NICKEL BASED SUPERALLOY, RAW MATERIAL, COMPONENT AND METHOD

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is the US National Stage of International Application No. PCT/EP2023/081291 filed 9 Nov. 2023, and claims the benefit thereof, which is incorporated by reference herein in its entirety. The International Application claims the benefit of United Kingdom Application No. GB 2218300.8 filed 6 Dec. 2022.

FIELD OF INVENTION

[0002]The invention relates to a Nickel based superalloy, a raw material, a component and method to cast it.

BACKGROUND OF INVENTION

[0003]When later stage blades are designed for aero efficiency and against creep, the design work is constrained by LCF in the disc attachment due to the mass of said blades. A reduced density in the blade alloy will, everything else being equal, enable designs with better aero efficiency and/or allow higher metal temperatures, as it eases the LCF constraint. Improved Aero efficiency and/or increased allowable metal temperatures will e.g., enable an increased thermal efficiency in a combined cycle power plant.

[0004]Later stage blades operate at temperatures where hot corrosion agents are particularly aggressive. In order to enable fuel flexibility, including use of corrosive fuels including biofuels and, handle air borne contaminants such as sea salt, it is advantageous to have a good hot corrosion resistance.

[0005]The general rule of thumb for good hot corrosion resistance is to have a nominal composition with at least about 12 wt % Chromium (Cr), at most about 2 wt % Molybdenum (Mo) and no Vanadium (V).

[0006]About 12 wt % Cr enables formation of a continuous layer of Cr₂O₃ in the oxide layer to slow down further oxidation, and, to retard the influx of corrosive elements into the alloy.

[0007]Molybdenum (Mo) and no Vanadium (V) can react with and accelerate the attack from corrosive agents.

[0008]Oxidation is normally not a major issue in the later stage blades since the metal temperatures are moderate by the standards of gas turbine hot stage components. Later stage blades typically operate in the 600 to 800° C. range. It is then sufficient to have the ability to form a continuous and adherent Cr₂O₃ layer.

[0009]IN792 with a nominal composition, in wt %, of Ni-8.5Co-12.5Cr-1.8Mo-4W-3.4Al-4Ti-4Ta-0.08C-0.02Zr-0.015B has a good combination of creep and hot corrosion resistance, a gamma prime particle content of about 50 mol %, and is extensively used for later stage blades and other turbine components. Molybdenum (Mo) and no Vanadium (V) are used to strengthen the gamma matrix while Titanium (Ti) and Tantalum (Ta) are used to strengthen the gamma prime particles.

SUMMARY OF INVENTION

[0010]It is therefore the aim of the invention to improve the properties of a nickel-based superalloy.

[0011]The problem is solved by an alloy according to claim 1, by a raw material according to claim 13, a component according to claim 14 and a method according to claim 17.

[0012]In the dependent claims further advantageous features are listed which can be combined with each other to yield further advantages.

[0013]While an increased gamma prime content above the relatively high level already seen in IN792 can be detrimental to the long-term creep strength as it can exacerbate the long-term coarsening of the gamma prime structure, coarsening is not a critical issue at the metal temperatures associated with later stage blades. While an increased gamma prime content can also be detrimental to all properties as it can exacerbate the tendency for precipitation of unwanted phases such as Laves and Sigma, in the invention the increased gamma prime content is balanced by the reduction in strengthening elements such that the phase stability is retained.

[0014]A blade alloy having creep and corrosion resistance on the IN792 level and a reduced density would enable higher performance. It is accordingly the aim of the invention to provide an alloy with IN792 level creep and corrosion resistance and a reduced density. Relative to IN792, the levels of the strengthening elements are reduced, which is detrimental for the creep strength but will reduce the density. Al is increased which will further reduce the density and increase the gamma prime particle content which is beneficial for the creep strength. Ta is replaced by Nb which will further reduce the density. Si is included at a low measured level while nominally set to zero in most specifications for IN792. This inclusion is to avoid the risk for the reduction in oxidation and hot corrosion resistance which might occur if the production process for a component turned out to result in an unusually low level of Silicon (Si) since Silicon (Si) has a beneficial catalytic effect on the selective oxidation of protective Cr₂O₃ even at low levels. Hafnium (Hf) values from up to 2.0 wt %, the range seen in commercial alloys, can be added as necessary. The higher Hafnium (Hf) values would be used if the alloy is directionally cast.

[0015]Hafnium (Hf) up to about 2.0 wt % is included by some casting vendors to improve the casting yield. Whether Hafnium (Hf) is an advantage, and if so at what level, depends on the component geometry, and, the specific casting methodology, mold materials and rigs used by the vendor. When Hafnium (Hf) is added, the levels of one or more of the other alloy elements are reduced somewhat to ensure that this does not cause an increased propensity for precipitation of unwanted phases such as Laves and Sigma.

[0016]Tantalum (Ta) is often added for oxidation resistance as well as strength in alloys made for hot stage blades while Niobium (Nb) provides the same strengthening effect per at % but does not boost the oxidation resistance. Hence a replacement of Tantalum (Ta) by Niobium (Nb) is beneficial in later stage blades where a reduced density at the cost of some oxidation resistance is a reasonable bargain.

DETAILED DESCRIPTION OF INVENTION

[0055]Especially it consists of these elements,

[0056]Especially only the elements are used for this alloy.

[0057]One possible composition is given, in wt %, by Ni-5Co-12.5Cr-2Mo-2W-5Al-3.5Ti-1.5Nb-0.12C-0.015B-0.01Zr and has a density in the 7.8 to 8 kg/dm3 range and a gamma prime particle content (at 850 C) of 67 mol %

[0058]It can be noted that the alloy is free of Rhenium (Re) and Tantalum (Ta), two elements which are commonly used in modern Nickel base superalloys, but which unfortunately significantly increase their densities. Boron (B), Carbon (C) and Zirconium (Zr) are used for grain boundary strengthening and are included at normal values for high strength Nickel base gamma prime strengthened superalloys.

[0059]The alloy should furthermore preferably be cast with a clean process such that e.g., the resulting sulfur content is below 5 ppm. Sulfur(S) can severely reduce the protective capability and adherence of the Cr₂O₃ layer, hence a strict cap on this element is prudent despite the moderate service temperatures.

[0060]One possible embodiment is given, in wt %, by Ni-5Co-12.5Cr-1.8Mo-2.2W-5Al-3.5Ti-1.5Nb-0.1C-0.015B-0.01Zr-0.01Si. It has a predicted density of about 7.9 kg/dm3 using the extended Caron formula below, and, a predicted gamma prime particle content, at 850° C., of 67 mol % based on ThermoCalc with TTNi8 as data base. Relative to IN792 the levels of the gamma matrix elements Co and W are reduced. The reduction in gamma matrix strength is however small since the amount of gamma matrix to be strengthened by Mo and W is reduced relative to IN792. 1.5 wt % Niobium (Nb) corresponds to about 3 wt % Tantalum (Ta) on an at % basis. Hence the sum of the gamma prime strengthening elements Titanium (Ti), Tantalum (Ta) and Niobium (Nb) is reduced relative to the very high level of gamma prime strengthening elements used in IN792. Al is increased to enable a higher gamma prime content.

[0061]Hafnium (Hf) can be added as required w.r.t. the production process requirements, as done today for IN792, in which case one or more of the alloy elements need to be adjusted by those skilled in the art such that e.g., the propensity for precipitation of unwanted phases does not increase. Relative to IN792, a higher upper limit of 2.0 wt % is chosen to also allow for directional solidification casting. Furthermore, Iron (Fe) can be added as necessary to reduce the gamma prime Solvus in order to increase the heat treatment window, e.g., to manage inclusion of Hafnium (Hf) which tends to reduce the heat treatment window.

[0062]Tungsten (W) in this range provides a useful strengthening effect and low enough to avoid excessive formation of undesired phases.

[0063]Aluminum (Al) is included at between about 4 wt % and 6 wt %. In this range it high enough to reduce the risk that the joint should be a weak link w.r.t. oxidation and corrosion, high enough to assist in the formation of gamma prime particles, and low enough to avoid excessive formation of undesired phases.

[0064]Tantalum (Ta) is preferably not included.

[0065]Molybdenum (Mo) is a potent gamma matrix strengthening element with a low diffusivity.

[0066]Carbon (C) is an element with a high diffusivity and can at a first glance be assumed to equilibrate easily with surrounding parts. In-house experience does however sometimes show essentially carbide free joints (which is not useful) when Carbon (C) free BFM's have been used. Our interpretation is that the carbon close to the joint is at least temporarily bound in carbo-borides as Boron (B) from the joint diffuses from the joint. There should be a lower limit on Carbon (C) to avoid completely Carbon (C) free joints, but too high levels of Carbon (C) implies a risk for formation of embrittling carbide films and elongated carbides. Carbon (C) is included at between 0.03 wt % and 0.15 wt %.

[0067]Zirconium (Zr) is an element which provides grain boundary strengthening and acts as a sulfur gatherer at low measured levels. It needs to be capped at a low level since it tends to segregate which could lead to incipient melting since it is a melt depressant. It is included at between 0.005 wt % and 0.03 wt %.

[0068]Further examples of the invention are listed here: