I recall, vaguely, that there is a particular type of stainless steel filler rod that can join all dissimilar ferrous metals, like tool steel to stainless, tool steel to steel, steem to stainless, all stainless, etc. I wanted to double check if my memory serves me right and that it is type 312 stainless.
The rule is that you want more chrome and nickel in the filler if you are joining stainless to non stainless. There is a nice chart at
It shows that 312 is one of the choices for joining mild steel to any stainless, but other good choices are 309 and 310.
What is your opinion about nickel rod, the kind you get when you clean the flux off of a stick electrode. That is what I use as kind of half way between a tig compatible bronze and steel wire as for heat input. I think it is a good choice for dis similar metals. If you buy out someone's stick collection you generally get some of this kind of rod. I guess some is just high nickel not pure nickel the original intention I expect is/was cast iron.
I am no expert, so hesitated to answer this. But my guess is that the nickel rod would work well. The only drawback is that it might not have the strength or hardness of a stainless rod. I am not very familar with nickel.