Chapter 12 - Thermal Cutting
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Transcript of Chapter 12 - Thermal Cutting
12.
Thermal Cutting
12. Thermal Cutting 166
2005
Thermal cutting processes
are applied in different
fields of mechanical engi-
neering, shipbuilding and
process technology for the
production of components
and for the preparation of
welding edges. The ther-
mal cutting processes are
classified into different
categories according to DIN
2310, Figure 12.1.
Figure 12.2 shows the clas-
sification according to the physics of the cutting process:
- flame cutting – the material is mainly oxidised (burnt)
- fusion cutting – the material is mainly fused
- sublimation cutting – the material is mainly evaporat
The gas jet and/or evaporation expansion is in all processes responsible for the ejection of
molten material or emerging reaction products such as slag.
The different energy carriers for the thermal cutting are depicted in Figure 12.3:
- gas,
- electrical gas discharge
and
- beams.
Electron beams for ther-
mal cutting are listed in the
DIN-Standard, they pro-
duce, however, only very
small boreholes. Cutting
is impossible.
Classification of Thermal CuttingProcesses acc. to DIN 2310-6
Classification of thermal cutting processes
- physics of the cutting process
- degree of mechanisation
- type of energy source
- arrangement of water bath
br-er12-01e.cdr
Figure 12.1
Classification of Processes bythe Physics of Cutting
Flame cutting
Fusion cutting
Sublimation cutting
The material is mainly oxidised;the products
are blown out by an oxygen jet.
The material is mainly fused and blown out
by a high-speed gas jet.
The material is mainly evaporated.
It is transported out of the cutting groove by
the created expansion or by additional gas.
br-er12-02e.cdr
Figure 12.2
12. Thermal Cutting 167
2005
Figure 12.4 depicts the different methods of thermal cutting with gas according to DIN
8580. These are:
- flame cutting
- metal powder
flame cutting
- metal powder
fusion cutting
- flame planing
-oxygen-lance cutting
- flame gouging or scarf-
ing
-flame cleaning
In flame cutting (principle
is depicted in Figure 12.5)
the material is brought to
the ignition temperature by
a heating flame and is then
burnt in the oxygen stream.
During the process the igni-
tion temperature is main-
tained on the plate top side
by the heating flame and
below the plate top side by
thermal conduction and
convection.
However, this process is
suited for automation and is, also easy to apply on site. Figure 12.6. shows a commercial
torch which combines a welding with a cutting torch. By means of different nozzle shapes
the process may be adapted to varying materials and plate thicknesses. Hand-held torches
or machine-type torches are equipped with different cutting nozzles: Standard or block-
type nozzles (cutting-oxygen pressure 5 bar) are used for hand-held torches and for torches
which are fixed to guide carriages.
Classification of Thermal Cutting Processes acc. to DIN 2310-6
-
- - sparks - arc - plasma
- - laser beam (light) - electron beam - ion beam
gas
electrical gas discharge
beams
thermal cutting by:
br-er12-03e.cdr
Figure 1.3
Thermal Cutting Processes Using Gas
thermal cutting processes using gas:
� metal powder
flame cutting
� oxygen cutting � metal powder
fusion cutting
� flame planing �
�
�
oxygen-lance cutting
flame gouging
scarfing
� flame cleaning
br-er12-04e.cdr
Figure 1.4
12. Thermal Cutting 168
2005
The high-speed cutting nozzle (cutting-oxygen pressure 8 bar) allows higher cutting
speeds with increased cutting-oxygen pressure. The heavy-duty cutting nozzle (cutting-
oxygen pressure 11 bar) is
mainly applied for eco-
nomic cutting with flame-
cutting machines. A further
development of the heavy-
duty nozzle is the oxygen-
shrouded nozzle which
allows even faster and
more economic cutting of
plates within certain thick-
ness ranges. Gas mixing is
either carried out in the
torch handle, the cutting
attachment, the torch head
or in the nozzle (gas mix-
ing nozzle); in special
cases also outside the
torch – in front of the noz-
zle. As the design of cutting
torches is not yet subject to
standardisation, many
types and systems exist on
the market.
Principle of Oxygen Cutting
cutting oxygenheating oxygengas fuel
cutting jet
heating flame
workpiece
br-er12-05e.cdr
Figure 12.5
Cutting Torch and Nozzle Shapes
block-typenozze
gas mixingnozzle
manual cutting equipmentas a cutting and weldingtorch combination
cutting oxygen
mixing chamber
gas fuel
heating oxygen
br-er12-06e.cdr
Figure 12.6
12. Thermal Cutting 169
2005
The selection of a torch or
nozzles important and de-
pends mainly on the cutting
thickness, the desired cut-
ting quality, and/or the ge-
ometry of the cutting edge.
Figure 12.7 gives a survey
of the definitions of flame-
cutting.
In flame cutting, the ther-
mal conductivity of the ma-
terial must be low enough
to constantly maintain
the ignition temperature,
Figure 12.8. Moreover, the
material must neither
melt during the oxidation
nor form high-melting
oxides, as these would
produce difficult cutting
surfaces. In accordance,
only steel or titanium mate-
rials fulfill the conditions for
oxygen cutting., Figure
12.9
Function of the Flame During Flame Cutting
The heating flame has to perform the following tasks:
- rapid heating of the material (about 1200°C)
- substitution of losses due to heat conduction
in order to maintain a positive heat balance
- preheating of cutting oxygen
- stabilisation of the cutting oxygen jet; formation
of a cylindrical geometry over a extensive length
and protection against nitrogen of the surrounding air
br-er12-08e.cdr
Figure 12.8
Flame Cutting Terms
cut lengthcutting length
end of the cut
cut
thic
kne
ss
start
kerf
kerf width
heating and cutting nozzle
torch
cutting jet
nozzle
-to
-wo
rkd
ista
nce
br-er12-07e.cdr
Figure 12.7
12. Thermal Cutting 170
2005
Steel materials with a C-content of up to approx. 0.45% may be flame-cut without preheating,
with a C-content of approx. 1.6% flame-cutting is carried out with preheating, because an
increased C-content demands more heat. Carbon accumulates at the cutting surface, so a
very high degree of hardness is to be expected. Should the carbon content exceed 0.45%
and should the material not have been subject to prior heat treatment, hardening cracks on
the cutting surface are re-
garded as likely.
Some alloying elements
form high-melting oxides
which impair the slag ex-
pulsion and influence the
thermal conductivity.
The iron-carbon equilibrium
diagram illustrates the car-
bon content-temperature
interrelation, Figure 12.10.
As the carbon content
increases, the melting
temperature is lowered.
That means: from a certain
carbon content upwards,
the ignition temperature is
higher than the melting
temperature, i.e., this
would be the first violation
to the basic requirement in
flame cutting.
Conditions of Flame Cutting
The material has to fulfill the following requirements:
- the ignition temperature has to be lower than the
melting temperature
- the melting temperature of the oxides has to be lower
than the melting temperature of the material itself
- the ignition temperature has to be permanently maintained;
i. e. the sum of the supplied energy and heat losses due to
heat conduction has to result in a positive heat balance
br-er12-09e.cdr
Figure 12.9
Ignition Temperature in theIron-Carbon-Equilibrium Diagram
liquid
Liquidus
Solidus
solid
solid
pasty
steel cast iron
tem
pera
ture
[°C
]
1500
1000
ignition curve
2,0 carbon content [%]
br-er12-10e.cdr
Figure 12.10
12. Thermal Cutting 171
2005
Steel compositions may
influence flame cuttability
substantially - the individual
alloying elements may
show reciprocate effects
(reinforcing/weakening),
Figure 12.11. The content
limits of the alloying con-
stituents are therefore only
reference values for the
evaluation of the flame cut-
tability of steels, as the cut-
ting quality is substantially
deteriorating, as a rule al-
ready with lower alloy con-
tents.
By an arrangement of one
or several nozzles already
during the cutting phase a
weld preparation may be
carried out and certain
welding grooves be pro-
duced. Figure 12.12 shows
torch arrangements for
- the square butt weld,
- the single V butt weld,
- the single V butt weld with root face,
- the double V butt weld and
- the double V butt weld with root face.
Flame Cutting Suitability in Dependance of Alloy-Elements
Maximum allowable contents of alloy-elements:
carbon: up to 1,6 %
silicon: up to 2,5 % with max. 0,2 %C
manganese: up to 13 % and 1,3 % C
chromium: up to 1,5 %
tungsten: up to 10 % and 5 % Cr, 0,2 % Ni, 0,8 % C
nickel: up to 7,0 % and/or up to 35 % with min. 0,3 % C
copper: up to 0,7 %
molybdenum: up to 0,8 %, with higher proportions of W, Cr and C
not suitable for cutting
br-er12-11e.cdr
Figure 12.11
Weld-Groove Preparation by Oxygen Cutting
square butt weld single-V butt weld single-V butt weldwith rootface
double-V butt weld double-V butt weldwith root face
br-er12-12e.cdr
Figure 12.12
12. Thermal Cutting 172
2005
Possible Flame Cutting Defects
edge defect:
cut face defects:
edge rounding chain of fused globules edge overhang
kerf constriction or extension angular deviation step at lower edge of the cut excessive depth of cutting grooves
cratering:
adherent slag:
cracks:
sporadic craterings connected craterings cratering areas
slag adhearing to bottom cut edge
face cracks cracks below the cut face
br-er12-13e.cdr
Figure 12.13
It has to be considered that, particularly in cases where flame cutting is applied for weld
preparations, flame cutting-related defects may lead to increased weld dressing work. Slag
adhesion or chains of molten globules have to be removed in order to guarantee process
safety and part accuracy for the subsequent processes. Figure 12.13 gives a survey of pos-
sible defects in flame cutting.
In order to improve the
flame-cutting capacity
and/or cutting of materials
which are normally not to
be flame-cut the powder
flame cutting process may
be applied.
Here, in addition to the cut-
ting oxygen, iron powder is
blown into the cutting gap.
In the flame, the iron pow-
der oxidises very fast and
adds further energy to the
process. Through the addi-
tional energy input the
high-melting oxides of
the high-alloy materials
are molten. Figure 12.14
shows a diagrammatic rep-
resentation of a metal
powder cutting arrange-
ment.
Metal Powder Flame Cutting
waterseperator
powderdispenser
acetylene
oxygencompressed
air
br-er12-14e.cdr
Figure 12.14
12. Thermal Cutting 173
2005
Figure 12.15 shows the
principle of flame gouging
and scarfing. Both meth-
ods are suited for the weld
preparation; material is re-
moved but not cut. This
way, root passes may be
grooved out or fillets for
welding may be produced
later.
Figure 12.16 shows the
methods of thermal cut-
ting processes by electri-
cal gas discharge:
- plasma cutting with non-transferred arc
- plasma cutting with transferred arc
- plasma cutting with transferred arc and secondary gas flow
- plasma cutting with transferred arc and water injection
- arc air gouging (represented diagrammatically)
- arc oxygen cutting (represented diagrammatically)
Flame Gouging and Scarfing
flame gouging scarfing
gougingoxygen scarfing
oxygen
gas-heatoxygen mixture gas-heat
oxygen mixture
br-er12-15e.cdr
Figure 12.15
Thermal Cutting Processesby Electrical Gas Discharge
Thermal cutting processes by electrical gas discharge:
arc air gouging arc oxygen cuttingplasma cutting
- with non-transferred arc
- with transferred arc -with secondary gas flow -with water injection
carbonelectrode
compressedair =
tube
electrodecoating
cuttingoxygen
arc
br-er2-16e.cdr
Figure 12.16
12. Thermal Cutting 174
2005
In plasma cutting the en-
tire workpiece must be
heated to the melting tem-
perature by the plasma jet.
The nozzle forms the
plasma jet only in a re-
stricted way and limits thus
the cutting ability of plate to
a thickness of approx.
150 mm, Figure 12.17.
Characteristic for the
plasma cut are the cone-
shaped formation of the
kerf and the rounded
edges in the plasma jet entry zone which were caused by the hot gas shield that envelops
the plasma jet. These process-specific disadvantages may be significantly reduced or limited
to just one side of the plate (high quality or scrap side), respectively, by the inclination of the
torch and/or water addition. With the plasma cutting process, all electrically conductive ma-
terials may be separated. Nonconductive materials, or similar materials, may be separated by
the emerging plasma flame, but only with limited ability.
In order to cool and to re-
duce the emissions,
plasma torches may be
surrounded by additional
gas or water curtains
which also serve as arc
constriction, Figure 12.18.
In dry plasma cutting
where Ar/H2, N2, or air are
used, harmful substances
always develop which not
only have to be sucked off
very carefully but which
Water Injection Plasma Cutting
electrodeplasma gas
nozzle
workpiece
water curtain
cone of water
cutting waterswirl chamber
water bath
br-er12-18e.cdr
Figure 12.18
Plasma Cutting
R
HFpowersource
-
+
electrodeplasma gas
nozzle
workpiece
coolingwater
br-er12-17e.cdr
Figure 12.17
12. Thermal Cutting 175
2005
also must be disposed of.
In water-induced plasma
cutting (plasma arc cutting
in water or under water)
gases, dust, also the noise,
and the UV radiation are,
for the most part, held back
by the water. A further,
positive effect is the cooling
of the cutting surface, Fig-
ure 12.18. Careful disposal
of the residues is here in-
evitable.
Figure 12.19 gives a survey
of the different cutting meth-
ods using a water bath.
Figure 12.20 shows a torch
which is equipped with an
additional gas supply, the
so-called secondary gas.
The secondary gas shields
the plasma jet and in-
creases the transition resis-
tance at the nozzle front.
The so-called “double and/or parasite arcs” are avoided and nozzle life is increased.
Types of Water Bath Plasma Cutting
cutting with water bath
plasma cutting with workpieceon water surface
underwater plasma cutting
water injection plasma cuttingwith water curtain
br-er12-19e.cdr
Figure 12.19
Plasma Cutting With Secondary Gas Flow
electrodeplasma gas
nozzle
workpiece
secondary gas
br-er12-20e.cdr
Figure 12.20
12. Thermal Cutting 176
2005
Thanks to new electrode
materials, compressed air
and even pure oxygen
may be applied as plasma
gas – therefore, in flame
cutting, the burning of unal-
loyed steel may be used for
increased capacity and
quality. The selection of the
plasma forming gases
depends on the require-
ments of the cutting proc-
ess. Plasma forming media
are argon, helium, hydro-
gen, nitrogen, air, oxygen or water.
The advantage of the use of oxygen as plasma gas is in the achievable cutting speeds
within the plate thickness range of approx. 3 – 12 mm (400 A, WIPC). In the steel plate thick-
ness range of approx. 1 – 10 mm the application of 40 A-compressed air units is recom-
mended. In comparison with 400 A WIPC systems, these allow vertical and significantly nar-
rower cutting kerfs, but with
lower cutting speeds. Fig-
ure 12.21 shows different
cutting speeds for different
units and plasma gases.
In the thermal cutting
processes with beams
only the laser is used as
the jet generator for cutting,
Figure 12.22.
Cutting Speeds of Different PlasmaCutting Equipment for Steel Plates
cuttin
g s
peed [m
/min
]
plate thickness [mm]
5 10 15 20
2
4
8
6
machine type and plasma medium1 WIPC, 400 A, O
2 WIPC, 400 A, N
3 200 A, s < 8 mm: N
4 40 A, compressed air
2
2
2
1
2
3
4
s > 8 mm: Ar/H2
br-er12-21e.cdr
Figure 12.21
Thermal Cutting With Beams
Thermal cutting processes
by laser beam
- laser beam combustion cutting
- laser beam fusion cutting
- laser beam sublimation cutting
br-er12-22e.cdr
Figure 12.22
12. Thermal Cutting 177
2005
Variations of the laser beam cutting process:
- laser beam combustion cutting, Figure 12.25
- laser beam fusion cutting, Figure 12.26
- laser beam sublimation cutting, Figure 12.27.
The process sequence in laser beam combustion cutting is comparable to oxygen cut-
ting. The material is heated to the ignition temperature and subsequently burnt in the oxy-
gen stream, Figure 12.23. Due to the concentrated energy input almost all metals in the
plate thickness range of up to approx. 2 mm may be cut. In addition, it is possible to achieve
very good bur-free cutting
qualities for stainless steels
(thickness of up to approx.
8 mm) and for structural
steels (thickness of up to
12 mm). Very narrow and
parallel cutting kerfs are
characteristic for laser
beam cutting of structural
steels.
In laser beam cutting, ei-
ther oxygen (additional en-
ergy contribution for oxidis-
ing materials) or an inactive
cutting gas may be applied
depending on the cutting
job. Besides, the very high
beam powers
(pulsed/superpulsed mode
of operation) allow a direct
evaporation of the material
(sublimation). In laser
beam combustion cutting
and laser beam sublima-
Laser Beam Cutting
cutting oxygen
lens
workpiece
slag jet
laser focus
thin layer of cristallisedmolten metal
br-er12-23e.cdr
Figure 12.23
Qualitative Temperature Dependencyon Absorption Ability
melting point Tm boiling point Tb
temperature
λ =µ
1,06 m (Nd:YAG-laser)
λ =µ
10,06 m (CO -laser)2
he
atin
g-u
p
me
ltin
g
evap
ora
tin
g
20
40
60
80
absorp
tion
facto
r
br-er12-24e.cdr
Figure 12.24
12. Thermal Cutting 178
2005
tion cutting the reflexion of the laser beam of more than 90 % on the workpiece surface de-
creases unevenly when the process starts. In laser beam fusion cutting remains the reflex-
ion on the molten material, however, at more than 90%! Figure 12.24 shows the absorption
factor of the laser light in
dependence on the tem-
perature. This factor mainly
depends on the wave
length of the used laser
light. When the melting
point of the material has
been reached, the absorp-
tion factor increases un-
evenly and reaches values
of more than 80%.
During laser beam combus-
tion cutting of structural
steel high cutting speeds are achieved due to the exothermal energy input and the low laser
beam powers, Figure 12.25. In the above-mentioned case (dependent on beam quality, fo-
cussing, etc.), above a
beam power of approx.
3,3 kW, spontaneous
evaporation of the material
takes place and allows
sublimation cutting. Signifi-
cantly higher laser powers
are necessary to fuse the
material and blow it out
with an inert gas, as the
reflexion loss remains con-
stant.
Characteristics of the LaserBeam Cutting Processes I
laser cutting (with oxygen jet)
- the laser beam is focused on the workpiece surface and the material burns in the oxygen jet starting from the heated surface
materials: - steel aluminium alloys, titanium alloys
cutting gas: - O N Ar
criteria: - high cutting speed, cut faces with oxide skin
2, 2,
br-er12-25e.cdr
Figure 12.25
laser fusion cutting:
- the laser beam melts the entire plate thickness (optimum focus point 1/3 below plate surface) - high reflection losses (>90%)
materials: - metals, glasses, polymers
cutting gas: - N , Ar, He
criterions: - cutting speed is only 10-15% in comparison to cutting with oxygen jet, characteristics melting drag lines
2
Characteristics of the LaserBeam Cutting Processes II
br-er12-26e.cdr
Figure 12.26
12. Thermal Cutting 179
2005
Important influence quantities for the cutting speed and quality in laser beam cutting are
the focus intensity, the position of the focus point in relation to the plate surface and the
formation of the cutting gas flow. A prerequisite for a high intensity in the focus is the high
beam quality (Gaussian intensity distribution in the beam) with a high beam power and suit-
able focussing optics.
Laser beam cutting of contours, especially of pointed corners and narrow root faces, requires
adaptation of the beam power in order to avoid heat accumulation and burning of the mate-
rial. In such a case the
beam power might be re-
duced in the continuous
wave (CW) operating
mode. With a decreasing
beam efficiency decreases
the cuttable plate thickness
as well. Better suited is the
switching of the laser to
pulse mode (standard
equipment of HF-excited
lasers) where pulse height
can be selected right up to
the height of the continuous
wave. A super pulse
equipment (increased exci-
tation) allows significantly
higher pulse efficiencies to
be selected than those
achieved with CW. Further
fields of application for the
pulse and super pulse op-
eration mode are punching
and laser beam sublimation
cutting.
Characteristics of the LaserBeam Cutting Processes III
laser evaporation cutting:
- spontaneous evaporation of the material starting from 10 W/cmwith high absorption rate and deep-penetration effect
- metallic vapour is pressed from the cavity by own vapour pressure and by a supporting gas flow
materials: - metals, wood, paper, ceramic, polymer
cutting gas: - N , Ar, He (lens protection)
criteria: - low cutting speed, smooth cut edges, minimum heat input
5 2
2
br-er12-27e.cdr
Figure 12.27
Fields of Application of Cutting Processes
laser 600 W 1500 W 600 W 1500 W 1500 W
plasma 50 A 5 kW 250 A 25 kW 500 A 150 kW
oxy-flame
10 100 10001
steel
Cr-Ni-steel
aluminium
steelCr-Ni-steelaluminium
StahlCr-Ni-Stahl
plate thickness [mm]
br-er12-28e.cdr
Figure 12.28
12. Thermal Cutting 180
2005
Laser beam cutting of aluminium plates thicker than appx. 2 mm does not produce bur-free
results due to a high reflex-
ion property, high heat
conductivity and large
temperature differences
between Al and Al2O3. The
addition of iron powder al-
lows the flame cutting of
stainless steels (energy
input and improvement of
the molten-metal viscosity).
The cutting quality, how-
ever, does not meet high
standards.
Figure 12.28 shows a comparison of the different plate thicknesses which were cut using
different processes. For the plate thickness range of up to 12 mm (steel plate), laser beam
cutting is the approved precision cutting process. Plasma cutting of plates > 3 mm allows
higher cutting speeds, in comparison to laser beam cutting, the cutting quality, however, is
significantly lower. Flame cutting is used for cutting plates > 3 mm, the cutting speeds are, in
comparison to plasma cutting, significantly lower. With an increasing plate thickness the dif-
ference in the cutting
speed is reduced. Plates
with a thickness of more
than 40 mm may be cut
even faster using the flame
cutting process.
Figure 12.29 shows the
cutting speeds of some
thermal cutting processes.
Cutting Speeds of Thermal Cutting Processes
cutt
ig s
peed
s
[m/m
in]
10
10
1001
1
0,1
plate thickness [mm]
oxygen cutting(Vadura 1210-A)
plasma cutting(WIPC, 300-600 A)
CO2-laser(1500 W)
br-er12-29e.cdr
Figure 12.29
Thermal Cutting Costs - Steal
costs
[D
M/m
cu
t le
ngth
]
plate thickness [mm]
5 10 15 20 25 30 35 40
1
2
3
4
5
6
laser
plasmaflame cuttingwith 3 torches
total costs
machine costs
br-er12-30e.cdr
Figure 12.30
12. Thermal Cutting 181
2005
Apart from technological aspects, financial considerations as well determine the application
of a certain cutting method. Figures 12.30 and 12.31 show a comparison of the costs of
flame cutting, plasma arc
and laser beam cutting –
the costs per m/cutting
length and the costs per
operating hour. The high
investment costs for a laser
beam cutting equipment
might be a deterrent to ex-
ploit the high cutting quali-
ties obtainable with this
process. Cost Comparison of Cutting Processes
plasma cutting(plasma 300A)
flame cutting(6-8 torches)
laser beam cutting(laser 1500W)
investment total(replacement value)
170,000.00€
€/h
€/h
€/h
€/h
220,000.00 500,000.00
1 shift, 1600h/year, 80% availability, utilisation time 1280h/year
calculation for a 6-year-accounting depreciation 23.50 29.00 65.00
maintenance costs 3.50 4.00 10.00
production cost unit ratecosts/1 operating hour 65.00 75.00 130.00
energy costs 1.00 2.50 2.50
extract from a costing acc. to VDI 3258
br-er12-31e.cdr
Figure 12.31