Tig welding carbon steel is often done using er70s-2 tig rod… and for good reason. It works. It’s cheap. And it’s predictable.
But there are times when using 309L tig welding rod works great.
309 stainless tig rod is more expensive than er70s-2. Like around 4-5 times as expensive.
Where I live, you can get er70 tig rod for less than 3 bucks a pound. 309 stainless costs more than 10 bucks a pound.
Now, some random thoughts on using 309 rod for carbon steel jobs.
I don’t think its a good idea to use 309 rod for everything because under certain conditions it might be prone to hot cracking…like on full penetration groove welds, and highly restrained welds too.
309 tig rod is designed to be used to dissimilar welds of carbon steel to stainless.
309 works for all kinds of odd jobs…like tool and die repair where high hardness is not required, tig welding 4130 chromoly to stainless, welding over hot rolled mill scale, sealing a hole when water is present, and more.
Using 309 indiscriminately where er70s-2 works just fine is not only more expensive, but is not as predictable.
But honestly, just being real, I would be hard pressed to describe an odd job on carbon steel, where it would not work.