An Estimation of Sufficient Impact Toughness for the Material of a Turbine Shaft

The fracture of a turbine shaft in the case of overloading can exhibit brittle or plastic failure, depending on the material properties, the turbine-shaft geometry and the form of loading. Usually, when the toughness increases, the stiffness of the shaft material is reduced, which can lead to the plastic twist of the shaft. If the fatigue crack appears in the critical region of the shaft then the low impact-toughness value may induce a brittle fracture. During the retrofit of a hydro-power plant a new turbine shaft was produced by quenching-and-tempering technology. Charpy impact-toughness tests showed lower values for the shaft material than those prescribed by the project documentation. Since the turbine shaft for a hydro-power plant is a massive and expensive component, it is necessary to determine a sufficient impact toughness for the material in terms of the geometry and the manner of loading for the turbine shaft. Since only the yield strength and the impact toughness of the material were prescribed, the level 0 of the SINTAP should be applied. The minimum impact toughness values that ensure the ductile fracture of the shaft, cracked circumferentially was also estimated. We also analyzed the variation of the shaft’s carrying capacity resulting from a reduction of the non-cracked ligament in the transversal direction.