Tweco 170 HF Weldskill - Manual (Page 33)

WELDSKILL 170 HF INVERTER
4-11 April 23, 2008
4.20 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.
4.21 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. Figures 4-20 and 4- 21
illustrate how distortion is created.
Art # A-07705
Figure 4-20: Parent metal expansion
Art # A-07706
Figure 4-21: Parent metal contraction
4.22 Overcoming Distortion Effects
There are several methods of minimising 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.
B. Distribution of Stresses
Distortion may be reduced by selecting a welding
sequence which will distribute the stresses suitably
so that they tend to cancel each other out. See Figures
4-25 through 4-28 for various weld sequences. Choice
of a suitable weld sequence is probably the most
effective method of overcoming distortion, although
an unsuitable sequence may exaggerate it.
Simultaneous welding of both sides of a joint by two
welders is often successful in eliminating distortion.
C. Restraint of Parts
Forcible restraint of the components being welded is
often used to prevent distortion. Jigs, positions, and
tack welds are methods employed with this in view.