Tweco 201 ts Thermal Arc - Manual (Page 37)

OPERATION THERMAL ARC 201 TS
Manual 0-5149 4-11 Operation
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 at-
tempts 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 tempera-
ture 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 defor-
mation), 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
Art # A-07704
Figure 4-20: Overhead fillet weld
4.20 Distortion
Distortion in some degree is present in all forms of
welding. In many cases it is so small that it is barely
perceptible, but in other cases allowance has to be made
before welding commences for the distortion that will
subsequently occur. The study of distortion is so com-
plex that only a brief outline can be attempted hear.
4.21 The Cause of Distortion
Distortion is cause by:
A. Contraction of Weld Metal:
Molten steel shrinks approximately 11 per cent in vol-
ume 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 (5/64") sheet, the contracting weld metal may
cause the sheet to become distorted.