ER70S-2 or ER70S-6? for TIG welding Carbon Steel

What is the best filler rod for TIG welding carbon steel?

Is it good old ER70s-2?

or ER70s-6 with higher silicon content?

The differences between ER70S-2 and ER70S-6 are pretty subtle for fillet welds on clean metal.

And even on light mill scale, i don't see a huge difference in the two when TIG welding.

It is true that ER70s-6 contains more silicon and is often promoted as being better for MIG welding over ER70s2 where there is rust or mill scale.

The higher silicon content seems to float out the oxides.

But just because ER70s2 has a lower silicon content, does not make it inferior when it comes to TIG welding. 

ER70s2 TIG rod is a triple deoxidized wire with added deoxidizers like Zr, Ti, and Al in addition to Silicon and manganese...and those elements can also aid in scavenging impurities.

Both ER70S-2 and ER70S-6 are fairly inexpensive filler rods so I recommend trying both and deciding which works better in your specific application.

I will say that ER70s6 seems to perform better for open root welds or full penetration butt welds where the back side is exposed to air.

I like the way ER70s6 flows even on simple fillet welds but I can see some benefit to ER70s2 being slightly less fluid.

ER309 is a stainless rod that is designed for welding carbon to stainless as well as other disimilar metal combinations and comes in really handy for certain situations where porosity occurs  like when repairing dirty injection molds where cracks and crevices are contaminated with rubber, plastic or crud.

Weldmonger.com has plenty of TIG filler rod in stock at great prices.