Patent Application
20250137099
This invention relates to wroughtable high strength alloys for use at elevated temperatures. In particular, it is related to alloys which possess sufficient mechanical strength, toughness/containment, and thermal stability for service in gas turbine cases and other gas turbine components.
To meet the demand for increased operating efficiency, gas turbine engine designers would like to employ higher and higher operating temperatures. However, the ability to increase operating temperatures is often limited by material properties. One application with such a limitation are gas turbine cases. Turbine cases surround the turbine section of the engine and provide protection. They are typically produced as forgings and thus require good forgeability. Often gamma-prime strengthened alloys are used in turbine cases due to their mechanical strength at elevated temperatures. In particular, they require high low-cycle fatigue (LCF) strength as well as high creep strength. High LCF strength is considered a good indicator of an alloy's resistance to thermal fatigue which results from the engine temperatures cycling throughout the life of the engine. However, current commercially available forgeable gamma-prime strengthened alloys do not possess adequate LCF strength beyond around 1400° F. High creep strength is a measure of an alloy's ability to withstand loads over sustained periods of time at intermediate to high temperatures.
Wrought gamma-prime strengthened alloys are often based on the nickel-chromium-cobalt system, although other base systems are also used. These alloys will typically have additions of one or more of the elements aluminum, titanium, niobium, and tantalum which are responsible for the formation of the gamma-prime phase, Ni3 (X) where X=Ti, Al, Nb, Ta, etc. An age-hardening heat treatment is used to develop the gamma-prime phase into the alloy microstructure. This heat treatment is normally given to the alloy when it is in the annealed condition. The presence of gamma-prime phase leads to a considerable strengthening of the alloy over a broad temperature range. Other elemental additions may include molybdenum or tungsten for solid solution strengthening, carbon for carbide formation, and boron for improved high temperature ductility. A certain amount of iron can be tolerated in these alloys and will typically be present as a result of the manufacturing process.
Many gas turbine engine components (including turbine cases) require high mechanical strength at elevated temperatures. There are multiple types of strength which may be required depending on the component use temperature and the nature of the loading. At elevated temperatures these include low-cycle fatigue strength, creep strength, and tensile strength. Another key property for turbine cases and certain other components is the toughness of the material. Toughness is a measure of an alloy's ability to absorb energy before failing. Both strength and ductility contribute to alloy toughness. An alloy with high toughness is also said to have high containment as further discussed later in this specification.
Thermal stability is a measure of whether the alloy microstructure remains relatively unaffected during a thermal exposure. Many high-temperature alloys can form brittle intermetallic or carbide phases during thermal exposure. The presence of these phases can dramatically reduce the room-temperature ductility of the material. This loss of ductility can be effectively measured using a standard room temperature tensile test.
In addition to the material properties required of a gas turbine engine alloy during service, it is necessary that the alloy have requisite properties which enable the fabrication of complex components. These may include such qualities as hot forgeability, cold formability, and weldability. For gamma-prime strengthened alloys it is often weldability which limits their use or which necessitates high fabrication costs. The weldability of a gamma-prime strengthened alloy is tied in large part to its ability to resist strain-age cracking as described later in this specification.
Many wrought gamma-prime strengthened alloys are available in today's marketplace. The Rene-41 or R-41 alloy (U.S. Pat. No. 2,945,758) was developed by General Electric in the 1950's for use in turbine engines. It has excellent creep strength, but is limited by poor thermal stability and resistance to strain age cracking. A similar General Electric alloy, M-252 alloy (U.S. Pat. No. 2,747,993), was also developed in the 1950's. The M-252 alloy has good creep strength and resistance to strain age cracking, but like R-41 alloy is limited by poor thermal stability. A Pratt & Whitney developed alloy known commercially as WASPALOY alloy (apparently having no U.S. patent coverage) is another gamma-prime strengthened alloy intended for use in turbine engines and available in sheet form. However, this alloy has inadequate LCF strength at 1500° F. (816° C.), marginal creep strength, and has only moderate resistance to strain age cracking. The alloy commercially known as 263 alloy (U.S. Pat. No. 3,222,165) was developed in the late 1950's and introduced in 1960 by Rolls-Royce Limited. This alloy has excellent thermal stability and resistance to strain age cracking, but has very poor LCF and creep strength at 1500° F. (816° C.). HAYNES® 282® alloy (U.S. Pat. No. 8,066,938 B2) was introduced in 2005. This alloy has a combination of resistance to strain-age cracking, good thermal stability and good creep strength. However, the LCF strength is not adequate at 1500° F. (816° C.) for certain advanced turbine case applications. HAYNES® 233® alloy (U.S. Pat. No. 10,577,680 B2) was introduced in 2016. This alloy has a combination of excellent oxidation resistance, good creep strength, good thermal stability, and good fabricability. However, similarly to Waspaloy and 282 alloy, the LCF strength of 233 alloy is not adequate at 1500° F. (816° C.) for advanced turbine case applications.
As suggested by these examples, the established commercial alloys often considered for turbine case applications do not have sufficiently high mechanical strength (LCF and/or creep) at 1500° F. (816° C.) in combination with high containment and good thermal stability to meet the increasing demands of the next generation of gas turbine engines.
British Patent Publication GB 1 029 609 discloses an alloy adapted for use in the manufacture of gas turbine engines. However, the claimed compositional range of the British patent includes compositions which would be expected to not possess at least one of the three key properties for turbine cases described above based on the relationships taught by the present invention. The aluminum ranges for compositions disclosed in this patent publication do not overlap the aluminum range for compositions of the present invention. Furthermore, the publication does not teach how to control the composition to obtain these desired properties. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
British Patent Publication GB 2 712 498 discloses alloys from which to make engineering parts capable of withstanding great stress at high temperatures, such parts including blades and others within internal combustion gas turbines. However, the claimed compositional range of the British patent includes compositions which would be expected to not possess at least one of the three key properties for turbine cases described above based on the relationships taught by the present invention. For example, commercial alloys Waspaloy and HAYNES 282 alloy and experimental alloys 3 and 7 are within the compositions disclosed in this patent publication that do not have all of the key properties. Furthermore, the publication does not teach how to control the composition to obtain these desired properties. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Japanese Published Patent Application JP 01129942 discloses a nickel-based alloy said to have excellent hot workability. This publication teaches that zirconium improves the hot workability of the alloy and should be present in an amount of from 0.02% to 0.1 wt. %. But, such levels of zirconium are likely to produce hot cracking problems during welding of the alloy. This patent application also teaches that tungsten is necessary for high temperature toughness, while I have found that very little tungsten can be used, but tungsten may be present as a partial substitute for molybdenum according to a specified relationship. Moreover, the claimed compositional range of JP 01129942 contains alloys which would be expected to not possess one or more of the key properties for gas turbine cases based on the relationships taught by the present invention. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Japanese Published Patent Application JP 06172900 discloses a nickel-chromium-cobalt-molybdenum alloy containing from 8 to 12 wt. % molybdenum. However, this patent (issued in the 1990's) appears to claim compositions of much earlier patented alloys such as R-41 alloy and M-252 alloy (described previously in this publication). The reference fails to recognize that molybdenum levels above 8.5 wt. % can be detrimental and lead to lower thermal stability, lower LCF strength, and/or lower containment in this type of alloy. The claimed compositional range of this Japanese reference includes compositions which would not be expected to have all of the key properties described above. HAYNES 282 alloy and experimental alloys 2, 4 and 5 in Table 1 below contain small amounts of iron (at levels which can be considered typical for iron impurities in commercially produced alloys of this type) and do not possess all of the key properties. Removal of that iron would create alloys that are within the compositional ranges of alloys disclosed in JP 06172900. But I would not expect removal of the iron to significantly change the key properties of those alloys. Furthermore, the publication does not teach how to control the composition to obtain these desired properties. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Consequently, there is a need for an alloy which has a combination of excellent mechanical strength, high containment, and good thermal stability at the intermediate temperature of 1500° F. (816° C.). In addition, for certain applications it would be beneficial if the alloy possessed good weldability, particularly good resistance to strain-age cracking.
The principal objective of this invention is to provide new forgeable, age-hardenable nickel-chromium-cobalt based alloys which are suitable for use in high temperature gas turbine cases and other gas turbine components that has higher low-cycle fatigue strength, good thermal stability and an acceptable containment factor.
It has been found that this objective can be reached with an alloy containing a certain range of chromium and cobalt, a certain range of solid-solution strengtheners (molybdenum and tungsten), and a certain range of gamma-prime formers (aluminum, titanium, niobium and tantalum) present, with a balance of nickel and various minor elements and impurities.
Specifically, the necessary ranges are 16 to 20 wt. % chromium, 8 to 13 wt. % cobalt, 4 to 8.5 wt. % molybdenum, up to 8 wt. % tungsten, 2.1 to 4.1 wt. % aluminum, titanium up to 1.9 wt. %, up to 3.7 wt. % niobium, up to 5 wt. % iron, carbon present up to 0.15 wt. %, up to 0.015 wt. % boron, up to 7.1 wt. % tantalum, up to 0.13 silicon, up to 1.0 manganese, and up to 0.06 wt. % zirconium, with a balance of nickel and impurities. Additionally, certain compositional relationships are necessary which place constraints on the total and relative amounts of both the gamma-prime formers and solid-solution strengtheners. As a result, there are certain experimental alloys which satisfy all individual elemental requirements but which are not considered part of the present invention due to not meeting one of said compositional relationships. Such alloys do not meet one or more of the key property targets.
I provide Ni—Cr—Co—(Mo,W)—(Al,Ti,Nb,Ta) base alloys which contain 16 to 20 wt. % chromium, 8 to 13 wt. % cobalt, 4 to 8.5 wt. % molybdenum, up to 8 wt. % tungsten, 2.1 to 4.1 wt. % aluminum, up to 1.9 wt. % titanium, up to 3.7 niobium, up to 7.1 wt. % tantalum, certain minor element additions, along with typical impurities, and a balance of nickel, subject to certain compositional relationships defined and disclosed herein, and which have properties useful for a variety of gas turbine engine components, such as turbine cases. Based on the understanding of the turbine case requirements of future gas turbine engines, an alloy with the following attributes would be highly desirable: 1) high containment at intermediate use temperatures such as 1200-1500° F. (649 to 816° C.), 2) good thermal stability at intermediate use temperatures, and 3) excellent mechanical strength at intermediate use temperatures. In many applications it would also be desirable to have moderate or better weldability. No commercial alloy is currently available in the marketplace with all of these qualities. One of the more commonly used turbine case alloys is Waspaloy alloy. Waspaloy alloy has high containment and good thermal stability, however its mechanical strength (both LCF and creep) at 1500° F. (816° C.) is fairly low and limits the upper use temperature of the alloy. As will be shown, the alloys of the present invention offer significantly improved mechanical strength over Waspaloy at 1500° F. (816° C.).
I tested forty-one experimental alloys whose compositions are set forth in Table 1. The experimental alloys were all nickel-based and had a chromium content which ranged from 16.80 to 19.65 wt. %, as well as a cobalt content ranging from 9.28 to 10.81 wt. %. The molybdenum content ranged from 3.73 to 8.59 wt. %. The aluminum content ranged from 1.35 to 3.90 wt. %. Iron was present from 0.87 up to 9.93 wt. %. Titanium was added to some of the alloys and ranged from less than 0.01 up to 2.96 wt. %. Niobium was added to some of the alloys and ranged from less than 0.02 up to 5.33 wt. %. Tantalum was added to some of the alloys and ranged from less than 0.01 up to 2.01 wt. %. Tungsten was added to some of the alloys and ranged from less than 0.01 up to 8.43 wt. %. Silicon was present as an impurity in some alloys and as an intentional addition to some alloys and ranged from 0.01 to 0.17 wt. %. Manganese was an intentional addition to some alloys and ranged from less than 0.01 up to 0.24 wt. %. Minor quantities of carbon and boron were also added to the experimental alloys and were present up to 0.081 and 0.006 wt. %, respectively. None of the alloys had an intentional addition of zirconium. Consequently, the measured zirconium levels of all the experimental alloys were less than 0.02 wt. % (for sake of brevity these results were not included in Table 1).
All testing of the alloys was performed on sheet material of 0.065″ to 0.125″ (1.6 to 3.2 mm) thickness. The experimental alloys were vacuum induction melted, and then electro-slag remelted, at a heat size of 30 to 50 lb. (13.6 to 27.2 kg). The ingots were hot forged to slab on an open die press generally without difficulty. Next, the slabs were hot rolled to intermediate gauge. The sheets were annealed, water quenched, and cold rolled to produce sheets of the desired gauge. Intermediate annealing of cold rolled sheet was necessary during production of the 0.065″ sheet (1.6 mm). The cold rolled sheets were annealed as necessary to produce a fully recrystallized, equiaxed grain structure with an ASTM grain size typically between 4 and 5. The sheet samples were water quenched or rapid air cooled after annealing. For certain tests the annealed samples were given age-hardening heat treatments appropriate for the specific alloy prior to testing. The details of the age-hardening heat treatments are provided later in this specification.
To evaluate the key properties (containment, thermal stability, mechanical strength) and a secondary property (weldability), up to five different types of tests were performed on experimental alloys to establish their suitability for the intended applications. The results of these tests are described in the following sections. Additionally, key and secondary property testing was performed on three commercially available alloys (282 alloy, Waspaloy alloy, and 233 alloy) to provide comparative information. Table 2 provides measured compositions of samples of the tested commercial alloys. Note that the sample chemistries for the commercial alloys were taken from actual samples of the commercial alloys and are considered representative, but may not correspond to the same heat(s) tested in this program.
The first key property of the alloys of this invention is containment. In certain components in gas turbine engines, particularly in aero engines, it is desired to have high containment (or toughness) at elevated operating temperatures. These components, which may include certain cases and rings, may be required to have high containment properties in the event of an engine failure. Such containment properties are highly dependent on the ductility of the alloy at the operating temperatures, in addition to high strength. While containment properties are best measured by costly special high strain rate tests, a reasonable measure of containment properties can be obtained by consideration of the ductility (elongation) values resulting from a standard tensile test at the relevant temperature. The yield strength (YS) and ultimate tensile strength (UTS) values from the tensile test are also considered. A containment factor, CF, can be calculated from the results of a tensile test and is defined as CF=½*(YS+UTS)*(Elongation). (Note: In customary units the containment factor is calculated using YS and UTS in ksi and the elongation is expressed as a percentage. The resulting unit for the containment factor is thus lbf-in/in3×10−1. For the sake of brevity, this unit descriptor will not be appended to the containment factor values in the remainder of this specification and will be assumed to be lbf-in/in3×10−1 unless otherwise noted. To covert the CF to metric units (MJ/m3) multiply by 0.06895.) For applications where containment properties are required, a high value of CF is desired. When comparing CF values of various alloys, it is important to use similar product forms and sizes and to use identical sample geometries, since tensile properties can be strongly dependent on product form and size as well as the geometry of the test sample. The containment factor is dependent on temperature given the fact that the underlying tensile properties are normally temperature dependent. For applications where containment properties are valued the use temperatures may fall in the “intermediate range” of approximately 1200° F. to 1500° F. (649 to 816° C.). For this reason, a temperature of 1400° F. (760° C.) was selected for testing.
The forty-one experimental alloys as well as three commercial alloys (HAYNES 282 alloy, HAYNES 233 alloy, and Waspaloy alloy) were tensile tested at 1400° F. to determine their strength and ductility. The results are provided in Table 3 along with the calculated containment factors. Note that all samples were tested in the as-heat treated condition as defined in Table 4.
The 1400° F. (760° C.) containment factors of the three commercial alloys were found to be 2305, 2924, and 3634 (159, 202, and 251 MJ/m3) for 282 alloy, 233 alloy, and Waspaloy alloy, respectively. To be considered for turbine case alloys, it would be desirable to have a 1400° F. (760° C.) containment factor as high or higher than 282 alloy, so this was used a target in the present invention. It was found that thirty-three of the forty-one experimental alloys had a 1400° F. (760° C.) containment factor higher than 282 alloy, i.e. CF≥2305 (159 MJ/m3). The eight alloys with a 1400° F. (760° C.) containment factor less than 2305 (159 MJ/m3) were experimental alloys 3, 4, 7, 12, 13, 14, 18, and 19. Therefore, these eight alloys are considered outside of the present invention.
The second key property of the alloys of this invention is thermal stability. Since applications such as turbine cases require prolonged use at high temperatures, the thermal stability of the materials with which they are constructed is of significant importance. The thermal stability of the experimental and commercial alloys was tested by applying a thermal exposure at 1400° F. (760° C.) for 1000 hours. Note that for all alloys the thermal exposures were applied to samples in the age-hardened condition. The room temperature (RT) tensile properties of the thermally exposed samples were then measured. The retained RT yield strength as well as the retained RT tensile elongation (ductility) after the thermal exposure is one measure of an alloy's thermal stability. Those measurements are in Table 5. The exposure temperature of 1400° F. (760° C.) was selected since many nickel-base alloys have the least thermal stability around that temperature range and it is a relevant temperature for turbine case applications. To have acceptable thermal stability for the applications of interest, it was determined that the retained RT yield strength should be as good or better than that of 282 alloy, i.e. ≥104.1 ksi (718 MPa) and the retained RT ductility should be at least 10%. The results for the forty-one experimental alloys and the three commercial alloys are shown in Table 5. It was found that thirty-nine of the forty-one experimental alloys had a retained yield strength greater than 104.1 ksi (718 MPa). Only experimental alloys 5 and 18 had insufficient retained yield strength to be considered part of the present invention. Both 233 alloy and Waspaloy alloy were found to have greater than 104.1 ksi (718 MPa) retained yield strength. Considering retained ductility, it was found that thirty-six of the forty-one experimental alloys had values greater than 10%. The five experimental alloys with less than 10% retained ductility were experimental alloys 11, 14, 15, 32, and 33 and these alloys are not considered part of the present invention. The three commercial alloys all had retained ductility greater than 20%.
The third key property of the alloys of this invention is high mechanical strength, particularly high LCF and/or creep strength. A common test to evaluate LCF strength is the strain-controlled test where a sample is cyclically strained to fixed strain level until failure and the number of cycles to failure (also termed the LCF life) is considered a measure of its LCF strength. In this specification LCF test results at 1500° F. (816° C.) are provided for most of the experimental alloys and all three of the commercial alloys. The tests were strain-controlled, fully-reversed (R=−1), with a triangular waveform, and a frequency of 20 cycles/min (0.33 Hz). Most of the tests were performed at a total strain range (TSR) of 0.6%. The results are provided in Table 6. Note that due to the effort and expense of such testing, not all of the experimental alloys were LCF tested.
As seen in Table 6, the LCF lives of the three commercial alloys under these test conditions were 6446, 6344, and 9507 cycles for 282 alloy, Waspaloy alloy, and 233 alloy, respectively. Since newer gas turbine engine designs will require a significant improvement in LCF strength over currently available alloys, I have selected a minimum of 10000 cycles to be a desirable target for alloys of the present invention, representing an improvement of more than 57% over Waspaloy. Given this 10000 cycle target, it can be seen from Table 6 that of the twenty-eight experimental alloys tested, only eighteen met the target while ten missed the target. The alloys which met the target were experimental alloys 3, 7, 8-10, 16, 17, 20, 22-25, 27, and 30-34. The alloys which missed the target were experimental alloys 2, 12-14, 18, 19, 21, 26, 28, and 29. The sixteen alloys which were not tested were experimental alloys 1, 4-6, 11, 15, 31-33, and 35-41.
In addition to the LCF test described above, eight experimental alloys were given an LCF test at the lower TSR value of 0.5%. These tests take longer and are more expensive to run, but in some ways are a better predictor of the long term fatigue strength of an alloy. The temperature and other test conditions were all held the same as the TSR=0.6% tests. The results are shown in Table 7 below.
As seen in Table 7, the LCF lives of the three commercial alloys under these test conditions were 13130, 31520, and 39957 cycles for 282 alloy, Waspaloy alloy, and 233 alloy, respectively. A target LCF life of 40000 cycles was selected for this TSR level, representing an increase of 27% over Waspaloy. Of the eight experimental alloys tested at this condition, three were found to miss the target (alloys 1, 5, and 6), while five met the target (7, 8, 23, 30, and 36). None of the three commercial alloys tested met the target LCF life.
The creep strength of fifteen experimental alloys as well as Waspaloy were measured with a creep test at 1500° F. (816° C.) with an applied stress of 30 ksi (207 MPa). The 1% creep life (i.e., the time to reach an elongation of 1%) was recorded for each test and the results are given in Table 8. The 1% creep life of Waspaloy was 49.5 hours. I have selected a target of 70 hours for the 1% creep life for alloys of this invention, which represents a 41% improvement over Waspaloy. All fifteen experimental alloys (3, 9, 17, 20, 22-23, 30-31, 35-41) subjected to the 1500° F. (816° C.), 30 ksi (207 MPa) creep test were found to meet this target.
When the criteria for all three key properties (containment, thermal stability, mechanical strength) were considered together, it was found that twenty-one experimental alloys met all three key property targets and are considered alloys of the present invention. These were experimental alloys 8-10, 16, 17, 20, 22-25, 27, 30, 31, and 34-41, as listed in Table 9. Note that since not all mechanical tests were performed on each alloy, I have counted alloys which met the target in one or more of the three tests (i.e. the two LCF test conditions and the creep test) as having satisfactory mechanical strength. It can also be seen in Table 9 that twenty of the experimental alloys and all three commercial alloys were found to not meet or more of the key property targets and are therefore not part of the present invention.
I have found that the major alloying elements of the acceptable alloys fall within certain compositional ranges as shown in Table 10 and that the acceptable alloys are subject to three additional compositional relationships which are now disclosed. (Note: the ranges of the minor alloying elements are discussed later in this specification). The first two compositional relationships pertain to the gamma-prime forming elements (aluminum, titanium, niobium, and tantalum). As implied, these elements form the strengthening gamma-prime phase (Ni3X where X=Al, Ti, Nb, and/or Ta) when subjected to the age-hardening heat treatment detailed in Table 4. It is critical to control the amount of these elements in the alloys of this invention as they can greatly affect all three key properties. I have found that the gamma-prime forming elements should be controlled using the following equations (where elemental compositions are in wt. %):
Note that by definition the upper limit of the R factor is 1. It is important to state that critical role that the R and G factors play in determining which alloy compositions provide the unique combination of the three key properties of the alloys of this invention was an unexpected and surprising finding.
The third compositional relationship relates to the solid-solution strengthening elements molybdenum and tungsten. Too much of these elements may lead to the presence of undesirable phases which could affect a number of alloy properties including, but not limited to, thermal stability and LCF. Conversely, too little of the solid-solution strengthening elements is undesirable since high mechanical strength is important for the alloys of this invention. Based on the present results and my understanding of similar alloys systems, I require, in addition to the acceptable limits for these elements individually (listed in Table 10), upper and lower limits on the sum of these two elements as described by the following equation (where elemental compositions are given in wt. %):
For convenience, the calculated values of the T, R, G, and Z factors for the experimental and commercial alloys in this specification are given in Table 11.
In Table 10 the upper limits of titanium, niobium, and tantalum are 1.9 wt. %, 3.7 wt. %, and 7.1 wt. %, respectively. These values were calculated from Equations 1 and 2 and the aluminum range of 2.1 to 4.1 wt. %. While the aluminum range is well supported by the example alloys, the calculated upper limit values for titanium, niobium, and tantalum are notably higher than the upper range of the satisfactory example alloys which were 0.50 wt. %, 2.20 wt. %, and 2.01 wt. %, respectively. Given this fact and the typical tolerances for these elements in melt practice, more conservative upper limits for these elements would be 1 wt. %. 2.7 wt. %, and 2.5 wt. % for titanium, niobium, and tantalum, respectively.
When the compositional requirements of the alloys of this invention (Table 10 plus Equations 1, 2, & 3) are considered, the twenty-one experimental alloys which met all three key property targets (experimental alloys 8-10, 16-17, 20, 22-25, 27, 30-31, and 34-41) were found to satisfy all compositional requirements. Conversely, the twenty experimental alloys and all three commercial alloys (experimental alloys 1-7, 11-15, 18-19, 21, 26, 28-29, 32, and 33 and commercial alloys 282 alloy, Waspaloy alloy, and 233 alloy) which did not meet one or more key property targets also did not satisfy one or more compositional requirements.
For certain gas turbine components it may be necessary to add secondary property targets. For example, if the component were to require welding then it would be desirable to have sufficient weldability. For alloys strengthened by the gamma-prime phase, weldability may be negatively affected by an effect known as strain age cracking. This phenomenon typically occurs when a welded part is subjected to a high temperature for the first time after the welding operation. Often this is during the post-weld annealing treatment given to most welded gamma-prime alloy fabrications. The cracking occurs as a result of the formation of the gamma-prime phase during the heat up to the annealing temperature. The formation of the strengthening gamma-prime phase in conjunction with the low ductility many of these alloys possess at intermediate temperatures, as well as the mechanical restraint typically imposed by the welding operation will often lead to cracking. The problem of strain age cracking can limit alloys to be used up to only a certain thickness since greater material thickness leads to greater mechanical restraint.
The resistance of the experimental and commercial alloys to strain-age cracking was measured using the modified CHRT test described by Metzler in Welding Journal supplement, October 2008, pp. 249s-256s. This test was developed to determine an alloy's relative resistance to strain-age cracking. It is a variation of the test described in U.S. Pat. No. 8,066,938 and in the paper by Rowe in Welding Journal supplement, February 2006, pp. 27s-34s. The CHRT test was originally developed in the late 1960's as a method to determine the susceptibility of various heats of Rene 41 (R-41) alloy to strain-age cracking (Fawley, R. W., Prager, M., Carlton, J. B., and Sines, G. 1970, WRC Bulletin No. 150. Welding Research Council, NY). In the CHRT test, a solution annealed tensile-test specimen is heated at a controlled rate to a test temperature within the gamma-prime precipitation temperature range, then pulled to failure. The CHRT elongation is indicative of an alloy's resistance to strain-age cracking/weldability. During the CHRT test, the sample undergoes gamma-prime precipitation that simulates the effect of welding and consequent cooling. The CHRT test thus reliably predicts the behavior of the material in the as-welded condition. The CHRT test was designed to be a relatively simple test to perform but the results agree well with reported strain-age cracking studies (for example, see Rowe in Welding Journal supplement, February 2006, pp. 27s-34s). Key variables found to affect performance in the CHRT test include composition and grain size. In the modified CHRT test, the width of the gauge section is variable and the test is performed on a dynamic thermo-mechanical simulator rather than a screw-driven tensile unit. The results of the two different forms of the test are expected to be qualitatively similar, but the absolute quantitative results will be different.
Modified CHRT tests were performed on the forty-one experimental alloys using solution annealed and surface ground samples. The testing was conducted at 1450° F. (788° C.), and the reported CHRT ductility values were measured as elongation over 1.5 inches (38 mm). The results are shown in Table 12. Similar tests were performed on the three commercial alloys (282 alloy, Waspaloy alloy, and 233 alloy) in the mill annealed condition and the results are also shown in Table 12. The CHRT elongation values for 282 alloy and 233 alloy were 13.0% and 12.5%, respectively. These relatively high values indicate that both alloys have good resistance to strain-age cracking. This is consistent with the reported field experience for these two alloys. However, the reputation of Waspaloy alloy is quite different. While Waspaloy is moderately weldable, it can be susceptible to strain-age cracking under certain conditions. The CHRT test was performed on Waspaloy in two conditions, the mill annealed condition and the mill annealed plus surface ground condition. The CHRT elongation values for these two conditions were 6.8% and 6.0%, respectively. To ensure a certain amount of weldability in the alloys of this invention, it is preferred that the CHRT elongation be greater than Waspaloy in the surface ground condition. That is, greater than 6.0%. More preferred would be to have a CHRT ductility greater than that of Waspaloy in the as-mill annealed condition, that is, greater than 6.8%.
It was found that a CHRT elongation greater than 6.0% can be achieved through further constraints on the amounts of gamma-prime forming elements. Specifically, I have found that Al should be limited to the range of 3.1 to 4.1 wt. %, Ti to up to 0.4 wt. %, Nb to up to 1.3 wt. %, and Ta to up to 2.5 wt. %. In addition, the R factor should be further limited as follows (where elemental compositions are given in wt. %):
Thus, when a CHRT elongation >6.0% is desired, the preferred compositional requirements of the major elements are summarized in Table 13. Note that the preferred requirements of the minor alloying elements are discussed later in this specification. The eighteen experimental alloys meeting the preferred compositional requirements were experimental alloys 9, 17, 20, 22-25, 27, 30-31, and 34-41. All eighteen alloys had a CHRT elongation >6.0%.
Additional compositional restraints can result in even further improvements to the weldability as well as other properties. Specifically, I have discovered alloy compositions where the CHRT elongation is greater than that of Waspaloy alloy (surface ground) and there are significant and simultaneous improvements to all three key properties (containment, thermal stability, and mechanical strength). These improved compositions are hereby referred to as the “more preferred” compositions and are summarized in Table 14 for the major alloying elements. The more preferred compositions involved tightening the elemental ranges of several elements, including adding a minimum required value for tungsten and iron-both of which were found to be beneficial in maximizing alloy properties. In addition, the R and G factors should be further limited as follows (where elemental compositions are given in wt. %):
Note that the more preferred requirements of the minor alloying elements are discussed later in this specification. I found that alloys with the more preferred compositions had CHRT elongation values greater than 6.8%, 1400° F. (760° C.) containment factors greater than 2500 (172 MJ/m3), retained RT yield strength and RT elongation values after 1400° F. (760° C.)/1000 h of greater than 115 ksi (793 MPa) and 15%, respectively, and one or more of the following three mechanical strength measures: 1) 1500° F. (816° C.), TSR=0.6% LCF lives greater than 19500 cycles, 2) 1500° F. (816° C.), TSR=0.5% LCF lives greater than 80000 cycles, 3) 1500° F. (816° C.), 30 ksi (207 MPa) 1% creep lives greater than 125 hours. This combination of properties represents a considerable improvement over Waspaloy alloy, the most commonly used turbine case alloy making these alloys strong candidates for the next generation of gas turbine engines. The eight experimental alloys meeting the more preferred compositional requirements in Table 14 were experimental alloys 23, 30-31, 36, and 38-41. These eight alloys all had a CHRT ductility greater than 6.8%, as well as meeting the improved containment, thermal stability, and mechanical strength targets defined in this paragraph.
The alloys of this invention are nickel-base alloys as are the vast majority of the so-called superalloys, a class of alloys known for providing high strength, environmental resistance, etc. under the demanding conditions common in gas turbine engines. A brief mention of the benefits of the major alloying elements in the alloys of this invention follows, but should not be considered exhaustive. Chromium provides useful oxidation and hot corrosion resistance as well as some strengthening. It is present in relatively high amounts in many wrought superalloys and the range of 16-20 wt. % chromium in the alloys of the present invention is fairly typical. Specification ranges for chromium in nickel-base superalloys can be quite large. For example, one specification for chromium in Waspaloy has a range of 3 wt. % (or ±1.5 wt. %), whereas one specification for 282 alloy has a range of 2 wt. % (or ±1 wt. %). Cobalt provides some strengthening and regulates the gamma-prime solvus temperature. However, due to its chemical similarity, cobalt can be substituted interchangeably for nickel in relatively large amounts without drastically affecting properties. In the alloys of this invention cobalt can range from 8 to 13 wt. % (or ±2.5 wt. %), more preferably from 9 to 11 wt. % (or 10±1 wt. %). These ranges compare favorably with industry specifications. Similar to chromium, the specification ranges for cobalt in nickel-base superalloys can be quite large. For example, one specification for cobalt in Waspaloy has a range of 3 wt. % (or ±1.5 wt. %), whereas one specification for 282 alloy has a range of 2 wt. % (or ±1 wt. %). Iron may be an intentional addition or present as typical impurity in nickel-base alloys. I have found that intentional iron additions of at least 1.5 wt. % provide beneficial effects on fabricability. However, iron should be kept to no more than 5 wt. % in order to meet all key property targets. More preferably iron should be kept to 4 wt. % or less. Aluminum, titanium, niobium, and tantalum provide significant strengthening through the formation of the gamma-prime phase during heat treatment. Concerning the relative amounts of these four gamma-prime forming elements, I have found that higher aluminum levels are beneficial and that titanium should preferably be kept to 0.4 wt. % or less. An even lower titanium level of 0.1 wt. % or less should provide further benefits. Aluminum must be present at a minimum of 2.1 wt. %, but should preferably be at least 3.1 wt. %. Molybdenum and tungsten are effective solid-solution strengthening elements. A minimum of 4 wt. % molybdenum is required. While tungsten is not required in the present invention, more preferred compositions would have at least 1 wt. % since tungsten was found to provide beneficial effects. A tungsten level of a least 2 wt. % is better still. However, tungsten should be kept to no more than 8 wt. % to avoid thermal stability issues. More preferably, tungsten should be limited to no more than 6 wt. %.
As described previously, the amounts of both the gamma-prime forming elements and solid-solution strengthening elements must be carefully controlled (using the compositional relationships I have discovered and described herein) in order to provide the necessary balance of alloy properties. As a consequence of the requirement to meet these compositional relationships, certain alloys may meet all individual elemental requirements but still fall outside of the present invention. For example, alloys 7, 28, 29, and 32 are within the broad elemental ranges for the present invention but the G factors for these alloys are too high (>2.40) and as stated in Table 9 do not meet one of the key property targets. Similarly, alloys may meet the compositional relationship requirements but fall outside of one or more of the individual elemental ranges. For example, alloys 3, 4, and 21 meet all the compositional relationship requirements, but alloys 3 and 4 have too high molybdenum and alloy 21 has too high iron. In addition to the case of the broad claims just described, similar consequences could arise with regard to the preferred or more preferred claims.
To graphically describe the R, G, and T factors and the limits on these factors described by the compositional relationships given in Equations 1, 2, 4, 5, and 6, I have constructed
In addition to carbon, which is required in these alloys, other minor element additions may include boron, manganese, silicon, zirconium, magnesium, calcium, hafnium and one or more rare earth elements (including, but not limited to, yttrium, lanthanum, and cerium). The acceptable ranges of the minor elements are described below and summarized in Table 15. These acceptable ranges are considered the same whether the minor element was intentionally added or only present as an impurity.
Carbon is intentionally added to the alloys of this invention to provide interstitial strengthening, carbide strengthening, and improved grain size control. Carbon is limited to a maximum of 0.15 wt. % in the alloys of this invention, to a maximum of 0.1 wt. % for preferred compositions, and to a maximum of 0.08 wt. % for the more preferred compositions. Boron may be added in a small, but effective trace content up to 0.015 wt. % to obtain certain benefits known in the art. For the preferred and more preferred compositions, the boron is limited to maximum of 0.008 wt. % to ensure good weldability. While boron was present in all of the experimental alloys and is believed to have mostly beneficial effects, it is an optional addition. To enable the removal of oxygen and sulfur during the melting process, these alloys may contain small quantities of manganese, up to about 1 wt. %, and possibly traces of magnesium, calcium, and rare earth elements (such as yttrium, cerium, and lanthanum) up to about 0.05 wt. % each. Although silicon is sometimes similarly added to alloys for oxygen and sulfur control, I have found that silicon is detrimental to the LCF strength of these alloys. Silicon should therefore be limited to 0.13 wt. %, preferably to 0.12 wt. %, and even more preferably to less than 0.1 wt. %. Zirconium may be present in these alloys as an impurity or intentional addition (for example, to improve creep life), but should be kept to 0.06 wt. % or less in these alloys to maintain reasonable fabricability. To ensure sufficient resistance to hot cracking during welding, it is preferable that zirconium be limited to 0.03 wt. % or less, more preferably to less than 0.02 wt. %. Hafnium may be present in these alloys as an impurity or intentional addition, but should be kept to 0.5 wt. % or less.
A summary of the tolerance for certain impurities is provided in Table 16. Additional unlisted impurities may also be present and tolerated if they do not degrade the key properties below the defined standards.
For convenience, the experimental and commercial alloys are listed in Table 17 and it is indicated which alloys meet the broad compositional ranges, preferred ranges, and more preferred ranges.
From the information presented in this specification I expect that all of the prophetic alloy compositions set forth in Table 18 would also have the desired properties of the present invention (with the exception of alloys XX. YY, and ZZ which are outside the broad claims of the invention).
It is useful to further consider the three commercial alloys: 282 alloy, Waspaloy alloy, and 233 alloy. In terms of the key properties for turbine cases identified here, all three alloys were found to have acceptable containment and thermal stability. All three alloys also had sufficient weldability/strain-age cracking resistance (although Waspaloy is marginal). However, the 1500° F. (816° C.) mechanical strength of the three alloys was not acceptable. Looking at the compositions of these three alloys provides an understanding of why this occurred. All three commercial alloys have compositions outside of those of the present invention. The 282 alloy composition does not meet the compositional requirements of the present invention mainly due to the gamma-prime forming elements. The alloy does not meet Equation (1) and both the aluminum and titanium are outside of the broad ranges. Additionally, the molybdenum, tungsten, and iron levels are outside of the more preferred ranges. Similarly, the Waspaloy alloy composition does not meet Equation (1) and again both the aluminum and titanium are outside of the broad ranges. Additionally, the cobalt, tungsten, and iron levels are outside the more preferred ranges. The 233 alloy is closer to the alloys of the present invention; however, the cobalt level is well above the broad range for the alloys of the present invention. Additionally, the titanium level is outside the preferred range, while the tungsten, iron, carbon, silicon, and zirconium levels are outside the more preferred ranges.
In addition to the three key properties described above and the secondary property of good weldability/strain-age cracking resistance, other desirable properties for the alloys of this invention could include: high room-temperature tensile strength and ductility in the age-hardened condition, good hot cracking resistance during welding, good oxidation/hot corrosion resistance, good hot forgeability, good cold formability, and others.
Even though the samples tested were limited to wrought sheet, the alloys having a composition within the ranges in Tables 10, 13, and/or 14 should exhibit comparably improved properties in other wrought forms (such as plates, bars, tubes, pipes, forgings, and wires) and in cast, spray-formed, additive manufactured (include the powder to produce such), or powder metallurgy forms, namely, powder, compacted powder and sintered compacted powder. Consequently, the present invention encompasses all forms of the alloy composition. It should be noted that the key property targets defined in this specification were specific to the product forms, processing parameters, and microstructures (e.g. grain size) used to develop the supporting data. These variables were kept as constant as possible between experimental alloys to better facilitate their fair comparison. If these variables are changed (for example, if the product form were hot rolled rings instead of sheet, or if the material was annealed to produce a different grain size) then the absolute values of the key property values may change. However, it is expected that the relative improvements offered by the alloys of this invention would still be valid.
The alloys of this invention have a unique combination of three key properties: high containment, good thermal stability, and excellent mechanical strength, and can be further controlled to provide good resistance to strain-age cracking. These properties make the alloys of this invention useful for gas turbine engine components and particularly useful for turbine cases in these engines. Such components and engines containing these components can be operated at higher temperatures without failure and should have a longer service life than those components and engines currently available. It is recognized that this unique combination of properties may also be attractive for other high temperature applications not described herein.
Although I have disclosed certain preferred embodiments of the alloy, it should be distinctly understood that the present invention is not limited thereto, but may be variously embodied within the scope of the following claims:
The present application claims priority to U.S. Provisional Application No. 63/593,400, filed Oct. 26, 2023. The entirety of the U.S. Provisional Application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63593400 | Oct 2023 | US |
