Tweco ArcMaster 401S - Manual (Page 45)

ARCMASTER 401S POWER SOURCE
Manual 0-5345 4-7 BASIC WELDING GUIDE
2. Vertical Down
The electrode makes welding in this position particularly easy. Use a electrode at 100 amps. The tip of the
electrode is held in light contact with the work and the speed of downward travel is regulated so that the
tip of the electrode just keeps ahead of the slag. The electrode should point upwards at an angle of about
45º.
3. Overhead Welds
Apart from the rather awkward position necessary, overhead welding is not much more difficult that
downhand welding. Set up a specimen for overhead welding by first tacking a length of angle iron at right
angles to another piece of angle iron or a length of waste pipe. Then tack this to the work bench or hold in
a vice so that the specimen is positioned in the overhead position as shown in the sketch. The electrode
is held at 45º to the horizontal and tilted 10º in the line of travel (Figure 4-18). The tip of the electrode may
be touched lightly on the metal, which helps to give a steady run. A weave technique is not advisable for
overhead fillet welds. Deposit the first run by simply drawing the electrode along at a steady rate. You will
notice that the weld deposit is rather convex, due to the effect of gravity before the metal freezes.
Art # A-07704
Figure 4-18: Overhead Fillet Weld
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 imperial dimensions 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 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