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 dur ing fabrication of assemblies and weld repair of service damaged rotors. Oc currence of a "softening" ("tempered") region in the grain refined heat-aff ected 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 therm al cycles have on the development of this softened region. Work was conduct ed on real weldments and with simulated heat-affected red zones produced wi th the Gleeble thermomechanical simulator and by isothermal furnace heat tr eatments. The thermal cycle at the softening region in the actual weldment was measured and reproduced during simulation; it was estimated that the pe ak temperature at this location was just above the intercritical A(1) tempe rature. Softening occurred before any changes in microstructure could be de tected 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.