Manufacturing process for a high-strength steel turbine shaft

Abstract

A method for manufacturing a high-strength steel turbine shaft (1), the method comprising the steps of: forging (9) a metal blank to obtain a forged blank, performing a first heat treatment (13) on the forged blank so that said forged blank has a first state of mechanical properties, drilling (16) the forged blank having the first state of mechanical properties to obtain a semi-finished part, performing a second heat treatment (17) on the semi-finished part so that said semi-finished part has a second state of mechanical properties distinct from the first state of mechanical properties, machining (22) the semi-finished part according to target dimensions to obtain a finished part, characterized in that the drilling step (16) is carried out prior to the second heat treatment step (17). Fig. 2.

Description

Title of the invention: Method for manufacturing a high-strength steel turbine shaft technical field

[0001] The present description relates to the field of turbomachinery, for example turboprops or turbojets. More particularly, the invention relates to the manufacture of a turbine shaft for a turbomachine, and more specifically to the manufacture of a turbine shaft for a turbomachine made of high-strength steel. Prior art

[0002] A turbine shaft for a turbomachine is manufactured from a metal billet which undergoes several transformation steps in order to form the desired turbine shaft in one piece.

[0003] Initially, a conventional process for manufacturing a turbine shaft for a turbomachine includes a hot forging step of the billet to obtain a cylindrical metal forged blank. This forged blank then undergoes various heat treatments to give the metal the desired mechanical properties for the shaft. Typically, in the case of high-strength steel, the blank undergoes a quenching and tempering treatment.

[0004] Once these heat treatments have been carried out, the forged blank is then drilled, or bored, to a constant internal diameter. Drilling is a machining technique that combines linear displacement and rotation of the tool. Several parameters must be taken into account in the machined surface finish: cutting speed, depth of cut, feed rate, tool life, lubrication, etc. Such drilling makes it possible to machine the inside of the shaft by removing material while guaranteeing a constant web thickness, that is, a constant shaft wall thickness.

[0005] Depending on the techniques employed, drilling makes it possible to create evolving internal shapes that are inaccessible with standard tools. For example, a first hole is drilled with a constant diameter, and a second hole is drilled with a drill head, that is, with a tool that can deploy cutters to modify the cutting diameter. Different internal diameter profiles for the shaft can thus be obtained depending on whether the portions are machined only during the first drilling or both during the first and second drilling. These drilled profiles, which are constant and cylindrical, can be either blind or through. In both cases, the cutting tool is moved along the blank to perform the internal machining.

[0006] However, in the case of turbine shafts made of high-strength steels, drilling is complex due to the material hardness, which directly impacts the lifespan of the cutting tools. Indeed, such high-strength steels exhibit a very low machinability rate, determined by measuring the weighted averages of the normal cutting speed, surface finish, and tool life for each material.

[0007] Thus, when drilling in high-strength steel, the forged blank material is difficult to drill with a single machining insert because the insert degrades rapidly. This difficulty is increased when the forged blank to be drilled is of significant length.

[0008] Changing the machining insert on the same surface generates imperfections (bumps) that can lead to fracture initiation. It is essential to machine the internal profile without bumps. In other words, changing the insert in case of premature wear is not feasible.

[0009] Furthermore, the hardness of high-strength steel is a source of significant vibrations during drilling, these vibrations causing relative movements between the forged blank and the drilling tools. In other words, these vibrations can lead to drilling defects and the rejection of the forged blank.

[0010] The conventional turbine shaft manufacturing process is therefore not suitable for manufacturing high-strength steel turbine shafts.

[0011] To enable the manufacture of turbine shafts from high-strength steels, a friction welding process has been developed. This process consists of preparing two separate parts through conventional machining and joining them by inertial friction welding. This welding process involves rotating two separate portions of the shaft at high speed and joining them by applying high pressure so that the friction of one portion on the other generates sufficient heat for the material to liquefy and bond between the two portions.

[0012] However, this process is complex and costly. It requires heavy investments (special machine), significant machine setup phases to ensure precise welding, and machining phases to rework the weld beads once the two parts of the shaft are assembled.

[0013] There is therefore a need for a simple and reliable method for manufacturing turbine shafts for turbomachinery from high-strength steel. In particular, there is a need for such a manufacturing method that allows for obtaining such a shaft with the desired dimensions without exhibiting structural weaknesses or requiring a complex process. Description of the invention

[0014] One idea underlying the invention is to enable the manufacture of turbine shafts for turbomachinery from high-strength steel. In particular, one idea underlying the invention is to provide a simple and reliable method for manufacturing turbine shafts for turbomachinery from high-strength steel.

[0015] To this end, the invention provides a method for manufacturing a turbine shaft from high-strength steel, the method comprising the steps of: - forging a metal blank to obtain a forged blank, - performing an initial heat treatment on the forged blank of in such a way that the said forged blank exhibits a first state of mechanical properties, - drilling of the forged blank exhibiting the initial state of mechanical properties in order to obtain a semi-finished part, - performing a second heat treatment on the semi-finished part so that said semi-finished part exhibits a second state of mechanical properties distinct from the first state of mechanical properties, - Machining the semi-finished part according to target dimensions in order to obtain a finished part, in which the drilling stage is carried out prior to the second heat treatment stage.

[0016] The method according to the invention allows drilling on the forged blank while the latter has mechanical properties, and in particular a hardness, compatible with the tool life. In other words, the method according to the invention allows a drilling step in a steel whose machinability is sufficiently high to perform this drilling without requiring replacement of the cutting tools or a complex assembly process for sections of shaft made of high-strength steel.

[0017] According to different embodiments, the manufacturing process according to the invention may include one or more of the following features, taken alone or in combination.

[0018] According to one embodiment, the process further comprises the step of providing a metal billet, preferably made of steel. According to another embodiment, the process comprises a step of cutting said metal billet.

[0019] According to one embodiment, the first state of mechanical properties includes a first hardness.

[0020] According to one embodiment, the second state of mechanical properties includes a second hardness, said second hardness being greater than the first hardness.

[0021] According to one embodiment, the process further comprises a step of removing scale from the forged blank prior to the drilling step. This scale removal consists of removing the outer crust of the forged blank. The aim is to obtain surfaces with adequate dimensional tolerances in order to carry out the drilling.

[0022] According to one embodiment, the forged blank comprises a barrel and a trunnion. In one embodiment, the barrel is cylindrical. Thus, prior to the drilling step, and where applicable, the deburring step, the barrel is a solid cylinder of revolution and, after the drilling step, and where applicable, the deburring step, the barrel is a hollow cylinder of revolution.

[0023] According to one embodiment, the forged blank has, prior to the drilling stage, a length-to-external-diameter ratio greater than 10.

[0024] According to one embodiment, the drilling step of the forged blank comprises a first drill hole and a second drill hole. Preferably, the first drill hole is made at a constant diameter. Preferably, this first drill hole is made at a first diameter over all or part of the length of the forged blank. According to one embodiment, the second drill hole includes a step of modifying the internal diameter of the forged blank. Thus, according to one embodiment, the second drill hole is made over all or part of the forged blank so that the semi-finished part obtained at the end of the drilling process has a finished internal profile corresponding to the geometry and thickness of the shaft.For example, this finished internal profile may include a first internal diameter on a first portion of said semi-finished part, this first diameter resulting from the first drilling, and a second internal diameter, larger than the first internal diameter, on a second portion of said semi-finished part, this second internal diameter resulting from both the first drilling and the second drilling.

[0025] The use of high-strength steels, due to their mechanical properties, makes it possible to obtain a turbine shaft with the desired mechanical properties. To limit the weight of the turbine shaft, the web thickness of a high-strength steel turbine shaft can be reduced. Thus, according to a preferred embodiment, the shaft has a web thickness of less than 1 centimeter. Such a web thickness with high-strength steel is sufficient to meet the mechanical and weight requirements, despite the web thickness being considered thin.

[0026] According to one embodiment, the process further comprises a shot peening step of the finished part. This shot peening step is performed after the second heat treatment, and preferably after the machining step. Such shot peening increases the service life of the turbine shaft.

[0027] However, high-strength steel turbine shafts with a thin web thickness may be damaged by deformation during the shot peening stage.

[0028] Thus, according to one embodiment, the process includes a rectification step, i.e., the creation of reference diameters on the shaft, subsequently during the shot blasting stage. The subsequent grinding stage, following shot blasting, corrects any deformations resulting from the blasting process. These deformations are more likely to occur with thinner cloth. In other words, this grinding after shot blasting ensures that the shaft has the desired diameters at the end of all hardening treatments, including shot blasting.

[0029] According to one embodiment, the process further comprises a pre-finishing step of the semi-finished part. Preferably, this pre-finishing step precedes the second heat treatment.

[0030] In this description, the terms "longitudinal", "lower", "upper" and their derivatives are defined with respect to the principal direction of, depending on the stage of the process, the billet, blank, forged blank, semi-finished part, finished part or turbine shaft. The terms "axial", "radial", "tangential", "inner", "outer" and their derivatives are defined with respect to the principal axis of, depending on the stage of the process, the billet, blank, forged blank, semi-finished part, finished part or turbine shaft.

[0031] The aforementioned features and advantages, as well as others, will become apparent from the detailed description that follows. This detailed description refers to the accompanying drawings. Brief description of the drawings

[0032] The accompanying drawings are schematic and are intended primarily to illustrate the principles of the exposition. In these drawings, from one figure to another, and in the description below, identical elements (or parts of elements) or elements fulfilling the same function are identified by the same reference symbols. [Fig.1] Fig.1 is a cross-sectional view of a turbine shaft for a turbomachine; [Fig.2] Fig.2 represents a manufacturing process diagram of a turbine shaft for a turbomachine from [Fig.1]; [Fig.3] The [Fig.3] is a cross-sectional view of a turbine shaft for a turbomachine according to an alternative embodiment of the [Fig.1]. Description of the implementation methods

[0033] Figure 1 shows a cross-sectional view, along a vertical plane passing through its longitudinal axis X, of a turbine shaft 1 for a turbomachine, for example, a low-pressure turbine shaft 1 for a turbojet or aircraft turboprop. This shaft 1 is made in one piece from a metallic alloy such as steel. Furthermore, this shaft 1 is hollow and includes a through bore extending along its entire length. The shaft 1 has a length, measured along its longitudinal axis X, on the order of a few meters.

[0034] The tree 1 has a central tubular portion 2, a first end 3 and a second end 4 opposite the first end 3.

[0035] The first end 3 has a trunnion 5. This trunnion 5 has an annular flange 6 intended to be fixed to a first element of the turbine rotor (not shown).

[0036] The second end 4 has grooves intended to cooperate in drive with a second element of the turbine rotor (not shown).

[0037] In the embodiment illustrated in [Fig. 1], the central portion 2 comprises a first cylindrical portion 7 and a second cylindrical portion 8.

[0038] The first cylindrical portion 7 is axially intercalated between the first end 3 and the second cylindrical portion 8. The second cylindrical portion 8 is axially intercalated between the second end 4 and the first cylindrical portion 7.

[0039] In the embodiment illustrated in [Fig. 3], the central portion 2 has a profile that varies along the axial direction. Thus, the central portion 2 comprises sections 23 with a reduced internal diameter and sections 24 with an increased internal diameter. Similarly, this central portion may comprise sections 25 with a reduced external diameter and sections 26 with an increased external diameter. It will be understood that the concepts of reduced or increased diameter refer to the diameters of the sections axially flanking said sections with reduced or increased diameter.

[0040] Conversely, in an embodiment not illustrated, the central portion 2 could have a single internal diameter extending over its entire length.

[0041] The shaft 1 is made from a metal billet (not shown), this metal billet having a generally cylindrical shape. [Fig. 2] schematically illustrates the different stages of formation of the shaft 1 from the metal billet.

[0042] During a first step in the manufacturing process of the shaft 1, hereafter referred to as the forging step 9, the billet is subjected to hot forging to form a forged blank.

[0043] This forging step 9 comprises a first sub-step 10 of forging during which a first portion of the billet is drawn out to form a shaft of the blank. Due to this drawing out, the shaft thus formed has a length greater than the initial length of the billet. In addition, this shaft also has a diameter smaller than the initial diameter of the billet. This shaft is intended to form the central portion 2 of the shaft 1.

[0044] A second sub-step 11 of the forging step 9 comprises drawing a second portion of the billet, this second portion being separate and contiguous from The first portion of the billet was used to create the shaft. Stretching this second portion allows for the formation of a pre-trunk.

[0045] Finally, a third sub-step 12 of the forging step 9 involves forging the pre-trunnion to form the trunnion 5 of the shaft 1.

[0046] Typically, the forging step 9 allows the billet to be deformed to give the forged blank the general shape of the shaft 1.

[0047] Following this forging step 9, the process according to the invention includes a first heat treatment step 13. This first heat treatment 13 is carried out on the forged blank resulting from the forging step 9. This first heat treatment 13 comprises a quench 14 and a first temper 15. The quench 14 is, for example, carried out by heating the blank to a high temperature and then cooling the blank. The first temper 15 is carried out by reheating the blank following the quench 14.

[0048] In order to make the shaft 1 hollow, it is necessary to perform a pre-machining step 27 on the forged blank. The pre-machining step 27 includes a drilling step 16 which allows the inside of the forged blank to be machined to form a semi-finished part, or semi-finished blank, of the shaft 1.

[0049] However, in the case of high-strength steel turbine shafts, drilling becomes complex due to the hardness of the material, which directly impacts the lifespan of the cutting tools. Furthermore, this hardness can generate undesirable vibrations between the drill bit and the forged blank, which can lead to unsatisfactory drilling and the rejection of the resulting semi-finished blank. This drilling process becomes increasingly difficult and uncertain as the length of the forged blank increases.

[0050] In order to avoid having to carry out drilling in high-strength steel, the process according to the invention provides for carrying out the drilling step 16 before all the treatments enabling the production of such high-strength steel have been carried out.

[0051] In particular, the applicant has found that the machinability rate of the forged blank decreases sharply after a second heat treatment, typically a second tempering 17.

[0052] For example, the applicant found that with a steel having a composition of 0.03% Carbon, 18% Nickel, 8% Cobalt, 5% Molybdenum and 0.5% Titanium (M250 type steel), the machinability rate is 40% before the second tempering 17 and drops to 15% after the second tempering 17. Similarly, the applicant found that with a steel having a composition of 0.23% Carbon, 13% Nickel, 6% Cobalt, 3.25% Chromium, 1.5% Molybdenum, 1.5% Aluminum and 0.25% Vanadium (ML340 type steel), the machinability rate is from 20 to 25% before the second income 17 and drops to 10% after the second income 17.

[0053] Thus, in the manufacturing process according to the invention, the pre-machining step 27, and therefore the drilling step 16, is advantageously carried out prior to the second heat treatment step, typically the second tempering 17. For example, the drilling step 16 is carried out directly following the first heat treatment 13, after the first tempering 15.

[0054] The drilling 16 prior to the second regrinding 17 makes it possible to produce internal diameters of the shaft 1 over a large length, typically with a length / diameter ratio greater than 10, in a simple and reliable manner, without causing premature wear of the cutting tool or requiring a change of the cutting tool or a complex process of assembling several portions of the shaft 1.

[0055] The drilling 16 can be continuous, i.e. with a constant internal diameter, or in several stages. Typically, continuous drilling makes it possible to obtain a constant internal diameter over the entire length of the shaft 1. Conversely, drilling in several stages makes it possible to obtain a shaft 1 with different internal diameters, for example as illustrated in [Fig. 3].

[0056] For example, in the case of the shaft 1 illustrated in [Fig. 1], a first hole is drilled to pre-machine the forged blank with a constant internal diameter along its entire length. This first internal diameter corresponds to the internal diameter of the second cylindrical portion 8. A second hole is then drilled with a suitable drill head, that is, with a tool that can deploy cutters to modify the cutting diameter and create different profiles. This second hole pre-machines the first cylindrical portion 7 with a second internal diameter such that the internal diameter of the first cylindrical portion 7 is greater than the internal diameter of the second cylindrical portion 8.

[0057] The pre-machining 27 of the forged blank may further include a deburring step 18 of the forged blank, as illustrated in [Fig. 2]. This deburring step 18 allows the removal of a crust of material formed on the forged blank following the first heat treatment 13. This deburring 18 also allows the maximum amount of material to be removed in order to approximate the finished profile of the part. It is therefore carried out after the first heat treatment 13.

[0058] The pre-machining 27 of the forged blank may also include a semi-finishing step 19. During this semi-finishing step 19, the forged blank is machined to prepare the reference surfaces for achieving the desired final dimensions of the shaft 1. This semi-finishing step 19 may include, for example, an internal reboring step and other machining steps. This semi-finishing 19 makes it possible to obtain a semi-finished part whose dimensions correspond to the desired dimensions of shaft 1 to within 1 or 2 mm, that is to say there remains a layer of 1 or 2 mm to be removed to obtain the desired dimensions of the finished part intended to form shaft 1.

[0059] When the pre-machining step 27 does not include deburring 18, the drilling 16 is carried out directly after the first heat treatment 13, typically after the first tempering 15. When the pre-machining step 27 includes deburring 18, said deburring 18 is carried out directly after the first heat treatment 13, typically after the first tempering 15, prior to drilling 16. When the pre-machining step 27 does not include a semi-finishing step 19, the second tempering 17 is carried out directly after drilling 16. When the pre-machining step 27 includes the semi-finishing step 19, the second tempering 17 is carried out directly after said semi-finishing step 19.

[0060] This second tempering 17 gives the steel forming the shaft 1 the desired final hardness. A machining step 22, also called the finishing step 22, allows the finished profile following this second tempering 17 to be machined to the desired dimensions, in other words, to obtain a finished part with the desired dimensions.

[0061] The process according to the invention may also include a shot peening step 20. This shot peening 20, or prestressing shot peening, consists of bombarding the finished part obtained after the finishing step 22 with a medium such as steel, ceramic, glass, or other shot. This bombardment compresses the outer surface of the shaft 1 in order to improve its service life.

[0062] However, shot blasting 20 generates significant stresses on surfaces and can generate more or less significant deformations depending on the thickness of material impacted by this shot blasting 20.

[0063] However, in the case of a high-strength steel shaft 1, the thickness of the material forming the central portion 2, also called the web thickness, can be relatively thin. Indeed, due to the high strength of the steel, it is possible to reduce the web thickness in order to minimize the weight of the shaft 1 while maintaining satisfactory mechanical properties for the intended use of the shaft 1.

[0064] To ensure that the shaft 1 has the desired dimensions at the end of the manufacturing process, shot blasting 20 is carried out after the second tempering 17. Preferably, shot blasting 20 is carried out after the finishing step 22, and a grinding step 21 is carried out after shot blasting 20. This grinding step 21 compensates for any deformations resulting from shot blasting. This grinding step 21 is carried out according to the reference dimensions to ensure that the shaft 1 retains the desired dimensions after all the hardening treatments carried out during the manufacturing process of the shaft 1.

[0065] Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various embodiments illustrated / mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than a restrictive sense.

Claims

Demands

1. A method for manufacturing a high-strength steel turbine shaft (1), the method comprising the steps of: - forging (9) a metal blank to obtain a forged blank, - carrying out a first heat treatment (13) on the forged blank so that said forged blank has a first state of mechanical properties, - drilling (16) the forged blank having the first state of mechanical properties to obtain a semi-finished part, - carrying out a second heat treatment (17) on the semi-finished part so that said semi-finished part has a second state of mechanical properties distinct from the first state of mechanical properties, - machining (22) the semi-finished part according to target dimensions to obtain a finished part, characterized in that the drilling step (16) is carried out prior to the second heat treatment step (17).

2. A method for manufacturing a turbine shaft (1) according to claim 1, wherein the first state of mechanical properties comprises a first hardness and the second state of mechanical properties comprises a second hardness, said second hardness being greater than the first hardness.

3. Method of manufacturing turbine shaft (1) according to claim 1 or 2, further comprising a step of deburring (18) the forged blank prior to the drilling step (16).

4. Method of manufacturing turbine shaft (1) according to any one of claims 1 to 3, further comprising a pre-finishing step (19) of the semi-finished part prior to the second heat treatment step (17).

5. A method for manufacturing a turbine shaft according to any one of claims 1 to 4, wherein the forged blank has, prior to the drilling step (16), a length-to-outer-diameter ratio greater than 10.

6. A method for manufacturing a turbine shaft according to any one of claims 1 to 5, wherein the drilling step (16) of the forged blank comprises a first drilling and a second drilling, the first drilling being carried out to a first diameter over all or part of the length of the forged blank, the second drilling being carried out over all or part of the forged blank so that the semi-finished part obtained at the end of the entire drilling has a finished internal profile corresponding to the geometry and thickness of the shaft.

7. Method of manufacturing turbine shaft (1) according to any one of claims 1 to 6, wherein the shaft (1) has a web of a thickness less than 1 centimeter.

8. Method of manufacturing turbine shaft (1) according to any one of claims 1 to 7, comprising a shot peening step (20) of the finished part.

9. Method of manufacturing turbine shaft according to claim 8, comprising a grinding step (21) subsequent to the shot peening step (20).

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