JP2002235135A - Nickel based superalloy having extremely high temperature corrosion resistance for single crystal blade of industrial turbine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a nickel based superalloy which is used for producing a stationary or movable single crystal blade of an industrial gas turbine by directional solidification. SOLUTION: The nickel based superalloy has extremely high temperature corrosion resistance, and is used for producing a single crystal blade of an industrial turbine. The alloy has a composition containing, by mass, 4.75 to 5.25% Co, 15.5 to 16.5% Cr, 0.8 to 1.2% Mo, 3.75 to 4.25% W, 3.75 to 4.25% Al, 1.75 to 2.25% Ti, 4.75 to 5.25% Ta, 0.006 to 0.04% C, <=0.01% B, <=0.01% Zr, <=1% Hf, <=1% Nb, and the balance Ni with impurities. The alloy is suitable for single crystal solidification.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nickel-based superalloy used for producing fixed and movable single crystal blades of an industrial gas turbine by directional solidification.

[0002]

BACKGROUND OF THE INVENTION Nickel-based superalloys are the highest performing materials currently utilized in the manufacture of mobile and stationary blades for industrial gas turbines. Two key features currently required for these alloys for such specific applications are excellent creep resistance at temperatures up to 850 ° C. and very good hot corrosion resistance. Some standard alloys currently utilized in this field are IN738, IN939 and IN792.

[0003] Blades manufactured using these related alloys are manufactured by ordinary casting using a lost wax method, and the structure thereof is polycrystalline. In other words, it is a structure in which crystals called crystal grains are juxtaposed with each other and randomly oriented. These crystal grains itself is constituted by an austenitic gamma (gamma) matrix based on nickel, in the matrix, gamma and based intermetallic compound Ni ₃ Al-prime (gamma
') The cured particles of the phase are dispersed. This structure of crystalline particles imparts these alloys a high level of creep resistance at temperatures up to about 850 ° C. and generally ranges from 50,000 to 1
Guarantees the life of blades that require durability of 00000 hours.

[0004] The chemical composition of the alloys IN939, IN738 and IN792 is also excellent in the resistance to combustion gas environments, especially against high temperature corrosion which is a severe phenomenon in industrial gas turbines. It has been determined to be resistant. Therefore, it is necessary to add significant amounts of chromium, for example 12 to 22% by weight, in order to impart the required hot corrosion resistance to these alloys in the intended application. When ranking in terms of creep resistance, IN939 <IN738 <IN7
92. The order from the point of high temperature corrosion resistance is opposite. That is, IN792 <IN738 <IN939.

One method of improving the power and wear characteristics of an industrial gas turbine is to increase the gas temperature at the turbine outlet. For this, the alloys for turbine blades must be able to withstand increasingly high operating temperatures while maintaining the same mechanical properties, especially creep resistance, or they will achieve the same durability It is not possible.

The same problem has been observed in the past for turbojet gas turbines and turbo engine gas turbines in aviation applications. In this case, a blade known as a single crystal blade, a blade known as a polycrystalline blade manufactured by ordinary casting,
That is, the problem was solved by changing to a blade constituted by metallurgical single crystal particles.

[0007] These single crystal blades are manufactured by directional solidification using lost wax casting. Removing the grain boundaries where creep deformation is likely to occur at high temperatures dramatically improves the properties of the nickel-base superalloy.
When single crystal solidification is applied, a suitable growth direction of the single crystal component can be selected. That is, the optimum <001> orientation can be selected in view of creep resistance and thermal fatigue. The worst stresses for a turbine blade are these two types of mechanical stress.

However, chemical superalloy compounds developed for single crystal turbine blades for aeronautical applications are not suitable for blades for terrestrial and marine applications known for industrial applications. In the case of these alloys, the purpose is to improve the mechanical resistance at a temperature of 1100 ° C. or higher,
This is detrimental to hot corrosion resistance. in this case,
The chromium concentration in superalloys intended for aviation single crystal turbine blades is generally less than 8% by mass, in other words, γ ′
The phase volume ratio is on the order of 70%, which is advantageous for creep resistance at high temperatures.

[0009] An alloy rich in chromium and known as a nickel-based superalloy suitable for single crystal solidification of the components of industrial gas turbines is SC16, which is described in French Patent Specification No. 2643085. ing. The chromium concentration is 16% by mass. Regarding the creep resistance properties of alloy SC16, the operating temperature is approximately 30 ° C. (830 ° C. instead of 800 ° C.) to approximately 50 ° C. (950 ° C. instead of 900 ° C.) when compared to polycrystalline alloy IN738.
° C). A comparative corrosion cycle test performed using Na ₂ SO ₄ in air at 850 ° C. under atmospheric pressure showed that the high temperature corrosion resistance of alloy SC16 was at least equivalent to the reference polycrystalline alloy IN738.

[0010] Manufacturers of industrial turbines also perform high temperature corrosion tests on SC16 using their own test benches. In very harsh environments exhibiting extreme operating conditions, it has been found that the hot corrosion resistance of alloy SC16 is inferior to that of alloy IN738. In addition, there is a high demand for manufacturers to improve the operating temperature of gas turbines, and thus the need for blade superalloys with higher creep resistance.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide at least high temperature corrosion resistance in a highly corrosive combustion gas environment of an industrial turbine at least equal to that of a reference polycrystalline superalloy IN738, and a creep resistance of 950. It is to provide a nickel-based superalloy comparable to or higher than the reference alloy IN792 at temperatures up to ° C. This superalloy is particularly suitable for the production of fixed and movable single crystal blades of large dimensions (several tens cm or less) of industrial gas turbines by directional solidification.

Further, the superalloy must exhibit excellent microstructural stability with respect to the precipitation of a chromium-rich brittle intermetallic phase when maintained at elevated temperatures for extended periods of time. More specifically, an object of the present invention is to obtain an alloy material having the following characteristics.

Optimized hot corrosion resistance. In any case, this hot corrosion resistance is at least comparable to the polycrystalline superalloy IN738, which is referenced in an environment that represents the combustion gas environment of an industrial turbine. Hardened precipitate of γ 'phase with maximum capacity ratio. The purpose is to improve the creep resistance at high temperatures.
Creep resistance at up to 950 ° C. is at least comparable to the referenced polycrystalline alloy IN792.

The present invention realizes homogeneity due to complete solid solution of γ ′ phase including γ / γ ′ eutectic phase in solid solution particles. It is the prevention of the precipitation of a brittle intermetallic layer rich in chromium from the gamma matrix, which occurs when maintained at high temperatures for a long time. 8.4g
A density of less than cm ^-3 . The purpose is to minimize the mass of the single crystal blade and to limit the centrifugal stress acting on the blade and on the turbine disk holding the blade. Excellent single crystal solidification of turbine blades with a height of several tens of cm and a mass of several kg.

[0015]

SUMMARY OF THE INVENTION A superalloy according to the present invention, suitable for single crystal solidification, has the following composition by mass. 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 impurities: 100% in addition to each component.

The alloy of the present invention is an alloy in which creep resistance and hot corrosion resistance are extremely balanced. This alloy produces a single crystal component, ie, a component consisting of metallurgical single crystal particles. This particular texture is produced, for example, by a conventional directional solidification method applying a thermal gradient, using a helical or chicane type device to select crystal grains or single crystal nuclei. The invention also relates to industrial turbine blades produced by single crystal solidification of the superalloys.

[0017]

DETAILED DESCRIPTION OF THE INVENTION An alloy of the present invention, referred to herein as SCA425, was produced with the nominal composition shown in Table 1. Table 1 shows reference alloys IN939, IN738, IN79.
Nominal concentrations of main elements in 2 and SC16 are also shown.

[0018]

[Table 1] Table 1: Concentration (%) by mass of main elements Alloy NiCoCrMoWAlTiTaNb IN939 balance 19 22.5-2 1.9 3.7 1.4 1 IN738 balance 8.5 16 1.7 2.6 3.4 3.4 1.7 0.9 IN792 balance 9 12.4 1.9 3.8 3.1 4.5 3.9-SC16 remaining-16 3-3.5 3.5 3.5-SCA425 remaining 5 16 1 4 4 2 5-

Chromium is an element that has an advantageous and dominant effect on the high-temperature corrosion resistance of nickel-based superalloys. According to the test, the hot corrosion resistance is comparable to that of the reference alloy IN738 under the conditions of the hot corrosion test described below, which represents the environment produced by the combustion gases of certain industrial turbines. In order to obtain chromium, it is necessary to add about 16% by mass of chromium to the alloy of the present invention. Chromium is also an element that contributes to the hardening of the gamma matrix in which this element is preferentially distributed.

Molybdenum is an element having a great effect on the hardening of the γ matrix in which this element is preferentially distributed. Note that there is a limit on the amount of molybdenum that can be incorporated into the alloy. This is because molybdenum is an element that has an adverse effect on the high-temperature corrosion resistance of a nickel-based superalloy. Molybdenum added to the alloy of the present invention at a concentration of about 1% by mass does not adversely affect corrosion resistance and is an element that greatly contributes to hardening.

Cobalt is also an element that contributes to the hardening of the γ matrix as a solid solution. The concentration of cobalt is γ
'The solid solution temperature of the hardened phase (γ' solvus temperature) is affected.
Therefore, it is advantageous to increase the cobalt concentration in order to lower the solvus temperature of the γ ′ phase and promote homogenization of the alloy by heat treatment in a state where solid solution does not start to occur. In addition, to increase the solvus temperature of the γ 'phase and better utilize the greater stability of the γ' phase at high temperatures, which leads to enhanced creep resistance, it is also advantageous to lower the concentration of cobalt. is there. When blended with the alloy of the present invention at a concentration of about 5% by mass, the balance between excellent homogenization ability and excellent creep resistance can be optimized.

Tungsten incorporated in the alloy of the present invention at a concentration of about 4% by mass is an element that is substantially equally distributed between the γ phase and the γ ′ phase and thus contributes to each hardening process. There is a limit to the tungsten concentration to be added to the alloy because this element is heavy and has a negative effect on high-temperature corrosion resistance.

The concentration of aluminum in the alloy of the present invention is about 4% by mass. When this element is present, γ '
A hardened phase precipitates. Aluminum is also an element that promotes oxidation resistance. In order to strengthen the γ ′ phase, elemental titanium and tantalum may be added to the alloy of the present invention. In this case, these two elements will replace the element aluminum. The concentration of these two elements in the alloy of the present invention is about 2% by mass for titanium and about 5% by mass for tantalum.

According to tests performed under the high temperature corrosion test conditions described below corresponding to the intended use, tantalum is more advantageous for high temperature corrosion resistance than titanium, but tantalum heavier than titanium is Is disadvantageous when viewed from the density of The volume ratio of the γ 'hardened phase is roughly determined by the total concentration of tantalum, titanium and aluminum. Considering the fact that the γ and γ ′ phases are stabilized when maintained at high temperatures for a long period of time and the chromium concentration is fixed at approximately 16% by mass in order to obtain the desired corrosion resistance, the capacity of the γ ′ phase is taken into account. The concentrations of these three elements can be adjusted to optimize the ratio. Alloy SCA425 was manufactured as a single crystal with orientation <001>. The density of this alloy, according to measurements,
It was 8.36 gcm- ³ .

After directional solidification, the constituent phases of the alloy are two phases:
That is, there are an austenitic γ matrix that is a nickel-based solid solution and a γ ′ phase that is an intermetallic compound. The intermetallic compound has a basic formula of Ni ₃ Al and precipitates mainly in a γ matrix as fine particles of less than 1 μm during cooling to a solid state. Contrary to the general perception of single crystal superalloys for turbine blades, alloy SCA 425 has no gamma prime phase dendritic particles resulting from the eutectic transformation of the residual liquid after solidification is complete.

At a temperature of 1285 ° C., 3
A time homogenization heat treatment was performed, followed by cooling in air. This temperature is higher than the solvus temperature (solid solution temperature of the γ ′ phase) of the precipitate of the gamma ′ phase which is 1198 ° C., but lower than the solidus temperature which is 1300 ° C. The intention of this treatment is to dissolve all the precipitates of the γ 'phase whose distribution size is very wide in the coarse state of directional solidification, and to suppress the chemical heterogeneity accompanying the dendritic solidification structure. is there. Since the difference between the γ ′ solvus temperature and the solidus temperature of the alloy SCA425 is very wide, the homogenization treatment can be easily performed without fear of melting, and a uniform microstructure can be reliably obtained. The creep resistance can be optimized.

Following the above homogenization treatment, cooling was performed by curing in air. In actual application, it is necessary to increase the cooling speed, and the size of particles precipitated during the cooling process is less than 500 nm. The above-described homogenization heat treatment is an example in which intended results are obtained. That is, this is an example in which a homogeneous distribution of fine particles of the γ ′ phase having a particle size not exceeding 500 nm is obtained. Note that this does not exclude the possibility of obtaining similar results by applying different processing temperatures, performed under the condition that the temperature is set between the γ ′ solvus temperature and the solidus temperature.

After performing the homogenization treatment as described above, γ
Alloy SCA425 was tested at two annealing temperatures to stabilize the size and volume ratio of the 'phase precipitate. First
In the annealing process, the alloy is heated to 1100 ° C. for 4 hours and then cooled in air to stabilize the size of the γ ′ phase precipitate. Then, after performing the second annealing treatment at 850 ° C. for 24 hours, cooling is performed in the air to optimize the volume ratio of the γ ′ phase. In the case of alloy SCA425, γ
According to the evaluation, the capacity ratio of the 'phase is 50%. After all heat treatments, the particle size is 200 nm and 500
Most of the γ ′ phase precipitates as cubic particles between the nm and nm. The fine particles of the γ ′ phase having a low volume ratio and a particle size not exceeding 50 nm exist between the large precipitates.

Hot corrosion tests were performed at different temperatures using alloy SCA425. The test method is as follows.
The composition is 4.3% Na ₂ SO ₄ + 22.7% CaS by mass ratio.
O ₄ + 22.3% Fe ₂ O ₃ + 20.6% ZnSO ₄ +1
0.4% K ₂ SO ₄ + 2.8% MgO + 6.5% Al ₂ O ₃
Partially immersing the sample in a container the mixture was loaded in + 10.4% SiO ₂ in which the combustion residues. A mixture of air + 0.15% by volume SO ₂ is passed through the combustion residue mixture at a rate of 6 liters / hour. The combustion residue mixture is replenished every 500 hours. This environment represents a very harsh environment consisting of the combustion gases of some industrial turbines. For comparison, alloys IN738, IN939, IN792 and SC
Sixteen samples were also tested.

The sample was cut, and the depth of the metal destroyed by the fragment corrosion phenomenon was measured. The graphs of FIGS. 1-3 show the average corrosion depth of the different alloys at 700 ° C., 800 ° C. and 850 ° C., respectively, as a function of the length of the test time. The corrosion depth is shallow and the corrosion resistance is very good. 7
At 00 ° C. and 800 ° C., the corrosion resistance of alloy SCA425 is comparable to alloy IN738 and superior to alloy SC16. At 850 ° C., the corrosion resistance of alloy SCA425 corresponds to the reference alloys IN738 and IN939.

A tensile stress creep test was performed using a mechanically cut test piece as a single crystal rod of orientation <001>. The single crystal rod was previously homogenized and then annealed according to the above procedure. Table 2 shows the rupture time values at 750 ° C, 800 ° C and 950 ° C under different levels of stress.

[0032]

[Table 2] Durability temperature (° C) in creep test of alloy SCA425 Stress (MPa) Rupture time (h) 750 650 216 / 321.1 750 575 984 850 400 400 201/276 850 300 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

According to the graph shown in FIG. 4, the alloy SCA4
The creep rupture times obtained for 25, IN792 and SC16 can be compared. The horizontal axis represents the applied stress. The vertical axis is Larson-Miller.
The value of the parameter. An expression representing this parameter is as follows. P = T (20 + logt) × 10 ⁻³ where T is the creep temperature (Kelvin) and t is the rupture time (h). From this graph, it can be seen that the creep resistance of the alloy SCA425 is at least equal to the creep resistance of the target alloy IN792, and is higher than the creep resistance of the reference alloy SC16.

At the end of the creep test, the microstructure of the alloy SCA425 test piece was examined. In the case of a nickel-based superalloy rich in chromium and having a γ matrix supersaturated with an additive element, It was found that no precipitation of brittle intermetallic particles which could possibly occur was observed.

A production test was conducted on the single crystal component of the superalloy SCA425.
It has been found that components in a wide range of g or more and at various levels of complexity can be cast. Crystal orientation <001
> The growth of the components is promoted and dominant,
Also, the presence of randomly oriented crystal grains is reduced to a minimum. Liquid metals are stable in the sense that they do not react with the materials normally used in mold production. The recrystallization phenomenon, which tends to occur during the homogenization process at high temperature, is also described in SCA425.
Not allowed for alloys.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating characteristics of different superalloys.

FIG. 2 is a diagram illustrating characteristics of different superalloys.

FIG. 3 is a diagram illustrating characteristics of different superalloys.

FIG. 4 is a diagram illustrating characteristics of different superalloys.

 ──────────────────────────────────────────────────の Continuing on the front page (71) Applicant 501462826 Kestrel Way Exter, Devon EX2 7LG GREAT-BRITAIN (72) Inventor Brackler Michell UK Ex2 5 Elk Yk Exeter Granger Close 1591940 Leuri Alejandr Alejandr (72) Inventor Malcolm McCorbin Gordon E.L.N. 6 8 Deep Linen North Heikeham Broadway 41 (72) Inventor Bahi Raeshbar Prasad Germany 14167 Berlin Ninechempastrasse 42 Be (72) Inventor Escare André Marcel France 65100 Omex Luka Rail · Batsurugeru 7 (72) inventor Relais Lauren France 77140 Dabaru Ryu de la Baroderi 50 F-term (reference) 3G002 EA06

Claims (2)

[Claims] 1. A nickel-based superalloy characterized by the following mass ratio suitable for single crystal solidification, which is suitable for single crystal solidification. 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 impurities: 100% in addition to each component. 2. An industrial turbine blade produced by single crystal solidification of the superalloy according to claim 1.