Metallurgical Assessment of the Softened HAZ Region During Multipass Welding

CrMoV steels are used in high temperature and high stress sections of power plant members; their good creep resistance is impaired by welding done during fabrication of assemblies and weld repair of service damaged rotors. Occurrence of a “softening” (“tempered”) region in the grain refined heat-affected zone/intercritical heat-affected zone, has become the limiting factor in the life extension of weld repaired high pressure/intermediate pressure steam turbine rotors. This study focuses on the effect that multiple thermal cycles have on the development of this softened region. Work was conducted on real weldments and with simulated heat-affected zones produced with the Gleeble thermomechanical simulator and by isothermal furnace heat treatments. The thermal cycle at the softening region in the actual weldment was measured and reproduced during simulation; it was estimated that the peak temperature at this location was just above the intercritical $A1$ temperature. Softening occurred before any changes in microstructure could be detected with the light microscope. Carbide coarsening, shown by limited TEM analysis, and the likely dissolution of some of the carbides, most probable, contributed to reduce the microhardness values. [S1087-1357(00)70202-4]