Chapter 4 Section (8) Weld Pass Distortion - Single Pass, Multi-Pass or Small Pass
Weld Pass Number and Distortion Mechanics
A large number of small passes generally causes more distortion than a few large passes because the first pass establishes a hinge point and subsequent passes increase upset areas and transverse shrinkage.
Shortening Effect Exception: When longitudinal distortion is critical, a bead with a smaller cross-section exerts less contraction force against rigid plates and stretches; increasing the number of passes reduces distortion in these scenarios.
Inherent plate rigidity against longitudinal bending depends on plate thickness and Young's modulus; thick plates resist bending, whereas light gauge sheets buckle easily.
Arc Travel Speed and Joint Movement
Faster arc travel speeds reduce the spread of heat ahead of the weld point, minimizing distortion along the free joint edges.
SMAW (Slow Speed, Low Heat Input): Allows heat to conduct ahead of the arc, causing the far ends of the plate edges to contract together; root opening must be maintained using a wedge block.
SAW (Submerged Arc Welding): High heat input and fast travel speed keep plate edges expanding ahead of the arc, pushing the far end open; heavy tack welds or tack-welded metal bars are required to preserve root opening.
Submerged arc welding produces times the heat input at times the travel speed of SMAW.
Heat Distribution and Preheating
Simultaneous and uniform heat application along the entire joint length provides the optimal condition for distortion control.
Uniform preheating across the entire structure reduces cooling thermal gradients and minimizes overall distortion.
Localized preheating placed solely around the weld zone introduces a non-uniform heat source, increasing distortion.