voestalpine Bohler Welding URANOS NX 2700 TLH Mode D'emploi page 36

OPERATING INSTRUCTIONS
Hot cracks
Cause
EN
» Incorrect welding parameters.
» Grease, varnish, rust or dirt on the workpieces to be
welded.
» Grease, varnish, rust or dirt on the filler material.
» Incorrect welding mode.
» Pieces to be welded have different characteristics.
Cold cracks
Cause
» Humidity in the filler metal.
» Particular geometry of the joint to be welded.
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9. OPERATING INSTRUCTIONS
9.1 Manual Metal Arc welding (MMA)
Preparing the edges
To obtain good welding joints it is advisable to work on clean parts, free from oxidations, rust or other contaminating agents.
Choosing the electrode
The diameter of the electrode to be used depends on the thickness of the material, the position, the type of joint and the type of
preparation of the piece to be welded.
Electrodes of large diameter obviously require very high currents with consequent high heat supply during the welding.
Type of coating
Rutile
Acid
Basic
Choosing the welding current
The range of welding current related to the type of electrode used is specified by the manufacturer usually on the electrode
packaging.
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).
Solution
» Decrease the welding current.
» Use a smaller diameter electrode.
» Clean the pieces accurately before welding.
» Always use quality materials and products.
» Keep the filler metal always in perfect condition.
» Carry out the correct sequence of operations for the type of joint to
be welded.
»
Solution
» Always use quality materials and products.
» Keep the filler metal always in perfect condition.
» Pre-heat the workpieces to be welded.
» Carry out post-heating.
» Carry out the correct sequence of operations for the type of joint to
be welded.
Property
Easy to use
High melting speed
High quality of joint
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Use
All positions
Flat
All positions