Page 108 - Ship Construction.DJ Eyres 6Ed

Ch10-H8070.fm Page 97 Wednesday, October 18, 2006 7:37 AM

10
Welding Practice and Testing Welds

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The strongest welded joint which may be produced in two plates sub-
sequently subjected to a tensile pull is the butt joint. A butt joint is one where
the two joined plates are in the same plane, and in any welded structure it is
desirable that butt joints should be used wherever possible.
In mild steel the weld metal tends to have a higher yield strength than
the plate material (see Figure 10.1). Under tension it is found that initial
yielding usually occurs adjacent to a butt weld in the plate when the yield
strength of the plate material is reached locally. Since a good butt weld in
tension has a strength equivalent to that of the mild steel plate it is not con-
sidered as a line of structural weakness.
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Lapped joints, where fillet welds are used to connect the plates, should
be avoided in strength members of a welded structure. As the fillet welds
are in shear when the plates are in tension the strength of the joint is very
much less than that of the plate material or butt joint. Fillet welds are
unavoidable where sections or plates are connected at an angle to an adjacent
plate, but often there is not the same problem as the loading is different.
The fatigue strength of fillet welds is also inferior to that of a butt weld.

Welding Practice

In making a butt weld with manual arc welding, where the plate thickness
exceeds say 5 to 6 mm it will become necessary to make more than one
welding pass to deposit sufficient weld metal to close the joint. With the higher
current automatic welding processes thicker plates may be welded with a
single pass, but at greater thicknesses multi-pass welds become necessary.
In ship work unless a permanent backing bar is used, or a ‘one sided’
welding technique or process (see Chapter 9) is used, a back run of weld is
required to ensure complete weld penetration. This is made on the reverse
side of the joint after cleaning out the slag, etc., by chipping or gouging. Per-
manent backing bars may conveniently be introduced where it is desired to
weld from one side only during erection at the berth. A good example is the
use of a cut-down channel bar used as a deck beam, the upper flange pro-
viding the backing bar for a deck panel butt weld, made by machine above.