15.4 Welding Processes: SMAW, GMAW, GTAW, OFW & Equipment Care
Key Takeaways
- In the AWS electrode classification E7018, the 70 means 70,000 psi tensile strength, the 1 means all positions, and the 8 identifies a low-hydrogen iron-powder coating.
- E7018 low-hydrogen electrodes must be kept in a heated rod oven at roughly 120 C after the sealed container is opened, because absorbed moisture causes hydrogen cracking.
- GMAW short-circuit transfer suits thin material and out-of-position work, while spray transfer requires higher current and argon-rich gas and is limited to flat and horizontal positions.
- GTAW uses direct current electrode negative for steel and stainless, and alternating current for aluminum so the reverse-polarity half cycle breaks up the refractory oxide layer.
- Cast iron repair welding requires 260–320 C preheat, nickel-based electrodes, short stringer beads peened while hot, and a very slow insulated cool-down.
Sub-tasks A-6.02 through A-6.06 cover joining and welding with oxy-fuel, SMAW, GMAW and GTAW equipment, plus maintaining that equipment. Millwrights are not welders by trade, but the interprovincial standard expects a journeyperson to perform routine welding competently, recognize defects, and know when a job requires a certified welder under a qualified procedure.
Process Comparison
| Process | Common name | Electrode | Shielding | Best for |
|---|---|---|---|---|
| SMAW | Stick | Consumable, flux-coated | Flux decomposition gas and slag | Field repair, dirty or rusty steel, wind, all positions, heavy sections |
| GMAW | MIG | Consumable solid wire | External gas (CO2, Ar/CO2, Ar/O2) | Fast production welding indoors on clean steel |
| FCAW | Flux core | Consumable tubular wire | Flux, with or without external gas | High deposition; self-shielded FCAW-S works outdoors |
| GTAW | TIG | Non-consumable tungsten | Argon or Ar/He | Highest quality, thin material, stainless, aluminum, root passes |
| OFW | Gas welding | Filler rod | Oxy-acetylene flame | Thin steel, brazing, torch brazing of tubing; largely superseded |
SMAW and AWS Electrode Classification
The AWS A5.1 code E XX Y Z is decoded left to right:
| Position | Meaning | Example: E7018 |
|---|---|---|
| E | Electrode | Electrode |
| XX | Tensile strength in thousands of psi | 70 = 70,000 psi |
| Y | Welding positions | 1 = all positions (2 = flat and horizontal fillet; 4 = includes vertical down) |
| Z | Coating type and current | 8 = low-hydrogen, iron powder; AC or DCEP |
| Electrode | Character | Millwright use |
|---|---|---|
| E6010 | Cellulose, DCEP, deep digging penetration, forceful arc | Root passes on pipe, dirty or rusty steel |
| E6011 | E6010 chemistry runnable on AC | Same as 6010 where only an AC machine is available |
| E6013 | Rutile, soft arc, shallow penetration, easy slag | Sheet metal, light fabrication, poor fit-up |
| E7018 | Low hydrogen, iron powder, smooth arc, high toughness | Structural steel, high-carbon and alloy steels, code work |
| ENi-CI / ENiFe-CI | Nickel-based | Cast iron repair |
| E308L / E309L | Stainless | 308L for 300-series to itself; 309L for stainless to carbon steel (dissimilar joints) |
Low-hydrogen storage is a hard rule. E7018 coatings absorb atmospheric moisture, and that moisture becomes hydrogen in the weld pool, driving delayed cracking in hardenable steels. Once a sealed container is opened, the electrodes go into a heated rod oven at about 120 C, and only the rods needed for the immediate task come out. Rods left out beyond the manufacturer's exposure limit must be re-conditioned or discarded.
Polarity conventions: DCEP (reverse polarity) puts about two thirds of the heat in the electrode, giving deeper penetration and better cleaning — the standard for E7018 and E6010. DCEN (straight polarity) puts more heat in the work, favouring faster melt-off on thin material and used for GTAW on steel.
GMAW: Gas and Transfer Mode
| Shielding gas | Behaviour |
|---|---|
| 100% CO2 | Deepest penetration, cheapest, most spatter, short-circuit only |
| 75% Ar / 25% CO2 | The general-purpose mix: good penetration, low spatter, stable arc |
| Ar with 1–5% O2 or 8% CO2 | Enables spray transfer on carbon steel |
| 100% Argon | Aluminum and non-ferrous only |
| Ar / He / CO2 tri-mix | Stainless steel |
| Transfer mode | Current | Characteristics | Position |
|---|---|---|---|
| Short circuit (dip) | Low | Wire touches the pool and short-circuits ~100 times per second; low heat input | All positions, thin material |
| Globular | Medium | Large irregular droplets; high spatter; generally avoided | Flat |
| Spray | High, argon-rich gas | Fine droplets propelled across the arc; high deposition, excellent fusion | Flat and horizontal only — the pool is too fluid otherwise |
| Pulsed spray | Pulsed | Spray quality at a lower average heat | All positions with the right equipment |
The classic GMAW defect is cold lap / lack of fusion from short-circuit transfer used at too low a heat on thick material: the weld looks acceptable but has not fused to the base metal.
GTAW
GTAW uses a non-consumable tungsten electrode with filler added separately by hand.
- DCEN for carbon steel, stainless, copper and titanium — maximum penetration, minimal tungsten heating.
- AC for aluminum and magnesium. Aluminum carries a refractory oxide film melting near 2,050 C over metal melting near 660 C. The electrode-positive half cycle provides cathodic cleaning that breaks up that oxide, while the electrode-negative half cycle provides penetration.
- High-frequency start avoids contaminating the tungsten by scratch-starting.
- Tungsten types: 2% thoriated (red — mildly radioactive, avoid grinding dust), 2% lanthanated (blue) and 2% ceriated (grey) are the common non-radioactive substitutes, and pure tungsten (green) for AC on older machines.
- Grind the tungsten longitudinally, with the grinding marks running along the electrode axis, so the arc stays stable.
Joints, Weld Types and Positions
Five basic joints: butt, lap, tee, corner, edge.
Weld types: fillet (triangular, at a tee or lap), groove (square, V, bevel, U, J), plug and slot, surfacing (hardfacing).
Positions are examined constantly:
| Code | Plate | Pipe |
|---|---|---|
| 1F / 1G | Flat | Pipe rotated, welded at the top |
| 2F / 2G | Horizontal | Pipe vertical, weld horizontal |
| 3F / 3G | Vertical | — |
| 4F / 4G | Overhead | — |
| 5G | — | Pipe horizontal and fixed, welder moves around it |
| 6G | — | Pipe fixed at 45 degrees — the all-position qualification test |
F denotes a fillet weld and G a groove weld. Qualifying on 6G qualifies a welder for essentially all positions, which is why it is the benchmark pipe test.
Weld Defects and Their Causes
| Defect | Appearance | Principal causes |
|---|---|---|
| Undercut | Groove melted into the base metal at the toe, not filled | Excessive current, too long an arc, wrong angle, too fast a travel speed |
| Porosity | Gas holes in the weld | Moisture, oil, paint, rust, galvanizing; lost shielding gas from wind or a blocked nozzle |
| Slag inclusion | Trapped non-metallic material | Poor interpass cleaning, wrong electrode angle, too low current |
| Lack of fusion / cold lap | Weld metal not bonded to base metal | Heat input too low, dirty surface, incorrect technique |
| Incomplete penetration | Root of the joint not reached | Root opening too small, land too thick, current too low, travel too fast |
| Cracks — hot | In the weld while solidifying | High sulphur or phosphorus, deep narrow bead, high restraint |
| Cracks — cold (hydrogen) | In the HAZ, delayed hours to days | Hydrogen from moisture, fast cooling, hardenable steel, no preheat |
| Overlap | Weld metal rolled over the base without fusing | Too slow travel, too low current |
| Arc strike | A stray melted spot outside the weld | Careless electrode contact; a fatigue crack initiator that must be ground out and inspected |
| Distortion | Warping, angular change, shrinkage | Uncontrolled heat input and sequence |
Controlling Distortion
Weld metal shrinks as it cools, and that shrinkage bends the work. Standard controls:
- Tack weld with adequate size and spacing, and use tacks that will be consumed or ground.
- Back-step welding — weld short segments in the direction opposite to overall progression.
- Skip / intermittent welding — distribute heat rather than laying one continuous pass.
- Balanced welding — alternate sides of a joint or member so shrinkage forces oppose.
- Pre-setting / pre-bending the parts so shrinkage pulls them into alignment.
- Fixturing and strongbacks to restrain the work — but recognize that restraint raises residual stress and cracking risk in hardenable steel.
- Minimum heat input — the smallest bead that meets the specification, with controlled interpass temperature.
Welding Equipment Maintenance (A-6.06)
| Equipment | Checks |
|---|---|
| Welding machine | Clean cooling air passages; check fan operation; inspect the primary cord, plug and disconnect; verify the case ground |
| Cables and connections | Look for cuts, exposed conductor, hot or discoloured lugs; a loose work-clamp connection causes arc instability, low voltage at the arc and overheating |
| Work (ground) clamp | Clean bare metal contact, close to the weld; never route the return through bearings, gearboxes or rolling elements — arc current through a bearing causes electrical fluting damage |
| Electrode holder / gun | Insulation intact, jaws clean; replace a cracked holder |
| GMAW gun consumables | Contact tip sized to the wire and replaced when worn oval; nozzle cleaned of spatter and anti-spatter applied; liner matched to wire size and blown out or replaced — a worn liner is the leading cause of erratic wire feed |
| Drive rolls | Groove matched to wire size and type (V for solid, knurled for flux-cored); tension set to the minimum that stops slipping |
| Shielding gas train | Regulator and flowmeter accuracy; leak-test hoses and connections with soap solution; check flow at the nozzle, not just the flowmeter |
| Rod oven | Verify actual temperature with a thermometer, not just the dial |
Special Case: Welding Cast Iron
- Clean the area and drill stop holes at the ends of the crack to prevent it running.
- Vee out the crack, removing all the graphite-contaminated metal.
- Preheat to 260–320 C slowly and uniformly.
- Weld with nickel electrodes (ENi-CI or ENiFe-CI) using short stringer beads about 25–50 mm long, not long continuous runs.
- Peen each bead while hot to relieve shrinkage stress.
- Allow the casting to cool very slowly, buried in lime or wrapped in insulating blankets. A fast cool cracks the casting beside the weld every time.
Welding Safety
- Fume: stainless steel produces hexavalent chromium, a confirmed carcinogen with a very low exposure limit; manganese fume is a neurotoxin; galvanized coating produces zinc oxide fume and metal fume fever. Local exhaust ventilation at the arc, or an air-supplied respirator, is required — not just a fan.
- Arc radiation: ultraviolet light causes arc eye and skin burns within minutes. Use the correct shade, cover all skin, and screen the area to protect others.
- Electric shock: open-circuit voltage is dangerous, especially when a welder is sweating or working in a damp confined space. Never drape cables over the body; keep dry gloves and dry insulation underfoot.
- Fire: hot work permit, cleared and protected combustibles, and a fire watch for at least 30 minutes after work ends.
- Never weld on a sealed vessel or a container that has held flammables until it is cleaned, purged and tested. A sealed vessel can also explode from expanding trapped gas even if the contents were inert.
A millwright pulls E7018 electrodes from a rod oven at the start of the shift, uses a few, and leaves the remainder in the open toolbox for two days before finishing the repair on a 4140 shaft collar. Why is this a problem?
Why must GTAW on aluminum normally use alternating current rather than direct current electrode negative?
A cracked grey cast iron gearbox housing is repaired by veeing out the crack and running long continuous beads with a mild steel electrode, then letting the casting cool in the shop air. New cracks appear alongside the weld the next morning. Which combination of errors caused this?