Tweco 130i Transarc Operating Manual - Manual (Page 44)

TRANSARC 130i
BASIC WELDING GUIDE 4-8 Manual 0-5283
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.
The Cause of Distortion
Distortion is caused 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. Figures 4-19 and 4- 20 illustrate how distortion is created.
Art # A-07705_AB
Hot Hot
Weld Upsetting
Expansion with
compression
Cool
Figure 4-19: Parent Metal Expansion
Art # A-07706_AB
Weld Permanent Upset
Contraction
with tension
Figure 4-20: Parent Metal Contraction
Overcoming Distortion Effects
There are several methods of minimizing distortion effects.
A. Peening
This is done by hammering the weld while it is still hot. The weld metal is flattened slightly and because of
this the tensile stresses are reduced a little. The effect of peening is relatively shallow, and is not advisable
on the last layer.