Method for manufacturing a high-strength steel turbine shaft

Abstract

The invention relates to a method for manufacturing a high-strength steel turbine shaft (1), the method including the steps of: - forging (9) a metal blank so as to obtain a forged blank, - performing a first heat treatment (13) on the forged blank such that the forged blank has a first state of mechanical properties, - drilling (16) the forged blank having the first state of mechanical properties so as to obtain a semi-finished part, - performing a second heat treatment (17) on the semi-finished part such that the semi-finished part has a second state of mechanical properties different from the first state of mechanical properties, and - machining (22) the semi-finished part according to target dimensions so as to obtain a finished part, characterised in that the drilling step (16) is carried out prior to the second heat treatment step (17).

Description

Description Title of the invention: Method for manufacturing a high-strength steel turbine shaft Technical Field

[0001] This presentation concerns 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. Previous technique

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

[0003] Initially, a conventional turbine shaft manufacturing process for turbomachinery involves a hot forging step of the billet to obtain a cylindrical metal 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 quenching and tempering.

[0004] Once these heat treatments are completed, 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 considered in the machined surface finish: cutting speed, depth of cut, feed rate, tool life, lubrication, etc. Such drilling allows the inside of the shaft to be machined by removing material while guaranteeing a constant web thickness, that is, a constant shaft wall thickness.

[0005] Depending on the techniques used, drilling allows for the creation of evolving internal shapes inaccessible with standard tools. For example, a first borehole is drilled with a constant diameter and a second borehole is This is achieved using a drill head, meaning a tool that can deploy cutters to change the cutting diameter. Different internal diameter profiles for the shaft can thus be obtained depending on whether the sections are machined only during the first drilling operation or both during the first and second drilling operations. 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, when drilling high-strength steel turbine shafts, the process 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 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 compounded when the forged blank to be drilled is long.

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

[0009] Furthermore, the hardness of high-strength steel generates significant vibrations during drilling. These vibrations cause relative movement 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 involves preparing two separate parts through conventional machining and joining them by inertial friction welding. This welding process consists of rotating two separate sections of the shaft at high speed and then welding them together. assemble by applying strong pressure so that the friction of one portion on the other generates enough heat for the material to liquefy and bond between the two portions.

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

[0013] Therefore, there is a need for a simple and reliable manufacturing process for high-strength steel turbine shafts for turbomachinery. Specifically, there is a need for such a manufacturing process that allows for the production of such a shaft with the desired dimensions without exhibiting structural weaknesses or requiring complex manufacturing procedures. 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, another idea 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 high-strength steel turbine shaft, the method comprising the following steps: - forging a metal blank to obtain a forged blank, - performing an initial heat treatment on the forged blank so that said forged blank exhibits an initial 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 it possesses 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 steel with a sufficiently high machinability to perform this drilling without requiring replacement of the cutting tools or a complex assembly process for high-strength steel shaft sections.

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

[0018] In one embodiment, the process further comprises the step of providing a metal billet, preferably made of steel. In 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] In one embodiment, the process further includes a step of removing the outer layer of the forged blank prior to the drilling step. This removal consists of removing the outer layer of the forged blank. The aim is to obtain surfaces with adequate dimensional tolerances for drilling.

[0022] In one embodiment, the forged blank comprises a barrel and a trunnion. In another embodiment, the barrel is a cylinder of revolution. Thus, prior to the drilling stage, and where applicable, the deburring stage, the barrel is a solid cylinder of revolution, and after the drilling stage, and where applicable, the deburring stage, 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-outer-diameter ratio greater than 10.

[0024] In 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 drilled at a constant diameter. Preferably, this first drill hole is drilled at a first diameter over all or part of the length of the forged blank. In one embodiment, the second drill hole includes a step of modifying the internal diameter of the forged blank. Thus, in one embodiment, the second drill hole is drilled 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 internal finished 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, greater 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, allows for the production of a turbine shaft with the desired mechanical characteristics. To limit the weight of the turbine shaft, the web thickness of a high-strength steel turbine shaft can be reduced. Thus, in 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 relatively thin.

[0026] In one embodiment, the process further includes a shot peening step on 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 can be damaged by deformation during the shot peening stage.

[0028] Thus, according to one embodiment, the process includes a rectification step, that is, the creation of reference diameters on the shaft, after the shot peening step. This rectification step after shot peening compensates for any deformations resulting from the shot peening, deformations that are more likely to occur when the fabric is thin. In other words, this rectification after shot peening ensures that the shaft has the desired diameters at the end after all the hardening treatments, including shot peening.

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

[0030] In this document, 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 upon reading the detailed description that follows. This detailed description refers to the attached drawings. Brief description of the drawings

[0032] The accompanying drawings are schematic and are primarily intended to illustrate the principles of the exposition. In these drawings, from one figure to the next, and in the description below, identical elements (or parts of elements) or those performing the same function are identified by the same reference symbols. [Fig. 1] Figure 1 is a cross-sectional view of a turbine shaft for a turbomachine; [Fig. 2] Figure 2 represents a process diagram for manufacturing a turbine shaft for the turbomachine of Figure 1; [Fig. 3] Figure 3 is a cross-sectional view of a turbine shaft for a turbomachine according to an alternative embodiment of Figure 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 engine. This shaft 1 is made from a single piece of a metal 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] 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 designed to cooperate in drive with a second element of the turbine rotor (not shown).

[0037] In the embodiment illustrated in Figure 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 Figure 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 are relative to the diameters of the portions axially framing said portions of reduced or increased diameter.

[0040] Conversely, in an unillustrated embodiment, the central portion 2 could have a single internal diameter extending along its entire length.

[0041] Tree 1 is made from a metal billet (not shown), which is generally cylindrical in shape. Figure 2 schematically illustrates the different stages of forming tree 1 from the metal billet.

[0042] In the first stage of the manufacturing process of shaft 1, subsequently referred to as forging stage 9, the billet is subjected to hot forging to form a forged blank.

[0043] This forging step 9 includes a first sub-step 10 during which a first portion of the billet is drawn out to form a blank shaft. Due to this drawing out, the shaft thus formed has a length greater than the initial length of the billet. Furthermore, 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 involves drawing out a second portion of the billet, this second portion being separate from and joined to the first portion of the billet used to create the barrel. Drawing out this second portion forms 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, 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 quenching 14 and a first tempering 15. The quenching 14 is, for example, carried out by heating the blank to a temperature raised and then cooling the blank. The first tempering 15 is carried out by reheating the blank following the quenching 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 material's hardness, which directly impacts the lifespan of the cutting tools. Furthermore, this hardness can generate unwanted vibrations between the drill bit and the forged blank, potentially leading to unsatisfactory drilling and the rejection of the resulting semi-finished blank. This drilling process becomes even more difficult and uncertain as the length of the forged blank increases.

[0050] In order to avoid having to drill into 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 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 and drops to 15% after the second tempering. 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 20 to 25% before the second tempering and drops to 10% after the second tempering.

[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, 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 regrind 17 allows internal diameters of the shaft 1 to be produced over a long 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] Drilling 16 can be continuous, i.e., with a constant internal diameter, or in several stages. Typically, continuous drilling results in a constant internal diameter along the entire length of the shaft 1. Conversely, multi-stage drilling results in a shaft 1 with different internal diameters, for example, as illustrated in Figure 3.

[0056] For example, in the case of shaft 1 illustrated in Figure 1, a first hole is drilled to pre-machine the forged blank to a constant internal diameter along its entire length. This first internal diameter corresponds to the internal diameter of the second cylindrical section 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 section 7 with a second internal diameter, such that the internal diameter of the first cylindrical section 7 is larger than the internal diameter of the second cylindrical section 8.

[0057] The pre-machining 27 of the forged blank may further include a deburring step 18 of the forged blank, as illustrated in Figure 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 for the removal of as much material as possible 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 produces a semi-finished part whose dimensions correspond to the desired dimensions of the shaft 1 to within 1 or 2 mm; that is, a layer of 1 or 2 mm remains to be removed to obtain the desired dimensions of the finished part intended to form the shaft 1.

[0059] When the pre-machining step 27 does not include deburring 18, 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 step 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 to be machined following this second tempering step 17 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 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, may be relatively thin. Indeed, due to the high strength of the steel, it is possible to reduce the thickness of the web in order to minimize the weight of shaft 1 while maintaining satisfactory mechanical properties for the intended use of shaft 1.

[0064] To ensure that the shaft 1 has the desired dimensions at the end of the manufacturing process, shot blasting 20 is performed after the second tempering 17. Preferably, shot blasting 20 is performed after the finishing step 22, and a grinding step 21 is performed after shot blasting 20. This grinding step 21 compensates for any deformations resulting from shot blasting. This grinding step 21 is performed 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 restrictive sense.

Claims

Demands

1. A method for manufacturing a high-strength steel turbine shaft (1), the method comprising the steps of: - forging (9) of a metal blank so as to obtain a forged blank, - carrying out a first heat treatment (13) on the forged blank so that said forged blank exhibits a first state of mechanical properties, - drilling (16) of the forged blank exhibiting the first state of mechanical properties in order to obtain a semi-finished part, - carrying out a second heat treatment (17) 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 (22) of the semi-finished part according to target dimensions so as to obtain a finished part, characterized in that the drilling step (16) is carried out prior to the second heat treatment step (17), the first heat treatment (13) comprising a quench (14) and a first tempering (15), the second heat treatment comprising a second tempering (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 of 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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