CORRECTING DISTORTION
If the frame is warped, it must be cut out and
replaced.
Distortion (buckling or twisting) of structural steel
members is caused by uneven expansion and
If removing the frame for a watertight door, do
contraction of the metal during welding. Distortion can
not cut the bulkhead. Watertight doors are
be corrected by flame shrinkage (also known as flame
welded in place using a lap joint. Only the weld
straightening). Flame shrinkage is accomplished by
metal should be removed.
directing a flame from a gas torch on an area where
excess metal has accumulated as a result of earlier
Weld repair any gouges or nicks in the
welding. The metal is displaced in the desired direction
bulkhead plating before reinstalling the door.
and the distortion is corrected or reduced. The operator
of the torch must be able to recognize the areas of excess
When fitting the new frame and door in place,
metal and determine the necessary amount of heat to
do not force the frame to the bulkhead. Instead,
apply.
straighten the bulkhead. If the door is forced
Figure 13-56 shows how flame shrinkage is used to
to the bulkhead, you will warp the frame.
control expansion and contraction. In view A, the flame
is directed toward a spot that is centrally located on a
When welding in the new watertight fixture,
distorted steel plate. The spot heats up quickly and must
ensure that the door is dogged tightly to the
expand. The cooler plate surrounding the distorted area
frame to prevent warping the door during
prevents the spot from expanding along the plane of the
welding.
plate, as shown in view B. This, in turn, will cause it to
expand abnormally through its thickness, as shown in
view C. Up to this point, the plate has actually
Perform a chalk test on the knife edge to the
thickened where the heat is applied. Upon cooling, the
seal after the watertight door is installed
plate will contract uniformly in all directions, as shown
according to the PMS card to ensure that the
in view D. When flame shrinkage is applied carefully,
door was not warped during installation.
it will give a planned shrinkage that is useful for
correcting distortion that was caused by previous
heating and cooling cycles.
STOPPING CRACKS IN PLATE
A high temperature is not required for flame
shrinkage; however, a large torch tip should be used.
In an emergency, you may stop a crack by drilling
The effectiveness of this process depends on your
a hole that is 1/2T (where T is equal to the thickness of
ability to rapidly change the temperature of the plate to
the plate) or 3/8 inch in diameter, whichever is the
a high degree at a given spot. A large torch will help to
largest, and welding an overlay, as shown in figure
attain the required rapid temperature change. When a
13-55, to arrest the crack. Drill the hole in the end of
long piece of metal, such as the flange of a beam, needs
the crack, making sure that none of the crack extends
to be flame shrunk, one of two methods should be used.
beyond the drilled hole. An overlay of one layer is
You should either (1) move the torch progressively
applied in the form of a trapezoid. This overlay is not
along the length, or (2) heat the metal at selected spots
to be any closer than one-half inch to the hole. No
and then let it cool. You should make intermediate
welding should be done on the crack nor within l/2 inch
checks of the effect of the flame on the correction of the
of the hole. The long dimension should be about 15
distortion as you go through this process.
inches, with the deposit tapering away from the drilled
hole to 10 inches at its 3 inch width. This will require
Distortion Problems
ten to twelve passes. If possible, an overlay should be
applied on both sides of the plate. A MIL-309 or
310-15/16 stainless steel 3/16-inch electrode is
Figure 13-57 shows a panel that has buckled due to
recommended. Care should be taken to fill craters and
welding stiffeners to it. There is an excess of metal
minimize arc strikes to prevent stress concentrations.
between the stiffeners. If you apply the necessary heat
After this temporary overlay has been accomplished,
in the proper areas, and allow it to contract upon
actual repair of cracks may be accomplished according
cooling, you provide the shrinkage needed to straighten
to the requirements of the applicable fabrication
the panel. If the panel is under restraint (as is the case
document.
with many weldments), too much heating could cause
13-49
