Assuming you have a autogenous weld with equal amounts of S30400 and mild steel . This would result in a martensitic structure and will not be acceptable.( Ref Schaeffler de long diagram ) If you use a higher alloyed consumable like S30900 and assuming it forms 1/3 rd of the weld pool the resultant composition will be in the austenitic region.

The big thing about this particular weld is that the chemistry of any stainless (300 series)
must be kept above the magical 18/8 chrome/nickel ratio for good mechanicals and max corrosion resistance.

If we weld with a lower grade filler, say, 308-the dilution (ratio of filler metal and base material) may be very well lower in chemistry than this 18/8 requirement.

ER309, with it's elevated chrome and nickel content can produce a weld chemistry above the magical 18/8, even with normal dilution.

Hope this helps

Good Luck

brande