HD Manual Arc Welders
25
©2000 CIGWELD All rights reserved
run by simply drawing the electrode along at a steady rate. You will notice that
the weld deposit is rather convex, due to the effect of gravity before the metal
freezes.
12. 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 hear.
12.1 The Cause of Distortion
Distortion is cause by:
a) Contraction of Weld Metal:
Molten steel shrinks approximately 11 per cent in volume on cooling to room temperature.
This means that a cube of molten metal would contract approximately 2.2 per cent 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 very great, as, for
example, in a heavy section of plate, the weld metal may crack. Even in cases where the
weld metal does not crack, there will still remain stresses “locked-up” in the structure. If
the joint material is relatively weak, for example, a butt joint in 2.0mm sheet, the
contracting weld metal may cause the sheet to become distorted.
b) 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 (i.e., “through the weld”), but when it
attempts to expand “across the weld” or “along the weld”, it meets considerable resistance,
and to fulfil the desire for continued expansion, it has to deform plastically, that is, the
metal adjacent to the weld is at a high temperature and hence rather soft, and, by
expanding, pushes against the cooler, harder metal further away, and tends to bulge (or 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. Several things can
then happen. The metal in the weld area is stretched (plastic deformation), the job may be
pulled out of shape by the powerful contraction stresses (distortion), or the weld may
crack, in any case, there will remain “locked-up” stresses in the job. Figure 19 and Figure
20 illustrate how distortion is created.
Figure 19 - Parent metal expansion
Figure 20- Parent metal contraction
