Tweco 181i Thermal Arc Fabricator - Manual (Page 74)

Fabricator 181i
BASIC WELDING GUIDE 4-16 Manual 0-5191
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 fulfill 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 4-29 and 4- 30 illustrate how distortion is created.
Art # A-07705_AB
Hot Hot
Weld Upsetting
Expansion with
compression
Cool
Figure 4-29: Parent Metal Expansion
Art # A-07706_AC
Weld Permanent Upset
Contraction
with tension
Figure 4-30: 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 4-30 through 4-33 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 4-31.
E. Preheating
Suitable preheating of parts of the structure other than the area to be welded can be sometimes used to reduce
distortion. Figure 4-32 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.