ER70S-6 vs ER70S-3: Stop Ruining Your Welds – Full Comparison Guide

When MIG welding mild steel, few decisions affect weld quality as much as your filler wire choice. ER70S-3 and ER70S-6 look nearly identical. But their subtle chemical differences can mean the difference between a clean, strong weld and a porous failure.

This guide covers everything you need to know about ER70S-6 and ER70S-3: chemical makeup, performance, ideal applications, and a simple framework for choosing the right wire.

Both wires fall under the AWS A5.18 specification for carbon steel filler metals:

• E – Electrode
• R – Rod (can be used as either an electrode or a filler rod)
• 70 –70,000 psi minimum tensile strength
• S – Solid wire
• 3 or 6 – Chemical composition variant

Both are designed for MIG welding of mild and carbon steels. The number indicates alloy content differences and that’s where the real distinction lies.

The primary difference is deoxidizer levels specifically silicon and manganese. These elements bind with oxygen in the weld pool, preventing porosity. ER70S-6 contains significantly higher silicon and manganese than ER70S-3.

ER70S-6 (Higher Deoxidizer Content)

ER70S-3 (Lower Deoxidizer Content)

The extra deoxidizers in ER70S-6 act like a cleaning agent. They “boil out” impurities before the weld solidifies.

ER70S-6 is the most widely used MIG wire for mild steel. Its popularity comes from one key advantage: tolerance to surface contamination.

When to Use ER70S-6

• The base metal has mill scale, light rust, or surface oils.

• You want a universal wire for varying material conditions.

• Fit-up is imperfect (the fluid puddle bridges gaps well).

• Working outdoors or in less controlled environments.

• Performing single-pass welding.

Key Advantages

• Forgiving on contamination – Higher Mn/Si neutralizes light rust and oil, reducing porosity.

• Excellent wetting action – Produces smooth, flat beads with good fusion.

• Versatile shielding gas – Works with both C25 and 100% CO₂.

• All-position capability – Suitable for flat, horizontal, vertical, and overhead.

• Good mechanical properties – High tensile strength (≥480 MPa) and elongation (22–30%).

Primary Applications

• Structural steel - Beams, columns, trusses for building and bridge construction

• Shipbuilding – Hull sections, decks, bulkheads where mill scale is common

• Automotive manufacturing – Body panels, chassis components, repair work

• General manufacturing – Frames, enclosures, machinery across various material thicknesses

• Maintenance and repair (MRO) – Field repairs where surface conditions vary

• Pipe and tube welding – Root runs and joint completion

Potential Drawbacks

• Silicon islands – The higher silicon can leave glass-like deposits. If paint is applied, these may cause flaking. In multi-pass welding, they can appear as slag inclusions on X-ray.

• Moderate spatter – Slightly more than ER70S-3, especially with 100% CO₂.

• Slightly higher cost – Due to additional alloying elements.

ER70S-3 is designed for general fabrication of mild steels. Its lower silicon and manganese make it better for applications where reduced silicon content is preferred.

When to Use ER70S-3

• The base metal is clean, oil-free, and rust-free.

• Multi-pass welding is required – Lower silicon content reduces the risk of silicon island accumulation between passes.

• Paint adhesion is critical – Minimal silicon islands mean better paint bonding.

• You want lower spatter – Produces a cleaner arc with less spatter than ER70S-6.

• Cost is a primary consideration – Generally more affordable than ER70S-6.

• Welding thin materials - Narrower bead profile offers better control.

Key Advantages

• Low silicon island formation – Best for painted finishes or X-ray quality multi-pass welds.

• Low spatter – Cleaner welds with less post-weld cleanup.

• Excellent for thin materials – Narrower bead profile and good penetration control.

• Lower cost – More economical for high-volume production on clean material.

Primary Applications

• Automotive Sheet Metal – Body panels, thin-gauge fabrication

• Metal Furniture – Clean welds with minimal post-weld finishing

• General Fabrication on Clean Steel – Where surface preparation is consistent

• Pressure Vessel Fabrication – For high-integrity root runs and joint completion

• Applications with Paint Requirements – Where silicon islands could cause coating adhesion problems

Potential Drawbacks

• No tolerance for contamination – Mill scale, light rust, or oils will likely cause porosity.

• Not recommended with 100% CO₂ – Best with C25; pure CO₂ causes arc instability and increased spatter.

• Narrower bead profile – Less “wash-in” at the weld toes, which may matter for fatigue-critical parts.

• Poorer performance on rusty materials – Lower deoxidizers can’t neutralize contamination effectively.

For ER70S-6

• C25 (75% Ar / 25% CO₂) – Smooth, stable arc, minimal spatter, clean bead appearance.

• 100% CO₂ – Deeper penetration, harsher arc with more spatter, lower cost. Suitable for thick plates and outdoor welding.

ER70S-6 performs well with both. For best appearance and minimal cleanup, use C25.

For ER70S-3

• C25 – Recommended. Low spatter, smooth arc, good bead appearance.

• 100% CO₂ – Not recommended. Higher spatter, rougher arc, may cause weld defects.

Stick with C25 for ER70S-3.

Both wires meet AWS A5.18 requirements with similar strength and elongation:

ER70S-6 offers superior low-temperature impact strength (-29°C vs. -18°C for ER70S-3), making it better for cold-weather applications.

• Using ER70S-3 on dirty steel – Insufficient deoxidizers cause porosity.
• Using ER70S-6 where silicon islands are a problem – For painted or X-ray multi-pass work, ER70S-3 may be better.
• Using 100% CO₂ with ER70S-3 – Rough arc, high spatter, poor quality.
• Assuming wire choice doesn’t affect weld quality – It absolutely does.

1. Can I use ER70S-6 and ER70S-3 interchangeably?

Not without understanding the consequences. While both will produce a weld, ER70S-3 on dirty steel will likely cause porosity. ER70S-6 on clean steel will work fine but may leave silicon islands that affect paint adhesion or X-ray quality.

2. Which wire is stronger?

Both wires meet the same minimum tensile strength requirement of 70,000 psi. The difference is not in strength but in contamination tolerance and weld puddle behavior.

3. Is ER70S-6 always better?

No. ER70S-6 is more versatile, but ER70S-3 has specific advantages: lower spatter, fewer silicon islands, and lower cost. For clean material with painting requirements or multi-pass X-ray work, ER70S-3 is often the better choice.

4. What about ER70S-2?

ER70S-2 contains even higher deoxidizer levels (including zirconium, aluminum, and titanium) and is designed for the most demanding applications, including welding over rusty surfaces. However, it can produce more slag formation. For most general fabrication, ER70S-6 strikes the best balance.

5. Can I use either wire for TIG welding?

Yes, both classifications can be used as filler rods for GTAW (TIG) welding, though ER70S-2 is often preferred for TIG applications due to its superior deoxidation

Selecting between ER70S-6 and ER70S-3 comes down to surface condition and finish requirements.

For most shops with varying material quality, ER70S-6 is the safe, versatile choice. Its higher deoxidizers handle real-world contamination. That’s why it’s the most widely used MIG wire for mild steel.

But for pristine paint adhesion, X-ray quality multi-pass welds, or minimal spatter on clean material, ER70S-3 remains excellent and is often more economical.

Know your material. Know your application. When in doubt about surface condition, choose ER70S-6.

Ronsco is a trusted global supplier of nickel alloy and stainless steel welding wires, including Inconel, Hastelloy, and Monel series. Our products are manufactured to meet industry standards and are widely used in demanding applications such as aerospace, oil & gas, chemical processing, and power generation. Explore our resource center for the latest industry insights, product knowledge, and technical references.

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