Patent Application
20100329883
The present invention generally relates to nickel-base superalloys and methods for processing such superalloys. More particularly, this invention relates to a nickel-base superalloy and a method of forging an article from the nickel-base superalloy to promote a more controlled grain growth during supersolvus heat treatment, such that the article is characterized by a microstructure with a finer uniform grain size and exhibits improved low cycle fatigue behavior.
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 nickel 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 such as peening 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 to Krueger et al.) and René 104 (R104; U.S. Pat. No. 6,521,175 to Mourer et al.), 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 0.5.0-1.0% 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. R104 has a nominal composition of, by weight, about 16.0-22.4% cobalt, about 6.6-14.3% chromium, about 2.6-4.8% aluminum, about 2.4-4.6% titanium, about 1.4-3.5% tantalum, about 0.9-3.0% niobium, about 1.9-4.0% tungsten, about 1.9-3.9% molybdenum, about 0.0-2.5% rhenium, about 0.02-0.10% carbon, about 0.02-0.10% boron, about 0.03-0.10% zirconium, the balance nickel and incidental impurities. Another notable gamma prime nickel-base superalloy is disclosed in European Patent Application EP1195446, and has a composition of, by weight, about 14-23% cobalt, about 11-15% chromium, about 0.5-4% tantalum, about 0.5-3% tungsten, about 2.7-5% molybdenum, about 0.25-3% niobium, about 3-6% titanium, about 2-5% aluminum, up to about 2.5% rhenium, up to about 2% vanadium, up to about 2% iron, up to about 2% hafnium, up to about 0.1% magnesium, about 0.015-0.1% carbon, about 0.015-0.045% boron, about 0.015-0.15% zirconium, 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. (Reference throughout to ASTM grain sizes is in accordance with the scale established in ASTM Standard E 112.) In order to improve fatigue crack growth resistance and mechanical properties at elevated temperatures, these alloys are then heat treated above their gamma prime solvus temperature (generally referred to as supersolvus heat treatment), to cause significant, uniform coarsening of the grains.
Forged gas turbine engine components often contain grains with sizes of about ASTM 9 and coarser, such as ASTM 2 to 9, though a much tighter range is typically preferred, such as grain sizes within a limited range of 2 to 3 ASTM units. Such a limited range can be considered uniform, which as used herein refers to grain size and growth characterized by the substantial absence of non-uniform critical grain growth. As used herein, critical grain growth (CGG) refers to localized excessive grain growth in an alloy that results in the formation of grains outside typical uniform grain size distributions whose size sufficiently exceeds the average grain size in the alloy (such as regions as coarse as ASTM 00 in a field of ASTM 6-10) to negatively affect the low cycle fatigue (LCF) properties of an article formed from the alloy, manifested by early preferential crack nucleation in the CGG regions. Critical grain growth can also have a negative impact on other mechanical properties, such as tensile strength. Critical grain growth occurs during supersolvus heat treatment following hot forging operations in which a wide range of local strains and strain rates are introduced into the material. Though not wishing to be held to any particular theory, critical grain growth is believed to be driven by excessive stored energy within the worked article, and may involve individual grains, multiple individual grains within a small region, or large areas of adjacent grains. The grain diameters of the effected grains are often substantially coarser than the desired grain size. Disks and other critical gas turbine engine components forged from billets produced by powder metallurgy and extrusion consolidation have appeared to exhibit a lesser propensity for critical grain growth than if forged from billets produced by conventional cast and wrought processing or spraycast forming techniques, but in any event are susceptible to critical grain growth during supersolvus heat treatment.
The above-noted U.S. Pat. No. 4,957,567 to Krueger et al. teaches a process for eliminating critical (abnormal) grain growth in fine grained component formed of R88DT by controlling the localized strain rates experienced during the hot forging operation. Strain rate is defined as the instantaneous rate of change of geometric strain with time. Krueger et al. teach that local strain rates must generally remain below a critical value, {dot over (ε)}c, in order to avoid detrimental critical grain growth during subsequent supersolvus heat treatment. According to Krueger et al., the maximum strain rate is composition, microstructure, and temperature dependent, and can be determined for a given superalloy by deforming test samples under various strain rate conditions, followed by a suitable supersolvus heat treatment. The maximum (critical) strain rate is then defined as the strain rate that, if exceeded during deformation and working of a superalloy and accompanied by a sufficient amount of total strain, will result in critical grain growth after supersolvus heat treatment.
Another processing limitation identified by Krueger et al. as avoiding critical grain growth in a nickel-base superalloy having a gamma prime content of, for example, 30-46 volume percent and higher, is to ensure superplastic deformation of the billet during forging. For this purpose, the billet is processed to have a fine grain microstructure that achieves a minimum strain rate sensitivity (m) of about 0.3 or greater for the superalloy within the forging temperature and strain rate ranges. As known in the art, the ability of a fine grain billet to deform superplastically is dependent on strain rate sensitivity, and superplastic materials exhibit a low flow stress as represented by the following equation:
σ=K{dot over (ε)}m
where σ is the flow stress, K is a constant, {dot over (ε)} is the strain rate, and m is the strain rate sensitivity, with higher values of m corresponding to greater superplasticity.
Further improvements in the control of final grain size have been achieved with the teachings of commonly-assigned U.S. Pat. No. 5,529,643 to Yoon et al. and U.S. Pat. No. 5,584,947 to Raymond et al. In addition to the requirement for superplasticity during forging (in other words, maintaining a high m value), Raymond et al. teach the importance of a maximum strain rate in combination with chemistry control, particularly the carbon and/or yttrium content of the alloy to achieve grain boundary pinning in alloys having a gamma prime content of up to 65 volume percent. In a particular example, Raymond et al. cites an upper limit strain rate of below about 0.032 per second (s−1) for R88DT (identified by Raymond et al. as Alloy D). In addition to maintaining a high m value, Yoon et al. also identifies a maximum strain rate of not more than about 0.032 s−1, particularly in reference to forging R88DT (identified in Yoon et al. as Alloy A). Yoon et al. further place an upper limit on the maximum strain rate gradient during forging, and requires extended annealing of the forging at a subsolvus temperature to remove stored strain energy prior to performing a supersolvus heat treatment. Finally, Yoon et al. achieve optimum superplasticity by forming the billet to have a grain size of finer than about ASTM 12, and maintaining the billet microstructure to achieve a minimum strain rate sensitivity of about m=0.3 within the forging temperature range.
In addition to the absence of critical grain growth, mechanical properties of components forged from fine grain nickel-base superalloys further benefit from improved control of the grain size distribution to achieve a distribution and average grain size that are, respectively, as narrow and fine as possible. Such a capability is particularly beneficial for high temperature, high gamma prime content (e.g., about 30 volume percent and above) superalloys, such as R88DT and R104, for which a desired uniform grain size is generally not coarser than ASTM 6 for gas turbine disks. Though prior forging practices of the type described above have achieved grain sizes in a range of ASTM 5 to 8, less than optimal mechanical properties can still result. For example,
The present invention provides a gamma prime precipitation-strengthened nickel-base superalloy and a method of forging an article from the superalloy to promote a more controlled grain growth during supersolvus heat treatment, such that the article is characterized by a microstructure with a finer uniform grain size and exhibits improved low cycle fatigue behavior.
The method includes formulating the superalloy to have a composition of, by weight, about 16.0-22.4% cobalt, about 6.6-14.3% chromium, about 2.6-4.8% aluminum, about 2.4-4.6% titanium, about 1.4-3.5% tantalum, about 0.9-3.0% niobium, about 1.9-4.0% tungsten, about 1.9-3.9% molybdenum, about 0.0-2.5% rhenium, greater than 0.05% and in certain embodiments greater than 0.1% carbon, at least 0.1% hafnium, about 0.02-0.10% boron, about 0.03-0.10% zirconium, the balance nickel and incidental impurities. The superalloy is similar in composition to R104, with the notable exceptions that R104 does not contain hafnium and has a carbon content of 0.02-0.10 weight percent. A billet is formed of the superalloy and worked at a temperature below the gamma prime solvus temperature of the superalloy so as to form a worked article. In particular, the billet is worked while maintaining strain rates as high as possible to control average grain size, but below an upper strain rate limit of greater than 0.03 per second to avoid critical grain growth. The worked article is then heat treated at a temperature above the gamma prime solvus temperature of the superalloy for a duration sufficient to uniformly coarsen the grains of the worked article, after which the worked article is cooled at a rate sufficient to reprecipitate gamma prime within the worked article. The cooled worked article has an average grain size of not coarser than ASTM 7 and preferably not coarser than ASTM 8, and is substantially free of grains in excess of three ASTM units coarser than the average grain size.
In view of the above, the superalloy has a sufficiently high carbon content and is forged at sufficiently high local strain rates so that, following a supersolvus heat treatment, the resulting forged component is characterized by a fine and substantially uniform grain size distribution. Also preferably avoided is critical grain growth that would produce individual grains or small regions of grains having grain sizes of more than five and preferably three ASTM units coarser than the average grain size in the component, or large regions that are uniform in grain size but with a grain size coarser than a desired grain size range of about two ASTM units. As a result, the forged component is capable of exhibiting improved mechanical properties, particularly low cycle fatigue behavior. Though not wishing to be held to any particular theory, it is believed that formulating a superalloy to have a chemistry similar to R104 but formulated to contain relatively high carbon levels, especially carbon levels above the upper limit of R104 (0.10 weight percent), allows the use of high strain rates, resulting in a forged component capable of exhibiting a more refined average grain size and substantially free of critical grain growth, which together improve the low cycle fatigue life of the component. Low cycle fatigue life can be particularly improved within a temperature range of about 400° F. to about 800° F. (about 200° C. to about 425° C.) relative to R104 with a conventional carbon content of up to 0.10 weight percent. Other benefits of the finer average grain size achieved with this invention include improved sonic inspection capability due to lower sonic noise, and improved yield behavior in service due to improved yield strength with finer grain size.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
The present invention is directed to gamma prime nickel-base superalloys, and particular those suitable for components produced by a hot working (e.g., forging) operation to have a polycrystalline microstructure. A particular example represented in
Disks of the type shown in
In the case of the nickel-base superalloy R104, a supersolvus heat treatment of a type described above has typically yielded an acceptable but not wholly optimal average grain size range of about ASTM 5 to 7, with the result that the low cycle fatigue behavior of the resulting turbine disk is less than optimal, particularly at temperatures of about 400° F. to about 800° F. (about 200° C. to about 425° C.). The present invention provides modifications to the chemistry of R104 to control and limit grain growth during supersolvus heat treatment to achieve and maintain a finer grain size following supersolvus heat treatment, as well as avoid critical grain growth. According to one aspect of the invention, a finer and more controllable average grain size can be achieved by modifying the R104 alloy to have a relatively high carbon content, for example, greater than 0.05 weight percent carbon and in some cases greater than 0.1 weight percent carbon. According to a second aspect of the invention, improved high temperature dwell behavior can be achieved by modifying the R104 alloy to contain at least 0.1 weight percent hafnium. According to additional aspects of the invention, grain refinement can be further promoted by utilizing relatively high strain rates and relatively low temperatures during forging. The teachings of U.S. Pat. Nos. 4,957,567 to Krueger et al., 5,529,643 to Yoon et al., and 5,584,947 to Raymond et al. are incorporated herein by reference, particularly regarding the use of high strain rates during forging and the placement of an upper limit on the strain rate (critical strain rate) to avoid critical grain growth during supersolvus heat treatment.
In an investigation leading to the present invention, a series of targeted alloy compositions were defined (by weight percent) as set forth in a table in
Following the supersolvus heat treatment, the specimens were cooled at rates that ensured re-precipitation of gamma prime within the gamma matrix or at grain boundaries. A controlled air cooling was employed to yield an approximately constant cooling rate of about 200° F./minute for all specimens. Finally, the specimens were aged at about 1550° F. (about 845° C.) for about four hours, followed by about eight hours at about 1400° F. (about 760° C.).
As noted above and well known in the art, in addition to grain recrystallization and solutioning gamma prime precipitates, the supersolvus heat treatment also resulted in grain growth (coarsening), typically resulting in grain sizes coarser than the original billet grain size.
A relationship between ASTM grain size and tensile behavior of the forged specimens is evidenced in
In view of the above results, broad, narrower, and preferred compositions and weight percent ranges were devised for the purpose of obtaining improvements in low cycle fatigue resistance and dwell crack growth behavior over the conventional R104 superalloy. These compositions and ranges are set forth below in Table I.
While the invention has been described in terms of particular processing parameters and compositions, the scope of the invention is not so limited. Instead, modifications could be adopted by one skilled in the art, such as by modifying the disclosed processing by substituting other processing steps or including additional processing steps. Accordingly, the scope of the invention is to be limited only by the following claims.
