voestalpine Bohler Welding URANOS 2000 SMC Mode D'emploi page 39

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 stop droplets of molten material short-circuiting the electrode to the weld pool and extinguishing the arc if the electrode
accidentally approaches the pool too closely, it is useful to increase welding current (Arc Force) temporarily in order to break a
short circuit.
If the electrode sticks to the workpiece, the short circuit current should be reduced to the minimum (antisticking).
Removing the slag
The slag is removed by a small hammer or is brushed away if friable.
9.2 TIG welding (continuos arc)
Description
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 melting temperature of 3370 °C) and the workpiece. An inert gas
(argon) atmosphere protects the weld pool.
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.
arc is established and the current increases to the set welding value.
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
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.
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 compared with metals.
High currents cannot be used, since they would cause excessive wear on the electrode.
Carrying out the welding
Electrode angle depends on the number of runs. The
electrode should normally be moved in a weaving
motion with pauses at the sides of the bead to avoid
an excessive accumulation of filler material at the
centre.
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OPERATING INSTRUCTIONS
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