Selco Saber 70 chp Manuel D'instructions page 46

Cause The weld pool solidifies too quickly.
Solution Decrease the travel speed while welding/cutting.
Pre-heat the workpieces to be welded/cut.
Increase the welding/cutting current.
Hot cracks
Cause Incorrect welding/cutting parameters.
Solution Decrease the welding/cutting voltage.
Use a smaller diameter electrode.
Cause Grease, varnish, rust or dirt on the workpieces to
be welded/cut.
Solution Clean the workpieces carefully before welding/cut-
ting.
Cause Grease, varnish, rust or dirt on the filler metal.
Solution Always use quality materials and products.
Keep the filler metal always in perfect condition.
Cause Incorrect welding/cutting mode.
Solution Carry out the correct sequence of operations for
the type of joint to be welded/cut.
Cause Pieces to be welded/cut have different characteris-
tics.
Solution Carry out buttering before welding.
Cold cracks
Cause Humidity in the filler metal.
Solution Always use quality materials and products.
Keep the filler metal always in perfect condition.
Cause Particular geometry of the joint to be welded/cut.
Solution Pre-heat the pieces to be welded/cut.
Carry out post-heating.
Carry out the correct sequence of operations for
the type of joint to be welded/cut.
Excessive dross
Cause Insufficient air pressure.
Solution Adjust the air flow.
Read the paragraph "Installation ".
Cause Incorrect welding/cutting mode.
Solution Increase the travel speed while welding/cutting.
Cause Worn nozzle and/or electrode.
Solution Replace the faulty component.
The nozzle overheats
Cause Insufficient air pressure.
Solution Adjust the air flow.
Read the paragraph "Installation".
Cause Worn nozzle and/or electrode.
Solution Replace the faulty component.
For any doubts and/or problems do not hesitate to contact
your nearest customer service centre.
46
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7 PLASMA CUTTING THEORY
A gas becomes a plasma when it is brought to an extremely
high temperature and ionizes wholly or partly, thus becoming
electrically conductive.
Although the plasma exists in every electric arc, by the term
"plasma arc" we refer specifically to a torch for welding/cutting
that uses an electric arc passing through the constricting neck of
a suitable nozzle in order to heat a gas coming out of this same
nozzle, so as to bring it to the plasma state.
7.1 Plasma cutting process
The cutting action is performed when the plasma arc, made
very hot and highly concentrated by the design of the torch,
transfers onto the conductive piece to be cut, closing the elec-
trical circuit of the power source. The material is melted at the
high temperature of the arc, and then removed by the high
pressure flow of the ionized gas from the nozzle.
The arc can have two different states: the transferred arc,
when the current passes through the workpiece to be cut, and
the pilot arc or non-transferred arc, when this is established
between the electrode and the nozzle.
Manual plasma cutting unit