ER70S-2 vs ER70S-6: Choosing a TIG Filler Rod for Mild Steel
Walk into any welding supply store and the mild steel TIG rod rack usually has two options: ER70S-2 and ER70S-6. They look identical, they cost about the same, and both are rated for the same minimum tensile strength. So which one goes in your rod tube?
The honest answer is that for a lot of hobby work either will do. But they behave differently in the puddle and tolerate surface condition differently, and once you know why, choosing becomes easy.
Reading the classification
Both names follow the same AWS pattern for solid carbon steel filler:
ER
Usable as an Electrode (MIG wire) or Rod (TIG filler).
70
Minimum tensile strength of the deposited weld metal, in thousands of psi.
S
Solid wire, as opposed to tubular (flux-core or metal-core).
-2 / -6
The chemistry designation, which mainly reflects the deoxidizers added.
So the strength rating is the same. What changes is the small percentage of elements added to scavenge oxygen from the puddle. Those elements affect how the puddle wets, how fluid it is, and how it handles surface contamination.
What deoxidizers do
When steel melts, oxygen from mill scale, rust, or the atmosphere can combine with it. If that oxygen isn't dealt with, it can cause porosity and poorer weld quality. Deoxidizers are elements that react with oxygen first, forming small silicate islands that float to the surface of the bead.
That's why you sometimes see small glassy brown dots on a TIG or MIG bead. They're the products of deoxidation, and they're a clue to how much work the filler had to do.
Side by side
| ER70S-2 | ER70S-6 | |
|---|---|---|
| Main deoxidizers | Manganese and silicon, plus small aluminum, titanium and zirconium | Higher manganese and silicon |
| Puddle feel (common report) | A bit stiffer, easier to control out of position | More fluid, wets out and flows easily |
| Tolerance for mill scale and light rust | Good | Very good, which is why it dominates MIG wire |
| Silicon islands on the bead | Usually fewer | Often more |
| Typical TIG uses | Clean steel, root passes, work where a tidy bead and controllable puddle are wanted | General fabrication, steel with less-than-perfect prep |
| Availability | Common at welding suppliers | Extremely common everywhere |
The puddle-feel row reflects what many welders report rather than a lab number, and it can be subtle. Try both if you can.
When ER70S-6 is the better pick
- Your steel has mill scale you didn't fully remove. Hot-rolled steel comes with a dark scale layer, and S-6 handles traces of it more gracefully.
- You're welding in the garage on random stock of uncertain origin and cleanliness.
- You want a fluid puddle that wets out to the toes of a fillet without much coaxing.
- You only want to stock one rod that matches the MIG wire most people already use.
When ER70S-2 makes sense
- You prep to bright metal every time and want fewer silicon islands to clean.
- You weld out of position and prefer a slightly firmer puddle.
- You're doing root passes on clean pipe or plate where a procedure or instructor calls for it.
- A procedure specifies it. If a welding procedure specification or a drawing names a filler, use that one. The spec wins over preference every time.
If the part is load-bearing or governed by a code, filler choice is set by the engineer, the drawing, or the WPS, not by a general article. Our guide to AWS certification, WPS and PQR explains how those documents work.
Prep matters more than the rod
TIG doesn't have flux, and argon shielding doesn't clean anything. Whatever is on the metal ends up in or on the weld. The deoxidizer difference between S-2 and S-6 is real, but it's small compared to the difference between dirty and clean steel.
- Remove mill scale from the joint and a margin on each side with a flap disc or grinder.
- Degrease the joint and the filler rod with a suitable solvent and a clean rag. Let solvent fully evaporate before striking an arc; some chlorinated solvents can form toxic gases when exposed to the arc's UV, so check the product's safety data sheet.
- Keep rods clean. Store them in a closed tube, not loose on the bench. Wipe each rod before use if it's been handled with oily gloves.
- Keep the hot end of the filler inside the gas shield between dabs so it doesn't oxidize.
Choosing a diameter
Classification is only part of the purchase. Rod diameter affects how easily you control heat and how fast you add metal.
| Rod diameter | Typical use |
|---|---|
| 0.035 or 0.045 in | Very thin sheet, small parts, delicate edges |
| 1/16 in | Thin sheet to light gauge; a common first size |
| 3/32 in | General hobby work on thicker sheet and light plate |
| 1/8 in | Heavier material and larger fillets at higher amperage |
These are general starting points, not rules. Joint type, position, and your machine settings all shift the right size, so check the manufacturer's recommendations and adjust from there.
How to tell rods apart once they're out of the box
Most quality TIG rods are stamped or flagged with their classification on one or both ends. Cheap rods sometimes aren't. Once a bundle is mixed in a tube with leftovers, you lose track fast. A few habits help:
- Keep one labeled tube per classification and diameter.
- Don't cut off the stamped end until the rod is short.
- Keep mild steel, stainless, and aluminum rods physically separate. Mixing steel filler into a stainless weld causes problems.
Verdict: For most people, ER70S-6 is the safe default for mild steel TIG, especially on hot-rolled stock. ER70S-2 is a good choice if you prep to bright metal and prefer a slightly stiffer puddle. When a drawing or procedure names one, use that.
FAQ
Is ER70S-2 or ER70S-6 stronger?
Both classifications require the same 70 ksi minimum tensile strength in the as-welded test condition. Real strength in your joint depends far more on penetration, fusion and joint design than on choosing one over the other.
Can I TIG weld with MIG wire straightened out?
Some people do in a pinch, but MIG wire is thin and coiled, so it's awkward to feed by hand. TIG cut-lengths are straight, labeled on the end or the rod, and come in diameters suited to hand feeding.
Which TIG filler should I use on rusty steel?
Clean it first. TIG is the least forgiving process for contamination. If you can't get it perfectly clean, ER70S-6's extra deoxidizers help, but it won't fix poor prep.
What about chromoly tubing?
Filler selection for 4130 chromoly depends on the application and the governing code or design. Follow the frame or part designer's specification rather than a general rule.
What diameter TIG rod should I buy?
Match rod diameter roughly to your material thickness and amperage. 1/16 and 3/32 inch are the most common for hobby sheet and light plate. Thinner rod is easier on thin material; thicker rod adds metal faster.
Bottom line
If you buy one tube of mild steel TIG rod, buy ER70S-6 in 1/16 and 3/32. It's widely available and forgiving of real-world steel. If you weld mostly clean, prepped steel and want to compare, pick up some ER70S-2 and see which puddle you prefer. Either way, your prep and torch technique will matter more than the classification.
Related: welding stainless steel covers stainless filler, and gas lens vs standard collet body covers improving gas coverage.