Hi waps75

This is a rather open ended question, because theoretically you can do anything you want, if you try hard enough. I do however sense that you have a question behind the question. If so, try asking it!

At any rate, here is my 2cents worth: 309L filler is made to weld 309L parent metal, or to weld typical 18-8 stainless steels to carbon steel. It is however one of the rods that some people use as a "cure-all" for all kinds of problems. This is because it is relatively hot crack resistant due to the reasonably high ferrite content in the deposit. As such, it is often used in place of the 29-9 (312) filler that is generally made as the "cure-all".

You could therefore weld carbon steel to carbon steel successfully, depending on the dilution during welding. (High dilution and you get too much martensite, leading to cracking problems.) You will however tend to pick up galvanic corrosion right next to the weld. You can also weld stainless to stainless, but in some instances it will be overalloyed costing money, and in some instances it will not give the same corrosion resistance to certain media. This again depends on the particular base metals you are welding.

I would not really want to weld C/Iron with this rod. Rather use a Ni based rod for this.

Hope this helps.

Regards
Niekie Jooste
Fabristruct Solutions