Characterisation of cracks in high strength steel weldments

After the big accident in the Cleuson-Dixence hydropower plant in December 2000 several investigations were performed to clarify questions concerning properties and behaviour of high strength steel S890 and its weldments. This contribution summarises the efforts undertaken to answer the question which mechanism caused the catastrophic failure that was originated in the weld deposit of a longitudinal weldment. Original material was taken from the shaft near the zone of the accident and was investigated. By extensive non-destructive testing four plates were decided to be removed by flame cutting and then retested in the laboratory. From these results some specimens were taken for further investigations using light microscope as well as scanning electron microscope. It was found that the investigated cracks were caused by the hydrogen induced cold cracking mechanism after welding, which is also called delayed cracking. By applying a proper pre and post weld heat treatment these cracks can be avoided, and by a diligent use of non-destructive testing methods the required quality of the welding can be assured.