Fractured Turbine Blades

In an electric power station, seven turbine blades out of 112 broke or cracked within 8 to 14 months after commencement of operation. The blades in question were all located on the last running wheel in the low pressure section of a 35,000 kW high pressure condensing turbine. They were milled blades without binding wires and cover band. They did not fracture at the fastening, i.e. the location of highest bending stress, but in a central region which was 165 to 225 mm away from the gripped end. The blades were fabricated from a stainless heat-treatable chromium steel containing 0.2C and 13.9Cr. Microstructural examination showed the blades were destroyed by flexural vibrations which evidently reached their maximum amplitude at the location of fracture. Erosion of the inlet edge, possibly in connection with vibration-induced corrosion cracking, contributed to fracture.