Tweco 95 S Thermal Arc(Jan2013) - Manual (Page 35)

OPERATION THERMAL ARC 95 S
Operation 4-9 Manual 0-5086
Art # A-07706_AB
Weld Permanent Upset
Contraction
with tension
Figure 4-21: Parent metal contraction
4.21 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 4-25
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.
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 4-22.
4.20 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 his 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_AB
Hot Hot
Weld Upsetting
Expansion with
compression
Cool
Figure 4-20: Parent metal expansion