Residual stresses in a quenched superalloy turbine disc: Measurements and modeling
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I. INTRODUCTION
THE production of modern gas turbine discs for aerospace applications involves multiple manufacturing steps carried out to rigorous specification. A typical processing route involves casting and refinement of the ingot via hot deformation with open and closed dies, followed by heat treatment and then a series of machining operations. The robust design of critical rotating aeroengine components requires these materials to possess exceptional structural integrity, and tight control is maintained over the material microstructure, defect content, and mechanical properties. To assist with quality control and cost savings, computational simulations of the various processing steps are often utilized to optimize the processing parameters (e.g., References 1 through 4), and significant progress has now been made with the development of integrated models covering several manufacturing stages.[5,6] Validation of such models has concentrated mainly on microstructural changes occurring during each stage of the manufacturing process. On the other hand, proper assessment of deep residual stresses created during postforging heat treatment of superalloys has proven rather more elusive. The difficulty arises because conventional diffraction methods, used for deducing internal stresses from crystallographic lattice strains, have hitherto proved somewhat ineffective—even neutrons have relatively weak penetration in nickel superalloys when compared to steels, for example. The near-surface stress state can be deduced using laboratory X-rays or hole-drilling methods, for instance, but deeper material removal methods and throughthickness ultrasonic probing techniques are prone to high M.A. RIST, Lecturer, and J.A. JAMES, Researcher, are with the Department of Materials Engineering, The Open University, Milton Keynes, MK7 6AA, United Kingdom. S. TIN, Assistant Director of Research, and B.A. RODER, Student, are with the Rolls-Royce University Technology Partnership, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. M.R. DAYMOND, Research Chair, is with the Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, Canada, K7L 3N6. Manuscript submitted April 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
degrees of inaccuracy. Now, using ENGIN-X, the new engineering diffractometer at the ISIS pulsed neutron source in the United Kingdom,[7] internal stresses can be precisely measured to greater depths than was previously possible. This has enabled through-thickness probing of a full-scale, nickel superalloy compressor disc to be carried out. Accurate quantification of the residual stress field in the heat-treated and quenched disc crucially allows elastic relaxation to be accounted for during subsequent machining operations, which can then be carried out to maintain optimum dimensional tolerances. Although a significant number of experimental and numerical studies have been undertaken to examine heat-treatment residual stresses in stee
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