WO2014170566A1

WO2014170566A1 - Thermochemical treatment method comprising a single nitriding phase before carburising

Info

Publication number: WO2014170566A1
Application number: PCT/FR2014/050581
Authority: WO
Inventors: Philippe Lapierre; Jerome Lardinois
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2013-04-18
Filing date: 2014-03-13
Publication date: 2014-10-23
Also published as: FR3004731A1; FR3004731B1; CN105264106A; EP2986750A1

Abstract

A method for thermochemically treating steel parts to improve the mechanical properties of same, comprising the diffusion of carbon by carburising and of nitrogen by nitriding in a low pressure furnace following a temperature cycle, characterised in that it first comprises an increase to a first relatively low temperature (T1) at which it is possible to start nitriding the metal, then a single nitriding phase (N) at a temperature between said first temperature (T1) and a second carburising temperature (T2), then a single carburising phase (C), followed by diffusion (D) at said second temperature (T2) which remains constant, and finally the phase of quenching (T) the metal.

Description

THERMO-CHEMICAL PROCESSING METHOD COMPRISING A SINGLE NITRIDING PHASE BEFORE CEMENTATION

The present invention relates to a method of thermochemical treatment of steel parts to reinforce them, and a gearbox gear for a motor vehicle reinforced with such a type of treatment.

To obtain a better hardness of steel parts over a certain thickness from the surface, in particular for motor vehicle gearbox gears comprising highly stressed toothing, it is known to perform a thermochemical treatment of this type of parts. by carbonitriding.

These treatments comprise first a carbon and nitrogen temperature diffusion over a certain thickness of the material, before quenching the part making it possible to increase the surface hardness while keeping certain original characteristics at heart. .

These types of treatment are carried out in a low-pressure furnace, with operations comprising, following a defined temperature profile, successive cementation phases to add carbon, nitriding to add nitrogen, and diffusion to diffuse these elements. At the end of the cycle, the final quenching is carried out.

One type of known treatment method, presented in particular by the document FR-B1-284523, comprises a first stage of temperature rise to reach 930 ° C., then a second stage of equalization of this temperature, followed subsequently by a succession similar cycles comprising first a cementation phase and then a nitriding phase, and finally the thermal quenching. It remains well during all cycles of supply elements at a constant temperature.

A problem with this type of process is that with the carburization temperatures usually used, which are around 900 ° C, the nitriding efficiency is low. It is then necessary to have significant nitriding times to introduce enough nitrogen into the room. In addition nitriding the workpiece with a reducing gas such as ammonia causes a decarburization of the metal surface by the hydrogen released during the decomposition of this ammonia, which is reducing and consumes the carbon. This decarburization reduces the hardness, and degrades the functional performance of the room.

Moreover, the control of the nitriding at the end of the cycle is delicate because the austenitic steel grains are already saturated with carbon and nitrogen by the preceding cementation phases. Grains can no longer absorb nitrogen and release it into grain boundaries. This nitrogen combines with the additive elements such as chromium and nickel, which charges the gaskets into precipitates of nitride of chromium and silicon, as well as carbonitrides.

Another type of known process of treatment, presented in particular by the document JP-B2-4655528, comprises a first stage of rise in temperature to reach 800 ° C followed by a first phase of nitriding at this temperature, then a second step of temperature rise to 980 ° C, then a succession of cycles each comprising a cementation phase followed by diffusion by maintaining this temperature, then a temperature descent step at 850 ° C followed by a second nitriding phase at this temperature. temperature, and finally the quenching phase of the metal.

For this process, a cementation time is obtained which comprises a succession of cementation and diffusion cycles, which is relatively long. In addition, the second nitriding phase after cementation also causes a decarburization of the layer on the surface of the metal.

The present invention is intended to avoid these disadvantages of the prior art.

It proposes for this purpose a method of thermochemical treatment of steel parts to improve the mechanical characteristics, comprising a carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it first has a rise to a first temperature relatively low to begin the nitriding of the metal, then a single nitriding phase at a temperature between this first temperature and a second carburizing temperature, then a single carburizing phase, then a diffusion at this second temperature which remains constant, and finally the quenching of the metal.

An advantage of this treatment process is that the cementation is not followed by a nitriding phase, there is no decarburization of the surface of the workpiece, and its hardness remains at the best level. The metallurgical quality is also better controlled, avoiding the formation of various nitrides in the grain boundaries that would reduce the mechanical characteristics of the part.

In addition, the only nitriding phase being made between the first relatively low temperature allowing this operation, and the second carburizing temperature, it is possible in certain cases to produce temperature rises simultaneously during this nitriding, which saves time on the complete cycle of treatment.

The thermochemical treatment process according to the invention may further comprise one or more of the following characteristics, which may be combined with each other.

According to one embodiment, the nitriding phase is carried out at the first temperature which remains constant.

The nitriding phase can be followed before the carburizing phase, by a diffusion phase comprising a rise in temperature from the first temperature to the second carburizing temperature.

Alternatively, the nitriding phase can be followed immediately after the carburising phase, first comprising a temperature rise from the first temperature to the second carburizing temperature.

According to another embodiment, the nitriding phase comprises a rise in temperature from the first temperature to the second carburizing temperature. The nitriding phase can be followed before the carburizing phase by a diffusion phase carried out at the second temperature which remains constant.

Advantageously, the first relatively low temperature for starting the nitriding of the metal is of the order of 700 ° C.

Advantageously, the second carburizing temperature is of the order of 940 ° C.

The invention also relates to a steel gear for a motor vehicle gearbox, reinforced by a thermochemical treatment comprising any one of the preceding features.

The invention will be better understood and other features and advantages will appear more clearly on reading the following description, given by way of example and without limitation with reference to the accompanying drawings in which:

FIG. 1 is a micrographic section of an austenitic steel treated by a carbonitriding process according to the prior art;

FIG. 2 is a micrographic section of this steel treated by a carbonitriding process according to the invention;

FIG. 3 is a graph showing, as a function of time, the treatment method according to the invention;

FIG. 4 is a graph showing the treatment method according to a first variant; and

FIG. 5 is a graph showing the treatment method according to a second variant.

FIG. 1 shows the micrographic section 6 of a low carbon austenitic steel, made to a height of a few tens of micrometers below the surface 2. This steel has undergone a low pressure carbonitriding treatment in accordance with the process presented by FIG. FR-B1 -2884523, comprising successive phases of carburizing and nitriding before final quenching. At the joint of the grains, large precipitates of nitrides 4 forming black spots are observed, following the supply of ammonia throughout the duration of the treatment phase giving an enrichment of carbon and nitrogen. These precipitates reduce the mechanical strength of the surface layer, which can cause premature wear of highly stressed parts with stress cycles, such as gears of gearboxes for motor vehicles.

Figure 2 shows the micrographic section of a similar austenitic steel, having undergone a low pressure carbonitriding treatment according to the process according to the invention. For the layer a few tens of micrometers below the surface 2, there is an absence of nitride precipitate in the grain boundaries.

FIG. 3 shows a first carbonitriding treatment cycle according to the invention carried out in a low pressure furnace, indicating the temperature T ° as a function of time t. The piece of steel to be treated is at time t0, at room temperature.

Firstly, up to the first time t1, a first step M of progressive temperature rise of the furnace is made to reach a first temperature level T1 of nitriding, which in this example is 700 ° C.

Then there is a second nitriding step N, which continues until the second time t2 remaining at this first temperature level T1.

It should be noted that the ideal nitriding temperatures are between 750 and 800 ° C. However, it can be started at 700 ° C and continued up to 900 ° C if you tolerate a little less nitrogen absorption in the room, to save processing time by reducing the rise time in temperature.

Then, until the third time t3, a third diffusion step D comprises a rise in temperature under a neutral atmosphere, to reach a second temperature plateau T2 of carburizing, which in this example is 940 ° C. During this third stage, diffusion D of nitrogen is carried out in the metal surface layer.

Then there is a fourth cementation step C, which continues until the fourth time t4 remaining at this second level of cementation temperature T2.

It will be noted that the carburizing temperature may vary, with a treatment depth which increases as a function of this temperature. For example, to obtain the same processing depth corresponding to the standardized treatment "E650", it takes a time of 3h to 880 ° C, and 2h to 940 ° C. It is not recommended to carry out a cementation above 950 ° C because it creates a large magnification of martensitic grains, and generates losses of functional performance of the parts.

Then there is a fifth step of second diffusion D in a neutral gas, which continues until the fifth time t5 remaining at this second level of cementation temperature T2. This second diffusion step D is optional, it depends on the desired carbon content at the end of treatment.

Finally, there is the last quench step T of the part which can be made in a gas, or in a liquid such as an oil or a polymer.

This gives both a relatively short treatment time that reduces production costs, and a rate of carbon and nitrogen absorption interesting without, however, producing a nitride precipitate at the grain boundaries of the material.

FIG. 4 alternatively shows a second carbonitriding treatment cycle according to the invention.

First, there is the first step M of temperature rise which is identical, to reach at the first time t1 the temperature T1 allowing nitriding.

Then there is the second nitriding step N which is carried out at the same time as a slow and gradual rise in temperature, to reach the second time t2 the second temperature plateau T2 of carburizing. Then there is the third diffusion step D of the nitrogen in the metal surface layer, which continues until the third time t3 remaining at this second level of cementation temperature T2.

The following steps, identical to those presented in FIG. 3, comprise the fourth cementation step C up to the fourth time t4 and the fifth diffusion step D up to the fifth time t5, remaining at this second cementation temperature step T2. Finally, we have the last stage of tempering T.

This variant makes it possible to optimize the treatment time, by reducing the temperature step between the nitriding phase N and the cementation phase C. However, the nitriding being carried out with a variable temperature rising above its optimum level, the concentration Nitrogen on the part is weaker with the same nitriding time.

FIG. 5 alternatively presents a third carbonitriding treatment cycle according to the invention.

Firstly, the first step M of temperature rise is identical, to reach at the first time t1 the first nitriding temperature T1.

Then there is the second nitriding step N which continues until the second time t2, remaining at the level of the first nitriding temperature T1 as for the process shown in FIG.

There is then no nitrogen D diffusion step, but instead a third cementation start step C which begins immediately, with up to a third time t3 a steady temperature rise to reach the second level of temperature of cementation T2.

The following steps are identical to those presented in FIG. 3, comprising the fourth step which is the continuation of carburizing C to the fourth time t4 and the fifth diffusion step D to the fifth time t5, remaining at this second level of T2 temperature. Finally, we have the last stage of tempering T. This variant also makes it possible to optimize the treatment time, by reducing or eliminating the diffusion step D between the nitriding N and the carburizing C, without, however, affecting the level of nitrogen in the part.

These different variants thus provide a better control of the process, comprising a nitrogen enrichment only before the carburizing phase, which allows a good diffusion of the carbon in the treated layer.

In addition, it is possible to mix the different variants presented, comprising, for example, a nitriding phase N comprising a temperature which initially remains at the first temperature stage T1, and which then gradually rises to reach the second temperature stage T2.

In addition, this process, specially developed by the company carrying out the invention, makes it possible not to use particular carbonitriding processes which may be the property of suppliers of low pressure furnaces intended for this type of treatment.

Claims

1 - Process for the thermochemical treatment of steel parts to improve the mechanical characteristics, comprising carbon diffusion by carburizing and nitrogen nitriding in a low pressure furnace according to a temperature cycle, characterized in that it comprises first a rise to a first temperature (T1) relatively low of the order of 700 ° C to begin the nitrurâion of the metal, then a single nitriding phase (N) at a temperature between this first temperature (T1) and a second carburizing temperature (T2), then a single cementation phase (C), then a diffusion (D) at this second temperature (T2) which remains constant, and finally the quenching phase (T) of the metal.

2 - Process according to claim 1, characterized in that the nitriding phase (N) is carried out at the first temperature (T1) which remains constant.

3 - Treatment process according to claim 2, characterized in that the nitriding phase (N) is followed before the carburizing phase (C) by a diffusion phase (D) comprising a rise in temperature from the first temperature (T1) at the second carburizing temperature (T2).

4 - Process according to claim 2, characterized in that the nitriding phase (N) is followed immediately after the carburizing phase (C) comprising first a temperature rise from the first temperature (T1) to the second carburizing temperature (T2).

5 - Process according to claim 1, characterized in that the nitriding phase (N) comprises a temperature rise from the first temperature (T1) to the second cementation temperature (T2).

6 - Process according to claim 5, characterized in that the nitriding phase (N) is followed before the carburizing phase (C), by a diffusion phase (D) carried out at the second temperature (T2) which remains constant.

7 - Process according to any one of the preceding claims, characterized in that the second carburizing temperature (T2) is of the order of 940 ° C.

8 - Steel pinion for a motor vehicle gearbox reinforced by a thermochemical treatment, characterized in that this treatment is carried out according to any one of the preceding claims.