As operating temperatures of gas turbine engines increase, there is a need for superalloy turbine disks that can operate with rim temperatures in excess of 1300 °F. To meet this need, a new generation of nickel-base superalloys, such as ME3, Alloy 10, and LSHR, has been developed. These alloys all contain a high percentage of gamma-prime precipitates, Ni3Al, and refractory element additions to achieve strength at temperature. Although they provide advantages over older alloys, the continuing need for high tensile strength at intermediate temperatures in the bore of a disk, which runs much cooler than the rim, as well as high creep strength in the rim also demands innovative heat treatments that can optimize bore and rim properties.
Traditional heat treatments produce fine-grain disks when the solution temperature is maintained below the gamma-prime solvus and produce coarse-grain disks when the solution temperature is maintained above the gamma-prime solvus. Fine-grain disks yield high strength at intermediate temperatures, whereas coarse-grain disks yield high creep strength at elevated temperatures. Recently, several advanced heat-treatment technologies have been developed that can produce a superalloy disk with a fine-grain bore and a coarse-grain rim. Tradeoff studies by GE Aircraft Engines and Allison Advanced Development Company (AADC) have identified the potential advantages offered by superalloy disks that employ a dual grain structure for advanced gas turbine engine applications. The GE Aircraft Engines report cited reduced fatigue for a disk with a dual grain structure in comparison to a coarse-grain disk, whereas the AADC report cited a creep benefit for a disk with a dual grain structure in comparison to a fine-grain disk.
Top: Etched sections of DMHT forgings showing dual grain structure. Bottom: Hardware and furnace used to produce DMHT disks.
One of the advanced heat-treatment technologies for producing superalloy disks with a dual grain structure was developed by the NASA Glenn Research Center to minimize cost and production problems. This process, known as dual microstructure heat treatment, or DMHT, has been demonstrated for a variety of disk shapes and sizes at the Ladish Company and Pratt & Whitney’s Forging Part Center. The basic concept behind the DMHT process utilizes the thermal gradient between the interior and exterior of the forging during the initial phase of a conventional heat treatment to develop a dual grain structure. By enhancing the thermal gradient with insulated heat sinks placed on the top and bottom surface of the forging, researchers can obtain the desired dual grain structure by using a conventional gas-fired furnace maintained at a temperature above the gamma prime solvus of the alloy. The forging and heat sink package is placed in the hot furnace; it is removed when the outer periphery of the forging has exceeded the solvus but before the center of the forging reaches the solvus, thereby producing the desired dual grain structure. The effect of DMHT processing on tensile and creep properties has been demonstrated using test coupons machined from disk forgings as well as spin testing of DMHT disks. These tests have confirmed the strength benefits of a fine-grain bore and the enhanced creep resistance of a coarse-grain rim.
Top: DMHT disk used in cyclic spin testing to verify the fatigue durability of the grain-size transition zone. Bottom: Experimental and predicted fatigue lives for DMHT disks.
Long description of figure 2.
More recently, the fatigue performance of DMHT disks was studied using test coupons machined from disk forgings and cyclic spin testing of DMHT disks. The coupon testing confirmed the enhanced fatigue resistance associated with a fine-grain bore. Cyclic spin testing of DMHT disks was performed with two goals in mind. First, the grain-size transition zone was intentionally loaded to high stress levels by employing web holes in the disk that bisected the grain-size transition zone. Second, the fatigue lives of the DMHT disks were estimated using advanced elastic-plastic finite-element techniques. The results of these tests and the analyses showed that the grain-size transition zone of the DMHT disk could withstand significant loads that exceeded the predicted fatigue lives. The next step in the development of this technology calls for demonstrating a DMHT disk in a ground-based engine to assess safety and reliability under realistic operating conditions.
John Gayda, 216-433-3273, John.Gaydaemail@example.com
Authors: Dr. John Gayda, Dr. Timothy P. Gabb, and Pete T. Kantzos
Headquarters program office: Aeronautics Research
Programs/Projects: Aviation Safety
Last updated: October 16, 2006
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