Picking the right filler metal for mild steel comes down to four questions: what process are you running, how clean is the base metal, does the joint need to meet a structural code, and what position are you welding in? The answers narrow the field to one or two options out of the dozen or so filler classifications available for carbon steel.
All standard mild steel fillers produce 70 ksi minimum tensile strength deposits. That overmatches every mild steel grade (A36, 1018, 1020, A500, A513, A53) by a comfortable margin. You won’t find a situation where 70 ksi filler is too weak for a mild steel joint. The differences between fillers are about usability, cleanliness, hydrogen level, and cost.
MIG Wire (GMAW Solid Wire)
MIG wire for mild steel falls under the AWS A5.18 specification. The designation ER70S-X tells you: ER = electrode/rod, 70 = 70 ksi tensile, S = solid wire, and the number after S identifies the chemistry.
ER70S-6: The Default
ER70S-6 is the most popular MIG wire in the world for carbon steel. Its higher silicon (0.80-1.15%) and manganese (1.40-1.85%) content provides strong deoxidation. This means it tolerates light mill scale, surface rust, and contamination better than cleaner wires. The silicon also improves wetting and produces a flatter, more uniform bead profile.
Use ER70S-6 for:
- General fabrication on hot-rolled steel
- Material with mill scale you haven’t fully removed
- All-position welding (with short-circuit transfer)
- Production work where stopping to grind every joint isn’t practical
The downside of ER70S-6 is silica islands. The excess silicon in the wire forms small glassy deposits on the bead surface. These need to be chipped or wire-brushed off before painting or running additional passes. On multi-pass welds, silica islands trapped between layers create inclusions.
ER70S-3: The Clean Option
ER70S-3 has lower deoxidizer content (0.90-1.40% Mn, 0.45-0.75% Si). Less deoxidizer means fewer silica islands and a cleaner bead surface. The tradeoff is reduced tolerance for surface contamination. Run ER70S-3 on blasted, ground, or pickled steel for the cleanest results.
Use ER70S-3 for:
- Material that’s been cleaned (blasted, ground, or pickled)
- Multi-pass welds where silica islands would be a problem
- Applications where paint adhesion matters
- Automotive work on clean sheet
ER70S-2: The Triple-Deoxidized Wire
ER70S-2 contains silicon, manganese, aluminum, titanium, and zirconium as deoxidizers. That triple-deoxidizer package makes it the most tolerant of surface contamination. It’s primarily used as TIG rod for carbon steel, but it’s also available as MIG wire. It costs more than S-6 and is harder to find in spool form.
Wire Diameter Selection
| Wire Diameter | Material Thickness Range | Machine Requirement | Notes |
|---|---|---|---|
| .023" | 24 ga - 16 ga | 110V or 220V | Auto body, thin sheet only |
| .030" | 20 ga - 3/16" | 110V or 220V | Most versatile size for light work |
| .035" | 16 ga - 1/2" | 220V preferred | General fabrication standard |
| .045" | 3/16" - unlimited | 220V+, industrial | High deposition, production work |
Shielding Gas for MIG on Mild Steel
75% Argon / 25% CO2 (C-25): The all-around standard. Smooth arc, good bead appearance, minimal spatter, works in all positions. This is what most shops run.
100% CO2: Deeper penetration, more spatter, lower cost. Preferred for structural work where penetration matters more than appearance. Produces a more convex bead profile. Higher heat input at the same wire feed speed compared to 75/25.
90% Argon / 10% CO2: Flatter bead, less spatter than C-25, slightly less penetration. Popular for sheet metal and automotive work. Good choice with pulse MIG machines.
Stick Electrodes (SMAW)
Stick electrodes for mild steel follow AWS A5.1 (carbon steel) classification. The E-XXXX format tells you: E = electrode, first two digits = tensile strength in ksi, third digit = welding positions, fourth digit = flux coating type and current.
E7018: Structural Standard
E7018 is the low-hydrogen electrode that dominates structural steel welding. The “18” means it runs in all positions (1) and has an iron powder low-hydrogen coating (8). Low-hydrogen means less than 8 mL of diffusible hydrogen per 100 grams of deposited metal (many brands achieve H4 levels).
Properties of E7018:
- 70 ksi tensile, 58 ksi yield minimum
- All-position (3/32 and 1/8 inch sizes)
- DCEP or AC
- Smooth, quiet arc with medium penetration
- Slag peels in long strips on flat welds
- Mandatory rod oven storage at 250-300F after opening
E7018 is prequalified by AWS D1.1 for all structural joints. When in doubt about which rod to use on mild steel, E7018 is always a safe answer.
E6013: Easy All-Purpose
E6013 uses a rutile (titanium dioxide) coating that produces an easy-to-strike, stable arc with a smooth, fine-rippled bead. It runs on AC, DCEP, or DCEN, making it compatible with any welding machine. The slag releases easily and the bead appearance is among the prettiest of any stick rod.
Limitations: E6013 is not low-hydrogen. It’s not prequalified for structural code work under D1.1. Penetration is shallow compared to 6010/6011. It’s a fabrication and repair rod, not a structural one.
E6010: Deep Penetration, DCEP Only
E6010 has a cellulose sodium coating that produces a deep-digging, forceful arc. It’s the rod pipeline welders use for root passes because it burns through scale, rust, and contamination while driving deep penetration. Runs on DCEP only, which means it won’t work on AC-only (transformer) machines.
E6011: Deep Penetration, AC Compatible
E6011 is the AC version of 6010. Cellulose potassium coating allows it to run on AC or DCEP. Same deep penetration and ability to handle dirty material. Slightly less aggressive arc than 6010. This is the rod for field work on AC buzz boxes.
E7024: High Deposition, Flat Only
E7024 has a thick iron powder coating that melts off rapidly, producing the highest deposition rate of any common mild steel stick rod. The catch: it’s flat and horizontal fillet only. No vertical, no overhead. It’s a production rod for shops running long, flat welds.
| Electrode | Tensile (ksi) | Positions | Current | Low-H | Best For |
|---|---|---|---|---|---|
| E6010 | 60 | All | DCEP | No | Root passes, dirty steel, pipe |
| E6011 | 60 | All | AC, DCEP | No | Field work, AC machines, dirty steel |
| E6013 | 60 | All | AC, DCEP, DCEN | No | General repair, beginners, light fab |
| E7018 | 70 | All | AC, DCEP | Yes | Structural, code work, all-around |
| E7024 | 70 | Flat, Horiz | AC, DCEP | No | High-deposition flat/horizontal |
TIG Filler Rod (GTAW)
TIG filler rods for carbon steel follow the same AWS A5.18 specification as MIG wire but come as 36-inch straight rods in 1/16, 3/32, and 1/8 inch diameters.
ER70S-2: Best for TIG
ER70S-2 is the premier TIG rod for mild steel. The triple deoxidizer package (Si, Mn, Al, Ti, Zr) produces a fluid, clean puddle that flows smoothly and wets out evenly. It handles surface contamination that would cause porosity with other rods. Most professional TIG welders keep ER70S-2 as their carbon steel rod.
ER70S-6: Budget Alternative
ER70S-6 works fine for TIG on clean mild steel. It’s cheaper and more available than S-2. The puddle is slightly stiffer and doesn’t flow as smoothly, but on ground or blasted material the results are nearly identical.
Rod Diameter Selection
| Material Thickness | Filler Diameter | Tungsten Size | Amperage Range |
|---|---|---|---|
| 20-16 ga sheet | 1/16" (1.6 mm) | 1/16" | 40-80A |
| 14-11 ga | 1/16" or 3/32" | 3/32" | 80-130A |
| 3/16" - 1/4" | 3/32" | 3/32" | 120-180A |
| 1/4" - 3/8" | 1/8" | 1/8" | 170-250A |
All TIG on mild steel runs DCEN (straight polarity) with 100% argon shielding at 15-20 CFH. Use 2% lanthanated or 2% ceriated tungsten.
Flux-Cored Wire (FCAW)
Flux-cored wires for mild steel come in two types: gas-shielded (FCAW-G, requires external gas) and self-shielded (FCAW-S, no external gas needed). Both follow AWS A5.20 classification.
E71T-1: Gas-Shielded Production Wire
E71T-1 is the structural flux-core wire for carbon steel. It requires external shielding gas (100% CO2 or 75/25 Ar/CO2). High deposition rates, excellent all-position capability, good mechanical properties including Charpy toughness. This is what ironworkers and structural fabricators run.
Use 100% CO2 for maximum penetration on thick structural connections. Use 75/25 for better bead appearance, less spatter, and improved all-position handling.
E71T-11: Self-Shielded Field Wire
E71T-11 generates its own shielding gas from the flux core, eliminating the need for external gas bottles and regulators. That makes it ideal for field work, outdoor welding, and windy conditions where MIG shielding gas would blow away.
Tradeoffs: lower deposition rate than gas-shielded wire, rougher bead appearance, higher diffusible hydrogen levels, and not all sizes handle all positions as well. E71T-11 is a convenience wire, not a production wire.
E70T-1: Flat and Horizontal Only
E70T-1 is a flat and horizontal gas-shielded wire with the highest deposition rates in the flux-core family. The “0” in the third position means flat and horizontal only. It produces large, fast welds on thick plate in the flat position. Not for structural all-position work.
| Wire Classification | Shielding | Positions | Hydrogen Level | Best Application |
|---|---|---|---|---|
| E71T-1 | CO2 or 75/25 | All | H8 typical | Structural fabrication, high production |
| E71T-11 | Self-shielded | All | H16 typical | Field, outdoor, no gas available |
| E70T-1 | CO2 or 75/25 | Flat/Horiz | H8 typical | Maximum deposition flat welds |
| E71T-1C/M H8 | CO2 (C) or mix (M) | All | H8 certified | Code structural, hydrogen controlled |
Quick Selection Guide
Not sure which filler to grab? Here’s the decision tree for mild steel:
Structural or code work? E7018 stick, E71T-1 flux-core, or ER70S-6 MIG. All are prequalified under AWS D1.1 for structural joints on Group I steels.
General shop fabrication? ER70S-6 MIG with 75/25 gas. Fastest, easiest, and cheapest for day-to-day work.
Dirty, rusty, or painted steel? E6011 stick or E71T-11 self-shielded flux-core. Both burn through contamination better than other options.
Thin sheet metal? .023 or .030 ER70S-6 MIG on low settings, or TIG with 1/16 ER70S-2 rod for precision.
Outdoor or windy conditions? E71T-11 self-shielded flux-core or stick (any type). Avoid MIG unless you can set up wind screens.
Highest quality, appearance matters? TIG with ER70S-2 rod. Slowest process but cleanest, most precise welds.
Maximum speed on thick plate? E71T-1 gas-shielded flux-core with CO2. Highest deposition rate of any semi-automatic process on mild steel.
All these fillers are compatible across the mild steel family: A36, 1018, 1020, A500 tubing, A513 tubing, and A53 pipe. For medium-carbon steels like 1045, the same fillers work but preheat and hydrogen control become mandatory due to the base metal’s increased hardenability.