Tweco 161 S Inverter Arc Welder - Manual (Page 38)

THERMAL ARC 161 S OPERATION
Operation 4-12 Manual 0-5199
The metal in the weld area is stretched (plastic deforma-
tion), 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 4-21: Parent metal expansion
Art # A-07706_AB
Weld Permanent Upset
Contraction
with tension
Figure 4-22: Parent metal contraction
4.22 Overcoming Distortion Effects
There are several methods of minimizing distortion ef-
fects.
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 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.
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