EP2510131B1 - Method for manufacturing inconel 718 nickel superalloys - Google Patents

Description

The present invention relates to a process for manufacturing nickel superalloys of the Inconel 718 type.

The Nickel base superalloy Inconel 718 (NC19FeNb) is commonly used for the manufacture of parts in advanced applications, in particular in aeronautics for rotating parts of turbomachines, housings, rings. The mechanical characteristics of use of these parts depend on the intrinsic characteristics of the alloy (chemical composition) of this part but also on the microstructure of the part, in particular on the grain size. In fact, the grain size governs in particular the characteristics of oligo-cyclic fatigue, of tensile strength, and of creep. A microstructure of which the grains are fine (for example with a grain size of substantially between 5 to 20 μm) makes it possible to obtain the best fatigue and tensile properties, while guaranteeing good creep resistance.

This fine grain size is currently achieved by means of ranges of heat treatments and part forgings which produce grain recrystallization mechanisms.

However, with the current ranges, one frequently observes on nickel superalloy parts areas of coarse grains (that is to say grains whose size is much greater than the size of fine grains). These areas are undesirable because they are the cause of a reduction in the mechanical properties of these parts. Other examples of ranges are described in the documents FR 2,089,069 , EP 792 945 and publication DU Furrer: "Forging of Nickel-Base Alloys" in: "Metalworking: Bulk Forming", January 1, 2005, ASM international, XP055581727, ISBN: 978-1-62708-185-6, pages 324-330, DOI: 10.31399 / asm.hb.v14a.a0003999 .

These coarse-grained areas appear even when the ranges include forging operations below the δ-solvus temperature (temperature for re-dissolving the precipitates δ), when such operations are deemed to have no effect on the final microstructure of the alloy, because they are supposed in theory to guarantee an absence of grain growth.

The invention aims to provide a manufacturing method which makes it possible to limit the appearance of coarse grains during the manufacture of the part.

où δ_i est la distance initiale entre le point M et un point M' voisin de M et δ_f est la distance entre les points M et M' après forgeage, est au moins égal à une valeur minimale D_m égale à 0,7, et en ce que ledit superalliage de nickel ne subit pas de traitement thermique à une température supérieure à une température seuil T_s égale à 750°C après ladite trempe.This object is achieved by means of a method according to claim 1, and in particular by virtue of the fact that the last forging step which the said nickel superalloy undergoes is such that it is carried out at a temperature T lower than the δ-solvus temperature, that at any point M of this superalloy of nickel the local deformation rate D, defined by $\mathrm{D}=\mathrm{Ln}\left(\frac{\mathrm{\delta i}}{\mathrm{\delta f}}\right)$

where δ _i is the initial distance between point M and a point M 'close to M and δ _f is the distance between points M and M' after forging, is at least equal to a minimum value D _m equal to 0.7 , and in that said nickel superalloy does not undergo heat treatment at a temperature above a threshold temperature T _s equal to 750 ° C. after said quenching.

Thanks to these arrangements, the coarse grains still present in the superalloy are transformed back into fine grains, and no new coarse grains are formed within the superalloy.

Advantageously, the nickel superalloy also undergoes tempering directly after the quenching following the last forging step.

Thus, the resilience properties of the superalloy are improved while its other mechanical properties are not significantly reduced. The tempering operation takes place at a sufficiently low temperature so that coarse grains do not recreate within the superalloy.

• la figure 1 montre schématiquement le procédé de fabrication selon l'invention,
• la figure 2 montre schématiquement un exemple du procédé de fabrication selon l'invention.

The invention will be well understood and its advantages will appear better on reading the following detailed description of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:

• the figure 1 schematically shows the manufacturing process according to the invention,
• the figure 2 schematically shows an example of the manufacturing process according to the invention.

In the present invention, nickel superalloys of the Inconel 718 type are considered.

In the process according to the invention, one starts with a billet which has already undergone thermomechanical treatments, intended to give this billet a structure and a shape conforming to the specifications.

By examining nickel superalloys of the prior art, the microstructure of which has coarse grains, the inventors have observed that these coarse grains were of two different types.

Thus, one distinguishes on the one hand the coarse equiaxial grains, which result from a static enlargement of the fine grains, for example because the alloy is maintained at a temperature higher than the temperature δ-solvus. Such magnification can be avoided by performing the forging operations of the alloy processing range at temperatures below the δ-solvus temperature.

Unexpectedly, the inventors have also observed the presence within the alloy of coarse “split” grains, the contours of which are very irregular. It is assumed that these grains are generally formed at temperatures below the temperature of δ-solvus, and that this is the energy stored following the work hardening during the previous forging operation, when the deformation is carried out. at temperatures below the temperature of δ-solvus (for example below 1000 ° C), which causes this bursting of the grains. This stored energy is then "released" in the form of an early and uncontrolled migration of grain boundaries, which generates these "burst" grains.

According to the invention, as shown in figure 1 , we ensure that in the forging range that the superalloy undergoes, the last forging step (referenced 1 on the figure 1 ) is carried out at a temperature T lower than the δ-solvus temperature, and furthermore that at any point M of this nickel superalloy the local deformation rate D is at least equal to a minimum value D _m .

où δ_i est la distance initiale entre ce point M et un point M' voisin de M, et δ_f est la distance entre les points M et M' après forgeage.The local strain rate D characterizes the local strain at a point M of a material. It is defined by the relation $$\mathrm{D}=\mathrm{Ln}\left(\frac{\mathrm{\delta i}}{\mathrm{\delta f}}\right)$$

where δ _i is the initial distance between this point M and a point M 'close to M, and δ _f is the distance between points M and M' after forging.

Carrying out this last forging step at a temperature T below the δ-solvus temperature makes it possible to avoid the formation of coarse equiaxial grains.

In addition, the condition that the local deformation rate D is at least equal to a minimum value D _m in a region of the superalloy makes it possible to recrystallize the “split” grains into fine grains in this region. During the forging step, depending on the final shape given to the part, certain regions of this part may undergo greater deformations than other regions. The fact that the above condition on the local strain rate D is valid at any point M of the superalloy makes it possible to ensure that the "burst" grains are recrystallized into fine grains throughout the volume of the superalloy.

The minimum value D _m is equal to 0.7.

Alternatively, the minimum value D _m is equal to 0.8 or 0.9.

After this forging, the superalloy undergoes quenching from the forging temperature T to ambient temperature T _A.

The quenching is advantageously carried out at a speed of the order of 15 ° C./min, the tests carried out by the inventors having demonstrated that the mechanical characteristics were best optimized at this quenching speed. Preferably, a water quenching is carried out.

In addition, after this quenching following this final forging step, the superalloy does not undergo heat treatment at a temperature above a threshold temperature T _s equal to 750 ° C.

In fact, a heat treatment at a temperature above the threshold temperature T _s is likely to create “split” grains within the superalloy.

In particular, the superalloy does not undergo dissolution because this takes place at a temperature above the threshold temperature T _s .

On the other hand, the superalloy can directly undergo tempering (step referenced 2 on the figure 1 ) after quenching following the last forging.

For example, the superalloy is heated to a temperature of 720 ° C for 8 hours, then cooled to a temperature of 620 ° C for 8 hours, before being cooled to room temperature. This situation is shown on the figure 2 .

Before the last forging step according to the invention, the superalloy may have undergone no other, another, or several other forging steps, with for each, several, one, or none of these steps a forging temperature higher than the temperature. δ-solvus.

Advantageously, all the forging steps preceding the last forging step are carried out at temperatures below the δ-solvus temperature.

Numerical simulations were carried out by the inventors and show that, at the end of the method according to the invention, the size of the grains is effectively reduced.

For example, at the end of the process according to the invention, the size of all the grains of the superalloy is between 5 and 30 μm.

Advantageously, at the end of the process according to the invention, the size of all the grains of the superalloy is between 5 and 20 μm. This finer average grain size results in a superalloy whose fatigue life and elastic limit are further improved.

Claims (3)

1. A fabrication method of fabricating Inconel 718 type nickel superalloys characterized in that the last forging step to which said nickel superalloy is subjected is such that it takes place at a temperature T lower than the δ-solvus temperature, that at all points M of the nickel superalloy the local deformation ratio D, defined as $\mathrm{D}=\mathrm{Ln}\left(\frac{\mathrm{\delta i}}{\mathrm{\delta f}}\right)$

   

   where δ_i is the initial distance between the point M and a point M' neighboring the point M, and δ_f is the distance between the points M and M' after forging, is not less than a minimum value D_m equal to 0.7, and in that said nickel superalloy is subjected to quenching after following that final forging step and is not subjected to any heat treatment at a temperature higher than a threshold temperature T_s equal to 750°C after said quenching.
2. A fabrication method according to claim 1 characterized in that said nickel superalloy is also subjected to tempering directly after the quenching following said forging step.
3. A fabrication method according to claim 1 characterized in that all of the forging steps preceding said last forging step are performed at temperatures lower than the δ-solvus temperature.

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