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| OFW | Gas welding | Filler rod | Oxy-acetylene flame | Thin steel, brazing, torch brazing of tubing; largely superseded |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?
The flux coating dries out and the arc becomes unstable
The low-hydrogen coating absorbs atmospheric moisture, which introduces hydrogen into the weld pool and causes delayed cracking in the hardenable base metal
Exposure to air oxidizes the core wire so the weld metal loses tensile strength
The electrodes become magnetized and cause arc blow
Why must GTAW on aluminum normally use alternating current rather than direct current electrode negative?
Alternating current reduces the argon consumption required for aluminum
Alternating current produces a hotter arc that melts aluminum faster
The electrode-positive half cycle provides cathodic cleaning that breaks up the refractory aluminum oxide film, which melts near 2,050 C while the base metal melts near 660 C
Aluminum is non-magnetic and direct current cannot maintain an arc on it
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?
The crack was not drilled at the ends before welding, which is the only required precaution
Grey cast iron cannot be welded under any circumstance and must always be replaced
The vee was too shallow and the electrode current was too high
Wrong filler metal, no preheat, over-long beads with no peening, and an uncontrolled cool-down — all of which impose shrinkage stress on brittle cast iron
Sections you finish are checked off in the contents.