Study of dual grain microstructure heat treatment in steel specimen representing gas turbine rotor disc

Abstract

Gas turbine discs which support the rotor blades operate at relatively high speeds and high rim temperatures and are therefore subjected high creep damage at the rim area and to very high low-cycle fatigue damage at the bore. Any heat treatment which results in homogenous grain structure and one small band of grain size will limit the life of the disc either by creep at the rim or by the fatigue at the bore. If the heat treatment parameters are optimized to increase the grain size at the rim area closer to the ASTM 6.5 and to finer grain size at the bore area closer to ASTM 11.5, the heat treated disc is likely to have much superior operational life as compared to that of heat treated which results in homogenous grain structure and size throughout the disc. There is therefore the need for exploring possibilities of obtaining variable grain structure during heat treatment operation.This study is aimed at evaluating two possible heat treatment processes that are likely to result in distinctly different grain size in a single test specimen such as a circular rod. These test rods represent rotating gas turbine discs and the objective of achieving differential grain size is to optimize and balance the creep life at one half portion of the test rod (representing the disc rim or Outer Diameter) and fatigue life at another half portion of the test rod(representing the bore or Inner Diameter).