Tweco 161 S UK - Manual (Page 36)

THERMAL ARC 161 S OPERATION
Operation 4-12 4-12 Manual 0-5183
4.22 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 se-
quence 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 ef-
fective 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 experi-
ence 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.
E. Preheating
Suitable preheating of parts of the structure other than
the area to be welded can be sometimes used to reduce
distortion. Figure 4-23 shows a simple application. By
removing the heating source from b and c as soon as
welding is completed, the sections b and c will contract
at a similar rate, thus reducing distortion.
Art # A-07707
Figure 4-23: Principle of presetting
Art # A-07708
B
Preheat Preheat
Dotted lines show effect if no preheat is used
Weld
C
Figure 4-24: Reduction of distortion by preheating
Art # A-07709
Figure 4-25: Examples of distortion
Art # A-07710
Figure 4-26: Welding sequence
Art # A-07711
Figure 4-27: Step back sequence