MX2007005560A

MX2007005560A - Nickel-based superalloy

Info

Publication number: MX2007005560A
Application number: MXMX/A/2007/005560A
Authority: MX
Inventors: Youssef Nazmy Mohamed
Original assignee: Alstom Technology Ltd; Youssef Nazmy Mohamed
Priority date: 2004-11-18
Filing date: 2007-05-09
Publication date: 2008-10-03

Abstract

The invention relates to a nickel-based superalloy. The inventive alloy is characterised by the following chemical composition (amount in wt.%):7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 AI, 1.3-1.4 Ti, 0.11 -0.15 Si, 0.11 -0.15 Hf, 200-750 ppm C, 50-400 ppm B, 0.1 -5 ppmS, 5-100 ppm Y and/or 5-100 ppm La, and the remainder is Ni and impurities arising from the production thereof. Said nickel-based superalloy is characterised in that it is very pourable, is highly resistant to oxidation and has a good compatibility to the TBC layers applied to the surface thereof.

Description

NICKEL BASE SUPERALEATION FIELD OF THE INVENTION The present invention relates to the field of materials science. It relates to a nickel-based superalloy, in particular for the production of monocrystalline components (SX alloy) or components with a directionally solidified microstructure (DS alloy), such as, for example, blades or vanes for gas turbines. However, the alloy according to the invention can also be used for cast components in a conventional manner. BACKGROUND OF THE INVENTION Nickel-based superalloys of this type are known. The monocrystalline components produced from these alloys have a very good material resistance at elevated temperatures. This makes it possible, for example, to increase the inlet temperature of gas turbines, which increases the efficiency of the gas turbine. Nickel-based superalloys for monocrystalline components, as known from US 4,643,782, EP 0 208 645 and US 5,270,123 for this purpose contain alloying elements that reinforce the solid solution, for example, Re, W, Mo, Co, Cr, as well as elements that form phases and ', for example, Al, Ta, and REF .: 181814 Ti. The proportion of high-melting alloy elements (W, Mo, Re) in the base matrix (austenitic phase) increases continuously with the increase in the temperature at which the alloy is exposed. For example, standard nickel-based superalloys for single crystals contain 6-8% W, up to 6% Re and up to 2% Mo (amounts in% by weight). The alloys disclosed in the aforementioned documents have a high resistance to progressive deformation, good properties of LCF (low cyclic fatigue) (LCF) and HCF (high cyclic fatigue) (HCF for its acronym in English ), and a high resistance to oxidation. These known alloys were developed for axion turbines and therefore optimized for short-term and medium-term use, that is, the charge duration is designed for up to 20,000 hours. In contrast, the components of industrial gas turbines must be designed for a load time of up to 75,000 hours. By way of example, the CMSX-4 alloy of US 4,643,782, when tested for use in a gas turbine at a temperature above 1000 SC has a phase? ' which became considerably thick after a charging time of 300 hours, and this phenomenon is disadvantageously associated with an increase in the rate of progressive deformation of the alloy.

Therefore, it is necessary to improve the resistance of known alloys to oxidation at very high temperatures. Another problem of the known nickel-based superalloys, for example, the alloys known from US 5,435,861, is that in the case of large components, for example, blades or blades of gas turbines with a length greater than 80 mm, the casting properties leave something to be desired. The casting of a perfect, relatively large, monocrystalline, directionally solidified component of a nickel-based superalloy is extremely difficult because most of these components have defects, for example, small-angle, marbled borders, ie , defects caused by a series of grains directed identically with a high eutectic content, equiaxial variation limits, microporosity, etc. These defects weaken the components at elevated temperatures, and consequently the service life or turbine operating temperature desired is not obtained. However, because a perfectly cast monocrystalline component is extremely expensive, the industry tends to allow as many defects as possible without adversely affecting the service life or operating temperature.

One of the most common defects is reticular borders, which are particularly damaging to the high temperature properties of monocrystalline products. Whereas in the small components the small-angle reticular borders only exert a minor influence on the properties in relative terms, they are nevertheless highly relevant in terms of the casting properties and the oxidation properties at elevated temperatures of the SX components or Large DS The reticular borders are regions with a large local disorder of the crystalline network, because in these regions the adjacent grains influence, therefore there is a certain orientation error between the crystalline networks. The greater the orientation error, the greater the disorder, that is, the greater the number of dislocations required in the reticular borders so that the two grains adapt to each other. This disorder is directly related to the properties of the material at elevated temperatures. Weaken the material if the temperature rises beyond the echosocial temperature (= 0.5 x melting point in K). This effect is known from GB 2 234 521 A. For example, in a conventional nickel-based monocrystalline alloy, at a test temperature of 871aC, the fracture toughness severely decays if the orientation error of the grains is higher than 6S. This was also confirmed in monocrystalline components with a directionally solidified structure, and consequently the point of view is generally that orientation errors greater than 6 are unacceptable. It is also known from GB 2 234 521 A above that enriching nickel-based superalloys with boron or carbon during directional solidification produces microstructures having an equiaxed or prismatic grain structure. Carbon and boron reinforce the reticular borders, by virtue of which C and B cause the precipitation of carbides and borides in the reticular borders, and these compounds are stable at high temperatures. Moreover, the presence of these elements in and along the reticular borders reduces the diffusion process, which is a main cause of the weakness of the reticular frontier. It is therefore possible to increase the orientation errors to 10 e to 12a and still obtain good material properties at elevated temperatures. However, these small-angle grid boundaries have an adverse effect on the properties, in particular of the large monocrystalline components formed of nickel-based superalloys.

EP 1 359 231 A1 discloses a nickel-based superalloy having improved casting properties and a greater resistance to oxidation than known nickel-based superalloys. Furthermore, this alloy is, for example, particularly suitable for large monocrystalline components for gas turbines with a length > 80 mm It has the following chemical composition (amounts in% by weight): 7.7-8.3 of Cr 5.0-5.25 of Co 2.0-2.1 of Mo 7.8-8.3 of W 5.8-6.1 of Ta 4.9-5.1 of Al 1.3-1.4 of Ti 0.11- 0.15 of Si 0.11-0.15 of Hf 200-750 ppm of C 50-400 ppm of B rest nickel and impurities related to production. However, its compatibility with thermal barrier coating layers TBC (TBC = thermal barrier coating as it is known in English), which are used in particular in the sector of gas turbines to protect components exposed to thermal stresses particularly elevated still requires improvements. SUMMARY OF THE INVENTION The object of the invention is to avoid the disadvantages of the prior art mentioned above. The invention is based on the object of further improving the nickel-based superalloy known from EP 1 359 231 Al, in particular with a view to obtaining better compatibility with the TBC layers that should be applied to this superalloy in combination. with equally good casting properties and a high resistance to oxidation compared to the nickel-based superalloy which is known from EP 1 359231 A1 according to the invention, this object is achieved by the fact that the nickel-based superalloy is characterized by the following chemical composition (amounts in% by weight): 7.7-8.3 of Cr 5.0-5.25 of Co 2.0-2.1 of Mo 7.8-8.3 of W 5.8-6.1 of Ta 4.9-5.1 of Al 1.3-1.4 of Ti 0.11-0.15 of Si 0.11-0.15 of Hf 200-750 ppm of C 50-400 ppm of B <; 5 ppm of S 5-100 ppm of Y and / or 5-100 ppm of The rest nickel and impurities related to the production. The advantages of the invention lie in the fact that the alloy has very good casting properties, a high resistance to oxidation at high temperatures and is very compatible with the TBC layers that must be applied. It is convenient if the alloy has the following composition (amounts in% by weight): 7.7-8.3 of Cr 5.0-5.25 of Co 2.0-2.1 of Mo 7.8-8.3 of W 5.8-6.1 of Ta 4.9-5.1 of Al 1.3-1.4 0.11-0.15 Si 0.11-0.15 Hf 200-300 ppm C 50-100 ppm B maximum 2 ppm S 10-80 ppm Y and / or 10-80 ppm The rest is nickel and impurities related to production. One advantageous alloy according to the invention has the following chemical composition (amounts in% by weight): 7.7% Cr 5.1 Co 2.0 Mo 7.8% W 5.8% Ta 5.0 Al 1.4 Ti 0.12 Si 0.12 Hf 200 ppm of C 50 ppm of B 1 ppm of S 50 ppm of Y 10 ppm of The rest is nickel and impurities related to production. This alloy is eminently suitable for the production of large monocrystalline components, for example, blades or blades for gas turbines. DETAILED DESCRIPTION OF THE INVENTION The invention is explained in more detail below on the basis of an exemplary embodiment. The nickel-based superalloys known from the prior art (CAI to CA5 comparative alloys) and Al alloy according to the invention having the chemical composition listed in Table 1 (amounts in% by weight) were tested: The Al alloy is a nickel-based superalloy for monocrystalline components, the composition of which is covered by the claim of the present invention. The alloys CAI, CA2, CA3, CA4 are comparative alloys corresponding to the prior art, known by the designations CMSX-11B, CMSX-6, CMSX-2 and René N5. Among other things they differ from the alloy according to the invention primarily by virtue of the fact that they are not alloyed with C, B, Si and Y and / or La. The comparative alloy CA5 is known from EP 1 359 231 Al, and differs from the alloy according to the invention in terms of the content of S, Y and / or La. Carbon and boron reinforce the reticular borders, particularly also the small-angle reticular borders that occur in the < 001 > in SX or DS vanes or blades of gas turbines made of nickel-based superalloys, since these elements cause the precipitation of carbides and borides in the reticular borders, and these compounds are stable at elevated temperatures. Additionally, the presence of these elements in and along the reticular borders reduces the diffusion process, which is a main cause of the weakness of the reticular frontier. This considerably improves the casting properties of very long monocrystalline components, for example, turbine blades or gas turbine blades with a length of approximately 200 to 230 mm. The addition of 0.11 to 0.15% by weight of Si, in particular in combination with Hf in approximately the same order of magnitude, significantly improves the oxidation resistance at elevated temperatures compared to the nickel-based superalloys CAI to CA4 previously known. Restricting the composition according to the invention to a sulfur content < 5 ppm produces very good properties, in particular good adhesion to the TBC layer that was applied to the surface of the superalloy, for example, by thermal spraying. If the sulfur content is > 5 ppm, this has an adverse effect on the adhesion of TBC, and the layer quickly peels off in the case of fluctuating thermal stresses. The addition of Y and / or La in the specified interval (in each case 5 to 100 ppm), ie in total, ie Y + La, 10 to 200 ppm if both elements are present produces very good adhesion of the coating Thermal barrier ceramic (TBC layer) to be applied. The Y content of 50 ppm and the La content of 10 ppm specified for the Al alloy is particularly advantageous, in that Al is particularly compatible with the TBC layers to be applied. Additionally, these two elements also increase the resistance to environmental influences (environment). The addition of these elements in these reduced ranges stabilizes the aluminum oxide / chrome scale layer on the surface of the alloy and produces a significant resistance to oxidation. Y and La are elements with oxygen activity that improve the adhesion strength of the husk layer on the base material. The resistance to flaking during cyclic oxidation is the key factor for the stability of the TBC layer. Table 2 lists in each case the number of cycles it takes for the layer of AI2O3 and other oxide layers formed to be shelled by cyclic oxidation at 1050 eC / lh / cooling by air at room temperature for the alloys listed in the table 1: Table 2: number of cycles until chipping occurs The Al alloy according to the invention, compared to the alloys known by the prior art, reaches by far the highest number of cycles before the oxide layer is peeled off . This implies a high stability of a TBC layer that must be applied to the surface of the superalloy, for example, by thermal spraying. If, in other exemplary embodiments, nickel-based superalloys with higher contents of C and B (at most 750 ppm of C and at most 400 ppm of B) are selected in accordance with claim 1 of the invention, it is also possible that the components produced from these alloys are cast in a conventional manner, that is, without producing single crystals. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims

REI INDICATIONS Having described the invention as above, the content of the following claims is claimed as property:

1. Nickel-based superalloy, characterized because it has the following chemical composition, with the indicated amounts in% by weight: 7.7-8.3 of Cr, 5.0-5.25 of Co, 2.0-2.1 of Mo, 7.8-8.3 of W, 5.8-6.1 of Ta, 4.9-5.1 of Al, 1.3-1.4 of Ti, 0.11-0.15 of Si, 0.11-0.15 of Hf, 200-750 ppm of C, 50-400 ppm of B, < 5 ppm of S, 5-100 ppm of Y and / or 5-100 ppm of La, nickel residue and impurities related to the production.

2. Nickel-based superalloy according to claim 1, in particular for the production of monocrystalline components, characterized in that it has the following chemical composition, with the amounts indicated in% by weight: 7.7-8.3 of Cr, 5.0-5.25 of Co, 2.0-2.1 of Mo, 7.8-8.3 of W, 5.8-6.1 of Ta, 4.9-5.1 of Al, 1.3-1.4 of Ti, 0.11-0.15 of Si, 0.11-0.15 of Hf, 200-300 ppm of C , 50-100 ppm of B, maximum 2 ppm of S, 10-80 ppm of Y and / or 10-80 ppm of La, the rest is nickel of impurities related to production.

3. Nickel-based superalloy according to claim 2, characterized in that it has the following chemical composition, with the amounts indicated in% by weight: 7.7% Cr, 5.1% Co, 2.0% Mo, 7.8% W, 5.8% Ta , 5.0 of Al, 1.4 of Ti, 0.12 of Si, 0.12 of Hf, 200 ppm of C, 50 ppm of B, 1 ppm of S, 50 ppm of Y, 10 ppm of La, the rest is nickel and impurities related to the production.