Method for furnace hardening a steel part and method for manufacturing a steel part

Abstract

The invention relates to a method (100) for furnace hardening a steel part (P1) comprising a core (10) and a surface (20), the method (100) comprising the following steps: (120) austenitizing the part (P1) under vacuum at a first threshold temperature (Ts1) so as to transform the steel into austenitic steel; and (130) quenching the part (P1) under an atmosphere of neutral gas in three cooling sub-steps so as to transform the austenitic steel into martensitic steel and obtain a martensitic steel part (P2). The hardening method (100) according to the invention differs from the prior art in that it consists of a hardening method using modulated quenching.

Description

[0001] DESCRIPTION

[0002] TITLE: OVEN HARDENING PROCESS FOR A STEEL PART AND PROCESS FOR MANUFACTURING A STEEL PART

[0003] Technical field of the invention

[0004] The invention relates to the technical field of furnace hardening processes for steel parts. The invention also relates to the technical field of manufacturing processes for martensitic steel parts using such a hardening process.

[0005] Technical background

[0006] The manufacture of a part in case-hardened steel requires a well-known heat treatment which is divided into two main phases: a case-hardening phase and a hardening phase.

[0007] During carburizing, the steel part is subjected to a temperature of 900°C or higher and acquires a macroscopic structure consisting of a core surmounted by a carbon-enriched surface layer. This surface layer, resulting from the diffusion of carbon across the surface of the part, improves the wear resistance and mechanical properties of the steel component.

[0008] Hardening generally consists of two stages. In the first stage, austenitizing, the part is heated for a predetermined time to a target temperature and acquires an austenitic structure. Then, in the second stage, quenching, the part is cooled rapidly enough to allow the formation of a martensitic microstructure throughout, while preventing the formation of undesirable phases (carbides, bainite, etc.). Quenching is therefore a forced cooling process. The cooling rate depends on the actual composition of the steel. Some steels require quenching in cold water, while others can simply be cooled in still air. This obviously depends on the cooling rate observed on the part, and therefore on the part's mass.In any case, the conditions under which quenching is carried out play a critical role in obtaining the desired mechanical properties, e.g. its hardness, and the characteristics of the steel in terms of alloy.

[0009] Notably, manufacturers have observed the appearance of numerous distortions in parts treated during hardening. These distortions most often result from thermal gradients within the part, especially in complex components. These unwanted distortions must be corrected by grinding to bring the part into compliance with the standards of the relevant industry, such as those for the aerospace sector. That said, excessive grinding is technically and industrially undesirable.

[0010] The state of the art has proposed numerous methods for reducing distortion in a martensitic steel part, thereby preventing its deformation. One approach reduces distortion by adjusting the quenching conditions via atmospheric pressure. A second approach reduces distortion by adjusting both the heating and quenching conditions of the part. These conditions are sometimes very demanding and may require adjusting the heating and quenching conditions based on the geometry and / or the speed of the part, making some processes unsuitable for industrial application. A third approach constrains the part using one or more formers during quenching, preventing deformation.However, depending on the complexity of the part, the formers can be very complex in order to maintain appropriate clearances with the part, not to mention the deformations that said formers are themselves subjected to due to the processing temperatures.

[0011] The present invention uses the first approach described above and therefore aims to reduce distortions by adjusting the quenching conditions. It is therefore useful to understand the mechanisms involved during this quenching phase.

[0012] In the first step, the part undergoes austenitization. For this, the part is heated under vacuum at a suitable temperature to transform the steel into the austenitic (face-centered cubic) phase while preventing oxidation. Once the austenitization is complete, the chamber containing the part is filled with gas at a pressure typically between 1 and 20 bar. The chamber is then rapidly cooled, resulting in the transformation of the austenitic steel into martensitic steel at the phase change temperature of the specific alloy steel. Every chemical element, except cobalt, lowers the part's temperature. This phase change also leads to a change in microstructure, as the steel transitions from a face-centered cubic to a body-centered cubic microstructure.Because the resulting structure is less dense than the initial structure, this generates swelling of the part. This swelling is spatially homogeneous if the microstructural change occurs simultaneously throughout the entire part. However, it should be noted that only infinitely slow cooling, i.e., in practice less than 1 °C / min, allows this condition to be achieved, but many undesirable transformations occur before the phase change to martensitic steel. Conversely, rapid cooling creates more or less pronounced thermal gradients within the part, at a minimum between the core and the surface layer, depending on the part's geometry, the furnace, the cooling method, etc. When the martensitic transformation occurs gradually between 350°C and 200°C, the first areas to transform will generate stresses in other areas of the part.Unprocessed areas, which still exhibit a face-centered cubic microstructure, are easily deformed. They will therefore deform and transform later during cooling. This results in a part with a geometry different from the initial geometry, with varying degrees of distortion. These distortions lead to a more complex grinding range and / or a longer grinding range and / or longer carburizing cycles and / or greater variation in properties across the part. It is therefore important to minimize them as much as possible.

[0013] Document JP 2005 060760 A discloses a process for hardening a steel part in a furnace.

[0014] The invention aims to overcome at least some of the aforementioned problems and proposes in this regard a hardening process which includes a quenching step which makes it possible to considerably reduce distortions in the part to be treated without degrading the mechanical properties of said part.

[0015] Summary of the invention

[0016] The invention proposes for this purpose a process for hardening in a furnace a steel part comprising a core and a surface, the process comprising the following steps:

[0017] - to perform austenitizing of the part under vacuum at a first threshold temperature Tsi in order to transform the steel into austenitic steel,

[0018] - to perform a quenching of the part under a neutral gas atmosphere in three sub-steps in order to transform the austenitic steel into martensitic steel and obtain a martensitic steel part, the quenching step comprising the following sub-steps:

[0019] > cool the part at a first speed v ri cooling greater than or equal to 200°C per minute such that the surface temperature changes from the first threshold temperature Tsi to a second threshold temperature Ts2,

[0020] > cool the part at a second speed v r 2. Cooling rate between 2°C per minute and 10°C per minute such that the surface temperature changes from the second threshold temperature TS2 to a third threshold temperature Tsa, the third threshold temperature Tsa being less than or equal to a temperature T ma r of initiation of the transformation, at least partial, of austenitic steel into martensitic steel, the surface reaching the temperature Tmar of initiation of the transformation, at least partial, of austenitic steel into martensitic steel during the second cooling sub-stage,

[0021] > cool the room at a third speed vr 3. Cooling rate greater than or equal to 10°C per minute such that the surface temperature drops from the third threshold temperature TS3 to an extraction temperature T E from the oven.

[0022] The process according to the invention solves the aforementioned problems of the prior art. In this respect, the hardening process according to the invention comprises a quenching step with three cooling sub-steps that differ significantly from one another in the cooling rates implemented.

[0023] The first gear v ri The cooling rate must be very high to prevent the precipitation of carbides and, more generally, undesirable phases during cooling. In this case, this rate is greater than or equal to 200°C per minute. If this rate is insufficient, the phase change may not occur.

[0024] The second speed v rThe second cooling stage aims to homogenize the room temperature before the phase change occurs. This second stage v r A second cooling rate, ranging from 2°C per minute to 10°C per minute, is applied as soon as the second threshold temperature TS2 is reached. This rate is both low and sufficient to absorb the existing thermal gradients in the room before the phase change occurs. This second rate v r 2. Cooling is maintained until the third threshold temperature Tss.

[0025] The third gear v r A cooling rate of 3°C helps to minimize carbon migration between the austenitic and martensitic phases. It must therefore be sufficiently high to prevent such migration and allow for structural stabilization.

[0026] The hardening process according to the invention therefore comprises a quenching step carried out in three sub-steps, each associated with a cooling rate, instead of one or two steps, each associated with a cooling rate, which makes it possible to obtain a steel part with reduced distortions while preserving the mechanical properties of said steel part.

[0027] Depending on various characteristics of the invention, which may be considered together or separately:

[0028] - the second threshold temperature TS2 is between 400°C and 700°C,

[0029] - the third threshold temperature Tss is between 250°C and 400°C,

[0030] - the extraction temperature T E is between 40°C and 60°C, preferably around 50°C, - we switch from the first speed v ri cooling at second speed v r2. Cooling by lowering the neutral gas mixing speed in the furnace and / or the pressure in the furnace from 60% to 95%,

[0031] - we adjust the first speed v ri cooling, the second speed v r 2 cooling and the third speed v r 3. Cooling by modifying the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace.

[0032] - we switch from second gear v r 2. Cooling at third speed v r 3. Cooling by increasing the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace.

[0033] - The neutral gas is chosen from nitrogen, helium, argon, and hydrogen.

[0034] - Concurrently with austenitization, the oven is placed under vacuum during the austenitization stage.

[0035] - Following the austenitization stage, the furnace is filled with inert gas,

[0036] - During the supply stage, the core comprises a mass percentage of: carbon strictly greater than 0% and strictly less than 3%, and / or chromium strictly greater than 2% and strictly less than 20%, and / or molybdenum strictly greater than 1% and strictly less than 10%, and / or vanadium strictly greater than 0% and strictly less than 5%, and / or cobalt strictly greater than 0% and strictly less than 20%, and / or nickel strictly greater than 0% and strictly less than 10%, and / or tungsten strictly greater than 0% and strictly less than 10%, and / or a manganese and silicon alloy strictly greater than 0% and strictly less than 3%

[0037] - during the supply stage, the surface comprises a mass percentage of carbon between 0.5% and 1.4%,

[0038] - the process further comprises a cementation step, said cementation step being carried out before austenitizing or simultaneously with austenitizing,

[0039] - the steel part is a turbomachine part, and in particular a transmission part such as a pinion or a bearing.

[0040] The invention further relates to a method for manufacturing a part made of martensitic steel comprising the following steps:

[0041] - to carry out a vacuum curing process as previously described,

[0042] - treat the martensitic steel part obtained at the end of the previous step,

[0043] - Optionally, perform finishing touches. Brief description of the figures

[0044] Other objects, features and advantages of the invention will become clearer in the following description, made with reference to the accompanying figures, in which:

[0045] - Figure 1 is a schematic view illustrating the different stages of a hardening process according to one embodiment of the invention,

[0046] - Figure 2 illustrates a steel part that can be used in the context of the invention,

[0047] - Figure 3 illustrates a cross-sectional view of the part shown in Figure 2,

[0048] - Figure 4 illustrates a cooling curve according to a continuous quenching method as implemented in the prior art and a cooling curve according to a quenching method implemented in the hardening process that is the subject of the present invention,

[0049] - Figure 5 is a schematic view illustrating the different stages of a manufacturing process for a martensitic steel part according to an embodiment of the present invention.

[0050] In the illustrated figures, optional steps are indicated by dotted lines.

[0051] Detailed description of the invention

[0052] The invention relates to a process for hardening a steel part P1 in a furnace.

[0053] Part P1 can be any steel component. For example, but not limited to, part P1 could be a turbomachine component, and in particular a transmission component, specifically one subjected to mechanical stresses. The transmission component could be a case-hardened steel gear or case-hardened steel bearings.

[0054] The present invention is of particular interest for steel parts where distortions must be reduced during a phase change from an austenitic to a martensitic phase. The present invention is even more particularly relevant for such steel parts that also have a complex shape. It allows for the application of hardening to case-hardened steels in order to avoid excessive grinding of the case-hardened surface layer that gives the part its properties.

[0055] In the context of the invention, "steel" means a metallic alloy comprising a mass percentage of: carbon strictly greater than 0% and strictly less than 3%, and / or chromium strictly greater than 2% and strictly less than 20%, and / or molybdenum strictly greater than 1% and strictly less than 10%, and / or vanadium strictly greater than 0% and strictly less than 5%, and / or cobalt strictly greater than 0% and strictly less than 20%, and / or nickel strictly greater than 0% and strictly less than 10%, and / or tungsten strictly greater than 0% and strictly less than 10%, and / or an alloy of manganese and silicon strictly greater than 0% and strictly less than 3%.

[0056] For example, M50NIL steel falls within such a definition insofar as it comprises, by mass percentage, between 0.11% and 0.15% carbon, between 4.00% and 4.25% chromium, between 4.00% and 4.50% molybdenum, between 1.13% and 1.33% vanadium, between 0% and 0.25% cobalt, between 3.20% and 3.60% nickel, between 0% and 0.15% tungsten, and between 0.25% and 0.60% of a silicon-manganese alloy. The invention is in no way limited to the use of such steel, and those mentioned above. However, it is of greater interest in such situations.

[0057] The hardening process 100 according to the invention is advantageously implemented using an oven (not illustrated). The oven comprises at least one chamber in which the part P1 to be treated can be placed.

[0058] The chamber has an inlet for at least one inert gas connected to one or more gas sources and a gas outlet connected to a pumping device. The gas source(s) and the pumping device allow the pressure inside the chamber to be regulated. The higher the pressure, the faster the cooling rate, with the pressure advantageously adjustable between 0.3 bar and 20 bar. The gas mixing speed can be adjusted by controlling a turbine speed. The gas then circulates through the furnace and a heat exchanger to cool it before being reinjected. The higher the mixing speed, the faster the cooling rate.

[0059] In addition, the oven may advantageously feature a control interface allowing adjustment of the temperature, pressure and gas mixing speed in the chamber.

[0060] With reference to Figure 1 and Figures 2 and 3, the hardening process 100 typically includes a first step 110 of supplying the steel part P1. The part P1 may have previously undergone a conventional case hardening step during which its surface was enriched with carbon. In practice, the part P1 therefore has a steel core 10 of a given composition and a surface 20 made of carbon-enriched steel. That being said, this case hardening step can be carried out simultaneously with a second step of the process, namely austenitizing 120, instead of being carried out before the austenitizing 120.

[0061] Advantageously, surface 20 is made of steel containing a carbon content by mass of between 0.5% and 1.4%. As will be seen in more detail later in this description, such a quantity of carbon makes it possible to obtain a surface 20 with a hardness of between 60 HRC and 68 HRC after the hardening process 100, which makes the surface 20 of part P1 a very high-hardness surface and therefore particularly suitable for the manufacture of bearings, for example for turbomachinery.

[0062] The hardening process 100 includes the second austenitizing step 120 of part P1 under vacuum at a first threshold temperature Tsi, thereby transforming the steel into austenitic steel. More precisely, following this austenitizing step 120, the entire microstructure of part P1, that is, the entire crystallographic structure of part P1, is transformed into an austenitic structure. When in the austenitic phase, the steel exhibits a face-centered cubic crystallographic structure. The first threshold temperature Tsi varies depending on the initial steel composition of part P1. This composition can vary significantly depending on the application. The first threshold temperature Tsi is typically between 800°C and 1200°C and typically corresponds to the highest temperature to which the part is subjected during process 100.For example, for a reference steel part M50NIL, the first threshold temperature Tsi can advantageously be 1100°C.

[0063] Preferably, during austenitization 120, the vacuuming of the furnace can be carried out concurrently with the heating of the furnace, which helps to avoid oxidation of part P1 and degradation of its properties, in particular its mechanical properties.

[0064] With further reference to Figure 1, the hardening process 100 comprises, following this second austenitizing step 120, a third quenching step 130 of the part P1 under a neutral gas atmosphere. According to the invention, this third quenching step 130 is carried out in three sub-steps, at the end of which the austenitic microstructure is transformed into a martensitic microstructure, resulting in a martensitic steel part P2. The invention thus lies in the fact that this third quenching step 130 is modified compared to the hardening processes described in the prior art.

[0065] Implementing this third quenching step 130 under a neutral gas atmosphere prevents oxidation of the steel part P1 during cooling. The neutral gas used is therefore inert with respect to the steel. The neutral gas is preferably chosen from nitrogen, helium, argon, and hydrogen. Nitrogen has the advantage of being less expensive than helium, argon, and hydrogen. The third quenching step 130 comprises a first substep 132 consisting of cooling the part P1 at a first rate v ri cooling greater than or equal to 200°C per minute such that the surface temperature 20 drops from the first threshold temperature T si at a second threshold temperature T s2 A first speed v ri Cooling at a rate of 200°C or higher per minute helps to prevent the precipitation of carbides and, more generally, undesirable phases during cooling.

[0066] Indeed, if the first speed v ri If the cooling is insufficient, the cooling dynamics of surface 20 of part P1 are altered, and the microstructural changes at surface 20 and in the core 10 of the part cannot occur. The first cooling substep 132 therefore creates the conditions that allow the subsequent transformation of austenitic steel into martensitic steel by inducing microscopic mechanisms that permit such a transformation while also preventing further carbide precipitation.

[0067] As illustrated in Figure 4, such a cooling rate makes it possible to lower the temperature of the surface 20 of part P1 from nearly 1100°C to less than 550°C in less than three minutes. It should be noted that in Figure 4, the temperature curve representing the prior art is designated by the reference numeral EA, while the temperature curve associated with an embodiment of the invention is designated by the reference numeral INV. The reference numerals 132, 134, and 134, associated respectively with the first, second, and third substeps of the quenching step 130 of the process according to the invention, relate only to the temperature curve INV.

[0068] In this regard, it is advantageous that the entire surface 20 reaches the second threshold temperature Ts2 at the end of the first sub-stage 132 of cooling to ensure the homogeneity of conditions at surface 20.

[0069] Preferably, the second threshold temperature Ts2 is between 400°C and 700°C. This second threshold temperature Ts2 depends not only on the initial composition of the steel from which part P1 is made but also on the mass percentage of carbon present on the surface of part P1. For example, if surface 20 has a mass percentage of carbon of 0.7%, the second threshold temperature Ts2 is preferably around 600°C. Thus, taking the example of a part P1 made of M50NIL steel, which also has a mass percentage of carbon of 0.7% on its surface 20, the first sub-step 132 of cooling would therefore consist of reducing the temperature of surface 20 from 1100°C to 600°C at a first rate v ricooling rate greater than or equal to 200°C per minute. It would therefore take approximately 2 minutes and 30 seconds to reach the second threshold temperature Ts2 with a rate of 200°C / min. There are various methods to achieve such a cooling rate. That being said, it is very advantageous to adjust the first rate v ri Cooling is achieved by modifying the mixing speed of the inert gas in the furnace and / or the pressure in the furnace. In this regard, and by way of non-limiting example, the pressure in the furnace can be set to a value greater than 6 bar. In practice, increasing the pressure in the furnace can be implemented immediately after the austenitizing step 120. To do this, at the end of the austenitizing step 120, the furnace is filled with the desired inert gas at a pressure greater than 6 bar. In conjunction with the pressure in the furnace, it is also possible to increase the mixing speed to obtain an initial velocity v ricooling greater than or equal to 200°C / min.

[0070] The third quenching step 130 further comprises a second cooling substep 134 consisting of cooling the part P1 at a second rate v r 2. Cooling rate between 2°C per minute and 10°C per minute such that the surface temperature 20 changes from the second threshold temperature Ts2 to a third threshold temperature Tsa, the third threshold temperature Tsa being less than or equal to a temperature T ma r of initiation of the transformation, at least partial, of austenitic steel into martensitic steel, the surface 20 reaching the temperature T ma r of initiation of the transformation, at least partial, of austenitic steel into martensitic steel during the second sub-stage 134 of cooling. Figure 4 illustrates a case in which the second speed v rThe cooling rate is 4°C / min. The slope of the temperature profile is much lower than during the first sub-stage 132 of cooling, illustrating the significant decrease in the cooling rate.

[0071] As can be deduced from the above, during the second cooling substep 134, it is not necessary to stop the second cooling substep 134 when the transformation phase of the surface 20 is complete. Nor is it necessary to stop the second cooling substep 134 when the transformation phase of the core 10 is complete or has even begun. However, the second cooling substep 134 can be stopped as soon as at least a portion of the surface 20 begins its transformation from austenitic steel to martensitic steel. Preferably, the second cooling substep 134 can be stopped as soon as an area near the core 10 begins its transformation from austenitic steel to martensitic steel.

[0072] The second cooling substep 134 allows for a temperature difference between the surface 20 of part P1 and the core 10 of part P1 of no more than 15°C ± 5°C, whereas in prior art hardening processes, when the surface of the part reaches a temperature between 80°C and 100°C, the core is still at a temperature of approximately 300°C because the surface cools much more rapidly than the core. Thanks to the hardening process 100 according to the invention, the temperature difference between the core 10 and the surface 20 is therefore reduced by a factor of 10 to 100 compared to the prior art.

[0073] In this regard, it is worth recalling that carbon lowers the transformation point, namely the temperature at which austenitic steel is transformed into martensitic steel. Thus, the duration of the second cooling sub-step 134 can vary considerably from one steel to another.

[0074] For example, when part P1 is made of M50NIL steel, the temperature at which the austenitic steel of the core 10 is transformed into martensitic steel is 350°C. However, slow cooling, such as that carried out in the second substep 134, can be implemented down to 300°C. This depends on the carbon content of part P1. The higher the carbon content of part P1, the more effectively the segregation phenomenon that one seeks to avoid can be controlled by adjusting the duration of the second cooling substep 134.

[0075] The second cooling sub-step 134 therefore significantly slows down the cooling process to eliminate existing thermal gradients in part P1, thus homogenizing the part temperature before the transformation of austenitic steel into martensitic steel occurs. This significantly reduces distortions while maintaining the performance of the hardening process 100 with respect to mechanical properties.

[0076] In this regard, the hardening process 100 according to the invention uses a speed v r 2. Cooling two orders of magnitude lower than the first speed v ri cooling used during the first sub-stage 132 of cooling. This second speed v rA second cooling rate, ranging from 2°C per minute to 10°C per minute, is applied as soon as the second threshold temperature TS2 is reached, and therefore at the end of the first sub-stage 132 of cooling. This second rate v r 2. Cooling is maintained until the surface 20 of the part reaches the third threshold temperature TS3. It should be noted again that while it is advantageous for the entire surface 20 to reach the third threshold temperature Tsa at the end of the second sub-stage 134 of cooling to ensure the homogeneity of conditions at the surface 20, it is not mandatory that the entire surface 20 has begun its transformation into martensitic steel and only a part of the surface may have begun its transformation.

[0077] Taking the example in which the surface 20 of the part P1 made of M50NIL steel was cooled from 1100°C to 600°C at a first speed v riwhere the cooling rate is greater than or equal to 200°C per minute during the first sub-stage 13 of cooling, the second cooling rate Vr2 can advantageously be set at 3°C ​​per minute during the second sub-stage 134 of cooling.

[0078] Preferably, the third threshold temperature Tsa is between 250°C and 400°C. Indeed, the temperature T ma The temperature at which the transformation, at least partial, of austenitic steel into martensitic steel begins is also within this temperature range and depends on the initial composition of the steel from which part P1 is made.

[0079] Continuing with the example in which surface 20 of part P1 made of M50NIL steel was cooled from 1100°C to 600°C at a first speed v ri for cooling greater than or equal to 200°C per minute, the second speed v r2. Cooling can be maintained at 3°C ​​per minute between 600°C and 350°C, so in this example the second sub-stage of cooling lasts approximately 83 minutes.

[0080] Similar to what was seen in the description relating to the first speed v ri For cooling, it is possible to adjust the second speed v r 2. Cooling is achieved by modifying the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace. Advantageously, one moves from the first speed v ri cooling at second speed v r 2. Cooling by lowering the neutral gas mixing speed in the furnace and / or the pressure in the furnace from 60% to 95% compared to the speed and / or pressure used in the first cooling sub-step 132. This allows switching from the first speed v ri cooling rate greater than or equal to 200°C / min at a second speed vr 2. Cooling rate between 2°C / min and 10°C / min, thus reducing the cooling rate by nearly two orders of magnitude. For example, the pressure in the oven can be reduced to a value close to 1 bar. In practice, the pressure and mixing rate are reduced as soon as the first cooling sub-stage 132 is completed.

[0081] The third quenching step 130 further comprises a third substep 136 of cooling the workpiece P1 at a third speed v r 3. Cooling rate greater than or equal to 10°C per minute such that the surface temperature 20 changes from the third threshold temperature Tss to an extraction temperature T Eof the oven. The oven extraction temperature TE is the temperature at which part P1 can be extracted from the oven using basic gripping means and / or basic personal protective equipment, such as gloves. The extraction temperature T E is advantageously between 40°C and 60°C, preferably equal to 50°C.

[0082] This third cooling sub-step 136 minimizes carbon migration between the austenitic and martensitic phases by increasing the third velocity v r 3. Cooling. This therefore prevents the carbon from partitioning in part P1. The third speed v r 3. Cooling must therefore be high enough for the alloy forming the steel to stabilize permanently in a martensitic microstructure.

[0083] At this point, it is worth recalling that the surface temperature 20 at the beginning of the third sub-stage 136 of cooling is not necessarily the same as the temperature T ma r of initiation of the transformation of austenitic steel into martensitic steel. Indeed, when the third sub-stage 136 of cooling begins, the third threshold temperature Tsa can be less than or equal to the temperature T ma r of initiation of the transformation, at least partial, of austenitic steel into martensitic steel at the level of surface 20, as seen previously.

[0084] As previously seen for the first v ri and second v r 2 cooling speeds, it is possible to adjust the third speed v r2 of cooling by modifying the speed of mixing of the neutral gas in the furnace and / or the pressure in the furnace compared to the values ​​used during the second sub-step 134 of cooling.

[0085] The implementation of such a hardening process does not reduce the core hardness of part P1 when it is made of M50NIL steel because the bainitic transformation nose is not penetrated. This is also the case for the steels whose composition was explained at the beginning of the detailed description. Following the hardening process according to the invention, a martensitic steel part P2 is obtained with homogeneous core and surface microstructures, and the surface exhibits a hardness between 60 HRC and 68 HRC for a part P1 made of M50NIL steel with a surface carbon mass percentage of 0.7%. A bearing made of steel with such hardness exhibits improved wear resistance compared to other bearings.

[0086] That being said, as can be understood, the experimental conditions, particularly the pressure, chamber temperature, and mixing speed, depend on the furnace used. These experimental conditions are therefore likely to vary from one furnace to another without departing from the inventive concept underlying the present invention. Furthermore, during the austenitizing step 120, it is possible to define temperature ramps and / or intermediate heating plateaus without departing from the scope of the invention.

[0087] With reference to Figure 5, the invention further relates to a method 200 for manufacturing a part P2 out of martensitic steel

[0088] The manufacturing process 200 comprises a first step 210 consisting of carrying out the vacuum hardening process 100 as previously defined. At the end of this step, a martensitic steel part P2 with improved hardness is obtained. The manufacturing process 200 comprises a second step 220 consisting of treating the martensitic steel part P2 obtained at the end of step 210. Such treatments may, for example, consist of a cryogenic step or tempering. Other treatment steps, known to those skilled in the art and carried out after prior art hardening processes, may also be carried out without prejudice to the inventive concept underlying the present invention.

[0089] The manufacturing process 200 may optionally include a third step 230 consisting of performing finishing work on the part P2. The configurations shown in the cited figures are only possible examples, in no way limiting, of the invention which, on the contrary, encompasses design variants within the grasp of a person skilled in the art.

Claims

DEMANDS 1. A process (100) for hardening in a furnace a steel part (P1) comprising a core (10) and a surface (20), the process (100) comprising the following steps: (120) carrying out austenitization of the part (P1) under vacuum at a first threshold temperature (Tsi) so as to transform the steel into austenitic steel, (130) to quench the part (P1) under a neutral gas atmosphere in three sub-steps so as to transform the austenitic steel into martensitic steel and obtain a part (P2) in martensitic steel, the quenching step (130) comprising the following sub-steps: (132) cool the part (P1) at a first speed (v r i) cooling greater than or equal to 200°C per minute such that the surface temperature (20) changes from the first threshold temperature (Tsi) to a second threshold temperature (TS2), (134) cool the part (P1) at a second speed (v r(g) cooling rate of between 2°C per minute and 10°C per minute such that the surface temperature (20) changes from the second threshold temperature (TS2) to a third threshold temperature (TS3), the third threshold temperature (TS3) being less than or equal to a temperature (T ma r) initiation of a transformation, at least partial, of austenitic steel into martensitic steel, the surface (20) reaching the temperature (T ma r) initiation of the transformation, at least partial, of austenitic steel into martensitic steel during the second sub-stage (134) of cooling, (136) cool the part (P1) at a third cooling rate (Vrs) greater than or equal to 10°C per minute so that the surface temperature (20) goes from the third threshold temperature (Tss) to an extraction temperature (TE) of the furnace.

2. Vacuum curing process (100) according to claim 1, wherein the second threshold temperature (TS2) is between 400°C and 700°C.

3. A hardening process (100) according to any one of claims 1 or 2, wherein the third threshold temperature (TS3) is between 250°C and 400°C.

4. A hardening process (100) according to any one of the preceding claims, wherein the extraction temperature (TE) is between 40°C and 60°C, preferably about 50°C.

5. A hardening method (100) according to any one of the preceding claims, wherein the first speed (v) is changed r i) Second speed cooling (Vr2) by lowering the neutral gas stirring speed in the furnace and / or the pressure in the furnace from 60% to 95%.

6. A hardening method (100) according to claim 5, wherein the first speed (v) is adjusted ri) cooling, the second speed (v r g) cooling and the third speed (Vrs) of cooling by changing the speed of stirring of the neutral gas in the furnace and / or the pressure in the furnace.

7. A hardening method (100) according to any one of the preceding claims, wherein the neutral gas is selected from nitrogen, helium, argon and hydrogen.

8. A hardening process (100) according to any one of the preceding claims, wherein, concurrently with austenitizing, the furnace is placed under vacuum during the austenitizing step (120), and, at the end of the austenitizing step (120), the furnace (125) is filled with neutral gas.

9. A hardening process (100) according to any one of the preceding claims, wherein, at the supply step (110), the core (10) comprises a mass percentage of: carbon strictly greater than 0% and strictly less than 3%, chromium strictly greater than 2% and strictly less than 20%, and / or molybdenum strictly greater than 1% and strictly less than 10%, and / or vanadium strictly greater than 0% and strictly less than 5%, and / or cobalt strictly greater than 0% and strictly less than 20%, and / or nickel strictly greater than 0% and strictly less than 10%, and / or tungsten strictly greater than 0% and strictly less than 10%, and / or a manganese and silicon alloy strictly greater than 0% and strictly less than 3%.

10. A hardening process (100) according to claim 9, wherein, during the supply step (110), the surface (20) comprises a mass percentage of carbon between 0.5% and 1.4%.

11. A hardening process (100) according to any one of the preceding claims, further comprising a carburizing step, said carburizing step being carried out before the austenitizing step (120) or simultaneously with the austenitizing step (120).

12. Hardening method (100) according to any one of the preceding claims, wherein the steel part (P1) is a turbomachine part, and in particular a transmission part.

13. Method (200) for manufacturing a part (P2) of martensitic steel comprising the following steps: (210) carry out a vacuum curing process (100) according to any of the claims 1 to 12, (220) treat the martensitic steel part obtained at the end of step (210), (230) optionally, perform finishing work.

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