GMAW Solid Wire: ER70S-6 vs ER70S-3 Under AWS D1.1

When a fab shop sets up a GMAW WPS for structural steel, one of the first questions is whether to spec ER70S-3 or ER70S-6 solid wire. The two look identical on a spool and both meet the 70 ksi minimum tensile strength required for most A36 and A572 Grade 50 applications. But they behave differently at the arc, respond differently to surface contamination, and carry different implications for your WPS qualification program. Getting the choice wrong costs rework — or worse, a rejected PQR.

What the AWS A5.18 Classification Actually Tells You

AWS A5.18 is the specification covering carbon steel electrodes and rods for gas-shielded arc welding. The "ER" prefix means electrode or rod; "70" is the minimum deposited weld metal tensile strength in ksi; "S" means solid wire; and the suffix digit identifies the chemical composition group.

The mechanical minimums — 70 ksi tensile, 58 ksi yield, 22% elongation — are the same for both ER70S-3 and ER70S-6. The difference is in chemistry, specifically deoxidizer levels:

ER70S-6 carries substantially more manganese and silicon, both of which are deoxidizers. That extra deoxidizer load is what drives most of the practical differences.

Deoxidizers and Why They Matter in Production

Deoxidizers combine with oxygen in the weld pool before the oxygen can form porosity or carbon monoxide bubbles. When you weld over mill scale, light rust, or surface oxides — conditions common in any real fab shop — the weld pool gets an extra oxygen charge. ER70S-3's lower deoxidizer content may not keep up. The result is scattered porosity, sometimes just sub-surface, that only shows up on UT or radiographic examination.

ER70S-6's higher silicon and manganese can scavenge more oxygen per unit time. That is why most structural shops default to ER70S-6 for general production. The tradeoff is a more prominent silicate island (glassy slag bead) on the weld surface, which must be brushed off between passes to avoid slag entrapment.

For prepped joint surfaces that are clean, ground bright, and free of mill scale, ER70S-3 can run equally well and produces a slightly cleaner bead profile with less silicate residue. Shops doing precision code work on prepped plate sometimes prefer S-3 for this reason.

WPS and PQR Implications Under AWS D1.1:2025

Under AWS D1.1:2025, Table 6.6 lists essential variables for SMAW, SAW, GMAW, FCAW, and GTAW processes. For GMAW, the filler metal specification and classification constitute an essential variable. That means:

• If your WPS was qualified using ER70S-6 (with a PQR run under Clause 6), substituting ER70S-3 without a separate PQR is a violation.
• If your WPS was qualified using ER70S-3, you cannot switch to ER70S-6 and remain compliant without requalification.

The practical fix is straightforward: pick the wire you intend to use in production, run the PQR with that exact wire, and list the classification explicitly on the WPS. If your shop runs both wire types on different work scopes, you need separate WPS documents (or separate PQRs) for each classification.

Rule library based on AWS D1.1:2025; verify against your governing edition — the AHJ or contract may specify 2020 or earlier.

For prequalified WPS developed under Clause 5, the constraint is different. A prequalified WPS must use filler metals that conform to the applicable A5 specification and meet the strength requirements. A5.18 ER70S-3 and ER70S-6 are both listed and both prequalified. You can use either on a prequalified WPS as long as you document it correctly and don't switch mid-project without updating the WPS.

Shielding Gas Pairing

Wire selection does not exist in isolation. Both ER70S-3 and ER70S-6 are typically run with a mixed gas — C25 (75% Ar / 25% CO₂) is the most common structural shop gas, though 90/10 and higher-argon blends are used for spray transfer or pulsed GMAW. 100% CO₂ is allowed but produces a rougher bead profile and requires attention to spatter.

The shielding gas composition is its own essential variable under Table 6.6. A change in gas mixture percentage (for example, from C25 to 98% Ar / 2% O₂) requires requalification even if you keep the same wire. So when you're setting up your WPS, pin down both the wire classification and the shielding gas blend you plan to use in production. Changing one without the other still triggers a requalification requirement.

For more on shielding gas variables, see GMAW shielding gas change and WPS requalification.

The Filler Metal Substitution Trap

A common audit finding in fab shops is a WPS listing one wire classification with production records showing a different one. This happens when purchasing switches wire brands or grades mid-project — sometimes without telling the QC department. The CMTR for the wire on the spool may show the correct chemistry, but if the AWS classification on the spool label doesn't match the WPS, the inspector has no choice but to flag a nonconformance.

The fix is a change-management procedure that routes any filler metal substitution through the QC manager before wire hits the floor. The QC manager checks whether the proposed substitute is covered by the existing WPS; if not, a revised WPS or supplemental PQR is required before welding continues. For a deeper discussion of this process, see filler metal substitution and WPS requalification under AWS D1.1.

Yield and Tensile Strength: Both Meet the Bar

One concern that surfaces occasionally is whether ER70S-6's higher silicon content affects mechanical properties. In practice, the tensile and Charpy results from ER70S-3 and ER70S-6 tested to A5.18 are very similar. Both classes deposit weld metal with at minimum 70 ksi tensile and 58 ksi yield, and both routinely exceed those minimums when run with appropriate parameters.

If your WPS covers demand-critical welds under AWS D1.8 or if CVN impact testing (Supplementary Essential Variables, Table 6.8) is required, the specific heat-and-lot combination of wire must be tracked and documented. A wire classification change under CVN supplementary essential variable coverage is its own requalification trigger under Table 6.8 Row 1 — see CVN supplementary essential variables and the Table 6.8 program for how that program works.

Overmatching and Base Metal Compatibility

Both ER70S-3 and ER70S-6 produce 70-ksi weld metal, which overmatches ASTM A36 (58 ksi yield minimum) and provides approximate strength match to A572 Grade 50 (65 ksi yield). For A572 Grade 65 or A992 (65 ksi yield, 65-90 ksi tensile), verify that the deposited mechanical properties from your actual PQR test results are acceptable.

The GMAW WPS for A572 Grade 50 is among the most commonly developed structural WPS types — GMAW WPS for A572 Grade 50 walks through a complete WPS setup for that base metal.

Making the Right Call for Your Shop

For a shop doing general structural fabrication — beam-column connections, base plates, shear tabs, moment frame work — ER70S-6 is the practical default because it tolerates the reality of fabrication surfaces without requiring spotless prep on every joint. Run it with C25 or C10 shielding gas, document the exact classification on your WPS, verify your PQR was run with the same wire, and you're covered.

ER70S-3 has its place on high-quality work where base metal is mechanically cleaned and inspected before welding. Shops that invest in consistent surface prep sometimes prefer S-3 because the bead profile is cleaner and silicate cleanup is simpler. Either choice is valid — but pick one, write it into your WPS, run your PQR with it, and enforce it in production.

If you're building or updating a GMAW WPS library and want to ensure your essential variable documentation is complete, see what a GMAW WPS software platform handles for you — including essential variable tracking, PQR linkage, and qualification range coverage.