Turbine blade distortion after heat treatment: Preliminary experimental investigation and FEM analysis

During the production process, turbine blades are subjected to a solubilization heat treatment, followed by tempering treatment, in order to obtain better mechanical properties. It is observed that, in some cases, permanent distortion can occur during the high temperature treatment (austenitising temperature). In this work, a high temperature creep resisting steel blade with a simplified geometry is considered. A finite element model is developed considering: the material properties depending on temperature, phase transformation and viscoplasticity (Nabarro-Herring and bilinear kinematic models). A nonlinear transient thermo-mechanical analysis is performed to simulate a standard thermal cycle. Material properties are partially calibrated based on dilatometric tests and partially from data available in literature. Adopting a laser scanner system, the blades geometry is measured before and after the heat treatment to calculate the permanent deflection. Comparing numerical results with experiments, it has been observed that the distortion phenomenon is mainly affected by the low-stress diffusional creep. This effect is due to the fact that, during the heat treatment, the blade is held at high temperature for a relatively long time according to a particular supporting lay-out. To minimize the permanent distortion, the numerical model permits an appropriate supporting system to be set-up, whose validity has been confirmed experimentally.