Tweco 175 TE Arcmaster - Manual (Page 39)

oPeration arc master 175 te
Manual 0- 5116 4-11 Operation
4.21 Distortion
Distortion in some degree is present in all forms of welding. In many cases it is so small that it is barely per-
ceptible, 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 here.
4.22 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
Art # A-07706_AC
Weld Permanent Upset
Contraction
with tension
Figure 4-21: Parent metal contraction
4.23 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.