HEAT TREATMENT FOR DUAL ALLOY TURBINE WHEELS

Abstract: A dual alloy gas turbine rotor is heat treated by HIP bonding a cast superalloy blade ring to a consolidated. powdered-metal hub. After bonding, the assembly is solution treated and aged at certain specific temperatures so as to optimize the mechanical properties of the dual alloy assembly for use in a high performance gas turbine engine.

Description: TECHNICAL FIELD This invention relates generally to the metallurgical arts and more specifically to a method of heat-treating certain components made from two different nickel-base superalloys. BACKGROUND OF THE INVENTION Radial turbine rotors or wheels in gas turbine engines are subjected to very high temperatures, severe thermal gradients, and very high centrifugal forces. The turbine blades are located directly in and are directly exposed to the hot gas-stream. The inducer tips of the blades therefore experience the highest temperatures and consequently are most susceptible to creep rupture failure that could result in an inducer tip striking the surrounding nozzle enclosure, causing destruction of the turbine. The turbine hub is subjected to very high radial tensile forces and also has a life limit imposed by low-cycle-fatigue crack initiation and growth. In order to achieve optimum blade and hub material properties, dual alloy structures have been developed in which the hub portion is formed of wrought superalloy material having high tensile strength and high low-cycle fatigue strength, while the blade ring portion, including the blades (i.e., airfoils) and blade rim, is formed of a cast superalloy material having high creep rupture strength at very high temperatures. The dual alloy approach has been used where very high performance turbine rotors are required because those materials that have optimum properties for the turbine blades do not have sufficiently high tensile strength and sufficiently high low-cycle fatigue strength for use in the turbine hubs. U.S. Patent No. 4,581,300 issued April 8, 1986 to Hoppin et al and U.S. Patent No. 4,659,288 issued April 21, 1987 to Clark et al, both assigned to the assignee of the present invention, disclose prior art methods for manufacturing a turbine rotor from two portions each having different superalloy composition. One problem in manufacturing such dual alloy components is in selecting the proper heat treating cycle to optimize the mechanical properties of both superalloy components. Obviously, selecting the thermal treatment employed to maximize strength of one of the alloys would not be expected to be optimum for a component containing a second alloy. Further, it would be apparent to those skilled in this art that merely "splitting the difference" between the temperatures and times of the two alloys' usual thermal treatment would be even less satisfactory and may even be totally useless (i.e., both components may have poor mechanical properties). The aforementioned U.S. Patent No. 4,659,288 teaches one method to heat treat a dual alloy turbine wheel in column 6, lines 5 to 35. However, the procedure is lengthy (about 36 to 48 hours) and complex (5 furnace cycles). In view of the foregoing, it should be apparent that there is an unmet need in the art for improvements in the heat treating of dual alloy components for use as turbine rotors in high performance gas turbine engines. It is therefore an object of the present invention to provide a novel method for improving the mechanical properties of certain dual alloy components. It is a further object of the present invention to provide a new and improved method of heat treating allow turbine rotors for use in high performance gas turbine engines. SUMMARY OF THE INVENTION The present invention aims to overcome the disadvantages of the prior art as well as offer certain other advantages by providing a faster and simpler method of heat treating dual allow turbine rotors of the type having a MAR M-247 cast superalloy blade ring and a powder metal ASTROLOY superalloy hub. According to a first aspect of the present invention there is provided a method of heat-treating a dual allow component of the type having a first portion made from a first nickel base superalloy nominally containing 15% Cr, 17% Co, 5.3% Mo, 4% Al and 3.5% Ti and a second portion made from a second nickel base superalloy nominally containing 8.2% Cr, 10% Co, 0.6% Mo, 10% W, 3% Ta, 5.5% Al and 1% Ti, comprising the steps of: heating the component at 1115°C (2040°F) for two hours, rapidly air cooling the component to room temperature, reheating the component to 870°C (1600°F) for 16 hours, allowing the component to cool, reheating the component to 760°C (1400°F) for 16 hours, and allowing the component to cool. According to a second aspect of the present invention there is provided a method of manufacturing a dual alloy turbine rotor for a high performance gas turbine engine, comprising the steps of: providing a hub portion made from a first nickel base superalloy nominally containing 15% Cr 17% Co, 5.3% Mo, 4% Al and 3.5% Ti; providing a blade portion made from a second nickel base superalloy nominally containing 8.2% Cr, 10% Co, 0.6% Mo, 10% W, 3% Ta, 5 5% Al and 1% Ti; bonding said hub portion to said blade portion by hot isostatic pressure; solution treating the bonded portions at 1115°C (2040°F) for 2 hours; reheating the bonded portions to 870°C (1600°F) for 16 hours; and again reheating the bonded portions to 760°C (1400°F) for another 16 hours. This new heat treatment produces superior stress-rupture life in the blade ring and good strength and ductility in the hub as compared to prior art processing methods. BRIEF DESCRIPTION OF THE DRAWINGS While this specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the objects, features, and advantages thereof may be better understood from the following detailed description of a presently preferred embodiment when taken in connection with the accompanying drawings in which: FIG. 1 is a perspective illustration of a dual alloy turbine wheel assembly after bonding; FIG. 2 is an illustration of the inner hub portion of the turbine wheel before bonding; and FIG. 3 is an illustration of the outer blade ring portion of the turbine wheel. BEST MODE FOR CARRYING OUT THE INVENTION A radial flow turbine wheel (1) shown in FIG. 1 before final machining, includes a central hub portion (2) and an outer blade ring portion (3). The generally conical blade ring (3) includes a plurality of thin, curved blades or airfoils (5) each having an inducer tip (6), extending radially from the largest diameter portion of the wheel, and an exducer tip (7) extending outwardly from the smaller diameter portion of the wheel. In use, hot gases impinge on the inducer tips (6), flow down the blade surfaces (5) causing the wheel to rotate, and leave the wheel in a generally axial direction past the exducer tips (7). In a dual alloy wheel, the hub (2), best seen in FIG. 2, is formed from a superalloy material having high tensile strength and good low-cycle fatigue strength in order to withstand the high centrifugal and thermal stresses during operation and imposed by prolonged cyclic operation. A preferred superalloy material is consolidated, low carbon, ASTROLOY powder having a nominal composition of about: 15% Cr, 17% co, 5.3% Mo, 4% Al, 3.5% Ti, 0.03% C, 0.2% B and the balance nickel plus impurities. Preferably, this allow is consolidated by hot isostatic pressing (HIP) the powder to near final shape at about 2230°F under 10,3421.4 kPa (15,000 psi) pressure for about 4 hours followed by slow furnace cooling. Usually, unitary components made from this alloy would be heat treated by: solutionizing at 2040°F (1115°C) for 2 hours and rapid air cooling, stabilization at 1600°F (870°C) for 8 hours with air cooling, and again at 1800°F (980°C) for 4 hours, followed by precipitation hardening at 1200°F (650°C) for 24 hours with air cooling, and again at 1400°F (760°C) for another 8 hours. This is the so-called "yo-yo" heat treatment originally developed for forged components made of the higher carbon version of this alloy. The blade ring portion (3) of a dual alloy wheel, as shown in Figure 3, is formed from a different superalloy material having good high-temperature creep strength and resistance to thermal fatigue. A preferred material is a fine grain casting of MAR M-247 which has a nominal composition of about: 8.2% Cr, 10% Co, 0.6% Mo, 10% W, 3% Ta, 5.5% Al, 1% Ti, 0.16% C, 0.02% B, 0.09 % Zr, 1.5% Hf and the balance nickel plus impurities. Typically, this casting is consolidated by HIPing at about 2165°F (1185°C) under about 17,2369kPa (25,000 psi) pressure fora bout 4 hours followed by slow furnace cooling. Usually, cast components made entirely from this alloy have been heat treated by solutionizing at 2165°F (1185°C) for 2 hours and rapid air cooling followed by aging at 1600°F (870°C) for about 20 hours and air cooling to room temperature. However, to manufacture a dual alloy wheel (1), the hub (2) must be bonded to the blade ring (3) before the final heat treatment of the assembly. Typically, the outer surface (4) of the hub (2) and the inner surface (8) of the blade ring (3) are both machined to provide a clean, smooth, close-fitting bonding surface. The two portions are assembled and diffusion bonded under pressure for several hours at about 2000° to 2300°F (1090° to 1260°C). The unitary bonded assembly is then ready for a final heat treatment to fully develop the desired mechanical properties in each portion of the wheel. It should be apparent that the previously used heat treating cycles for each of the two materials are so significantly dissimilar from one another that neither cycle would be expected to maximize mechanical properties in the other alloy. Several tests were performed to substantiate, and determine the severity of, this perceived incompatability. Individual test components of the two superalloy compositions were procured in the HIP - consolidated condition and subjected to a simulated thermal bonding cycle of 2225°F (1218°C) for 4 hours in preparation for the series of tests set out below. EXAMPLE I To provide a basis for comparison, several ASTROLOY components were heat treated according to the usual temperature and times set forth above (i.e. the "yo-yo" heat treatment). Those foregoing processing steps produced ASTROLOY components having an average yield strength of 8.76 x 10³ kg/cm² (124,700 psi) and an ultimate tensile strength of 1.31 x 10⁴ kg/cm² (186,200 psi). Creep-rupture testing of similar components at 1300°F (700°C) under a 7.03 x 10³ kg/cm² (100,000 psi) load, gave a time to failure of 163.6 hours and an elongation of 26.6 percent. Likewise, MAR M-247 components were heat-treated according to the usual cycle for such castings as set forth above. Such a heat treating cycle produced MAR M-247 components having an average yield strength of 8.30 x 10³ kg/cm² (118,100 psi) and an ultimate tensile strength of 1.01 x 10⁴ kg/cm² (144,000 psi). Creep-rupture testing of the components, at 1500°F (815°C) under 5.27 x 10³ kg/cm² (75,000 psi) load, gave a time to failure of 46.6 hours and an elongation of about 1.5 to 1.7 percent. EXAMPLE II In order to determine the detrimental effects of heat treating both components of a dual alloy wheel by either one of the previously recommended processes, ASTROLOY components where heat treated according to the recommended MAR M-247 cycle and MAR M-247 components were treated according to the usual cycle for ASTROLOY. Testing of these components indicated that their yield and tensile strengths were not significantly reduced and the creep-rupture properties were even improved somewhat. These ASTROLOY components averaged 8.3 x 10³ kg/cm² (118,000 psi) yield strength (down 5-1/2%), 1.3 x 10⁴ kg/cm² (186,800 psi) tensile strength (same as Example I), 191.6 hours to rupture (up 17%) and 27.9% creep elongation (up 5%). The MAR M-247 castings averaged (122,000 psi) yield strength (up 3-1/2%), 1.03 x 10⁴ kg/cm² (147,000 psi) tensile strength (up 2-1/2%), 110.3 hours to rupture and 2.9% creep elongation (both about doubled from Example I). While these test results were better than expected, a close examination of the creep test curves indicated that both heat treatments (Examples I and II) of the MAR M-247 castings caused the specimens to fail during second-stage creep; i.e., prematurely. Further testing was undertaken to try to overcome this defect and to find a single heat treating cycle which produced improved properties in both components of a dual alloy turbine wheel. EXAMPLE III Test components of both allows were solutionized at 2040°F (1115°C) for 2 hours and rapidly air cooled to room temperature. They were then treated at 1600°F (870°C) for 16 hours and allowed to air cool. A final treatment at 1400°F (760°C) for 16 hours, followed by air cooling, prepared the components for testing. The data below indicates that their yield and tensile strengths were not significantly different rom the baseline data of Example I but the creep-rupture strength of the MAR M-247 alloy was greatly improved. More importantly, examination of the creep test curves showed that this improved heat treating cycle allowed the MAR M-247 test components to proceed to third stage creep and fail "normally". This improvement was quite unexpected and the exact reasons for such improvements has not yet been exactly determined. The tests of the Astroloy components showed: 8.5 x 10³ kg/cm² (121,300 psi) yield strength (down 3%); 1.32 x 10⁴ kg/cm² (187,500 psi) tensile strength (same), 158.9 hours to rupture (down 3%) and 30.5% creep elongation (up 15%). The MAR M-247 castings averaged 8.5 x 10³ kg/cm² (121,600 psi) yield strength (up 3%), 10.4 x 10⁴ kg/cm² (147,400 psi) tensile strength (up 2-1/2%), 227.7 hours to rupture and 7.4% creep elongation (both increased about 4-1/2 times over Example I). The foregoing heat treating procedure produces a dual allow turbine rotor assembly suitable for final machining, having extremely high material strengths optimized in both the hub and blade portions at relatively lower costs than the prior art methods.

Claims: A method of heat-treating a dual alloy component of the type having a first portion made from a first nickel base superalloy nominally containing 15% Cr, 17% Co, 5.3% Mo, 4% Al and 3.5% Ti and a second portion made from a second nickel base superalloy nominally containing 8.2% Cr, 10% Co, 0.6% Mo, 10% W, 3% Ta, 5.5% Al and 1% Ti, comprising the steps of: heating the component at 1115°C (2040°F) for two hours, rapidly air cooling the component to room temperature, reheating the component to 870°C (1600°F) for 16 hours, allowing the component to cool, reheating the component to 760°C (1400°F) for 16 hours, and allowing the component to cool. The method of Claim 1 further including the preliminary step of bonding said first portion to said second portion by hot isostatic pressing the two portions toegether at 1218°C (2225°F) under 100 MPa (15,000 psi) pressure for four hours. The method of Claim 2 wherein said first portion is consolidated from powders of said first superalloy prior to bonding. The method of Claim 2 wherein said second portion is cast from said second superalloy prior to bonding. A method of manufacturing a dual alloy turbine rotor for a high performance gas turbine engine, comprising the steps of: providing a hub portion made from a first nickel base superalloy nominally containing 15% Cr 17% Co, 5.3% Mo, 4% Al and 3.5% Ti; providing a blade portion made from a second nickel base superalloy nominally containing 8.2% Cr, 10% Co, 0.6% Mo, 10% W, 3% Ta, 5.5% Al and 1% Ti; bonding said hub portion to said blade portion by hot isostatic pressure; solution treating the bonded portions at 1115°C (2040°F) for 2 hours; reheating the bonded portions to 870°C (1600°F) for 16 hours, and again reheating the bonded portions to 760°C (1400°F) for another 16 hours. The method of Claim 5 wherein said bonding step includes heating the two portions to 1220°C (2230°F) for 4 hours under sufficient pressure and time to bond said hub portion to said blade portion.

Inventors: HOPPIN III, GEORGE S.
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1989
Main classification: C22F1
IPC codes: C22C19, F01D5, C22F1