Fluid structure interaction modeling of expansion-contraction deformation during welding in a spacer-band-blade assembly of a HP steam turbine diaphragm

An important stage in the process of manufacturing diaphragms for steam turbines is the application of the seal welding in bands and blades. The geometry of the assembly, the weld constraint and thermal deformation lead to a complex distribution of residual stresses, which reduce the service life of the component. In this research work the expansion-contraction phenomenon was studied, as well as the residual stress distribution in the seal welding of a 12Cr-1Mo steel band-blade assembly. The One-Way Fluid Structure Interaction approach was considered in the numerical solution of the mathematical model through Finite Volume (FV) and Finite Element (FE) methods. Microstructural analysis and experimental tests were carried out to measure the thermal history and postwelding mechanical properties. The results indicated that the complex distribution of the transversal residual stresses generated by temperature gradients in the assembly are responsible for the expansion-contraction phenomenon occurring during the welding process. The volumetric expansion of the 12Cr-1Mo steel by phase change was a localized phenomenon. The microstructural study related to the FE analysis of the residual stresses suggest the possibility of dismissing a post-weld heat treatment during the manufacture of these components.