Miller Multimatic 255 Owner's Manual page 83

Distortion - contraction of weld metal during welding that forces base metal to move.
Illustration: Base metal moves in the direction of the weld bead.
Possible Causes Corrective Actions
Excessive heat input. Use restraint (clamp) to hold base metal in position.
Make tack welds along joint before starting welding operation.
Select lower amperage for electrode.
Increase travel speed.
Weld in small segments and allow cooling between welds.
OM-278174 Page 79