Page 208 - CITS - Welder - Trade Theory

WELDER - CITS

The voltage sensitive system operates on the principle of feed back control. The wire is driven by a variable speed
D.C. motor. The arc voltage is monitored by a sensing element, amplified and fed to the armature of the wire feed
motor. Thus, the wire feed rate increases when the arc voltage rises up and vice versa. The arc voltage remains
constant during the process. The system is used for high current density arcs.
The constant wire feed system operates through a constant speed motor, usually an induction motor. No feed
back control is used in this system. This system is used in conjunction with a constant voltage or constant
potential characteristic power source. The arc voltage is constant in this system due to the self adjusting nature of
the constant voltage power source. This system is suitable for wire diameter not exceeding 4mm.
Mounting of The Welding Head : The welding head may be fixed by any of the following three methods:

1 Fixed Type
2 Carriage Mounted Type
3 Boom Mounted Type.
1 Fixed Type: In this set up, the welding head is kept fixed and movement is provided in the jobs to be welded.
A typical application of this arrangement is in the welding of circumferential joints in cylindrical vessels. The
job is mounted on powered roller positioners and the welding speed is maintained by controlling the speed of
rotation of the job.
2 Carriage Mounted Type : In this set up, the welding head is mounted on a movable carriage. The carriage is
mounted on rails and powered by a variable speed electric motor. The main controls such as carriage speed,
current, voltage, etc., are provided on the carriage itself for facilitating ease of operation. Carriage mounted
welding heads are most suitable for butt and fillet welding of plates.
3 Boom Mounted Type : These are the most versatile type of welding heads. Normally the boom is supported
by means of a column. The welding head along with carriage is mounted on to the boom and longitudinal
movement is provided on the carriage. The boom itself is provided with a drive, to move in the transverse
direction. Because of the X and Y movements available to this set up, they find extensive applications in
longitudinal welding of internal and external surfaces of cylinders and in the manufacture of platformed tubes
and shells.
Power Sources: Submerged arc welding can be carried out with both Direct Current and Alternating Current
power sources. Hence, a D.C. generator, a Transformer-Rectifier or a Transformer can be used as the power
supply unit. The main requirement of the submerged arc welding power source is that it should be capable of
supplying heavy current at high duty cycle.
Direct current powers source, with flat characteristics, provides more versatile control over the bead shape,
penetration and speeds; difficult contours can be welded at high speeds. Control of bead shape is found to be
best with direct current reverse polarity (electrode positive), while high deposition rates are obtained with DC
straight polarity (electrode negative) with less penetration. However, Direct Current welding at higher currents
create the phenomenon of arc blow.
Alternating current is usually preferred for high current arcs or when the diameter of the wire is 4 mm or above.
Multiple are welding is possible using AC power source. The bead shape and penetrations with AC arc are
intermediate between DCRP and DCSP.

Flux feeding & recovery system: The flux feeding is from a hopper mounted on the welding head through a tube
into the welding zone. The food is normally through gravity in the automatic machines while pneumatic system
sare used for semi-automatic machines. The conveyor tube is also provided with stop valve to control the flow of
the flux. The conveyer tube is always kept ahead of the contact tube to ensure adequate supply of flux ahead of
the arc.
A pressurised system is used for the recovery of the flux. This system sucks the un melted flux from the weld
puddle for reuse. It is important that the flux recovery be carried out after the weld pool has completely solidified.
Welding variables: The major process variables in the approximate order of importance are:

1 Welding current
2 Welding voltage
3 Welding speed

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CITS : C G & M - Welder - Lesson 61-76