12. thermal cutting - kau.ac.kr

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


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


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


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


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


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


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


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.


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


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


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


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


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


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


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


br-er12-28e.cdr

Figure 12.28


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.


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


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

]


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


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

12. thermal cutting - kau.ac.kr

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