GMAW NC II cheat sheet

OHS hierarchy of control, most to least effective: elimination, substitution, engineering controls (fume extraction, screens), administrative controls (rotation, signage), then PPE as the last barrier.

GMAW fume generation rises with current and wire feed rate; spray transfer produces finer fume but higher emission rates than short-circuiting, so extraction must be rated for the actual duty cycle.

Shielding gas hazards: argon and CO2 are asphyxiants that displace oxygen; CO2 is heavier than air and collects in pits and low areas. Confined-space entry requires oxygen testing before and during work.

Cylinder handling rules: store upright and secured, keep caps on when not in use, separate fuel gas from oxygen by 6 m or a 30-minute fire-rated barrier, and keep oil and grease off oxygen fittings.

Arc flash and radiation: GMAW emits UV and IR; filter shade selection follows amperage (roughly shade 10-12 for typical 100-300 A work). Nearby workers need screens and eye protection.

5S sequence: Sort (seiri), Set in order (seiton), Shine (seiso), Standardize (seiketsu), Sustain (shitsuke); each step is audited before the next is claimed as complete.

Hot work permit conditions: fire watch maintained at least 30 minutes after welding ends, combustibles removed 11 m (35 ft) or protected, and extinguishers charged and accessible.

Electrical safety: GMAW open-circuit voltage typically 15-45 V; ensure the work lead clamp is on clean bare metal close to the joint so welding current does not travel through slings, bearings, or structural members.

ISO 2553 vs AWS A2.4: on an ISO symbol the reference line is drawn with a dashed or solid identification line and the arrow side is the solid-line side; AWS places the weld below the reference line for arrow-side welds.

Symbol elements in order along the reference line: weld type, size (left of symbol), length and pitch (right), then tail with process reference (e.g., 135 for GMAW per ISO 4063).

A fillet weld size is leg length by default under AWS; an isosceles triangle with a dimension left of the symbol sets leg size, and a dimension in parentheses indicates a concave profile.

Supplementary symbols: flush contour is a straight line, convex contour is a curved line, and the all-around symbol is a circle at the reference-line/arrow junction meaning weld the joint on all sides.

WPS essential variables for GMAW include process, base metal specification and thickness range, filler classification, shielding gas composition and flow range, current type and polarity, voltage range, travel speed, and transfer mode.

A PQR supports the WPS; the WPS must fall within the qualified ranges of the PQR. A WPS without a supporting PQR is not a qualified procedure.

Drawing dimension conventions: read the title block for units, scale, and revision; welded assemblies often show pre-weld dimensions plus allowance for shrinkage, so do not add or subtract allowance on your own.

Joint preparation symbols: single-V groove 60-degree included angle is common for GMAW plate; root gap, root face, and bevel angle are separate dimensions that must all be met at fit-up.

Clean base metal at least 25 mm (1 in) back from the joint on both faces: remove mill scale, rust, oil, paint, and moisture, since GMAW is far more sensitive to surface contamination than SMAW.

Fit-up tolerances: root gap and mismatch control cap distortion and burn-through; tack welds must be of qualified quality and either fused into the final weld or removed per the WPS.

Jigs and fixtures: copper or aluminum backing absorbs heat and controls root profile; over-clamping restrains shrinkage and raises residual stress, so release sequence matters.

GMAW setup order: connect work lead to clean metal, set polarity per WPS (DCEP for solid wire; DCEN used for some self-shielded and basic flux-cored wires), install correct drive rolls and liner size for the wire diameter, then set gas flow.

Wire drive checks: drive roll groove must match wire diameter, tension set so the wire deforms slightly but does not crush; a worn liner or wrong contact tip causes erratic feed and porosity.

Shielding gas flow typically 12-20 L/min (25-40 cfh); too low causes porosity, too high causes turbulence and aspiration of air into the plume.

Contact tip-to-work distance (CTWD) is a parameter, not a preference: increasing CTWD raises resistance heating and lowers current, so the WPS value must be maintained.

Preheat is required by the WPS for thick sections or high-carbon-equivalent steels; verify with a contact thermometer or temperature-indicating crayon before the arc starts.

Transfer modes: short-circuiting (low current, low voltage, all positions, high spatter), globular (to be avoided), spray (high current above transition, flat and horizontal only, argon-rich gas), and pulsed spray (spray-like at lower average current, all positions).

Voltage sets arc length and bead width; current (wire feed speed) sets penetration and deposition rate. Increasing WFS without increasing voltage shortens the arc and can stub the wire.

Inductance in short-circuiting controls the rate of current rise: more inductance softens the arc and reduces spatter; too little causes spatter and stubbing.

Polarity: DCEP puts more heat into the workpiece and gives deeper penetration; DCEN puts more heat into the wire and melts the wire faster with shallower penetration.

Work and travel angles: push (forehand) gives a flatter, wider bead with less penetration; drag (backhand) gives deeper penetration and a narrower bead; a 10-15 degree travel angle is typical.

Position rules: flat uses highest deposition; vertical-up gives better penetration and bead control on thicker plate; vertical-down is limited to thin sheet; overhead requires lower current and shorter arc.

Stringer beads versus weave: weave width should not exceed about three times the electrode diameter; excessive weaving causes lack of fusion at the sidewalls.

Shielding gas selection: 100% CO2 gives deep penetration and is cheapest but spatters; 75-85% Ar/CO2 balances spray transfer and penetration; Ar/O2 mixes suit stainless steel.

Porosity causes: low gas flow, drafts, contaminated base metal, wet wire, or excessive CTWD. Correction: restore gas coverage and clean the joint; remove the defect by grinding before repair welding.

Lack of fusion causes: travel too fast, current too low, wrong angle, or a weave too wide for the parameters. Correction: raise current, slow travel, or reduce weave width.

Undercut causes: excessive current, excessive voltage, or travel angle too steep. Correction: reduce voltage/current and correct the angle; undercut exceeding the acceptance limit must be repaired.

Burn-through and melt-through indicate excessive heat input or root gap too large; correct by reducing current, increasing travel speed, or adding backing.

Spatter is a process indicator, not only cosmetic: excessive spatter points to wrong inductance, wet wire, or gas problems; remove spatter before painting or inspection.

Inspection sequence: visual (100% of welds) first, then surface NDT (PT/MT) where specified, then volumetric NDT (UT/RT) per the drawing; do not paint or coat before inspection.

Weld gauges: fillet weld gauge checks leg and throat; undercut gauge and pit gauge quantify depth; a hi-lo gauge measures internal mismatch on pipe.

Distortion correction: mechanical straightening or localized heating must follow approved procedures; never hammer a weld to shape without engineering approval.

Post-weld cleaning: remove slag and spatter, then wire-brush or grind; for stainless steel use dedicated stainless brushes to avoid iron contamination and follow with pickling or passivation if specified.

Heat treatment: post-weld heat treatment relieves residual stress and is specified by procedure; it must be recorded with time-temperature charts and the ramp rate.

Machine shutdown: close the gas valve at the cylinder, purge the regulator, and switch off the power supply; leaving gas pressurized in the line wastes gas and stresses the regulator.

Preventive maintenance schedule: inspect contact tip and nozzle daily, liner and drive rolls weekly or per duty cycle, and replace any component showing wear that affects arc stability.

Documentation set: WPS, PQR, welder qualification record, material certificates, consumable batch certificates, inspection reports, and NDT reports are all traceable to the weld.

Welder performance qualification must be revalidated when the welder changes process, position, or essential variables beyond the qualified ranges, or after a specified lapse in welding.

Daily report content: date, WPS number, joint identification, filler batch, gas type and flow, parameters used, ambient conditions, and inspector sign-off.

Traceability: record the heat number of the base metal and the batch number of the filler so any failure can be traced back to the material lot.