Method for nitriding a steel
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SAFRAN TRANSMISSION SYST
- Filing Date
- 2024-07-17
- Publication Date
- 2026-06-03
AI Technical Summary
The existing steel nitruration process for secondary hardening steels faces challenges such as significant deformation during quenching, leading to a reduced thickness of the carbon-depleted surface layer that can be nitrured, and limited depth decaruration when austenitization and decaruration stages are concomitant, which affects the quality and thickness of the nitrured layer.
A process involving superficial cementation, austenitization, quenching, rectification, superficial cleaning, decaruration, and nitruration stages, where the rectification occurs before decaruration to maintain a thicker carbon-rich layer for effective nitruration, and using a nitruration oven for both decaruration and nitruration steps to control the carbon level and prevent intergranular precipitate formation.
This approach ensures a thicker nitrured layer with controlled carbon content, reducing deformation and maintaining the original composition deep within the steel part, thereby enhancing the mechanical properties and surface hardness of the steel.
Smart Images
Figure FR2024050982_30012025_PF_FP_ABST
Abstract
Description
PROCESS FOR NITRIDING STEEL Technical field
[0001] This 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 bearings. Prior art
[0002] Steels, especially secondary hardening steels, have good core mechanical properties. After 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 process for nitriding a steel part comprising a step of decarburizing the surface of the part, followed by a step of quenching and tempering the part partially decarburized on a surface layer and a step of nitriding the surface layer depleted in carbon.
[0005] Heat treatment involves a hardening step followed by a tempering step. The hardening step generally generates a relatively significant deformation of the part. The part must therefore be ground before the nitriding step, which results in removing a certain thickness of the carbon-depleted surface layer, consequently reducing the thickness of the layer that can be nitrided.
[0006] Decarburization of the part surface is carried out before or concurrently with the austenitization step. When the decarburization and austenitization steps are concurrent, the decarburization depth achieved is limited by the time required for the austenitization step of the part. Statement of the invention
[0007] This presentation aims to remedy at least part of these drawbacks.
[0008] For this purpose, the present disclosure relates to a process for nitriding a secondary hardening steel part, the process 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 an austenitic steel layer; - 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 returned 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] A "rough part" is a part whose functional surfaces have been created and taking into account an excess thickness which will be removed during the grinding stage.
[0010] Surface cementation can be carried out under conventional conditions.
[0011] Surface carburizing is performed by injecting carbon gases into a carburizing furnace. The carbon gases decompose on the surface of the part to provide a carbon flux.
[0012] The duration of the surface carburizing stage 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.
[0013] It is understood that the cemented layer is present on the entire exterior surface of the rough part on the first thickness.
[0014] Thus, in a cutting plane, moving from the surface of the part towards the center of the part, after the cemented layer, the part includes 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] As a 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 stage allows the hardening of the surface layer of the martensitic steel part by precipitation of carbides.
[0019] By way of non-limiting example, the revenue stage may include revenue sub-stages.
[0020] By way of non-limiting example, the income stage may comprise between two and four income sub-stages.
[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 allows to remove the deformations due to quenching and to approach the dimensions of the finished part, that is to say 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] Rectification can be carried out under conventional conditions.
[0024] As a non-limiting example, 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 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 removes residues that could prevent the adsorption of decarburization and nitriding gas on the part. These residues can include, for example, 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. Therefore, there is no specific furnace for decarburization. There is also no need 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 grinding takes place before the surface decarburization, the thickness of the decarburized layer is not modified by the grinding 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 than the carbon content of the core part. In a section plane of the part, starting from the outer surface towards the center 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 returning to the carbon content of the original part. The composition of the part no longer varies to the center 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] As a 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: 0.1-0.4% carbon, 0-6.0% chromium, 0-6.0% molybdenum, 0-3.0% vanadium, 0-20% cobalt, 0-10% nickel, and 0-3.0% 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 the secondary hardening steel commonly called M50NÎL (AMS 6278).
[0040] In some embodiments, the surface decarburization may be performed with a mixture of hydrogen gas and water vapor.
[0041] Hydrogen gas and water vapor form an atmosphere that allows the surface of the clean part to be decarburized to a thickness that varies depending on the decarburization time. Water vapor content can be controlled, for example, by measuring the dew point, which is a function of pressure and humidity.
[0042] In addition, the presence of hydrogen gas 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 balance of the carbon partial pressure of the gaseous atmosphere and the carbon concentration of the surface layer, to control the carbon rate in the decarburized layer of the decarburized part on the surface.
[0045] Depending on the nitriding process of the surface nitriding step, the carbon gas can be carbon dioxide (CO2) when the surface nitriding is gaseous nitriding or methane (CH4) when the surface nitriding is ionic nitriding.
[0046] Thus, there is no need 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] As a non-limiting example, cryogenic treatment is typically carried out between -70°C and -100°C.
[0050] Typically, this step can be completed within two hours of soaking.
[0051] In some embodiments, the cryogenic treatment may be performed for a cryogenic time of 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 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 carburizing may be gas carburizing or low pressure carburizing.
[0057] As a non-limiting example, 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, 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] Figure 1 is a schematic longitudinal sectional view of a turbomachine.
[0061] [Fig. 2] Figure 2 is a flowchart showing the steps in a nitriding process for a secondary hardening steel part.
[0062] Throughout the figures, common elements are identified by identical numerical references. Detailed description
[0063] Figure 1 represents, in section along a vertical plane passing through its main axis A, a double-flow turbojet engine 10. The double-flow turbojet engine 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 enable rotation around the main axis A of the moving blades, the turbojet engine includes gears and bearings. The parts of these gears and bearings are subjected to significant mechanical stresses 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 gear or bearing part.
[0069] Figure 2 is a flowchart showing the steps of a nitriding process 100 of a secondary hardening 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 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 remove 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 on 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] As a non-limiting example, the secondary hardening steel part may be an M50NÎL 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 or different plateau temperature 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 discussed 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 in secondary hardening steel; - surface cementation (104) of the rough part to obtain a cemented layer on a first thickness of the rough part; - austenitization (106) of the cemented layer to obtain a layer of austenitic steel; - quenching (108) of the austenitic steel layer to obtain a martensitic steel layer; - tempering (110) of the martensitic steel layer to obtain a tempered layer; - rectification (112) of the tempered layer to obtain a rectified tempered layer having a second thickness; - surface cleaning (114) of the rectified returned 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 on the third thickness.
2. The nitriding method (100) according to 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).
6. The nitriding method (100) of claim 5, wherein the cryogenic treatment (120) is carried out for a cryogenization time greater than or equal to 1 hour and less than or equal to 10 hours.
7. 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.
8. A nitriding method (100) according to any one of claims 1 to 7, wherein the austenitization (106) is carried out in a vacuum.
9. A nitriding method (100) according to any one of claims 1 to 8, wherein the surface nitriding (118) is gas nitriding or ion nitriding.
10. 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.