Heat treatment process for a case-hardened low-carbon steel aircraft part

Abstract

The invention relates to a heat treatment process for a case-hardened low-carbon steel aircraft part that has previously undergone a machining operation on a surface of the part, generating a burnt zone extending from the surface towards the interior of the part, wherein the process comprises: - a heating step of the part to a heating temperature Tc higher than the AC3 transformation point temperature of the part structure TAC3 increased by 50°C and lower than the AC3 transformation point temperature of the part structure TAC3 increased by 100°C so as to form an austenitic structure in the burnt zone, - a quenching step of the part to transform the austenitic structure into a martensitic structure, and - a tempering step of the part. Fig. 2.

Description

Title of the invention: Heat treatment process for a case-hardened low-carbon steel aircraft part. Technical field

[0001] The present invention relates to the field of aircraft turbomachinery, and more particularly to a heat treatment method for a steel part of a turbine. Previous technique

[0002] An aircraft turbomachine, such as a helicopter turboshaft engine or an aircraft turbojet engine, includes transmission components such as gears for driving a rotating rotor. These transmission components are subjected to high wear and friction conditions and must have a surface with sufficient hardness to resist fatigue and wear.

[0003] To this end, the transmission parts are made of low-carbon steel which undergoes a case hardening treatment before quenching. Case hardening is a diffusion process that enriches the surface of a ferrous alloy with carbon by heating it to a high temperature in a carbon-rich atmosphere. Case hardening then generates a carbon gradient from the surface to the core of the part, resulting in a hardness gradient after the part has been hardened by quenching. The case-hardened part then undergoes a tempering treatment before being machined by grinding.

[0004] Grinding the part is a finishing and shaping operation by abrasion that takes place during the final manufacturing phase of the part. Machining consists of repeatedly removing thin layers of material from the surface of the part. Machining improves the surface finish of the part and ensures compliance with the tight dimensional tolerances of parts intended for the manufacture of helicopter engines, for example.

[0005] The removal of material layers is carried out using a grinding wheel, which is a bonded abrasive tool composed of high-hardness grains held together by a binder. By applying pressure to this tool in the direction of the workpiece and moving the grinding wheel relative to the workpiece, the removal of material from the surface of the workpiece results in the production of micro-chips at the tips of the abrasive grains.

[0006] During grinding, the energy required to produce micro-chips is converted into heat. If key machining parameters (lubricant, grinding wheel feed speed, grinding wheel condition, for example) are not properly controlled, this can lead to high temperatures at the grinding wheel / workpiece interface and generate metallurgical transformations at the workpiece surface and towards the core of the workpiece.

[0007] However, the case-hardened structure of the part is sensitive to these local temperature increases in an oxidizing environment. Local overheating (called burn-through) can occur, leading to an alteration of the mechanical and metallurgical properties in the machined area of ​​the part.

[0008] Below the AC3 transformation point of the workpiece structure, and more specifically at a temperature between 200°C and 720°C, an over-tempering burn forms where the workpiece is machined and extends towards the core of the workpiece, resulting in carbide precipitation and thus localized softening. This over-tempering burn forms a burnt zone in which the hardness decreases proportionally with the temperature reached.

[0009] Beyond the AC3 transformation point of the part structure, i.e. at a temperature above approximately 720°C, a re-quench burn is formed where the part is machined, extending towards the core of the part and resulting in a localized increase in hardness in the part.

[0010] Classically, parts with burns on functional areas are subject to machining by fine grinding if the dimensions of the part allow it, that is to say if the dimension of the part remains within the desired tolerances.

[0011] In the event that the depth of the burn proves to be too great and / or the dimensional tolerances do not allow it, the parts are rejected. Description of the invention

[0012] The invention therefore aims to resolve at least in part these drawbacks by proposing a heat treatment process for a case-hardened and machined low-carbon steel aircraft part, making it possible to reduce the rate of rejected parts.

[0013] The invention relates to a heat treatment method for a case-hardened low-carbon steel aircraft part that has previously undergone a machining operation on a surface of the part, generating a burnt zone extending from the surface towards the interior of the part, wherein the method comprises: - a heating step of the part to a heating temperature Te higher than the AC3 transformation point temperature of the part structure increased by 50°C and lower than the AC3 transformation point temperature of the part structure increased by 100°C so as to form an austenitic structure in the burned zone, - a quenching step of the part to transform the austenitic structure into a martensitic structure, and - a revenue stage for the piece.

[0014] The invention thus provides a heat treatment method for a case-hardened low-carbon steel aircraft part, enabling the hardness gradient of a machined burnt area to be raised in order to restore the mechanical properties of the part.

[0015] The heat treatment process makes it possible to avoid rejecting machined and non-conforming parts when a rework operation by fine grinding of the part is not possible if the depth of the burn is too great or if the dimensional tolerances of the part do not allow it.

[0016] The rate of rejected parts after machining is thus reduced, reducing manufacturing costs.

[0017] In some embodiments, the heating temperature Te is between 820°C and 870°C.

[0018] In some embodiments, the heating step of the part includes a first sub-step of raising the temperature to a temperature Te' greater than or equal to a temperature TACi of the transformation point AC1 of the structure of the part for a time t1, a second sub-step of raising the temperature from the temperature TAci to the heating temperature Te for a time t2 and a sub-step of maintaining at the heating temperature Te for a time t3.

[0019] This two-stage temperature increase allows for gradual heating in order to limit deformation of the parts and to homogenize the temperature within the parts in order to homogenize the heating times on all parts.

[0020] In some embodiments, the time t3 is between 30 min and 1h30.

[0021] In certain embodiments, the rate of temperature rise during the the first sub-stage of temperature rise is greater than the rate of temperature rise during the second sub-stage of temperature rise.

[0022] In some embodiments, the quenching step of the part includes a first quenching substep during which the temperature of the part is lowered from the heating temperature Te to the ambient temperature and maintained at the ambient temperature for a time t5, and a second quenching substep during which the temperature of the part is lowered from the ambient temperature to a cryogenic temperature, lower than the ambient temperature.

[0023] At room temperature, the martensitic transformation is incomplete. Cryogenic quenching allows the martensitic transformation of the residual austenite to be completed and the residual austenite content to be minimized as much as possible.

[0024] In some embodiments, the cryogenic temperature is less than -40°C, preferably between -40°C and -100°C, and applied for at least 60 min.

[0025] In some embodiments, the tempering step is carried out at a temperature Tr between 130°C and 250°C.

[0026] The invention also relates to a case-hardened low-carbon steel aircraft part obtained by the heat treatment process as defined above.

[0027] The invention also relates to a method for manufacturing a case-hardened low-carbon steel aircraft part comprising:

[0028] Method for manufacturing a case-hardened low-carbon steel aircraft part comprising: - a machining step on a surface of the workpiece, - a step to inspect the machined part to detect if a burnt area has been generated on the surface of the part, - a step to estimate the feasibility of removing the burned area through a complementary machining step, and - if a burnt area has been detected during the inspection step and if it is estimated that the dimensions of the machined part do not allow for an additional machining operation to remove the burnt area, a step of carrying out a heat treatment process, as defined previously.

[0029] The aforementioned features and advantages, as well as others, will become apparent from the following detailed description and examples of embodiments of the process. This detailed description refers to the accompanying drawings. Brief description of the drawings

[0030] The attached drawings are schematic and are intended primarily to illustrate the principles of the exposition.

[0031] [Fig-1] Fig. 1 represents a graph showing two hardness evolutions in depth function for two case-hardened low-carbon steel parts having respectively a burnt area by over-tempering burn and a burnt area by re-quenching burn, before the application of a heat treatment process according to the invention;

[0032] [Fig.2] Fig.2 represents a thermal cycle of the heat treatment process according to one embodiment of the invention;

[0033] [Fig.3] Fig.3 represents a graph showing the evolution of hardness as a function of depth for a case-hardened low carbon steel part exhibiting an over-tempered burn after the application of the heat treatment process according to the invention. Description of the implementation methods

[0034] To make the explanation more concrete, an example of a heat treatment process is described in detail below, with reference to the accompanying drawings. It should be noted that the invention is not limited to this example.

[0035] The invention relates to a heat treatment process for a transmission component such as a pinion driving a rotor of a turbine in a helicopter turboshaft engine or an aircraft turbojet engine, for example. This component, subjected to temperatures between 100 °C and 160 °C, must retain its mechanical properties and resist fatigue and wear, for example.

[0036] The transmission parts are made of low-carbon steel which is case-hardened before being quenched. Low-carbon steel has a low carbon content of between 0.15% and 0.35% by mass.

[0037] Carburizing is a diffusion process that enriches the surface of a ferrous alloy with carbon by heating it to a high temperature in a carbon-rich atmosphere. Carburizing generates a carbon gradient from the surface to the core of the part, resulting in a hardness gradient after the part has been hardened by quenching. The carburized part then undergoes a tempering treatment before being ground to refine its dimensions.

[0038] Grinding the part is a finishing and shaping operation by abrasion that takes place during the final manufacturing phase of the part. Machining consists of repeatedly removing thin layers of material from the surface of the part. Machining improves the surface finish of the part and ensures compliance with the tight dimensional tolerances of parts intended for the manufacture of helicopter engines, for example.

[0039] The removal of material layers is carried out using a grinding wheel, which is a bonded abrasive tool composed of high-hardness grains held together by a binder. By applying pressure to this tool in the direction of the workpiece and moving the grinding wheel relative to the workpiece, the removal of material from the surface of the workpiece results in the production of micro-chips at the tips of the abrasive grains.

[0040] During grinding, the energy required to produce micro-chips, which is higher than with other cutting processes such as turning or milling, is converted into heat. If key machining parameters (lubricant, grinding wheel feed rate, grinding wheel condition, for example) are not properly controlled, this can lead to high temperatures at the grinding wheel / workpiece interface and generate metallurgical transformations on the workpiece surface that extend into the material.

[0041] However, the cemented structure of the part is sensitive to these local temperature increases in an oxidizing environment. Local overheating (called burn-in) sometimes occurs, leading to an alteration of the mechanical and metallurgical properties of the part.

[0042] Below the AC3 transformation point of the workpiece structure, and more specifically at a temperature between 200°C and 720°C, an overheat burn is formed where the workpiece is machined, and extends towards the core of the This process leads to the precipitation of carbides and therefore localized softening. This over-tempering burn forms a burnt zone in which the hardness drops proportionally with the temperature reached.

[0043] The AC3 transformation point corresponds to a transformation end temperature (when heated) for the steel structure, from ferrite to austenite.

[0044] Beyond the AC3 transformation point of the part structure, i.e. at a temperature above approximately 720°C, a re-quench burn is formed where the part is machined, and extends towards the core of the part, resulting in a localized increase in hardness in the part.

[0045] Figure 1 shows a graph with a first plot of square points 1 representing the evolution of hardness in an area burned by over-tempering of a first part as a function of depth, and a second plot of circular points 2 representing the evolution of hardness in an area burned by re-tempering of a second part, as a function of depth. The measurements are carried out under a load of 500 g (HV0.5).

[0046] The x-axis 3 represents the depth (in mm) and the y-axis 4 represents the hardness (in HV). A first line of dashed lines represents the minimum required surface hardness 5 for the part, which is between 675 HV and 770 HV, and preferably in the order of 675 HV. A second line of dashed lines represents a case hardening depth reference 6 corresponding to a hardness limit of 550 HV.

[0047] The first point plot 1 shows measured hardness values ​​that are all below the minimum required hardness 4 for the part, regardless of the depth. The first surface hardness value at 500 HV is well below the minimum required surface hardness 5 and even below the case hardening depth reference 6. The other hardness values ​​measured at greater depths show that the hardness increases slightly towards the core of the part and then decreases.

[0048] The second plot of points 2 shows high surface hardness values, including a first value of about 800 HV and a second value of about 775 HV, which are above the minimum required surface hardness 5.

[0049] To restore the mechanical properties of the burned area, a heat treatment process is carried out on the steel part in a furnace.

[0050] The heat treatment process comprises: - a heating step of the part to a heating temperature Te higher than a temperature TAC3 of the transformation point AC3 of the part structure increased by 50°C and lower than the temperature Tacs of the transformation point AC3 of the part structure increased by 100°C so as to form an austenitic structure in the burned zone, - a quenching step of the part to transform the austenitic structure into a martensitic structure, and - a revenue stage for the piece.

[0051] The heating step brings the steel into the austenitic range to a temperature between TAC3 + 50°C and TAC3 + 100°C, which varies depending on the carbon content of the steel, since the TAC3 temperature of the AC3 transformation point of the part varies with the carbon content of the steel. This heating step changes the crystallographic structure, transforming it into an austenitic structure.

[0052] In order to determine the temperature TAC3 of the transformation point AC3 of the part, it is necessary to determine the carbon content of the part.

[0053] For example, the temperature TAC3 of the AC3 transformation point of the part can be between 780°C and 860°C.

[0054] Fig. 2 represents an example of a thermal cycle 11 of a heat treatment process according to an embodiment of the invention.

[0055] The x-axis 7 represents time (in hours) and the y-axis 8 represents temperature (in degrees Celsius).

[0056] The heat treatment process includes a heating step of the part comprising a first sub-step of raising the temperature from the ambient temperature Tamb to a temperature Te' greater than or equal to a temperature TACi of the transformation point AC1 of the structure of the part in a time tl which can be between 2h and 2h30, for example.

[0057] The TACi temperature corresponds to the temperature at which austenitic transformation begins. It can be between 720°C and 730°C, for example. The TACi temperature also depends on the carbon content of the part.

[0058] The heating step includes a second sub-step of temperature rise, from the temperature TACi to the heating temperature Te for a time t2, less than tl, which can be between 30 min and 45 min, for example.

[0059] The rate of temperature rise during the first sub-stage of temperature rise is greater than the rate of temperature rise during the second sub-stage of temperature rise.

[0060] The heating temperature Te is between TAC3 + 50°C and TAC3 + 100°C. The heating temperature Te can be between 820°C and 870°C, if the temperature TAC3 is 770°C, for example.

[0061] The heating step also includes a sub-step of maintaining the heating temperature Te for a time t3. The time t3 is between t1 and t2. The time t3 is less than t1+t2. The time t3 can be between 30 min and 1h30. It is approximately 1h, for example.

[0062] In an alternative (not shown), the heating step may only include a single temperature rise step, from the ambient temperature Tamb to the heating temperature Te.

[0063] The heat treatment process also includes a quenching step of the part, carried out after the heating step, to transform the austenitic structure into a martensitic structure.

[0064] The quenching step of the part includes a first quenching substep during which the temperature of the part is rapidly lowered from the heating temperature Te to the ambient temperature Tamb in a time t4 less than t1, t2 and t3, which may be approximately 1 min, for example. The part is held at ambient temperature Tamb for a time t5, less than t3, which may be approximately 30 min, for example.

[0065] The quenching step of the part also includes a second quenching substep during which the temperature of the part is lowered from the ambient temperature Tamb to a cryogenic temperature Tcryo, lower than the ambient temperature, in a time t6 of approximately 10 min, for example. The temperature of the part is maintained at the cryogenic temperature Tcryo for a time t7 of at least 60 min, preferably between 60 min and 1h45, and advantageously of approximately 1h45.

[0066] The cryogenic temperature Tciyo is less than -40°C, preferably between -40°C and -100°C and advantageously about -80°C.

[0067] Cryogenic quenching allows the transformation of austenite into martensite to be continued and completed, and the proportion of residual austenite to be limited.

[0068] The room temperature then rises from the cryogenic temperature Tciyo to the ambient temperature Tamb in a time t8, which may be approximately 10 minutes, for example. The room temperature is maintained at ambient temperature for a time t9, which may be approximately 15 minutes, for example.

[0069] The heat treatment process includes a tempering step of the hardened part at a temperature Tr between 130°C and 250°C, applied after the quenching step. The part temperature rises from ambient temperature to the tempering temperature Tr in a time tl0, which can be approximately 10 minutes, for example. The tempering temperature Tr is maintained for a time tl1 between 1h and 3h, for example.

[0070] A metallographic analysis shows that the burn has disappeared from the burned area after heat treatment.

[0071] Figure 3 represents a graph showing a third plot of points 10 representing the evolution of hardness in the area burned by over-burning of The first part is measured according to its depth and after the application of the heat treatment process described previously. The measurements are taken under a load of 500g (HV0.5).

[0072] The x-axis 3 represents the depth (in mm) and the y-axis 4 represents the hardness (in HV). A first line of dashed lines represents the minimum required surface hardness 5 for the part, which is approximately 675 HV. A second line of dashed lines represents a case hardening depth reference 6 corresponding to a hardness limit of 550 HV.

[0073] The third point plot 10 shows that the surface hardness gradient has increased compared to the hardness gradient shown on the first point plot 1 of [Fig.1] obtained for the first part before heat treatment.

[0074] In particular, the first three surface hardness values ​​measured were above the minimum required surface hardness 5. These values ​​ranged from 675 HV to 700 HV for a depth of less than 0.2 mm. The other hardness values ​​measured beyond 0.2 mm gradually decreased.

[0075] The hardness values ​​of the steel part have been successfully restored. The heat-treated steel part exhibits a hardness gradient identical or close to that of the hardness gradient of the part obtained before machining. The mechanical properties of the part are therefore also restored.

[0076] Optionally, a machining operation can be performed on the part to refine its dimension.

[0077] According to a more comprehensive manufacturing process example, a thermochemical carburizing treatment is performed on a low-carbon steel part and is followed by finishing machining by grinding a surface of the part to obtain a finished part. An inspection step is then performed on the finished part. The inspection can be visual, for example.

[0078] If the part is non-conforming, i.e. if it has a burn mark, an additional machining step, called rework by fine grinding, can be applied to the part to remove the burn mark if this machining does not reduce the dimensions or dimensions of the part below an acceptable tolerance.

[0079] More specifically, the manufacturing process includes a step to estimate the feasibility of removing the burnt area by the additional machining step, in which the depth of the burnt area is determined. An estimation of the dimensions of the part that would be obtained after additional machining is then performed to obtain estimated dimensions. The estimated dimensions are compared to the dimensional tolerances to verify whether they would conform.

[0080] If the estimated dimensions are within tolerances, the additional machining step can be applied.

[0081] If the burn is too deep, the amount of material to be removed to eliminate the burn would be too great, resulting in a reduction of the part's dimensions at the burn site below the tolerances. The heat treatment process according to the invention is then implemented, making it possible to avoid rejecting non-conforming parts with estimated dimensions after machining that are below the tolerances.

[0082] 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.

[0083] It is also evident that all the characteristics described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a process.

Claims

Demands

1. A heat treatment process for a case-hardened low-carbon steel aircraft part that has previously undergone a machining operation on a surface of the part generating a burnt zone extending from the surface towards the interior of the part, the process comprising: - a heating step of the part to a heating temperature Te greater than a temperature TAC3 of the AC3 transformation point of the structure of the part increased by 50°C and less than the temperature TAC3 of the AC3 transformation point of the structure of the part increased by 100°C so as to form an austenitic structure in the burnt zone, - a quenching step of the part to transform the austenitic structure into a martensitic structure, and - a tempering step of the part.

2. Heat treatment process according to claim 1, wherein the heating temperature Te is between 820°C and 870°C.

3. A heat treatment method according to any one of claims 1 or 2, wherein the heating step of the part comprises a first substep of raising the temperature to a temperature Te' greater than or equal to a temperature TACidu transformation point AC1 of the structure of the part for a time t1, a second substep of raising the temperature from the temperature TACi to the heating temperature Te for a time t2 and a substep of holding at the heating temperature Te for a time t3.

4. Heat treatment process according to claim 3, wherein the time t3 is between 30 min and 1h30.

5. A heat treatment process according to any one of claims 3 or 4, wherein the rate of temperature rise during the first sub-stage of temperature rise is greater than the rate of temperature rise during the second sub-stage of temperature rise.

6. A heat treatment process according to any one of claims 1 to 5, wherein the quenching step of the workpiece comprises a first quenching substep during which the temperature of the workpiece is lowered from the heating temperature Te at room temperature and maintained at room temperature for a time t5, and a second quenching sub-step during which the room temperature is lowered from room temperature to a cryogenic temperature, below room temperature.

7. A heat treatment process according to claim 6, wherein the cryogenic temperature is less than -40°C, preferably between -40°C and -100°C, and applied for at least 60 min.

8. Heat treatment process according to any one of claims 1 to 7, wherein the tempering step is carried out at a temperature Tr between 130°C and 250°C.

9. Case-hardened low-carbon steel aircraft part obtained by the heat treatment process according to any one of claims 1 to R

10. d O. A method for manufacturing a case-hardened low-carbon steel aircraft part comprising: - a machining step on a surface of the part, - a step of inspecting the machined part to detect if a burnt area has been generated on the surface of the part, - a step of estimating the feasibility of removing the burnt area by a further machining step, and - if a burnt area has been detected during the inspection step and if it is estimated that the dimensions of the machined part do not allow for a further machining operation to remove the burnt area, a step of carrying out a heat treatment process, as defined according to any one of claims 1 to 8.

Images