A manufacturing process, particularly for an aircraft turbomachine part, with nitriding reinforcement.

Abstract

The invention relates to a method for manufacturing a nitrided steel part, in which a semi-finished blank is produced from nitrided steel and a strengthening treatment is carried out on the resulting semi-finished blank, comprising a nitriding step. The blank, or a steel bar from which the blank is obtained, is first heat-treated. Prior to the nitriding step of the strengthening treatment, the semi-finished blank undergoes induction hardening. The subsequent nitriding process is a shallow nitriding process carried out for a duration of less than 100 hours (preferably less than 50 hours and even more preferably less than 30 hours) at a temperature between 400°C and 600°C (preferably less than 500°C). Figure 3

Description

Title of the invention: Method for manufacturing, in particular, a part of an aircraft turbomachine, with nitriding reinforcement General technical field and prior art

[0001] The present invention relates to the general field of manufacturing a part in nitrided steel.

[0002] Nitriding of low alloy steels is a classic solution for many parts, in particular power transmission parts in aircraft turbomachinery (gear teeth, splined shafts, bearings, crowns, etc.) whose operating temperature does not allow the use of case-hardened steels.

[0003] To ensure the expected mechanical resistance, these parts must exhibit very high hardness over depths of up to 2 or 3 times the depth of the stressed sub-layer.

[0004] This quality and depth of hardening can be obtained with steels containing alloying elements allowing hardening by nitriding.

[0005] Nitriding consists of the diffusion of atomic nitrogen N onto the surface of parts previously treated by quenching and tempering (N and C for nitrocarburizing). The insertion of N (or N and C), the formation of nitrides with the alloying elements of the steel, causes surface hardening, providing the desired properties (hardness and corrosion resistance).

[0006] However, due to the diffusion kinetics of nitrogen, the nitriding treatments required to achieve reinforcement depths compatible with the applications mentioned above (penetration depth greater than 1 mm) are generally lengthy (typically, more than 500 hours). These lengthy treatments are difficult to reconcile with industrial production rates and are expensive. General presentation of the invention

[0007] In order to facilitate industrialization, one aim of the invention is to propose a manufacturing process with a reduced reinforcement treatment time, while allowing the required resistance properties to be maintained.

[0008] In particular, according to one aspect, the invention proposes a method for manufacturing a nitrided steel part in which a semi-finished blank is manufactured from nitrided steel and a strengthening treatment comprising a nitriding step is carried out on the semi-finished blank thus obtained, said blank or a steel bar from which said blank is obtained being previously heat-treated,

[0009] in which, prior to the nitriding step of the strengthening treatment, an induction hardening step is carried out on the semi-finished blank, the nitriding subsequently carried out being a shallow nitriding carried out for a period of less than 250H (preferably less than 150H and even more preferably less than 100H) at a temperature between 400°C and 600°C (preferably between 450°C and 550°C).

[0010] This solution makes it possible to drastically reduce cycle times from a few hundred hours to a few tens of hours.

[0011] The combination of a surface treatment by induction, followed by a proposed nitriding treatment (shallow depth nitriding) makes it possible to considerably reduce the nitriding time required to obtain the desired mechanical properties.

[0012] Induction allows for rapid processing over a large depth (> 1 mm) in accordance with current design constraints (2 to 3 times the maximum loading depth).

[0013] Shallow depth nitriding makes it possible to further increase surface hardness and therefore resistance to surface fatigue and bending, particularly at the tooth root for a tooth.

[0014] Thus, the strengthening treatment ensures resistance to underlayer fatigue (location of maximum loading), surface fatigue (micro-scaling) and tooth root bending.

[0015] Furthermore, the distortions related to the strengthening treatment are less compared to deep nitriding alone, which facilitates the manufacture of parts and makes it possible to reduce the thickness of material to be reworked during the final machining (economic gains on material and limitation of intervention times related to the rework of the part at the end of manufacturing (going from several hundred µm to several tens of µm of thickness reworked on post-treatment part)).

[0016] The proposed method is advantageously complemented by the following characteristics.

[0017] Nitriding steel comprises nitriding alloying elements and a carbon content of between 0.20% and 0.45%, preferably greater than 0.25%.

[0018] The depth of the nitriding layer is less than 1.5 mm and is preferably between 0.1 mm and 1 mm and even more preferably between 0.2 and 0.8 mm.

[0019] The preliminary heat treatment carried out on a blank or on a steel bar from which said blank is obtained is, for example, a quenching and tempering treatment.

[0020] Also, prior to the nitriding step, surface preparation by sandblasting and / or phosphatizing is preferably implemented.

[0021] Furthermore, the part obtained is advantageously finished by grinding and / or electrochemical polishing and / or tribofinishing.

[0022] The part is advantageously a power transmission part of an aircraft turbomachine, such as a toothed or splined part, a gear part, a bearing raceway, etc...

[0023] In addition to saving treatment time, the combined induction and nitriding treatment ensures the correct fatigue resistance of the underlayer, improves resistance to surface fatigue (micro-scaling) and to bending at the base of the tooth.

[0024] It should also be noted that the fact of intervening nitriding after induction makes it possible to minimize residual austenite, which is an important characteristic in aeronautical applications, since residual austenite can have an impact on geometric distortions and on the metallurgical instability of the part.

[0025] The intervention of nitriding after induction also makes it possible to avoid the generation of nitrogen ferrite.

[0026] This process also allows for greater stress near the surface and an accumulation of residual stress profiles related to the two treatments. All of this is favorable for contact fatigue and flexural fatigue resistance, particularly at the tooth root.

[0027] Furthermore, induction generates fresh martensite after quenching. Such generation normally requires a tempering treatment: in this case, this is incorporated into the nitriding step. No further tempering step is then necessary. Brief description of the drawings

[0028] Other features and advantages of the invention will become apparent from the following description, which is purely illustrative and not limiting, and should be read in conjunction with the accompanying figures in which: - [Fig.1] is a schematic cross-sectional representation of two meshed wheels of a power transmission gear; - [Fig.2] schematically illustrates a tooth profile of the gear wheels of [Fig.1]; - [Fig.3] illustrates the expected hardness profile following surface quenching and nitriding treatment, with acceptable limits in hardness and depth of treatment also indicated.

[0029] Description of one or more implementation and embodiment methods Parts and applications

[0030] In general, the proposed process is advantageously applicable to any part with maximum load constraint in the sublayer.

[0031] It is particularly advantageous in the field of aeronautics and in particular for the reinforcement of power transmission parts of the toothed and / or splined type, gearing (especially pinions), bearing tracks, etc. in aircraft turbomachinery.

[0032] More generally, it can be applied to all parts which have to undergo a severe thermal environment and which are subjected to strong mechanical stress on the surface (flexural fatigue, contact fatigue, fretting, wear, etc.).

[0033] Figure [1] shows two wheels la, 1b of a straight-toothed cylindrical gear E and Figure [2] shows a tooth D of one of the wheels la or 1b of this gear E.

[0034] Such teeth are subjected to both bending fatigue stresses at the tooth root (zone P) and surface pressure stresses likely to generate spalling at the contact surface of the teeth (zone S) or even to generate tooth breakage.

[0035] Parts 1a and 1b can be manufactured according to the process as described below which ensures hardness and resistance to bending compatible with the application of the gear. Alloys

[0036] The part manufactured by the proposed process is made of a low-alloy nitriding steel with nitrurigenic alloying elements such as Cr, V, Mo and Al, (non-exhaustive list) allowing hardening by nitriding (precipitation of submicroscopic nitrides from these nitrurigenic elements, present in solid solution in the treated steel, etc...).

[0037] Such a nitriding steel typically has a carbon content of between 0.15% and 0.8%, preferably between 0.15% and 0.65%, enabling the base material to acquire its core mechanical properties after heat treatment.

[0038] Such steels are for example the following: 32CDV13 (33CrMoV12-9), 40CDV12 (40CrMoV13-9), 300M steel, etc...

[0039] The manufacturing process involves producing a blank of the part in this steel, heat treating this blank, and then semi-finishing the blank. The resulting semi-finished part 5 is then subjected to induction hardening and shallow nitriding. Creating a rough draft

[0040] In a first step, a rough draft of the steel part is manufactured to give a first shape to the part concerned.

[0041] This rough blank is obtained by successive "coarse" machining steps on a steel bar. These steps make it possible to obtain the general shape of the part. At this At this stage, excess material (approximately 0.5mm from the minimum dimensions) is retained on the surface for the subsequent finishing machining phase, which allows the desired final dimensional dimensions of the part to be achieved (step 1).

[0042] Other techniques for obtaining rough parts could of course also be considered: additive manufacturing in particular, in the case of parts with complex shapes. Heat treatment of the blank or steel

[0043] The blank thus produced is subjected to heat treatment, by quenching and tempering.

[0044] The quenching treatment ensures austenitization of the steel. It is carried out by heating to a temperature between 800°C and 1200°C, typically between 900°C and 1100°C for a few hours.

[0045] The income occurs at a temperature between 200°C and 650°C, typically between 520°C and 650°C for a few hours, for example 620°C for 2 to 4 hours.

[0046] Alternatively, it may be provided that the rough quenching and tempering treatment takes place before the machining of the rough, on the steel bar. Induction Hardening

[0047] Electromagnetic induction hardening makes it possible to obtain uniform, rapid heating over a controlled and reproducible depth from 1 mm to several centimeters.

[0048] Induction hardening can be performed on all teeth simultaneously or locally, for example, tooth by tooth. During induction hardening on all teeth simultaneously, the workpiece is placed inside a single- or multi-turn inductor coaxially surrounding the workpiece and carrying an alternating current at high, medium, or low frequency. This inductor behaves with the workpiece like a transformer and develops an induced current within it. The alternating magnetic field in the workpiece heats the outer surface.

[0049] In the case of localized quenching, the area of ​​interest is heated with an inductor through which an alternating current flows.

[0050] The power delivered is chosen to be sufficient to ensure austenitization over the desired functional depth.

[0051] For contour hardening on teeth, the alternating field supplying the gear is typically high frequency (10 to 600kHz), with a current generator whose power is greater than 10 kW.

[0052] Other frequencies and powers are of course possible depending on the desired reinforcement depth.

[0053] Since the heating effect at the periphery of the part is very rapid, the duration of the induction can be short: from a few tenths of a second to a few seconds.

[0054] This treatment allows for example a hardness of up to 700 HV - (Vickers hardness)) over a great depth (> 1mm).

[0055] Typically, the hardness obtained is 600 HV for 32CDV13 and 700 HV for 40CDV12. Surface preparation for nitriding

[0056] This nitriding treatment is preceded by surface preparation by sandblasting and / or phosphating. Shallow Nitriding

[0057] This quenching treatment is followed by a shallow nitriding treatment (step 3b).

[0058] Nitriding can, in a traditional way, consist of immersing the part in a medium capable of releasing nitrogen on the surface, at a temperature allowing the nitrogen to diffuse from the surface to the core of the part.

[0059] This nitriding can be gas nitriding, ionic nitriding or nitriding by salt baths.

[0060] It operates at fairly low temperatures (between 400°C and 600°C and (preferably below 500°C to avoid losing the advantage provided by the induction treatment).

[0061] The duration is restricted (a few tens or tens of hours - 20 to 30 hours, for example and in any case less than 100 hours (preferably less than 50 hours)) and is a function of the desired total nitriding layer depth, nitriding conditions and intended applications.

[0062] For examples of nitriding processes, reference may be made to the thesis

[0063] TS O. Skiba “Development of a nitriding process for aeronautics. Study of hardening mechanisms on nitrided iron-chromium alloys”.

[0064] In practice, however, nitriding may be chosen according to industrial applications and the functional need for strengthening the mechanical material under the surface.

[0065] Typically, the depth of the nitriding layer can reach up to 1.5 mm. It is preferably between 0.1 mm and 1 mm and even more preferably between 0.2 and 0.8 mm.

[0066] The level of hardness obtained is greater than that at the exit of the induction hardening step (typically greater than 800 HV).

[0067] This is illustrated in [Fig. 3]: surface hardening treatment allows hardnesses of 600 HV or higher (part of the TS curve); additional nitriding increases this hardness and allows values ​​greater than 800 HV. Finishes

[0068] A finishing stage is then planned, followed by a superfinishing stage.

[0069] The finishing stage consists, for example, of reworking the machining by grinding the geometric distortions and a possible white layer.

[0070] It is noteworthy that with the proposed process, this finishing step generates less material chips (from the order of hundreds of µm to the order of tens of µm).

[0071] It is observed that induction generates fewer geometric changes than deep nitriding.

[0072] This finishing step can then be followed by a superfinishing step, which for example implements electrochemical polishing and / or tribofinishing in a granule bath, in order to give the desired surface condition to the part.

[0073] The part produced by this process differs from that obtained by a conventional process due to typical percentages of carbon and nitrogen resulting from the process. The absence of a carbon gradient can be observed in the zone resulting from induction hardening (unlike carburizing).

Claims

Demands

1. A method for manufacturing a nitrided steel part in which a semi-finished blank is manufactured from nitrided steel and a strengthening treatment comprising a nitriding step is carried out on the semi-finished blank thus obtained, said blank or a steel bar from which said blank is obtained being previously heat-treated, said prior heat treatment comprising a heat quenching treatment ensuring austenitization of the steel and tempering at a temperature between 200°C and 650°C, in which a treatment consisting of surface induction hardening followed by a shallow nitriding treatment is carried out on the semi-finished blank, this nitriding being carried out for a period of less than 100 hours, at a temperature between 400°C and 600°C.

2. A process according to claim 1, wherein the nitriding temperature is between 450°C and 550°C.

3. A process according to any one of the preceding claims, wherein the nitriding steel comprises nitriding alloying elements and a carbon content of between 0.15% and 0.8%, preferably between 0.15% and 0.65%.

4. A method according to any one of the preceding claims, wherein the depth of the nitriding layer is less than 1.5 mm and is preferably between 0.1 mm and 1 mm.

5. A method according to any one of the preceding claims, wherein the pre-heat treatment carried out on a blank or on a steel bar from which said blank is obtained, is a quenching and tempering treatment.

6. A method according to any one of the preceding claims, wherein prior to the nitriding step, surface preparation is carried out by sandblasting and / or phosphating.

7. A method according to any one of the preceding claims, wherein the resulting part is finished by machining by grinding and / or electrochemical polishing and / or tribofinishing.

8. A method according to any one of the preceding claims, wherein the part has teeth and induction hardening takes place on all the teeth simultaneously or in a localized manner.

9. 9 Power transmission part of an aircraft turbomachine, characterized in that it is obtained by a manufacturing process according to one of the preceding claims.

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