Method for manufacturing toothed parts hardened by nitriding, in particular parts for an aircraft turbine engine

Abstract

The invention relates to a method (1) for manufacturing a toothed part from a nitriding steel part, comprising the following successive steps: a step (2) of heat-treating the steel part to a first austenitising temperature of the steel and a step (3) of quenching the part, a step (4) of intermediate tempering of the steel part up to a second temperature lower than 350°C, a step (5) of cutting teeth in a surface of the part by wire electrical discharge machining, a step (7, 8) of nitriding the part at a third temperature higher than the second temperature and lower than the first temperature.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for manufacturing toothed parts hardened by nitriding, in particular parts for aircraft turbomachines

[0003] TECHNICAL FIELD This disclosure relates generally to the field of metallurgy. It relates more specifically to processes for manufacturing steel parts comprising heat treatment of steel parts by nitriding. It advantageously finds application for the manufacture of power transmission parts, such as, for example, pinion gears, crowns or bearings used in aircraft turbomachines.

[0004] STATE OF THE ART

[0005] In order to increase the mechanical surface hardness of a steel part, it is common to use heat treatment processes called "nitriding", during which nitrogen atoms are diffused into the surface to be treated, in particular by applying a nitrogen-rich gaseous environment around the part. The gases, which generally include ammonia, are adsorbed by the surface, and precipitate in the form of iron nitrides on the surface, forming a surface layer called the combination layer or "white" layer. The excess nitrogen will then diffuse into the steel matrix and precipitate in the form of nitrides with certain alloying elements (for example chromium, vanadium or molybdenum), forming a diffusion layer. The combination of these two layers forms a hardened layer which increases the hardness of the treated surface.It is generally desirable to remove the combination layer following nitriding, as this layer is too fragile for certain applications, particularly when surface loading of the part is planned.A metal part is often subjected to nitriding taking place at temperatures of the order of 480 to 580 °C after other heat treatment steps, namely: a so-called austenitization heat treatment which results in a homogeneous and entirely austenitic microstructure - quenching, i.e. a rapid drop in the temperature of the part, which transforms at least part of the austenite obtained following heating into martensite whose hardness is particularly high, tempering, which consists of a heat treatment at a temperature typically between 600 and 620 °C which softens the effects of quenching by increasing the ductility and toughness of the part while removing the residual stresses of thermal origin resulting from quenching.Such processes allow the surface of a part to be treated efficiently, however the time required to implement the nitriding step increases considerably when a deeper part of the part is to be treated, reaching several hundred hours depending on the depth of treatment. This makes it incompatible with mass production of parts.

[0006] In addition to the industrial and technical difficulties associated with such treatment times in themselves, maintaining the part at a high temperature for a long time during the nitriding step produces the same effects as a tempering step, i.e. a reduction in the hardness of the part, not only in its surface but also in the core of the part. Thus, the greater the depth that one wishes to treat by nitriding, the longer the nitriding step, so that the reduction in hardness in the core of the part becomes significant if the nitriding temperature is high.

[0007] French patent FR 3 082 529 B1 discloses a nitriding treatment process in which the tempering step carried out between the quenching step and the nitriding step is a low-temperature tempering step, or "stress relief". This step, like the tempering step in conventional nitriding processes, consists of a rise in temperature followed by a slow cooling of the part, however the maximum temperature reached is less than 350°C so that the hardness of the part after stress relief in the process of patent FR 3 082 529 B1 is greater than its hardness after tempering in a conventional treatment process. After the stress relief step of the patent, the part is in a so-called "tempered-stressed" state, as compared to the "tempered-tempered" state of a part following the tempering step in conventional nitriding processes.The nitriding step according to this patent is therefore carried out on a part with higher hardness, and can take place at a high temperature, the drop in hardness which occurs in the core of the part during the nitriding step becoming acceptable with regard to the target hardness which is desired to be achieved.

[0008] For the specific case of transmission parts, this nitriding process on the part in the "stressed hardened" state gives the part a higher surface hardness than that of a part treated using a conventional nitriding process (approximately 500 to 600 HV versus approximately 400 HV respectively), so that it becomes difficult to cut teeth in the part by conventional machining.

[0009] EXPOSED

[0010] An aim of the present disclosure is therefore to enable the manufacture of a toothed part, such as a mechanical transmission part, which has a high surface hardness without compromising the hardness at the core of the part, by means of a nitriding process whose implementation time is limited. To this end, according to a first aspect, a method is proposed for manufacturing a toothed part from a nitriding steel part comprising the following successive steps: a step of heat treatment of the steel part at a first austenitization temperature of the steel and quenching of the part, a step of intermediate tempering of the steel part up to a second temperature below 350°C, a step of cutting teeth in a surface of the part by wire EDM, a step of nitriding the part at a third temperature higher than the second temperature and lower than the first temperature.

[0011] Thus, the proposed process allows the manufacture of a steel toothed part with high hardness while maintaining a process implementation time compatible with the production of parts on an industrial scale.

[0012] According to one implementation, the method comprises, after cutting the teeth, a step of grinding the surface of the part by removing material.

[0013] According to one implementation of the method, the nitriding of the part successively comprises: a low-temperature nitriding step carried out at a temperature greater than or equal to 450°C and less than or equal to 500°C, a high-temperature nitriding step carried out at a temperature greater than or equal to 520°C and less than or equal to 550°C.

[0014] According to one implementation of the method, the low-temperature nitriding comprises maintaining the steel part in an atmosphere containing gaseous ammonia and gaseous hydrogen, successively for a first time range at a nitriding potential KNJ greater than 3, and for a second time range at a nitriding potential Kw 2 greater than a transition nitriding potential at which a formation of y'-phase iron nitride takes place, the nitriding potential being defined by the equation where p(NH s ), is the partial pressure of ammonia in Pascals and p(Hs)i is the partial pressure of hydrogen in Pascals during the i-th time range, and

[0015] I = 1 or 2. According to an implementation of the method, the first time range corresponds substantially to 25% of a duration of the low temperature nitriding, and the second time range corresponds substantially to 75% of a duration of the low temperature nitriding.

[0016] According to one implementation of the method, high temperature nitriding comprises maintaining the steel part in an atmosphere containing ammonia and hydrogen gas, successively for a third time range at a nitriding potential 3and during a fourth time range at a nitriding potential KNJ strictly lower than the nitriding potential KN_3, the nitriding potential being defined by the equation where p(NHs)i is the partial pressure of ammonia in Pascals and p(Hz)i is the partial pressure of hydrogen in Pascals during the i-th time range, and i = 3 or 4.

[0017] According to one implementation of the process, high temperature nitriding is carried out for a duration greater than or equal to 100 hours and less than or equal to 300 hours.

[0018] According to one implementation of the process, the intermediate tempering is carried out at a temperature greater than or equal to 180°C and less than or equal to 300°C.

[0019] Furthermore, a turbomachine is proposed comprising a toothed part obtained by implementing the method defined above, the toothed part being in particular a mechanical transmission part.

[0020] DESCRIPTION OF FIGURES

[0021] Other characteristics, aims and advantages of the invention 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:

[0022] Figure 1 illustrates a method of manufacturing a toothed part according to the present disclosure.

[0023] Throughout the figures, similar elements have identical references.

[0024] DETAILED DESCRIPTION

[0025] A method 1 for manufacturing a toothed part from a nitriding steel part is illustrated in Figure 1. The method is suitable for the treatment of any nitriding steel part, typically a low-alloy steel containing elements allowing the formation of nitrides, such as chromium, molybdenum, vanadium or aluminum. The steel has a carbon content which may preferably be from 0.15 to 0.8% by mass. The method allows in particular the manufacture of toothed parts for the aeronautical industry, such as pinions or teeth, which ensure transmission of mechanical forces or which must more generally withstand the mechanical stresses to which they are subjected, these stresses being concentrated essentially on the surface and near the surface,

[0026] The manufacturing process 1 comprises a first step 2 of heat treatment of the part at an austenitizing temperature of the steel, at which the steel has a fully austenitic microstructure. An austenitizing temperature corresponds to a temperature which allows the transformation of alpha iron into gamma iron. It can be determined for a given steel from the phase transformation diagrams. The heating step 2 can in particular be carried out up to a temperature greater than or equal to 900 °C, for example in the range of temperatures greater than or equal to 920 °C and less than or equal to 940 °C. The duration of the heating is such that it allows the carbides (carbon and alloying elements) to be dissolved and homogenized in the austenitic matrix. It can be for example between 30 minutes and 1 hour 30 minutes.

[0027] Heat treatment step 2, commonly called the austenitization step, is a step well known to those skilled in the art who will be able to determine the operating conditions for a given steel in order to give the steel initial hardness properties.

[0028] Heat treatment step 2 is followed by a quenching step 3, which is a rapid cooling, carried out for example at a sufficiently high cooling rate, given by the TRC diagram of the steel considered, to allow the complete transformation of austenite into martensite, a crystallographic phase with a particularly high hardness.

[0029] Quenching step 3 is a step well known to those skilled in the art, which will therefore not be detailed in the present application.

[0030] The method 1 comprises, subsequent to the quenching step 3, a low-temperature intermediate tempering step 4. This tempering is carried out up to a temperature below 350°C, for example greater than or equal to 150°C and less than or equal to 350°C, preferably greater than or equal to 180°C and less than or equal to 300°C. This low-temperature tempering allows relaxation of the part, i.e. relaxation of part of the residual stresses resulting from the quenching and a slight softening of the material allowing it to be machined without risking the appearance of cracks. The hardness of the core of the part always remains quite high compared to parts having undergone “conventional” tempering at higher temperatures of the order of 570-620°C. Following the intermediate tempering step 4, a tooth cutting step 5 is carried out in the part, so as to obtain a toothed part.This may include a transmission part, such as a gear for a reducer. The cutting of teeth 5 is carried out by wire EDM. EDM processes consist of eroding a conductive part to give it a predetermined shape by applying electrical discharges. For wire EDM, a live metal wire and the workpiece are immersed in an insulating liquid. An electric arc is formed between the workpiece, which forms a first electrode, and the live wire, which forms a second electrode - the two electrodes generally not touching (although infrequent contacts may occur without negative consequences on the result obtained). The wire is typically arranged between two coils, so as to allow the wire to be unwound during the EDM process, so as to change the active part of the wire continuously.The wire is guided so as to erode the part by forming small craters in it, the size of the craters depending on the parameters of the EDM (tension in the wire, width of the wire, etc.). The EDM allows the cutting of teeth 5 in the steel part after the intermediate tempering 4, despite the high hardness of the surface of such a part in the "hardened and relaxed" state which can be up to approximately 600HV, in comparison with the parts in the "hardened and tempered" state of the art, whose surface is less hard.

[0031] The cutting of teeth 5 is followed by a nitriding step 7, 8 during which the steel part is placed in an atmosphere containing a gas with a high nitrogen content, typically ammonia, which releases nitrogen to the surface of the steel during its adsorption (by catalytic cracking with iron oxide). A surface layer, called a combination layer, then forms on the surface of the steel due to the formation of iron nitrides. Then, the nitrogen diffuses into the matrix of the steel and metal nitrides are formed with certain alloying elements of the steel (for example chromium, vanadium or molybdenum) at the grain boundaries, thus forming a so-called "diffusion" layer. This diffusion layer comprises nitrides formed by the nitrogen atoms in contact with the metals contained in the steel, which give it increased hardness compared to the surface of the part before nitriding.The assembly formed by the diffusion layer and the combination layer forms a hardened layer, which increases the surface hardness of the part. The duration of the nitriding step 7, 8 depends on the depth to which the part is to be treated.

[0032] Preceding nitriding 7, 8 with low-temperature intermediate tempering 4 makes it possible to obtain, between these two stages, a part whose hardness is higher than that of a part which would have been treated with high-temperature tempering, in particular with regard to the hardness at the core of the part. Thus, the drop in hardness which inevitably occurs at the core of the part during nitriding, and which is all the more significant when the nitriding stage is carried out at high temperature or for a long time, becomes acceptable: for a part treated by means of low-temperature tempering, the hardness of the core at the start of nitriding is sufficiently high so that the hardness of the core at the end of nitriding remains above a desired threshold.In other words, the low-temperature tempering prior to nitriding compensates for the drop in hardness associated with a high nitriding temperature, this high temperature being advantageous in itself because it accelerates diffusion. It is therefore possible to increase the surface hardness of the part by means of a relatively long nitriding without compromising the hardness in the core of the part, or alternatively, and in accordance with the Hollomon-Jaffe law, to reduce the necessary treatment time while maintaining comparable hardness over the same depth, because the nitriding can then be carried out at a higher temperature.

[0033] According to one embodiment, a grinding step 6 is implemented following the tooth cutting step 5. The grinding step 6 is preferably implemented before the nitriding 7, 8 so as to be carried out when the surface hardness of the part has not yet reached its post-nitriding value, but can possibly be carried out after the nitriding 7, 8. The cutting of teeth 5 by electroerosion can disturb the metallurgical structure on the surface of the part over a few hundred microns, and in particular increase the geometric irregularity of the surface of the part. Such a grinding step 6 therefore makes it possible to obtain a part surface which has dimensions included within predefined tolerances.

[0034] According to one embodiment, the nitriding 7, 8 is carried out in two separate steps. A first low-temperature nitriding step 7 is implemented at a temperature greater than or equal to 450°C and less than or equal to 500°C. The duration of this stage is set according to the depth of the part that one wishes to treat, as well as according to the thickness of the surface that one must remove during the grinding 6. The low-temperature nitriding 7 makes it possible to generate small-sized metal nitrides in the surface of the part so as to increase its hardness, the so-called “white layer” or “combination layer” containing the nitrides and of very low thickness being formed in the extreme surface of the part. The low-temperature nitriding 7 is followed by a high-temperature nitriding step 8, implemented at a temperature greater than or equal to 520°C and less than or equal to 550°C.The duration of this step depends on the nitriding temperature, the depth of the part to be treated, and the desired surface and core hardness of the part, and may in particular be between 100 and 300 hours. The high-temperature nitriding temperature 8 may be chosen so as to be significantly lower than the temperature at which the first austenite grains appear in the part, so as not to compromise the beneficial effect of the increase in surface hardness obtained during quenching 3. Indeed, nitriding carried out in the austenitic range would generate nitrogen ferrite, which is particularly brittle. High-temperature nitriding 8 allows the diffusion of the nitrogen atoms contained in the white layer towards the interior of the part, so as to form a layer called the "diffusion layer", adjacent to the white layer.The division of the nitriding step into two steps of low temperature nitriding 7 and high temperature 8 allows a good compromise between, on the one hand, the nitriding duration, and on the other hand the mechanical performance of the treated part. Indeed, the inventors have found that nitriding carried out entirely at high temperature, for example at 550 °C, negatively affects the hardness at the core of the part, despite the use of a low temperature intermediate tempering 4, while nitriding carried out entirely at low temperature, for example at 450 °C, must be carried out for unacceptable durations for the efficient production of transmission parts.

[0035] As is known, a nitriding step is characterized by a quantity called nitriding potential, defined by the equation: where p(NHs) is the partial pressure of ammonia and p(H?) the partial pressure of hydrogen, both expressed in Pascals. This quantity effectively reflects the nitriding nature of an atmosphere.

[0036] According to one embodiment, the low-temperature nitriding step 7 is itself carried out in two successive sub-steps, both carried out at a temperature in the range of 450 - 500 °C but at different nitriding potentials KN. The first low-temperature nitriding sub-step 7 is carried out at a nitriding potential KNJ greater than 3, while the second low-temperature nitriding sub-step 7, which follows the first sub-step, is carried out at a nitriding potential K N-2greater than a transition nitriding potential, at which the formation of y'-phase iron nitrides begins, i.e. with the chemical formula FesN^ The first low-temperature nitriding sub-step 7, thanks to its high nitriding potential, ensures the formation of a white layer of sufficient thickness to act as a reservoir of nitrogen atoms during the rest of the nitriding. During the second sub-step, the growth of nitrides in the white layer is significantly less significant than during the first sub-step due to the lower nitriding potential KN. This makes it possible to reduce the growth kinetics of the combination layer, or even to consume it in order to avoid a rectification step of this layer that is too large at the end of manufacturing.

[0037] With regard to the duration of the sub-steps, the first low-temperature nitriding sub-step 7 may for example be implemented during the first quarter of the low-temperature nitriding step 7 while the second low-temperature nitriding sub-step 7 may be implemented during the last three-quarters of the low-temperature nitriding step 7.

[0038] For high temperature nitriding 8 carried out in depth, the white layer can reach a few tens of micrometers at the extreme surface, the thickness of the white layer evolving with the depth of nitriding. However, the white layer is very hard, reaching 1000 HV, and very fragile. This makes it unusable when the surface of the part is intended to be mechanically loaded, which is the case for mechanical transmission parts. In the state-of-the-art nitriding processes carried out at high temperature, it is therefore necessary to remove this white layer at the end of the heat treatment of the part by nitriding, which can be complex and expensive depending on the geometry of the affected area.Furthermore, during nitriding, the white layer and the diffusion layer have different expansion coefficients, so that significant heating of the part occurring during high-temperature nitriding 8 can result in crazing of the part. One way to overcome this problem is to vary the nitriding potential KN during the high-temperature nitriding step 8. For example, high-temperature nitriding 8 may comprise a first sub-step, at a nitriding potential KN_3 and a second sub-step at a nitriding potential KN_4 lower than the potential KN_3, so as to maintain a sufficient nitrogen reserve in the combination layer to ensure the diffusion of nitrogen into the diffusion layer and to allow the growth of the diffusion layer while limiting the growth of the combination layer by formation of iron nitride.The first sub-step may last most of the time allocated to the high-temperature nitriding 8, while the second sub-step may be shorter, for example a few hours or a few tens of hours. The first sub-step of high-temperature nitriding 8 allows both a continuation of the adsorption of nitrogen atoms by the surface initiated during the low-temperature nitriding 7, as well as a start of diffusion of the nitrogen atoms from the white layer to the diffusion layer, while the second sub-step of high-temperature nitriding 8, which is carried out at a nitriding potential K. N _4 very low, largely consumes the white layer by diffusion of nitrogen towards the diffusion layer. Thus, the presence of a white layer at the end of high temperature nitriding 8 is avoided, so that a step of removing such a layer is not necessary.

[0039] Decarburization of the surface may take place during high temperature nitriding 8. A second grinding may be carried out to treat the affected areas, so as to prevent a reduction in the hardness and mechanical strength of these areas. According to another aspect, the present disclosure relates to a turbomachine comprising a toothed part, in particular a mechanical transmission part, for example a reducer, the toothed part being obtained by implementing the method for manufacturing a toothed part described above.

Claims

CLAIMS Method (1) for manufacturing a toothed part from a nitriding steel part comprising the following successive steps: a step of heat treatment (2) of the steel part at a first austenitization temperature of the steel and quenching (3) of the part, a step of intermediate tempering (4) of the steel part up to a second temperature lower than 350°C, a step of cutting teeth (5) in a surface of the part by wire electroerosion, a step of nitriding (7, 8) of the part at a third temperature higher than the second temperature and lower than the first temperature.

2. Manufacturing method according to the preceding claim, comprising, after cutting the teeth (5), a step of grinding (6) the surface of the part by removing material.

3. Manufacturing method according to any one of claims 1 and 2, in which the nitriding of the part successively comprises: a low temperature nitriding step (7) carried out at a temperature greater than or equal to 450°C and less than or equal to 500°C, a high temperature nitriding step (8) carried out at a temperature greater than or equal to 520°C and less than or equal to 550°C.

4. Manufacturing method according to the preceding claim, in which the low-temperature nitriding (7) comprises maintaining the steel part in an atmosphere containing gaseous ammonia (NHs) and gaseous hydrogen (H2), successively for a first time range at a nitriding potential KNJ greater than 3, and for a second time range at a nitriding potential Kw_2 greater than a transition nitriding potential at which a formation of iron nitride of phase y' (FeaN^) takes place, the nitriding potential being defined by the equation where p(NHs): is the partial pressure of ammonia in Pascals and p(Hz)i is the partial pressure of hydrogen in Pascals during the i-th time range, and i = 1 or 2.

5. Manufacturing method according to the preceding claim, in which the first time range corresponds substantially to 25% of a duration of the low temperature nitriding (7), and the second time range corresponds substantially to 75% of a duration of the low temperature nitriding (7).

6. Manufacturing method according to any one of claims 3 to 5, in which the high temperature nitriding (8) comprises maintaining the steel part in an atmosphere containing gaseous ammonia (NH3) and hydrogen (H2), successively for a third time range at a nitriding potential KN_3 and for a fourth time range at a nitriding potential KNJ strictly lower than the nitriding potential KN_3, the nitriding potential being defined by the equation where p(NHs)i is the partial pressure of ammonia in Pascals and p(Hz)i is the partial pressure of hydrogen in Pascals during the i-th time range, and i = 3 or 4.

7. Manufacturing method according to any one of claims 3 to 6, in which the high temperature nitriding (8) is carried out for a duration greater than or equal to 100 hours and less than or equal to 300 hours.

8. Manufacturing method according to any one of claims 1 to 7, in which the intermediate tempering (4) is carried out at a temperature greater than or equal to 180°C and less than or equal to 300°C.

9. Turbomachine comprising a toothed part obtained by implementing the method according to any one of claims 1 to 8, the toothed part being in particular a mechanical transmission part.

Images