Method for producing nitrided component of aircraft turbomachine

Abstract

The invention relates to a method for producing a nitrided steel component, in which a semi-finished blank made of a nitrided steel material is produced and the semi-finished blank thus obtained is subjected to a strengthening treatment comprising a nitriding step, in which the blank or the steel bar from which the blank is obtained is previously subjected to a heat treatment, according to the invention, the semi-finished product blank is subjected to an induction hardening step prior to the nitriding step carried out as part of the strengthening treatment, the subsequent nitriding being light nitriding, the light nitriding being carried out for less than 100 hours (preferably less than 50 hours, even more preferably less than 30 hours) at a temperature between 400 DEG C and 600 DEG C (preferably less than 500 DEG C).

Description

Technical Field

[0001] The present invention relates to the general field of manufacturing nitrided steel components. Background Art

[0002] Nitriding of low-alloy steels is a conventional solution for many components, especially power transmission components (gear teeth, spline shafts, bearings, crown gears, etc.) in aircraft turbines, where the operating temperature does not allow the use of case-hardened steels.

[0003] To ensure the expected mechanical resistance, these components must have a very high hardness, reaching a depth of 2 or 3 times the depth of the stress sublayer.

[0004] This hardening quality and depth can be achieved with steels containing alloying elements that allow hardening by nitriding.

[0005] Nitriding consists of diffusing atomic nitrogen N into the surface of a component that has been previously hardened and tempered (N and C are used for nitrocarburizing). The insertion of N (or N and C), which forms nitrides with the alloying elements of the steel, results in surface hardening, providing the required properties (hardness and corrosion resistance).

[0006] However, due to the diffusion kinetics of nitrogen, the nitriding treatments required to achieve a hardening depth compatible with the above applications (penetration depth greater than 1 mm) are usually very long (usually more than 500 hours). These long treatments are difficult to reconcile with industrial productivity and are expensive. Summary of the Invention

[0007] To facilitate industrialization, an object of the present invention is to propose a production method with a reduced hardening treatment time while still allowing the required resistance to be maintained.

[0008] Specifically, according to one aspect, the present invention proposes a method for manufacturing nitrided steel components, in which a semi-finished blank of nitrided steel material is produced, and the obtained semi-finished blank is subjected to a hardening treatment, including a nitriding step, and the blank or the steel bar from which the blank is obtained is pre-heat-treated.

[0009] Wherein, before the nitriding step carried out as part of the hardening treatment, an induction hardening step is performed on the semi-finished blank, and the subsequent nitriding is a shallow nitriding carried out at a temperature between 400 °C and 600 °C (preferably between 450 °C and 550 °C) for a time of less than 250 hours (preferably less than 150 hours, even more preferably less than 100 hours).

[0010] This solution can significantly reduce the cycle time from hundreds of hours to dozens of hours.

[0011] Combining surface induction treatment with the proposed nitriding treatment (shallow-depth nitriding) can significantly reduce the nitriding time required to obtain the desired mechanical properties.

[0012] According to current design constraints (2 to 3 times the maximum load depth), induction allows for rapid treatment at a relatively large depth (>1 mm).

[0013] Shallow nitriding further increases surface hardness, thus improving surface fatigue resistance and bending resistance, especially at the tooth roots of a set of teeth.

[0014] Therefore, the strengthening treatment ensures resistance to subsurface fatigue (maximum load position), surface fatigue (microchipping), and tooth root bending.

[0015] In addition, compared with deep nitriding alone, there is less deformation associated with the strengthening treatment, which is beneficial for component production and allows for a reduction in the thickness of the material to be reworked during the final machining process (economic savings in materials and limitations in the intervention time associated with reworking components at the end of production (the thickness of the reworked post-treatment components ranges from a few hundred micrometers to a few tens of micrometers)).

[0016] The proposed method is advantageously supplemented by the following features.

[0017] The nitriding steel includes nitriding alloying elements and has a carbon content between 0.20% and 0.45%, preferably greater than 0.25%.

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

[0019] The preliminary heat treatment performed on the blank or the steel bar from which the blank is obtained is, for example, hardening and tempering treatment.

[0020] In addition, preferably, surface pre-treatment is carried out by sandblasting and / or phosphating before the nitriding step.

[0021] Moreover, advantageously, finishing operations are performed on the obtained components by grinding and / or electrochemical polishing and / or burnishing.

[0022] Advantageously, the component is a power transmission component of an aircraft turbine, such as a component with teeth or splines, a pinion component, a bearing race, etc.

[0023] In addition to saving processing time, the combined induction and nitriding treatment ensures that the subsurface has sufficient fatigue resistance and improves resistance to surface fatigue (microchipping) and tooth root bending.

[0024] It should also be noted that nitriding is used after induction to minimize retained austenite, which is an important feature in aerospace applications as retained austenite can have an impact on the geometric distortion and metallurgical instability of components.

[0025] The intervention of nitriding after induction also allows the avoidance of the formation of nitrided ferrite.

[0026] The method also allows for more constraint near the surface, as well as the accumulation of the residual stress distribution associated with the two treatments. All of these are beneficial for resistance to contact fatigue and bending fatigue, especially at the tooth root.

[0027] In addition, induction after hardening generates new martensite. This generation typically requires a tempering treatment: in this case, the tempering treatment is incorporated into the nitriding step. Thus, no additional tempering step is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Other features and advantages of the present invention will become apparent from the following description, which is illustrative and not restrictive and must be read in conjunction with the accompanying drawings, in which:

[0029] Figure 1 is a schematic cross-sectional view of two meshing wheels of a power transmission gear;

[0030] Figure 2 schematically shows Figure 1 a set of teeth of the gear of

[0031] Figure 3 shows the expected hardness curve after nitriding following a case hardening treatment, and also shows the acceptable limits of hardness and treatment depth. DETAILED DESCRIPTION

[0032] Components and Applications

[0033] Generally speaking, the proposed method is advantageously applicable to any component with maximum load constraint in the sub-layer.

[0034] It is particularly advantageous in the aerospace field, especially for strengthening power transmission components such as toothed and / or splined components, gears (especially pinions), raceways, etc. in aircraft turbines.

[0035] More generally, it can be applied to all components that must withstand a harsh thermal environment and high mechanical stresses (bending fatigue, contact fatigue, fretting wear, wear, etc.) on the surface.

[0036] Figure 1 shows two wheels 1a, 1b of a cylindrical gear E with straight teeth, Figure 2Shows a set of teeth D of one of the wheels 1a or 1b of the gear E.

[0037] Such teeth are subject to bending fatigue stress and surface pressure stress at the tooth root (region P), which may create notches on the contact surface (region S) of the teeth and even lead to tooth fracture.

[0038] Components 1a and 1b can be produced by the method described below, which provides hardness and bending strength compatible with gear applications.

[0039] Alloy

[0040] Components produced by the proposed method are made of low-alloy nitriding steel, which has nitriding alloying elements such as Cr, V, Mo, and Al, (non-exhaustive list), allowing hardening by nitriding (precipitation of submicroscopic nitrides from these nitriding elements occurs in the solid solution in the treated steel, etc.).

[0041] The carbon content of such nitriding steel is generally between 0.15% and 0.8%, preferably between 0.15% and 0.65%, allowing the base material to obtain its core mechanical properties after heat treatment.

[0042] Examples of such steels are: 32CDV13 (33CrMoV12-9), 40CDV12 (40CrMoV13-9), 300M steel, etc.

[0043] The production of the component includes producing a blank of the component from such steel, heat-treating the blank, and then semi-finishing the rough component. Then, the obtained semi-finished component 5 is induction hardened and then shallow nitrided.

[0044] Producing the blank

[0045] In the first step, a blank of the steel component is produced to give the first shape of the component.

[0046] Such a blank is obtained by successive "rough" machining steps on a steel bar. These steps allow obtaining the approximate shape of the component. At this stage, surplus material (minimum size about 0.5 mm) is left on the surface for finishing in subsequent stages, which allows obtaining the desired final size of the component (step 1).

[0047] Of course, other techniques for obtaining the blank can also be considered: especially additive manufacturing in the case of components with complex shapes.

[0048] Heat treatment of the blank or steel

[0049] The blank thus produced is heat-treated by hardening and tempering.

[0050] Hardening ensures the austenitization of the steel. It is carried out by heating to a temperature between 800 °C and 1200 °C, usually between 900 °C and 1100 °C, for several hours.

[0051] Tempering is carried out at a temperature between 200 °C and 650 °C, usually between 520 °C and 650 °C, for several hours, for example, at 620 °C for 2 to 4 hours.

[0052] Alternatively, the blank hardening and tempering of the steel bar can be carried out before machining the blank.

[0053] Induction hardening

[0054] Electromagnetic induction hardening can provide uniform and rapid heating within a controllable and repeatable depth range of 1 mm to several centimeters.

[0055] Induction hardening can be carried out simultaneously or locally on all teeth, for example, tooth by tooth. When carrying out induction hardening on all teeth simultaneously, the part is placed inside a single-turn or multi-turn inductor that coaxially surrounds the part, and alternating current of high, medium or low frequency is passed through. The inductor behaves like a transformer with the part and induces an electric current in it. The alternating magnetic field in the part heats the outer surface.

[0056] In the case of local hardening, the area of interest is heated with an inductor through which alternating current passes.

[0057] The transmitted power is selected to be sufficient to ensure austenitization at the desired functional depth.

[0058] For the profile hardening of a set of teeth, the supplied alternating field is usually high frequency (10 to 600 kHz), and the power of the current generator is greater than 10 kW.

[0059] Depending on the required hardening depth, other frequencies and powers are of course possible.

[0060] Due to the very rapid heating effect around the part, the induction duration can be very short: from tenths of a second to several seconds.

[0061] This treatment allows, for example, to reach a hardness of 700 HV (Vickers hardness) at a large depth (> 1 mm).

[0062] Typically, for 32CDV13, the obtained hardness is 600 HV, and for 40CDV12, the obtained hardness is 700 HV.

[0063] Surface pretreatment for nitriding

[0064] Before the nitriding treatment, surface pretreatment is carried out by sandblasting and / or phosphating.

[0065] Shallow nitriding

[0066] This hardening treatment is followed by a light nitriding treatment (step 3b).

[0067] Traditionally, nitriding can consist of immersing the part in an environment capable of releasing nitrogen on the surface and carrying it out at a temperature allowing nitrogen to diffuse from the part surface to the core.

[0068] This nitriding can be gas, ion or salt bath nitriding.

[0069] It operates at a rather low temperature (between 400 °C and 600 °C, preferably below 500 °C to avoid losing the advantages provided by the induction treatment).

[0070] The duration is limited (about ten or dozens of hours - for example, 20 to 30 hours and in any case less than 100 hours (preferably less than 50 hours)) and depends on the required total depth of the nitrided layer, the nitriding conditions and the intended application.

[0071] For an example of a nitriding method, reference can be advantageously made to the following paper: TS O.Skiba “Development of anitriding method for aeronautics.Study of hardening mechanisms on nitridediron-chromium alloys”.

[0072] However, in practice, nitriding can be selected according to industrial applications and the functional need to strengthen mechanical materials under the surface.

[0073] Typically, the depth of the nitrided layer can reach 1.5 mm. It is preferably between 0.1 mm and 1 mm, even more preferably between 0.2 mm and 0.8 mm.

[0074] The hardness level obtained (usually higher than 800 HV) is higher than the hardness level output by the induction hardening step.

[0075] This is shown in Figure 3 : the surface hardening treatment allows a hardness of 600 HV or higher (a part of the TS curve); the additional nitriding increases this hardness, allowing values higher than 800 HV.

[0076] Finishing operations

[0077] Then a finishing step is planned, followed by a superfinishing step.

[0078] For example, the finishing step consists of reworking to correct geometric deformations and possible white layers.

[0079] It is worth noting that, using the proposed method, this finishing step produces fewer material fragments (from an order of magnitude of 100 μm to an order of magnitude of 10 μm).

[0080] In fact, it is observed that induction produces fewer geometric changes than deep nitriding.

[0081] This finishing step can be followed by a superfinishing step that uses, for example, electrochemical polishing and / or abrasive finishing in a particle bath to impart the desired surface finish to the part.

[0082] The parts obtained by this method differ from those obtained by conventional methods in the typical percentages of carbon and nitrogen produced by the method. It can be observed that there is no carbon gradient in the areas produced by induction hardening (as opposed to carburizing).

Claims

1. A method for producing a nitrided steel component, wherein, Producing a semi-finished blank made of nitriding steel and subjecting the obtained semi-finished blank to a strengthening treatment, the method comprising a nitriding step, and pre-heat treating the blank or the steel bar from which the blank is obtained, wherein, prior to the nitriding step which is carried out as part of the strengthening treatment, an induction hardening step is carried out on the semi-finished blank, and the subsequent nitriding is carried out at a temperature between 400 °C and 600 °C for a time of less than 250 hours.

2. The method according to claim 1, wherein The nitriding step lasts for less than 150 hours.

3. The method according to claim 2, wherein The nitriding step lasts for less than 100 hours.

4. The method according to any one of the preceding claims, wherein, The nitriding temperature is between 450 °C and 550 °C.

5. The method according to any one of the preceding claims, wherein, The nitriding steel comprises nitriding alloying elements and has a carbon content between 0.15% and 0.8%, preferably between 0.15% and 0.65%.

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

7. The method according to any one of the preceding claims, wherein, The preliminary heat treatment carried out on the blank or the steel bar from which the blank is obtained is a hardening and tempering treatment.

8. The method according to any one of the preceding claims, wherein, Prior to the nitriding step, surface pre-treatment is carried out by sandblasting and / or phosphating.

9. The method according to any one of the preceding claims, wherein The obtained component is subjected to a finishing operation by grinding and / or electro-chemical polishing and / or burnishing.

10. The method according to any one of the preceding claims, wherein, The component is toothed and induction hardening occurs simultaneously or locally on all teeth.

11. A power transmission component of an aircraft turbine, characterized in that, The power transmission component is obtained by the production method according to one of the preceding claims.

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