Numerical and experimental analysis of the residual stress field in cladded components

The inner surface of a ferritic reactor pressure vessel is protected against corrosion by an austenitic cladding that is usually performed by a double-pass welding in order to avoid under-clad defects und to improve the microstructural properties of the cladding material. On the other hand the different thermal expansion coefficients of the cladding and the base metal induce a complex residual stress field. This has a non-negligible influence on the fracture mechanical assessment of postulated flaws within or under the cladding. The determination of the residual stress field was achieved by numerical simulation of the cladding process. The calibration of the used equivalent heat sources for the modelling of the heat input within the simulation was performed using measured data of the temperature field in a KTA compliant cladding process of test plates made of plant-representative materials. The simulation of the welding process used the temperature dependent material characteristics taking into account the transformation behaviour of the ferritic base metal. The resulting residual stress field shows significant tensile stresses within the cladding with a subsequent compressive stress field under the cladding. The calculated residual stress field is in good agreement with the experimental data. A comparison of the calculated residual stress field using the process simulation with the results of a simplified modelling assuming an increased stress-free temperature in the range of the operation temperature shows also a good agreement, esp. using the materials characteristics of KTA