Distortion is the unavoidable side effect of pouring concentrated heat into a restrained plate. As the weld and its surroundings cool, they shrink against the cold metal around them, bending the part and shortening it along the joint. This estimator gives planning-level figures for angular and longitudinal distortion so you can compare joint designs and choose countermeasures before striking an arc.
How it works
Distortion scales with the heat poured in and shrinks with thickness. The estimator uses calibrated empirical relations of the form:
angular (deg) ~ C x HI / t^1.3 summed with diminishing per-pass effect
longitudinal (mm/m) ~ 0.9 x HI x passes / t (capped)where HI is heat input per pass in kJ/mm, t is thickness in mm, and C is larger for a single-V butt than a fillet because its weld metal sits more eccentrically across the thickness. Spreading the same fill over more passes reduces each pass’s contribution, which is why the per-pass effect diminishes.
Tips and countermeasures
- Preset: tilt the parts to the opposite angle before welding so shrinkage pulls them flat.
- Backstep: lay short weld increments in the reverse direction of overall progress to cancel longitudinal pull.
- Balance: alternate passes either side of the neutral axis, or use a double-sided prep, so opposing shrinkage offsets.
- Restraint: strongbacks and stiff fixtures hold geometry while welding, though they raise residual stress.
- Less heat: smaller, faster beads with lower heat input shrink less; this is usually the highest-leverage change.
Treat the output as relative guidance for comparing options. A trial weld on a representative assembly is the only way to get a reliable number for production.
Understanding the two types of distortion separately
Angular distortion — the plate rotates about the weld line as it cools, creating a “V” shape when viewed from the end of the joint. This is the dominant distortion mode for fillet welds and single-sided groove welds. It increases with heat input and decreases with plate thickness. Thicker plate resists rotation because the thermal gradient through the section is smaller relative to the stiffness.
Longitudinal shrinkage — the part shortens along the direction of the weld as the deposited metal contracts. On a 1-metre weld, the shrinkage is usually a fraction of a millimetre for steel at typical heat inputs, but on thin sheet metal or long structures it accumulates and can pull joints out of alignment at one end while the other end was clamped.
How multi-pass welding changes the picture
Adding more passes at lower heat input per pass does not necessarily eliminate distortion — it redistributes it. The first pass sets up a temperature gradient and begins to distort the plate. Each subsequent pass adds more heat but into a part that is already partially restrained by the cooling of the previous passes. The estimator models this with a diminishing per-pass contribution: the first few passes create most of the total distortion, and later passes add progressively less.
This is why very thick groove welds (20+ passes) often distort less than the single-pass equivalent would if such a thing were feasible — the part builds up restraint during the fill sequence. Conversely, trying to reduce pass count by using a higher heat input per pass usually increases total distortion even though fewer passes are made.
Typical severity thresholds for structural work
What counts as “acceptable” distortion depends on the application:
- For structural steelwork: tolerances in AWS D1.1 allow angular distortion of roughly 1 in 80 (about 0.7°) over a 150 mm span before straightening is required.
- For pressure vessels: ASME codes set tighter dimensional tolerances on shell roundness and seam alignment.
- For thin-gauge fabrication (under 6 mm): even 1–2° of angular distortion can make assemblies unclosable, so preset and fixturing are essential rather than optional.
Use the severity rating in the estimator as a flag for whether you need to plan countermeasures actively. A “low” severity result may require only a simple clamp; a “high” severity result warrants preset angle calculations and possibly a full fixture design before the first arc is struck.