2026-10-10 · 5 min read
ER70S-6 and ER70S-3 are two of the most widely used carbon steel solid MIG wires. They share a minimum 70,000 psi tensile designation, they are both classified to AWS A5.18, and they both run on DC+ with CO2 or an argon/CO2 blend. Buyers sometimes treat them as interchangeable. They are not, and the reason is a difference in alloy content that you can measure.
What the designations mean
In the AWS A5.18 designation ER70S-6: ER marks a solid electrode or rod, 70 is the minimum as-welded tensile strength in ksi, S indicates a solid wire for gas-shielded and similar processes, and the final digit records the chemistry group. So ER70S-3 and ER70S-6 are the same strength class, with different deoxidiser levels.
The actual difference: manganese and silicon
ER70S-6 carries more manganese and more silicon than ER70S-3. Those two elements act as deoxidisers: they combine with oxygen in the weld pool and float out as slag, protecting the deposit from porosity. Our own typical as-deposited figures:
| ER70S-6 | ER70S-3 | |
|---|---|---|
| Carbon (C) | 0.07 | 0.074 |
| Manganese (Mn) | 1.53 | 1.01 |
| Silicon (Si) | 0.85 | 0.62 |
| Yield strength | 425 MPa | 435 MPa |
| Tensile strength | 540 MPa | 540 MPa |
| Elongation | 31% | 29% |
| Impact | 89 J at −30 °C | 90 J at −30 °C |
For reference, the AWS A5.18 requirement for the ER70S-6 classification sets manganese at 1.40–1.85% and silicon at 0.80–1.15%, with a minimum tensile strength of 483 MPa and a minimum elongation of 22%. ER70S-3 sits below that range, by design.
What that means at the bench
More deoxidiser means the wire tolerates a dirtier plate. Rust, mill scale, oil and moisture all introduce oxygen into the weld pool, and the extra manganese and silicon in ER70S-6 neutralise more of it before it can form porosity. That is why ER70S-6 is the default choice in fabrication where plate arrives from the yard and cannot be brought to bare white metal before every pass.
ER70S-3 does the opposite trade. With less manganese and silicon, less silicate slag forms on the surface of the bead, so there is less to chip, grind and wire-brush between passes. On clean plate that is a genuine productivity gain. Our catalogue states its use plainly: thin low-carbon steel plate and low-carbon steel parts with a thoroughly treated surface.
Both wires run all positions with DC+ and CO2 or an argon/CO2 mix, so the choice is not a positional one. It is a question of plate condition and of how much time you want to spend cleaning between passes.
Choosing
- Plate with rust, mill scale or oil, or general fabrication → ER70S-6
- Thin, clean, well-prepared sheet where post-weld cleaning is the bottleneck → ER70S-3
- Mixed shop where one wire has to cover most work → ER70S-6
- High-speed welding of clean sheet → either; ER70S-3 leaves less slag to remove
The Chinese standard equivalents
Specifying to GB/T 8110 instead of AWS, the counterparts are ER50-6 for ER70S-6, ER50-3 for ER70S-3, and likewise ER50-2 and ER50-4 for ER70S-2 and ER70S-4. The strength designation differs between the two systems — GB quotes the level in a way that reads differently from the AWS ksi figure — so it is worth confirming grade by grade rather than assuming the numbers line up.
If you tell us the material thickness, the surface condition and whether the work is single or multi-pass, we will confirm the grade and the diameter for the job.