Experimental study of effect of gas nature on plasma arc cutting of mild
steel.
Tetyana Kavka1, Alan Maslani1, Milan Hrabovsky1, Thomas Stehrer2 and Heribert Pauser2
1
Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 3, Prague 8, 18200, Czech Republic
2
Fronius International GmbH, 4600 Wels-Thalheim, Austria
Abstract: The present paper studies influence of the gas nature on the arc
behavior and cutting performance of mild steel. A plasma cutting system, usually
operated on steam originating from a water-ethanol mixture, has been modified
to enable usage of different plasma gases. Experimental results obtained during
cutting of 16 mm thick mild steel plates at 60 A with steam, air, nitrogen and
oxygen plasma gases are presented. The results discuss effect of the plasma gas
nature and flow rate on arc characteristics, melting rate of the material, energy
balance of the cutting process and kerf shape.
Keywords: transferred arc, plasma arc cutting
1. Introduction
Utilization of a transfer arc for cutting of metallic
materials is a rather old and well-established
technology. An electric arc is established between a
cathode inside the plasma torch and a piece of
material to be cut. The narrow nozzle constricts the
arc forming a narrow and hot plasma jet, which
results in fast material melting and removal. The arc
is stabilized by the flow of a plasma forming gas.
Originally, non-oxidizing or inert gases and their
mixtures were used as plasma medium. Nowadays
the preferences changed towards application of air or
oxygen as a plasma gas due to reaction of oxygen
with cut material, which improve the cut quality and
reduce energy consumption [1]. In R&D activities
the general attention has been paid to phenomena
taking place during arc cutting by conventional gas
torches, e.g. electrode phenomena [2], pilot arcing
[3], kerf [4] and dross [5] formation. However,
despite wide application in industry, there are still
big gaps in understanding the physical mechanisms,
which govern the process and determine the plasma
– material interaction and how the gas nature affects
them. Especially if liquid is utilized as a plasma
medium.
The paper provides some experimental results of
effect of plasma medium on properties of arc and cut
performance during cutting of mild steel plates. The
plasma cutting system usually operated on steam has
been modified to enable usage of different plasma
gases. The results discuss effect of the plasma gas
nature and flow rate on arc characteristics, melting
rate of the material, energy balance of the cutting
process and kerf shape.
2. Experimental setup
The experiments were conducted on the plasma
cutting system Fronius TransCut 300 (Fronius Int.
GmbH, Austria). The torch is usually working on a
steam originating from a mixture of water with
alcohol provided by the producer. The schematic
diagram of the torch is shown in Fig. 1. The liquid is
pumped up into the torch, is heated and evaporated
by the heating element. Steam is flowing under
pressure along the cathode towards the nozzle,
where it is further heated and ionized consuming
energy coming from the arc. The standard TransCut
300 system was modified in such a way to enable
usage of different plasma gases and apply arc current
of 60 A without shielding gas. The nozzle has a bore
diameter of 1 mm. Several gases commonly used in
commercial cutting torches have been applied with
different flow rates to examine their effect on arc
behavior and cutting performance. The advantage of
the present setup is a possibility to change plasma
medium keeping the arc chamber geometry constant
and thus examine an effect of the gas nature on
processes taking place during arc cutting. A
required flow rate of N2, O2 and air was supplied as
between 6 and 20 g/min. The experiments confirmed
strong linking between the plasma gas flow rate and
pressure in the arc chamber: the higher the flow rate,
the higher the chamber pressure. The chamber
pressure almost did not depend on the type of the
plasma gas and varied between 4 and 7 bars
depending on the gas flow rate. Fig. 2 shows
dependence of the nozzle voltage on plasma gas
flow rate. The voltage increased with the gas flow
rate for all studied gases. The voltage increase can
be connected to two phenomena: more energy is
required to heat up higher amount of the supplied
gas as well as the fact that the arc is more
constricted. The voltage level was similar for gases,
while it was much higher for liquid, which is
connected with big difference in enthalpy and
thermal conductivity of media, which are much
higher for steam. The total voltage curves showed
the same tendency besides for oxygen, which was
lower than for both N2 and air.
90
80
Voltage , V
plasma forming gas. The flow rate of liquid could
not be set directly and was varied by modification of
water pressure in the water line of the torch. Total
arc voltage, nozzle potential, gas mass flow rate and
pressure in the arc chamber were monitored and
controlled. The plasma jet images were captured by
the fast shutter CCD camera Sensicam. The
temperature profiles at the torch exit were measured
with a Jobin-Yvon imaging spectrograph Triax 550.
The measured line-of-sight spectral intensities were
converted to the local spectrum emission coefficients
by means of an Abel inversion. Temperature was
calculated using Saha equation. In the case of
nitrogen plasma atomic and ionic nitrogen lines were
analyzed, while for oxygen containing plasma
oxygen lines were considered. Electron density for
steam plasma was determined from Stark broadening
of Hβ line, while for gases was calculated from Stark
broadening of nitrogen and oxygen ionic lines.
The experiments have been conducted for two
configurations: on a water-cooled rotating disc and
normal cutting conditions. Tests were performed on
16 mm thick mild steel plates (S235JR). The torch
was at the same position, while the material was
attached to the moving system. The following
cutting parameters have been applied: cutting speed
– 30 cm/min, stand-off distance – 2 mm. After the
cutting tests the metal piece was cut at some distance
from the edge by a saw to enable analyses of the kerf
geometry and cut surface.
70
60
steam
50
N2
O2
40
Air
30
0
5
10
15
gas flow rate, g/min
20
25
Figure 2. Voltage drop on a stabilized part of the arc for
different plasma gases as a function of gas flow rate.
The images of the arc show an under-expanded
supersonic flow for all studied conditions (Fig. 3)).
The high brightness zones at the exit of the torch
show that the plasma core does not fill completely
the nozzle, which confirms existence of a two-zone
Figure 1. Schematic diagram of TransCut 300 torch.
3. Arc plasma properties
In the present experiments three molecular gases
(N2, O2 and air) as well as Fronius liquid were used
as plasma medium. The gas flow rate was varied
Figure 3. Imaging of arcs for different gases at 60 A arc current
and 8 g/min plasma gas flow rate (images related to the highest
intensity level of the individual picture, exposure – 10 µs, gray
filter, the bottom edge corresponds to the workpiece position).
flow: the highly luminous conducting arc zone
surrounded by a cold gas layer. The conducting
regions of N2 and air plasma gases were narrower
than for steam and O2. The plasma flow expands
towards the work-piece.
Temperature profiles of the arc column at the torch
exit obtained by means of optical emission
spectroscopy are shown in Fig. 4. The profiles were
analyzed for different gases at the same mass flow
rate of 12 g/min. The profiles correspond to the
region with the sonic conditions. The temperatures
were above 20 kK at the jet centerline and were
reducing towards the edges. The highest temperature
was observed for nitrogen plasma, the smallest – for
steam. Despite owning different thermodynamic
and transport properties temperature profiles for O2
and air plasma were very similar.
4
2.4 x 10
N
Temperature, K
2.3
2
Air
O2
2.2
steam
2.1
2
1.9
1.8
0
0.1
0.2
0.3
radial distance, mm
0.4
Figure 4. Temperature profiles at the nozzle exit for different
gases (gas flow rate – 12 g/min).
4. Cutting performance
As the plasma torch is moved along the material the
arc energy is transferred to the material, which
results in its melting and formation of a kerf. To see
an effect of the gas nature on the kerf geometry and
cutting performance the gas flow rate (8 g/min),
stand-off distance (2 mm) and cutting speed (30
cm/min) were kept constant. These values are not
optimal for cutting, however are sufficient to provide
a comparative study.
The resulting kerf geometries are shown in Fig. 5.
There is some similarity in the kerf shape of N2 and
air as well as O2 and steam. For N2 and air the kerf is
wider at the top and narrows towards the bottom. In
the case of O2 and steam the kerf is the narrowest at
the top, then next couple of millimeters it widens
and then starts narrowing as well. Such a kerf shape
indicates that the heat distribution along the cut
surface is not uniform. The kerf was the narrowest
for nitrogen, which corresponds to the narrowest
plasma jet with the best energy density concentration
in the smallest volume among studied cases. The
shape of the kerf for oxygen and steam cutting may
result from different factors. In the literature it is
explained by too close position of the torch with
respect to material [6]. It may lead to the situation
when the plasma jet meets the material at the point
of the compression, which is characterized by the
smaller arc diameter. However, visualization of the
jets confirms that the material position corresponds
to the second expansion (Fig. 3)). Moreover, this
effect was not observed for nitrogen and air cutting.
Another explanation can be oxidation of the
material, while it interacts with oxygen containing in
plasma. This interaction is the most intensive
upstream close to the top where the highest amount
of oxygen molecules is available and energy
released due to iron oxidation facilitates melting of
the material. Deeper inside the material the amount
of the free oxygen molecule decreases and energy
also reduces. However, it is still not clear if oxygen
coming from the steam is in the form of free
molecules and can react or if it is bounded to
hydrogen. One more factor, which may cause such a
kerf shape, is an anode attachment, which is
positioned in the upper part of the material and adds
to the melting process as well. This question is still
open and more investigations should be done to
explain the phenomena conditioning the kerf shape.
Figure 5. Influence of the plasma gas on kerf geometry (gas
flow rate – 8 g/min).
From the analyses of the kerf geometry and size it
was possible to determine material removal rate and
thus the energy required for its melting. The kerf
shape was considered to be constant along the cut.
Fig. 6 shows dependence of the material removal
rate on the gas flow rate for all studied gases. The
general trends are the same for nitrogen, air and
steam: the higher the gas flow rate, the smaller the
material removal rate. This effect may be caused by
two factors: either by higher arc constriction or by
the increased layer of cold gas surrounding the jet
causing worse heat transfer between the hot plasma
core and the material. However, this is not valid for
oxygen cutting, where higher flow rates resulted in
increase of the amount of the melted material. For
oxygen cutting the available electrical power of the
arc is the smallest (Fig. 7), however, reaction of
oxygen with iron results in additional heat release.
For the higher gas flow rates more oxygen is
available to react with iron and thus more heat is
released promoting the melting process.
material removal rate, g/s
1.8
N2
Air
O2
steam
1.6
1.4
1.2
1
0.8
7
9
11
13
15
17
gas flow rate, g/min
Figure 6. Influence of the plasma gas nature and flow rate on
material removal during cutting.
For all studied conditions only less than 20 % of
total electrical power supplied by the arc was
utilized for the material melting. The rest of the
energy was either transferred to the work-piece or
was taken away with the jet flowing downstream the
material. Fig. 7 shows the electrical energy input and
the energy, which is convected downstream the
work-piece and represents energy losses. These
losses were measured calorimetrically in the vessel
placed below the work-piece at a leading edge point.
11.0
N2
10.0
Air
O2
steam
Energy, kW
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
7
9
11
13
15
17
gas flow rate, g/min
Figure 7. Effect of plasma gas nature and flow rate on energy
input (dotted line) and energy which is not involved into the
cutting process (solid line).
The energy losses behavior corresponds to the power
input. Indeed, the power input increases with the gas
flow rate and is the highest for the steam cutting.
However, the energy losses for the steam are
comparable to gas cutting conditions despite much
higher energy input. Not only more energy is used
for the cutting itself resulting in a wider kerf, but
also more energy is transferred to the work-piece in
form of the heat conduction losses.
5. Conclusions
The paper describes properties of the transferred
electric arc generated with different plasma media to
cut mild steel. Oxygen, air and nitrogen plasma
gases were tested at different flow rates and
compared to liquid medium. It was shown that the
steam arc provides more energy for the same arc
current level than the arcs generated in the gases due
to higher arc voltage. The energy involved into the
cutting process under studied conditions was the
highest also for steam. The energy losses convected
under the work-piece were comparable for all
applied gases and increased with the gas flow rate.
The plasma jets generated in nitrogen and air were
narrower resulting in smaller kerf size and material
removal rate. The kerfs typical for oxygen and steam
cutting exhibit similar features, they are bigger due
to higher amount of energy available for cutting,
which originates either from the arc for steam or is
released due to oxidation of iron.
The research was supported by Fronius Int. GmbH,
Austria and by GACR under project 205/11/2070.
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