Preparation of Materials, Joint Configuration and Fit-up
This domain covers what happens before the arc starts: cleaning, bevelling, joint geometry and tack-up. It is the highest-leverage domain for defect prevention, and exam questions often present a fit-up that is out of tolerance and ask what to do.
Surface preparation
Clean at least 25 mm each side of the joint - mill scale, rust, oil, paint, moisture and zinc coating all cause porosity and lack of fusion. Thermally cut bevels carry an oxide layer that must be ground back to bright metal. Galvanized or primed plate needs the coating removed and extraction running, because zinc fume causes metal fume fever.
Joint geometry
A single-V butt in plate is typically 60° included (30° per side) with a 1.5–3 mm root face and a root gap set by the WPS. Pipe work commonly uses a 37.5° bevel, giving 75° included. Too tight a gap means incomplete root fusion; too wide a gap invites burn-through and shrink.
Fit-up and tacks
Tack welds use the same classification as the production weld, are at least as thick as the root pass, and must be sound - a cracked tack will propagate. Alignment is checked with a hi-lo gauge on pipe and a straight edge plus taper gauge on plate. Control distortion by balanced sequence, backstep, presetting and correct restraint.
Worked example
A 12 mm plate single-V joint is specified at 60° included, 2 mm root face, 2 mm root gap. The fitter opens the gap to 4 mm so the welder can "get penetration." Answer: reject the fit-up. The 4 mm gap with a 2 mm land increases burn-through risk and changes the weld cross-section and heat input; re-fit within tolerance or raise a concession, never compensate by altering the welding parameters.
Traps that cost marks
Welding over oil or paint. Leaving cracked tacks in the root. Confusing per-side bevel angle with included angle. Fitting tight at one end and open at the other.