature rise of the nitriding phase is continued with the same temperature gradient as during the single temperature rise to a point at a temperature of 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 figure 3 the rise in temperature of the nitriding phase is continued from point Ni1 with a lower temperature gradient than in the first embodiment, preferably in a range of 2 ° C / min at 8 ° C / min. , up to a moment noted Ni4, here corresponding to a temperature of 850 ° C, from which a high temperature rise is made to reach the carburizing temperature, according to a similar gradient to that of the second embodiment.

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 the invention, the last nitriding step Nn, illustrated by a dashed line in the figure, the temperature is lowered continuously to a temperature in the optimum temperature range for nitriding while remaining high enough to allow effective quenching. In the example illustrated, the final temperature before quenching is 840 ° C. In practice satisfactory results 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 figure 2 , the last nitriding step Nn, illustrated by a long dashed line in the figure, the descent of temperature is first of all carried out in a strong way, with a gradient as strong as possible without causing undue stresses in the steel, up to the optimum nitriding temperature for the steel being treated, denoted Nn1 in the figure, here 840 ° C, then the temperature is maintained at a plateau until the beginning of quenching.

In practice, the process according to the invention can even terminate the treatment cycle in a conventional manner, that is to say with a quench carried out directly from the carburising temperature.

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 embodiment described and alternative embodiments 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. In this case, however, it will be noted that the nitriding phase has a shortened duration as illustrated by a mixed line in the figure.

Due to the reduced temperature gradient during the initial nitriding phase, it has been experimented that the temperature of the workpieces has time to equalize so that it is possible to eliminate the equalization step provided in the aforementioned document. If this is necessary, for example because of a particular configuration of the parts to be treated, it may however provide a temperature equalization step of reduced duration between the initial nitriding phase and the first cementation step.

Claims (5)

1. Method for low pressure nitrocarburizing steel parts, in particular entering into the production of motor vehicles, comprising alternate steps of cementation (C1-Cn) and of nitriding (N2-Nn), at a constant temperature preceded by a step of increasing temperature comprising a simple temperature increasing phase (M), and followed by a soaking (T) step, characterised in that the simple temperature increasing phase (M) is done with a temperature gradient of between 8 °C/min and 75 °C/min and is followed by an initial nitriding phase (Ni) done with a temperature gradient continuing the temperature increase, in that the initial nitriding phase is done from a temperature of between 700 °C and 750 °C, and until a temperature of between 860 °C and 1000 °C and in that during the initial nitriding phase (Ni), the temperature increase is done with a temperature gradient of between 3.5 °C/min and 16 °C/min.
2. Low pressure nitrocarburization method according to claim 1, characterised in that the initial nitriding phase is immediately followed by a first cementation step.
3. Low pressure nitrocarburization method according to claim 1, characterised in that it comprises a final nitriding step (Nn) accompanied by a fall in temperature immediately before the soaking (T).
4. Low pressure nitrocarburization method according to claim 3, characterised in that the fall in temperature is done until a temperature of between 900 °C and 800 °C.
5. Low pressure nitrocarburization method according to claim 3, characterised in that the fall in temperature is done with a temperature gradient of between 10 °C/min and 1 °C/min.

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