Bohler URANOS 1500 RC Instruction Manual page 24

Cause Grease, varnish, rust or dirt on the workpieces to
be welded.
Solution Clean the workpieces accurately before welding.
Cause Grease, varnish, rust or dirt on the filler material.
Solution Always use quality materials and products.
Keep the filler material always in perfect condi-
tions.
Cause Incorrect welding mode.
Solution Carry out the correct sequence of operations for
the type of joint to be welded.
Cause Pieces to be welded have different characteristics.
Solution Carry out buttering before welding.
Cold cracks
Cause Humidity in the weld material.
Solution Always use quality materials and products.
Keep the weld material always in perfect condi-
tions.
Cause Particular geometry of the joint to be welded.
Solution Pre-heat the pieces to be welded.
Carry out post-heating.
Carry out the correct sequence of operations for
the type of joint to be welded.
For any doubts and/or problems do not hesitate to contact
your nearest customer service centre.
7 WELDING THEORY
7.1 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 thick-
ness of the material, the position, the type of joint and the type
of preparation of the piece to be welded.
Electrodes with considerable diameter obviously require very
high currents with consequent high heat supply during the
welding.
Type of coating Property
Rutile Easy to use
Acid High melting speed
Basic High quality of joint
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 an initial cur-
rent spot is given in order to heat suddenly the tip of the elec-
trode 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.
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The external coating of the electrode is being consumed and
this supplies the protective gas for the weld pool, ensuring the
good quality of the weld.
To prevent the molten material globules cause the extinguishing
of the the arc by short-circuiting and sticking the electrode with
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, to the short circuit
current is being reduced to the minimum in order to aid the
electrode unsticking without damaging the electrode holder
(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 material at the centre.
Removing the slag
The 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.
Use
All positions
Flat
All positions