PROCESS FOR NITRIDING STEEL
The method for nitriding secondary hardening steel parts through controlled surface cementation, austenitization, quenching, and decarburization in a single furnace addresses deformation and depth limitations, maintaining the nitrided layer's integrity and hardness.
Patent Information
- Application Number
- FR2023007949
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-07-24
- Publication Date
- 2025-07-25
- Estimated Expiration
- 2043-07-24
AI Technical Summary
Existing methods for nitriding secondary hardening steel parts result in significant deformation during quenching, necessitating grinding that reduces the thickness of the nitridable layer, and the decarburization process is limited by austenitization time, restricting the depth of the nitrided layer.
A method involving surface cementation, austenitization, quenching, tempering, rectification, cleaning, surface decarburization, and nitriding in a single furnace, with controlled thickness adjustments to maintain the nitrided layer integrity and avoid intergranular precipitates.
The method effectively maintains the nitrided layer thickness and hardness while minimizing deformation and intergranular precipitates, ensuring consistent mechanical properties without additional grinding or furnace changes.
Smart Images
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Abstract
Description
Title of the invention: METHOD FOR NITRIDING STEEL Technical field
[0001] The present disclosure relates to a process for nitriding a steel part, in particular a secondary hardening steel, for example a secondary hardening steel part for gears or for 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 compared to the core hardness.
[0003] The properties of a surface layer of steel can be modified by a nitriding treatment which consists of diffusing nitrogen into the steel part.
[0004] FR3032723 discloses a method for nitriding a steel part comprising a step of decarburizing the surface of the part, followed by a step of quenching and tempering the partially decarburized part on a surface layer and a step of nitriding the carbon-depleted surface layer.
[0005] The heat treatment comprises a quenching step followed by a tempering step. The quenching step generally generates a relatively significant deformation of the part. The part must therefore be ground before the nitriding step, which has the consequence of removing a certain thickness of the carbon-depleted surface layer, consequently reducing the thickness of the layer that can be nitrided.
[0006] The decarburization of the surface of the part is carried out before or concomitantly with the austenitization step. When the decarburization and austenitization steps are concomitant, the decarburization depth achieved is limited by the time required for the austenitization step of the part. Statement of the invention
[0007] The present disclosure aims to remedy at least in part these drawbacks.
[0008] For this purpose, the present disclosure relates to a method of nitriding a secondary hardening steel part, the method comprising the following steps: - obtaining a rough part in secondary hardening steel; - surface cementation of the rough part to obtain a cemented layer on a first thickness of the rough part; - austenitization of the cemented layer to obtain a layer of austenitic steel; - quenching of the austenitic steel layer to obtain a martensitic steel layer; - tempering of the martensitic steel layer to obtain a tempered layer; - rectification of the tempered layer to obtain a rectified tempered layer having a second thickness; - surface cleaning of the rectified tempered layer to obtain a clean layer; - surface decarburization of the clean layer in a nitriding furnace to obtain a decarburized layer over a third thickness, the third thickness being strictly less than the second thickness; - surface nitriding of the decarburized layer in the nitriding furnace to form a surface nitrided part on the third thickness.
[0009] The term “rough part” means a part whose functional surfaces have been created and taking into account an excess thickness which will be removed during the grinding step.
[0010] Surface cementation can be carried out under conventional conditions.
[0011] Surface carburizing is carried out by injecting carbonaceous gases into a carburizing furnace. The carbonaceous gases decompose on the surface of the part to provide a carbon flux.
[0012] The duration of the surface carburizing step depends on the thickness of the desired surface carburizing layer as well as the carburizing temperature for a given steel. This thickness takes into account the deformations of the part during quenching.
[0013] It is understood that the cemented layer is present over the entire exterior surface of the rough part over the first thickness.
[0014] Thus, in a section plane, moving from the surface of the part towards the center of the part, after the cemented layer, the part comprises a layer in which the carbon content gradually decreases until it returns to the carbon content of the original part, that is to say that the surface cementation step does not modify the composition of the part at the core.
[0015] The duration of the austenitization step must be adapted according to the temperature in order to dissolve as many carbides as possible present on the surface while maintaining a reasonable grain size.
[0016] By way of non-limiting example, the duration of the austenitization step is generally less than 30 minutes.
[0017] Quenching allows the transformation of austenite into martensite.
[0018] The tempering step allows the hardening of the surface layer of the martensitic steel part by precipitation of carbides.
[0019] By way of non-limiting example, the income step may comprise income sub-steps.
[0020] By way of non-limiting example, the income step may comprise between two and four income sub-steps.
[0021] The quenching step causes deformations of the part which are generally greater than the thickness of the desired nitrided layer.
[0022] Also, the grinding step makes it possible to remove the deformations due to quenching and to approach the dimensions of the finished part, i.e. the surface nitrided part. Without this grinding step before surface decarburization and surface nitriding, the nitrided layer could be completely removed during a grinding step carried out after nitriding.
[0023] The rectification can be carried out under conventional conditions.
[0024] By way of non-limiting example, the grinding can be carried out with a grinding wheel whose parameters are adapted to the material and the lubrication.
[0025] The second thickness being 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 depend in particular on the geometry of the part and the deformations of the part following the quenching step.
[0026] By way of non-limiting example, the second thickness may be between 0.5 mm and 3 mm, for example between 0.7 mm and 2 mm.
[0027] The surface cleaning step makes it possible to eliminate residues which could prevent the adsorption of the decarburization and nitriding gas on the part. The residues may be, for example, residues of grease, dust, oxides, etc.
[0028] The surface cleaning step can be carried out by mechanical and / or chemical methods.
[0029] The surface decarburization and surface nitriding steps are carried out in the same furnace. There is therefore no specific furnace for decarburization. There is also no requirement to lower the temperature and transport the parts from a decarburization furnace to the nitriding furnace.
[0030] By way of non-limiting example, the surface decarburization step is carried out at a temperature close to the nitriding temperature or equal to the nitriding temperature. By "close" is meant a temperature difference less than or equal to 40°C, preferably less than or equal to 20°C.
[0031] Furthermore, since the rectification takes place before the surface decarburization, the thickness of the decarburized layer is not modified by the rectification step.
[0032] Surface decarburization on a third thickness strictly less than the second thickness makes it possible to avoid the formation of intergranular precipitates during the surface nitriding step.
[0033] The third thickness being strictly less than the second thickness, there remains a layer with a carbon content higher than the carbon content of the decarburized layer and the carbon content of the core part. In a plane of section of the part, starting from the outer surface towards the centre of the part, the part comprises the decarburized layer on the third thickness, the ground tempered layer on 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 until the centre of the part.
[0034] By way of non-limiting example, the surface decarburization step can be carried out at plate temperatures between 450°C and 550°C, for a duration which is a function of the third thickness which it is desired to obtain for the nitrided layer of the surface-nitrided part.
[0035] By way of non-limiting example, the carbon content of the decarburized layer is less than or equal to 0.75% by mass.
[0036] Surface nitriding can be carried out under conventional conditions.
[0037] By way of non-limiting example, the third thickness may be between 0.05 mm and 0.5 mm, for example between 0.2 mm and 0.4 mm.
[0038] In some embodiments, the secondary hardening steel may comprise, in mass percent: 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.
[0039] By way of non-limiting example, the secondary hardening steel may be a steel comprising, in mass percentage: 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, maximum 0.015% phosphorus, maximum 0.010% sulfur, maximum 0.10% copper, maximum 0.25% cobalt, maximum 0.15% tungsten, the remainder being made up of iron and unavoidable impurities, for example secondary hardening steel commonly called M50NiL (AMS 6278).
[0040] In some embodiments, the surface decarburization may be performed with a mixture of hydrogen gas and water vapor.
[0041] The hydrogen gas and the water vapor form an atmosphere allowing the surface of the clean part to be decarburized over a thickness varying according to the decarburization time. The control of the water vapor content can, for example, be carried out by measuring the dew point which is a function of the pressure and the humidity level.
[0042] In addition, the presence of gaseous hydrogen in the mixture makes it possible to limit, or even avoid, the formation of oxides on the surface of the part, oxides which must be eliminated.
[0043] In some embodiments, the mixture may further comprise a carbonaceous gas.
[0044] The addition of a carbon gas to the mixture allows, thanks to the pressure balance partial carbon content 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.
[0045] According to the nitriding process of the surface nitriding step, the carbon gas may be carbon dioxide (CO2) when the surface nitriding is gaseous nitriding or methane (CH4) when the surface nitriding is ionic nitriding.
[0046] Thus, it is not necessary to modify the nitriding furnace to supply it with additional gas.
[0047] In some embodiments, quenching may be followed by cryogenic treatment.
[0048] Cryogenic treatment allows the temperature to be lowered in order to transform as much austenite as possible into martensite.
[0049] By way of non-limiting example, the cryogenic treatment is typically carried out between -70°C and -100°C.
[0050] Typically, this step can be completed within two hours of quenching.
[0051] In some embodiments, the cryogenic treatment may be performed for a cryogenic time greater than or equal to 1 hour and less than or equal to 10 hours.
[0052] In certain embodiments, the cemented part may comprise at the surface a carbon content greater than or equal to 0.8% by mass.
[0053] In some embodiments, austenitization may be performed under vacuum.
[0054] Vacuum austenitization makes it possible to reduce, or even avoid, the formation of oxide and / or the decarburization of the cemented layer and the austenitic steel layer obtained after austenitization of the cemented layer.
[0055] In some embodiments, the surface nitriding may be gas nitriding or ion nitriding.
[0056] In some embodiments, the surface cementation may be gas cementation or low pressure cementation.
[0057] As a non-limiting example, the surface cementation can be carried out by low pressure cementation and the carbon gases can be acetylene or propane.
[0058] By way of non-limiting example, the surface cementation can be carried out by gas cementation and the carbon gases can be a mixture of nitrogen and methanol. Brief description of the drawings
[0059] Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given as non-limiting examples, with reference to the appended figures.
[0060] [Fig. 1] [Fig.l] is a schematic longitudinal sectional view of a turbomachine.
[0061] [Fig.2] [Fig.2] is a flowchart representing the steps of a nitriding process for a secondary hardening steel part.
[0062] Throughout the figures, the common elements are identified by identical numerical references. Detailed description
[0063] [Fig.l] represents, in section along a vertical plane passing through its main axis A, a double-flow turbojet 10. The double-flow turbojet 10 comprises, from upstream to downstream according to the circulation of the air flow, 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.
[0064] The high-pressure turbine 20 comprises a plurality of moving 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 arranged opposite the moving blades 20A of the turbine 20.
[0065] Similarly, the low pressure turbine 22 comprises a plurality of moving blades rotating with the rotor and rectifiers mounted on the stator.
[0066] In the following, the elements common to the different embodiments are identified by the same numerical references.
[0067] To allow rotation around the main axis A of the moving blades, the turbojet engine comprises gears and bearings. The parts of these gears and bearings are subjected to significant mechanical forces which can cause significant wear. The gear and bearing parts are generally surface treated to increase their hardness and mechanical strength.
[0068] By way of non-limiting example, the secondary hardening steel part may be a part for a gear or for a bearing.
[0069] [Fig.2] is a flowchart representing the steps of a nitriding process 100 of a secondary hardened steel part.
[0070] The nitriding process 100 comprises a first step 102 of obtaining a rough part made of secondary hardening steel.
[0071] The rough part then undergoes a surface cementation step 104 making it possible to obtain a cemented layer over a first thickness of the rough part.
[0072] The cemented layer then undergoes an austenitization step 106 allowing to obtain a layer of austenitic steel.
[0073] The austenitic steel layer then undergoes a quenching step 108 to obtain a martensitic steel layer.
[0074] The martensitic steel layer then undergoes a tempering step 110 to obtain a tempered layer on the first thickness.
[0075] The income step 110 may comprise several income sub-steps, for example three income sub-steps.
[0076] The tempered layer then undergoes a rectification step 112 to obtain a rectified tempered layer having a second thickness.
[0077] The rectified returned layer then undergoes a surface cleaning step 114 to obtain a clean layer on the second thickness.
[0078] The surface cleaning step 114 makes it possible to eliminate residues which could prevent the adsorption of the decarburization and nitriding gas on the part. The residues may be, for example, residues of grease, dust, oxides, etc.
[0079] The part is then placed in a nitriding furnace.
[0080] In the nitriding furnace, the clean layer then undergoes a surface decarburization step 116 to obtain a decarburized layer over a third thickness, the third thickness being strictly less than the second thickness.
[0081] In the nitriding furnace, the decarburized layer then undergoes a surface nitriding step 118 to form a surface nitrided part on the third thickness.
[0082] By way of non-limiting example, the secondary hardening steel part may be an M50NiL steel part.
[0083] For M50NiL steel, the surface carburizing step 104 can be carried out at plateau temperatures between 900°C and 1000°C, for example at 950°C for 10 hours.
[0084] The austenitization step 106 can be carried out at plateau temperatures between 1050°C and 1100°C, for example, for 30 minutes.
[0085] The quenching step 108 may be followed by a cryogenic treatment 120 at temperatures between -70°C and -100°C for 1 to 10 hours.
[0086] The tempering step 110 may comprise several tempering sub-steps, for example three tempering sub-steps. The tempering sub-steps may for example be carried out at plateau temperatures between 500°C and 550°C, for 1 to 4 hours each.
[0087] The sub-steps can be carried out at the same plateau temperature or not and for equal or different times.
[0088] The grinding step 112 can remove material to a thickness of between 0.1 and 0.5 mm.
[0089] The surface decarburization step 116 can be carried out at plate temperatures between 450°C and 550°C, for a duration which is a function of the third thickness which it is desired to obtain for the nitrided layer of the surface nitrided part.
[0090] The surface nitriding step 118 can be carried out at plate temperatures between 450°C and 550°C, for a duration which is a function of the third thickness.
[0091] Although the present disclosure has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments recited may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.
[0092] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may 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 illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.
[0093] It is also obvious that all the characteristics described with reference to a method 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 method.
Claims
Claims
1. A method of nitriding (100) a secondary hardening steel part, the method comprising the following steps: - obtaining (102) a rough part made of secondary hardening steel; - surface carburizing (104) the rough part to obtain a carburized layer over a first thickness of the rough part; - austenitizing (106) the carburized layer to obtain a layer of austenitic steel; - quenching (108) the layer of austenitic steel to obtain a layer of martensitic steel; - tempering (110) the layer of martensitic steel to obtain a tempered layer; - grinding (112) the tempered layer to obtain a ground tempered layer having a second thickness; - surface cleaning (114) of the ground tempered layer to obtain a clean layer;- surface decarburization (116) of the clean layer in a nitriding furnace to obtain a decarburized layer over a third thickness, the third thickness being strictly less than the second thickness; - surface nitriding (118) of the decarburized layer in the nitriding furnace to form a surface nitrided part over the third thickness.;
2. The nitriding method (100) of claim 1, wherein the secondary hardening steel comprises, in mass percentage: 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. A nitriding method (100) according to claim 1 or 2, wherein the surface decarburization (116) is carried out with a mixture of hydrogen gas and water vapor.
4. The nitriding method (100) of claim 3, wherein the mixture further comprises a carbonaceous gas.
5. A nitriding method (100) according to any one of claims 1 to 4, wherein the quenching (108) is followed by a cryogenic treatment (120).
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10. Nitriding method (100) according to claim 5, in which the cryogenic treatment (120) is carried out for a cryogenic time greater than or equal to 1 hour and less than or equal to 10 hours. Nitriding method (100) according to any one of claims 1 to 6, in which the cemented part comprises on the surface a carbon content greater than or equal to 0.8% by mass. Nitriding method (100) according to any one of claims 1 to 7, wherein the austenitization (106) is carried out under vacuum. A nitriding method (100) according to any one of claims 1 to 8, wherein the surface nitriding (118) is gas nitriding or ion nitriding. A nitriding method (100) according to any one of claims 1 to 9, wherein the surface carburizing (104) is gas carburizing or low pressure carburizing.