There are two schools of thought on this type of dissimilar weld. The Power Industry has generally favored filler metals matching the higher alloy side of the joint, while the petrochemical industry has generally favored the filler metals matching the lower alloy side for making butt welds.
For me it depends on the situation. When weld hardness is a criteria as in B31.3 or other petrochemical services or hydrogen induced cracking from welding is more of a potential(field welding where preheat may not be controlled as well as it should be), the lower alloyed filler material is preferred as it is more forgiving. When welding lower alloy non-pressure parts to higher alloy pressure parts, the higher alloy filler metal would be recommended.
In some cases logistics and concern for the potential of placing the lower alloy fiiler metal in welds of two higher alloy welds, the higher alloy filler metal would be used.
