In 2013, cracks was found in the radius of the dove tail slots of the rotor rim sheets in generator 1 of Juktans hydro power plant in Västerbotten, Sweden. The cracks were estimated to be too deep to be able to repair and Alstom made an investigation on the fracture cause. The investigation came to the conclusion that the radius was too small and that the new rotor rim sheets should have a six times greater fillet radius. However, it was not been investigated whether the material structure or the manufacturing process may have an impact on the crack initiation and following propagation which is the focus of this report.
Parts of the dove tail slots was cut out and characterized with XRF, SEM and LOM. Further mechanical characterizations was done according to Vickers.
From the SEM and LOM micrographs a high amount of big (10µm) and cubic particles was found in the microstructure. The micrographs also showed a deformation of the microstructure and the hardness test showed a deformation hardening near the edge where the sheet had been punched. The edge surface of the sheet also had notches.
The big and hard particles in the microstructure affect the mechanical properties of the steel to the worse. Furthermore, the hardening effect combined with the notches the will make a good crack initiation point. Therefore, there is a higher possibility that a crack will initiate in the radius of the dove tale slots where there are large stresses occur.