Patent Application
20150167123
The present invention generally relates to alloy compositions, and more particularly to superalloys suitable for components requiring a polycrystalline microstructure and both high temperature dwell and/or creep capabilities, for example, turbine disks of gas turbine engines. Such alloys may also be useful in a multi-grain directionally solidified form or single crystal form.
The turbine section of a gas turbine engine is located downstream of a combustor section and contains a rotor shaft and one or more turbine stages, each having a turbine disk (rotor) mounted or otherwise carried by the shaft, and turbine blades mounted to and radially extending from the periphery of the disk. Components within the combustor and turbine sections are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor disks, blisks, and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, airfoil components such as blades and vanes are often formed of equiaxed, directionally solidified (DS), or single crystal (SX) superalloys, whereas turbine disks are typically formed of superalloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a polycrystalline microstructure having a controlled grain structure and desirable mechanical properties.
Turbine disks are often formed of gamma prime (γ) precipitation-strengthened nickel-base superalloys (hereinafter, gamma prime nickel-base superalloys) containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and/or vanadium as principal elements that combine with nickel to form the desirable gamma prime precipitate strengthening phase, principally Ni3(Al,Ti). Particularly notable gamma prime nickel-base superalloys include René 88DT (R88DT; U.S. Pat. No. 4,957,567) and René 104 (R104; U.S. Pat. No. 6,521,175), as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. R88DT has a composition of, by weight, about 15.0-17.0% chromium, about 12.0-14.0% cobalt, about 3.5-4.5% molybdenum, about 3.5-4.5% tungsten, about 1.5-2.5% aluminum, about 3.2-4.2% titanium, about 0.5-1.0% niobium, about 0.010-0.060% carbon, about 0.010-0.060% zirconium, about 0.010-0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium, the balance nickel and incidental impurities.
Disks and other critical gas turbine engine components are often forged from billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Gamma prime nickel-base superalloys formed by powder metallurgy are particularly capable of providing a good balance of creep, tensile, and fatigue crack growth properties to meet the performance requirements of turbine disks and certain other gas turbine engine components. In a typical powder metallurgy process, a powder of the desired superalloy undergoes consolidation, such as by hot isostatic pressing (HIP) and/or extrusion consolidation. The resulting billet is then isothermally forged at temperatures slightly below the gamma prime solvus temperature of the alloy to approach superplastic forming conditions, which allows the filling of the die cavity through the accumulation of high geometric strains without the accumulation of significant metallurgical strains. These processing steps are designed to retain the fine grain size originally within the billet (for example, ASTM 10 to 13 or finer), achieve high plasticity to fill near-net-shape forging dies, avoid fracture during forging, and maintain relatively low forging and die stresses. Such alloys may be heat treated below or above the gamma prime solvus. In order to improve yield strength and ductility at moderately elevated temperature these alloys may be heat treated below their gamma prime solvus temperature (generally referred to as subsolvus heat treatment) to maintain fine uniform grains. In order to improve fatigue crack growth resistance and mechanical properties at even more elevated temperatures, these alloys are heat treated above their gamma prime solvus temperature (generally referred to as supersolvus heat treatment) to cause significant, uniform coarsening of the grains.
Current alloys, including R88DT, have provided significant improvements in rotor performance capabilities. However, improvements in the economics and feasibility of producing these alloys are continuously sought. Key factors for realizing low-cost processing while maintaining premium quality alloy product include utilization of a high level of scrap and revert as input to melting processes and powder metallurgy processes often used in alloy production. Revert material is often in the form of solids or chips of the nominal composition of a material, whereas the nominal composition of a scrap material may be of a different composition and may contain elements not intended in the composition of the desired alloy to be produced. Alloys of different compositions may be used as input materials for a melting batch, either as revert materials or scrap materials, along with other elemental input materials. The requirement is that the aggregate chemistry of the input materials meets the allowable composition ranges for the alloy desired to be produced. Such combinations of input materials are guided by rule of mixtures and are well established in standard melting practices and currently utilized as state of the art.
Rotor-grade superalloys may be produced by a variety of processes including powder processing and melt processing. R88DT is typically manufactured using powder metal processing. Some other disk alloys, such as the nickel-base superalloy IN718 are typically produced using conventional melting processes. To improve cost effectiveness, it is desirable that R88DT and similar alloys be produced using conventional melting processes utilizing scrap and revert with the same melting and billet conversion equipment used to produce IN718. Nominal elemental composition ranges reported for IN718 are, by weight: 50-55% nickel, 17-21% chromium, 2.8-3.33% molybdenum, 4.75-5.5% niobium, 0-1.0% cobalt, 0.65-1.15 titanium, 0.2-0.8% aluminum, 0-0.35% manganese, 0-0.3% copper, 0-0.08% carbon, 0-0.006% boron, the balance iron (18.5% nominal) and incidental impurities. Among superalloys IN718 stands out for its pervasive use, reported to be at approximately 45% of total industrial production of wrought nickel base superalloys. With this level of usage, there is also the practical potential for IN718 to be mixed within many R88DT revert forms, especially chips and other superalloy scrap. However, the ability to tolerate the significant iron content of IN718 a limiting factor in cost-effective revert and scrap utilization for R88DT, because as described above, R88DT does not contain iron as a constituent. It is generally believed that R88DT when contaminated with iron can lead to formation of observable amounts of sigma phase which, in R88DT, is generally (Fe,Mo)x(Ni,Co)y, where x and y=1 to 7. Sigma phase is a well-known topologically close-packed (TCP) phase which can adversely affect the mechanical capabilities of a gamma prime nickel-base alloy. In the context of this discussion, an observable amount is considered to be any amount that can be seen in suitable etched metallographic samples at an optical magnification of 500×. Hence, scrap or revert utilization for use in the production of R88DT would carry with it a high probability of iron contamination and sigma phase formation that is not desirable in R88DT.
Prior attempts to prevent iron cross-contamination while utilizing high levels of IN718 scrap or revert include alloy segregation by physically keeping chips of different alloys separate. Unfortunately, such methods have significant limitations in terms of additional personnel training and maintaining separate chips or containers. In addition, there can be contamination from melting equipment or melt-handling or machining equipment that may have been previously used for the production of an iron-containing alloy, in which case extensive cleaning is required of equipment between switching over to production of different alloys.
The above mentioned methods of preventing iron contamination result in loss of valuable material and/or production efficiency. Thus it would be desirable if a gamma prime nickel-base superalloy could be developed capable of having properties similar to R88DT, yet could tolerate the presence of iron, so that the use of scrap and revert iron-containing alloys or the use of revert containing inadvertent iron contamination would be permissible. It would be further desirable if superalloy compositions that fall within the composition space of R88DT could be identified that would lend themselves to additions of measurable amounts of iron that would not lead to formation of sigma phase and hence would not adversely affect the mechanical properties of the superalloy.
The present invention provides gamma prime nickel-based superalloys suitable for use in forming components such as a turbine disk, compressor disk, blisk, seal, shaft or retainer, and processes for producing such superalloys, wherein the processes allow scrap and revert usage. The superalloys accommodate limited amounts of iron in their compositions and are particularly well suited for achieving physical and chemical properties similar to those of R88DT, yet allow for an iron content that was previously considered excessive and unallowable in R88DT, including powder-metallurgy processed versions of R88DT.
According to a first aspect of the invention, a gamma prime nickel-based superalloy has a composition that falls within a compositional space defined by the following ranges, by weight: 15.8-16.2% chromium, about 12.9-13.3% cobalt, about 3.95-4.1% molybdenum, about 3.9-4.1% tungsten, about 2.01-2.24% aluminum, about 3.6-3.9% titanium, about 0.5-1.0% niobium, about 0.010-0.060% carbon, about 0.02-0.06% zirconium, about 0.010-0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium, the balance nickel and incidental impurities, wherein the superalloy further contains iron in an amount exceeding an impurity level and up to 2.0%, and the superalloy being free of an observable amount of sigma phase.
According to a second aspect of the invention, structural components can be formed from the superalloy described above, particular examples of which include turbine disks, compressor disks and blisks, seals, shafts, and retainers of gas turbine engines.
A third aspect of the invention is a process for making the superalloy described above which includes scrap and revert usage of alloys that contain iron, either intentionally or inadvertently, and/or using melting and melt-handling equipment without extensive cleaning after producing an iron-containing alloy.
A technical effect of the invention is that the superalloy described above is capable of providing approximately the same properties and structural and chemical capabilities as R88DT or a similar superalloy designed and processed to a consistent microstructure for high temperature properties, and is obtainable by suitable processing to achieve a desirable microstructure, but allows for significant iron content. In this manner, the superalloy is capable of being produced more economically and efficiently, with less material waste, higher scrap and revert utilization, less cleaning of machining, scrap and chip handling equipment, melting and melt-handling equipment, and lower personnel and time requirements.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
The present invention is directed to nickel-base alloys, and particularly to gamma prime nickel-base alloys suitable for components produced by a hot working (e.g., forging) operation. A particular but non-limiting example represented in
Disks of the type shown in
The present invention discloses a set of compositions that share certain similarities with other nickel-base superalloys, including René 88DT (R88DT; U.S. Pat. No. 4,957,567). The present invention is particularly intended to maintain the structural and mechanical attributes of R88DT most advantageously in the subsolvus fine-grained condition, especially as produced in a cast and wrought form. However, in several of the current compositional ranges allowed in commercial formulations, R88DT is considered to be unable to accommodate significant iron contamination while maintaining desirable mechanical properties and avoiding an observable amount of sigma phase. More particularly, conventional wisdom has been that the introduction of iron into R88DT promotes the formation of sigma phase, which can be detrimental to mechanical properties of R88DT. Accordingly, conventional practice has been to avoid any iron contamination of R88DT so as to ensure absence of an observable amount of sigma phase and so that mechanical properties of R88DT are not compromised. As such, R88DT, in its widest composition ranges, cannot be guaranteed to be processed efficiently with the scrap and revert utilization methods described previously if there is a significant risk of iron contamination. In contrast, the superalloys of the current invention can accommodate a significant amount of iron contamination without sacrificing advantageous structural and chemical characteristics, particularly those of R88DT subsolvus fine-grained condition and especially as produced in a cast and wrought form. As a result, the superalloys of this invention are capable of allowing scrap and revert usage of alloys that contain iron, either intentionally or inadvertently, and can also advantageously allow the use of machining equipment, scrap and revert handling equipment, and melting and melt-handling equipment without extensive cleaning after producing an iron-containing alloy. A notable example of an iron-containing alloy is IN718.
To identify a gamma prime nickel-base superalloy composition that contains iron but with properties similar to those of R88DT, a targeted allowable iron content was initially identified by modeling alloy phase stability. Phase stability modeling showed that the largest impact of increasing iron content is an increased sigma solvus temperature. Alloy phase stability modeling suggested that formation of sigma phase is thermodynamically possible at zero iron content. However, experience has shown that lower solvus temperatures with zero iron content make the kinetics of the formation to be a primary controlling factor and no observable sigma formation occurs. At an addition of 2.0% iron, by weight, to R88DT, the sigma solvus temperature would be approximately 1400° F. (760° C.), coincident with a preferred heat-treat aging temperature, indicating that sigma phase formation is thermodynamically possible. However it is known that, at a sufficiently low temperature, kinetics of phase formation are not often favorable for the formation of all thermodynamically predicted phases. This phenomenon suggested that a potential maximum iron content exists below which kinetics control the sigma phase formation and are not favorable for an observable amount of sigma phase formation.
Several heats were produced utilizing compositions in the R88DT compositional space, which is defined herein to be superalloy compositions that fall within the ranges described above for R88DT, but were produced to further contain intentional amounts of iron, for example, iron contents of about 0.6, 1.3, or 1.34 weight percent. The alloys were evaluated in the as heat-treated condition and confirmed to be free of observable amounts of sigma phase. Based on the analysis of this data, +/−three standard deviations were taken to provide acceptable ranges for the elemental compositions of these iron-containing alloys to be free of observable amounts of sigma phase. Experimentally, long-term exposure tests of up to 10,000 hours at temperatures up to 1400° F. (760° C.) confirmed the lack of observable amount of sigma formation confirming the acceptability of this iron limit. Sigma phase formation was evaluated by an optical examination at a minimum of 500× utilizing suitably etched metallographic samples. Any microstructural features suspected of being sigma phase were subjected to additional chemical composition analysis and crystallographic analysis for final determination of the absence of sigma phase. Based on this analysis, a suitable composition comprises, and more preferably consists of, by weight, 15.8 to 16.2% chromium 12.9 to 13.3% cobalt, 3.95 to 4.1% molybdenum, 3.9 to 4.1% tungsten, 2.01 to 2.24% aluminum, 3.6 to 3.9% titanium, 0.67 to 0.74% niobium, 0.012 to 0.02% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium 0.005 to 0.011% carbon, 0.02 to 0.06% zirconium, and iron in an amount of about 0.6 to about 1.3%, the balance nickel and incidental impurities. More broadly, on the basis of the investigations it was concluded that gamma prime nickel-base superalloys of this invention can have a composition that falls within a compositional space of, by weight, about 15.0-17.0% chromium, about 12.0-14.0% cobalt, about 3.5-4.5% molybdenum, about 3.5-4.5% tungsten, about 1.5-2.5% aluminum, about 3.2-4.2% titanium, about 0.5-1.0% niobium, about 0.010-0.060% carbon, about 0.010-0.060% zirconium, about 0.010-0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium, the balance nickel and incidental impurities, the superalloy further containing iron in an amount exceeding an impurity level and up to 2.0%, the superalloy being free of an observable amount of sigma phase.
It should be noted that typical impurity levels for iron in R88DT can be up to about 0.1%. In addition, the alloy may contain up to 0.0035% nitrogen. Also, it is generally recognized that carbon and nitrogen levels together influence the degree of carbo-nitride inclusions. Higher levels of carbon and nitrogen and higher scrap input may be tolerated if higher levels of carbo-nitride inclusions are acceptable for a specific application. A particular embodiment of the alloy, contains, by weight, about 13% cobalt, 16% chromium, 4% molybdenum, 4% tungsten, 2.1% aluminum, 3.7% titanium, 0.7% niobium, 0.008% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, about 0.0-0.01 yttrium 0.005 to 0.011% carbon, 0.03 to 0.06% zirconium, up to 0.0035% nitrogen and more preferably up to 0.0018% nitrogen, the balance nickel, incidental impurities, and iron in an amount greater than impurity levels and up to about 1.3%. This particular embodiment of the alloy, as well as other alloys of the invention indicated through the composition ranges above, can be produced using scrap and revert usage of alloys that contain iron, intentionally or in advertently. Further, these alloys can be advantageously produced in melt equipment previously used for the production of iron-containing alloys without the need for significant decontamination or expensive alloy segregation procedures.
A nonlimiting example of a method capable of producing a superalloy of this invention includes combining at least one iron-containing alloy with raw materials that do not contain intentional additions of iron, wherein the iron-containing alloy(s) and raw materials are combined in appropriate amounts and then melted to produce the desired composition for the superalloy and its intentional but limited addition of iron. At least one iron-containing scrap alloy can be used in place of or in addition to the iron-containing alloy(s). Alternatively or in addition, intentional additions of iron can be made present in a superalloy of this invention as a result of melting raw materials, and/or iron-containing alloys(s), and/or iron-containing scrap alloy(s) using melt equipment that had been immediately previously used to melt an iron-containing alloy and without cleaning the melt equipment to remove remnants of the iron-containing alloy. It should be apparent that the presence of iron can occur through any combination of the above techniques in a manner that may promote efficiencies and/or reduce material and processing costs.
It should be stressed that, while the alloys of the present invention, without considering the iron content, fall into the compositional space of R88DT, several alloys in the R88DT compositional space do not lend themselves to significant introduction of iron without compromising their mechanical properties. This phenomenon is because phases formed in multicomponent systems (for example, superalloys) are a complex function of the elemental composition of the system. It should also be stressed that, in addition to the alloys of the present invention described above, certain other compositions within the general compositional space of R88DT (as defined by its compositions reported herein) may also lend themselves to iron addition without a significant compromise in properties compared to R88DT. This is due to the complex thermodynamic interactions prevailing among the elements in a multicomponent system in an n-dimensional space, where n is the number of significant elements in the composition of the alloy. The effects of these interactions create situations wherein, at the same percentage content of an element, different phases can occur as the percentage contents of the other constituent elements vary, even when temperature and pressure are fixed. Due to this complex nature of the multicomponent systems, it is not readily apparent as to what compositional ranges within R88DT would lend themselves to additions of iron and simultaneously the required phase stability and properties for the alloy. However, in investigations leading to the present invention, alloy phase stability modeling indicated the potential absence of an observable amount of sigma phase in the iron-containing superalloys having the compositions described above. These alloy compositions were produced using known melting processes, the absence of an observable mount of sigma phase was verified, and the observed phases were of the same chemistry as those observed in similarly-produced R88DT. The microstructural features and the structural and chemical properties of the iron-containing alloys of the present invention were also evaluated and found to be similar to those of R88DT. The coefficient of thermal expansion (CTE) was measured over a suitable temperature range and shown to be essentially the same among the iron-containing alloys and essentially iron-free forms of the alloys that were prepared. Also, Young's Modulus was measured over a suitable temperature range and found to be essentially the same among the iron-containing and essentially iron-free forms of the alloys. Test samples were also processed in the subsolvus heat-treat condition and tested in tension over a suitable temperature range, yielding nominally equivalent values for 0.2% yield strength and tensile strength that demonstrated no loss in strength for the iron-containing alloys compared to that of the essentially iron-free forms of the alloys when processed to yield similar microstructures. These results evidenced that superalloys of the invention can be utilized to produce structural components and in particular, as non-limiting examples, turbine disks, compressor disks and blisks, seals and shaft retainers of gas turbine engines.
In view of the above, the superalloys of this invention are capable of exhibiting comparable properties to similar high temperature superalloys, including R88DT, while accommodating significant iron content with negligible or no loss in advantageous properties. This ability to accommodate significant levels of iron contamination allows superalloys to be produced with scrap and revert usage of alloys that contain iron, either intentionally or inadvertently, and can also allow for the advantageous use of melting and melt-handling equipment without extensive cleaning after an iron-containing alloy, for example, IN718, is produced with the equipment. This flexibility can lead to significant reduction in production costs of the superalloy.
Additional potential benefits include the ability to reduce or eliminate the need for special training of personnel and the expense of extensive cleaning of melting process equipment when the equipment is switched between iron-containing alloy compositions and superalloys of this invention. As an example, the invention can reduce or eliminate the need to segregate iron from a recycling stream with specialized equipment capable of isolating an iron-containing material as well as reduce or eliminate the need for operator training in order to strictly maintain such isolation. Additionally, the invention can promote recycling economics by allowing superalloys to be produced with the use of machining chips or recycled material from articles having multi-alloy constructions that include iron-containing alloys, a notable example of which are compressor spools that often have multi-alloy constructions.
While the invention has been described in terms of specific embodiments, including particular compositions and properties of the superalloys, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, it should be understood that the invention is not limited to the specific disclosed embodiments, and the scope of the invention is to be limited only by the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/670,634, filed Jul. 12, 2012, the contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2013/049999 | 7/11/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61670634 | Jul 2012 | US |
