JP2002194467A - Nickel based superalloy having high temperature corrosion resistance for single crystal blade of industrial turbine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nickel based superalloy used for producing the fixed and movable single crystal blades of an industrial gas turbine by directional solidification. SOLUTION: The nickel based superalloy suitable for single crystal solidification has a composition containing, by mass, 4.75 to 5.25% Co, 11.5 to 12.5% Cr, 0.8 to 1.2% Mo, 3.75 to 4.25% W, 3.75 to 4.25% Al, 4 to 4.8% Ti, 1.75 to 2.25% Ta, 0.006 to 0.04% C, <=0.01% B, <=0.01% Zr, <=1% Hf and <=1% Nb, and the balance Ni with impurities.

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 alloys currently used in this area are:
IN738, IN939 and IN792.

[0003] Blades manufactured using these alloys are manufactured by ordinary casting using the 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 themselves are constituted by an austenitic gamma (γ) matrix based on nickel, which includes a gamma prime (γ ′) based on an intermetallic compound Ni ₃ Al.
The cured particles of the phase are dispersed. This structure of crystalline particles imparts these alloys a high level of creep resistance at temperatures around 850 ° C., generally between 50,000 and 100,000.
Guarantees the life of blades that require durability of 000 hours. Alloys IN939, IN738 and I
The chemical composition of N792 has been determined such that these alloys have excellent resistance to combustion gas environments, especially to hot corrosion, which is a severe phenomenon in industrial gas turbines. Thus, in order to impart the required high temperature corrosion resistance to these alloys in the intended application, a considerable amount of, for example, 12-22% by weight,
Of chromium must be added. When ranking in terms of creep resistance, IN939 <IN738 <IN792
It is. The order from the point of high temperature corrosion resistance is opposite. That is, IN792 <IN738 <IN939.

One way to improve 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 recognized in the past in 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.

[0006] These single crystal blades are manufactured by directional solidification utilizing 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 turbine blades 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 volume percentage of the phase is on the order of 70%, which is advantageous for creep resistance at high temperatures.

An alloy rich in chromium and known as a nickel-based superalloy suitable for single crystal solidification of components of industrial gas turbines is SC16, FR2,64.
3,085A. The chromium concentration is 16% by weight. Regarding the creep resistance properties of alloy SC16, the operating temperature rises from 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. It is. Na ₂ S in air at 850 ° C under atmospheric pressure
From the comparative corrosion cycle test performed using O ₄ , the high temperature corrosion resistance of the alloy SC16 was determined to be the reference polycrystalline alloy IN738.
Was found to be at least equivalent to

[0009] 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, the high temperature corrosion resistance of alloy SC16 is higher than that of alloy IN738.
It turned out to be worse. 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.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a polycrystalline superalloy IN792 which at least refers to high-temperature corrosion resistance in highly corrosive combustion gas environments of industrial turbines.
And provide a nickel-based superalloy with a creep resistance comparable to or greater than alloy IN792 at temperatures up to 1000 ° C. This superalloy is particularly suitable for the production of large fixed and movable single crystal blades (of several tens cm or less in height) of industrial gas turbines by directional solidification.

Furthermore, the superalloy must exhibit excellent microstructural stability with respect to the precipitation of a chromium-rich brittle intermetallic phase when maintained at high temperatures for long periods of time. More specifically, an object of the present invention is to obtain an alloy compound having the following properties.

Optimized hot corrosion resistance. In any case, this hot corrosion resistance is at least comparable to the polycrystalline superalloy IN792 in each environment representing 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. 1000
Creep resistance at temperatures up to 100 ° C. is the reference polycrystalline alloy I
It is better than N792.

This is the realization of homogeneity due to complete solid solution of the γ ′ phase including the γ / γ ′ eutectic phase in the solid solution particles. From a γ matrix, which occurs when maintained for a long time at a high temperature,
Prevention of precipitation of a brittle intermetallic layer rich in chromium.

A density of less than 8.4 g · 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]

The superalloy suitable for single crystal solidification of the present invention has the following composition by weight. Co: 4.75 to 5.25% Cr: 11.5 to 12.5% Mo: 0.8 to 1.2% W: 3.75 to 4.25% Al: 3.75 to 4.25% Ti: 4 to 4.8% Ta: 1.75 to 2.25% 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 is suitable for producing single crystal components, ie components 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 SCB444, was produced with the nominal compositions listed in Table 1. Table 1 shows the reference alloys IN939, IN738, IN792.
The nominal concentrations of main elements in SC16 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-SCB444 remaining 5 12 1 4 4 4.4 2-

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 test conditions described below show a hot corrosion resistance comparable to the hot corrosion resistance of alloy IN792 under conditions representing the environment created by the combustion gases of certain industrial turbines. In order to obtain it, it is necessary and sufficient conditions to add about 12% by mass of chromium to the alloy of the present invention. The higher the chromium concentration, the higher the γ ′ required for the excellent creep resistance of the alloy at temperatures up to 1000 ° C.
If the phase volume ratio cannot be realized, and if it is realized, a brittle intermetallic compound rich in chromium is precipitated in the γ matrix, and the alloy becomes unstable. 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 blended in the alloy of the present invention at a concentration of about 1% by mass has no effect on 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. Note that 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 4.4% by mass for titanium and about 2% by mass for tantalum. Tests performed under the hot corrosion test conditions described below, corresponding to the intended application, show that tantalum has a higher hot corrosion resistance than titanium, but the concentration of titanium, on the other hand, Due to the fact that elements tend to have a negative effect on oxidation resistance, and on the other hand, if the concentration of titanium is too high,
There is a limitation due to the instability of the γ 'phase. 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 a high temperature for a long period of time, and that the chromium concentration is fixed to approximately 12% 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.

The alloy SCB444 was manufactured as a single crystal having the orientation <001>. The density of this alloy was 8.22 g · cm ^{−3 as} measured.

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 mainly precipitates in the γ matrix as fine particles of less than 1 μm during cooling to a solid state. The γ 'phase has a small capacity, and liquid eutectic transformation after solidification is complete →
It is also present inside the solid particles resulting from γ + γ ′. In addition,
The volume ratio of the γ / γ ′ eutectic phase is about 1.4%.

At a temperature of 1270 ° C., 3
A time homogenization heat treatment was performed, followed by cooling in air. This temperature is higher than the solvus temperature of the gamma-phase precipitate (solid solution temperature of the gamma-phase precipitate) which is 1253 ° C., but is 1285 ° C.
It is lower than the solidus temperature which is ° C. The intention of this treatment is to dissolve all γ ′ phase precipitates having a very wide distribution size in the coarse state of directional solidification, to eliminate solid γ / γ ′ eutectic particles, and to form a dendritic solidified structure. The purpose is to suppress the accompanying chemical heterogeneity.

Since the difference between the γ ′ solvus temperature of alloy SCB444 and its solidus temperature is very wide, homogenization can be easily performed without fear of melting, and a homogeneous microstructure can be reliably obtained. And therefore the creep resistance can be optimized. Following the homogenization process described above,
Cooling was performed by curing in air. In practical applications, the cooling rate must be increased, and the size of the 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, when the particle size is 50
This is an example in which a homogeneous distribution of fine particles of the γ ′ phase not exceeding 0 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 SCB444 was tested at two annealing temperatures to stabilize the size and volume ratio of the 'phase precipitate. First
In the annealing treatment, 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 SCB444, γ '
The phase volume ratio is 57% according to the evaluation. After all heat treatments, the particle size is 200 nm and 500
The γ ′ phase was precipitated as cubic particles having a particle size between the γ ′ phase and the nm.

A high temperature corrosion cycle test of alloy SCB444 was performed at 900 ° C. on an industrial corrosion bench equipped with a burner. The cycle used is as follows. A test piece in a corrosive atmosphere created by a burner was
Hold for a time, then remove from oven and hold at ambient temperature for 15 minutes. The burner fuel contained 0.20% sulfur. 0.5 g · l ⁻¹ NaCl saline was evaporated on the specimen at a rate of 2.2 m ³ · h ⁻¹ . 1
0.5 mg · cm ⁻² Na ₂ SO ₄ was adhered to the test piece every 00 hours. For comparison, alloys IN738 and I
N792 was tested simultaneously. The criterion for corrosion resistance is the number of cycles until the first corrosion pit appears on the specimen surface.

The results of the corrosion test are shown in the graph of FIG. 9
Corrosion initiation at 00 ° C. was based on alloys SCB444 and IN7
It has the same number of cycles as 92 and achieves its initial purpose.

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.

[0033]

Table 2 Durability temperature (° C) Stress (MPa) Rupture time (h) 750 725 134 750 650 612 750 750 600 1152 850 500 43.1 850 425 168.5 850 300 3545 /> 3456 in creep test of alloy SCB444 950 250 115/135 950 200 551/544 950 180 578 950 140 2109 950 120 3872

According to the graph of FIG. 2, the alloy SCB444,
The creep rupture times obtained for IN738 and SC16 can be compared. The horizontal axis represents the applied stress. The vertical axis is the value of the Larson-Miller 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). It can be seen that the creep resistance of this graph alloy SCB444 is much better than alloy IN792.

At the end of the creep test, the microstructure of the specimen of alloy SCB444 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 SCB444.
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>
Is promoted and dominant, and 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 that tends to occur during the homogenization process at high temperatures is also not observed with SCB444 alloy.

[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.

 ────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 501462826 Kestrel Way Exter, Devon EX2 7LG GREAT-BRITAIN (72) Inventor Caron Pierre France 91940 Le Uri Are Goya 3 Residence Hermitage (72) Inventor Blackler Michell United Kingdom Ek2 5 RL Exeter Granger Close 1591940 Leuri Alle Goya 3 Residens Hermitage (72) Inventor Malcolm McCorbin Gordon United Kingdom Elene 6 8 Deep Linkern North Heikeham Broadway A 41 (72) Inventor Bahi Raeshbar Prassa Berlin 167 Nienchempashtra C42by (72) Inventor Escarle André Marcel France 65100 Omex Ryu Carrère de Batulguere 7 (72) Inventor Relais Lauren France 77140 Darval Rue de la Barroudeli 50 F term (reference) 3G002 BA06 BA10 BB04 BB05 CA11 CA15

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: 11.5 to 12.5% Mo: 0.8 to 1.2% W: 3.75 to 4.25% Al: 3.75 to 4.25% Ti: 4 to 4.8% Ta: 1.75 to 2.25% 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.