EP4045689A1

EP4045689A1 - Method for treating a part made of ferrous metal, and part made of ferrous metal

Info

Publication number: EP4045689A1
Application number: EP20851331.7A
Authority: EP
Inventors: Luc HERMANN; Vincent MONTEUX
Original assignee: Hydromecanique et Frottement SAS
Current assignee: Hydromecanique et Frottement SAS
Priority date: 2019-12-24
Filing date: 2020-12-23
Publication date: 2022-08-24
Also published as: KR20220115576A; BR112022010610A2; WO2021130460A1; ZA202205809B; MX2022007942A; AU2020411028A1; FR3105262A1; CN114846160A; FR3105262B1; CA3160425A1; JP2023508956A; US20230029324A1

Abstract

The invention relates principally to a method for treating a part (P) made of ferrous metal, comprising: a nitriding operation forming on the part (P) a combination layer (2) having a thickness of between 5 and 30 pm, and a diffusion region (3), arranged beneath and in contact with the combination layer (2), having a thickness of between 100 pm and 500 pm; then an operation of quenching the part (P) by high-frequency induction, over an induction depth that is greater than or equal to 0.5 mm, thereby hardening the part (P) and lending said part (P): • a surface hardness greater than or equal to 50 HRC, • a hardness of the combination layer (2) greater than or equal to 400 HV0.05, • a hardness of the part of greater than or equal to 500 HV0.05 at a depth of 500 pm, and wherein the high-frequency induction quenching operation is carried out without the application of a protective film on the part (P) prior to the induction quenching operation. The invention also relates to a part (P) made of ferrous metal, having significant resistance to wear by abrasion and adhesion, improved friction properties and improved resistance to scaling, and good corrosion behavior.

Description

PROCESS FOR TREATING A FERROUS METAL PART AND A PART OF

FERROUS METAL

TECHNICAL AREA

The field of the invention is that of the surface treatment of ferrous metal parts, in particular very low or low alloy steel.

PRIOR ART

In automotive, aeronautical or industrial applications, mechanical parts are generally subjected to significant stresses in service.

Conventionally, the parts can receive one or more treatments making it possible to improve some of their performances, among which the frictional properties, the wear resistance, the fatigue resistance, the chipping resistance, the resistance. corrosion, etc.

However, it is difficult to achieve a good compromise between the different properties of the part.

By way of example, document WO2011013362A1 describes a method of treating a part, comprising a nitriding operation, a coating operation with a chemical conversion film (sol-gel), and an induction hardening operation. However, such a process is prohibitively expensive, due to the cost of the film and the need to carry out three successive operations.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to remedy the above drawbacks, while maintaining a good compromise between the different properties of the part.

To this end, the invention relates to a process for treating a piece of ferrous metal, comprising:

a nitriding operation forming on the part a combination layer having a thickness between 5 μm and 30 μm, and a diffusion zone arranged under and in contact with the combination layer having a thickness between 100 μm and 500 μm; then - a high-frequency induction hardening operation of the part, to an induction depth greater than or equal to 0.5 mm, causing hardening of the part providing said part:

• a surface hardness greater than or equal to 50 HRC,

• a hardness of the combination layer greater than or equal to 400 HV0.05,

• a hardness of the part greater than or equal to 500 HV0.05 at a depth of 500 μm, and in which the high-frequency induction hardening operation is carried out without applying a protective film to the part prior to the induction hardening operation.

The method of the invention makes it possible to obtain a part exhibiting both high resistance to wear by abrasion and adhesion, improved friction properties and resistance to chipping, as well as good corrosion resistance. . The method of the invention is also simpler to implement and less expensive than the methods of the prior art because it saves the arrangement of a protective film of the combination layer, as well as the possible removal of said layer. protective film.

The protective film can be of any type suitable for preventing degradation of the combination layer during hardening by high frequency induction, this degradation being able to manifest itself by chipping, cracking, or fracturing of the combination layer.

In particular, the protective film can be a sol-gel film. Therefore, the high-frequency induction hardening operation is performed without a sol-gel film.

According to other aspects, the treatment process according to the invention has the following different characteristics taken alone or according to their technically possible combinations: the nitriding operation is carried out with gas, or by plasma, or by molten salts; the nitriding operation is carried out at a temperature between 500 ° C and 630 ° C, for a period of between 15 minutes and 3 hours; the induction hardening operation is not followed by a tempering operation; the workpiece high-frequency induction hardening operation is carried out so as to retain ferrite in the workpiece between the diffusion zone / combination layer interface and a depth of 500 μm, preferably between the zone interface diffusion / combination layer and a depth of 300 µm. The diffusion zone / combination layer interface means the contact surface between the diffusion zone and the overlying combination layer. Quenching is quick and does not not completely transform the part's ferrite into martensite over its processing depth, so that ferrite remains over the depth treated by HF quenching at the end of the process. The depth of 500 μm corresponds to an induction depth where hardening and / or changes in the metallurgical structure of the part are observed; the high-frequency induction hardening operation of the part is carried out so as to have a residual ferrite level in the part, between the diffusion zone / combination layer interface and a depth of 500 μm, between 1% and 50% by volume, preferably between 1% and 30%, and more preferably between 5% and 30%. The level of residual ferrite is volume, and corresponds to the ratio of the volume of ferrite to the volume of the rest of the part in the zone considered; the high-frequency induction hardening operation of the part is carried out so as to have a residual ferrite level in the part, between the diffusion zone / combination layer interface and a depth of 500 μm, between 5% and 20% by volume, preferably between 5% and 15%; the method comprises an impregnation step subsequent to the high-frequency induction hardening operation. If a tempering step is performed, impregnation is performed after tempering. It can be done for example by soaking or by spraying. Impregnation protects the part because it makes it possible to delay the onset of corrosion, reduce the corrosion rate and thus increase the life of the part; the process provides the part with corrosion resistance greater than 80 hours during a neutral salt spray test. Corrosion resistance is measured by a neutral salt spray test, sometimes also called a standard salt spray, according to EN ISO 9227; the high-frequency induction hardening operation is carried out with the following parameters:

• a frequency between 50 and 400 kHz,

• linear energy between 4.6 and 5.8 J / mm.

This dual condition on frequency and linear energy makes it possible to obtain a ferrous metal part whose mechanical properties are greatly improved compared to parts of the state of the art, in particular resistance to wear by abrasion and adhesion, friction, resistance to chipping, while maintaining good corrosion resistance. The frequency and linear energy are adjusted according to the morphology of the part, for example its diameter; the high-frequency induction hardening operation is carried out with a running speed of between 5 and 40 mm / s.

The invention also relates to a part made of ferrous metal, comprising a combination layer having a thickness between 5 μm and 30 μm, and a diffusion zone arranged under and in contact with the combination layer, having a thickness between 100 pm and 500 pm, said part having:

- a surface hardness greater than or equal to 50 HRC,

- a hardness of the combination layer greater than or equal to 400 HV0.05,

- a hardness of the part greater than or equal to 500 HV0.05 at a depth of 500 μm, said part comprising ferrite and martensite between the diffusion zone / combination layer interface and a depth of 500 μm.

According to other aspects, the ferrous metal part according to the invention has the following different characteristics taken alone or according to their technically possible combinations: the hardness of the part at a depth of 0.5 mm is greater than or equal to a hardness of core + 100 HV0.05; the hardness of the part at a depth of 0.25 mm is greater than or equal to a core hardness + 350 HV0.05; the part is made of very low alloy steel, of the C10-C70 family, with a manganese content of less than 1%. Under these conditions, the steel does not exhibit any significant addition element, that is to say an element which would exceed 5% by mass relative to the total mass of the steel. Preferably, the part is made of C45 grade steel. The term “grade”, commonly used in the field of steels, designates a specific steel in a family. In particular, this is the C45 grade chosen from the C10 to C70 family of steels; the part is made of low alloy steel, with no addition element exceeding 5% by mass. More preferably, the part is made of 31CrMo4 grade steel; the part comprises ferrite and martensite between the diffusion zone / combination layer interface and a depth of 300 µm; the part comprises a level of ferrite, between the diffusion zone / combination layer interface and a depth of 500 μm, between 1% and 50% by volume, preferably between 1% and 30%, and more preferably between 5% and 30%; the part comprises a level of ferrite, between the diffusion zone / combination layer interface and a depth of 500 μm, between 5% and 20% by volume, preferably between 5% and 15%; the part exhibits a corrosion resistance greater than 80 hours in a neutral salt spray test.

In this text, "thickness" is understood to mean the distance between the upper limit and the lower limit of a layer or a given area within the piece of ferrous metal. The thickness is perpendicular to the average surface of said upper and lower limits.

The term “depth” designates the distance between the surface of the part, also called the free surface, and a given point within the part. The depth is perpendicular to the mean surface of the free surface. For example, "a hardness of the diffusion zone greater than or equal to 500 HV0.05 at a depth of 500 µm" means that at a distance of 500 µm within the part, counted from the free surface of the part. part, the hardness of the diffusion zone is greater than or equal to 500 HV0.05.

Terms such as "over", "above", "above" and "below", "below", "below" refer to the positions of layers or areas relative to each other within the part. . These terms do not necessarily imply that there is contact between the layers or areas considered.

In a manner known per se, nitriding consists of immersing a piece of ferrous metal in a medium liable to give up nitrogen. In this text, nitriding includes nitrocarburizing, which is a variant of nitriding, in which carbon enters the part in addition to nitrogen. The ARCOR process described in the remainder of this text is a preferred example of a nitrocarburizing process.

Within the treated part, the diffusion zone is arranged under the combination layer, and extends towards the core of the part (away from the free surface) from said combination layer. The combination layer for its part can be on the surface of the part or at a given depth.

An induction depth greater than or equal to 0.5 mm means that the hardening and / or changes in the metallurgical structure of the part, caused by the induction hardening step, extend from the surface of the part to 'to a depth of at least 0.5 mm. Beyond a certain depth, the thermal effect gradually diminishes until there is no longer any measurable effect on the microstructure and hardness of the part.

The high-frequency induction hardening operation provides a hardness of the part greater than or equal to 500 HV0.05 at a depth of 500 µm, and preferably a corrosion resistance greater than 80 hours when tested at. standard salt spray. In fact, surprisingly, the hardening by high-frequency induction according to the invention makes it possible to reinforce the mechanical characteristics, in particular the hardness, of the part nitrided beforehand, while retaining the combination layer. Thus the corrosion resistance of the parts is maintained without resorting to an additional device such as, for example, a sol-gel film or a paint. Without a sol-gel film, processing costs are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the accompanying drawings in which:

FIG. 1 is a graph illustrating the hardness profile of two parts, respectively compliant (ARCOR FLASH, that is to say ARCOR nitriding treatment followed by high frequency induction hardening) and not in accordance with the invention ( ARCOR alone, without high frequency induction hardening).

Figure 2 is a table describing a series of tests carried out on steel parts, in order to characterize the process according to the invention.

Figure 3 is a graph illustrating a series of tests corresponding to the table in Figure 2.

FIG. 4 is a micrograph of a part treated by the method according to the invention.

Figure 5 is a close-up view of Figure 4.

Figure 6 is a micrograph of a part treated according to the prior art (ARCOR treatment followed by induction hardening according to the prior art).

Figure 7 is a micrograph of a part treated after ARCOR treatment, without induction hardening.

Figure 8 is a micrograph of a ferrous metal part according to the invention (ARCOR FLASH treatment).

FIG. 9 is an assembly of a micrograph of a ferrous metal part according to the invention and of a hardness profile obtained by measurement on this same part.

FIG. 10 is a graph illustrating the evolution of the friction coefficient of rings, for a ring in accordance with the invention (ARCOR FLASH treatment) and a ring of the state of the art (ARCOR treatment alone). FIG. 11 is a photograph of a ferrous metal part having undergone an ARCOR treatment alone.

Figure 12 is a photograph of a ferrous metal part according to the invention, having undergone an ARCOR FLASH treatment (ARCOR nitriding followed by high frequency induction hardening).

Figure 13 is a close-up view of the micrograph of Figure 4, centered on the induction layer.

DETAILED DESCRIPTION OF THE INVENTION

The inventors' approach was to carry out several series of tests using different treatments of a ferrous metal part.

In particular, the inventors have studied the effects of the following two treatments.

The ARCOR nitrocarburizing treatment (trademark registered by the Applicant) provides, from the surface to the core of the part, a combination layer 2 and a diffusion zone 3 juxtaposed (see FIG. 4). The combination layer 2 typically has a thickness of about 20 µm, while the diffusion zone 3 typically has a thickness of a few tens or hundreds of microns, for example 300 µm.

High frequency hardening (frequency> 20 kHz) provides a martensitic structure on the surface of the part, on an induction layer generally having a depth of around 1 mm. In other words, the induction hardening extends from the surface of the part to a depth of the order of 1 mm, and is superimposed on the hardening profile already obtained by the nitriding. The induction layer comprises Fe (a ') martensite resulting from the transformation of Fe (a) ferrite, as well as the remaining untransformed Fe (a) ferrite, and offers a significant hardness, admitted as very favorable to resistance to abrasive wear and fatigue.

The combination layer 2 offers, among other things, good frictional properties, high adhesive wear resistance and good corrosion resistance.

The diffusion zone 3 offers a hardness gradient, between the combination layer 2 and the base material 1 located under the diffusion zone 3, favorable to a certain wear resistance (abrasive and adhesive) and a resistance to the tired.

Table 1 below describes different series of tests:

Caption:

0: non-existent property +: moderate property improvement ++: good property +++: excellent property -: degraded property

Comments on the results of the test series:

- Series 1: Low depth of hardness in the underlayment ("0.3 mm), therefore resistance to abrasive wear and moderate fatigue.

- Series 2: Lack of anti-seize property and corrosion resistance. - Series 3: The ARCOR nitriding temperature ( ^“ 590 ° C) has a tempering effect on the martensitic structure provided by the HF quenching. This results in a significant drop in hardness. The results are comparable to those of Series 1.

- Series 4: The time / temperature parameter of the HF quenching degrades the ARCOR combination layer. The corrosion resistance properties and the tribological behavior are therefore degraded.

- Series 5: Surprisingly, the HF FLASH quenching makes it possible to minimize or even eliminate the degradation of the ARCOR combination layer 2 (oxidation or spalling which induces the loss of corrosion resistance properties and the tribological properties associated with the combination layer 2). In comparison with the Series 4, part P retains its basic properties provided by ARCOR. Compared to Series 1, HF FLASH quenching increases the hardness below combination layer 2, as well as the depth of hardening.

The development of the invention required, firstly, to identify the unexpected advantages of HF FLASH quenching compared to conventional HF quenching, then secondly, to characterize the parameters of HF FLASH quenching. in order to be able to implement the ARCOR treatment process + HF FLASH quenching = ARCOR FLASH on all types of ferrous parts.

Figure 1 is a graph for comparing the hardness profile of two parts, including a part receiving ARCOR treatment alone (Series 1) and a part receiving ARCOR FLASH treatment according to the invention (Series 5). The ARCOR FLASH treatment increases the hardness below the combination layer 2, especially in the diffusion zone, as well as the depth of cure. For the sample of Figure 1, the diffusion zone 3 has a thickness of between 400 µm and 500 µm and the induction depth is approximately 1 mm.

Figure 2 is a table describing a series of tests carried out on steel parts, in order to characterize the ARCOR FLASH treatment process according to the invention.

The parts are steel bars with a diameter of 38 mm, having received an ARCOR treatment creating a combination layer with a thickness between 15 and 20 µm.

The E1-E9 tests are carried out on C45 steel bars, the E10 and E11 tests on C10 steel bars, and the E12 test on a C70 steel bar and the E13 test on a 42CD4 steel bar. The tests consist of high-frequency induction hardening operations, carried out with variable parameters. The travel speed is that of the mobile magnetic inductor in translation along the part.

Comments on the test results:

- E1 (comparative): Low frequency and high power. Induction degraded combination layer.

- E2 (according to the invention): Optimal linear energy. Satisfactory results.

- E3 (comparative): Scrolling a little too fast. Linear energy a little too low. Too low surface hardness and induction depth.

- E4 (according to the invention): Results less good than E2 but better than E3.

- E5 (comparative): Scroll a little too slow. Linear energy a little too high. Satisfactory surface hardness and induction depth, but combination layer degraded by induction.

- E6, E7, E8 and E9 (all in accordance with the invention): Tests aimed at determining the influence of the frequency and the scrolling speed. Satisfactory results.

- E10, E11 and E12: Tests illustrating the influence of the steel grade on the results of the treatment.

- E10 (comparative): The parameters of test E5, tested on a C10 steel, give a non-compliant result.

- E11 (in accordance with the invention): The parameters of test E2, tested on a C10 steel, allow satisfactory results to be obtained.

- E12 (according to the invention): The parameters of test E2 also allow satisfactory results to be obtained with a C70 steel.

- E13 (in accordance with the invention): The parameters of test E8, applied to a 42CD4 steel, allow satisfactory results to be obtained.

Figure 3 is a graph showing the results of tests E1-E9 of Figure 2, performed on C45 steel bars.

In the graph, the linear energy (in W.s / mm) is shown on the abscissa and the induction frequency (in kHz) is shown on the ordinate.

Linear energy is defined as being the power of the induction reduced to the running speed of the parts P during the induction. This quantity is linked to the geometry of the P parts processed. Another more general quantity could be a surface power density applied over a certain time, that is to say the power of the induction divided by the surface area of the part absorbing the induction, and divided by the running speed. It would thus be possible, from optimal hardening parameters for a part of a first dimension, to easily find the optimal hardening parameters for a part of a second dimension (for example of larger diameter), the other parameters being otherwise equal (same material, same nitriding).

From figures 2 and 3, it can be seen that the tests carried out on C45, C10, C70 and 42CD4 steel for which the frequency (F) is between 50 kHz and 400 kHz and the linear energy (E) is between 4.6 and 5.8 J / mm (we are then in the area modeled by the rectangle in dotted lines in Figure 3), allow to obtain after induction: a combination layer of satisfactory quality, a combination layer with a hardness greater than or equal to 400 HV0.05, an induction depth greater than or equal to 0.5 mm, a surface hardness greater than or equal to 50 HRC, and satisfactory corrosion resistance.

In addition, these results are obtained without it being necessary to first coat the part in a protective film before hardening by high-frequency induction, such as a sol-gel film, which makes it possible to reduce the complexity and treatment costs.

For tests 2, 4, and 6-9 and 11-12, all in accordance with the invention, the following advantageous properties: the hardness of the diffusion zone at a depth of 0.25 mm is greater than or equal to a hardness core + 350 HV0.05, and the hardness of the diffusion zone at a depth of 0.5 mm is greater than or equal to the core hardness + 100 HV0.05.

The treatment according to the invention is therefore effective up to a great depth in the diffusion zone.

These tests were carried out on C45, C10, C70 and 42CD4 steel bars. In practice, the frequency (F) and the linear energy (E) of the high-frequency induction hardening are adapted to the ferrous metal of the part P. It may be necessary to proceed by tests in order to determine the appropriate parameters. For the production of micrographs of the metal parts illustrated in FIGS. 4 to 8, and described below, the parts were subjected to a chemical attack by a solution of nitric acid and alcohol called “Nital”. The Nital thus plays the role of revealing the microstructure of the part, and makes the latter visible under an optical microscope.

FIGS. 4 and 5 are micrographs of a part P in C45 steel having received the ARCOR FLASH treatment (ARCOR + HF induction hardening, according to the invention) with a combination layer 2 of 18 μm, a diffusion zone 3 of about 300pm and an induction depth of about 0.5mm.

Part P comprises a steel substrate 1, an induction layer 4, a combination layer 2 and diffusion zone 3. An aluminum foil 5 and a coating 6 have been added in order to make the cut necessary to make micrography. In figure 4, the segment [AB] represents the distance (the thickness) between an average surface of the combination layer 2 (interface between the diffusion zone 3 and the combination layer 2), and an average surface of the substrate steel 1.

The combination layer 2 and the diffusion zone 3 are here obtained by ARCOR nitrocarburizing.

The induction layer 4 is obtained by high-frequency induction. It is composed of fine martensite Fe (a ’) and ferrite Fe (a). FIG. 5 clearly shows the presence of Fe (a) ferrite remaining in the quenching zone of the part obtained at the end of the process, after quenching. It is a microstructure which conforms to the invention.

Figure 6 illustrates a micrograph of a steel that was nitrided and then received a conventional HF quenching: all the Fe (a) ferrite was transformed into Fe (a ’) martensite during the quenching. There is therefore no longer any ferrite left in the treated area. This microstructure is therefore not in accordance with the invention.

FIG. 7 illustrates a part made of ferrous metal having received an ARCOR nitrocarburizing alone (without quenching), and FIG. 8 illustrates a part according to the invention, therefore having received a nitrocarburizing then an HF quenching (ARCOR + hardening by HF induction, according to invention).

In FIG. 8, it is observed that the combination layer 2 of the part P comprises an upper layer 2a of black color and measuring around ten micrometers. This upper layer 2a has been made porous by the HF quenching, and is clearly revealed by the Nital. This demonstrates that at the end of the treatment process according to the invention, the combination layer is slightly degraded following the HF quenching, but remains present, and retains its structural integrity at least on its lower part 2b. Such an upper layer 2a is not observed in FIG. 7. The structure of the combination layer has in fact not been modified since there has been no quenching.

Part P according to the invention therefore indeed has a combination layer 2 providing the part with properties of resistance to wear and friction, and of resistance to corrosion, despite the fact that the HF hardening has been carried out. without protective film.

FIG. 9 is an assembly juxtaposing a micrograph of a part P according to the invention, and a hardness profile obtained by measurements on this same part. Hardness measurement points are visible on the micrograph and measurement steps corresponding to the different layers have been framed.

In this figure, the partially oxidized combination layer 2 and the induction layer 4 are particularly visible. Hardness measurements taken just below the suit layer show a hardness of up to 900HV. As you move away from the surface of the part and down towards the core of the part, the hardness decreases in an almost linear fashion, which makes it possible to estimate the thickness of diffusion zone 3 at about 175 μm, where the depth is. hardness is 775HV.

For depths ranging from 200pm to 500pm, the hardness is generally stable at values between 550 and 600HV. These depths are located in the induction treatment zone, which is visually detectable on the micrograph from the crystallography of the part.

Measurements taken from a depth of 600pm and beyond are located in the base material of the part, i.e. the core of the part, which has not received any treatment. The hardnesses recorded are around 250HV.

With reference to FIGS. 10 to 12, the Applicant then proceeded to mechanical tests for the aging of parts in order to characterize the performance of the parts obtained. A smooth 42CD4 steel ring with ARCOR nitrocarburizing alone, hereinafter referred to as "ARCOR ring", is compared with a smooth 42CD4 steel ring with ARCOR nitrocarburizing and HF quenching according to the invention.

These two rings were mounted on 16NC6 CT steel axles, with the addition of commercial lubricant. The applied load induced a contact pressure of 50MPa, and the speed of rotation of the rings relative to the axis was 7.8 mm / s.

FIG. 10 is a graph illustrating the evolution of the coefficient of friction of these two rings as a function of the number of revolutions performed. On the ordinate is the coefficient of friction m (without unit), and on the abscissa is the number of revolutions Rev (in turns) that the ring undergoes. We see that the ARCOR ring alone has a new coefficient of friction m of about 0.15, and that it begins to increase regularly from only 2000 revolutions, until reaching high values, of the order of 0.6 for about 9000 turns.

Part P according to the invention has, when new, a coefficient of friction slightly lower than that of the ARCOR ring alone, of the order of 0.1, and remains stable up to approximately 11,000 revolutions. It is only from this value that the coefficient of friction begins to increase, reaching a value of 0.6 at approximately 125,000 revolutions, similar to that of the ARCOR ring alone.

Figures 11 and 12 are photographs respectively of the ARCOR ring alone and of the part P according to the invention, after these tests. It can be seen that the ARCOR ring alone shows marked wear, material having been torn off by seizing. Part P for its part exhibits less pronounced wear.

FIG. 13 is a close-up view of the micrograph of FIG. 4, centered on the induction layer 4. The thickness of the induction layer 4 is represented by the segment [AB] An image processing of the figure 13 makes it possible to estimate the proportion of the zones made up of ferrite Fe (a) in the induction layer, that is to say compared to the sum of the zones of ferrite Fe (a) and the zones of martensite Fe (at'). More precisely, by defining lower and upper gray level thresholds, we can estimate the air occupied by the medium gray zone of the martensite phase and thus go back to the ferrite level. It is advisable to use two thresholds and to vary them in order to arrive at this estimate, because although the ferrite appears in clear, the phase interfaces can appear dark and for small ferrites this cannot be negligible. .

In the example of FIG. 13, the rate of residual ferrite with respect to the rest of the layer delimited by the segment [AB] is between 1% and 15%, it being understood that this rate tends towards 1% near the combination layer (point A), and tends to 15% near the heart (point B). The level of residual ferrite is volume.

In general, the treatment method according to the invention makes it possible to obtain a level of residual ferrite in the part, between the diffusion zone 3 / combination layer 2 interface and a depth of 500 μm (segment [AB]) , greater than or equal to 1%, preferably greater than or equal to 5%.

Likewise, the treatment method according to the invention makes it possible to obtain a level of residual ferrite in the part, between the diffusion zone 3 / combination layer 2 interface and a depth of 500 μm (segment [AB]), less than or equal to 50%, preferably less than or equal to 30%, more preferably less than or equal to 20%, and more preferably less than or equal to 15%.

Preferably, the level of residual ferrite is between 1% and 20%, preferably between 5% and 15%.

The manufacturing process can optionally include an impregnation step, in order to improve the corrosion resistance of part P.

Preferably, the impregnation takes place after induction hardening.

Impregnation in itself is a technique well known to those skilled in the art, and a particular method is described, for example, in document EP3237648. Impregnation can be done by dipping or spraying.

Impregnation protects the part because it helps to delay the onset of corrosion, reduce the corrosion rate and thus increase the life of the part.

An evaluation of the resistance of the parts to corrosion can be made by tests in a corrosive atmosphere, for example a salt spray. Standard EN ISO 9227 "Corrosion tests in artificial atmospheres - Salt spray tests" describes such tests. By adding an impregnation step to the process according to the invention, it is possible to obtain a part P whose corrosion resistance during a neutral salt spray test is greater than 80 hours.

In view of the above, and unexpectedly, many advantages can be obtained by carrying out a nitriding operation followed by a high frequency induction hardening operation according to the invention. These operations make it possible to obtain parts made of ferrous materials exhibiting significant resistance to wear by abrasion and adhesion, and an improvement in frictional properties, resistance to chipping combined with correct corrosion resistance, without the need for '' coat the part before HF quenching.

Claims

A method of treating a part (P) made of ferrous metal, comprising: a nitriding operation forming on the part (P) a combination layer (2) having a thickness between 5 µm and 30 µm, and an area diffusion (3) arranged under and in contact with the combination layer (2) having a thickness between 100 µm and 500 µm; then a high-frequency induction hardening operation of the part (P), to an induction depth greater than or equal to 0.5 mm, causing hardening of the part (P) providing said part (P):

• a surface hardness greater than or equal to 50 HRC,

• a hardness of the combination layer (2) greater than or equal to 400 HV0.05,

• a hardness of the part greater than or equal to 500 HV0.05 at a depth of 500 μm, and in which the high-frequency induction hardening operation is carried out without applying a protective film to the part (P) beforehand. to the induction hardening operation.

2. Method according to claim 1, characterized in that the induction hardening operation is not followed by a tempering operation.

3. Method according to any one of claims 1 or 2, characterized in that the high-frequency induction hardening operation of the part is carried out so as to keep the ferrite in the part (P) between the interface. diffusion zone (3) / combination layer (2) and a depth of 500 µm, preferably between the diffusion zone (3) / combination layer (2) interface and a depth of 300 µm.

4. Method according to any one of the preceding claims, characterized in that the high-frequency induction hardening operation of the part is carried out so as to have a residual ferrite level in the part (P), between the part. diffusion zone (3) / combination layer (2) interface and a depth of 500 µm, between 1% and 50% by volume, preferably between 1% and 30%, and more preferably between 5% and 30 %.

5. Method according to any one of the preceding claims, characterized in that the high-frequency induction hardening operation of the part is carried out so as to have a residual ferrite level in the part (P), between the diffusion zone (3) / combination layer (2) interface and a depth of 500 μm, between 5% and 20% by volume, preferably between 5% and 15%.

6. Method according to any one of the preceding claims, characterized in that it further comprises an impregnation step subsequent to the high-frequency induction hardening operation.

7. Method according to claim 6, characterized in that it provides the part (P) with a corrosion resistance greater than 80 hours during a neutral salt spray test.

8. Method according to any one of the preceding claims, characterized in that the high-frequency induction hardening operation is carried out with the following parameters:

• a frequency (F) between 50 and 400 kHz,

• a linear energy (E) between 4.6 and 5.8 J / mm.

9. Part (P) of ferrous metal, comprising a combination layer (2) having a thickness between 5 μm and 30 μm, and a diffusion zone (3) arranged under and in contact with the combination layer (2). , having a thickness between 100 μm and 500 μm, said part (P) having:

• a surface hardness greater than or equal to 50 HRC,

• a hardness of the part greater than or equal to 500 HV0.05 at a depth of 500 μm, said part (P) comprising ferrite and martensite between the diffusion zone (3) / combination layer (2) interface ) and a depth of 500 µm.

10. Part (P) according to claim 9, characterized in that the hardness of the part (P) at a depth of 0.5 mm is greater than or equal to a core hardness + 100 HV0.05.

11. Part (P) according to claim 9 or claim 10, characterized in that the hardness of the part (P) at a depth of 0.25 mm is greater than or equal to a core hardness + 350 HV0.05.

12. Part (P) according to any one of claims 9 to 11, characterized in that the Ile is made of very low alloy steel, of the C10-C70 family, having a manganese content of less than 1%.

13. Part (P) according to any one of claims 9 to 12, characterized in that the Ile comprises ferrite and martensite between the diffusion zone interface (3) / combination layer (2) and a depth of 300 µm.

14. Part (P) according to any one of claims 9 to 13, characterized in that the Ile comprises a level of ferrite, between the diffusion zone interface (3) / combination layer (2) and a depth 500 µm, between 1% and 50% by volume, preferably between 1% and 30%, and more preferably between 5% and 30%.

15. Part (P) according to any one of claims 9 to 13, characterized in that the Ile comprises a level of ferrite, between the diffusion zone interface (3) / combination layer (2) and a depth 500 μm, between 5% and 20% by volume, preferably between 5% and 15%.

16. Part (P) according to any one of claims 9 to 15, characterized in that the Ile has a corrosion resistance greater than 80 hours during a neutral salt spray test.