FR3013060A1 - SUPERALLIAGE BASED ON NICKEL FOR A TURBOMACHINE PIECE - Google Patents

Abstract

The invention relates to a method of heat treatment (S) of a part in a nickel-based superalloy, such as a turbomachine part, comprising the following steps: - dissolving super-solvus (S1) of the part at a temperature higher than the solvus γ 'of its constituent material, - slow cooling (S2) of the workpiece to a temperature below the solvus γ', - dissolving subsolvus (S3) of the workpiece at a temperature below the solvus γ 'of its constituent material, and - rapid cooling (S4) so as to precipitate the intragranular phase y'.

Description

FIELD OF THE INVENTION The invention relates to nickel-based superalloy parts, and more particularly to a heat treatment process applicable to such parts in order to improve, in particular, their creep and tensile strength. BACKGROUND OF THE INVENTION In order to respond to the significant increase in temperatures and stresses in the hot parts of new-generation civil and military turbojets as well as turboshaft engines in general, it is necessary to improve the mechanical characteristics of the current alloys, and in particular the creep resistance which becomes very dimensioning and first order at high temperature. The alloys used for the parts of these hot parts such as disks, rings, flanges, labyrinths or casings are conventionally nickel-based superalloys. Nickel-based superalloys are those alloys in which nickel enters at least 50% by weight in their composition (all percentages given in this text will be percentages by weight), and comprising a gamma y matrix with precipitates. intermetallic y '(Ni3 (Al, Ti)). The phase y is a solid solution having a centered-face cubic crystal and different species of atoms distributed in the crystal. In comparison, the phase y 'has an ordered crystalline mesh, also cubic with L12 centered faces, where the nickel occupies the center of the faces of the cube and the titanium and the aluminum the vertices of the cube. In the hottest parts, especially for rotating parts of the disc and ring type, the alloys used are y-type nickel superalloys having a high y-phase content, that is to say at less than 25% of phase y '. The mechanical strength of these materials is provided mainly by structural hardening of the material, obtained in particular by precipitation of the phase y 'during a specific heat treatment. The material will be all the more hardened as the phase y 'will be fine and of a homogeneous distribution. This controlled precipitation occurs during the heat treatment during cooling after dissolution. The phase y 'will be all the more fine and homogeneous as the cooling after dissolution is fast. The grain size, which is controlled by the forging but also by the heat treatment undergone by the part, is also an important microstructural parameter and of the first order vis-à-vis the mechanical properties. Conventionally, fine grains will be favorable for tensile and fatigue properties, while a high grain size will be favorable for creep and crack resistance. The classical microstructures of these nickel-based superalloys of y / y 'type consist of relatively fine grains (5 to 20 μm). The grain size is controlled by the presence of y 'called primary (y'p), from the upstream processing range (ingot conversion) and then preserved and formatted during forging and heat treatment to control the grain size (Typically a content of 15% of y 'primary size of the order of 3 to 5pm provides a grain size of about 10pm). The secondary (y's) and tertiary (y ') y's that precipitate upon cooling after dissolution will precipitate inside the grain and harden the material. To increase the temperature capacity of these nickel-base superalloys y / y 'and in particular to improve their creep properties, it is necessary to increase the grain size of the material on the part, while maintaining a fine and homogeneous precipitation of the y-phase. . The increase in the grain size is conventionally performed on this type nickel base alloy y / y 'by dissolving the superalloy above the solvus temperature y'. The primary phase which blocks the grain boundaries is then completely dissolved so that the grain can grow. Accordingly, in this case, the volume fraction of phase available to harden the material by precipitation is maximum. However, the combination of a high grain size, a high content of y 'and high cooling rates during quenching after dissolution required to obtain the mechanical properties, generate significant residual stresses that can locally leading to intergranular decohesions then to cracks (also called cracks) prohibitive when these stresses are close to or greater than the intrinsic rupture limit of the material.

SUMMARY OF THE INVENTION An objective of the invention is therefore to improve the known heat treatments in order to obtain a part made of a nickel-based superalloy which exhibits both good resistance to creep, cracking, traction and fatigue, while reducing the level of residual stresses in the final piece. For this purpose, the invention proposes a method for the heat treatment of a part in a nickel-based superalloy comprising a gamma matrix and gamma '(y') intermetallic precipitates (Ni3 (Al, Ti)), such as a turbomachine part, comprising a super-solvus dissolution step during which the piece is raised to a temperature greater than the solvus y 'of its constituent material, the method being characterized in that it comprises the following additional steps: slow cooling of the room to a temperature below the solvus y ', in order to reduce the thermal gradients in the room; dissolving the room at a temperature below the solvus y' of its constituent material, and rapid cooling of the room so as to precipitate the intragranular phase. Some preferred but non-limiting characteristics of the heat treatment process are as follows: the super-solvus dissolution is carried out at a temperature below the burn temperature of the constituent material of the part. the slow cooling is carried out according to a cooling kinetics of less than or equal to 80 ° C./min in the precipitation temperature range of the γ 'phase, preferably 60 ° C./min, for example 50 ° C./min; the dissolving solution is carried out at a temperature below 10 ° C to 30 ° C at the temperature of the solvus y ', - the rapid cooling is carried out according to a cooling kinetics greater than 80 ° C / min in the temperature range precipitation of the phase y ', preferably greater than 100 ° C / min, - rapid cooling is performed using a fluid such as forced air, oil, water additivée of a polymer or water, - during slow cooling, the part is at least cooled to a minimum temperature of the precipitation temperature range of the phase y ', - the method further comprises an additional step of room income after the stage of rapid cooling, to reduce the stresses in the room, and - the superalloy of the room is one of the following superalloys: AD730TM, N18, UDIMET 720 or Astroloy.

The invention also proposes a turbomachine part, heat treated according to a heat treatment process as described above. BRIEF DESCRIPTION OF THE DRAWINGS Other features, objects and advantages of the present invention will appear better on reading the detailed description which follows, and with reference to the appended drawings given as non-limiting examples and in which: FIG. 1 FIG. 2 schematically illustrates the steps of an exemplary method according to a second embodiment of the invention, and FIG. 3 illustrates the typical structure of a nickel-based superalloy of y / y 'type in which primary (YiD), secondary (y's) and tertiary (y' t) precipitates are visible. . DETAILED DESCRIPTION OF AN EMBODIMENT In what follows, the invention will be more particularly described in relation to the heat treatment of a part of a turbomachine in a nickel-based superalloy. This is however not limiting, since this heat treatment S can be applied to any part comprising such a superalloy having a good resistance to creep, tensile, cracking and fatigue. Here, the part is preferably in a nickel-based superalloy, comprising a matrix gamma y with intermetallic precipitates y '. Preferably, the nickel-based superalloy comprises intermetallic precipitates y 'based on aluminum and titanium (Ni3 (Al, Ti)). The superalloy may further comprise cobalt, chromium and / or molybdenum.

By way of example, the part may for example be made in a superalloy of the AD730TM, N18, UDIMET 720 or Astroloy type. The method S according to the invention proposes to split the heat treatment solution and cooling in two sequences with a heat treatment comprising a double dissolution. This heat treatment is preferably applied directly to the part, that is to say once it has been forged. Thus, during a first step S1, the part is put in solution at a temperature higher than the solvus y 'of its constitutive material (solubilization super-solvus).

By solvus, here is understood the temperature from which the precipitates y 'are completely returned to solution (that is to say dissolved). After this temperature, the superalloy then no longer includes phase y ', whose elements are in solution.

For example, for a part made in a superalloy type AD730TM, N18, UDIMET 720 or Astroloy, the solvus y 'is respectively 1100 ° C, 1195 ° C, 1150 ° C or 1145 ° C. Preferably, the temperature of the dissolution stage remains below the burn temperature of the superalloy, that is to say the temperature from which the alloy begins to locally melt. For example, for the N18 superalloy, the temperature of the dissolving step Si preferably remains below 1220 ° C. For example, the solution treatment may be carried out at a temperature of from 10 ° C. to 30 ° C., for example 20 ° C., to the solvus y 'of the superalloy. Thus, for a superalloy of the AD730TM, N18, UDIMET 720 or Astroloy type, the temperature of the super-solvus solution treatment can be respectively 1120 ° C, 1205 ° C, 1170 ° C or 1165 ° C. The kinetics of the rise in temperature of step 51 has no influence on the metallurgical quality of the part. Moreover, the super-solvus dissolution is carried out for a sufficient time to ensure that the entire phase y 'is dissolved, for example for a period of between 1 h and 4h. Indeed, it is necessary that the phase y 'is completely dissolved in order to allow the grains to grow. However, it is better not to heat too long (more than 10 hours for example) the room so that the grains do not grow too much. In a second step S2, the workpiece is then cooled slowly in the region of precipitation temperatures of the phase Y 'to a temperature below the solvus y'. S2 cooling must be slow enough to reduce thermal gradients in the room by minimizing residual stresses that may lead to quenching. According to one embodiment, the cooling kinetics of the part during step S2 is less than or equal to 80 ° C / min in the temperature range of precipitation of phase y ', preferably less than 60 ° C / min, for example of the order of 50 ° C / min. This cooling kinetics is important in the precipitation domain of the phase y ', which corresponds to the temperature range in which the phase y' precipitates. For the examples of superalloys mentioned in the description (AD730TM, N18, UDIMET 720 or Astroloy), the precipitation domain of the phase y 'is between approximately 700 ° C. and the solvus y'. Thus, the cooling kinetics of the part during step S2 is preferably slow, for example less than 80 ° C./min between the super-solvus solution temperature and the minimum temperature of the precipitation domain of the reactor. phase y ', about 700 ° C, then can be maintained or accelerated to the final temperature chosen of step S2. For example, the cooling S2 can be carried out in air. In any case, following the cooling step S2, the zo temperature of the room must be lower in every point than the solvus y ', and preferably at the minimum temperature of the precipitation domain of the phase y'. Thus, according to a first embodiment, the part can be cooled S2 to room temperature. Alternatively, as illustrated in FIG. 2, the cooling time of the workpiece can be adjusted so that the hottest point of said workpiece reaches a temperature below the solvus y ', and preferably at the minimum temperature of the workpiece. precipitation domain of the phase y '(about 700 ° C for the aforementioned superalloys). This cooling time depends in particular on the geometry of the part, its constituent material as well as the temperature reached during the super-solvus dissolution stage.

Solubilization super-solvus S1 makes it possible to enlarge the grain thanks to the dissolution of the precipitates y 'primary. In addition, the slow kinetics of the cooling S2 after this dissolution D1 reduces the thermal gradients in the room during cooling and thus minimize the residual stresses that lead to quenching taps, when the cooling is too high. However, a low cooling rate after placing in solution super-solvus S1 does not make it possible to obtain a size of precipitates y 'sufficiently fine to obtain the required tensile and fatigue characteristics, in particular in the field of aeronautics, as for example for turbomachine disks that are particularly loaded. During a third step S3, the part is again put in solution, at a temperature lower than the solvus y 'of its constituent material (dissolving solution subsolvus) to partially put in solution the phase y'. For example, the solution treatment S3 can be carried out at a temperature of from 10 ° C to 30 ° C, for example 20 ° C, to the solvus y 'of the superalloy. Thus, for a superalloy of the AD730TM, N18, UDIMET 720 or Astroloy type, the temperature of the solution dissolving treatment S3 can be respectively 1080 ° C, 1165 ° C, 1120 ° C or 1125 ° C. The kinetics of the rise in temperature of step S3 has no influence on the metallurgical quality of the part. Moreover, the solution dissolution subsolvus is carried out for a time sufficient to put a fraction of the phase y 'in solution, for example for a period of between 1 h and 4h.

During a fourth step S4, the part is then rapidly cooled in the region of precipitation temperatures of the phase y '. For example, the cooling can be carried out by pulsed air or with the aid of a fluid (oil, water additive of a polymer, water, etc.). According to one embodiment, the cooling kinetics of the part during step S4 is greater than 80 ° C./min in the precipitation temperature range of phase y ', preferably greater than 100 ° C./min. . This cooling kinetics is important in the precipitation domain of the phase y '. Thus, the cooling kinetics of the part during step S4 is preferably rapid, for example of the order of 100 ° C./min, between the solution dissolution temperature subsolvus and the minimum temperature of the precipitation domain. of the phase y ', about 700 ° C, then can be maintained or slowed down to the selected final temperature of step S4 (eg room temperature).

This dissolution of subsolvus solution S3 makes it possible to obtain the desired precipitation for the intragranular phase, and thus to reinforce the mechanical strength of the part, in particular in tension and in fatigue. Furthermore, this dissolution S3 reduces the level of residual stresses in comparison with a rapid cooling from a temperature above the solvus y 'through the conjunction of the following three effects: - The dissolution of subsolvus solution S3 makes it possible to precipitate a fraction of phase y 'at the grain boundaries, which makes it possible to reinforce their behavior at the end of the step of rapid cooling in the presence of stresses, - the dissolving of the solution subsolvus S3 makes it possible to reduce the fraction of phase y 'precipitating during quenching and in fact the residual stresses related to the precipitation of this phase, and - The cooling S4 operates from a lower temperature which also contributes to the reduction of residual stresses related to the thermal gradient.

Therefore, the cooling S4 operates on a less charged structure in Y ', which allows, despite the rapid cooling, to avoid quenching taps. In addition, the rapid cooling S4 after dissolving S3 makes it possible to format the size and the distribution of the phase y 'in order to reach the required mechanical properties. Finally, after double dissolving S1-S4, the heat treatment S may comprise a conventional treatment S5 income of the room, in order to reduce the internal stresses in the room. This income, successively comprising a low temperature heating (typically between 650 ° C. and 850 ° C.), for a long time (several hours), a maintenance in temperature and a slow cooling (typically cooling air), also participates in the obtaining mechanical properties without generating residual stresses.

Claims (10)

REVENDICATIONS1. A method of heat treating (S) a workpiece in a nickel-based superalloy comprising a gamma matrix (y) and gamma '(y') intermetallic precipitates (Ni3 (Al, Ti)) such as a workpiece turbomachine, comprising a super-solvus dissolution step (S1) during which the workpiece is raised to a temperature greater than the solvus y 'of its constituent material, the process being characterized in that it comprises the steps following additional: - slow cooling (S2) of the room to a temperature below the solvus y ', to reduce the thermal gradients in the room - dissolving subsolvus (S3) of the room at a temperature below 15 solvus y 'of its constituent material, and - fast cooling (S4) of the part so as to precipitate the intragranular phase y'. 2. Heat treatment process (S) according to claim 1, wherein the solution super-solvus (S1) is carried out at a temperature below the burn temperature of the constituent material of the part. 3. Heat treatment process (S) according to one of claims 1 or 2, wherein the slow cooling (S2) is carried out with a cooling kinetics lower than or equal to 80 ° C / min in the temperature range of precipitation of the phase y ', preferably 60 ° C / min, for example 50 ° C / min ,. 30 4. Heat treatment process (S) according to one of claims 1 to 3, wherein the dissolving subsolvus (S3) is performed at a temperature of 10 ° C to 30 ° C below the temperature of the solvus y '. 5. heat treatment process (S) according to one of claims 1 to 4, wherein the rapid cooling (S4) is carried out according to a cooling kinetics greater than 80 ° C / min in the temperature range of precipitation of the phase y ', preferably greater than 100 ° C / min. 6. heat treatment process (S) according to one of claims 1 to 5, wherein the rapid cooling (S4) is performed using a fluid such as forced air, oil, water with a polymer additive or water. 7. heat treatment process (S) according to one of claims 1 to 6, wherein during the slow cooling (S2), the room is at least cooled to a minimum temperature of the precipitation temperature range of the phase y '. 8. heat treatment process (S) according to one of claims 1 to 7, further comprising an additional step of income (S5) of the workpiece after the step of rapid cooling (S4), to reduce the constraints in the room. 9. Heat treatment process (S) according to one of claims 1 to 8, wherein the superalloy of the part is one of the following superalloys: AD730TM, N18, UDIMET 720 or Astroloy. 10. Turbomachine part, characterized in that it is heat-treated according to a heat treatment process (S) according to one of claims 1 to 9.