Patent Application
20130129522
Disclosed are single crystal nickel-base superalloys exhibiting excellent high temperature creep resistance, while being substantially free of rhenium, without deleteriously affecting other relevant characteristics.
Because of a worldwide growing demand for products that have customarily required substantial quantities of relatively scarce metal elements, both the demand and prices of rare metal elements have sharply increased. As a result, manufacturers are searching for new technologies that will reduce or eliminate the need for these metal elements.
Rhenium is an example of a truly rare metal that is important to various industries. It is recovered in very small quantities as a by-product of copper-molybdenum and copper production. In addition to its high cost, use of rhenium presents a supply chain risk of both economic and strategic consequence.
Rhenium has been widely employed in the production of nickel-base superalloys used to cast single crystal gas turbine components for jet aircraft and power generation equipment. More specifically, rhenium is used as an alloying additive in advanced single crystal superalloys for turbine blades, vanes and seal segments, because of its potent effect at slowing diffusion and thus slowing creep deformation, particularly at high temperatures (e.g., in excess of 1,000 degrees C.) for sustained periods of time. High temperature creep resistance is directly related to the useful service life of gas turbine components and engine performance such as power output, fuel burn and carbon dioxide emissions.
Typical nickel-base superalloys used for single crystal castings contain from about 3% rhenium to about 7% rhenium by weight. Although rhenium has been used as only a relatively minor additive, it has been regarded as critical to single crystal nickel-base superalloys to inhibit diffusion and improve high temperature creep resistance, it adds considerable to the total cost of these alloys.
From the foregoing discussion, it is apparent that it would be extremely desirable to develop single crystal nickel-base superalloys that exhibit excellent high temperature creep resistance, while reducing or eliminating the need for rhenium additions, and while retaining other desirable properties such as good castability and phase stability.
The rhenium-free single crystal nickel-base superalloys disclosed herein rely on, among other things, balancing the refractory metal elements (tantalum, tungsten and molybdenum) at a total amount of about 17% to 20% in order to achieve good creep-rupture mechanical properties along with acceptable alloy phase stability, in particular, ensuring freedom from excessive deleterious topological close-packed (TCP) phases that are rich in tungsten, molybdenum and chromium, while substantially eliminating rhenium from the alloy.
It has been discovered that a rhenium-free single crystal nickel-base superalloy exhibiting excellent high temperature creep resistance and other properties well suited for used in casting gas turbine components can be achieved in an alloy composition containing 5.60% to 5.85% aluminum by weight, 9.4% to 9.9% cobalt by weight; 5.0% to 6.0% chromium by weight, 0.08% to 0.35% hafnium by weight, 0.50% to 0.70% molybdenum by weight, 8.0% to 9.0% tantalum by weight, 0.60% to 0.90% titanium by weight, 8.5% to 9.8% tungsten by weight, and the balance comprising nickel and minor amounts of incidental elements, the total amount of incidental elements being substantially less than 1% by weight.
In accordance with certain embodiments, the incidental elements of the alloy is controlled to maximums of 100 ppm carbon, 0.04% silicon, 0.01% manganese, 3 ppm sulfur, 30 ppm phosphorous, 30 ppm boron, 0.1% niobium, 150 ppm zirconium, 0.15% rhenium, 0.01% copper, 0.15% iron, 0.1% vanadium, 0.1% ruthenium, 0.15% platinum, 0.15% palladium, 200 ppm magnesium, 5 ppm nitrogen, and 5 ppm oxygen, with each of any other incidental elements being present as a trace element as a maximum of about 25 ppm.
In accordance with certain embodiments, the trace elements in the incidental impurities of the disclosed nickel-base superalloys is controlled to maximums of 2 ppm silver, 0.2 ppm bismuth, 10 ppm gallium, 25 ppm calcium, 1 ppm lead, 0.5 ppm selenium, 0.2 ppm tellurium, 0.2 ppm thallium, 10 ppm tin, 2 ppm antimony, 2 ppm arsenic, 5 ppm zinc, 2 ppm mercury, 2 ppm cadmium, 2 ppm germanium, 2 ppm gold, 2 ppm indium, 20 ppm sodium, 10 ppm potassium, 10 ppm barium, 30 ppm phosphorous, 2 ppm uranium, and 2 ppm thorium.
In accordance with certain embodiments in which enhanced oxidation resistance and/or coating and thermal barrier coating (TBC) life are desired, sulfur is present at a maximum amount of 0.5 ppm, and lanthanum and yttrium are added to target an amount of total lanthanum and yttrium of from about 5 ppm to about 80 ppm in single crystal components cast from the alloy.
In accordance with certain embodiments used for large industrial gas turbine (IGT) single crystal component applications requiring low angle boundary (LAB) strengthening up to 12 degrees, carbon is added in an amount from about 0.02% to about 0.05% by weight and boron is added in an amount from about 40 ppm to about 100 ppm.
In addition to achieving excellent high temperature creep resistance in a substantially rhenium-free composition, certain embodiments of the disclosed single crystal nickel-base superalloys have a desirably not excessive density that is about 8.8 gms/cc or less, such as 8.79 gms/cc (kg/dm3).
The alloys disclosed herein will be referred to as “CMSX®-7” alloys. This is the designation that will be used commercially, the expression “CMSX” being a registered trademark of the Cannon-Muskegon Corporation used in connection with the sale of a family or series of nickel-base single crystal (SX) superalloys.
The alloys disclosed herein are alternatively described as being rhenium-free, or substantially free of rhenium. As used herein, these terms means that the alloys do not contain any added rhenium and/or that the amount of rhenium present in the alloy is a maximum of 0.15% by weight.
Unless otherwise indicated, all percentages are by weight, and all amounts in parts per million (ppm) refer to parts per million by weight based on the total weight of the alloy composition.
Single crystal superalloys and castings have been developed to exhibit an array of outstanding properties including high temperature creep resistance, long fatigue life, oxidation and corrosion resistance, solid solution strengthening, with desired casting properties and low rejection rates, and phase stability, among others. While it is possible to optimize a single additive alloying elements for a particular property, the effects on other properties are often extremely unpredictable. Generally, the relationships among the various properties and various elemental components are extremely complex and unpredictable such that it is surprising when a substantial change can be made to the composition without deleteriously affecting at least certain essential properties.
With the embodiments disclosed herein, refractory metal elements (tantalum, tungsten and molybdenum) were maintained at a total amount of from about 17% to about 20% by weight, while balancing the amounts of the refractory elements to achieve good creep-rupture mechanical properties along with acceptable alloy phase stability (freedom from excessive deleterious topological close-packed (TCP) phase—normally tungsten, molybdenum and chromium rich in this type of alloy). Chromium and cobalt were also adjusted to ensure the required phase stability. The high amount of tantalum (approximately 8%) was selected to provide excellent single crystal castability, such as freedom from “freckling” defects. The amount of titanium (approximately 0.8%) and tantalum (approximately 0.8%) were adjusted to provide low negative γ/γ′ mismatch for high temperature creep strength and acceptable room temperature density (e.g., about 8.8 gms/cc, such as 8.79 gms/cc). Aluminum, titanium and tantalum were adjusted to attain a suitable γ′ volume fraction (Vf), while the combination of aluminum, molybdenum, tantalum and titanium were selected to provide good high temperature oxidation resistance properties. The amount of hafnium addition was selected for coating life attainment at high temperatures.
Typical chemistry for the alloys disclosed and claimed herein are listed in Table 1. However, there are certain minor variations. First, in order to achieve enhanced oxidation resistance and/or enhanced thermal barrier coating life, it is desirable to add lanthanum and/or yttrium in amounts such that the total of lanthanum and yttrium is targeted to provide from about 5 to 80 ppm in the single crystal castings made from the alloys. As another variation, in the case of large industrial gas turbine (IGT) single crystal applications where low angle boundary (LAB) strengthening is required up to 12 degrees, carbon and boron additions are targeted in the range from about 0.02% to 0.05% and 40-100 ppm, respectively.
The invention will be described with respect to certain illustrative, non-limiting embodiments that will facilitate a better understanding.
A 400 lb 100% virgin initial heat of CMSX®-7 alloy was melted in January 2011 in the CM V-5 Consarc VIM furnace using aim chemistry to CM KH Jan. 3, 2011 (CM CRMP #81-1700 Issue 1). The heat (5V0424) chemistry is shown in Table 2.
Two molds (#s 912 and 913) of SX NNS DL-10 test bars were cast to CMSX-4® casting parameters by Rolls-Royce Corporation (SCFO). DL-10 test bar yield at 23 fully acceptable out of a total 24 cast was excellent. A mold (#53701) of solid HP2 turbine blades were also SX cast by SCFO using CMSX-4® casting parameters with typical casting yields for this production component.
These DL-10 test bars and turbine blades were solutioned/homogenized+double aged heat treated at CM as follows—based on solutioning/homogenization studies on CMSX®-7 test bars.
Acceptable microstructure attainment is evident in FIGS. 1-2—complete γ′ solutioning, some remnant γ/γ′ eutectic, no incipient melting and approximately 0.5 μm average cubic, aligned γ′, indicating appropriate γ/γ′ mis-match and γ/γ′inter-facial chemistry, following the 4 hr/2050° F. (1121° C.) high temperature age.
Creep—and stress-rupture specimens were low stress ground and tested by Joliet Metallurgical Labs, with the results to date shown in Table 3 and Table 4. Larson-Miller stress-rupture graphs (
Phase stability is surprisingly good with absolutely no TCP phases apparent in the post-test creep/stress rupture bars examined to date (
Burner rig dynamic, cyclic oxidation and hot corrosion (sulfidation) testing is currently scheduled at a major turbine engine company. The MFB 0.020″ thick gage mini-flat results at 12 ksi/2000° F. (Table 4,
CMSX-7 Tensile Properties
The alloy shows very high tensile strength (up to 200 ksi (1379 MPa) UTS at 1400° F. (760° C.)) and 0.2% proof stress (up 191 ksi (1318 MPa) at the same temperature and good ductility (Table 5,
A further heat (5V0459) of 100% Virgin (470 lbs) designated CMSX®-7 Mod B was melted in May 2011 in the CM V-5 Consarc VIM furnace using aim chemistry to CM KH Apr. 13, 2011 (CM CRMP #81-1703 Issue 1). The heat (5V0459) chemistry is shown in Table 6.
Two molds (#s 923 & 924) of SX NNS DL-10 test bars were cast to CMSX-4® casting parameters by Rolls-Royce Corporation (SCFO). DL-10 test bar yield at 22 fully acceptable out of a total 24 cast was excellent.
These DL-10 test bars were solutioned/homogenized+double aged heat treated at Cannon-Muskegon Corporation as follows—based on solutioning/homogenization studies on CMSX®-7 Mod B test bars.
Acceptable microstructure attainment is evident
The creep-rupture properties of CMSX®-7 Mod B are very similar to that of CMSX®-7, with no apparent advantage (Table 7).
Post-test microstructures from a longer term, high temperature stress-rupture test [2000° F./12 ksi (1093° C./83 MPa)/1176.5 hours] are shown (
The embodiments disclosed herein are non-limiting examples that are provided to illustrate and facilitate a better understanding, the scope of the invention being defined by the appending claims as properly construed under the patent laws, including the doctrine of equivalents.
