Method for nitriding a steel

Abstract

A nitriding method for a secondary hardening steel part, the method including the following steps: obtaining a secondary hardening steel rough-machined part; surface carburizing the rough-machined part to obtain a carburized layer over a first thickness of the rough-machined part; austenitizing the carburized layer to obtain an austenitic steel layer; quenching the austenitic steel layer to obtain a martensitic steel layer; tempering the martensitic steel layer to obtain a tempered layer; grinding the tempered layer to obtain a ground tempered layer having a second thickness; surface cleaning the ground tempered layer to obtain a clean layer; surface decarburizing the clean layer in a nitriding furnace to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness; surface nitriding the decarburized layer in the nitriding furnace to form a surface nitrided part.

Description

TECHNICAL FIELD

[0001] The present disclosure concerns a method for nitriding a steel part, in particular a secondary hardening steel, for example a secondary hardening steel part for gears or bearings.PRIOR ART

[0002] Steels, especially secondary hardening steels, have good mechanical properties at the core. After a surface carburizing treatment, the surface hardness is increased relative to the core hardness.

[0003] The properties of a surface layer of steel can be modified by a nitriding treatment that consists of diffusing nitrogen into the steel part.

[0004] A method of nitriding a steel part is known from FR3032723, comprising a step of decarburizing the surface of the part, followed by a quenching and tempering step of the partially decarburized part on a surface layer and a nitriding step of the carbon-depleted surface layer.

[0005] The heat treatment comprises a quenching step followed by a tempering step. The quenching step usually leads to a relatively large deformation of the part. The part must therefore be ground before the nitriding step, which removes a certain thickness from the carbon-depleted surface layer, thereby reducing the thickness of the nitrided layer.

[0006] Decarburization of the surface of the part is carried out before or at the same time as the austenitizing step. When the decarburizing and austenitizing steps are concurrent, the decarburization depth reached is limited by the time required for the step of austenitizing the part.DISCLOSURE OF THE INVENTION

[0007] The present description aims to remedy these disadvantages at least in part.

[0008] To this end, this disclosure concerns a nitriding method for a secondary hardening steel part, the method comprising the following steps:

[0009] obtaining a secondary hardening steel rough-machined part;

[0010] surface carburizing the rough-machined part to obtain a carburized layer over a first thickness of the rough-machined part;

[0011] austenitizing the carburized layer to obtain an austenitic steel layer;

[0012] quenching the austenitic steel layer to obtain a martensitic steel layer;

[0013] tempering the martensitic steel layer to obtain a tempered layer;

[0014] grinding the tempered layer to obtain a ground tempered layer having a second thickness;

[0015] surface cleaning the ground tempered layer to obtain a clean layer;

[0016] surface decarburizing the clean layer in a nitriding furnace to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness;

[0017] surface nitriding the decarburized layer in the nitriding furnace to form a surface nitrided part on the third thickness.

[0018] “Rough-machined part” means a part whose functional surfaces have been created and takes into account an excess thickness that will be removed during the grinding step.

[0019] Surface carburizing can be carried out under conventional conditions.

[0020] The surface carburizing is carried out by injecting carbon-containing gases into a carburizing furnace. The carbon-containing gases decompose on the surface of the part to provide a flow of carbon.

[0021] The duration of the surface carburizing step depends on the thickness of the desired surface carburizing layer and the carburizing temperature for a given steel. This thickness takes into account the deformations of the part during quenching.

[0022] It is understood that the carburized layer is present over the entire outer surface of the rough-machined part over the first thickness.

[0023] Thus, in a cutting plane, moving from the surface of the part to the center of the part, after the carburized layer, the part comprises a layer in which the carbon content gradually decreases until it returns to the carbon content of the original part, i.e., the superficial carburizing step does not change the composition of the core piece.

[0024] The duration of the austenitizing step must be adapted according to the temperature in order to dissolve the most carbides present on the surface while maintaining a reasonable grain size.

[0025] By way of non-limiting example, the duration of the austenitizing step is generally less than 30 minutes.

[0026] Quenching allows transforming austenite into martensite.

[0027] The tempering step allows the hardening of the surface layer of the martensitic steel part by precipitation of carbides.

[0028] By way of non-limiting example, the tempering step may comprise tempering substeps.

[0029] By way of non-limiting example, the tempering step may comprise between two and four tempering substeps.

[0030] The quenching step causes deformations of the part which are generally greater than the thickness of the desired nitrided layer.

[0031] Also, the grinding step removes the deformations due to quenching and approaches the dimensions of the finished part, that is to say the surface-nitrided part. Without this grinding step prior to surface decarburizing and surface nitriding, the nitrided layer could be completely removed in a grinding step performed after nitriding.

[0032] Grinding can be carried out under conventional conditions.

[0033] As a non-limiting example, grinding can be carried out with a grinding wheel whose parameters are adapted to the material and lubrication.

[0034] Since the second thickness is obtained after grinding the secondary hardening steel part, it is understood that the second thickness is less than or equal to the first thickness. The second thickness may not be constant. It may especially depend on the geometry of the part and on the deformations of the part following the quenching step.

[0035] By way of non-limiting example, the second thickness may be comprised between 0.5 mm and 3 mm, for example between 0.7 mm and 2 mm.

[0036] The surface cleaning step removes residues that can prevent the adsorption of the decarburizing and nitriding gas on the part. The residues can be grease residues, dust residues, oxides, etc.

[0037] The surface cleaning step can be carried out by mechanical and / or chemical methods.

[0038] The surface decarburizing and surface nitriding steps are carried out in the same furnace. There is therefore no specific furnace for decarburization. It is also not necessary to lower the temperature and transport the parts from a decarburizing furnace to the nitriding furnace.

[0039] By way of non-limiting example, the surface decarburizing step is carried out at a temperature close to the nitriding temperature or equal to the nitriding temperature. “Close” is understood to mean a temperature difference of less than or equal to 40° C., preferably less than or equal to 20° C.

[0040] In addition, since the grinding takes place before the surface decarburization, the thickness of the decarburized layer is not changed by the grinding step.

[0041] Surface decarburization on a third thickness strictly smaller than the second thickness avoids the formation of intergranular precipitates during the surface nitriding step.

[0042] Since the third thickness is strictly less than the second thickness, there remains a layer with a carbon content higher than the carbon content of the decarburized layer and higher than the carbon content of the core part. In a cutting plane of the part, starting from the outer surface toward the center of the part, the part comprises the decarburized layer on the third thickness, the tempered layer ground to a thickness equal to the difference between the second thickness and the third thickness, a layer in which the carbon content gradually decreases until it returns to the carbon content of the original part. The composition of the part no longer varies up to the center of the part.

[0043] By way of non-limiting example, the surface decarburizing step can be carried out at plateau temperatures comprised between 450° C. and 550° C., for a period that depends on the third thickness that it is desired to obtain for the nitrided layer of the nitrided part on the surface.

[0044] By way of non-limiting example, the carbon content of the decarburized layer is less than or equal to 0.75% by mass.

[0045] Surface nitriding can be carried out under conventional conditions.

[0046] By way of non-limiting example, the third thickness may be comprised between 0.05 mm and 0.5 mm, for example between 0.2 mm and 0.4 mm.

[0047] In some embodiments, the secondary hardening steel may comprise, in percent by mass: 0.1 to 0.4% carbon, 0 to 6.0% chromium, 0 to 6.0% molybdenum, 0 to 3.0% vanadium, 0 to 20% cobalt, 0 to 10% nickel and 0 to 3.0% tungsten.

[0048] By way of non-limiting example, the secondary hardening steel may be a steel comprising, in percent by mass: 0.11 to 0.15% carbon, 0.10 to 0.25% silicon, 0.15 to 0.35% manganese, 4.00 to 4.25% chromium, 3.20 to 3.60% nickel, 4.00 to 4.50% molybdenum, 1.13 to 1.33% vanadium, a maximum of 0.015% phosphorus, a maximum of 0.010% sulfur, a maximum of 0.10% copper, a maximum of 0.25% cobalt, a maximum of 0.15% tungsten, the balance being composed of iron and unavoidable impurities, for example the secondary hardening steel commonly known as M50NIL (AMS 6278).

[0049] In some embodiments, surface decarburization may be performed with a mixture of hydrogen gas and water vapor.

[0050] Hydrogen gas and water vapor form an atmosphere allowing the clean part to be decarburized on the surface to a thickness varying according to the decarburization time. The water vapor content can be monitored, for example, by measuring the dew point, which is a function of the pressure and humidity.

[0051] In addition, the presence of gaseous hydrogen in the mixture limits or even prevents the formation of oxides on the surface of the part, which oxides must be eliminated.

[0052] In some embodiments, the mixture may further comprise a carbon-containing gas.

[0053] The addition of a carbon-containing gas to the mixture makes it possible, by means of the equilibrium of the carbon partial pressure of the gaseous atmosphere and the carbon concentration of the surface layer, to control the carbon content in the decarburized layer of the surface decarburized part.

[0054] Depending on the nitriding process of the surface nitriding step, the carbon-containing gas can be carbon dioxide (CO2) when the surface nitriding is a gaseous nitriding or methane (CH4) when the surface nitriding is an ionic nitriding.

[0055] Thus, it is not necessary to modify the nitriding furnace to supply it with an additional gas.

[0056] In some embodiments, quenching may be followed by a cryogenic treatment.

[0057] The cryogenic treatment makes it possible to lower the temperature in order to transform the most austenite into martensite.

[0058] By way of non-limiting example, the cryogenic treatment is typically carried out between −70° C. and −100° C.

[0059] Typically, this step can be carried out within two hours following quenching.

[0060] In some embodiments, the cryogenic treatment may be performed for a cryogenization time greater than or equal to 1 hour and less than or equal to 10 hours.

[0061] In some embodiments, the carburized part may comprise a carbon content of greater than or equal to 0.8% by mass on the surface.

[0062] In some embodiments, austenitization may be performed under vacuum.

[0063] Vacuum austenitization reduces or even prevents oxide formation and / or decarburization of the carburized layer and the austenitic steel layer obtained after austenitizing the carburized layer.

[0064] In some embodiments, the surface nitriding may be gas nitriding or ion nitriding.

[0065] In some embodiments, the surface carburizing may be gas carburizing or low pressure carburizing.

[0066] By way of non-limiting example, the surface carburizing can be carried out by low-pressure carburizing and the carbon-containing gases can be acetylene or propane.

[0067] By way of non-limiting example, the surface carburizing can be carried out by gas carburizing and the carbon-containing gases can be a mixture of dinitrogen and methanol.BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given by way of non-limiting examples, with reference to the attached figures.

[0069] FIG. 1 is a schematic longitudinal sectional view of a turbomachine.

[0070] FIG. 2 is a flowchart showing the steps of a nitriding method for a secondary hardening steel part.

[0071] In all the figures, the elements in common are identified by identical numerical references.DETAILED DESCRIPTION

[0072] FIG. 1 shows, in section along a vertical plane passing through its main axis A, a turbofan engine 10. The turbofan engine 10 comprises, from upstream to downstream according to the flow of air, a fan 12, a low pressure compressor 14, a high pressure compressor 16, a combustion chamber 18, a high pressure turbine 20, and a low pressure turbine 22.

[0073] The high pressure turbine 20 comprises a plurality of blades 20A rotating with the rotor and rectifiers 20B mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 disposed opposite the blades 20A of the turbine 20.

[0074] Likewise, the low pressure turbine 22 comprises a plurality of blades rotating with the rotor and rectifiers mounted on the stator.

[0075] In the following, the elements common to the various embodiments are identified by the same numerical references.

[0076] To allow rotation around the main axis A of the blades, the turbojet engine comprises gears and bearings. The parts of these gears and of these bearings are subjected to high mechanical forces which can cause high wear. Gear and bearing parts are usually surface treated to increase their hardness and mechanical strength.

[0077] By way of non-limiting example, the secondary hardening steel part may be a gear or bearing part.

[0078] FIG. 2 is a flowchart showing the steps of a nitriding method 100 for a secondary hardening steel part.

[0079] The nitriding method 100 comprises a first step 102 of obtaining a secondary hardening steel rough-machined part.

[0080] The rough-machined part then undergoes a surface carburizing step 104 to obtain a carburized layer over a first thickness of the rough-machined part.

[0081] The carburized layer then undergoes an austenitizing step 106 to obtain an austenitic steel layer.

[0082] The austenitic steel layer then undergoes a quenching step 108 to obtain a martensitic steel layer.

[0083] The martensitic steel layer then undergoes a tempering step 110 to obtain a tempered layer on the first thickness.

[0084] The tempering step 110 may comprise several tempering substeps, for example three tempering substeps.

[0085] The tempered layer then undergoes a grinding step 112 to obtain a ground tempered layer having a second thickness.

[0086] The ground tempered layer then undergoes a surface cleaning step 114 to obtain a clean layer on the second thickness.

[0087] The surface cleaning step 114 removes residues that may prevent the adsorption of decarburization and nitriding gas on the part. The residues can be grease residues, dust residues, oxides, etc.

[0088] The part is then placed in a nitriding furnace.

[0089] In the nitriding furnace, the clean layer then undergoes a surface decarburizing step 116 to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness.

[0090] In the nitriding furnace, the decarburized layer then undergoes a surface nitriding step 118 to form a surface nitrided part on the third thickness.

[0091] By way of non-limiting example, the secondary hardening steel part may be an M50NIL steel part.

[0092] For M50NIL steel, the surface carburizing step 104 may be carried out at plateau temperatures comprised between 900° C. and 1000° C., for example at 950° C. for 10 hours.

[0093] The austenitizing step 106 may be carried out at plateau temperatures comprised between 1050° C. and 1100° C. for example for 30 minutes.

[0094] The quenching step 108 may be followed by a cryogenic treatment 120 at temperatures comprised between −70° C. and −100° C. for 1 to 10 hours.

[0095] The tempering step 110 may comprise several tempering substeps, for example three tempering substeps. The tempering substeps may, for example, be carried out at plateau temperatures comprised between 500° C. and 550° C., for 1 to 4 hours each.

[0096] The substeps may be carried out at the same or different plateau temperatures and for equal or unequal durations.

[0097] The grinding step 112 may remove material to a thickness comprised between 0.1 and 0.5 mm.

[0098] The surface decarburizing step 116 may be carried out at plateau temperatures comprised between 450° C. and 550° C., for a duration that depends on the third thickness that it is desired to obtain for the nitrided layer of the nitrided part on the surface.

[0099] The surface nitriding step 118 may be carried out at plateau temperatures comprised between 450° C. and 550° C., for a period that depends on the third thickness.

[0100] Although the present disclosure has been described with reference to a specific embodiment, it is obvious that various modifications and changes can be made to these examples without exceeding the general scope of the invention as defined by the claims. In addition, individual characteristics of the various embodiments mentioned may be combined into additional embodiments. Therefore, the description and drawings must be considered in an illustrative rather than restrictive sense.

[0101] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes can be made to these examples without exceeding the general scope of the invention as defined by the claims. In particular, individual characteristics of the various embodiments illustrated or mentioned can be combined into additional embodiments. Therefore, the description and drawings must be considered in an illustrative rather than restrictive sense.

[0102] It is also obvious that all the characteristics described with reference to a method can be transposed, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device can be transposed, alone or in combination, to a method.

Examples

Embodiment Construction

[0072]FIG. 1 shows, in section along a vertical plane passing through its main axis A, a turbofan engine 10. The turbofan engine 10 comprises, from upstream to downstream according to the flow of air, a fan 12, a low pressure compressor 14, a high pressure compressor 16, a combustion chamber 18, a high pressure turbine 20, and a low pressure turbine 22.

[0073]The high pressure turbine 20 comprises a plurality of blades 20A rotating with the rotor and rectifiers 20B mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 disposed opposite the blades 20A of the turbine 20.

[0074]Likewise, the low pressure turbine 22 comprises a plurality of blades rotating with the rotor and rectifiers mounted on the stator.

[0075]In the following, the elements common to the various embodiments are identified by the same numerical references.

[0076]To allow rotation around the main axis A of the blades, the turbojet engine comprises gears and bearings. The parts of the...

TECHNICAL FIELD

[0001] The present disclosure concerns a method for nitriding a steel part, in particular a secondary hardening steel, for example a secondary hardening steel part for gears or bearings.PRIOR ART

[0002] Steels, especially secondary hardening steels, have good mechanical properties at the core. After a surface carburizing treatment, the surface hardness is increased relative to the core hardness.

[0003] The properties of a surface layer of steel can be modified by a nitriding treatment that consists of diffusing nitrogen into the steel part.

[0004] A method of nitriding a steel part is known from FR3032723, comprising a step of decarburizing the surface of the part, followed by a quenching and tempering step of the partially decarburized part on a surface layer and a nitriding step of the carbon-depleted surface layer.

[0005] The heat treatment comprises a quenching step followed by a tempering step. The quenching step usually leads to a relatively large deformation of the part. The part must therefore be ground before the nitriding step, which removes a certain thickness from the carbon-depleted surface layer, thereby reducing the thickness of the nitrided layer.

[0006] Decarburization of the surface of the part is carried out before or at the same time as the austenitizing step. When the decarburizing and austenitizing steps are concurrent, the decarburization depth reached is limited by the time required for the step of austenitizing the part.DISCLOSURE OF THE INVENTION

[0007] The present description aims to remedy these disadvantages at least in part.

[0008] To this end, this disclosure concerns a nitriding method for a secondary hardening steel part, the method comprising the following steps:

[0009] obtaining a secondary hardening steel rough-machined part;

[0010] surface carburizing the rough-machined part to obtain a carburized layer over a first thickness of the rough-machined part;

[0011] austenitizing the carburized layer to obtain an austenitic steel layer;

[0012] quenching the austenitic steel layer to obtain a martensitic steel layer;

[0013] tempering the martensitic steel layer to obtain a tempered layer;

[0014] grinding the tempered layer to obtain a ground tempered layer having a second thickness;

[0015] surface cleaning the ground tempered layer to obtain a clean layer;

[0016] surface decarburizing the clean layer in a nitriding furnace to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness;

[0017] surface nitriding the decarburized layer in the nitriding furnace to form a surface nitrided part on the third thickness.

[0018] “Rough-machined part” means a part whose functional surfaces have been created and takes into account an excess thickness that will be removed during the grinding step.

[0019] Surface carburizing can be carried out under conventional conditions.

[0020] The surface carburizing is carried out by injecting carbon-containing gases into a carburizing furnace. The carbon-containing gases decompose on the surface of the part to provide a flow of carbon.

[0021] The duration of the surface carburizing step depends on the thickness of the desired surface carburizing layer and the carburizing temperature for a given steel. This thickness takes into account the deformations of the part during quenching.

[0022] It is understood that the carburized layer is present over the entire outer surface of the rough-machined part over the first thickness.

[0023] Thus, in a cutting plane, moving from the surface of the part to the center of the part, after the carburized layer, the part comprises a layer in which the carbon content gradually decreases until it returns to the carbon content of the original part, i.e., the superficial carburizing step does not change the composition of the core piece.

[0024] The duration of the austenitizing step must be adapted according to the temperature in order to dissolve the most carbides present on the surface while maintaining a reasonable grain size.

[0025] By way of non-limiting example, the duration of the austenitizing step is generally less than 30 minutes.

[0026] Quenching allows transforming austenite into martensite.

[0027] The tempering step allows the hardening of the surface layer of the martensitic steel part by precipitation of carbides.

[0028] By way of non-limiting example, the tempering step may comprise tempering substeps.

[0029] By way of non-limiting example, the tempering step may comprise between two and four tempering substeps.

[0030] The quenching step causes deformations of the part which are generally greater than the thickness of the desired nitrided layer.

[0031] Also, the grinding step removes the deformations due to quenching and approaches the dimensions of the finished part, that is to say the surface-nitrided part. Without this grinding step prior to surface decarburizing and surface nitriding, the nitrided layer could be completely removed in a grinding step performed after nitriding.

[0032] Grinding can be carried out under conventional conditions.

[0033] As a non-limiting example, grinding can be carried out with a grinding wheel whose parameters are adapted to the material and lubrication.

[0034] Since the second thickness is obtained after grinding the secondary hardening steel part, it is understood that the second thickness is less than or equal to the first thickness. The second thickness may not be constant. It may especially depend on the geometry of the part and on the deformations of the part following the quenching step.

[0035] By way of non-limiting example, the second thickness may be comprised between 0.5 mm and 3 mm, for example between 0.7 mm and 2 mm.

[0036] The surface cleaning step removes residues that can prevent the adsorption of the decarburizing and nitriding gas on the part. The residues can be grease residues, dust residues, oxides, etc.

[0037] The surface cleaning step can be carried out by mechanical and / or chemical methods.

[0038] The surface decarburizing and surface nitriding steps are carried out in the same furnace. There is therefore no specific furnace for decarburization. It is also not necessary to lower the temperature and transport the parts from a decarburizing furnace to the nitriding furnace.

[0039] By way of non-limiting example, the surface decarburizing step is carried out at a temperature close to the nitriding temperature or equal to the nitriding temperature. “Close” is understood to mean a temperature difference of less than or equal to 40° C., preferably less than or equal to 20° C.

[0040] In addition, since the grinding takes place before the surface decarburization, the thickness of the decarburized layer is not changed by the grinding step.

[0041] Surface decarburization on a third thickness strictly smaller than the second thickness avoids the formation of intergranular precipitates during the surface nitriding step.

[0042] Since the third thickness is strictly less than the second thickness, there remains a layer with a carbon content higher than the carbon content of the decarburized layer and higher than the carbon content of the core part. In a cutting plane of the part, starting from the outer surface toward the center of the part, the part comprises the decarburized layer on the third thickness, the tempered layer ground to a thickness equal to the difference between the second thickness and the third thickness, a layer in which the carbon content gradually decreases until it returns to the carbon content of the original part. The composition of the part no longer varies up to the center of the part.

[0043] By way of non-limiting example, the surface decarburizing step can be carried out at plateau temperatures comprised between 450° C. and 550° C., for a period that depends on the third thickness that it is desired to obtain for the nitrided layer of the nitrided part on the surface.

[0044] By way of non-limiting example, the carbon content of the decarburized layer is less than or equal to 0.75% by mass.

[0045] Surface nitriding can be carried out under conventional conditions.

[0046] By way of non-limiting example, the third thickness may be comprised between 0.05 mm and 0.5 mm, for example between 0.2 mm and 0.4 mm.

[0047] In some embodiments, the secondary hardening steel may comprise, in percent by mass: 0.1 to 0.4% carbon, 0 to 6.0% chromium, 0 to 6.0% molybdenum, 0 to 3.0% vanadium, 0 to 20% cobalt, 0 to 10% nickel and 0 to 3.0% tungsten.

[0048] By way of non-limiting example, the secondary hardening steel may be a steel comprising, in percent by mass: 0.11 to 0.15% carbon, 0.10 to 0.25% silicon, 0.15 to 0.35% manganese, 4.00 to 4.25% chromium, 3.20 to 3.60% nickel, 4.00 to 4.50% molybdenum, 1.13 to 1.33% vanadium, a maximum of 0.015% phosphorus, a maximum of 0.010% sulfur, a maximum of 0.10% copper, a maximum of 0.25% cobalt, a maximum of 0.15% tungsten, the balance being composed of iron and unavoidable impurities, for example the secondary hardening steel commonly known as M50NIL (AMS 6278).

[0049] In some embodiments, surface decarburization may be performed with a mixture of hydrogen gas and water vapor.

[0050] Hydrogen gas and water vapor form an atmosphere allowing the clean part to be decarburized on the surface to a thickness varying according to the decarburization time. The water vapor content can be monitored, for example, by measuring the dew point, which is a function of the pressure and humidity.

[0051] In addition, the presence of gaseous hydrogen in the mixture limits or even prevents the formation of oxides on the surface of the part, which oxides must be eliminated.

[0052] In some embodiments, the mixture may further comprise a carbon-containing gas.

[0053] The addition of a carbon-containing gas to the mixture makes it possible, by means of the equilibrium of the carbon partial pressure of the gaseous atmosphere and the carbon concentration of the surface layer, to control the carbon content in the decarburized layer of the surface decarburized part.

[0054] Depending on the nitriding process of the surface nitriding step, the carbon-containing gas can be carbon dioxide (CO2) when the surface nitriding is a gaseous nitriding or methane (CH4) when the surface nitriding is an ionic nitriding.

[0055] Thus, it is not necessary to modify the nitriding furnace to supply it with an additional gas.

[0056] In some embodiments, quenching may be followed by a cryogenic treatment.

[0057] The cryogenic treatment makes it possible to lower the temperature in order to transform the most austenite into martensite.

[0058] By way of non-limiting example, the cryogenic treatment is typically carried out between −70° C. and −100° C.

[0059] Typically, this step can be carried out within two hours following quenching.

[0060] In some embodiments, the cryogenic treatment may be performed for a cryogenization time greater than or equal to 1 hour and less than or equal to 10 hours.

[0061] In some embodiments, the carburized part may comprise a carbon content of greater than or equal to 0.8% by mass on the surface.

[0062] In some embodiments, austenitization may be performed under vacuum.

[0063] Vacuum austenitization reduces or even prevents oxide formation and / or decarburization of the carburized layer and the austenitic steel layer obtained after austenitizing the carburized layer.

[0064] In some embodiments, the surface nitriding may be gas nitriding or ion nitriding.

[0065] In some embodiments, the surface carburizing may be gas carburizing or low pressure carburizing.

[0066] By way of non-limiting example, the surface carburizing can be carried out by low-pressure carburizing and the carbon-containing gases can be acetylene or propane.

[0067] By way of non-limiting example, the surface carburizing can be carried out by gas carburizing and the carbon-containing gases can be a mixture of dinitrogen and methanol.BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given by way of non-limiting examples, with reference to the attached figures.

[0069] FIG. 1 is a schematic longitudinal sectional view of a turbomachine.

[0070] FIG. 2 is a flowchart showing the steps of a nitriding method for a secondary hardening steel part.

[0071] In all the figures, the elements in common are identified by identical numerical references.DETAILED DESCRIPTION

[0072] FIG. 1 shows, in section along a vertical plane passing through its main axis A, a turbofan engine 10. The turbofan engine 10 comprises, from upstream to downstream according to the flow of air, a fan 12, a low pressure compressor 14, a high pressure compressor 16, a combustion chamber 18, a high pressure turbine 20, and a low pressure turbine 22.

[0073] The high pressure turbine 20 comprises a plurality of blades 20A rotating with the rotor and rectifiers 20B mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 disposed opposite the blades 20A of the turbine 20.

[0074] Likewise, the low pressure turbine 22 comprises a plurality of blades rotating with the rotor and rectifiers mounted on the stator.

[0075] In the following, the elements common to the various embodiments are identified by the same numerical references.

[0076] To allow rotation around the main axis A of the blades, the turbojet engine comprises gears and bearings. The parts of these gears and of these bearings are subjected to high mechanical forces which can cause high wear. Gear and bearing parts are usually surface treated to increase their hardness and mechanical strength.

[0077] By way of non-limiting example, the secondary hardening steel part may be a gear or bearing part.

[0078] FIG. 2 is a flowchart showing the steps of a nitriding method 100 for a secondary hardening steel part.

[0079] The nitriding method 100 comprises a first step 102 of obtaining a secondary hardening steel rough-machined part.

[0080] The rough-machined part then undergoes a surface carburizing step 104 to obtain a carburized layer over a first thickness of the rough-machined part.

[0081] The carburized layer then undergoes an austenitizing step 106 to obtain an austenitic steel layer.

[0082] The austenitic steel layer then undergoes a quenching step 108 to obtain a martensitic steel layer.

[0083] The martensitic steel layer then undergoes a tempering step 110 to obtain a tempered layer on the first thickness.

[0084] The tempering step 110 may comprise several tempering substeps, for example three tempering substeps.

[0085] The tempered layer then undergoes a grinding step 112 to obtain a ground tempered layer having a second thickness.

[0086] The ground tempered layer then undergoes a surface cleaning step 114 to obtain a clean layer on the second thickness.

[0087] The surface cleaning step 114 removes residues that may prevent the adsorption of decarburization and nitriding gas on the part. The residues can be grease residues, dust residues, oxides, etc.

[0088] The part is then placed in a nitriding furnace.

[0089] In the nitriding furnace, the clean layer then undergoes a surface decarburizing step 116 to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness.

[0090] In the nitriding furnace, the decarburized layer then undergoes a surface nitriding step 118 to form a surface nitrided part on the third thickness.

[0091] By way of non-limiting example, the secondary hardening steel part may be an M50NIL steel part.

[0092] For M50NIL steel, the surface carburizing step 104 may be carried out at plateau temperatures comprised between 900° C. and 1000° C., for example at 950° C. for 10 hours.

[0093] The austenitizing step 106 may be carried out at plateau temperatures comprised between 1050° C. and 1100° C. for example for 30 minutes.

[0094] The quenching step 108 may be followed by a cryogenic treatment 120 at temperatures comprised between −70° C. and −100° C. for 1 to 10 hours.

[0095] The tempering step 110 may comprise several tempering substeps, for example three tempering substeps. The tempering substeps may, for example, be carried out at plateau temperatures comprised between 500° C. and 550° C., for 1 to 4 hours each.

[0096] The substeps may be carried out at the same or different plateau temperatures and for equal or unequal durations.

[0097] The grinding step 112 may remove material to a thickness comprised between 0.1 and 0.5 mm.

[0098] The surface decarburizing step 116 may be carried out at plateau temperatures comprised between 450° C. and 550° C., for a duration that depends on the third thickness that it is desired to obtain for the nitrided layer of the nitrided part on the surface.

[0099] The surface nitriding step 118 may be carried out at plateau temperatures comprised between 450° C. and 550° C., for a period that depends on the third thickness.

[0100] Although the present disclosure has been described with reference to a specific embodiment, it is obvious that various modifications and changes can be made to these examples without exceeding the general scope of the invention as defined by the claims. In addition, individual characteristics of the various embodiments mentioned may be combined into additional embodiments. Therefore, the description and drawings must be considered in an illustrative rather than restrictive sense.

[0101] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes can be made to these examples without exceeding the general scope of the invention as defined by the claims. In particular, individual characteristics of the various embodiments illustrated or mentioned can be combined into additional embodiments. Therefore, the description and drawings must be considered in an illustrative rather than restrictive sense.

[0102] It is also obvious that all the characteristics described with reference to a method can be transposed, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device can be transposed, alone or in combination, to a method.

Examples

Embodiment Construction

[0072]FIG. 1 shows, in section along a vertical plane passing through its main axis A, a turbofan engine 10. The turbofan engine 10 comprises, from upstream to downstream according to the flow of air, a fan 12, a low pressure compressor 14, a high pressure compressor 16, a combustion chamber 18, a high pressure turbine 20, and a low pressure turbine 22.

[0073]The high pressure turbine 20 comprises a plurality of blades 20A rotating with the rotor and rectifiers 20B mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 disposed opposite the blades 20A of the turbine 20.

[0074]Likewise, the low pressure turbine 22 comprises a plurality of blades rotating with the rotor and rectifiers mounted on the stator.

[0075]In the following, the elements common to the various embodiments are identified by the same numerical references.

[0076]To allow rotation around the main axis A of the blades, the turbojet engine comprises gears and bearings. The parts of the...

Claims

1. A nitriding method for a secondary hardening steel part, the method comprising the following steps:obtaining a secondary hardening steel rough-machined part;surface carburizing the rough-machined part to obtain a carburized layer over a first thickness of the rough-machined part;austenitizing the carburized layer to obtain an austenitic steel layer;quenching the austenitic steel layer to obtain a martensitic steel layer;tempering the martensitic steel layer to obtain a tempered layer;grinding the tempered layer to obtain a ground tempered layer having a second thickness;surface cleaning the ground tempered layer to obtain a clean layer;surface decarburizing the clean layer in a nitriding furnace to obtain a decarburized layer on a third thickness, the third thickness being strictly less than the second thickness;surface nitriding the decarburized layer in the nitriding furnace to form a surface nitrided part on the third thickness.

2. The nitriding method according to claim 1, wherein the secondary hardening steel comprises, in percent by mass: 0.1 to 0.4% carbon, 0 to 6.0% chromium, 0 to 6.0% molybdenum, 0 to 3.0% vanadium, 0 to 20% cobalt, 0 to 10% nickel and 0 to 3.0% tungsten.

3. The nitriding method according to claim 1, wherein the surface decarburizing is carried out with a mixture of gaseous hydrogen and water vapor.

4. The nitriding method according to claim 3, wherein the mixture further comprises a carbon-containing gas.

5. The nitriding method according to claim 1, wherein the quenching is followed by a cryogenic treatment.

6. The nitriding method according to claim 5, wherein the cryogenic treatment is performed for a cryogenization time greater than or equal to 1 hour and less than or equal to 10 hours.

7. The nitriding method according to claim 1, wherein the carburized part comprises a carbon content of greater than or equal to 0.8% by mass on the surface.

8. The nitriding method according to claim 1, wherein the austenitizing is carried out under vacuum.

9. The nitriding method according to claim 1, wherein the surface nitriding is gas nitriding or ion nitriding.

10. The nitriding method according to claim 1, wherein the surface carburizing is gas carburizing or low pressure carburizing.

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