How to estimate welding shrinkage and distortion
A weld contracts as it cools, so the finished member ends up shorter and narrower than the layout. This calculator uses Blodgett's empirical transverse formula, 0.10 times the weld area divided by plate thickness plus 0.05 times the root opening, together with Okerblom's longitudinal relation based on heat input and member cross-sectional area. A 0.39 in² weld on 3/4 in plate with a 1/8 in root opening shrinks roughly 0.058 in across the joint.
Both expressions are empirical, so the numbers are estimates. Restraint, welding sequence, pass count and fixturing can push the real movement from half to double the calculated value, so weld and measure a test joint before committing a shrinkage allowance on critical members.
The most reliable way to cut distortion is to deposit less weld metal: tighten the groove angle and root opening within the qualified range of the procedure. Balanced welding, presetting the parts in the opposite direction, back-step sequencing and rigid fixtures all reduce movement as well. Angular distortion varies too widely with joint geometry and restraint to reduce to a single number, so verify it on a mock-up and have a welding engineer review the fabrication sequence.
Frequently Asked Questions
Transverse shrinkage uses Blodgett's empirical expression, 0.10 times weld area divided by thickness, plus 0.05 times the root opening. Longitudinal shrinkage uses Okerblom's relation, strain equals 0.335 times the thermal expansion coefficient times heat input per unit length, divided by specific heat times density times member area. Both are empirical, so results are estimates.
Restraint, welding sequence, number of passes and fixturing can move actual shrinkage from half to double the estimate. For critical members, weld a test joint and measure the movement before locking in a shrinkage allowance.
Reducing the weld metal volume itself works best: tighten the included angle and root opening within the qualified range. Balanced or alternating welds, presetting, back-step sequencing and rigid fixturing all help as well.