Hi Ed
It will definately make a difference. With C/Steel backing, you will get a much higher dilution of the filler metal with Fe. With Monel backing rings, it will be much lower. With C/Steel backing rings, it will be very possible to get to the 30% level of Fe, if the welder is getting a lot of penetration into the backing ring. If this is the problem, then the cracking should only be limited to the root. By the time the second run is made, the dilution should be below the 30% level.
Just as a matter of interest, with TIG and MIG welding, the maximum Fe dilution that can be accomodated is only around 15 - 20%.
Regards
Niekie Jooste
Fabristruct Solutions