43 Multipass
welding is often employed with the submerged arc process. When plate thicknesses
exceed the limitation
of two pass techniques, or where inability to provide accurate joint fit-up prevents
the use of high current
- multiple pass submerged arc welding should be used. Where
possible a split pass procedure as shown in Figure 39 should be used to allow
easy flux removal and to
prevent weld cracking. Each weld pass should be slightly convex as shown to assist
in slag removal and preventing
weld cracking. Multipass
welding procedures also enable a variety of weld joints and plate thicknesses
to be welded with the
same procedures and materials. In certain base materials, the multiple pass welding
technique must be used
to maintain adequate properties in the base HAZ. An
example of the use of the multiple pass technique for joints which present difficult
alignment problems is in
Figure 40. For pressure vessel circumferential welds such as head-to-shell and
shell butts, this double bevel
plate preparation with semiautomatic or automatic Mig used to handle varying fit-up
in the root area is an
excellent combination procedure. Fill passes are then welded with submerged arc
to provide consistent quality
low cost welds. The gas metal-arc welding process is the best choice for manual
or automatic root or first
pass procedures. The resulting weld metal is free from internal slag and external
slag is minimal making the
subsequent submerged arc welds free from defects. For
welding plates above two inches thick, multipass procedures must be used. Figures
41 and 42 provide examples
of joint preparation and both single and two wire welding conditions for thick
plates. Welding
flux and wire must be selected with multipass procedures in mind. Control of weld
chemistry is especially
important to insure crack free deposits. Multipass
Welding
