Tweco 186 DC Inverter - Manual (Page 62)

186 AC/DC INVERTER BASIC WELDING
BASIC WELDING GUIDE 4-8 Manual 0-5237
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 5/64" (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_AC
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.
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 4-20 through 4-23 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.
D. Presetting
It is possible in some cases to tell from past
experience or to find by trial and error (or less
frequently, to calculate) how much distortion will
take place in a given welded structure. By correct
pre-setting of the components to be welded,
constructional stresses can be made to pull the
parts into correct alignment. A simple example is
shown in Figure 4-21.