Firepower MST 180i Operating Manual page 68

Firepower MST 180i
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 complex that only a brief outline can be attempted hear.
The Cause of Distortion
Distortion is caused by:
A. Contraction of Weld Metal:
Molten steel shrinks approximately 11 per cent in volume 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 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 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
Figure 4-29: Parent Metal Expansion
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.
BASIC WELDING GUIDE
Upsetting
Weld
Expansion with
compression
Cool
Hot
Art # A-07706_AC
Figure 4-30: Parent Metal Contraction
4-14
Weld
Permanent Upset
Contraction
with tension
Manual 0-5339