The Harris ER70S-6 Mild Steel TIG Welding Rod (part number E70S650) is a copper-coated mild steel filler metal classified ER70S-6 per AWS A5.18/A5.18M. Available in a 3/32 in (2.38 mm) diameter, 36-inch cut length, 10 lb box, this rod provides premium deoxidized weld deposits on mild steel, low-carbon steel, and structural steel plate using the gas tungsten arc welding (GTAW) process. Harris Products Group, a Lincoln Electric company, produces this ER70S-6 rod with tight manganese (1.40–1.85%) and silicon (0.80–1.15%) chemistry — the highest deoxidizer levels of the ER70S series — which makes it exceptionally tolerant of mill scale, slight rust, and surface oxides that would cause porosity with lower-deoxidized ER70S-2 or ER70S-3 rods. The result is a rod that produces smooth, bright, crack-free weld deposits even on less-than-perfect base metal surfaces, and provides reliable mechanical properties (≥70,000 psi tensile, ≥58,000 psi yield) in the as-welded condition.
While ER70S-6 is best known as the dominant MIG wire classification for structural steel, the cut-rod TIG version is widely used for premium root-pass welding on carbon steel pipe, structural tubing welds requiring X-ray quality, and job shop TIG work on mild steel plate and fabrications where MIG setup is impractical. The 3/32 in diameter is the production size for 1/8 in–3/8 in carbon steel plate, structural section, and pipe wall applications.
| Attribute | Value |
|---|---|
| AWS Classification | ER70S-6 (AWS A5.18/A5.18M) |
| Harris Part Number | E70S650 |
| Diameter | 3/32 in (2.38 mm) |
| Cut Length | 36 in (914 mm) |
| Package Weight | 10 lb box |
| Carbon Content | 0.06–0.15% |
| Manganese | 1.40–1.85% |
| Silicon | 0.80–1.15% |
| Phosphorus (max) | 0.025% |
| Sulfur (max) | 0.035% |
| Copper (max) | 0.50% (includes copper coating) |
| Tensile Strength (as welded) | ≥70,000 psi (483 MPa) |
| Yield Strength (as welded) | ≥58,000 psi (400 MPa) |
| Elongation (as welded) | ≥22% |
| Impact Toughness (as welded) | ≥27 J at −29°C (−20°F) |
| Polarity | DCEP or DCEN (DCEN preferred for TIG) |
| Shielding Gas | 100% Argon or Ar/CO₂ blends |
| Process | GTAW (TIG) |
| Base Metals | ASTM A36, A53, A106, A516, A572, API 5L carbon and structural steels |
- Carbon Steel Pipe Root Passes: The primary structural TIG application for ER70S-6 3/32 in rod is root pass welding on carbon steel pipe (API 5L Grade B, X42, X52, A106 Grade B) in 1–6 in NPS for process piping, fire protection, utilities, and general service. TIG root passes produce full-penetration, radiographically sound welds that SMAW or MIG fill/cap passes are completed over — the most common pipe welding sequence on construction and shutdown projects.
- Structural Steel Fabrication: TIG welding A36 and A572 structural steel plate, tube, and section in shop fabrication environments where weld quality exceeds typical MIG requirements. Column connections, moment frames, and crane runway connections sometimes specify TIG roots for quality assurance.
- Pressure Vessel & Boiler Plate: ASME Section VIII and Section I vessels in P-No. 1 (carbon steel) — GTAW root passes on nozzle connections, flange-to-shell, and head-to-shell seams where full penetration radiographic quality is required.
- Repair & Maintenance: Repair welding on carbon steel components, buildup of worn surfaces on mild steel machinery parts, and structural repair of beams and frames. TIG with ER70S-6 is particularly useful for repairs in confined spaces where MIG gun access is limited.
- Aerospace Structural Steel: Low-alloy carbon steel hardware and structural members in aircraft support equipment, tooling fixtures, and ground support equipment where weld quality documentation is required.
- Automotive & Racing Fabrication: Chassis tubes, suspension components, and roll cage fabrication in 1010–1020 mild steel tube and sheet where TIG's clean deposit and precise heat control provide the cosmetic and mechanical quality required for competition vehicle certification.
Mild steel TIG welding with 3/32 in ER70S-6 uses DCEN polarity (same as stainless) with argon or argon-CO₂ shielding:
Polarity — DCEN: DC electrode negative is standard for GTAW on carbon steel. DCEN concentrates 70% of the arc energy at the workpiece (better penetration) while keeping the tungsten cool. Do not use AC for mild steel TIG — it provides no benefit and produces an unstable arc.
Shielding Gas: 100% argon is standard for mild steel GTAW and provides the cleanest arc starts and best bead appearance. Argon-CO₂ blends (75/25 Ar/CO₂) are sometimes used to increase deposition rate on heavy sections, but produce slightly higher spatter and darker bead color than pure argon. 100% argon at 18–22 CFH is the standard recommendation for 3/32 in rod work.
Tungsten: 3/32 in 2% ceriated or 2% lanthanated tungsten, sharpened to a 30–45° included angle point. For high-amperage heavy-plate work (above 200 A), 1/8 in ceriated tungsten. The copper coating on ER70S-6 can deposit trace copper onto tungsten if the rod contacts the tungsten — if this occurs, replace or grind the tungsten tip back to clean metal.
| Material Thickness | Amperage (DCEN) | Travel Speed | Passes | Ar Flow CFH |
|---|---|---|---|---|
| 1/8 in A36 | 100–140 A | 5–8 in/min | 1 | 18–20 |
| 3/16 in carbon steel | 140–180 A | 4–6 in/min | 1–2 | 20 |
| 1/4 in plate | 170–210 A | 4–5 in/min | 2–3 | 20–22 |
| Pipe root pass (Sch 40) | 70–110 A | 3–4 in/min | Root only | 18–20 |
Interpass Cleaning: Unlike stainless, mild steel TIG weld deposits produce relatively little slag or silica islands. Light wire brushing or light grinding between passes on multi-pass work removes any surface silicate deposits and improves fusion on subsequent passes. For code-required radiographic quality (X-ray or UT), ensure full fusion at each pass and maintain specified preheat for the base metal thickness and chemistry.
- Rust Prevention: ER70S-6 rod has a thin copper coating to protect against rust during storage. Store in original sealed box in a dry location (below 60% RH, above 40°F). Despite the copper coating, rust will develop on stored rods in humid environments. Rusty rods must not be used — the rust introduces hydrogen and oxygen into the weld pool, causing porosity and inclusions.
- Temperature: Store at room temperature. Rod that has been stored in cold, damp conditions and brought indoors may condensate moisture on the copper coating — allow rods to reach room temperature before use.
- Do Not Use Rusty Rod: Inspect rods before use. Light surface tarnish on the copper coating is acceptable. Red-orange rust spots on the underlying steel indicate the coating has been compromised — discard these rods or use only on non-code work where porosity inspection is not required.
- Separate from Stainless Rods: Do not store ER70S-6 mild steel rods in the same container as stainless TIG rods. Cross-contamination of mild steel rod with stainless weld joints, or vice versa, creates quality defects. Label storage clearly.
- Low-Hydrogen Classification: ER70S-6 is not a low-hydrogen electrode in the same classification sense as E7018 stick electrodes. However, keeping rods dry and free from contamination is good practice for all filler metals.
ER70S-6 TIG rod works with any DC GTAW machine:
- Lincoln Electric Invertec V155-S: Compact, portable DC TIG/SMAW machine suitable for pipe root passes and light structural TIG with ER70S-6.
- Lincoln Electric Square Wave TIG 200: Full AC/DC machine; DC mode for ER70S-6 mild steel TIG. Suitable for shop structural and pipe root-pass work.
- Lincoln Electric Precision TIG 225 / 275: Higher-capacity machines for production structural steel TIG work at 175–250 A continuous duty.
| Base Metal | ER70S-6 Compatibility | Notes |
|---|---|---|
| ASTM A36 structural steel | ✅ Primary | Standard structural steel; ER70S-6 is default pre-qualified filler per AWS D1.1 |
| ASTM A53 / A106 pipe | ✅ Primary | Carbon steel pipe; standard root pass choice |
| ASTM A516 Gr.70 (pressure vessel) | ✅ Primary | Normalized carbon steel; ER70S-6 meets strength requirements |
| ASTM A572 Gr.50 HSLA | ✅ Acceptable | Higher-strength base metal; ER70S-6 provides adequate weld metal strength for most joints |
| API 5L Gr.B / X42 / X52 pipe | ✅ Primary (with WPS) | Pipeline root pass; verify WPS and CE (carbon equivalent) requirements |
| 1010–1020 mild steel tube | ✅ Primary | Automotive / chassis; excellent choice for TIG on thin-wall mild tube |
| Cast iron | ❌ | Use ENi-CI or ENiFe-CI for cast iron repair |
| Stainless steel | ❌ | Use ER308L/ER316L for stainless; ER309L for dissimilar carbon-to-stainless |
Q1: What is the difference between ER70S-6 and ER70S-2 or ER70S-3 TIG rod?
The number suffix designates the deoxidizer level. ER70S-6 has the highest Mn (1.40–1.85%) and Si (0.80–1.15%) levels of the common ER70S grades — making it the most tolerant of scale, rust, and surface contamination on the base metal. ER70S-2 and ER70S-3 have lower deoxidizer levels and produce cleaner (less silica) welds on very clean, freshly machined or ground base metal. For general structural steel work and pipe, ER70S-6 is the standard choice because real-world base metal surfaces are rarely perfectly clean. ER70S-2 is specified for some pipe applications (API requirements) requiring specific notch toughness at very low temperatures.
Q2: Can I use ER70S-6 TIG rod for root passes on carbon steel pipe?
Yes. ER70S-6 is the most common GTAW filler for carbon steel pipe root passes (open butt, consumable insert, or backing ring configurations). Its high deoxidizer content produces porosity-resistant, radiographically sound root beads even on mill-scaled pipe OD/ID surfaces. Verify the applicable WPS — most ASME and AWS pre-qualified procedures for P-No. 1 carbon steel allow ER70S-6 as an acceptable filler metal.
Q3: Do I need to preheat carbon steel before TIG welding with ER70S-6?
Preheating depends on the base metal thickness and carbon equivalent (CE). For A36 plate below 3/4 in thickness and ambient temperature above 50°F, no preheat is typically required. For plate above 3/4 in, A106 Grade B pipe above 1.5 in wall, A572 Grade 50, or any carbon steel in cold weather (below 40°F ambient), consult AWS D1.1 or ASME IX preheating requirements for the specific carbon equivalent. ER70S-6's good impact toughness (27 J at −29°C) ensures the weld metal is adequate even when moderate preheat is applied.
Q4: What shielding gas should I use for ER70S-6 TIG rod?
100% argon is standard and recommended for most applications. Some welders use 75/25 Ar/CO₂ or 90/10 Ar/CO₂ blends on mild steel TIG to increase arc energy density and productivity on thick plate. These blends produce slightly darker bead color, more spatter, and higher silicon island formation, but provide adequate results on structural work. For code quality (X-ray/UT), 100% argon typically produces the cleanest welds.
Q5: Is ER70S-6 TIG rod suitable for welding A36 to A572 Grade 50 dissimilar mild steel?
Yes. ER70S-6 is an acceptable filler for welding A36 to A572 Gr.50 dissimilar mild steel joints. The minimum specified weld metal strength (70,000 psi) is "matching" strength for the lower-strength A36 member and "undermatching" for the A572 Gr.50 member. Per AWS D1.1, using a filler that matches the lower-strength base metal in a dissimilar joint is acceptable, as the joint design accounts for the lower-strength connection. Verify with the project structural engineer if the specific joint is critical.
Q6: Can I weld galvanized mild steel with ER70S-6 TIG rod?
Technically yes, but with significant safety precautions. TIG welding galvanized steel at carbon steel melting temperatures vaporizes the zinc coating, generating zinc oxide fumes that cause zinc fume fever. Adequate ventilation or supplied-air respirator is mandatory. If minimum zinc fume is critical (automotive body panels), use silicon bronze TIG rod (ERCuSi-A) for braze-welding, which operates below steel melting temperature. ER70S-6 on galvanized is limited to structural applications where the zinc fume hazard is controlled.
Q7: What is the copper coating on ER70S-6 rod for?
The thin copper coating on ER70S-6 rod serves two purposes: (1) corrosion protection during storage and transit — the copper barrier slows rust formation on the steel rod surface, and (2) improved electrical contact in MIG wire feeders (the copper reduces drive roll wear and provides consistent contact-tip resistance). On TIG cut rods, the copper coating provides the same storage corrosion protection. The copper is consumed in the weld deposit — a maximum 0.50% Cu is allowed in the final chemistry per AWS A5.18, which is within the acceptable range for carbon steel weld metal.