EP2773789A1 - Carbonitriding method having a final nitridation step during temperature decrease - Google Patents

Abstract

The invention relates to a method for the carbonitriding of steel parts, in particular parts used in the manufacture of automobiles, comprising: a heating step that includes a simple heating phase (M) followed by an initial nitridation phase (Ni) from a temperature between 700°C and 750° C to a temperature between 860° C and 1000° C and is carried out using a reduced temperature gradient compared to the simple heating phase; and alternate cementing (C1-Cn) and nitridation (N1- Nn) steps at constant temperature; wherein the final nitridation step is accompanied with a decrease in temperature immediately before quenching (T).

Description

 METHOD FOR CARBONITURING AT FINAL NITRIDATION STEP DURING TEMPERATURE LOWERING

The present invention claims the priority of the French application 1 159878 filed October 31, 201 1 whose content (text, drawings and claims) is here incorporated by reference.

The present invention relates to a process for the carbonitriding of 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

[Ooo3] Document EP 1885904 discloses a process for the carbonitriding 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. 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 [0005] With a view to attaining this object, it is proposed according to the invention a carbonitriding process for steel parts, in particular parts used in the manufacture of motor vehicles, comprising alternating steps. carburizing and nitriding at constant temperature, preceded by a temperature rise step, and followed by a quenching step, wherein the final nitriding step is accompanied by a lowering temperature immediately before quenching. [ooo6] 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. [Ooo7] 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.

[Ooo8] According to another advantageous aspect of the invention the temperature rise step comprises a simple temperature rise phase followed by an initial nitriding phase with continued rise in temperature. 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.

[Ooo9] According to yet another advantageous aspect of the invention, the initial nitriding phase is carried out from a temperature between 700 ° C and 750 ° C, and up to a temperature between 860 ° C and 1000 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[Ooi o] Other features and advantages of the invention will appear on reading the following description of different modes of implementation non-limiting particulars of the low pressure carbonitriding process according to the invention, with reference to the 3 appended figures which are diagrammatic diagrams illustrating the different steps of the method according to the invention according to different embodiments. DETAILED DESCRIPTION OF THE INVENTION

[Ooi i] Referring to 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 room temperature to at a point at a temperature of 700 ° C, denoted Ni 1 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 minutes and 90 minutes. 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 method then comprises an initial nitriding phase Ni with continuation of the temperature rise step to a temperature of 940 ° C in the example shown. 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 embodiment of 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 known manner 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 in FIG. 2, the rise in temperature continues with the same temperature gradient as during the single temperature rise to a point at a temperature comprised between between 750 ° C and 850 ° C, here 800 ° C, denoted Ni2 in the figure. The temperature is then maintained in a step until a moment noted Ni3 in Figure 2 from which a high temperature rise is performed to reach the carburizing temperature.

The bearing temperature is chosen in a manner known per se to achieve 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 carried out 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 with the aid of FIG. 3, the rise in temperature continues from the point Ni 1 with a lower temperature gradient than in the first mode. embodiment, preferably in a range of 2 ° C / min to 8 ° C / min, up to a moment noted Ni4, here corresponding to a temperature of 850 ° C, from which a high temperature rise is achieved to reach the carburising temperature, according to a gradient similar to that of the second embodiment. Whatever the embodiment used for the initial nitriding phase, the method then comprises n alternating carburizing 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. On the FIG. 4, 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 temperature drop. 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 nitriding while remaining high enough to allow efficient 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 minutes and 60 minutes, 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. 2, the descent of temperature is first of all carried out in a strong manner, 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, noted Nn1 in the figure, here 840 ° C, and 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 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.

Of course, the invention is not limited to the embodiments described and can be made alternative embodiments 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

 1. Process for carbonitriding steel parts, in particular parts used in the manufacture of motor vehicles, comprising alternating steps of carburizing (C1-Cn) and nitriding (N1 -Nn-1) at temperature 5 constant, preceded by a step of temperature rise, and followed by a quenching step (T), characterized in that it comprises a final nitriding step (Nn) accompanied by a temperature drop immediately before the quenching (T). 2. Carbonitriding process according to claim 1, characterized in that the descent of temperature is carried out up to a temperature between 900 ° C and 800 ° C. 3. carbonitriding process according to claim 1, characterized in that the descent of temperature is carried out with a temperature gradient of between 10 ° C / min and 1 ° C / min. 5 4. carbonitriding process according to claim 1, characterized in that the final nitriding step comprises a temperature step (Nn2). 5. carbonitriding process according to claim 1, characterized in that the temperature rise step comprises a single temperature rise phase (M) followed by an initial nitriding phase (Ni) with o continuing the rise in temperature. 6. carbonitriding process according to claim 5, characterized in that the initial nitriding phase (Ni) is carried out from a temperature between 700 ° C and 750 ° C, and up to a temperature between 860 ° C and 1000 ° C. 5 7. carbonitriding process according to claim 5, characterized in that during the initial nitriding phase (N1) the rise in temperature is carried out with a reduced temperature gradient compared to the phase of single temperature rise (M). 8. carbonitriding process according to claim 7, characterized in that the initial nitriding phase (Ni) comprises a temperature step (Ni2-Ni3). 9. carbonitriding process according to claim 7, characterized in that the initial nitriding phase (N1) is immediately followed by a first cementation step (C1). 10. carbonitriding process according to claim 5, characterized in that the initial nitriding phase (N1) is carried out with a temperature gradient of between 3.5 ° C / min and 10 ° C / min.