voestalpine Bohler Welding URANOS NX 2200 SMC Classic Mode D'emploi page 38

OPERATING INSTRUCTIONS
10.2 TIG welding (continuos arc)
Description
EN
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.
To avoid dangerous inclusions of tungsten in the joint, the electrode 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.
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
Characteristics of TIG welds
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.).
Preparing the edges
Careful cleaning and preparation of the edges are required.
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.
D.C.S.P.-Pulsed (Direct Current Straight Polarity Pulsed)
welding pool width and depth.
The welding pool is formed by the peak pulses (Ip), while the base current (Ib) keeps the arc
and consequently a reduced risk of hot cracks and gas inclusion.
Increasing the frequency (MF) the arc becomes narrower, more concentrated, more stable and
the quality of welding on thin sheets is further increased.
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