There are occasions where it is prudent to under match the weld. I will often intentionally under match the weld when the repair involves a highly restrained joint when low carbon or high strength low alloy steels.
When the first weld bead is deposited, it will shrink as it solidifies and further contract as it cools to the preheat temperature (ambient if no preheat is utilized). When the joint is highly restrained either because of the mass of the base metal or the configuration of the joint (think of a circular patch in the wall of a vessel), something has to move as the weld shrinks and further contracts. If the weld is as strong as the base metal or possibly stronger than the adjacent base metal, the movement is accommodated initially by the hot weld metal. However, as the weld cools and recovers tensile strength, the adjacent heat affected zone is forced to accommodate the forces resulting from the contraction of the weld and surrounding base metal as the assembly cools. In many instances, the properties of the HAZ are over whelmed by the stresses introduced by the contraction of the much stronger weld and a crack develops in the HAZ.
It can be an advantage to use an under matched filler metal when the joint is highly restrained so the residual stressed resulting from the contraction that occurs during cooling is accommodated by the weld rather than the adjacent HAZ. The weld often has a geometry that permits plastic deformation to occur without cracking better than the adjacent HAZ. As the under matched weld is subjected to plastic deformation, there will be some stain hardening in the weld as it cools, thereby increasing the tensile strength somewhat at the expense of ductility. However, the effects of strain hardening and loss of ductility will be reduced somewhat by the second layer of weld deposited over the initial root bead. The process will continue, i.e., each weld bead is strain hardened as it cools, but the mechanical properties are improved by the tempering effects of successive weld beads.
If the weld joint is not highly restrained, there is little to be gained by under matching the weld.
Best regards - Al