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
General
Description
Joint
Design &
Preparation
Making the
Weld
Operating
Hints &
Procedures
Current, Voltage
and Speed
Granular
Joint Cleaning
Fit-Up of Joints
Material
Welding Wire
and Current
Current Circuit
Fillet Welding
Circumferential
Seams
Weld Faults
Index
Weld Costs
Conditions
Tables
Multipass
Welding