Investigation on crack propagation behaviour in welded waterwall tubes

Fracture mechanics based safety assessment of welded waterwall tubes is performed in order to quantify the propagation behaviour of existing or postulated defects in welds. The tube bundles are subjected to internal pressure, dead weight and welding residual stresses (WRS). The residual stresses were determined by the numerical simulation of the welding process and verified by X-ray measurements. At operation temperature above 500 °C, the lifetime of the tubes is mainly controlled by creep. Therefore, the relevant material properties of the base and weld metal, especially the relaxation behaviour and creep crack growth rates, were experimentally determined providing a basis for the numerical stress simulation and assessment of the propagation behaviour of postulated flaws at highly loaded locations. The experimental characterization of the weld metal included two types of welding employed in the manufacture process - sub-merged arc and tungsten inert gas (TIG) welding, representative of the longitudinal and circumferential welds, respectively. For the both weld types, high welding residual stresses with a magnitude about the yield stress of the base metal were determined, so that failure tolerance assessment was required to prove the component integrity during the design lifetime. The analyses demonstrated that due to superimposed residual and primary stresses, as well as due to high creep rates achieved at the operation temperature, considerable stress relaxation occurs. In particular, a fast relief of the welding residual stresses is predicted at the initial stage of operation, whereas these almost vanish at the end of the design lifetime. As a result, limited creep crack growth was estimated for all crack postulates in both weld types with a tendency of crack retardation or even arrest. For the utmost unfavourable double-sided crack postulate in the longitudinal weld, the crack is shown to be able propagating through the weld with a subsequent arrest after penetrating the base metal which exhibits considerably better creep properties than those of the weld metal. In all cases considered, no crack propagation through the tube wall is achieved.