Tweco 175 SE Arc Master - Manuel (Page 35)

OpERATION ARC MASTER 175 SE
Manual 0-5054 4-9 OPERATION
Art # A-07706_AC
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% in volume on
cooling to room temperature. This means that a cube
of molten metal would contract approximately 2.2%
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 excessive, e.g. a heavy section of
plate, then 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 5/64" (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