6 BASIC WELDING GUIDE
0463 815 001
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© ESAB AB 2021
Figure 65: Overhead fillet weld
1 45° to plate
3 Angle tacked to a pipe
2 Tilted 10° in line of travel
Distortion
Distortion in some degree is present in all forms of welding. In many cases it is so small that it is
barely perceptible, but in other cases allowance has to be made before welding commences for
the distortion that will subsequently occur. The study of distortion is so complex that only a brief
outline can be attempted here.
The cause of distortion
Distortion is caused by:
•
Contraction of weld metal:
Molten steel shrinks approximately 11% in volume on cooling to room temperature. This
means that a cube of molten metal would contract approximately 2.2% in each of its three
dimensions. In a welded joint, the metal becomes attached to the side of the joint and
cannot contract freely. Therefore, cooling causes the weld metal to flow plastically, that is,
the weld itself has to stretch if it is to overcome the effect of shrinking volume and still be
attached to the edge of the joint. If the restraint is too strong, for example in a heavy
section of a plate, the weld metal may crack. Even in cases where the weld metal does not
crack, stresses will remain "Locked up" in the structure. If the joint material is relatively
weak, for example, a butt joint in 5/64 in. (2.0 mm) sheet, the contracting weld metal may
cause the sheet to become distorted.
•
Expansion and contraction of parent metal in the fusion zone:
While welding is proceeding, a relatively small volume of the adjacent plate material is
heated to a very high temperature and attempts to expand in all directions. It is able to do
this freely at right angles to the surface of the plate (that is "through the weld", but when it
attempts to expand "across the weld" or "along the weld", it meets considerable resistance.
To enable expansion, it has to deform plastically, that is, the metal adjacent to the weld is
at a high temperature and rather soft, and by expanding, pushes the cooler, harder metal
further and tends to bulge (or it is "upset"). When the weld area begins to cool, the "upset"
metal attempts to contract as much as it expanded, but, because it has been "upset" it
does not resume its former shape, and the contraction of the new shape exerts a strong
pull on adjacent metal.
The metal in the weld area is stretched (plastic deformation), the workpiece may be pulled
out of shape by the powerful contraction stresses (distortion), or the weld may crack, in any
case "locked up" stresses will remain in the workpiece. Figures below illustrate how
distortion is created.
