EP1211335B1 - Nickel based superalloy having a very high resistance to hot corrosion for single crystal turbine blades of industrial turbines - Google Patents

Description

The invention relates to a nickel-based superalloy suitable for the controlled solidification production of stationary and mobile monocrystalline blades of industrial gas turbines.

Nickel-based superalloys are the most efficient materials used today for the manufacture of stationary and mobile blades for industrial gas turbines. The two main features required so far for these alloys for these specific applications are good creep resistance at temperatures up to 850 ° C and very good resistance to hot corrosion. Reference alloys commonly used in this field are those known as IN738, IN939 and IN792.

The blades made with these reference alloys are prepared by conventional lost-wax casting and have a polycrystalline structure, that is to say that they consist of the juxtaposition of randomly oriented crystals with respect to one another. and called grains. These grains are themselves constituted by a nickel-based austenitic gamma (γ) matrix in which gamma prime (γ ') phase hardening particles whose base is the Ni ₃ Al intermetallic compound are dispersed. These grains give these alloys high creep resistance up to temperatures close to 850 ° C., which guarantees the longevity of the blades for which life times of between 50 000 and 100 000 hours are generally sought. The chemical composition of the IN939, IN738 and IN792 alloys has been defined in order to give them an excellent resistance to the environment of the combustion gases, in particular with respect to hot corrosion, a particularly aggressive phenomenon in the case of gas turbines industrial. Important additions of chromium, typically between 12 and 22% by weight, are thus necessary to give these alloys the hot corrosion resistance required for the applications concerned. From the point of view of creep resistance the classification of these alloys is: IN939 <IN738 <IN792. From the point of view of resistance to hot corrosion, the classification is reversed, ie: IN792 <IN738 <IN939.

To improve the performance of industrial gas turbines, in terms of efficiency and consumption, one way is to increase the temperature of the gases at the inlet of the turbine. This therefore requires the availability of alloys for turbine blades that can withstand increasingly higher operating temperatures, while maintaining the same mechanical characteristics, particularly creep, in order to achieve the same lifetimes.

The same type of problem has arisen in the past in the case of gas turbines for turbojets and turbomachines for aeronautical applications. In this case the solution adopted was to pass so-called polycrystalline blades developed by conventional foundry so-called monocrystalline blades, that is to say consist of a single metallurgical grain.

These monocrystalline blades are manufactured by solidification directed in lost-wax foundry. The elimination of grain boundaries, which are preferred locations for creep deformation at high temperatures, has dramatically increased the performance of nickel-based superalloys. In addition, the monocrystalline solidification process makes it possible to select the preferential orientation of growth of the monocrystalline part and thus to choose the <001> orientation which is optimal from the point of view of resistance to creep and to thermal fatigue, these two mechanical stressing modes being the most harmful for turbine blades.

However, the superalloy chemical compositions developed for monocrystalline turbine blades for aeronautical applications are not suitable for blades for terrestrial or marine applications, so-called industrial applications. These alloys are indeed defined so as to favor their mechanical strength up to temperatures above 1100 ° C, and this to the detriment of their resistance to hot corrosion. Thus, the chromium concentration of the superalloys for single-crystal blades of aeronautical turbines is generally less than 8% by weight, which makes it possible to reach γ 'phase volume fractions of the order of 70%, favorable to resistance to high creep. temperature.

A chromium-rich nickel-based superalloy capable of monocrystalline solidification of industrial gas turbine parts is known under the name SC16 and described in FR 2 643 085 A. Its chromium concentration is equal to 16% by weight. The creep resistance characteristics of the SC16 alloy are such that this alloy provides, relative to the reference polycrystalline alloy IN738, an operating temperature gain of approximately 30 ° C. (830 ° C. instead of 800 ° C.). at about 50 ° C (950 ° C instead of 900 ° C). Comparative tests for cyclic corrosion at 850 ° C in air at atmospheric pressure with Na ₂ SO ₄ contamination showed that the hot-corrosion resistance of the SC16 alloy was at least equivalent to that of the alloy polycrystalline reference IN738.

Hot corrosion tests were conducted on the SC16 alloy by industrial turbine manufacturers in their own test benches. In very severe environments, representative of extreme operating conditions, it has been shown that the resistance to hot corrosion of this alloy remains lower than that of the IN738 alloy.

Moreover, the increasing demand of these manufacturers for an increase in the operating temperature of Gas turbines require even better creep resistance of blade superalloys.

The object of the invention is to provide a nickel-based superalloy having a resistance to hot corrosion, in the aggressive environment of the combustion gases of industrial gas turbines, at least equivalent to that of the reference polycristalline superalloy IN738 , with a creep resistance greater than or equal to that of the IN792 reference alloy in a temperature range of up to 950 ° C.

This superalloy must in particular be suitable for the production by directed solidification of fixed and mobile monocrystalline blades of large dimensions (up to several tens of centimeters in height) of industrial gas turbines.

This superalloy must also show good microstructural stability with respect to the precipitation of chromium-rich brittle intermetallic phases during long-term high temperature holdings.

• Une résistance à la corrosion à chaud optimisée, dans tous les cas au moins égale à celle du superalliage polycristallin de référence IN738, et ce dans un environnement représentatif de celui des gaz de combustion des turbines industrielles;
• Une fraction volumique maximale de précipités durcissants de phase γ' afin de favoriser la résistance au fluage à haute température;
• Une résistance au fluage jusqu'à 950 °C au moins égale à celle de l'alliage polycristallin de référence IN792;
• Une aptitude à l'homogénéisation par remise en solution totale des particules de phase γ', y compris les phases eutectiques γ/γ';
• L'absence de précipitation de phases intermétalliques fragiles riches en chrome, à partir de la matrice γ, au cours de maintiens de longue durée à haute température;
• Une masse volumique inférieure à 8,4 g.cm^-3 afin de minimiser la masse des aubes monocristallines et par conséquent de limiter la contrainte centrifuge agissant sur ces aubes et sur le disque de turbine sur lequel elles sont fixées;
• Une bonne aptitude à la solidification monocristalline d'aubes de turbines dont la hauteur peut atteindre plusieurs dizaines de centimètres et la masse plusieurs kilogrammes.

More specifically, it has been sought an alloy composition ensuring:

• An optimized hot corrosion resistance, in all cases at least equal to that of the reference polycristalline superalloy IN738, and this in an environment representative of that of the combustion gases of industrial turbines;
• A maximum volume fraction of γ 'phase hardening precipitates to promote creep resistance at high temperature;
• Creep resistance up to 950 ° C at least equal to that of the reference polycrystalline alloy IN792;
• An ability to homogenize by total dissolution of the γ 'phase particles, including the eutectic phases γ / γ';
• The absence of precipitation of chromium-rich brittle intermetallic phases, from the γ matrix, during long-term high temperature maintenance;
• A density of less than 8.4 gcm ^{-3 in} order to minimize the mass of the single-crystal blades and consequently to limit the centrifugal stress acting on these vanes and on the turbine disc on which they are fixed;
• Good ability to monocrystalline solidification of turbine blades whose height can reach several tens of centimeters and the mass several kilograms.

• Co: 4,75 à 5,25 %
• Cr: 15,5 à 16,5 %
• Mo: 0,8 à 1,2 %
• W: 3,75 à 4,25 %
• Al: 3,75 à 4,25 %
• Ti: 1,75 à 2,25 %
• Ta: 4,75 à 5,25 %
• C: 0,006 à 0,04 %
• B: ≤ 0,01 %
• Zr: ≤ 0,01 %
• Hf: ≤ 1 %
• Nb: ≤ 1 %
• Ni et impuretés éventuelles: complément à 100 %.

The superalloy according to the invention, capable of monocrystalline solidification, has the following weight composition:

• Co: 4.75 to 5.25%
• Cr: 15.5 to 16.5%
• Mo: 0.8 to 1.2%
• W: 3.75 to 4.25%
• Al: 3.75 to 4.25%
• Ti: 1.75 to 2.25%
• Ta: 4.75 to 5.25%
• C: 0.006 to 0.04%
• B: ≤ 0.01%
• Zr: ≤ 0.01%
• Hf: ≤ 1%
• Nb: ≤ 1%
• Ni and possible impurities: 100% complement.

The alloy according to the invention has an excellent compromise between creep resistance and hot corrosion resistance. It is suitable for the manufacture of monocrystalline parts, that is to say made of a single metallurgical grain. This particular structure is obtained for example by means of a conventional method of solidification directed in a thermal gradient, using a helical or baffled grain selection device or a monocrystalline seed.

The invention also relates to an industrial turbine blade made by monocrystalline solidification of the superalloy above.

The features and advantages of the invention will be set forth in more detail in the description below, with reference to the accompanying drawings.

Figures 1 to 4 are graphs illustrating the properties of different superalloys.

An alloy according to the invention called SCA425 was developed by aiming at the nominal composition presented in Table I. In this table are also reported the nominal concentrations in major elements of the reference alloys IN939, IN738, IN792 and SC16. Table I: Concentrations by weight in major elements (%) Alloy Or Co Cr MB W al Ti Your Nb IN939 Based 19 22.5 - 2 1.9 3.7 1.4 1 IN738 Based 8.5 16 1.7 2.6 3.4 3.4 1.7 0.9 IN792 Based 9 12.4 1.9 3.8 3.1 4.5 3.9 - SC16 Based - 16 3 - 3.5 3.5 3.5 - SCA425 Based 5 16 1 4 4 2 5 -

Chromium has a beneficial and preponderant effect on the resistance to hot corrosion of nickel-based superalloys. The experiment has thus shown that a concentration close to 16% by weight was necessary in the alloy of the invention to obtain a resistance to hot corrosion equivalent to that of the reference alloy IN738 under the conditions of the tests. described below, which are representative of the environment created by the combustion gases of certain industrial turbines. Chromium also participates in the hardening of the γ matrix in which this element is distributed preferentially.

Molybdenum strongly hardens the matrix y in which this element is distributed preferentially. The amount of molybdenum that can be introduced into the alloy is however limited because this element has a detrimental effect on the resistance to hot corrosion of the superalloys based on nickel. A concentration close to 1% by weight in the alloy of the invention is not detrimental to its resistance to corrosion and significantly contributes to its hardening.

Cobalt also participates in solid solution hardening of the γ matrix. The cobalt concentration has an influence on the solution temperature of the hardening phase γ '(solvus temperature γ'). It is thus advantageous to increase the cobalt concentration to lower the solvus temperature of the γ 'phase and to facilitate the homogenization of the alloy by heat treatment without risk of causing a start of melting. Moreover, it may also be advantageous to reduce the cobalt concentration in order to increase the solvus temperature of the γ 'phase and thus to benefit from greater stability of the γ' phase at high temperature, which is favorable to the creep resistance. The concentration of 5% by weight of cobalt in the alloy of the invention leads to an optimal compromise between good homogenization ability and good creep resistance.

The tungsten whose concentration is around 4% by weight in the alloy of the invention is distributed substantially equally between the γ and γ 'phases and thus contributes to their respective hardenings. Its concentration in the alloy is however limited because this element is heavy, and has a negative effect on the resistance to hot corrosion.

The concentration of aluminum is around 4% by weight in the alloy of the invention. The presence of this element causes the precipitation of the hardening phase γ '. Aluminum also promotes resistance to oxidation. The titanium and tantalum elements are added to the alloy of the invention in order to reinforce the γ 'phase in which they substitute for the aluminum element. The respective concentrations of these two elements in the alloy of the invention are close to 2% by weight for titanium and 5% by weight for tantalum. In the test conditions of Hot corrosion described below, corresponding to the intended application, experience has shown that the presence of tantalum was more favorable to the resistance to hot corrosion than that of titanium. However, tantalum is heavier than titanium which is unfavorable vis-à-vis the density of the alloy. The sum of the concentrations of tantalum, titanium and aluminum roughly defines the hardening phase volume fraction γ '. The concentrations of these three elements have been adjusted in such a way as to optimize the γ 'phase volume fraction, while keeping the stable γ and γ' phases during the long-term hold at high temperature, and taking into account that the Chromium concentration was set at about 16% by weight so as to achieve the desired corrosion resistance.

The SCA425 alloy was developed as <001> orientation single crystals. The density of this alloy was measured and found to be 8.36 g.cm ^-3 .

After directed solidification, the alloy consists essentially of two phases: the austenitic matrix γ, a nickel-based solid solution, and the γ 'phase, an intermetallic compound whose basic formula is Ni ₃ Al, which precipitates for the most part at within the matrix y in the form of fine particles smaller than one micrometer during the solid state cooling. Contrary to what is generally encountered in monocrystalline superalloys for turbine blades, the SCA425 alloy does not contain massive interdendritic particles of γ 'phase resulting from a eutectic transformation of the residual liquid at the end of solidification.

The SCA425 alloy underwent a homogenization heat treatment at a temperature of 1285 ° C. for 3 hours with cooling in air. This temperature is higher than the solvus temperature of the γ 'phase (dissolution temperature of the γ' phase precipitates), which is equal to 1198 ° C., and lower than the melting start temperature, equal to 1300 ° C. The purpose of this treatment is to dissolve all of the γ 'phase precipitates, the size distribution of which is very extensive in the raw state of directed solidification and to reduce the chemical heterogeneities associated with the solidification dendritic structure.

The difference between the solvate temperature γ 'of the SCA425 alloy and its melting start temperature is very large, which allows the easy application of the homogenization treatment without risk of melting and with the certainty of obtaining a homogeneous microstructure allowing optimized creep resistance.

The cooling following the homogenization treatment described above was carried out by quenching in air. In practice, the speed of this cooling must be sufficiently high so that the size of the particles having precipitated during this cooling is less than 500 nm.

The homogenization heat treatment procedure that has just been described is an example that makes it possible to obtain the desired result, ie a homogeneous distribution of fine γ 'phase particles whose size does not exceed 500 nm. This does not exclude the possibility of obtaining a similar result by using another processing temperature provided that it is in the range separating the solvate temperature γ 'and the melting start temperature.

The SCA425 alloy was tested after having been subjected to a homogenization treatment as described above, then to two tempering treatments making it possible to stabilize the size and the volume fraction of the γ 'phase precipitates. A first treatment of income was to heat the alloy at 1100 ° C for 4 hours with cooling in air which has the effect of stabilizing the size of γ phase precipitates. A second treatment of income at 850 ° C for 24 hours, followed by cooling in the air, allows to optimize the volume fraction of phase γ '. This γ 'phase volume fraction is estimated at 50% in the SCA425 alloy. At the end of all the heat treatments, most of the γ 'phase precipitated in the form of cuboidal particles whose size is between 200 and 500 nm. A small fraction of γ 'phase fine particles whose size does not exceed 50 nm is present between the large precipitates.

Hot corrosion tests were carried out at different temperatures on the SCA425 alloy using the following procedure: samples are partially immersed in a crucible containing a mixture of ash whose weight composition is as follows: 4.3% Na ₂ SO ₄ + 22.7% CaSO ₄ + 22.3% Fe ₂ O ₃ + 20.6% ZnSO ₄ + 10.4% K ₂ SO ₄ + 2.8% MgO + 6.5% Al ₂ O ₃ + 10.4% SiO ₂ . A mixture of air + 0.15% SO ₂ by volume passes through the ash mixture at the rate of 6 liters per hour. The ash mixture is renewed every 500 hours. This environment is representative of the very aggressive environment of the combustion gases of certain industrial turbines. For comparison alloy samples IN738, IN939, IN792 and SC16 were tested simultaneously.

The samples were sectioned and the metal depth destroyed by the corrosion phenomenon was measured. The graphs in Figures 1-3 show average corrosion penetration depths for the various alloys at 700 ° C, 800 ° C and 850 ° C respectively, depending on the test time. The corrosion resistance is better the lower the depth of penetration. At 700 ° C and 800 ° C the alloy SCA425 shows a corrosion resistance equivalent to that of the alloy IN738 and better than that of the alloy SC16. At 850 ° C, the corrosion resistance of the SCA425 alloy is comparable to that of the IN738 and IN939 reference alloys.

Tensile creep tests were carried out on specimens machined in monocrystalline bars Orientation <001>. The bars were previously homogenized and then returned according to the procedures described previously. Break time values obtained at 750, 850 and 950 ° C for different levels of applied stress are reported in Table II. Table II: Creep lifetimes of SCA425 alloy Temperature (° C) Constraint (MPa) Break time (h) 750 650 216 / 321.1 750 575 984 850 400 201/276 850 300 2121/2945/3220 850 250 6161 950 250 73/76 950 200 261/291 950 180 578 950 160 1098 950 140 2109 950 120 3872

The graph in FIG. 4 makes it possible to compare the creep rupture times obtained for the SCA425, IN792 and SC16 alloys. On the abscissa is the applied stress. On the ordinate is the value of the Larson-Miller parameter. This parameter is given by the formula P = T (20 + log t) × 10 ^-3 where T is the creep temperature in Kelvin and t is the break time in hours. This graph shows that the creep resistance of the SCA425 alloy is at least equivalent to that of the IN792 alloy, which is the fixed objective, and higher than that of the reference alloy SC16.

Control of microstructure of SCA425 alloy specimens after creep testing demonstrated the absence of precipitation of chromium-rich brittle intermetallic particles that may occur during long-term high temperature holdovers in superalloys based on nickel where the matrix is supersaturated with addition elements.

Manufacturing trials of SCA425 superalloy monocrystalline parts have shown that it is possible to cast a wide range of components ranging in weight from a few grams to over 10 kg, with varying degrees of complexity. Particle growth in the <001> crystallographic orientation is favored and dominant and the presence of randomly oriented grains is minimized. The liquid metal is stable in that it does not react with materials commonly used for mold making. The recrystallization phenomenon that can occur during the high temperature homogenization treatment is absent in the case of the SCA425 alloy.

Claims (2)

1. A nickel-based superalloy, suitable for single crystal solidification, characterized in that its composition by weight is as follows: Co: 4,75 à 5,25 % Cr: 15,5 à 16,5 % Mo: 0,8 à 1,2 % W: 3,75 à 4,25 % Al: 3,75 à 4,25 % Ti: 1,75 à 2,25 % Ta: 4,75 à 5,25 % C: 0,006 à 0,04 % B: ≤ 0,01 % Zr: ≤ 0,01 % Hf: ≤ 1 % Nb: ≤ 1 % Ni and possible impurities: to 100%.
2. An industrial turbine blade produced by single crystal solidification of a superalloy according to claim 1.

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