Reading the designation
- ER — electrode/rod: the filler can be used as a MIG electrode or a TIG rod.
- 70 — minimum tensile strength of the deposited weld metal, 70 000 psi.
- S — solid wire, as opposed to a flux-cored or metal-cored product.
- 6 — the chemistry class, which is where the practical difference lives.
Why the "6" matters
The final digit describes deoxidiser content, principally silicon and manganese. ER70S-6 carries more of both than ER70S-3, and those elements scavenge oxygen out of the weld pool as it forms.
The practical consequence is tolerance. Mill scale, light surface rust and less-than-perfect preparation are exactly the conditions that put oxygen into a weld, and a wire with more deoxidiser copes with them without porosity. That is the whole reason ER70S-6 is the general-purpose choice in a working shop rather than a laboratory.
Where ER70S-3 still belongs
On clean, prepared plate — and particularly on multi-pass work where a slightly lower silicon content gives a cleaner bead appearance and less silicon island formation between passes — ER70S-3 is a perfectly good and slightly cheaper choice.
The mistake is treating them as interchangeable when the material is not clean. If plate arrives as-rolled and gets welded as-received, specify the -6.
Practical ordering
Grade is only part of an order. Diameter (commonly 0.8, 0.9, 1.0 and 1.2 mm), spool or pack size, and quantity all matter, and the spool size should suit how the shop actually consumes wire — small spools on a portable set, larger spools where the machine runs all day.
Where a welding procedure names a consumable, send the procedure sheet rather than a description. Specifying against the procedure is faster and it removes the substitution conversation entirely.