Heat treatment process for power transmission parts, in particular for aircraft turbomachines

Abstract

The present disclosure relates to a method for heat treating a self-hardening steel part, in particular for an aircraft turbomachine, comprising steps of: carburizing (102) the part, comprising heating the part to a carburizing temperature (T102) and bringing the part into contact with an environment containing carbon for at least part of the heating, cooling (103) the part at a first cooling rate less than or equal to 8°C per second, induction heating (106) a surface of the cooled part to an induction temperature (T106), and quenching (107) the part at a second cooling rate, the second cooling rate being greater than or equal to the first cooling rate. Figure for abstract: Fig. 1

Description

Title of the invention: Method for heat treatment of power transmission parts, in particular for aircraft turbomachines Technical field

[0001] The present disclosure relates generally to the field of metallurgy. It relates more specifically to heat treatment processes for steel parts. It advantageously finds application for the hardening of power transmission parts, such as, for example, pinion teeth, crowns or bearings used in aircraft turbomachines. STATE OF THE ART

[0002] In the field of aeronautical turbomachines, it is common to implement heat treatment or thermochemical processes in order to increase the mechanical resistance of the surfaces of steel components. Indeed, certain components, such as for example the crowns used to transmit power in a fan reducer, between a shaft of a fan rotor and a low pressure turbine shaft, are subjected to a high loading on their surface.

[0003] Among the treatment processes used, deep nitriding consists of incorporating nitrogen atoms into the surface layer of the steel forming the component, so as to increase its surface hardness without affecting the mechanical properties of the part elsewhere than on its surface. This process is very slow and the treatment time increases with the depth of nitriding, being able to reach several hundred hours depending on the depth of the treatment. The production of nitrided parts to significant depths therefore constitutes a significant constraint due to the required duration of the treatments.

[0004] Other treatments used include carburizing processes. These processes consist of subjecting a steel to a carbon-rich environment during a temperature increase, for example a gaseous environment or a plasma, in order to diffuse carbon atoms into the surface of the alloy so as to increase the hardness potential of this surface. The temperature increase of the steel can go up to a temperature between approximately 850 °C and 950 °C for most steels, this temperature depending in particular on the carbon content of the steel and the content of certain alloying elements. This increase in temperature causes austenite grains, a specific allotrope of steel, to appear in the steel. A very rapid quenching is then carried out in a high drasticity fluid, so as to cool the material quickly to a very low temperature and thus ensure a trans formation of austenite into martensite, another allotrope of steel which generates a hardening of the surface layer of the component and residual compressive stresses. However, for large and relatively thin components - i.e. with a very low thickness compared to their other dimensions - this quenching causes significant distortions that cannot be corrected after treatment. These processes therefore do not allow precise control of the geometry of the components finally obtained, and are therefore poorly suited to the manufacture of large parts such as the gear rings commonly used in aircraft turbomachinery. EXPOSED

[0005] An aim of the present application is to enable the manufacture of large steel components, without generating excessive distortions and obtaining satisfactory surface hardness and residual compressive stresses compared to the requirements of the aeronautical industry.

[0006] Another aim is to enable the industrial scale manufacturing of steel components, with limited cost and processing times.

[0007] For this purpose, according to a first aspect, a method is proposed for the heat treatment of a self-hardening steel part, in particular for an aircraft turbomachine, comprising steps of: - carburizing the part, comprising heating the part to a carburizing temperature and contacting the part with a carbon-containing environment for at least part of the heating, - cooling of the part at a first cooling rate less than or equal to 8°C per second, - induction heating of a surface of the cooled part to an induction temperature, and - quenching the part at a second cooling rate, the second cooling rate being greater than or equal to the first cooling rate.

[0008] A combination of slow cooling after carburizing and induction hardening is thus implemented, both carried out on a self-hardening steel part, i.e. a steel whose critical hardening speed, which corresponds to the minimum cooling speed necessary for the formation of martensite, is less than or equal to the natural cooling speed of the steel in still air after heating has enabled austenitization of the steel. In a non-exhaustive manner, such steels include the following steels: M50NiL (described by AMS 6278), Ferrium C64 (ANS6509), Pyrowear 675 (AMS5930). This makes it possible to obtain a part core which has satisfactory mechanical properties - notably increasing its hardness and its yield strength - as well as functional areas on the surface of the part with high hardness, without generating significant distortions when it comes to a large part. The processing time is also greatly improved compared to certain processes according to the state of the art, in particular compared to nitriding processes, the implementation of the proposed process being up to 15 times faster than that of a conventional nitriding process.

[0009] According to one implementation of the method, quenching begins before induction heating is completed.

[0010] According to one implementation of the method, an inducing magnetic field allowing the electromagnetic induction of the part during induction heating has a frequency greater than or equal to 100 KHz and less than or equal to 300 KHz.

[0011] According to one implementation of the method, the induction heating successively comprises preheating the part to a temperature below 750°C and main heating of the part, an inducing magnetic field allowing the electromagnetic induction of the part having: - A frequency greater than or equal to 10 kHz and less than or equal to 50 kHz during preheating of the part, and - A frequency greater than or equal to 100 kHz and less than or equal to 300 kHz during the main heating of the room.

[0012] According to one implementation of the method, the induction temperature is greater than or equal to 1050°C and less than or equal to 1200°C.

[0013] According to one implementation of the method, the induction heating is configured to increase a temperature of the part over a depth of between 0.6 mm and 3.0 mm from an external surface of the part to at least an austenitizing temperature of the steel.

[0014] According to one implementation, the method further comprises: a. further cooling of the part to a cooling temperature at which the part is maintained for a predefined period of time, b. a step of relaxing the part, comprising heating the part to a relaxation temperature followed by maintaining the part at the relaxation temperature for a predefined period of time,

[0015] and the additional cooling and stress relief steps are implemented between the induction cooling and heating steps and / or after the second quenching.

[0016] According to one implementation of the method, the cooling temperature is greater than or equal to -100°C and less than or equal to -70°C and / or the expansion temperature is greater than or equal to 150°C and less than or equal to 250°C.

[0017] According to one implementation of the method, the part is maintained at the re- cooling for a period of one to three hours.

[0018] According to one implementation, the method comprises a tempering step, preferably three successive tempering steps, each tempering step successively comprising: a. a rise in temperature of the part to a tempering temperature lower than the carburizing temperature, the tempering temperature preferably being greater than or equal to 450°C and less than or equal to 600°C, b. maintaining the part at the tempering temperature for a predefined period, c. cooling of the room,

[0019] the tempering step being implemented after quenching.

[0020] According to one implementation of the method, the second cooling rate is greater than or equal to 10°C per second.

[0021] According to one implementation of the method, the cementation temperature is greater than or equal to 1050°C and less than or equal to 1100°C.

[0022] According to one implementation of the method, the contacting of the part with an environment containing carbon is carried out at temperatures between 900°C and 1050°C.

[0023] According to another aspect, the present disclosure relates to a method of manufacturing a part for an aircraft turbomachine, successively comprising: a. a step of machining the part from a raw part, notably comprising the manufacturing of teeth in the part, and b. the implementation of the heat treatment process as defined above.

[0024] According to another aspect, the present disclosure relates to an aircraft turbomachine comprising a part treated with the heat treatment method as defined above or a part obtained by means of the manufacturing method as defined above. DESCRIPTION OF FIGURES

[0025] Other characteristics, aims and advantages will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0026] [Fig. 1] is a block diagram illustrating a method of heat treating a steel part according to a first aspect.

[0027] [Fig.2] schematically illustrates a hardness profile obtained by a heat treatment process according to the state of the art and by the proposed process.

[0028] [Fig. 3] is a functional diagram illustrating a method of manufacturing a part in steel according to another aspect.

[0029] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION OF EMBODIMENTS

[0030] A method for heat treating a steel aircraft turbomachine part is shown in [Fig.l]. It can in particular be implemented on a toothed part, such as a reduction gear crown, or on a pinion. It can also be implemented on a bearing part. Generally speaking, the proposed method is suitable for any part for which a very high surface hardness is desired, and a part core having both a high hardness and a high elastic limit while retaining satisfactory ductility and toughness.

[0031] The proposed method is implemented on a part made of a so-called "self-hardening" or air-hardening steel, that is to say for which the critical hardening speed, which corresponds to the minimum cooling speed necessary for the formation of martensite, is less than or equal to the natural cooling speed of the steel in calm air after heating which has allowed austenitization of the steel. It is thus possible to carry out hardening of the steel forming the part in the open air, in contrast to steels which can only be hardened in a fluid of higher drasticity (water or oil in particular). It will be noted that the invention applies in particular to any case-hardening steel.

[0032] The method comprises a cementation step 102, during which the part is heated in a carbon-containing environment in order to diffuse carbon atoms into a surface of the part. The part may be brought into contact with the carbon-containing environment for the entire duration of the heating. Alternatively, the part may be brought into contact with the carbon-containing environment for only part of the heating. The cementation 102 may in particular be carried out in a furnace, in which carbon-rich gases are diffused and may come into contact with the part during its temperature rise. The depth to which the carbon atoms are diffused depends in particular on the duration of cementation as well as the maximum temperature reached during the cementation.

[0033] During the carburizing 102, the part is heated to a carburizing temperature T102, so as to form austenite, an allotrope of the steel obtained when heating it to high temperatures. The carburizing temperature may for example be greater than or equal to 900°C and less than or equal to 1100°C. The carburizing temperature may for example be greater than or equal to 1050°C and less than or equal to 1100°C. This corresponds substantially to the range of austenitization temperatures of self-hardening steels, and therefore capable of undergoing air quenching, depending on their composition. This ensures that an austenitic transformation has well placed in the heart of the part. The carburizing temperature can be chosen in particular according to the chemical composition of the alloy forming the part and its metallurgical state before carburizing, so as to obtain optimal austenitization of the part.

[0034] According to one implementation of the method, the contacting of the part with the carbon-containing environment is carried out at temperatures greater than or equal to 900°C and less than or equal to 1050°C.

[0035] The cementation 102 is followed by a first cooling step 103. This step allows the transformation of the austenite formed during the cementation 102 into martensite, an allotrope of steel having a particularly high hardness.

[0036] The cooling 103 is carried out at a first cooling rate of less than 8°C per second. Preferably, the cooling 103 is carried out in air.

[0037] The cementation 102 makes it possible to increase the hardness potential of the surface of the part. The cooling 103, due to its particularly slow first cooling rate and the high percentage of carbon in the surface following the cementation 102, allows a transformation of the austenite contained in the part into martensite, in its core as well as in its surface, and therefore increases the core and surface hardness of the part, without generating significant distortions in the part. This increase in hardness is particularly significant at the surface, due to the high carbon content thereof following the cementation 102.

[0038] Subsequent to the cooling 103, the treatment method comprises a step of induction heating 106 of the surface of the part up to an induction temperature Ti06, as well as a quenching 107 starting either simultaneously with the induction heating 106, on other parts of the surface of the part than those subjected to the induction heating 106, or subsequently to it. These steps make it possible to further treat the surface of the part, in order to increase its hardness beyond that obtained after the cooling 103, without however affecting the core of the part, and in particular without reducing its ductility and its toughness. The induction heating step 106 again generates austenite in the surface of the part, treated during this step, and the quenching 107 makes it possible to carry out a transformation into martensite of this austenite generated in the surface during the induction heating 106.Tempering 107 also allows the generation of residual stresses in the surface of the part, which contribute to increasing its mechanical resistance.

[0039] The induction heater 106 heats the surface of the part by electromagnetic induction therein of eddy currents by an inductor coil, these currents transferring heat from the part to the outside by Joule effect, and the depth at which these currents are generated in the part can easily be controlled. by modifying the treatment duration, the placement of the inductor coil, the geometry of the coil according to the geometry of the treated part, and the frequency of the inductor alternating magnetic field. The induction heating 106 allows a new austenitic transformation in the surface of the part, while the quenching 107, implemented at a cooling rate higher than that of the first quenching 103, ensures the transformation of the austenite thus formed into martensite, giving the surface its increased hardness. The induction heating 106 and quenching 107 steps may in particular be configured to heat the part from its external surface to a depth of between 0.6 mm and 3.0 mm, preferably of the order of 1.0 mm from the external surface in a direction normal to a central axis of the part, up to at least an austenitization temperature of the steel forming the part.In this way, it is ensured that austenite has formed in a predefined depth of the part, thus allowing the formation of martensite during quenching 107. .

[0040] Induction heating 106 and quenching 107 have the advantage of allowing precise control of the depth of the part and the specific areas thereof that are to be subjected to treatment. In prior art heat treatment processes that include carburizing followed by quenching, sufficiently rapid quenching is necessary, on the one hand to allow martensitic transformation in the surface of the part so as to give the surface a desired hardness, and on the other hand to generate residual stresses in the part that allow its mechanical strength to be increased. However, this rapid quenching is the cause of geometric distortions of the part.By comparison, in the proposed method, the cooling step 103, which is slower than conventional quenching, makes it possible to obtain the desired mechanical properties in the core of the part without deforming it, the induction heating steps 106 and quenching 107 giving the surface its superior hardness without affecting the core of the part - that is to say without causing allotropic transformation in the steel, and without causing distortions in the part. Induction heating 106 also has the advantage of allowing good control of the residual stresses introduced into the part, which can be used to increase the mechanical strength - particularly fatigue strength - of the part, depending on the type of loading it is intended to undergo.

[0041] According to one implementation of the method, the alternating magnetic field of the induction coil, applied during induction heating 106, may have a frequency of between 100 kHz and 300 kHz.

[0042] According to another particular implementation of the method, the induction heating 106 is carried out in two sub-steps, namely a first sub-step during which the alternating magnetic field of the induction coil has an average frequency greater than or equal to 10 kHz and less than or equal to 50 kHz, the first substep being carried out up to a temperature below 750°C, and being followed by a second substep during which the alternating magnetic field of the induction coil has a high frequency greater than or equal to 100 kHz and less than or equal to 300 kHz. In this case, the first substep allows preheating of the part, while the second substep allows the majority of the increase in temperature of the surface of the part during induction heating 106.

[0043] According to one implementation of the method, the part is cooled during quenching 107 at a second cooling rate greater than 10°C per second.

[0044] According to one implementation of the method, the latter comprises a first step of additional cooling and expansion between the cooling 103 and the induction heating 106. Such a step comprises a first additional cooling 104 of the part to a first cooling temperature T104, maintaining the part at the first cooling temperature, then a first expansion 105 of the part. The first cooling temperature Ti04 may be less than or equal to -70 and greater than or equal to -100°C. Maintaining the part at the first cooling temperature Ti^ may be carried out for a period of 1 to 3 hours. The first expansion 105 comprises heating the part to a first expansion temperature T105, preferably lower than the carburizing temperature T102, for example a first expansion temperature T105 greater than or equal to 150°C and less than or equal to 250°C.The first expansion 105 comprises, after heating, maintaining the part at the first expansion temperature Ti05.

[0045] According to one implementation of the method, the latter comprises a step of second additional cooling and relaxation after quenching 107. Such a step comprises a second additional cooling 108 of the part to a second cooling temperature T|0X, maintaining the part at the second cooling temperature Ti08, then a second relaxation 109 of the part. The second cooling temperature Ti08 may be less than or equal to -70°C and greater than or equal to -100°C. Maintaining the part at the cooling temperature Tios may be carried out for a period of 1 to 3 hours. The second relaxation 109 comprises heating the part to a second relaxation temperature T 109, preferably lower than the carburizing temperature T102, for example a second relaxation temperature T105 greater than or equal to 150°C and less than or equal to 250°C.The second expansion 109 comprises, after heating, maintaining the part at the second expansion temperature T109.

[0046] The relaxation 105, 109 makes it possible to transform the residual austenite remaining after the cooling 103 and / or the quenching 107 into martensite, and thus further increase the hardness and yield strength of the part. In addition, stress relief 105, 109 reduces residual stresses that may be present in the core of the part, which are not affected by induction heating and which can negatively affect the mechanical strength of the part.

[0047] According to one implementation of the method, at least one tempering step 110 is implemented after the quenching 107. When the method comprises a second additional cooling 108 and a second stress relief 109, the tempering step 110 is implemented after the second additional cooling 108 and the second stress relief 109. Each tempering step 110 comprises a rise in temperature of the part to a tempering temperature Tn0 lower than the carburizing temperature T102. The tempering temperature Tn0 may in particular be greater than or equal to 400°C and less than or equal to 650°C. The part is then maintained at the tempering temperature T no, so as to allow the diffusion of carbon from a small percentage of the martensite obtained following the cooling 103 and quenching 107 steps. This transformation of martensite into ferrite and carbides increases the ductility and toughness of the part.It also slightly reduces the hardness of the part, to values ​​satisfactory with regard to aeronautical requirements. Each tempering step 110 also includes cooling of the part. In addition, each tempering step 110 makes it possible to further reduce both the residual stresses that may be present in the part and the quantity of residual austenite remaining in the part. Providing three tempering steps 110 makes it possible to reduce this quantity of residual austenite as much as possible.

[0048] [Fig.2] schematically illustrates a surface hardness profile obtained according to the depth by a heat treatment process according to the state of the art and by the proposed process, on a part made of steel of type M50Nil (AMS 6278 according to the notation of the American Society of Automotive Engine ers). The hardness considered is a Vickers hardness 0.3, expressed in kilograms-force per mm2. Curve I represents the results obtained by a heat treatment process comprising a low-pressure carburizing step and a quenching step, as well as three tempering steps at a tempering temperature of 535 °C. Curve II represents the results obtained with the proposed heat treatment process, the induction heating step 106 is carried out up to a Ti06 induction temperature of 1200 °C, and is followed by a quenching 107 up to a temperature of -90 °C and a tempering step 110 at a tempering temperature Tno of 535 °C.These curves show that the proposed process allows an increase in the hardness at the extreme surface of the part of the order of 15%.

[0049] Another aspect of the present disclosure, illustrated in [Fig. 3], relates to a method 150 of manufacturing a part for an aircraft turbomachine. The method of manufacturing comprises the implementation of the heat treatment method 100 as defined previously on a part, said part being obtained by machining 101 from a raw part. In particular, the part may be a toothed part, and the machining step 101 then comprises the manufacture of teeth in the part.

[0050] The reduction in distortions in the part makes it possible to manufacture a blank, obtained after machining 101 of a raw part, closer to the part that is desired to be obtained after completion of the heat treatment process 100. The grinding operations that are necessary after completion of the heat treatment process are consequently less cumbersome, which reduces the cost and time required for manufacturing a part. In particular, the blanks used to undergo a carburizing treatment according to the state of the art must have an excess thickness so as to accommodate the distortions that appear during the treatment. In comparison, the proposed method does not generate distortion and it is therefore not necessary to provide an excess thickness on the blank, allowing a saving of material.

[0051] Another aspect of the present disclosure relates to an aircraft turbomachine equipped with a part treated using the heat treatment method 100 defined previously, or a part manufactured using the manufacturing method 150 defined previously.

Claims

Claims

1. A method of heat treating a self-hardening steel part, in particular for an aircraft turbomachine, comprising steps of: carburizing (102) the part, comprising heating the part to a carburizing temperature (T102) and bringing the part into contact with an environment containing carbon for at least part of the heating, cooling (103) the part at a first cooling rate less than or equal to 8°C per second, induction heating (106) a surface of the cooled part to an induction temperature (Ti06), and quenching (107) the part at a second cooling rate, the second cooling rate being greater than or equal to the first cooling rate.

2. A heat treatment method according to claim 1, wherein the quenching (107) begins before the induction heating (106) is completed.

3. A heat treatment method according to any one of claims 1 and 2, wherein an inducing magnetic field enabling electromagnetic induction of the workpiece during induction heating (106) has a frequency greater than or equal to 100 KHz and less than or equal to 300 KHz.

4. A heat treatment method according to any one of claims 1 and 2, wherein the induction heating (106) successively comprises preheating the part to a temperature below 750°C and main heating of the part, an inducing magnetic field allowing electromagnetic induction of the part having: - A frequency greater than or equal to 10 kHz and less than or equal to 50 kHz during the preheating of the part, and - A frequency greater than or equal to 100 kHz and less than or equal to 300 kHz during the main heating of the part.

5. A heat treatment method according to any one of claims 1 to 4, wherein the induction temperature (Ti06) is greater than or equal to 1050°C and less than or equal to 1200°C.

6. A heat treatment method according to any one of claims 1 to 5, wherein the induction heater (106) is configured to increase a temperature of the workpiece to a depth of between 0.6 mm and 3.0 mm from an external surface of the workpiece to at least an austenitizing temperature of the steel.

7. A heat treatment method according to any one of claims 1 to 6, further comprising: a. further cooling (104, 108) the workpiece to a cooling temperature (T104, Ti08) at which the workpiece is held for a predefined time, b. a step of stress relieving (105, 109) the workpiece, comprising heating the workpiece to a stress relieving temperature (T105, Tu») followed by holding the workpiece at the stress relieving temperature for a predefined time, wherein the further cooling (104, 108) and stress relieving (105, 109) steps are carried out between the cooling (103) and induction heating (106) steps and / or after the second quenching (107).

8. A heat treatment method according to claim 7, wherein the cooling temperature (T104, Tios) is greater than or equal to -100°C and less than or equal to -70°C and / or the expansion temperature (Ti05, T109) is greater than or equal to 150°C and less than or equal to 250°C.

9. A heat treatment method according to any one of claims 7 and 8, wherein the part is maintained at the cooling temperature (T104, Ti08) for a period of one to three hours.

10. Heat treatment method according to any one of claims 1 to 9, comprising an aging step (110), preferably three successive aging steps, each aging step successively comprising: a. a rise in temperature of the part to a aging temperature (Tn0) lower than the carburizing temperature (T102), the aging temperature (T110) preferably being greater than or equal to 450°C and less than or equal to 600°C, b. maintaining the part at the aging temperature (Tn0) for a predefined period, i.e. cooling of the part, the tempering step (110) being implemented after the quenching (107).

11. A heat treatment method according to any one of claims 1 to 10, wherein the second cooling rate is greater than or equal to 10°C per second.

12. A heat treatment method according to any one of claims 1 to 11, wherein the carburizing temperature (TiO2) is greater than or equal to 1050°C and less than or equal to 1100°C.

13. A heat treatment method according to any one of claims 1 to 12, wherein the contacting of the part with a carbon-containing environment is carried out at temperatures between 900°C and 1050°C.

14. Method of manufacturing (150) a part for an aircraft turbomachine, successively comprising: a. a step of machining (101) the part from a raw part, including in particular the manufacturing of teeth in the part, and b. the implementation of the heat treatment method (100) according to any one of claims 1 to 13 on the part.

15. Aircraft turbomachine comprising a part treated with the heat treatment method (100) according to any one of claims 1 to 13 or a part obtained by means of the manufacturing method (150) according to claim 14.

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