Method for producing a nitrided part for an aircraft turbomachine

Abstract

The invention relates to a method for producing a nitrided steel part in which a semi-finished blank made of nitriding steel is produced and a reinforcing treatment is carried out on the semi-finished blank thus obtained, which treatment comprises a nitriding step, the blank or a steel bar from which the blank is obtained being heat-treated beforehand, wherein, prior to the step of nitriding performed as part of the reinforcing treatment, an induction hardening step is carried out on the semi-finished blank, the nitriding that is subsequently carried out being shallow nitriding carried out for a period of less than 100 hours (preferably less than 50 hours and even more preferably less than 30 hours) at a temperature of between 400°C and 600°C (preferentially less than 500°C).

Description

[0001] METHOD FOR MANUFACTURING AN AIRCRAFT TURBOMACHINE PART WITH NITRIDING REINFORCEMENT

[0002] Description

[0003] 5 General technical field and prior art

[0004] The present invention relates to the general field of manufacturing a nitrided steel part.

[0005] Nitriding of low-alloy steels is a classic solution for many parts, in particular power transmission parts in aircraft turbomachines (gear teeth, splined shafts, bearings, crowns, etc.) whose operating temperature does not allow the use of case-hardened steels.

[0006] To ensure the expected mechanical resistance, these parts must have very high hardnesses over depths of up to 2 or 3 times the depth of the stressed sub-layer.

[0007] This quality and depth of hardening can be achieved with steels containing alloying elements that allow hardening by nitriding.

[0008] Nitriding consists of a diffusion of atomic nitrogen N on the surface of the 20 parts previously treated by quenching and tempering (N and C for nitrocarburizing).

[0009] The insertion of N (or N and C), the formation of nitrides with the alloying elements of the steel, causes a surface hardening providing the desired properties (hardness and corrosion resistance).

[0010] 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 long (typically, more than 500 hours). These long treatments are difficult to reconcile with industrial production rates and are expensive.

[0011] 30

[0012] General presentation of the invention In order to facilitate industrialization, one aim of the invention is to propose a manufacturing method with a reduced reinforcement treatment time, while making it possible to maintain the required resistance properties.

[0013] 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 nitriding steel and a strengthening treatment is carried out on the semi-finished blank thus obtained, comprising a nitriding step, said blank or a steel bar from which said blank is obtained being previously heat-treated, in which an induction hardening step is carried out on the semi-finished blank, prior to the nitriding step of the strengthening treatment, the nitriding subsequently carried out being a shallow nitriding carried out over a period of less than 250 hours (preferably less than 150 hours and even more preferably less than 100 hours) at a temperature of between 400°C and 600°C (preferably between 450°C and 550°C).

[0014] This solution makes it possible to drastically reduce cycle times from a few hundred hours to a few dozen hours.

[0015] The combination of a surface induction treatment, 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.

[0016] Induction allows rapid treatment over a significant depth (> 1 mm) in accordance with current design constraints (2 to 3 times the maximum loading depth).

[0017] Shallow nitriding further increases surface hardness and therefore resistance to surface fatigue and bending, particularly at the root of the tooth for a set of teeth.

[0018] Thus, the strengthening treatment ensures resistance to sub-layer fatigue (location of maximum loading), surface fatigue (micro-chipping) and bending at the tooth root.

[0019] Furthermore, the distortions linked 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 final machining (economic savings on material and limitation of intervention times linked to the reworking of the part at the end of manufacturing (going from several hundred pm to several tens of pm of thickness reworked on the post-treatment part)).

[0020] The proposed method is advantageously supplemented by the following characteristics.

[0021] Nitriding steel comprises nitriding alloying elements and a carbon content of between 0.20% and 0.45%, preferably greater than 0.25%.

[0022] 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.

[0023] 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.

[0024] Also, it is preferential to carry out, prior to the nitriding step, a surface preparation by sandblasting and / or phosphating.

[0025] Furthermore, a finishing process is advantageously carried out on the part obtained by grinding and / or electrochemical polishing and / or tribofinishing.

[0026] The part is advantageously a power transmission part of an aircraft turbomachine, such as a toothed or splined part, a pinion part, a bearing track, etc.

[0027] In addition to saving processing time, the combined induction and nitriding treatment ensures that the underlayer is subject to fatigue resistance as needed, and improves resistance to surface fatigue (micro-chipping) and to bending at the tooth root.

[0028] It should also be noted that using nitriding after induction minimizes residual austenite, which is an important characteristic in aeronautical applications, since residual austenite can have an impact on geometric distortions and metallurgical instability of the part. Using nitriding after induction also prevents the generation of nitrogen ferrite.

[0029] This process also allows for more constraints near the surface as well as an accumulation of residual stress profiles linked to the two treatments. All of this is favorable for resistance to contact fatigue and bending fatigue, particularly at the tooth root.

[0030] In addition, induction generates fresh martensite after quenching. Such generation normally requires a tempering treatment: this is in this case incorporated into the nitriding step. No additional tempering step is then necessary.

[0031] Brief description of the drawings

[0032] Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended figures in which: Figure 1 is a schematic cross-sectional representation of two meshed wheels of a power transmission gear; Figure 2 schematically illustrates a toothing of the wheels of the gear of Figure 1; Figure 3 illustrates the expected hardness profile following the surface hardening treatment followed by nitriding, the acceptable limits in hardness and depth of treatment also being indicated.

[0033] Description of one or more methods of implementation and production

[0034] Parts and Applications

[0035] Generally speaking, the proposed method applies advantageously to any part with maximum loading constraint in the sub-layer.

[0036] It is particularly advantageous in the aeronautics field and in particular for the reinforcement of power transmission parts of the toothed and / or splined type, gears (pinions in particular), raceways, etc. in aircraft turbomachines. More generally, it can be applied to all parts which have to undergo a severe thermal environment and which are subjected to high mechanical stress on the surface (flexural fatigue, contact fatigue, fretting, wear, etc.).

[0037] Figure 1 shows two wheels 1a, 1b of a straight-toothed cylindrical gear E and Figure 2 shows a toothing D of one of the wheels 1a or 1b of this gear E.

[0038] Such teeth are subject to both bending fatigue stresses at the tooth root (zone P) and surface pressure stresses likely to generate chipping at the contact surface of the teeth (zone S) or even to cause tooth breakage.

[0039] Parts 1a and 1b can be manufactured by the process as described below which ensures hardness and bending strength compatible with the gear application.

[0040] Alloys

[0041] The part manufactured by the proposed process is made of a low-alloy nitriding steel with 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, present in solid solution in the treated steel, etc.).

[0042] Such a nitriding steel typically has a carbon content of 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.

[0043] Examples of such steels are: 32CDV13 (33CrMoV12-9), 40CDV12 (40CrMoV13-9), 300M steel, etc.

[0044] The manufacturing of the part involves the manufacturing of a rough part in this steel, a heat treatment of this rough part, then a semi-finishing of the rough part. The semi-finished part 5 obtained is then subjected to induction hardening, then to shallow nitriding.

[0045] Making a rough draft

[0046] In a first step, a rough cut of the steel part is manufactured to give a first shape to the part concerned. This rough cut is obtained by successive stages of "rough" machining on a steel bar. These stages make it possible to obtain the general shape of the part. At this stage, surplus material (approximately 0.5 mm of the minimum dimensions) is kept on the surface for the subsequent phase of finishing machining, which makes it possible to achieve the desired final dimensional dimensions of the part (stage 1).

[0047] Other roughing techniques could of course also be considered: additive manufacturing in particular, in the case of parts with complex shapes.

[0048] Heat treatment of the blank or steel

[0049] The blank thus produced is subjected to heat treatment, by quenching and tempering.

[0050] 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.

[0051] Tempering takes place 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.

[0052] Alternatively, it may be provided that the rough quenching and tempering treatment takes place before the rough machining, on the steel bar.

[0053] Induction Hardening

[0054] Electromagnetic induction hardening provides uniform, rapid heating over a controlled and reproducible depth of 1 mm to several centimetres.

[0055] Induction hardening can be performed on all teeth simultaneously or locally, for example tooth by tooth. In induction hardening on all teeth simultaneously, the part is placed inside a single-turn or multi-turn inductor coaxially surrounding the part and carrying a high, medium or low frequency alternating current. This behaves like a transformer with the part and develops an induced current in it. The alternating magnetic field in the part heats the outer surface.

[0056] In the case of localized quenching, the area of ​​interest is heated with an inductor through which an alternating current flows. The power delivered is chosen to be sufficient to ensure austenitization over the desired functional depth.

[0057] For contour hardening on teeth, the supplying alternating field is typically high frequency (10 to 600kHz), with a current generator with a power greater than 10 kW.

[0058] Other frequencies and powers are of course possible depending on the desired reinforcement depth.

[0059] Since the heating effect at the periphery of the part is very rapid, the duration of induction can be short: a few tenths of a second to a few seconds.

[0060] This treatment allows for example a hardness of up to 700 HV - (Vickers hardness)) over a great depth (> 1mm).

[0061] Typically, the hardness obtained is 600 HV for 32CDV13 and 700 HV for 40CDV12.

[0062] Surface preparation for nitriding

[0063] This nitriding treatment is preceded by surface preparation by sandblasting and / or phosphating.

[0064] Shallow nitriding

[0065] This quenching treatment is followed by a shallow nitriding treatment (step 3b).

[0066] Nitriding can, traditionally, consist of immersing the part in an environment capable of releasing nitrogen on the surface, at a temperature allowing the nitrogen to diffuse from the surface to the core of the part.

[0067] This nitriding can be gas, ionic or salt bath nitriding.

[0068] It takes place 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).

[0069] The duration is limited (around ten or a few dozen hours - 20 to 30 hours, for example, and in any case less than 100 hours (preferably less than 50 hours)) and depends on the depth of the total nitriding layer desired, the nitriding conditions and the intended applications. For examples of nitriding processes, it is advantageous to refer to the thesis

[0070] TS O. Skiba “Development of a nitriding process for aeronautics. Study of hardening mechanisms on nitrided iron-chromium alloys”.

[0071] In practice, however, nitriding can be chosen depending on industrial applications and the functional need for strengthening the mechanical material on the subsurface.

[0072] 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.

[0073] The hardness level obtained is higher than that at the output of the induction hardening stage (typically higher than 800 HV).

[0074] This is illustrated in Figure 3: the surface hardening treatment allows hardnesses of 600 HV or higher (part of the TS curve); the additional nitriding increases this hardness and allows values ​​higher than 800 HV.

[0075] Finishes

[0076] A finishing step is then planned, followed by a superfinishing step.

[0077] The finishing stage consists, for example, of re-machining to correct geometric distortions and a possible white layer.

[0078] It is notable that with the proposed process, this finishing step generates fewer material chips (we go from the order of a hundred pm to the order of ten pm).

[0079] In fact, we note that induction generates fewer geometric changes than deep nitriding.

[0080] This finishing step can then be followed by a superfinishing step, which for example uses electrochemical polishing and / or tribofinishing in a bath of granules, in order to give the desired surface finish to the part.

[0081] The part resulting from the process differs from that obtained by a conventional process by typical percentages of Carbon and Nitrogen due to the process. We can observe the absence of Carbon gradient in the zone resulting from induction hardening (unlike carburizing).

Claims

Claims 1. Method for manufacturing a nitrided steel part in which a semi-finished blank is manufactured from nitriding steel and a strengthening treatment is carried out on the semi-finished blank thus obtained, comprising a nitriding step, said blank or a steel bar from which said blank is obtained being previously heat-treated, in which an induction hardening step is carried out on the semi-finished blank, prior to the nitriding step of the strengthening treatment, the nitriding subsequently carried out being a nitriding carried out over a period of less than 250 hours, at a temperature of between 400°C and 600°C.

2. Method according to claim 1, in which the nitriding step lasts less than 150 hours.

3. Method according to claim 2, in which the nitriding step lasts less than 100 hours.

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

5. Method according to any one of the preceding claims, in which 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%.

6. Method according to any one of the preceding claims, in which the depth of the nitriding layer is less than 1.5 mm and is preferably between 0.1 mm and 1 mm.

7. Method according to any one of the preceding claims, in which the preliminary heat treatment carried out on a blank or on a steel bar from which said blank is obtained, is a quenching and tempering treatment.

8. Method according to any one of the preceding claims, in which a surface preparation by sandblasting and / or phosphating is carried out prior to the nitriding step.

9. Method according to any one of the preceding claims, in which a finishing by grinding and / or electrochemical polishing and / or tribofinishing is carried out on the part obtained.

10. Method according to any one of the preceding claims, in which the part is toothed and the induction hardening takes place on all the teeth simultaneously or locally.

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

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