voestalpine Bohler URANOS 2000 SMC Manuel D'instructions page 53

Striking and maintaining the arc
The electric arc is produced by scratching the electrode tip on
the workpiece connected to the earth cable and, once the arc
has been struck, by rapidly withdrawing the electrode to the
normal welding distance.
Generally, to improve the arc striking behaviour a higher initial
current is given in order to heat suddenly the tip of the electrode
and so aid the arc establishing(Hot Start).
Once the arc has been struck, the central part of the electrode
starts melting forming tiny globules which are transferred into
the molten weld pool on the workpiece surface through the
arc stream.
The external coating of the electrode is being consumed and
this supplies the shielding gas for the weld pool, ensuring the
good quality of the weld.
To prevent the molten material globules cause the extinguish-
ing of the arc by short-circuiting and sticking the electrode to
the weld pool, due to their proximity, a temporary increase of
the welding current is given in order to melt the forming short-
circuit (Arc Force).
If the electrode sticks to the workpiece, the short circuit current
should be reduced to the minimum (antisticking).
Carrying out the welding
The welding position varies depending on the number of runs;
the electrode movement is normally carried out with oscillations
and stops at the sides of the bead, in such a way as to avoid an
excessive accumulation of filler metal at the centre.
Removing the slag
Welding using covered electrodes requires the removal of the
slag after each run.
The slag is removed by a small hammer or is brushed away if
friable.
7.2 TIG welding (continuos arc)
The TIG (Tungsten lnert Gas) welding process is based on the
presence of an electric arc struck between a non-consumable
electrode (pure or alloyed tungsten with an approximate melt-
ing temperature of 3370°C) and the work-piece; an inert gas
(argon) atmosphere protects the weld pool.
To avoid dangerous inclusions of tungsten in the joint, the elec-
trode must never come in contact with the workpiece; for this
reason the welding power source is usually equipped with an
arc striking device that generates a high frequency, high voltage
discharge between the tip of the electrode and the workpiece.
Thus, thanks to the electric spark, ionizing the gas atmosphere,
the welding arc is struck without any contact between electrode
and workpiece.
Another type of start is also possible, with reduced tungsten
inclusions: the lift start, which does not require high frequency,
but only an initial short-circuit at low current between the elec-
trode and the workpiece; when the electrode is lifted, the arc
is established and the current increases until reaching the set
welding value.
All manuals and user guides at all-guides.com
To improve the quality of the filling at the end of the welding
bead it is important to control carefully the down slope of the
current and it is necessary that the gas still flows in the welding
pool for some seconds after the arc is extinguished.
Under many operating conditions, it is useful to be able to use
two preset welding currents and to be able to switch easily from
one to the other (BILEVEL).
Welding polarity
D.C.S.P. (Direct Current Straight Polarity)
This is the most used polarity and ensures limited wear of the
electrode (1), since 70% of the heat is concentrated in the
anode (piece).
Narrow and deep weld pools are obtained, with high travel
speeds and low heat supply.
Most materials, except for aluminium (and its alloys) and mag-
nesium, are welded with this polarity.
D.C.R.P. (Direct Current Reverse Polarity)
The reverse polarity is used for welding alloys covered with a
layer of refractory oxide with higher melting temperature com-
pared with metals.
High currents cannot be used, since they would cause excessive
wear on the electrode.
D.C.S.P.-Pulsed (Direct Current Straight Polarity Pulsed)
The use of pulsed direct current allows better control, in particu-
lar operating conditions, of the welding pool width and depth.
The welding pool is formed by the peak pulses (Ip), while the
basic current (Ib) keeps the arc ignited.
This operating mode helps to weld thinner metal sheets with
less deformations, a better form factor and consequently a lower
danger of hot cracks and gas penetration.
Increasing the frequency (MF) the arc becomes narrower, more
concentrated, more stable and the quality of welding on thin
sheets is further increased.
7.2.1 Steel TIG welding
The TIG procedure is very effective for welding both carbon and
alloyed steel, for first runs on pipes and for welding where good
appearance is important.
Straight polarity is required (D.C.S.P .).
53