A36 is the most common structural steel in North America. It’s got a minimum yield strength of 36 ksi, tensile strength of 58-80 ksi, and a maximum carbon content of 0.25%. That carbon level puts it firmly in the “easy to weld” category. You can run MIG, TIG, stick, or flux-core on A36 with standard 70 ksi filler metals and get full-strength joints without exotic procedures.
The ASTM A36 specification covers plates, bars, and shapes (I-beams, channels, angles) used in building construction, bridges, and general fabrication. Unlike grades specified by carbon content (like 1018 or 1045), A36 is a mechanical-property specification. That means the mill can adjust the chemistry as long as the steel meets the yield and tensile requirements. Carbon can range from 0.18% to 0.25% depending on thickness, with manganese up to 1.20% and silicon up to 0.40%.
A36 Composition and Weldability
A36’s weldability comes down to its carbon equivalent (CE). The formula most shops use is:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
Typical A36 runs a CE between 0.35 and 0.40. Anything under 0.45 is considered readily weldable without special precautions. Above 0.45, you start needing preheat and low-hydrogen procedures even on thinner material.
| A36 Property | Value |
|---|---|
| Minimum Yield Strength | 36 ksi (250 MPa) |
| Tensile Strength | 58-80 ksi (400-550 MPa) |
| Carbon (max) | 0.25% (plates to 3/4"), 0.26% (over 3/4") |
| Manganese (max) | 0.80-1.20% (varies by thickness) |
| Carbon Equivalent (typical) | 0.35-0.40 |
| Weldability Rating | Excellent |
The practical takeaway: A36 doesn’t harden significantly in the heat-affected zone during normal welding. You won’t get brittle martensite unless you’re welding thick plate with high-hydrogen consumables in freezing conditions. For most shop work, standard procedure with appropriate filler is all you need.
Filler Metal Selection for A36
Filler metals for A36 are classified by minimum tensile strength. A “70” series filler (ER70S-6, E7018, ER70S-2) produces deposits with 70 ksi minimum tensile strength, which overmatches A36’s 58-80 ksi range. Overmatching is the goal for structural joints because it forces failure into the base metal rather than the weld.
MIG (GMAW) Filler Wire
ER70S-6 is the industry default for MIG welding A36. The “S-6” designation means it has higher silicon (0.80-1.15%) and manganese (1.40-1.85%) content than other ER70S wires. These deoxidizers scavenge oxygen from the weld pool, which means ER70S-6 tolerates light mill scale and surface contamination better than cleaner wires. It produces a flatter bead profile with good wetting at the toes.
ER70S-3 is the cleaner alternative. Lower silicon and manganese mean less deoxidizer residue on the bead surface. Use it on ground or blasted A36 where surface cleanliness isn’t a concern. It produces slightly less silica islands on the weld surface, which matters for multi-pass welds and painted structures.
Stick (SMAW) Electrodes
E7018 is the structural standard. Low-hydrogen coating (less than 8 mL H2/100g of deposited metal) prevents hydrogen cracking, runs smooth on DCEP or AC, and produces excellent mechanical properties including good Charpy toughness at low temperatures. AWS D1.1 prequalifies 7018 for all structural joints on A36. Keep rods in an oven at 250-300F after opening the sealed can.
E6013 is easier to strike and produces a prettier bead, but it’s not a structural electrode. Fine for non-critical fabrication, light sheet metal, and general repair work on A36. Don’t use it on code-governed connections.
E6010 and E6011 provide deep penetration for root passes and dirty conditions. 6010 runs DCEP only. 6011 runs AC or DCEP, making it the pick for transformer-based (AC-only) machines. Both burn through mill scale, paint, and rust better than 7018.
TIG (GTAW) Filler Rod
ER70S-2 is the top choice for TIG on A36. Its triple deoxidizer package (silicon, manganese, and aluminum plus titanium and zirconium) handles mill scale and light contamination that would cause porosity with other rods. The weld pool flows smoothly and wets out to a flat, uniform bead.
ER70S-6 works for TIG too, but ER70S-2 produces a cleaner, easier-flowing puddle under argon shielding. On clean, ground A36, ER70S-6 is perfectly acceptable and cheaper.
Flux-Cored (FCAW) Wire
E71T-1 (gas-shielded, CO2 or 75/25) is the production workhorse for A36 structural welding. High deposition rates, good mechanical properties, all-position capability. This is what structural ironworkers run.
E71T-11 (self-shielded) eliminates the need for external shielding gas. It works outdoors in wind and on field erection where running gas lines isn’t practical. Deposition rates and bead appearance aren’t as good as gas-shielded wire, but the convenience is unmatched.
| Process | Filler Metal | Shielding | Best Application |
|---|---|---|---|
| MIG (GMAW) | ER70S-6 | 75/25 Ar/CO2 or 100% CO2 | General fabrication, production |
| MIG (GMAW) | ER70S-3 | 75/25 Ar/CO2 | Clean material, minimal spatter |
| Stick (SMAW) | E7018 | Self-shielded (flux coating) | Structural, code work |
| Stick (SMAW) | E6010/6011 | Self-shielded (flux coating) | Root passes, dirty steel |
| TIG (GTAW) | ER70S-2 | 100% Argon | Precision, thin material |
| Flux-Core (FCAW-G) | E71T-1 | 100% CO2 or 75/25 | High-deposition structural |
| Flux-Core (FCAW-S) | E71T-11 | Self-shielded | Outdoor, field erection |
Preheat Requirements by Thickness
A36 under 3/4 inch thick generally needs no preheat when welded with low-hydrogen consumables (E7018, ER70S-6) in normal shop conditions. The rules change with thicker material, high restraint, and cold weather.
AWS D1.1 Structural Welding Code (Table 3.3) sets mandatory minimums based on the thickness of the thickest part at the joint and the welding process group. For A36 (Group I steel), the requirements are:
| Thickness of Thickest Part | Minimum Preheat (Low-H Process) | Minimum Preheat (Non-Low-H Process) |
|---|---|---|
| Up to 3/4" | 32F (0C) - none required above freezing | 32F (0C) |
| 3/4" to 1-1/2" | 50F (10C) | 150F (66C) |
| 1-1/2" to 2-1/2" | 150F (66C) | 225F (107C) |
| Over 2-1/2" | 225F (107C) | 300F (149C) |
“Low-H process” means E7018 stick (with proper rod storage), GMAW (MIG), GTAW (TIG), or FCAW with H8 or lower diffusible hydrogen designation. Non-low-H covers E6010, E6013, and any stick rod that isn’t specifically low-hydrogen.
In practice, many shops preheat A36 over 1 inch regardless of code requirements. It costs almost nothing (a few minutes with a rosebud torch) and eliminates any risk of hydrogen cracking on restrained joints. Verify preheat temperature with a temperature-indicating crayon (Tempilstik) or contact thermocouple placed 3 inches from the weld joint on the side opposite the heat source.
Welding Process Settings for A36
MIG Settings (ER70S-6, 75/25 Ar/CO2)
| Material Thickness | Wire Diameter | Voltage | Wire Feed Speed (IPM) | Transfer Mode |
|---|---|---|---|---|
| 16 ga (0.060") | .023" | 15-16 | 200-250 | Short circuit |
| 14 ga (0.075") | .030" | 16-17 | 250-300 | Short circuit |
| 11 ga (0.120") | .030" | 18-19 | 300-350 | Short circuit |
| 3/16" | .035" | 20-22 | 350-400 | Short circuit / spray |
| 1/4" | .035" | 22-24 | 380-450 | Spray |
| 3/8" | .045" | 24-27 | 300-380 | Spray |
| 1/2" | .045" | 26-29 | 350-420 | Spray (multi-pass) |
Stick Settings (E7018, DCEP)
| Rod Diameter | Amperage Range | Typical Use |
|---|---|---|
| 3/32" | 65-100A | Thin material, root passes |
| 1/8" | 110-160A | General-purpose, most common size |
| 5/32" | 140-210A | Heavy fabrication, flat/horizontal |
| 3/16" | 200-275A | Thick plate, flat position only |
Joint Preparation for A36
Mill scale is the gray-blue oxide layer that forms on hot-rolled A36 during manufacturing. It’s the single biggest source of weld defects on this steel. MIG and flux-core can burn through light mill scale with silicon/manganese deoxidizers in the wire, but stick and TIG produce cleaner welds on ground steel.
For structural work to AWS D1.1, grind or blast the weld zone clean 1 inch back from the joint on both sides. Remove rust, paint, oil, and heavy mill scale. Light, tight mill scale on hot-rolled plate is acceptable for prequalified MIG and FCAW procedures, but grinding it off always produces better welds.
Bevel angles follow standard AWS conventions: 60-degree included angle for V-grooves (30 degrees per side), 45 degrees for single-bevel. Root opening of 1/16" to 1/8" with a 1/16" to 1/8" root face. Backing bars (steel or ceramic) let you run the root pass with higher heat without blowthrough.
Common Problems Welding A36
Porosity in MIG welds. Almost always a gas coverage problem. Check flow rate (20-25 CFH for 75/25), look for drafts, inspect the nozzle for spatter buildup, and verify the gas isn’t running out or contaminated. Dirty base metal is the second most common cause. Grind the mill scale and degrease with acetone.
Undercut on fillet welds. Caused by excessive voltage, wrong gun angle, or too-fast travel speed on the top toe. Reduce voltage 1-2 volts, point the wire into the joint root, and slow down. On vertical-up joints, use a slight weave or triangle technique to fill the toes.
Hydrogen cracking on thick plate. Shows up 24-48 hours after welding as cracks in the HAZ or weld metal. Three factors must be present simultaneously: hydrogen in the weld, tensile stress, and a susceptible (hardened) microstructure. Eliminate any one factor and cracking stops. Use low-hydrogen consumables with proper storage, preheat per code, and avoid highly restrained joints where possible.
Distortion on thin plate. A36 sheet gauge and thin plate warp easily. Clamp aggressively, backstep or skip-weld, keep heat input low, and alternate sides on double-sided joints. Tack every 4-6 inches before running continuous beads.
Post-Weld Treatment
A36 doesn’t need post-weld heat treatment (PWHT) for most applications. The low carbon content and low hardenability mean the HAZ stays ductile without stress relief.
PWHT is specified on A36 when code requires it for specific applications: pressure vessels (ASME Section VIII), certain piping codes, or when the engineer of record specifies it for highly-restrained, thick-section joints. Standard PWHT for A36 is 1100-1200F held for 1 hour per inch of thickness, followed by controlled furnace cooling.
For non-code work, the money spent on PWHT is better spent on proper preheat and procedure. If you control hydrogen, preheat thick sections, and use good fit-up, A36 welds are reliable as-welded.
A36 is the steel most welders learn on, and for good reason. It forgives sloppy technique, works with every process, and uses common, inexpensive consumables. Match the filler to the application (7018 for structural, ER70S-6 for general MIG, ER70S-2 for TIG), preheat the thick stuff, and you’ll produce sound joints every time.