Damage behaviour of Nickel-based transition welds under cyclic thermomechanical loading

The present study was performed on dissimilar nickel-based transition girth welds on superheater tubes. The results of a testing programme including tube tests under cyclic pressure, axial load and thermal loading with dwell times at 625 degrees C will be presented. The pressure and the axial load were varied between 290 and 480 bars, 0 and 42 kN respectively. For testing times below 8.000 hours, failure occurred in the X10CrMoNbV9-1 tube, however not at the fusion line to the NiCr20Nb weld metal. However, for the longest testing time of 20.000 hours high resolution scanning electron microscope investigations showed significant microstructural changes at the fusion line. Single creep cavities were observed in the interfacial region of the martensitic steel when a crack had propagated there from an oxide notch at the tube surface.