Method for forging a metal component

Abstract

The invention relates to a forging method comprising: - one or more forging steps in order to obtain a component comprising fine grains via a complete recrystallization of the entire component; - a solution heat treatment step; - a tempering heat treatment step; the forging method being characterized in that it further comprises, after the forging steps and before the solution heat treatment step, a specific forging step applied only to an outer portion of the component, the specific forging step being carried out at a temperature of between 1000°C and 1080°C, with a strain of between 0.1 and 0.5, at a strain rate of between 0.05 s-1 and 1.0 s-1.

Description

Forging process of a metal part Technical Field

[0001] The invention relates to a method for forging a metal part. Previous technique

[0002] Metallic alloys are useful for a wide range of applications, particularly in the aerospace industry.

[0003] However, in general, metals or metal alloys are known to have properties that depend on their thermomechanical history; that is, a part with the same shape and made of the same alloy will not have the same properties if it was produced by additive manufacturing or by casting. Even more specifically, the conditions applied during a given process also influence the characteristics of the final part.

[0004] Indeed, the microstructure, characterized in particular by grain size, the presence of twinning, and dislocation density, has a significant effect on the mechanical behavior of a given metal part. This microstructure depends on the manufacturing conditions of the part.

[0005] The problem of determining an optimal microstructure, and therefore a suitable manufacturing process, must be addressed according to the purpose of the part to be manufactured and must be optimized again if the characteristics of the part are changed.

[0006] This is particularly the case for aeronautical turbomachinery parts.

[0007] Indeed, with a view to improving the efficiency of new turbomachines, and thus reducing fuel consumption, there is a constant search for ways to increase the operating temperature and / or the rotational speed, particularly in certain turbine stages.

[0008] This constraint therefore requires reviewing the properties and thus the microstructures of the parts closest to the hot air flow, and optimizing their characteristics, if it turns out that these need to be improved.

[0009] Among the affected parts are the nickel-based superalloy turbine discs. These parts are annular and subjected to particularly high stress. Indeed, the disc rim, which is the peripheral area of ​​the part, operates at a high temperature due to its proximity to the hot air flow and at a high rotational speed. Elsewhere, the disc bore, which is the central area, operates at a lower temperature and is subjected to very high centrifugal forces when the disc rotates at high speed. Historically, fine-grained microstructures have been proposed, particularly suited to the yield strength and fatigue resistance constraints encountered by discs.

[0010] However, such microstructures are more susceptible to high-temperature cracking and creep, which becomes a disadvantage if one wishes to increase the operating temperature of turbomachines.

[0011] Nevertheless, a microstructure with large grains exhibits better creep and high-temperature crack resistance. Therefore, to improve the performance of highly stressed turbine disks, it has been proposed to adapt the microstructure, and thus the associated mechanical properties, to the local stresses of the disk. This has been achieved by introducing grain size gradients, where the disk rim is composed of large grains and the bore of fine grains, using different methods (GF Mathey (1994) Method of making superalloy turbine disks having graded coarse and fine grains, US5312497A; S. Ganesh, R. Tolbert (1995) Differentially heat treated article, and apparatus and process for the manufacture thereof, US5527020A; J. Gayda, D. Furrer (2003) Dual-micro structure heat treatment, Advanced Materials & Processes 161; Rl Mitchell, DU Furrer, JA Lemsky, MCHardy (2009) A method of heat treating a superalloy component and an alloy component, W02009019418A1; -Mr. Franchet, G. Klein (2017) Device for generating a structural-gradient microstructure on an axisymmetric part, WO2017077248A1).

[0012] However, many problems then arise.

[0013] Indeed, to obtain larger grains at the periphery of the part while maintaining fine grains at the center, it is generally recommended to apply a thermal gradient across the component during heat treatment, starting from fine microstructures. The rim of the disc is subjected to a temperature higher than the dissolution temperature of the phases that impede grain boundary movement (this threshold temperature is known as the solvus temperature). This so-called "super-solvus" heat treatment leads to the formation of large grains in the disc rim, due to the absence of grain boundary anchoring by y' phase precipitates in y / y' superalloys. Furthermore, the disc bore is treated at a temperature lower than this solvus temperature. Thus, the microstructure of this area can retain the finer grain size resulting from forging thanks to the grain boundary anchoring effect of y' phase precipitates.

[0014] However, applying temperature gradients across the part increases the likelihood of significant residual stresses present in the final part during cooling. Therefore, the cooling process after this gradient heat treatment is relatively slow to avoid generating high residual stresses due to the temperature difference between the rim and the bore, which could potentially lead to disc distortion.

[0015] This slow cooling also requires additional heat treatment to optimize the hardening precipitates and thus obtain the required mechanical properties.

[0016] It is known that grain growth is caused both by the capillary force resulting from the difference in grain sizes and the difference in internally stored energy across grain boundaries during high-temperature heat treatment.

[0017] All existing gradient microstructure generation processes are based on the same principle: maintaining a temperature gradient across the component during heat treatment for grain growth via capillary force only, but without playing on the difference in stored energy.

[0018] However, the limitations of existing solutions are related to their ability to be applied on a large scale, as an installation can only process one disk at a time.

[0019] This is why there remains a need for a process that would allow for a simplified production of a disc whose bore has grains of a different size than the rim grains and which presents a possibility of easier industrial application. Description of the invention

[0020] The invention is specifically designed to address this need.

[0021] To achieve this, it proposes a process for forging a metal part comprising: - one or more forging stages carried out at a sub-solvus temperature between 1000°C and 1080°C, with a strain greater than or equal to 0.5, at a strain rate less than or equal to 1.0 s -1 in order to obtain a piece containing fine grains via complete recrystallization of the entire piece; - a solution stage carried out by heat treatment at a temperature between 1050°C and 1080°C for a period of 30 minutes to 2 hours; - a tempering stage carried out by heat treatment at a temperature between 700 °C and 800 °C for a period of 6 to 12 hours; the forging process being characterized in that it further comprises, after the forging steps and before the solution treatment step, a specific forging step applied only to an external portion of the part, the specific forging step being carried out at a temperature between 1000°C and 1080°C, with a deformation between 0.1 and 0.5, at a deformation rate between 0.05 and 1.0 s. 1 .

[0022] A fine grain structure is understood to be a structure in which the grains are blocked by the y' or 5 phase, and a coarse grain structure is a structure in which the grains are no longer blocked by these phases. In general, the grain size in a structure is given according to the ASTM standard in which 1 ASTM corresponds to a grain of 225 pm, i.e. a large grain, and 10 ASTM corresponds to a grain of 10 pm, i.e. a fine grain. In general, for any type of alloy, a grain size is considered to be that of a fine grain if it is greater than 9 ASTM, advantageously greater than or equal to 10 ASTM. Depending on the alloy type, the grain size threshold for being considered coarse varies. Generally, it must be less than 8.0 ASTM, and specifically less than or equal to 7 ASTM. This is particularly true for powder metallurgy alloys (N18, N19). For conventional alloys such as AD730, R65, and U720, a grain is considered coarse if its size is less than 4 ASTM, and for Inco718, depending on the temperature reached, a grain is considered coarse if its size is between 3 and 6 ASTM.

[0023] Understanding the stresses experienced by the different parts of the metal piece allowed the inventors to propose the particular forging process.

[0024] The forging stage(s) make it possible to obtain throughout the piece a small-grained microstructure, similar to the microstructures of the earlier art.

[0025] However, the specific forging step, which is newly added compared to the processes of the prior art, is carried out under conditions that promote grain growth and is applied only to the external portion of the piece.

[0026] Thus, the proposed process makes it possible on the one hand to maintain a microstructure with a small grain size for the internal portion of the part and on the other hand to promote grain growth for the external portion of the part.

[0027] Furthermore, the invention's method of solution treatment at a single, homogeneous temperature eliminates the need for temperature gradients. This avoids the residual stresses observed in prior art processes that modify the microstructure. It also allows for the simultaneous treatment of multiple parts in a single conventional solution treatment furnace, which is not possible with pre-existing solutions.

[0028] The specific forging step is also performed under conditions that allow not only for larger grain sizes than those constituting the internal part of the component, but also for a high twin density with a clear predominance of inconsistent joints. According to the scientific literature, this particularity of the conditions applied for the specific forging step and the resulting local microstructure could ensure improved resistance to crack propagation and fatigue, and confer better resistance to intergranular corrosion.

[0029] Without wanting to be bound by theory, the inventors are of the opinion that the specific forging step allows the growth of grains in the rim portion by a particular mechanism and that one wishes to avoid in the processes of the prior art.

[0030] Indeed, it has been observed in deformed samples treated at a homogeneous sub-solvus temperature as part of a phenomenon called "grain bursting" (Charpagne, M.-A., Franchet, J.-M. and Bozzolo, N. (2018) 'Overgrown grains appearing during sub-solvus heat treatment in a polycrystalline yy' nickel-based superalloy', Materials & Design, 144, pp. 353-360).

[0031] The grain bursting phenomenon involves the generation of large grains via a driving force from the difference in stored energy, thus overcoming the anchoring effect of y' precipitates in nickel-based y / y' alloys.

[0032] Depending on the distribution of energy stored in the microstructure subjected to heat treatment, the grain bursting phenomenon can lead to very large grains in a finer grain microstructure or to grain size gradients comparable to those produced by the processes described previously.

[0033] Grain bursting is usually considered a problematic phenomenon, when large burst grains form in unwanted areas, such as the flange or bore of the part.

[0034] However, it is to the inventors' credit that they considered that by causing this bursting only at the rim, it could have a beneficial effect on the holding creep and cracking similar to that produced by existing solutions via gradient heat treatment.

[0035] In this way, we have a process free from the disadvantages of existing solutions while allowing access to parts with at least equivalent characteristics.

[0036] Furthermore, the proposed process allows for a complete procedure with a duration comparable to prior art forging processes, thus ensuring good industrial applicability. In this application, the deformation Σ obtained during a process step corresponds to the local deformation at point D within the workpiece. It is defined by the following relationship. [Math 1]

[0037] where dj is the initial distance between this point D and a point D' near D, and df is the distance between this point D and D' after forging.

[0038] The strain rate e is the first derivative with respect to time of the strain Σ.

[0039] In one embodiment, the local strain distribution ε and strain rate ε are calculated by a macroscopic forging simulation using the finite element method. This simulation includes a finite element mesh of the workpiece and takes into account the thermomechanical properties of the material, as well as a model that incorporates the thermomechanical properties of the superalloy under consideration. The model may also include heat exchange between the workpiece, the tooling, and / or the environment. The model may also include friction between the forged workpiece and the tooling.

[0040] In one embodiment, the part can be an aeronautical turbomachine disc.

[0041] Indeed, for such parts, the properties obtained allow for excellent performance.

[0042] In particular, the microstructures obtained by the proposed process ensure, on the one hand, that a microstructure with a small grain size is maintained for the internal portion of the disk which is subjected to tensile stresses and fatigue during operation, and on the other hand, to promote the growth of large grains for the external portion of the part which is subject to cracking and creep stresses.

[0043] The resulting part thus exhibits particularly suitable resistances for a turbomachine disk.

[0044] In one embodiment, the process can include between 2 and 5 forging steps.

[0045] In one embodiment, the forging steps may include crushing, punching and / or die-forging steps.

[0046] In one embodiment, the forging steps consist of a crushing step and a die-forging step.

[0047] These steps are particularly useful for obtaining the desired microstructure when the part is a turbomachine disk.

[0048] In one embodiment, the heating ramps required to reach the temperatures of the different stages can be between 5°C / min and 30°C / min.

[0049] In one embodiment, the required cooling ramps can be between -70°C / min and -250°C / min.

[0050] This embodiment offers an excellent compromise between cooling that is too slow, which would form precipitates that are too large and impair mechanical properties, and cooling that is too rapid, which could cause excessive residual stresses.

[0051] In one embodiment, the specific forging step can be carried out with a deformation between 0.1 and 0.3.

[0052] In one embodiment, the specific forging step can be carried out at a deformation rate between 0.05 s -1 and 0.1 s' 1 .

[0053] In one embodiment, the part undergoing the described steps can be a disk or an annular part.

[0054] If necessary, a hollowing step can be introduced into the process to allow the production of an annular part from a disc-shaped part.

[0055] Here and in the request we will understand that an annular piece has the shape of a hollowed-out disc.

[0056] The radial dimension will be the dimension of the part characterizing the extent of the part between its center and its outer circumference.

[0057] Here and in the application, "outer portion" or "rim" is understood as a portion comprising at least the outer circumference of the part.

[0058] Conversely, the "internal portion" is understood as the part of the piece that is not the external portion.

[0059] In one embodiment, the external portion extends from the external circumference over at least 5% or even over at least 50% of the radial dimension, advantageously remaining less than 85% of the radial dimension.

[0060] In one embodiment, the metal part can be made of a metal alloy chosen from nickel-based alloys, for example, from the Y / Y' alloys named Udimet 720, AD 730.

[0061] According to another aspect, the invention further relates to a metal part comprising an internal portion and an external portion, characterized in that the grains of the internal portion are smaller than the grains of the external portion.

[0062] In one embodiment, the internal portion of the part comprises grains whose grain size is greater than or equal to 10 ASTM.

[0063] The ASTM unit is understood as the determination of grain size according to the standard of the American standard ASTM El 12, classically used in the field of metallography.

[0064] Traditionally, grain size determination can be done by methods known as such for image analysis that statistically measure the number of grains per unit area.

[0065] In one embodiment, the outer portion of the part comprises grains whose grain size is less than or equal to 8.0 ASTM.

[0066] In one embodiment, the invention relates to a metal part comprising an inner portion and an outer portion, characterized in that the grains of the inner portion are smaller than the grains of the outer portion and characterized in that the grains of the inner portion have a grain size greater than or equal to 10 ASTM and in that the grains of the outer portion have a grain size less than or equal to 8.0 ASTM.

[0067] In one embodiment, the part can be obtained by a process described above. Brief description of the drawings

[0068] [Fig. 1] Figure 1 schematically represents a part obtained after the first stages of a conventional forging process according to an embodiment of the invention for generating fine grains.

[0069] [Fig. 2] Figure 2 schematically represents a part obtained after all the steps of a forging process, including solution heating and tempering according to an embodiment of the invention of generating gradient microstructures. Description of the implementation methods

[0070] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0071] The figures do not show parts to scale, nor even to relative scale. They serve only to illustrate embodiments to facilitate understanding of the invention.

[0072] Figure 1 illustrates a part 100 after one or more forging steps carried out at a sub-solvus temperature between 1000°C and 1080°C, with a strain greater than or equal to 0.5, at a strain rate less than or equal to 1.0 s -1 in order to obtain a classic fine grain disc via complete recrystallization of the entire piece.

[0073] As shown in Figure 1, part 100 has, after these steps, a microstructure in which the grains 11 are small in size.

[0074] In the figures, part 100 is annular but we do not go outside the scope of the invention with a part forming a disc.

[0075] For example, part 100 includes, after these steps, grains whose dimension is greater than or equal to 10 ASTM.

[0076] Essentially, the representation of grains 11 on figure 1 is made by hexagons, but it is understood that the actual grains may have more irregular dimensions.

[0077] The representation with hexagons is schematic and is only intended to offer an illustrative representation of the grains.

[0078] Grain size is measured by methods known as such and preferably by image analysis methods that statistically measure the number of grains per unit area.

[0079] Among them are the comparison procedure, the planimetric procedure, and the interception procedure, performed on a micrographic image of the sample.

[0080] The size of 10 ASTM obtained at the end of the forging steps ensures a satisfactory initial state for the specific forging step.

[0081] Once the forging steps have been completed, the process then includes a specific forging step applied only to an external portion of the part, the specific forging step being carried out at a temperature between 1000°C and 1080°C, with a deformation of less than 0.5 at a deformation rate of less than 1.0 s' 1 .

[0082] The specific forging step allows obtaining a coarse-grained microstructure, only for the external portion 22 of part 100 shown in figure 2.

[0083] More specifically, the conditions are precisely determined to allow the growth of the small grains 11 obtained at the end of the first forging stages to form larger grains 12.

[0084] This gives us a piece 100 having an internal portion 21 composed of small grains 11 and an external portion 22 composed of larger grains 12.

[0085] It is noteworthy that, in addition to their sizes, the specific forging step is carried out under conditions that are favorable to the establishment of a high number of twins, with inconsistent twinning joints in the microstructure after solution treatment.

[0086] This characteristic is directly linked to the specific forging step carried out under the identified conditions, and ensures an increase in fatigue resistance and resistance to intergranular corrosion of the external portion 22.

[0087] In addition, the large grains 12 of the external portion 22 obtained by the specific forging step have particularly tortuous grain boundaries which reduces the crack propagation speed in the external portion 22.

[0088] For example, the external portion 22 is understood as the portion D2 of the part 100 extending from the internal circumference over at least 5% or even over at least 50% of the radial dimension, advantageously remaining less than 85% of the radial dimension.

[0089] Conversely, the internal portion 21 is understood to mean the rest of the room.

[0090] The "radial dimension" of part 100 is therefore understood as the dimension D1+D2 identified in figure 2.

[0091] The process also includes, for part 100, steps called solution setting and tempering.

[0092] These steps are known in forging processes and notably help to reduce residual stresses that develop during cooling following hot forging. Solution heating combined with specific forging prior to the external portion could generate microstructures with large grains.

[0093] In one embodiment, the solution treatment step and the tempering step are both carried out at a homogeneous temperature in the room, which may nevertheless be different for each of the two heat treatments.

Claims

Demands

1. A method for forging a metal part (100) comprising: - one or more forging stages carried out at a sub-solvus temperature between 1000°C and 1080°C, with a strain greater than or equal to 0.5, at a strain rate less than or equal to 1.0 s -1 in order to obtain a piece containing fine grains via complete recrystallization of the entire piece; - a solution stage carried out by heat treatment at a temperature between 1050°C and 1080°C for a period of 30 minutes to 2 hours; - a tempering step carried out by heat treatment at a temperature between 700 °C and 800 °C for a duration of 6 to 12 hours; the forging process being characterized in that it further comprises, after the forging steps and before the solution heating step, a specific forging step applied only to an external portion of the workpiece, the specific forging step being carried out at a temperature between 1000 °C and 1080 °C, with a strain between 0.1 and 0.5, at a strain rate between 0.05 s -1 and 1.0 s' 1 .

2. A forging method according to claim 1, wherein the specific forging step is carried out with a deformation between 0.1 and 0.

3.

3. A forging method according to claim 1 or 2, wherein the specific forging step is carried out with a deformation rate between 0.05 s-1 and 0.1 s' 1 .

4. A forging process according to any one of claims 1 to 3, which comprises between 2 and 5 forging steps prior to the specific forging step.

5. A forging process according to any one of claims 1 to 4, wherein the forging steps include crushing, punching and / or die-forging steps.

6. A forging process according to any one of claims 1 to 5, wherein the heating ramps required to reach the temperatures of the different stages are between 5°C / min and 30°C / min.

7. A forging process according to any one of claims 1 to 6, wherein the cooling ramps required to reach the temperatures of the different stages are between -70°C / min and -250°C / min.

8. A forging method according to any one of claims 1 to 7, wherein the outer portion extends from the outer circumference over at least 5% or even over at least 50% of the radial dimension.

9. A forging process according to any one of claims 1 to 8, wherein the metal part is made of a metal alloy selected from nickel-based alloys, for example, from the Y / Y' alloys known as Udimet 720, AD 730.

10. Metal part (100) comprising an inner portion (21) and an outer portion (22), characterized in that the grains (11) of the inner portion are smaller than the grains (12) of the outer portion and characterized in that the grains of the inner portion have a grain size greater than or equal to 10 ASTM and in that the grains of the outer portion have a grain size less than or equal to 8.0 ASTM.

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