
14
PLASMA CUTTING FUNDAMENTALS
The Plasma arc process has always been seen as an alternative to
the oxy-fuel process. The plasma arc cutting process is
illustrated below. The basic principle is that the arc formed
between the electrode and the workpiece is constricted by a ne
bore, copper nozzle. This increases the temperature and velocity
of the plasma emanating from the nozzle. The temperature of the
plasma is in excess of 20 000°C and the velocity can approach
the speed of sound. When used for cutting, the plasma gas ow is
increased so that the deeply penetrating plasma jet cuts through the
material and molten material is removed in the efux plasma.
The process differs from the oxy-fuel process in that the plasma
process operates by using the arc to melt the metal whereas in the
oxy-fuel process, the oxygen oxidises the metal and the heat from the exothermic reaction melts the metal.
Thus, unlike the oxy-fuel process, the plasma process can be applied to cutting metals which form refractory
oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.
POWER SOURCE
The power source required for the plasma arc process must have a drooping characteristic and a high
voltage. Although the operating voltage to sustain the plasma is typically 50 to 60V, the open circuit voltage
needed to initiate the arc can be up to 400V DC.
On initiation, the pilot arc is formed within the body of the torch between the electrode and the nozzle.
For cutting, the arc must be transferred to the workpiece in the so-called ‘transferred’ arc mode.
The electrode has a negative polarity and the workpiece a positive polarity so that the majority of the arc
energy (approximately two thirds) is used for cutting.
GAS COMPOSITION
In the conventional system using a tungsten electrode, the plasma is inert, formed using argon, argon-H2
or nitrogen. However oxidising gases, such as air or oxygen, can be used but the electrode must be copper
with hafnium. The plasma gas ow is critical and must be set according to the current level and the nozzle
bore diameter. If the gas ow is too low for the current level, or the current level too high for the nozzle bore
diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to workpiece.
The effect of ‘double arcing’ is usually catastrophic with the nozzle melting.
CUT QUALITY
The quality of the plasma cut edge is similar to that achieved with the oxy-fuel process. However, as the
plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of
the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. As these
limitations are associated with the degree of constriction of the arc, several torch designs are available to
improve arc constriction to produce more uniform heating at the top and bottom of the cut.
Plasma is commonly known as the 4th matter of state the rst being solid, then liquid, gas and then plasma.
each matter changes from one state to another through the introduction of heat