Two dissimilar weldments between 9Cr-1Mo and 2.25Cr-1Mo ferritic steel
s have been characterized for their microstructural stability during v
arious postweld heat treatments (PWHTs). The samples for the investiga
tion were extracted from bead-on-plate weldments made by depositing 2.
25Cr-1Mo weld metal on 9Cr-1Mo base plate and vice versa. Subsequent a
pplication of PWHT resulted in the formation of a soft zone in the low
Cr ferritic steel weld or base plate. A carbide-rich hard zone, adjoi
ning the soft zone, was also detected in the high Cr side of the weldm
ent. Unmixed zones in the weld metal provided additional soft and hard
zones in the weld metals. The migration of carbon from low-Cr steel t
o high-Cr steel, driven by the carbon activity gradient, has been show
n to be responsible for the formation of soft and hard zones. A carbon
activity diagram for 2.25Cr-1Mo/9Cr-1Mo weldments has been proposed t
o aid in the selection of welding consumables for reducing or preventi
ng the soft zone formation.