Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Table 9.2 Narrow-gap submerged arc parameters [191]
Groove width Current Voltage Travel speed Flux height
(mm) (A) (V) (mm min –1 ) (mm)
12 425 27 180 45
14 500 28 250 50
16 550 29 250 50
18 600 31 250 50
20 625 32 230 50
22 625 34 230 50
Multipass per layer technique
The multipass technique requires a larger gap width for a similar wire diameter
(e.g. 18 instead of 10 mm with a 4 mm filler wire and two passes) and the
joint completion rate is consequently lower. The technique does, however,
allow greater control of sidewall fusion, weld metal refinement and improved
access for flux removal and inter-run cleaning. For these reasons, most
commercial applications of the process use the two pass per layer mode with
a single electrode. Typical values of gap width and welding parameters [191]
are given in Table 9.2. For thicknesses above 100 mm a special narrow-gap
torch is required; this is usually rectangular with provision for flux feed and
recovery, seam tracking and height control. A typical torch assembly uses
tactile sensors to control torch height and optoelectronic sensing of the torch
position within the gap. The process is commonly applied to longitudinal
and circumferential joints in large cylindrical components; in both cases, a
heavy column and boom will be required to carry the torch head and, in the
case of circumferential joints, the workpiece must be rotated under the welding
head. For these circumferential applications, it is essential to prevent fluctuation
in the lateral position of the seam as it rotates, and feedback control devices
are often incorporated in the roller bed to sense and correct the position of
the component.
The single filler wires used most commonly have diameters in the range
3.2–4.8 mm; smaller wires are prone to cause random arc wander, particularly
with the relatively long electrical stick-outs that are used, and larger diameter
wires are more difficult to feed. Although it is possible to use standard fluxes,
special flux formulations with improved slag release characteristics have
been devised for narrow-gap SAW. The power used is DC electrode positive
or AC; AC gives greater resistance to magnetic arc blow and square wave
AC power supplies have also been used to improve control of the process.
Developments
Increased productivity and improved process control may be achieved by