Hi AceMet
I interpret your statement "non code weld" to mean that this welding procedure is not qualified. Am I correct? If it is qualified, then the "powers that be" have obviously tried this out through procedure tests, and found good results, in which case you may wonder why it is being done, but changing it will not really be an option. At any rate, here is my thinking as to why they may be going this way:
A significant problem with this weld will be root cracking, because the carbon pick-up from the AISI 1045 will make a standard E7018 root deposit rather brittle, and the shrinkage stresses experienced in the first run or two will be high compared to the throat thickness of the deposit. By using the GMAW (presuming it is dip transfer) you will minimise the dilution with the base metal. (Possibly from around 50% for SMAW root, to around 25% for GMAW root, where the dilution from the AISI 1045 side will be half of the total dilution, assuming the same amount of penetration on both sides of the joint.) The ER80S-Ni 1 fillers are designed for improved toughness, (Normally used in impact test applications, and also often in applications where CTOD testing is to be done.) so they should have a better chance of surviving the excessive shrinkage stresses in the root run.
Once you have survived the biggest failure potential in the weld (the root) you will then turn to a filler and process that will give acceptable results once PWHT'ed. (The E7018)
I have not done any calcs to verify that the ER80S-Ni 1 will have a suitable toughness given the expected dilution, but my gutt feel is that it should do the trick. (Hopefully the welding engineer that specified this filler has done the calcs, or has experience doing this weld.)
Let us know how the job went, so that we can see if this was a good strategy.
Regards
Niekie