Properties for design consideration

Abstract

The criteria adopted in the design of high-temperature discs and blading vary according to industry, duty, and inspection requirements. These criteria are therefore discussed in some detail since they dictate the properties required. Current and potential disc and blading superalloys available to aero, marine, and industrial turbine makers are reviewed, together with their properties as supplied by the forgemasters in their promotional literature and quality acceptance test certification. The data are compared, where possible, with the properties more typically used by design engineers, these tending to be the results of component cutup tests performed under conditions approaching more closely those experienced during operation. The range of operating modes catered for by the turbine makers, and their individual levels of expertise, are notable. The scope is reflected by the variety of properties known to exist, even though for commercial reasons the details are not always freely available. Such data range from ultimate tensile strength, and plain specimen fatigue and creep resistances, to plane strain critical stress intensity factor and static and/or cyclic subcritical crack growth rates. The published data and their practical application are examined. The materials covered include the nickel-iron alloys (e.g. Nimonic 901, Inconel 718) and the traditional nickelbase alloys (e.g. Nimonic 80A, Nimonic 90, Waspaloy). Reference is also made to the more advanced 'atomized powder' disc and 'mechanically alloyed' blading alloys. The choice of alloy does not always depend entirely on properties. Factors such as economics and forgeability (often size dependent) are also important considerations.