Hardening process in a furnace of a steel part and manufacturing process of a steel part

Abstract

The invention relates to a process (100) for hardening a steel part (P1) comprising a core (10) and a surface (20) in a furnace. The process (100) comprises the following steps: (120) austenitizing the part (P1) under vacuum at a first threshold temperature (Ts1) to transform the steel into austenitic steel; (130) quenching the part (P1) under a neutral gas atmosphere in three sub-cooling stages to transform the austenitic steel into martensitic steel and obtain a martensitic steel part (P2). The hardening process (100) according to the invention differs from the prior art in that it consists of a hardening process with modulated quenching. Figure for the abstract: Fig. 4

Description

Title of the invention: METHOD FOR HARDENING A STEEL PART IN A FURNACE AND METHOD FOR MANUFACTURED A STEEL PART Technical field of the invention

[0001] 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. Technical background

[0002] 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.

[0003] 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, which results from the diffusion of carbon onto the surface of the part, improves the wear resistance and mechanical properties of the steel part.

[0004] Hardening generally consists of two stages. In the first austenitizing stage, the part is heated for a predetermined time to a target temperature and acquires an austenitic structure. Then, in the second quenching stage, 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 cold water quenching, while others can simply be cooled in still air. This obviously depends on the cooling rate observed on the part, and therefore on the mass of the part.In any case, the conditions under which quenching takes place play a critical role in obtaining the desired mechanical properties, e.g. its hardness, and the characteristics of the steel in terms of alloying.

[0005] Notably, manufacturers have observed the appearance of numerous distortions in parts treated during hardening. These most often result from thermal gradients within the part, especially when the part is complex. These unwanted distortions must be corrected by grinding in order to bring the part into conformity with the standards of the relevant field, by For example, those in the aeronautics industry. That being said, too significant a correction is technically and industrially undesirable.

[0006] The prior art has proposed numerous methods for reducing the distortion of a martensitic steel part, and thus preventing its deformation. According to a first approach, distortions are reduced by adjusting the quenching conditions via the atmosphere. According to a second approach, distortions are reduced 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 according to the geometry and / or the speed of the part, which means that some processes are not suitable for industrial application. According to a third approach, the part is constrained by one or more formers during quenching, which prevents it from deforming.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 the formers themselves undergo due to the processing temperatures.

[0007] 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.

[0008] In the first step, the part undergoes austenitization. For this purpose, the part is heated under vacuum in a manner appropriate to transform the steel into the austenitic (face-centered cubic) phase while preventing oxidation of the part. Once the austenitization of the part is complete, the chamber in which the part is located 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 steel in the austenitic phase into martensitic steel at the phase change temperature of the alloy steel in question. Each chemical element, with the exception of cobalt, lowers the temperature of the part. This phase change also results in a change in microstructure, as the steel transitions from a face-centered cubic microstructure 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 homogeneous in space if the change in microstructure 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, makes it possible to achieve this condition, but many undesirable transformations occur before the phase change to martensitic steel.

[0009] Conversely, rapid cooling creates more or less pronounced thermal gradients in the part, at a minimum between the core and the surface layer, depending on The geometry of the part, the furnace, the cooling method, etc., all play a role. 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. The untransformed areas, which still exhibit a face-centered cubic microstructure, are easily deformable. They will therefore deform and transform later during cooling. This results in a part with a different geometry than the initial one, 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.

[0010] 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. Summary of the invention

[0011] 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: - to perform austenitization of the part under vacuum at a first threshold temperature Tsi in order to transform the steel into austenitic steel, - to perform a quenching of the part under a neutral gas atmosphere in three sub-steps so as to transform the austenitic steel into martensitic steel and obtain a martensitic steel part, the quenching step comprising the following sub-steps: > cool the part at a first cooling rate vri greater than or equal to 200°C per minute so that the surface temperature goes from the first threshold temperature Tsi to a second threshold temperature Ts2, > cool the part at a second cooling rate vr2 of between 2°C per minute and 10°C per minute so that the surface temperature goes 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 Tmar of initiation of the transformation, at least partial, of the austenitic steel into martensitic steel, > cool the part at a third cooling rate vr3 greater than or equal to 10°C per minute so that the surface temperature goes from the third threshold temperature Ts3 to an extraction temperature TE of the furnace.

[0012] The process according to the invention makes it possible to solve the aforementioned problems of the prior art. In this respect, the hardening process according to the invention comprises a quenching stage with three cooling sub-stages which differ significantly from each other in the cooling rates implemented.

[0013] The initial cooling rate vH 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.

[0014] The second cooling rate vr2 aims to homogenize the room temperature before the phase change occurs. This second cooling rate vr2, ranging from 2°C per minute to 10°C per minute, is applied as soon as the second threshold temperature Ts2 is reached. It is both low and sufficient to eliminate the existing thermal gradients in the room before the phase change occurs. This second cooling rate vr2 is maintained until the third threshold temperature Ts3.

[0015] The third cooling rate vr3 minimizes carbon migration between the austenitic and martensitic phases. It must therefore be sufficiently high to prevent such migration and allow for structural stabilization.

[0016] 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.

[0017] According to various features of the invention which may be considered together or separately: • The second threshold temperature Ts2 is between 400°C and 700°C. • The third threshold temperature Ts3 is between 250°C and 400°C, • The extraction temperature TE is between 40°C and 60°C, preferably around 50°C • We switch from the first cooling speed vH to the second cooling speed vr2 by lowering the neutral gas mixing speed in the furnace and / or the pressure in the furnace from 60% to 95%, • The first cooling speed vr i, the second cooling speed vr 2, and the third cooling speed vr 3 are adjusted by modifying the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace. • We move from the second cooling speed vr 2 to the third cooling speed vr 3 by increasing the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace, • The neutral gas is chosen from nitrogen, helium, argon, and hydrogen. • Concurrently with austenitization, the oven is placed under vacuum during the austenitization stage. • Following the austenitizing step, the furnace is filled with inert gas, • During the supply stage, the core comprises a mass percentage: of carbon strictly greater than 0% and strictly less than 3%, and / or of chromium strictly greater than 2% and strictly less than 20%, and / or of molybdenum strictly greater than 1% and strictly less than 10%, and / or of vanadium strictly greater than 0% and strictly less than 5%, and / or of cobalt strictly greater than 0% and strictly less than 20%, and / or of nickel strictly greater than 0% and strictly less than 10%, and / or of tungsten strictly greater than 0% and strictly less than 10%, and / or of manganese and silicon alloy strictly greater than 0% and strictly less than 3% • During the supply stage, the surface comprises a mass percentage of carbon between 0.5% and 1.4%, • the process further comprises a cementation step, said cementation step being carried out before or simultaneously with austenitizing, • The steel part is a turbomachine part, and in particular a transmission part such as a pinion or a bearing.

[0018] The invention further relates to a method for manufacturing a part made of martensitic steel comprising the following steps: - carry out a vacuum hardening process as previously described, - treat the martensitic steel part obtained at the end of the previous step, - optionally, carry out finishing. Brief description of the figures

[0019] Other objects, features and advantages of the invention will become more apparent in the following description, made with reference to the accompanying figures, in which:

[0020] - Fig. 1 is a schematic view illustrating the different stages of a process of hardening according to an embodiment of the invention,

[0021] - Figure 2 illustrates a steel part that can be used in the context of the invention,

[0022] - [Fig.3] illustrates a sectional view of the part illustrated in [Fig.2],

[0023] - Figure 4 illustrates a cooling curve according to a continuous quenching process such that implemented in the prior art and a cooling curve according to a quenching process implemented in the hardening process that is the subject of the present invention,

[0024] - [Fig. 5] is a schematic view illustrating the different stages of a process manufacturing a part in martensitic steel according to an embodiment of the present invention.

[0025] In the figures illustrated, optional steps are indicated by boxes with dotted lines. Detailed description of the invention

[0026] The invention relates to a method of hardening a steel part PI in a furnace.

[0027] Part PI can be any steel part. For example, by way of non-limiting example, part PI can be a turbomachine component, and in particular a transmission component, specifically one involved in mechanical stresses. The transmission component can be a case-hardened steel pinion or case-hardened steel bearings.

[0028] The present invention is of particular interest for steel parts where it is desired to reduce distortions 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.

[0029] 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 of chromium strictly greater than 2% and strictly less than 20%, and / or of molybdenum strictly greater than 1% and strictly less than 10%, and / or of vanadium strictly greater than 0% and strictly less than 5%, and / or of cobalt strictly greater than 0% and strictly less than 20%, and / or of nickel strictly greater than 0% and strictly less than 10%, and / or of tungsten strictly greater than 0% and strictly less than 10%, and / or of manganese and silicon alloy strictly greater than 0% and strictly less than 3%.

[0030] 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% of 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.

[0031] 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 PI to be treated can be placed.

[0032] 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 faster the mixing speed, the faster the cooling rate.

[0033] In addition, the oven may advantageously have a control interface allowing adjustment of the temperature, pressure and gas mixing speed in the chamber.

[0034] With reference to [Fig. 1] and Figures 2 and 3, the hardening process 100 typically includes a first step 110 of supplying the steel part PI. The part PI may have previously undergone a conventional case hardening step during which its surface was enriched with carbon. In practice, the part PI thus 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 consisting of austenitizing 120 instead of being carried out before the austenitizing 120.

[0035] Advantageously, the surface 20 is made of a steel comprising a mass percentage of carbon 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 having a hardness between 60 HRC and 68 HRC after the hardening process 100, which makes the surface 20 of part PI a very high hardness surface and therefore makes it particularly suitable for the manufacture of bearings, for example for turbomachinery.

[0036] The hardening process 100 comprises the second step 120 of austenitizing the part PI under vacuum at a first threshold temperature Tsi so as to transform the steel into austenitic steel. More precisely, at the end of this austenitizing step 120, the entire microstructure of the part PI, that is to say the entire crystallographic structure of the part PI, is transformed into a structure Austenitic. When in the austenitic phase, steel exhibits a face-centered cubic crystallographic structure. The first threshold temperature (Tsi) varies depending on the initial composition of the steel part. 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 part made of M50NIL reference steel, the first threshold temperature (Tsi) can advantageously be 1100°C.

[0037] Preferably, during austenitization 120, the vacuuming of the furnace can be carried out concurrently with the heating of the furnace, which makes it possible to avoid oxidation of the part PI and a degradation of its properties, in particular its mechanical properties.

[0038] With further reference to [Fig. 1], the hardening process 100 comprises, following this second austenitizing step 120, a third quenching step 130 of the part PI under a neutral gas atmosphere. According to the invention, this third quenching step 130 takes place 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.

[0039] Implementing this third quenching step 130 under a neutral gas atmosphere prevents oxidation of the steel part PI during cooling. The neutral gas used is therefore neutral 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.

[0040] The third quenching step 130 comprises a first substep 132 consisting of cooling 132 the workpiece PI at a first cooling rate vri greater than or equal to 200°C per minute so that the surface temperature 20 drops from the first threshold temperature Tsi to a second threshold temperature Ts2. A first cooling rate vri greater than or equal to 200°C per minute prevents the precipitation of carbides and, more generally, undesirable phases during cooling.

[0041] Indeed, if the first cooling rate vri is insufficient, the cooling dynamics of the surface 20 of the part PI are altered, and the microstructure changes on the 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 the austenitic steel into steel martensitic by inducing microscopic mechanisms that allow such a transformation but also prevent further precipitation of the carbides.

[0042] As illustrated in Figure 4, such a cooling rate makes it possible to lower the temperature of the surface 20 of part PI 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 indicated by the reference numeral EA, while the temperature curve associated with an embodiment of the invention is indicated by the reference numeral INV. 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, refer only to the temperature curve INV.

[0043] In this regard, it is advantageous that the entire surface 20 reaches the second threshold temperature Ts2 at the end of the first sub-step 132 of cooling to ensure the homogeneity of conditions at the surface 20.

[0044] 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 PI is made but also on the mass percentage of carbon present on the surface of part PI. 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 PI made of M50NIL steel which also has a mass percentage of carbon of 0.7% on its surface 20, the first cooling substep 132 would therefore consist of reducing the temperature of surface 20 from 1100°C to 600°C at a first cooling rate vH 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 speed of 200°C / min.

[0045] There are various methods for achieving such a cooling rate. That being said, it is very advantageous to adjust the first cooling rate vri by modifying the mixing speed of the neutral 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 125 is filled with the desired neutral 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 a first cooling rate vri greater than or equal to 200°C / min.

[0046] The third quenching step 130 further comprises a second cooling substep 134 consisting of cooling the part PI at a second rate vr2 The cooling rate is between 2°C per minute and 10°C per minute, so that the surface temperature 20 drops from the second threshold temperature Ts2 to a third threshold temperature Ts3. This third threshold temperature Ts3 is less than or equal to the temperature Tmar at which the transformation, at least partial, of austenitic steel into martensitic steel begins. Figure 4 illustrates a case in which the second cooling rate vr2 is 4°C / min. The slope of the temperature profile is much less steep than during the first cooling substage 132, illustrating the significant decrease in the cooling rate.

[0047] 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. On the other hand, 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.

[0048] The second cooling substep 134 makes it possible to obtain a temperature difference between the surface 20 of part PI and the core 10 of part PI of at most 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.

[0049] In this regard, it should be noted that carbon lowers the transformation point, namely the temperature at which austenitic steel is transformed into martensitic steel. Thus, the duration of the second substep 134 of cooling can vary considerably from one steel to another.

[0050] For example, when part PI 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 proportions of carbon in part PL. The higher the proportions of carbon in part PI, the more advantageously the segregation phenomenon that one seeks to avoid can be controlled by adjusting the duration of the second cooling substep 134.

[0051] The second cooling substep 134 thus makes it possible to considerably slow down the cooling process in order to eliminate the existing thermal gradients in the PI part, and therefore to homogenize the temperature of the part before the transformation of the austenitic steel into martensitic steel occurs. This makes it possible to significantly reduce distortions while maintaining the performance of the hardening process 100 with respect to mechanical properties.

[0052] In this respect, the hardening process 100 according to the invention uses a cooling rate vr2 two orders of magnitude lower than the first cooling rate vri used during the first cooling substep 132. This second cooling rate vr2, between 2°C per minute and 10°C per minute, is applied as soon as the second threshold temperature Ts2 is reached, and therefore at the end of the first cooling substep 132. This second cooling rate vr2 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 Ts3 at the end of the second cooling substep 134 to ensure the homogeneity of conditions on the surface 20, this is not mandatory, and only a portion of the surface may have begun its transformation.

[0053] Taking up the example in which the surface 20 of the M50NIL steel part PI was cooled from 1100°C to 600°C at a first cooling rate vH greater than or equal to 200°C per minute during the first cooling sub-step 13, the second cooling rate vr2 can advantageously be set at 3°C ​​per minute during the second cooling sub-step 134.

[0054] Preferably, the third threshold temperature Ts3 is between 250°C and 400°C. Indeed, the temperature Tmar 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 the part PI is made.

[0055] Still according to the example in which the surface 20 of the PI part in M50NIL steel was cooled from 1100°C to 600°C at a first cooling rate vri greater than or equal to 200°C per minute, the second cooling rate vr2 can be maintained at 3°C ​​per minute between 600°C and 350°C, so that in this example the second cooling sub-stage lasts about 83 minutes.

[0056] Similar to what was seen in the description of the first cooling speed vri, the second cooling speed vr2 can be adjusted by changing the mixing speed of the neutral gas in the furnace and / or the pressure in the furnace. Advantageously, the first cooling speed vH is changed to the second cooling speed vr2 by lowering the mixing speed of the gas Neutral temperature in the furnace and / or a furnace pressure of 60% to 95% compared to the speed and / or pressure used during the first cooling sub-step 132. This allows a transition from the first cooling speed vH (greater than or equal to 200°C / min) to a second cooling speed vr2 (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 furnace can be reduced to a value close to 1 bar. In practice, the pressure and mixing speed are reduced immediately after the first cooling sub-step 132.

[0057] The third quenching step 130 further comprises a third substep 136 of cooling the workpiece PI at a third cooling rate vr3 greater than or equal to 10°C per minute, such that the surface temperature 20 drops from the third threshold temperature Ts3 to a furnace extraction temperature TE. The furnace extraction temperature TE is the temperature at which the workpiece PI can be removed from the furnace using basic gripping means and / or basic personal protective equipment, such as gloves. The extraction temperature TE is advantageously between 40°C and 60°C, preferably 50°C.

[0058] This third cooling substep 136 minimizes carbon migration between the austenitic and martensitic phases by increasing the third cooling rate vr3. This prevents carbon partitioning in the PL part. The third cooling rate vr3 must therefore be sufficiently high to ensure that the steel alloy stabilizes permanently in a martensitic microstructure.

[0059] At this stage, it should be noted that the surface temperature 20 at the beginning of the third cooling substep 136 is not necessarily the same as the Tmar temperature at which the transformation of austenitic steel into martensitic steel begins. Indeed, when the third cooling substep 136 begins, the third threshold temperature Ts3 may be less than or equal to the Tmar temperature at which the transformation of austenitic steel into martensitic steel begins, at least partially, at the surface 20, as seen previously.

[0060] As previously seen for the first vri and second vr2 cooling speeds, it is possible to adjust the third vr2 cooling speed by modifying the neutral gas stirring speed in the furnace and / or the pressure in the furnace relative to the values ​​used during the second sub-stage 134 of cooling.

[0061] Implementing such a hardening process 100 does not reduce the core hardness of part PI 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 100 according to the invention, a martensitic steel part P2 is obtained with homogeneous core and surface microstructures, and the surface 20 exhibits a hardness between 60 HRC and 68 HRC for a part PI made of M50NIL steel having a surface carbon mass percentage of 0.7%. A bearing made of steel with such hardness exhibits improved wear resistance compared to other bearings.

[0062] That being said, as can be understood, the experimental conditions, in particular 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. Moreover, 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.

[0063] With reference to [Fig. 5], the invention further relates to a method 200 for manufacturing a part P2 in martensitic steel

[0064] The manufacturing process 200 comprises a first step 210 consisting of implementing the vacuum hardening process 100 as previously defined. At the end of this step, a martensitic steel part P2 with improved hardness is obtained.

[0065] 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 a tempering step. Other treatment steps, known to those skilled in the art and carried out using prior art hardening processes, may also be implemented without prejudice to the inventive concept underlying the present invention.

[0066] The manufacturing process 200 may optionally include a third step 230 consisting of performing finishing work on the part P2.

[0067] The configurations shown in the cited figures are only possible examples, by no means limiting, of the invention which on the contrary encompasses the variants of designs within the reach of the person skilled in the art.

Claims

Demands

1. A process (100) for hardening a steel part (PI) comprising a core (10) and a surface (20) in a furnace, the process (100) comprising the following steps: (120) austenitizing the part (PI) under vacuum to a first threshold temperature (Tsi) so as to transform the steel into austenitic steel, (130) quenching the part (PI) under a neutral gas atmosphere in three substeps so as to transform the austenitic steel into martensitic steel and obtain a martensitic steel part (P2), the quenching step (130) comprising the following substeps: (132) cooling the part (PI) at a first cooling rate (vri) greater than or equal to 200°C per minute so that the temperature of the surface (20) changes from the first threshold temperature (Tsi) to a second threshold temperature (Ts2),(134) cool the part (PI) at a second cooling rate (vr2) 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 (Tmar) at which a transformation, at least partial, of the austenitic steel into martensitic steel begins, (136) cool the part (PI) at a third cooling rate (vr3) greater than or equal to 10°C per minute such that the surface temperature (20) changes from the third threshold temperature (Ts3) 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 cooling speed (vri) to the second cooling speed (vr2) by lowering a neutral gas stirring speed in the furnace and / or a pressure in the furnace from 60% to 95%.

6. A hardening method (100) according to claim 5, wherein the first cooling rate (vri), the second cooling rate (vr2) and the third cooling rate (vr3) are adjusted by changing the mixing rate 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. A hardening method (100) according to any one of the preceding claims, wherein the steel part (PI) is a turbomachine part, and in particular a transmission part.

13. A method (200) for manufacturing a part (P2) from martensitic steel comprising the following steps: (210) carrying out a vacuum hardening process (100) according to any one of claims 1 to 12, (220) treating the martensitic steel part obtained at the end of step (210), (230) optionally, carrying out finishing.