Integrated modeling for the manufacture of Ni-based superalloy discs from solidification to final heat treatment
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I. INTRODUCTION
INCREASINGLY stringent targets for fuel efficiency, engine performance, and emission control in modern gas turbines require structural components to operate under extreme conditions of temperature, atmosphere, and stress. This has led to the use of robust Ni-based superalloy discs as the critical rotating components. These discs need to have exceptional integrity, especially for aerospace applications, where safety is paramount. The quality of a gas-turbine disc is assessed in terms of critical features, such as its grain structure, defect content, mechanical properties, residual stress state, dimensional tolerance, and surface condition.[1] Each processing stage can influence these features and requires tight process control to ensure conformity to the engineering design requirements, significantly impacting the cost of a disc. This article demonstrates the ability of an integrated, through-process model to simulate the characteristic changes S. TIN, Assistant Director of Research, is with the Rolls-Royce University Technology Partnership, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Contact e-mail: [email protected] P.D. LEE, Reader, A. KERMANPUR, Associate Professor, and M. McLEAN, Professor, are with the Department of Materials, Imperial College London, London SW7 2BP, United Kingdom. M. RIST, Lecturer, is with the Department of Materials Engineering, Open University, Milton Keynes MK7 6AA, United Kingdom. Manuscript submitted November 18, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
in grain size/morphology associated with the commercial production of Ni-based superalloy discs. With the hightemperature properties of cast/wrought superalloys being highly sensitive to the resultant microstructures,[2] the ability to precisely engineer the grain structure enables further optimization of the structural component. For example, coarse-grained microstructures can be utilized to minimize creep deformation at the gas-turbine disc rim, while refinement of the grains at the bore of the finished disc increases the resistance to fatigue-crack growth/initiation and also provides a significant degree of Hall–Petch strengthening. In addition to the influence on the in-service mechanical performance, control of grain size also affects the thermalmechanical response during hot working, which can reduce operational costs and improve component integrity. Typical manufacturing routes for gas-turbine discs consist of seven successive stages.[3] First, vacuum induction melting (VIM) is used to produce an alloyed electrode ingot with tightly controlled composition and low impurity levels. With the resulting properties of these structural materials being highly sensitive to trace elements (such as O, N, S, and C) melting in vacuum assists in minimizing contamination of cast ingots, which often weigh in excess of 5000 kg. To minimize melt-related defects and yield a sound ingot with a microstructure that is amenable to subsequent thermomechanical processing, the ingot produced by VIM is remelted
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