Joint Preparation, Fit-Up and Assembly
Geometry that governs soundness
Five basic joint types: butt, lap, T, corner and edge. Groove shapes include square, single or double V, bevel, U and J. For a typical 10 mm V-groove butt joint: included angle 60-75 degrees, root face 1.5-2 mm, root gap 2-3 mm. An angle that is too narrow prevents access to the root; too wide increases distortion and fill volume.
Fit-up tolerances
Misalignment is normally limited to about 10% of the thinner plate, subject to a code cap (often 3 mm). Check root gap around the whole joint with a gap gauge, not just at the start. Excess gap causes burn-through and excess reinforcement; an under-size gap causes lack of penetration.
Tacks
Tack length is typically about three times plate thickness - roughly 10-15 mm on 10 mm plate - with sound, tapered ends. Tacks that will be consumed by the root pass must be clean, crack-free and fully fused; otherwise remove them. Cracked tacks are a rejection risk under most structural codes.
Distortion control
Use balanced sequences: backstep, skip welding, alternating sides, welding from the centre outward. Add preset, restraint, chill blocks or a copper backing where the WPS allows. Never run one long continuous bead on a large assembly without a sequence plan.
Worked example
A 3000 mm long 8 mm fillet on stiffened plate. Split it into 300 mm backstep increments, alternate sides of the stiffener, and weld away from the centre. This balances shrinkage and keeps angular distortion within tolerance without changing the WPS.
Traps
- Widening a gap to make fit-up easier - that changes the joint and may breach the WPS.
- Leaving arc strikes outside the joint; most codes treat them as stress raisers.
- Failing to clean and feather tacks before the root pass.