Abstract
In this study, the mechanical properties of last stage 680 mm blades from a steam turbine with actual service lives of 10 years (70 000 h) and 20 years (120 000 h) have been systematically analysed by comparing them with a virgin blade. Experiment results show that the effect of service damage on the static tensile properties was weak, but the effect on fatigue fracture properties was remarkably strong. The fatigue limit and impact ductility had decreased by a little for the blade with a 10 year service life and by more for the one with a 20 year service life. The rate of fatigue crack propagation had increased a little for the blade with a 10 year service life and a lot for the one with a 20 year sevice life. The degradation of the fatigue and fracture behaviour of a blade is not linear. It rapidly gets worse at the later stage of service life. Observations of tensile and impact fracture in the blades by SEM show that the bound strength of the interface between fine carbides and the matrix weakened during long term service. Therefore, it has been proposed that the microstructure was weakened before the geometrical damage of the last stage.
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