12. Thermal Cuttingdantn/WT/WT1-c12.pdfFigure 12.16 shows the methods ofthermal cutting processes by ele ctrical gas discharge: - plasma cutting with non-transferred arc - plasma cutting

2003

12.

Thermal Cutting

12. Thermal Cutting 160

Thermal cutting processes are applied in different fields of mechanical engineering,

shipbuilding and

process technology

for the production

of components and

for the preparation

of welding edges.

The thermal cut-

ting processes

are classified into

different categories

according to DIN

2310, Figure 12.1.

Figure 12.2 shows the classification according to the physics of the cutting proc-

ess:

- 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 ejec-

tion of molten material or emerging reaction products such as slag.

The different en-

ergy carriers for

the thermal cut-

ting are depicted in

Figure 12.3:

- gas,

- electrical gas

discharge and

- beams.

Electron beams

for thermal cutting

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


are listed in the DIN-Standard, they produce, however, only very small boreholes.

Cutting is impossible.

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 cut-

ting

- flame gouging or

scarfing

-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. Dur-

ing the process the ignition temperature is maintained on the plate top side by the

heating flame and

below the plate top

side by thermal

conduction and

convection.

However, this proc-

ess is suited for

automation and is,

also easy to apply

on site. Figure 12.6.

shows a commer-

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:

l metal powder

flame cutting

l oxygen cutting l metal powder

fusion cutting

l flame planing l

l

l

oxygen-lance cutting

flame gouging

scarfing

l flame cleaning

br-er12-04e.cdr

Figure 1.4


cial torch which combines a welding with a cutting torch. By means of different noz-

zle shapes the process may be adapted to varying materials and plate thicknesses.

Hand-held torches

or machine-type

torches are

equipped with dif-

ferent cutting noz-

zles: Standard or

block-type nozzles

(cutting-oxygen

pressure 5 bar) are

used for hand-held

torches and for

torches which are

fixed to guide car-

riages.

The high-speed cutting nozzle (cutting-oxygen pressure 8 bar) allows higher cut-

ting speeds with increased cutting-oxygen pressure. The heavy-duty cutting nozzle

(cutting-oxygen pressure 11 bar) is mainly applied for economic 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

thickness ranges.

Gas mixing is ei-

ther carried out in

the torch handle,

the cutting attach-

ment, the torch

head or in the noz-

zle (gas mixing

nozzle); in special

cases also outside

the torch – in front

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


of the nozzle. As the design of cutting torches is not yet subject to standardisation,

many types and systems exist on the market.

The selection of a

torch or nozzles

important and de-

pends mainly on

the cutting thick-

ness, the desired

cutting quality,

and/or the geome-

try of the cutting

edge. Figure 12.7

gives a survey of

the definitions of

flame-cutting.

In flame cutting, the thermal conductivity of the material must be low enough to con-

stantly 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 materi-

als fulfill the conditions for oxygen cutting., Figure 12.9

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, be-

cause an increased

Flame Cutting Terms

cut lengthcutting lengthend of the cut

cut t

hick

ness

start

kerf

kerf width

heating and cutting nozzletorch

cutting jetno

zzle

-to-

wor

kdi

stan

ce

br-er12-07e.cdr

Figure 12.7

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


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 cut-

ting surface are

regarded as likely.

Some alloying

elements form

high-melting ox-

ides which impair

the slag expulsion

and influence the

thermal conductiv-

ity.

The iron-carbon equilibrium diagram illustrates the carbon content-temperature inter-

relation, Figure 12.10. As the carbon content increases, the melting temperature

is lowered. That means: from a certain carbon content upwards, the ignition tem-

perature is higher than the melting temperature, i.e., this would be the first violation to

the basic requirement in flame cutting.

Steel compositions

may influence flame

cuttability substan-

tially - the individual

alloying elements

may show recipro-

cate effects (rein-

forcing/weakening),

Figure 12.11. The

content limits of the

alloying constitu-

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

solidpasty

steel cast iron

tem

pera

ture

[°C

]

1500

1000

ignition curve

2,0 carbon content [%]br-er12-10e.cdr

Figure 12.10


ents are therefore

only reference val-

ues for the evalua-

tion of the flame

cuttability of steels,

as the cutting qual-

ity is substantially

deteriorating, as a

rule already 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 produced. 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.

It has to be consid-

ered that, particu-

larly in cases where

flame cutting is ap-

plied for weld

preparations, flame

cutting-related de-

fects may lead to

increased weld

dressing work.

Slag adhesion or

chains of molten

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


globules have

to be removed

in order to

guarantee

process safety

and part accu-

racy for the

subsequent

processes.

Figure 12.13

gives a survey

of possible

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 cutting oxygen, iron powder is blown into the cutting gap. In

the flame, the iron powder 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 dia-

grammatic repre-

sentation of a metal

powder cutting

arrangement.

Metal Powder Flame Cutting

waterseperator

powderdispenser

acetyleneoxygen

compressedair

br-er12-14e.cdr

Figure 12.14

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


Figure 12.15 shows

the principle of

flame gouging and

scarfing. Both

methods are suited

for the weld prepa-

ration; material is

removed but not

cut. This way, root

passes may be

grooved out or fil-

lets for welding may

be produced later.

Figure 12.16 shows the methods of thermal cutting processes by electrical 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)

In plasma cutting

the entire workpiece

must be heated to

the melting tempera-

ture by the plasma

jet. The nozzle forms

the plasma jet only

in a restricted way

and limits thus the

cutting ability of

plate to a thickness

of approx. 150 mm,

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


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 disadvan-

tages 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 electri-

cally conductive

materials may be

separated. Non-

conductive materi-

als, or similar mate-

rials, may be sepa-

rated by the emerg-

ing plasma flame,

but only with limited

ability.

In order to cool and to reduce 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

also must be dis-

posed of.

In water-induced

plasma cutting

(plasma arc cutting

in water or under

water) gases, dust,

also the noise, and

Plasma Cutting

R

HFpowersource

-

+

electrodeplasma gas

nozzle

workpiece

coolingwater

br-er12-17e.cdr

Figure 12.17

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


the UV radiation are, for the most part, held back by the water. A further, positive ef-

fect is the cooling of

the cutting surface,

Figure 12.18. Care-

ful disposal of the

residues is here

inevitable.

Figure 12.19 gives

a survey of the dif-

ferent 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 increases the

transition resistance at the nozzle front. The so-called “double and/or parasite arcs”

are avoided and nozzle life is increased.

Thanks to new electrode materials, compressed air and even pure oxygen may be

applied as plasma gas – therefore, in flame cutting, the burning of unalloyed steel

may be used for

increased capacity

and quality. The

selection of the

plasma forming

gases depends on

the requirements of

the cutting process.

Plasma forming

media are argon,

helium, hydrogen,

nitrogen, air, oxy-

gen or water.

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


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 thickness range of approx. 1 – 10 mm the application of 40 A-compressed

air units is recom-

mended. In com-

parison with 400 A

WIPC systems,

these allow vertical

and significantly

narrower cutting

kerfs, but with lower

cutting speeds. Fig-

ure 12.21 shows

different cutting

speeds for different

units and plasma

gases.

In the thermal cut-

ting processes

with beams only

the laser is used as

the jet generator for

cutting, Figure

12.22.

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.

Cutting Speeds of Different PlasmaCutting Equipment for Steel Plates

cutti

ng s

peed

[m

/min

]

plate thickness [mm]

5 10 15 20

2

4

8

6

machine type and plasma medium1 WIPC, 400 A, O2 WIPC, 400 A, N3 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


The process sequence in laser beam combustion cutting is comparable to oxy-

gen cutting. The material is heated to the ignition temperature and subsequently

burnt in the oxygen 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 cut-

ting of structural steels.

In laser beam cut-

ting, either oxygen

(additional energy

contribution for oxi-

dising materials) or

an inactive cutting

gas may be applied

depending on the

cutting job. Besides,

the very high beam

powers

(pulsed/superpulse

d mode of opera-

tion) allow a direct

evaporation of the

material (sublima-

tion). In laser beam

combustion cut-

ting and laser

beam sublimation

cutting the reflex-

ion of the laser

beam of more than

90 % on the work-

piece surface de-

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

heat

ing-

up

mel

ting

evap

orat

ing

20

40

60

80

abso

rptio

n fa

ctor

br-er12-24e.cdr

Figure 12.24


creases unevenly when the process starts. In laser beam fusion cutting remains

the reflexion on the molten material, however, at more than 90%! Figure 12.24 shows

the absorption factor of the laser light in dependence on the temperature. This factor

mainly depends on

the wave length of

the used laser

light. When the

melting point of the

material has been

reached, the ab-

sorption factor

increases un-

evenly and

reaches values of

more than 80%.

During laser beam combustion 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, focussing, etc.),

above a beam power of approx. 3,3 kW, spontaneous evaporation of the material

takes place and allows sublimation cutting. Significantly higher laser powers are nec-

essary to fuse the

material and blow it

out with an inert

gas, as the reflex-

ion loss remains

constant.

Important influ-

ence quantities

for the cutting

speed and quality

in laser beam cut-

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


ting 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 suitable 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 material. In such a case the beam power might be reduced in the con-

tinuous 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 (stan-

dard equipment of

HF-excited lasers)

where pulse height

can be selected

right up to the

height of the con-

tinuous wave. A

super pulse

equipment (in-

creased excitation)

allows significantly

higher pulse effi-

ciencies to be se-

lected than those

achieved with CW.

Further fields of

application for the

pulse and super

pulse operation

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


Laser beam cutting of aluminium plates thicker than appx. 2 mm does not produce

bur-free results due to a high reflexion property, high heat conductivity and large

temperature dif-

ferences between

Al and Al2O3. The

addition of iron

powder allows the

flame cutting of

stainless steels

(energy input and

improvement of the

molten-metal vis-

cosity). The cutting

quality, however,

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 cut-

ting is used for

cutting plates

> 3 mm, the cut-

ting speeds are,

in comparison to

plasma cutting,

significantly

lower. With an

increasing plate

thickness the dif-

Cutting Speeds of Thermal Cutting Processes

cutti

g sp

eeds

[m

/min

]

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

cost

s [D

M/m

cut

leng

th]


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


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.

Apart from technological aspects, financial considerations as well determine the ap-

plication 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 invest-

ment costs for a

laser beam cutting

equipment might

be a deterrent to

exploit the high

cutting qualities

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 Cuttingdantn/WT/WT1-c12.pdfFigure 12.16 shows the methods ofthermal cutting processes by ele ctrical gas discharge: - plasma cutting with non-transferred arc - plasma cutting

Documents