Weld Joints, Positions, Distortion and Residual Stress Control
What this domain tests
Joint and weld taxonomy, position designations, distortion mechanisms, and the sequence and technique used to control shrinkage and residual stress.
Core rules
- Joints and welds. Five joint types - butt, lap, T, corner, edge. Weld types: groove, fillet, plug, spot, seam.
- Positions. ISO 6947: PA flat, PB horizontal-vertical, PC horizontal, PD horizontal-overhead, PE overhead, PF vertical up, PG vertical down. AWS uses 1G–4G for groove, 1F–4F for fillet, and 5G/6G for pipe fixed positions.
- Distortion types. Longitudinal and transverse shrinkage, angular distortion, bowing, buckling, and twisting (from a misplaced weld sequence).
- Controls. Balanced welding on both sides, backstep and skip sequence, pre-setting the parts in the opposite direction, rigid fixtures and strongbacks, minimum number of passes, low heat input, and keeping the weld near the neutral axis of the section.
- Residual stress. Relieved thermally at 550–650 °C for carbon steel, typically one hour per 25 mm of thickness; mechanically by peening (never on the root or final pass) or vibratory treatment.
Worked example
A 300 mm deep web is fillet welded to a flange, first one side fully then the other; the web leans 4 mm at the top. Cause: each fillet contracts transversely as it cools and pulls the web toward its own side; completing one side first allows unopposed angular distortion. Control: weld both sides alternately in short runs (about 150 mm) with a backstep sequence, and pre-set the web 2–3 mm to the opposite side before welding so shrinkage brings it back to square. Check with a square after each pair of runs, not only at the end.
Traps
Confusing PF (vertical up) with PG (vertical down); assuming peening removes stress everywhere; presetting in the same direction as the expected movement; counting passes without reference to heat input.