Chapter 4 Section (5) Distortion: Mechanisms and Control
Causes of Distortion and Residual Stresses
Distortion results from unbalanced residual stresses induced during primary production (such as cold rolling or casting) and fabrication operations (such as forming, cutting, and drilling machining).
Welding generates distortion through uneven heating, rapid cooling, and restricted expansion in localized zones.
Localized thermal expansion is restrained by cooler adjacent material with higher yield strength, causing plastic flow that becomes permanent deformation upon cooling.
Thermal Cutting and Stress Balancing
Flame cutting using a single torch heats only one edge, making bowing inevitable after cooling.
Cutting simultaneously with two torches balances residual stresses along the edges and equalizes them across the plate's neutral axis or center line, maintaining straightness.
Welding Thermal Gradients and Primary Distortion Modes
A moving welding arc creates a steep thermal gradient, with temperatures varying by up to within a few millimeters.
The magnitude of the temperature drop is proportional to heat input and travel speed.
Basic forms of welding distortion include:
Transverse shrinkage and longitudinal shrinkage.
Angular distortion (caused by transverse shrinkage).
Bending distortion (caused by longitudinal shrinkage).
Buckling (caused primarily by longitudinal shrinkage and secondarily by transverse shrinkage, occurring predominantly in large thin plates and sheets).
Distortion frequently appears in compounded forms, combining bending, twisting, and angular modes.
Joint Contraction and Restraint Mechanics
In unrestrained V-groove butt joints, angular distortion occurs because transverse contraction is non-uniform due to the weld face being wider than the root.
Depositing a uniform weld cross-section eliminates angular distortion in theory, producing only uniform transverse contraction across the joint.
Surrounding cold metal restrains localized longitudinal expansion, forcing the heated zone to thicken or upset at the plate edge.
Upon cooling, contraction shortens the edge and causes concave bending (shortening along the weld side both transversely and longitudinally).
Restraints from massive components, rigid clamps, or tack welds restrict movement and significantly elevate internal stresses between the heated zone, weld metal, and base plate.