EP0148688B1 - Method for the thermomechanical treatment of superalloys to obtain structures with great mechanical characteristics - Google Patents

Description

The present invention relates to a thermomechanical treatment process applicable to superalloys and making it possible to obtain structures with high mechanical characteristics.

The current economic conditions and the performances required for aeronautical turbomachines in development cause a renewed interest for nickel-based alloys, of the NC 19 Fe Nb type (commercial designation: Inconel 718).

Its relatively low cost, the absence of cobalt in its composition and the experience accumulated with this alloy for many years, both in development and forging and in use on engine, give it a preponderant position among the alloys with high characteristics. , for temperatures which can exceed 650 ° C in short periods.

Laboratory studies carried out by the applicant with a view to improving this alloy have shown that a significant increase in certain employment characteristics - in particular of its elastic limit, of its resistance to oligocyclic fatigue and to creep - could be obtained by the generation of fine, homogeneous microstructures, presenting a residual hardening associated with the absence of phase Ni ₃ Nb-3 (delta) in the form of platelets. However, under the usual forging conditions of this alloy, the structure of the parts often appears very heterogeneous: weakly wrought zones with large grains coexist with so-called duplex structure domains (large hardened grains and fine recrystallization grains) while the parts thick parts subjected to sufficient working and slower cooling after forging, have a recrystallized structure with fine equiaxed grains. This observation has therefore highlighted the importance of certain parameters of thermomechanical treatments, such as temperature, duration of heating, rate of deformation, cooling mode, etc. in the generation of the desired structure and therefore in obtaining improved mechanical characteristics.

We have therefore studied different ranges of thermomechanical treatments in order to define the parameters of the roughing and finishing sequences which make it possible to develop in the NC19 FeNb alloy homogeneous structures with fine and hardened grains, characterized by the absence of Ni platelets. ₃ Nb-S, these results having to be obtained by a process applicable to the scale of industrial production.

It is important to emphasize that the current forging conditions of the NC19 Fe Nb alloy, adopted by different blacksmiths, lead to structures presenting a compromise with regard to mechanical characteristics: the improvement of certain properties can, in fact, lead to the alteration of other characteristics.

Thus, during the finishing sequence, a too low reheating temperature leads to the conservation of the fine structure due to the previous sequence, with however, precipitation of the Ni ₃ Nb-ô phase in the grain boundaries or, for certain conditions, inside the grains, in the form of platelets increasing preferentially in the crystallographic planes of the type {111}. Phase 5, of orthorhombic structure, is harmful whatever its morphology because it fixes niobium and thus limits the formation of the hardening phase Ni ₃ Nb-y "(gamma second), metastable, of centered quadratic structure.

Finally, in the case of wafer morphology, phase 8 induces a more marked sensitivity to initiation in fatigue.

Conversely, the heating before forging at a too high temperature avoids the precipitation of phase ô in platelets, but on the other hand leads to an increase in the grain size, detrimental to the fatigue strength.

The advantage of the process according to the invention stems from the possibility of obtaining fine-grained structures, according to heating / forging sequences simultaneously guaranteeing the absence of phase 0 wafers and the existence of an essential residual work hardening. to the consolidation of the alloy.

The beneficial influence of a fine-grained structure on the fatigue life of the Inconel 718 is well known to those skilled in the art. Thus, US Pat. No. 3,660,177 proposes a method for refining the grain based on the precipitation of the Ni ₃ Nb-δ phase within the grains before the forging operation and the recrystallization treatment. The precipitation treatment of phase 8 carried out at approximately 900 ° C., before forging, leads to a subdivision of the grains by platelets of phase ô which are formed in planes of type {111}. The heat treatment, carried out after forging with a thickness reduction of 50 to 65%, leads to globulation of the deformed wafers of phase S and a recrystallization of the structure. This process makes it possible to obtain recrystallized structures, of 10 ASTM or more called "Minigrain", whose fatigue characteristics are improved, but whose creep resistance and tenacity are notoriously insufficient for a material with high characteristics, necessary for certain industrial applications.

The conditions sought in the particular case of an alloy of the NC 19 Fe Nb type with the commercial designation Inconel 718 pq are also found for precipitation hardening superalloys, in general, of which those based on nickel constitute a subclass.

• -structure fine et homogène;
• -grains écrouis;
• -diminution des contraintes propres de refroidissement;
• -absence de phase parasite.

Consequently, the present invention defines the thermomechanical parameters which make it possible to obtain an improvement in the set of mechanical characteristics for these superalloys. To ensure industrial reproducibility and obtaining optimal results, rigorous control is essential throughout the manufacturing process, in terms of forging parameters and cycles. heat treatment. In particular, the temperature and the rate of deformation of the finishing sequence must be perfectly defined in order to avoid the growth of the grain and the precipitation of a parasitic phase, but to generate within the grains a substructure of dislocations. In fact, in order to achieve these objectives, the method claimed by the invention must make it possible to satisfy four criteria which the previously known methods did not make it possible to obtain simultaneously:

• - fine and homogeneous structure;
• - hardened grains;
• -reduction of the own cooling constraints;
• - absence of parasitic phase.

• (a) une opération de chauffe dont les conditions de température et de durée sont connues en soi pour le superalliage considéré,
• (b) une opération de déformation à chaud par compression, le taux de déformation appliqué étant compris entre 30% et 60%, de manière à obtenir une structure de type duplex en cours de recristallisation,
• (c) un traitement thermique consistant en un maintien isotherme pour lequel dans l'application au superalliage considéré la température qui est inférieure à la température déterminée de l'étape (a) et le temps de maintien permettent d'obtenir une structure homogène, de 7 ASTM ou plus et dans laquelle n'apparaissent pas de précipités thermodynamiquement stables, sous forme de plaquettes constituant une phase fragilisante,
  puis, dans la séquence de finition, qui s'enchaîne avec les étapes précédentes de la séquence d'ébauche finale:
• (d) une opération de déformation par compression à chaud, le taux de déformation appliqué étant compris entre 8% et 25%, de manière à obtenir un écrouissage de la structure précédente, homogène, à grains fins, consolidant la structure sans produire de phénomène de recristallisation
• et, dans le traitement thermique final, comporte une étape unique qui est constituée par:
• (e) un traitement de revenu dont les conditions sont connues en soi pour le superalliage considéré de manière à conserver la structure écrouie et à provoquer la précipitation de la phase durcissante dudit superalliage sans l'apparition de précipités thermodynamiquement stables, sous forme de plaquettes constituant une phase fragilisante.

The process of thermomechanical treatments for precipitation hardening superalloy according to the invention comprises the following steps taken in sequence, in the final roughing sequence:

• (a) a heating operation whose temperature and duration conditions are known per se for the superalloy considered,
• (b) a hot deformation operation by compression, the deformation rate applied being between 30% and 60%, so as to obtain a duplex type structure during recrystallization,
• (c) a heat treatment consisting of an isothermal holding for which, in the application to the superalloy considered, the temperature which is lower than the determined temperature of step (a) and the holding time make it possible to obtain a homogeneous structure, 7 ASTM or more and in which thermodynamically stable precipitates do not appear, in the form of platelets constituting an embrittling phase,
  then, in the finishing sequence, which is linked with the previous steps of the final roughing sequence:
• (d) a hot compression deformation operation, the applied deformation rate being between 8% and 25%, so as to obtain a hardening of the previous structure, homogeneous, with fine grains, consolidating the structure without producing a phenomenon recrystallization
• and, in the final heat treatment, comprises a single step which consists of:
• (e) a tempering treatment the conditions of which are known per se for the superalloy considered so as to preserve the work hardened structure and to cause precipitation of the hardening phase of said superalloy without the appearance of thermodynamically stable precipitates, in the form of platelets constituting a weakening phase.

In the application of the process according to the invention to nickel-based precipitation hardening superalloys, the embrittling phase, the appearance of which is avoided during the process steps, is an Ni ₃ Nb type 8 phase in the form of platelets.

In the application of the method according to the invention to an alloy of average composition in weight percent C 0.02 to 0.08; Mn 0.35 max; If 0.35 max; P 0.015 max; S 0.015 max; Cr 17-21; Fe 15 to 21; Co 1 max; Mo 2.80 to 3.80; Nb + Ta 4.75 to 5.50; Ti 0.75 to 1.15; AI 0.30 to 0.70; B 0.001 to 0.006; Cu 0.15 max; No complement to 100, this alloy being obtained by applying techniques known per se of preparation and reflow under vacuum, the heating conditions in the final roughing sequence are 1040 ° C ± 10 ° C for fifty minutes, this time up to an hour and isothermal maintenance is carried out at 970 ° C for thirty minutes, this time up to an hour.

• -les figures 1 à 1A sont des microphotographies à deux grandissements, respectivement 50 fois et 300 fois, d'une pièce en Inconel 718 révélant un grain de 7 ASTM après un maintien isotherme suivant un forgeage avec taux de déformation de 25%,
• -les figures 2 et 2A sont des microphotographies analogues à celle des figures 1 et 1A lorsque le taux de déformation est de 45% pour un grain obtenu de 8-8,5 ASTM,
• -les figures 3 et 3A sont également des microphotographies analogues à celles des figures 1, 1A ou 2, 2A lorsque le taux de déformation est de 60% pour un grain obtenu de 8-8,5 ASTM,
• -les figures 4 et 4A sont des microphotographies analogues à celles des figures 1, 1A et suivantes et révèlent un grain de 6-6,5 ASTM à la suite d'un maintien isotherme de trente minutes à 980°C après un forgeage avec un taux de déformation de 45%,
• -les figures 5 et 5A sont des microphotographies obtenues dans les mêmes conditions que celles des figures 4 et 4A à l'exception de la température de maintien isotherme qui est de 970°C et conduit à un grain de 8 ASTM,
• -les figures 6 et 6A sont des microphotogrpahies obtenues dans les mêmes conditions que celles des figures 4, 4A et 5, 5A à l'exception de la température de maintien isotherme qui est de 960°C et conduit à un grain de 8 ASTM,
• -la figure 7 est une microphotographie électronique à grandissement 3200 fois d'une pièce en Inconel 718 ayant suivi les séquences d'ébauche et de finition conformes à l'invention avec un taux de déformation en finition de 10% montrant des grains et des sous-grains écrouis,
• -la figure 7A est une microphotographie à grandissement 25000 fois obtenue dans les mêmes conditions que celle de la figure 7 et montrant un exemple de sous-joints et de sous-grains avec la répartition du réseau de dislocations,
• -la figure 8 est une microphotographie à grandissement 6400 fois d'une pièce en Inconel 718 obtenue dans les mêmes conditions que celle de la figure 7 à l'exception du taux de déformation en finition qui est de 15% et montrant des grains écrouis et un petit grain de recristallisation,
• -la figure 8A est une microphotographie à grandissement 25000 fois montrant une structure, à sous-grains écrouis analogue à celle de la figure 7A et obtenue dans les mêmes conditions, à l'exception du taux de déformation en finition qui est de 15%,
• -les figures 9 et 9A sont des microphotographies à deux grandissements, respectivement 50 fois et 300 fois, d'une pièce en Inconel 718 obtenue selon un procédé connu par le brevet US 3 660 177 conduisant à une structure dite "minigrain" de taille 10-11 ASTM,
• -les figures 10 et 10A sont des microphotographies analogues à celles des figures 9 et 9A dont la structure est obtenue selon un procédé couramment utilisé conduisant à une structure à grains fins recristallisés, de taille 7-8 ASTM,
• -les figures 11 et 11A sont des microphotographies analogues à celles des figures 9 et 9A, 10 et 10A et correspondant à une structure obtenue en appliquant le procédé conforme à l'invention.

Other characteristics and advantages of the invention will be better understood with the aid of the description below, with reference to the appended drawings in which:

• FIGS. 1 to 1A are microphotographs with two magnifications, respectively 50 times and 300 times, of a part in Inconel 718 revealing a grain of 7 ASTM after an isothermal hold following a forging with deformation rate of 25%,
• FIGS. 2 and 2A are microphotographs similar to that of FIGS. 1 and 1A when the deformation rate is 45% for a grain obtained from 8-8.5 ASTM,
• FIGS. 3 and 3A are also microphotographs similar to those of FIGS. 1, 1A or 2, 2A when the deformation rate is 60% for a grain obtained from 8-8.5 ASTM,
• FIGS. 4 and 4A are microphotographs similar to those of FIGS. 1, 1A and following and reveal a grain of 6-6.5 ASTM following an isothermal hold of thirty minutes at 980 ° C. after forging with a 45% deformation rate,
• FIGS. 5 and 5A are microphotographs obtained under the same conditions as those of FIGS. 4 and 4A with the exception of the isothermal holding temperature which is 970 ° C. and leads to a grain of 8 ASTM,
• FIGS. 6 and 6A are microphotogrpahs obtained under the same conditions as those of FIGS. 4, 4A and 5, 5A with the exception of the isothermal holding temperature which is 960 ° C. and leads to a grain of 8 ASTM,
• FIG. 7 is an electronic photomicrograph with 3200 times magnification of an Inconel 718 part having followed the roughing and finishing sequences in accordance with the invention with a deformation rate in finishing of 10% showing grains and sub - hardened grains,
• FIG. 7A is a 25,000-fold magnification photomicrograph obtained under the same conditions as that of FIG. 7 and showing an example of sub-joints and sub-grains with the distribution of the dislocation network,
• FIG. 8 is a micrograph of 6400 times magnification of an Inconel 718 part obtained under the same conditions as that of FIG. 7 with the exception of the deformation rate in finishing which is 15% and showing hardened grains and a small grain of recrystallization,
• FIG. 8A is a photomicrograph with 25,000 times magnification showing a structure with hardened sub-grains similar to that of FIG. 7A and obtained under the same conditions, with the exception of the rate of deformation in finishing which is 15%,
• FIGS. 9 and 9A are microphotographs with two magnifications, respectively 50 times and 300 times, of a part made of Inconel 718 obtained according to a process known by US Pat. No. 3,660,177 leading to a structure called "minigrain" of size 10 -11 ASTM,
• FIGS. 10 and 10A are microphotographs similar to those of FIGS. 9 and 9A, the structure of which is obtained according to a commonly used process leading to a structure with fine recrystallized grains, of size 7-8 ASTM,
• FIGS. 11 and 11A are microphotographs similar to those of FIGS. 9 and 9A, 10 and 10A and corresponding to a structure obtained by applying the method according to the invention.

• -une structure fine et homogène,
• -des grains écrouis,
• -une diminution des contraintes propres de refroidissement,
• -l'absence de phase parasite éventuelle et en particulier, dans le mode de réalisation de l'invention concernant l'inconel 718, l'absence de plaquettes de phase Ni₃Nb-8,
  et par ces moyens, de réunir les avantages qui en découlent et, en particulier, de hautes caractéristiques mécaniques.

Structures fines et homogènesWe will develop the conditions which allow, by the process according to the invention, to provide an optimal solution to the problem posed which is to obtain simultaneously for a superalloy:

• -a fine and homogeneous structure,
• - hardened grains,
• -a decrease in the own cooling constraints,
• the absence of any parasitic phase and in particular, in the embodiment of the invention concerning inconel 718, the absence of platelets of phase Ni ₃ Nb-8,
  and by these means, to bring together the advantages which flow therefrom and, in particular, high mechanical characteristics.

Fine and homogeneous structures

• a) Incidence du taux de déformation

Research carried out to obtain a homogeneous, fine-grained structure has highlighted the influence of three parameters:

• a) Impact of the deformation rate

à partir d'une structure initiale de 3 1/2 ASTM.Tests carried out for three deformation rates ― 25―45―60%, carried out at 1040 ° C and followed by an isothermal maintenance of recrystallization, led to the following grain sizes (FIGS. 1 to 3A):

from an initial structure of 3 1/2 ASTM.

• b) Incidence de la température et du temps de maintien isotherme Pour les mêmes conditions de forgeage (1040°C-45%), des maintiens isothermes ont été réalisés dans le domaine 960-980°C, chacun pendant une demi-heure.

The structure obtained is moreover more homogeneous, following a reduction of the dead zones: in fact, the roller parts which cool prematurely in contact with the tool, present, before isothermal maintenance, a work hardened structure and grains of recrystallization, the migration of the joints of which has been hindered by cooling to temperatures below the recrystallization range. Isothermal maintenance then makes it possible to develop the microstructure by generalizing the recrystallization to the largest part of the part: the dead zones are therefore reduced and the structure refined, since the grain size of the internal parts of the roller passes for example from 3 1/2 ASTM to 8-8 1/2 AST after 45% or 60% of working. The intermediate rate of wrinkling (45%), associated with the isothermal maintenance of recrystallization, therefore ensures that a fine and homogeneous structure is obtained, the third characteristic of which resides in the absence of phase 6 in platelets. In the economic interest represented by the chain between deformation and isothermal maintenance, without going back to room temperature, is added the advantage of avoiding the formation of phase 8 germs which normally precipitate during cooling and then reheating of parts , by crossing the domain of existence of this phase (800-990 ° C).

• b) Incidence of temperature and isothermal hold time For the same forging conditions (1040 ° C-45%), isothermal holdings were carried out in the range 960-980 ° C, each for half an hour.

Between 960 and 980 ° C, the recrystallization grain goes from 8 to 6-6.5 ASTM, the intermediate temperature 970 ° C-conferring a fine and homogeneous structure, with a grain size of about 8 ASTM (Figures 4 to 6A ).

These results show the advantage of retaining the temperature of 970 ° C., for an isothermal maintenance lasting approximately half an hour. This temperature thus makes it possible to accept the operating tolerances of industrial ovens, a fluctuation of ± 10 ° C. then having only a limited impact on the grain size of the recrystallized structure.

With regard to the isothermal holding time, this factor has a moderate incidence which has been verified. Extending the hold time tends to cause grain size to increase. However, below one hour of maintaining defined temperatures, no decisive harmful influence with regard to the final result obtained on the finished product has been observed. The desired results are obtained for a holding time substantially close to thirty minutes and under industrial application conditions this period always remains less than one hour.

It will be noted that the refining of the grain according to the invention does not have the drawbacks of the method which was the subject of the patent US-A-3,660,177 mentioned above which consists, in part, in artificially fragmenting the grain by precipitation of phase 5 platelets.

In the process according to the invention, the Niobium element is used in full for the formation of the hardening phase Ni ₃ Nb-y "; only a few seeds of Ni ₃ Nb-δ phase can sometimes be detected by microscopy at high magnification The low volume fraction of these germs and their globular morphology then have no detrimental impact on the mechanical properties.

Grain hardening

At the end of the isothermal maintenance, a final deformation was carried out. Different wrought rates were tested between 8 and 45%, the cooling being carried out in the open air.

When the deformation rate exceeds 25% reduction in thickness, new recrystallization seeds are generated and the structure is then made up of a mixture of hardened fine grains and very fine recrystallized grains: their respective sizes are 8 and 10 / 11 ASTM.

One of the characteristics of the invention is to retain, for the final deformation, deformation rates not exceeding 25%. A homogeneous structure of 8 ASTMs is then obtained, the grains of which have the particularity of being provided with a network of dislocations which tend, in part, to rearrange into a very fine substructure (= 15 ASTM) also hardened, at in the vicinity of the deformed grain boundaries (see Figures 7 to 8A). These latter structures have the best mechanical characteristics, due to the consolidation of the alloy by dislocations and the substructure associated with them.

Reduction of the own quenching stresses

It is common practice in the ranges of implementation of the Inconel 718 alloy, in some blacksmiths, to cool with water at the end of the hot finish. This quenching is the source of significant stresses which are released in a heterogeneous manner during machining and can cause significant deformation, generating costly scrap.

However, in such thermomechanical ranges, the final deformation rates reach very high values approximately 60%) which require cooling with water in order to moderate the recrystallization of the deformed structure, intervening in part during the cooling of the raw parts.

• -séquence (a):
• -fin de forgeage
• -retour à la température ambiante
• -traitement de revenu
• -retour à la température ambiante
• -séquence (b):
• -fin de forgeage
• -traitement de revenu
• -retour à la température ambiante

Two types of sequences can be adopted within the framework of the invention according to the means available from the forging workshop:

• -sequence (a):
• - end of forging
• -back to room temperature
• - income processing
• -back to room temperature
• - sequence (b):
• - end of forging
• - income processing
• -back to room temperature

The first solution (a) consists in allowing the raw, forged parts to cool in the open air on refractory plates, without stacking them. After cooling, the parts undergo a heat treatment limited to the income R of precipitation of the phase y ".

In the second solution (b), the forged piece is placed directly in an oven, without going through the ambient temperature, to undergo the tempering treatment R.

For inconel 718, the tempering treatment applied is one of the known treatments and consists in maintaining for eight hours at 720 ° C followed by cooling at the speed of 50 ° C per hour to 620 ° C with maintenance eight hours at 620 ° C, terminated by air cooling.

No phase platelets δ

The thermomechanical range, object of the invention, made it possible to obtain at the end of forging, a hardened structure, with fine grains, free of phase 8 in platelets. The treatment T = 955 ° C1 h-air has been deliberately excluded from the ranges proposed. Indeed, the latter whose role was to ensure the homogenization of the alloy, before the treatment-R-precipitation of the phase y ", leads in fact, on the one hand to the more or less marked precipitation of platelets phase ô and, on the other hand, to a heterogeneous recrystallization, at the origin of a deconsolidation of the alloy.

It will be noted that the residual work hardening obtained by the invention makes it possible, among other things, to facilitate under certain conditions the germination of a minor phase such as Ni ₃ Nb-b or y ". Taking into account the objective which returns to avoid the precipitation of phase 5, it is thus necessary to suppress the treatment T whose temperature belongs to the domain of existence of phase 5. On the other hand, the application of the income, only, makes it possible to preserve work hardening residual of the structure; moreover, the tempering temperature range (720620 ° C) corresponds to the unique precipitation of the hardening phase y ". Example of ranges according to the invention for the Inconel 718

It is understood that these ranges relate only to final forging operations and in no way prejudge the upstream definition operations.

• -Chauffage de la pièce à 1040°C±10°C (50 minutes de maintien)
• -Déformation à la presse: 45%
• -Mise au four à 970°C pendant 30 minutes.

1) Roughing sequence

• -Heating of the part to 1040 ° C ± 10 ° C (50 minutes of holding)
• - Press training: 45%
• - Put in the oven at 970 ° C for 30 minutes.

At the end of this sequence, the rough part has a homogeneous, fine-grained structure.

2) Finishing sequence

At the end of isothermal maintenance, the part is taken out of the oven to be directly crushed with a deformation rate of 8 to 25%.

This low rate of deformation constitutes an important advantage of the method: it makes it possible to use less powerful tools, therefore more readily available and less expensive.

• -homogène
• fine
• -écrouie.

At the end of this sequence, the blank has a structure:

• -homogeneous
• fine
• - nut.

3) Air cooling

Air cooling can be carried out either at the end of forging, or at the end of the final heat treatment, on a refractory hearth (to avoid excessively rapid heat exchanges).

4) Income

• -un maintien de huit heures à 720°C suivi d'un refroidissement jusqu'à 620°C, à la vitesse de 50°C par heure, avec un maintien de huit heures à cette température puis un retour à la température ambiante en air calme.

It is carried out under the conditions of the standard income processing of the Inconel 718, that is to say:

• -a hold of eight hours at 720 ° C followed by cooling to 620 ° C, at the speed of 50 ° C per hour, with a hold of eight hours at this temperature then a return to ambient temperature in air calm.

Comparison of mechanical characteristics of parts

• A-"Minigrain" selon brevet US 3 660 177+T' R (pour une taille de grain de 10/11 ASTM) T' correspond à un traitement thermique d'une durée d'une heure à 980°C suivi d'un refroidissement à l'air.
• B-Recristallisée à grains fins+T.R. (pour une taille de grain de 7/8 ASTM) T correspond à un traitement thermique d'une durée d'une heure à 955°C suivi d'un refroidissement à l'air.
• C-selon gamme proposée par l'invention (pour une taille de grain de 8 ASTM).
  • a) Caracteristiques de traction a 20 et à 650°C (0=4,5 mm-lo=23 mm)
    
  • b) Caracteristiques de fluage-rupture à 650°C (0=4,5 mm-lo=23 mm) sous o=₇₅₀ Mpa
    
  • c) Caracteristiques de fatigue oligocyclique à 650°C:

In the tables below, we compare the main mechanical characteristics of three typical microstructures (Plate 4) for the Inconel 718:

• A- "Minigrain" according to US patent 3,660,177 + T 'R (for a grain size of 10/11 ASTM) T' corresponds to a heat treatment lasting one hour at 980 ° C followed by a air cooling.
• B-Crystallized fine grain + TR (for a grain size of 7/8 ASTM) T corresponds to a heat treatment lasting one hour at 955 ° C followed by air cooling.
• C-according to the range proposed by the invention (for a grain size of 8 ASTM).
  • a) Traction characteristics at 20 and 650 ° C (0 = 4.5 mm-lo = 23 mm)
    
  • b) Characteristics of creep-rupture at 650 ° C (0 = 4.5 mm-lo = 23 mm) under o = ₇₅₀ Mpa
    
  • c) Characteristics of oligocyclic fatigue at 650 ° C:

Limitation of endurance at the start in imposed deformation.

où e:lt est la déformation longitudinale totale (élastique+plastique).The oligocyclic fatigue tests with imposed total longitudinal deformation were carried out at 650 ° C according to a triangular cycle of frequency 0.05 Hz with:

where e: lt is the total longitudinal deformation (elastic + plastic).

The comparison was mainly established between structures B and C. The results showed a gain of 15 to 20% in endurance limit, of C compared to B.

Claims (8)

1. Method for thermomechanical treatments of precipitation hardened superalloys, comprising a final rough forming sequence, a finishing sequence and a final heat treatment, in which the steps of the method are carried out in succession and consist, in the final rough forming sequence, of:

(a) a heat treatment, for which the conditions of temperature and duration are known per se for the superalloy in question,

(b) a hot compressive strain, with an applied strain of between 30% and 60%, such as to obtain a duplex structure during recrystallization,

(c) a heat treatment consisting of a maintenance at a constant temperature for which, in the application to the superalloy in question, the temperature, which is below the temperature fixed for step (a), and the time during which the temperature is maintained make it possible to obtain a homogeneous structure with a grain size of ASTM 7 or more and in which there do not appear thermodynamically stable precipitates in the form of platelets constituting an embrittling phase, and then, in the finishing sequence which follows on from the preceding steps of the final rough forming sequence:

(d) a hot compressive strain, with an applied strain of between 8% and 25%, such as to cause a work-hardening of the preceding structure, fine-grained and homogeneous, by consolidating the structure without producing any recrystallization, and in which the final heat treatment consists of a single step consisting of:

(e) an anneal, for which the conditions are known per se for the superalloy in question, such as to preserve the work-hardened structure and to cause the precipitation of the hardening phase of said superalloy without the appearance of thermodynamically stable precipitates in the form of platelets constituting an embrittling phase.

2. Method for thermomechanical treatments of precipitation hardended superalloys according to Claim 1 in which step (e) of the method, when the final heat treatment is taking place, follows directly after the preceding step (d) of a hot compressive strain in the finishing sequence.

3. Method for thermomechanical treatments of precipitation hardened superalloys according to Claim 1 in which step (d) of the method, when the hot compressive strain in the finishing sequence is taking place, is followed, before the final heat treatment, by a return to ambient temperature obtained by cooling in still air.

4. Method for thermomechanical treatments of precipitation hardened superalloys according to any one of the preceding claims in which the strain applied in step (b) of the method, when the hot compressive strain of the final rough forming sequence is taking place, is preferably 45%.

5. Method for thermomechanical treatments of a superalloy defined by a composition in which the average concentrations by weight ar given as percentages in the following table:

C, 0.02 to 0.08; Mn, 0.35 (max); Si, 0.35 (max);

P, 0.015 (max); S, 0.015 (max);

Cr, 17 to 21; Fe, 15 to 21; Co, 1 (max);

Mo, 2.80 to 3.30; Nb+Ta, 4.75 to 5.50;

Ti, 0.75 to 1.15; Al, 0.30 to 0.70; B, 0.001 to 0.006;

Cu, 0.15 (max); Ni, making up to 100,

this alloy being obtained by applying techniques, known per se, of smelting and remelting under vacuum, - the method being such as is defined in any one of the preceding claims and in which step (a) of the method, consisting of a heat treatment in the final rough forming sequence, is carried out at a temperature maintained at 1040°C±10°C for a time of fifty minutes, which can go up to one hour.

6. Method for thermomechanical treatments of a superalloy defined by a composition in which the average concentrations by weight are given as percentages in the following table:

C, 0.02 to 0.08; Mn, 0.35 (max); Si, 0.35 (max);

P, 0.015 (max); S, 0.015 (max);

Cr, 17 to 21; Fe, 15 to 21; Co, 1 (max);

Mo, 2.80 to 3.30; Nb+Ta, 4.75 to 5.50;

Ti, 0.75 to 1.15; AI, 0.30 to 0.70; B, 0.001 to 0.006;

Cu, 0.15 (max); Ni, making up to 100,

this alloy being obtained by applying techniques, known per se of smelting and remelting under vacuum, the method being such as is defined in any one of the preceding claims and in which step (b) of the method, consisting of being held at a constant temperature in the final rough forming sequence, is carried out at a temperature maintained at 970°C±10°C for a time of thirty minutes, which can go up to one hour.

7. Method for thermomechanical treatments of a superalloy defined by a composition in which the average concentrations by weight are given as percentages in the following table:

C, 0.02 to 0.08; Mn, 0.35 (max); Si, 0.35 (max);

P, 0.015 (max); S, 0.015 (max);

Cr, 17 to 21; Fe, 15 to 21; Co, 1 (max);

Mo, 2.80 to 3.30; Nb+Ta, 4.75 to 5.50;

Ti, 0.75 to 1.15; AI, 0.30 to 0.70; B, 0.001 to 0.006;

Cu, 0.15 (max); Ni, making up to 100, 10

this alloy being obtained by applying techniques, known per se, of smelting and remelting under vacuum, the method being such as is defined in any one of the preceding claims and in which step (e) of the method, consisting of an anneal forming the final heat treatment, is carried out at a temperature maintained at 720°C for a time of eight hours, followed by cooling to 620°C at a rate of 50°C per hour, being held at the new temperature for eight hours, and terminated by cooling in air so that the structure obtained by the method applied to said superalloy is a structure with fine homogeneous grains of grain size ASTM 7 or more, in which the grains have preserved their work-hardening, said structure comprising a phase hardened by type y" Ni₃Nb precipitates without the simultaneous presence of the embrittling Ni₃Nb type 5 phase in the form of platelets.

Images