N. Enzinger, H. Cerjak, TU Graz, Graz, Austria
After the catastrophic failure of the pressurized shaft of the Cleuson-Dixence hydropower plant the main questions “why did it fail?” and “is the material applicable?” had to be answered to enable the application of proper measures for the reconstruction of the plant. Therefore material from the broken shaft, as far as available, was investigated very carefully to determine characteristic failures existing in the original joints to review the quality of the production process in the affected area. By applying a special process[1] for cleaning the oxidized fracture surfaces characteristic fracture features indicating hydrogen induced, delayed cold cracking could be demonstrated which coincide with the results from previous investigations[2]. Because the availability of this original material was limited, quality measurement records from the shaft erection were evaluated to enable the production of equivalent joints representing different steel suppliers and welding processes for further investigations. Based on the additionally generated material further comprehensive investigations started including the determination of mechanical and technological properties in tensile, fracture mechanic and Charpy tests[3] as well as its characterization under very harsh corrosive environmental conditions (H2S saturated water) to simulate a worst case situation. The main result is that the high strength steel S890 and its weld shows some limited behavior compared with lower strength materials. Nevertheless, by strictly obeying some quality requirements the higher strength of this material can be utilized as it also was shown on other areas of the shaft.
[1] M. Neubauer, S. Porner and R. Löhberg. Oxidablösung von Bruchflächen Hoch-legierter Stähle. Sonderbände der Praktischen Metallographie, 195-202, 1982 [2] Institute de Soudure, Paris, Report No. 36088, 25.5.2000
Summary: As a result of the Cleuson-Dixence failure case comprehensive investigations started on original material as well as on generated material to characterise the material and cause of the failure.
