Full Scale Three-Dimensional Moving Arc Weld Analyses of Steam Generator Vessel With Effect of Post Weld Heat Treatment

Abstract Computational weld analysis was performed for a steam generator design using the Virtual Fabrication Technology (VFT™) computer code. The weld residual stress (WRS) field that develops from the welding process using a prescribed weld sequence is provided for the three large welds considered (divider plate top and bottom and the bowl closure weld). Hoop (stresses in the direction of the weld) and in plane weld residual stresses are above yield for portions of the weld and heat affected zone because the weld process causes material hardening and because of the structural constraint in the steam generator geometry. The WRS developed in the divider plate welds are decreased following application of the bowl weld which is selected to be the final weld and results in the optimum weld sequence. An axisymmetric analysis was also performed of the bowl closure weld and the WRS field compared well with the full 3D solution for this weld for locations away from the weld start/stop locations. The effect of cladding on the bowl was also considered and this increases the WRS field in the clad region compared to no cladding. Some of the findings from this study include the following. • The WRS component field that develops in the steam generator are higher than yield within portions of the weld and heat affected zone. This is due to material hardening and structural constraint. • The weld sequence of completing the bowl weld last results in a reduced WRS field in the divider plate welds and it thus considered the optimum sequence to use. • The effect of hydrotest pressure of 24.8 MPa has a rather small effect on reducing the WRS field. Higher pressure would be required to reduce the WRS field more. • An axisymmetric solution of the bowl closure weld provides a reasonable prediction of the WRS field in this weld compared to full 3D solution. • The cladding applied to the bowl weld region increases the local hoop and axial stress in the bowl weld. However, the OD hoop stress is reduced and the axial stress is reduced through the thickness. The weld analysis of the steam generator system shows that the final residual stress field, after cool down, can be large may be of significance in a fitness for service assessment. It is found that performing a total PWHT, where the entire subassembly can be immersed in a large oven, reduces the WRS field significantly. By contrast, performing a local PWHT (LPWHT) of the closure weld region can result in high residual stresses at room temperature and at operation temperature if not designed properly. Caution should be taken when a large, complex structure is subjected to LPWHT.