Nitriding treatment process without a compound layer for a steel part

Abstract

This document describes a process for treating a steel part by nitriding, comprising the following steps: supplying a steel part comprising a volume to be nitrided, the volume to be nitrided including an external surface; deposition, on the external surface of the steel part, of a first coating layer comprising a material that does not react with nitrogen; nitriding the steel part for a predetermined time, so as to form a diffusion layer in the volume to be nitrided. Figure for the abstract: Fig. 3

Description

Title of the invention: Nitriding process without a compound layer for a steel part technical field

[0001] This presentation relates to the field of thermochemical treatments of metal parts. More specifically, it concerns a nitriding treatment process for these metal parts. A preferred application is the production of aircraft turbine components or helicopter transmission systems, and in particular the manufacture of various power transmission components such as gears, shafts, or pulleys. STATE OF THE ART

[0002] Parts made of metallic materials are usually subject to surface treatments. Indeed, certain parts of aircraft turbomachinery must undergo thermochemical treatments such as nitriding in order to withstand high mechanical and physical stresses.

[0003] Nitriding is a surface treatment that involves incorporating nitrogen into the surface layer of steel. Nitriding causes the diffusion of nitrogen and carbon, resulting in significant phase changes that modify the surface mechanical properties. These phase changes, particularly for parts made of low-alloy metallic materials, increase the surface hardness of the part and the development of compressive residual stresses, without affecting the intrinsic mechanical properties of the core.

[0004] The nitriding of these metallic materials causes the creation of three distinct zones in part 4, as seen in [Fig. 1a]: - a surface layer of combination 45, also called the "white layer" due to its color after Nital etching. This layer 45 is composed of iron nitrides of different crystallographic structures. - a diffusion layer 413 enriched in nitrogen by precipitation of nitrides with iron from the ferritic matrix and / or the addition elements. The M23C6 type carbides constitute nitride nucleation sites and thus cause the diffusion of carbon into the matrix. - the core material 4a not treated by the nitriding step

[0005] The combination layer 45 is detrimental to the overall properties of the parts 4 because it can generate surface flaking and cracking problems. One solution is to remove this surface layer at the end of the nitriding process, to obtain a part with a diffusion layer 413 but without the combination layer 45, as illustrated. by [Fig.lb], but this implies an additional machining step, for example by grinding. Moreover, such a step can lead to conformity problems, additional costs, or weakening of part 4. GENERAL STATEMENT

[0006] One object of the invention is to solve at least one of the problems mentioned above.

[0007] To this end, according to one aspect of this disclosure, a process for treating a steel part by nitriding is proposed, comprising the following steps: - supplying a steel part comprising a volume to be nitrided, the volume to be nitrided comprising an external surface; - deposition, on the external surface of the steel part, of a first coating layer comprising a material that does not react with nitrogen; - nitriding the steel part for a determined period of time, so as to form a diffusion layer in the volume to be nitrided.

[0008] The coating layer thus regulates the flow of nitrogen produced during nitriding, thereby preventing the formation of the previously described combination layer. Thus, the process described herein yields a nitrided part with a nitrogen-enriched diffusion layer due to nitride precipitation, but without a combination layer on its surface.

[0009] Furthermore, the coating layer is easier to remove from the part than a compound layer, thus simplifying part finishing. Indeed, since the materials that can constitute the coating layer are more ductile than the iron nitrides that make up the compound layer, the coating layer can be removed by simple sandblasting or chemical means, unlike the compound layer, which requires grinding for removal. This coating can also be retained if its impact on the functional properties is acceptable or even beneficial (improvement).

[0010] Thus, and more particularly for parts with complex shapes, the difficulty caused by the need to remove a combination layer from the surface of the part is therefore avoided.

[0011] Advantageously, but optionally, the described method includes at least one of the following features: - following the nitriding step, a step of removing the first coating layer;

[0012] - the first coating layer is deposited by electrolytic deposition, vapor phase or dry process;

[0013] - the material of the first coating layer is copper or any other material ensuring the limitation of the nitrogen flow entering the volume to be nitrided and which can be deposited by electrolytic deposition, dry process or vapor phase;

[0014] - the steel part comprises, in addition to the volume to be nitrided, at least one volume not to nitriding comprising an external surface, the process comprising the deposition of a second coating layer on the external surface of the volume not to be nitrided, the second coating layer comprising a thickness greater than the thickness of the first coating layer;

[0015] - the material of the second coating layer is copper and is deposited by electrolytic deposition;

[0016] - the volume to be nitrided comprises a first thickness of between 0.05 mm and 1.5 mm;

[0017] - the thickness of the first coating layer and the second coating layer clothing is between 0.5 pm and 40 pm.

[0018] According to another aspect, a turbomachine is proposed comprising at least one steel part obtained by the nitriding process. The steel part constitutes a power transmission element such as a gear, a shaft, or a bearing.

[0019] According to another aspect, an aircraft is proposed comprising at least one steel part obtained by the nitriding process. The steel part constitutes a power transmission component such as a gear, shaft, or bearing. DESCRIPTION OF FIGURES

[0020] Other features, purposes and advantages will become apparent from the following description, which is purely illustrative and not limiting, and which should be read in conjunction with the accompanying drawings on which:

[0021] Fig. 1a and Fig. 1b illustrate metallographic sections after a nitriding step, before and after the removal of a compound layer;

[0022] Figure 2 illustrates a section of a propulsion system according to an embodiment of the present invention;

[0023] Fig. 3 illustrates a perspective view of a power transmission part according to an embodiment of the present invention;

[0024] Figure 4 illustrates a flowchart of a treatment process, according to a particular implementation of the present invention;

[0025] Fig. 5a, Fig. 5b, Fig. 5c and Fig. 5d illustrate a metallographic cross-section of a part during the different stages of a treatment process, according to a particular embodiment of the present invention.

[0026] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION Aircraft

[0027] An aircraft is a device configured to rise and move through the air, and can, For example, it could be an airplane, civil or military, or even a helicopter. An aircraft comprises a fuselage, which in the case of an airplane consists of a fuselage, wings, tail assembly, control surfaces, and landing gear. An aircraft includes at least one propulsion system to enable it to take off. Propulsion system

[0028] The propulsion system 1, as illustrated in [Fig. 2], comprises an engine 2 (or turbine) and a nacelle 3, and has a main direction extending along a longitudinal axis XX. The propulsion system 1 is configured to be fixed to the aircraft's airframe, for example, under its wings in the case of an airplane, by means of a pylon (or mast). The propulsion system 1 can also be mounted on the wing of the airplane or at the rear of its fuselage, or even be integrated into its fuselage.

[0029] The engine 2 may be a twin-body, twin-flow, direct-drive ducted turbojet, as described below, but may also have a different number of bodies and / or flows, and / or be another type of turbojet, such as a geared turbojet or a turboprop, with or without afterburner, ducted or unducted.

[0030] Unless otherwise specified, the terms "upstream" and "downstream" refer to the overall direction of airflow through the propulsion system 1 in operation. Similarly, an axial direction corresponds to the direction of the longitudinal axis XX, and a radial direction is a direction perpendicular to the longitudinal axis XX and intersecting it. Furthermore, an axial plane is a plane containing the longitudinal axis XX, and a radial plane is a plane perpendicular to the longitudinal axis XX. A circumference is understood to be a circle lying on a radial plane and whose center lies on the longitudinal axis XX. A tangential or circumferential direction is a direction tangent to a circumference: it is perpendicular to the longitudinal axis XX but does not pass through it.Finally, the adjectives "interior" (or "internal") and "exterior" (or "external") are used with reference to a radial direction, such that the interior part of an element is, along a radial direction, closer to the longitudinal axis XX than the exterior part of the same element.

[0031] The engine 2 comprises, from upstream to downstream, a blower 20, a compression section 22, comprising a low pressure compressor 220 and a high pressure compressor 222, a combustion chamber 24 and a turbine section 26, comprising a high pressure turbine 262 and a low pressure turbine 260.

[0032] The compressor section 22 comprises a series of stages, each comprising a rotating impeller (rotor) rotating in front of a fixed impeller (stator). The Turbine section 26 also includes a succession of stages, each comprising a fixed blade wheel (stator) behind which rotates a moving blade wheel (rotor).

[0033] The blower 20, the rotor portion of the low-pressure compressor 220, and the rotor portion of the low-pressure turbine 260 are connected by a low-pressure shaft 280 extending along the longitudinal axis XX, thus forming a low-pressure housing (LP housing). The rotor portion of the high-pressure compressor 222 and the rotor portion of the high-pressure turbine 262 are connected by a high-pressure shaft 282 extending along the longitudinal axis XX, thus forming a high-pressure housing (HP housing). The low-pressure shaft 280 is generally housed, along a section of its length, within the high-pressure shaft 282 and is coaxial with the high-pressure shaft 172.

[0034] The compression section 22, the combustion chamber 24, and the turbine section 26 are surrounded by an engine casing 23, to which are connected the stator parts of the low-pressure compressor 220, the high-pressure compressor 222, the high-pressure turbine 262, and the low-pressure turbine 260, while the blower 20 is surrounded by a blower casing 25. The engine casing 23 and the blower casing 25 are connected to each other by radially extending profiled arms 27 forming straighteners (or OGVs for "Outlet Guide Vanes" in Anglo-Saxon terminology), which are distributed circumferentially around the longitudinal axis XX. At least some of these arms 27 may be structural.The longitudinal axis XX defines the axis of rotation for the blower 20, the rotor parts of the compression section 22 and the rotor parts of the turbine section 26, in other words for the LP body and the HP body, which are each capable of being driven in rotation around the longitudinal axis XX relative to the engine casing 23 and the blower casing 25.

[0035] The nacelle 3 extends radially outside the engine 2, all around the longitudinal axis XX, so as to surround both the fan housing 25 and the engine housing 23, and to define, with a downstream part of the engine housing 23, a downstream part of a secondary flow B, the upstream part of the secondary flow B being defined by the fan housing 25 and an upstream part of the engine housing 23. The upstream part of the nacelle 3 further defines an air inlet 29 through which the fan 20 draws in the airflow circulating through the propulsion system 1. The nacelle 3 is integral with the fan housing 25 and attached and fixed to the aircraft by means of the mast.

[0036] The propulsion system 1 further comprises power transmission parts 4. The propulsion system 1 may also include, among its power transmission parts 4, at least one accessory gearbox, called an AGB (for "Accessory gear box" in Anglo-Saxon terminology), typically housed in a cavity within the nacelle 3. The accessory gearbox comprises a set of elements Rotary gears, or gears, allow a plurality of shafts to be driven in rotation around their own axis, with accessories mounted on these shafts to derive useful mechanical power from their rotation. The gear set is itself driven by means of a power take-off shaft (or RDS for "Radial Drive Shaft" in Anglo-Saxon terminology) connecting, possibly via a transfer case, the accessory housing to at least one of the high-pressure body HP and the low-pressure body BP, typically by meshing with at least one of the high-pressure shaft 282 and the low-pressure shaft 280. In this respect, the power take-off shaft may extend within a longitudinal cavity formed within one of the arms 27.In this way, mechanical power is likely to be taken from at least one of the high-pressure (HP) and low-pressure (BP) bodies to be delivered to at least one of the accessories via the accessory box.

[0037] A controller may also be provided to interface between the propulsion system 1 and the aircraft, and also to control the propulsion system 1. Typically, the controller may perform the following functions: regulating the flow of the various fluids necessary for the operation of the propulsion system 1, starting the propulsion system 1, transmitting various measured parameters from the propulsion system 1 to the aircraft cockpit, managing thrust or reverse thrust, etc. Such a controller may implement digital control and include a computer, memory, and various data exchange channels that interact with each other. Typically, the controller may be of the FADEC type (for "Full Authority Digital Engine Control" in Anglo-Saxon terminology).

[0038] In operation, the fan 20 draws in an airflow, a portion of which, circulating within a primary stream A, is successively compressed within the compression section 22, ignited within the combustion chamber 24, and expanded within the turbine section 26 before being ejected from the propulsion system 1. The primary stream A passes completely through the engine casing 23. Another portion of the airflow circulates within the secondary stream B, which takes an elongated annular shape surrounding the engine casing 23. The air drawn in by the fan 20 is straightened by the straighteners 27 and then ejected from the propulsion system 1. In this way, the propulsion system 1 generates thrust. This thrust can, for example, be used to power the aircraft on which the propulsion system 1 is mounted.

[0039] Nitriding of power transmission parts

[0040] The power transmission parts 4 mentioned above are gears, splined shafts, bearing races, pistons, pins or various gears.

[0041] Such power transmission parts 4, as illustrated by [Fig. 3], born These components require a certain toughness, but also high resistance to wear and contact fatigue. They are therefore made of low-alloy steel, grades suitable for case hardening or nitriding. The power transmission component 4 comprises a core 4a and an outer layer 4b. To better meet the requirements of their function, only a portion of the outer layer 4b of the power transmission component 4 requiring specific wear resistance is hardened, in this case by nitriding. The nitriding treatment promotes surface hardening and the generation of high compressive residual stresses. The remaining portion of the outer layer 4b of the power transmission component 4, which is not to be hardened, is prevented from nitriding by masking; it thus retains its toughness.Preferably, the core 4a of the power transmission part 4 is not nitrided.

[0042] The power transmission parts 4 are nitrided, or partially nitrided, according to a process described below. Nitriding increases the surface hardness of the parts without affecting their intrinsic internal mechanical properties. Figure 4 illustrates steps in the nitriding process.

[0043] The process first comprises supplying (step 11) a steel power transmission part 4. The outer layer 4b of the supplied steel part 4 comprises at least one nitriding volume 41 and may include one or more non-nitriding volumes 42. The nitriding volumes 41 and 42 each comprise a clean outer surface 411, 421. The nitriding volume 41 comprises a thickness 412. The thickness 412 of the nitriding volume 41 is preferably less than the thickness of the steel part 4.

[0044] Figure 3 illustrates an embodiment in which the supplied steel part 4 is cylindrical and includes a casing 40 delimiting the external surface of the material. The volume to be nitrided 41 is contained within one axial portion of the cylinder, while the volume not to be nitrided 42 is contained within another axial portion of the cylinder, their respective external surfaces 411, 421 being respective portions of the casing 40.

[0045] The nitriding process, the steps of which are shown in [Fig. 4], then comprises a deposition (step E2) of a coating layer 43 onto the steel part 4. [Fig. 5a] illustrates a steel part 4 whose external surface 411 of the volume to be nitrided 41 is covered with a coating layer 43, referred to as the first coating layer. The coating layer 43 completely covers the external surface 411 of the volume to be nitrided 41.

[0046] Advantageously, the volume not to be nitrided 42 is also coated (step E21) with a coating layer 44, referred to as the second coating layer. Thus, a coating layer 43, 44, is deposited on each of the external surfaces 411, 421, respective volumes of the nitriding volume 41 and the non-nitriding volume 42. However, the coating layer 44 deposited on the external surface 421 of the non-nitriding volume 42 has a thickness 441 greater than the thickness 431 of the coating layer 43 deposited on the external surface 411 of the nitriding volume 41. Thus, both the nitriding volumes 41 and 42 are covered with a coating layer 43, 44, but these coating layers 43, 44 are of different thicknesses. The coating layer 44 deposited on the external surface 421 of the non-nitriding volume 42 may optionally be made of a different material than the coating layer 43 deposited on the external surface 411 of the nitriding volume 41.

[0047] The coating layer 43 deposited on the external surface 411 of the volume to be nitrided 41 is made of a material that does not react with nitrogen N2. The material constituting the coating layer 43 is copper or any other material ensuring the limitation of the nitrogen flux entering the nitriding zone 41, for example nickel, silver, tin, or TiN, TiC, or AlTiN... The material constituting the coating layer 43 is deposited by a thin-film deposition. The thin-film deposition is an electrolytic deposition, either dry process or vapor phase.

[0048] The coating layer 44 deposited on the external surface 412 of the volume 42 not to be nitrided is made of a material that does not react with nitrogen N2. The material constituting the coating layer 44 is preferably copper. The coating layer 44 is deposited on the external surface 421 by electrolytic deposition.

[0049] The deposition of the respective coating layers 43, 44 may further include degreasing with chlorinated or electrolytic solvents, rinsing with running or cold water, the deposition itself, alkaline neutralization, and drying. The deposited thickness is precisely controlled by the duration of the deposition (steps E2, E21) and by the intensity of the activation. The thicknesses 431, 441 of the deposited coating layers 43, 44 range from 0.5 µm to 40 µm. The thicknesses 431, 441 of the coating layer 43 on the external surface 411 of the volume to be nitrided 41 and of the coating layer 44 on the external surface 421 of the volume not to be nitrided 42 are interdependent and also depend on the duration of a nitriding step E3, as explained below.

[0050] The process then includes the nitriding step E3 illustrated in [Fig. 5b]. Nitriding E3 consists of carrying out a thermochemical treatment of the steel part 4. The steel part 4 is positioned in a medium 5 capable of releasing nitrogen N2, at a temperature between 300 °C and 580 °C. The steel part 4 can be heated in the presence of ammonia or any other compound capable of releasing nitrogen N2 under the action of heat. Over time, the nitrogen N2 diffuses into the volume to be nitrided 41 of the steel part 4 but is blocked by the coating layer. 44 covering the external surface 421 of the volume not to be nitrided 42 whose thickness 441 prevents nitrogen N2 from diffusing to the steel part 4.

[0051] After a certain time has elapsed, the steel part 4 is removed from the nitrogen-releasing medium 5 (N2). As illustrated in [Fig. 5c], the volume to be nitrided 41 has a diffusion layer 413, and the volume not to be nitrided 42 is free of a diffusion layer. The volume to be nitrided 41 now consists of a mixture of nitrides of alloying elements, such as chromium nitrides, vanadium nitrides, etc. The thickness 412 of the diffusion layer 413 of the volume to be nitrided 41 is between 0.05 mm and 1.5 mm. The nitriding time E3 depends on this thickness 412. At the end of nitriding E3, the entire volume to be nitrided 41 is nitrided.

[0052] Thus, the coating layer 43 deposited on the external surface 411 of the volume to be nitrided 41 allowed the volume to be nitrided 41 to be nitrided without developing on its external surface 411 a combination layer 45 supersaturated with iron nitrides and therefore very fragile. And, the greater thickness 441 of the coating layer 44 deposited on the external surface 421 of the volume not to be nitrided 42 allowed the volume not to be nitrided 42 to remain unnitrided. In other words, the coating layers 43 and 44, depending respectively on their difference in thickness and the duration of nitriding, either allow the diffusion layer 413 to be obtained by nitriding the volume to be nitrided 41 without the creation of a combination layer, or completely prevent the creation of a diffusion layer in the volume not to be nitrided 42.

[0053] According to a certain embodiment, the coating layers 43, 44, respectively of the external surfaces 411, 421, of the volume to be nitrided 41 and the volume not to be nitrided 42, are removed (step E4) or are partially removed. Figure 5d illustrates an embodiment in which the external surfaces of the volume to be nitrided 41 and the volume not to be nitrided 42 are completely removed. The coating layers 43, 44 can be removed by a sandblasting step or with chemical means. This step, although possible, is nevertheless easier to carry out than a step involving the removal of a compound layer 45. Indeed, since a compound layer 45 is composed of iron nitrides, it can only be removed by grinding. However, grinding is costly, difficult to perform in certain areas with complex shapes, and can generate defects such as crazing.

[0054] According to a certain embodiment, the coating layers 43, 44, respectively on the external surfaces 411, 421, the volume to be nitrided 41 and the volume not to be nitrided 42, are not removed from the steel part 4. This avoids a removal step E4 which entails additional costs and time. The coating layers 43, 44 can then be used for the purpose of protecting the part, for example against wear, fatigue or corrosion. 10

Claims

Demands

1. A method for treating a steel part (4) by nitriding comprising the following steps of: - supplying (E1) a steel part (4) comprising a volume to be nitrided (41), the volume to be nitrided (41) comprising an external surface (411) and at least one volume not to be nitrided (42) comprising an external surface (421); - deposition (E2), on the external surface (411) of the volume to be nitrided (41), of a first coating layer (43) comprising a material not reacting with nitrogen (N2); - deposition (E21) on the external surface (421) of the volume not to be nitrided (42), of a second coating layer (44) comprising a thickness (441) greater than a thickness (431) of the first coating layer (43); and - nitriding (E3) of the steel part (4) for a determined time, so as to form a diffusion layer (413) in the volume to be nitrided (41).

2. Processing method according to claim 1, comprising, following the nitriding step (E3), a removal step (E4) of the first coating layer (43).

3. Processing method according to any one of claims 1 to 2, wherein the deposition (E2) of the first coating layer (43) is obtained by electrolytic deposition, vapor phase or dry process.

4. A treatment method according to any one of claims 1 to 3, wherein the material of the first coating layer (43) is copper or any other material ensuring the limitation of the flow of nitrogen (N2) arriving in the volume to be nitrided (41) and which can be deposited by electrolytic deposition, dry process or vapor phase.

5. Processing method according to any one of claims 1 to 4, wherein the material of the second coating layer (44) is copper and is deposited (E21) by electrolytic deposition.

6. A treatment method according to any one of claims 1 to 5, wherein the volume to be nitrided (41) comprises a first thickness (412) of between 0.05 mm and 1.5 mm.

7. A treatment method according to any one of claims 1 to 6,

8.

9.

10. in which the thickness (431,441) of the first coating layer (43) and of the second coating layer (44) is between 0.5 pm and 40 pm. Steel part (4) for turbomachine, obtained by the nitriding process according to any one of claims 1 to 7, the steel part (4) constituting a power transmission element (4) such as a gear, shaft or bearing. turbomachine comprising at least one steel part (4) according to claim 8. Aircraft comprising at least one steel part (4) according to claim 8.

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