Sometimes 309 stainless tig rod is a good choice when you need to Tig weld carbon steel.
I worked a job once where simple carbon steel fixtures where fabricated daily. The welder would routinely tig weld them with super missile weld rod. ( a fancy marketing name for 309 or 312 stainless tig rod)
The reason? even though the rods were much more expensive than plain old er70s2, the welder like them better and could weld hot rolled steel easier without getting porosity. Also , there was a breeze when the door opened to the welding area and that caused lots of bubbling unless the super missile rod was used.
So, when is it not OK to use 309 tig rod to tig weld carbon steel?
How about coded welds where er70 rod is specified?
There is a firestorm on several forums over this issue.
It goes something like this...
"Hey guys, buttermilk-69 here...how do I get rid of porosity in a boiler tube weld?
The inside gets wet before I finish the root and it gets porosity, and no matter how I grind it out, it comes back."
Then rodBurner comments .."hey , just add a few drops of 309 filler rod and it will fix the porosity issue"
BuckEye76 adds, "I am an engineer with 26 years of coal power experience and I cant believe you think you can just use whatever rod you want for a high pressure weld...its hacks like you that put lives in danger and give real welders a bad name."
Then buttermilk-69 fires back... " yeah, while you engineers are high in your ivory towers, guys like me in the trenches are down in a nasty boiler figuring out how to make that weld that you designed in a corner 4 inches off the floor."
and it goes on and on...and at the end of the firestorm, the guy who wonders if 309 will work on a roll cage or on 4130 tubing is left wondering.
So what is the answer?
Where is it ok to use 309 tig rod on carbon steel?
After all, 309 tig rod is specifically designed to weld carbon to stainless.
The resulting weld is a mix of stainless steel and carbon and yet the welds withstand stress and thermal cycles.
All that said, its not OK to use 309 stainless tig rod on high pressure pipe or tube welds where it is not specified in the code.
But...there are all kinds of situations where using 309 stainless tig rod can work very well.
Here are a few that come to mind:
where water or moisture can not be completely removed and causes porosity when carbon steel tig rod is used.
when the parts you are welding have high enough carbon to cause hardening of the weld...especially when the weld deposit will be machined afterwards.
filling in a threaded hole that has contaminants that cant be cleaned out.
when you are not quite sure of the metal type like when modifying a wrench.
welding pretty much any stainless to any carbon steel
hot rolled mild steel shelves.
injection mold repair for areas where hardness is not required.
There is some risk of hot cracking when stainless tig rod is used for carbon steel. But in my experience, that most often happens in full penetration butt welds...or highly restrained welds.
And keep this fact in mind:
Carbon steel joints welded using 309 tig rod can be inconsistent.
The weld metal depends on heat used and techniques used. One guy mixes more base metal because he welds hotter. The other guy uses less heat and the weld metal is very different.