Structural investigations of candidate materials for turbine disc applications beyond 700 °C

Abstract

Three Ni-base wrought alloys with different hardening mechanisms (Inconel 706, Waspaloy and Inconel 617) are candidates for steam turbine disc applications with temperatures up to 700 °C and were examined in respect to their microstructure and microstructural stability. The materials were investigated after different heat treatments and after short and long term ageing by metallography, scanning and transmission electron microscopy. Special DTA experiments has been performed to confirm the known TTT diagrams and to study the initial precipitation of different phases and their growth, as function of cooling rate after the heat treatment. The amount and size of precipitations were evaluated by image analysis and stereological methods. The Nb containing alloy Inconel 706 shows a complex microstructure containing γ′, γ″ and η phases which are stable under long term service up to 620 °C. At higher temperatures a strong particle coarsening and phase transformation was observed. Waspaloy is hardened by γ′ particles with a bimodal size distribution. After ageing at 700 °C and higher a coarsening was observed by loss of the bimodal size distribution. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M23C6 carbides. After long term ageing at temperatures of 650 °C to 750 °C the carbides tended to form carbide films along the grain boundaries and at 700 °C to 750 °C γ′ precipitated to homogeneously distributed particles with low coarsening under long term service.