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TWI Training & Examination Services

Course Reference WIS 5

Course notes section reference 15

CSWIP 3.1 Welding Inspection

Cutting Processes

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A jet of pure oxygen reacts with iron, that has been preheated to

its ignition point, to produce the oxide Fe3O4 by exothermicreaction.This oxide is then blown through the material by the

velocity of the oxygen stream

Different types of fuel gases may be used for the pre-heating

flame in oxy fuel gas cutting: i.e. acetylene, hydrogen,propane. etc

By adding iron powder to the flame we are able to cut most

metals - Iron Powder Injection

The high intensity of heat and rapid cooling will cause

hardening in low alloy and medium/high C steels they are

thus pre-heated to avoid the hardening effect

Oxyfuel Gas Cutting Process

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The cutting torch

Neutral cutting flame

Neutral cutting flame with

oxygen cutting stream

Oxyfuel Gas Cutting Process

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Oxyfuel gas cutting equipment

Torch-mix cutting

torch

fuel and oxygen for

preheat flame are mixedwithin the torch

can be injector type (for

pressure less than 14

kPa) or mediumpressure type

Handle

Universalpressuremixer

Mixedpreheatgases

Tip nut

Cutting tip

Fuel gasinlet

Preheatfuel gasvalve

Oxygeninlet

Preheatoxygenvalve

Cuttingoxygenlever

Cutting oxygenTorch head

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Oxyfuel gas cutting equipment

Tip mix cutting

torch

fuel and oxygen

for preheat flamesare mixed in the

tip of the torch

Cutting

oxygen

Three tubedesign

Cuttingoxygenlever

PreheatoxygenPreheatfuel gas

Preheatvalves

Handle

Tip nut

Tip mix

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Oxyfuel gas cutting related terms

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Oxyfuel gas cutting quality

Good cut - sharp top edge, fine and even drag lines,

little oxide and a sharp bottom edge

Cut too fast -

pronounced break in the

drag line, irregular cut

edge

Cut too slow - top edge is melted,

deep groves in the lower portion,

heavy scaling, rough bottom edge

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Oxyfuel gas cutting quality

Good cut - sharp top edge, fine and even drag lines,

little oxide and a sharp bottom edge

Preheat flame too high - top

edge is melted, irregular cut,

excess of adherent dross

Preheat flame too low - deep

groves in the lower part of the cut

face

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Oxyfuel gas cutting quality

Good cut - sharp top edge, fine and even drag lines,

little oxide and a sharp bottom edge

Irregular travel speed - uneven space

between drag lines, irregular bottom

with adherent oxide

Nozzle is too high above the

works - excessive melting of the

top edge, much oxide

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Mechanised oxyfuel cutting

can use portable carriages or gantry type machines

high productivity accurate cutting for complicate shapes

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Mechanised oxyfuel cutting

Cutting and bevelling

head

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OFW/C advantages/disadvantages

Disadvantages:1) High skill factor

2) Wide HAZ

4) Slow process

5) Limited range of

consumables

3) Safety issues

Advantages:1) No need for power supply

portable

3) Low equipment cost

4) Can cut carbon and lowalloy steels

5) Good on thin materials

2) Versatile: preheat,

brazing, surfacing, repair,

straightening

6) Not suitable for reactive &

refractory metals

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Plasma cutting

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Plasma cutting

no need to promote

oxidation no preheatworks by melting and blowing

and/or vaporisation

gases: air, Ar, N2, O2, mix of Ar

+ H2, N2 + H2

air plasma promotes

oxidation increased speed

but special electrodes need

shielding gas - optional

applications: stainless steels,aluminium and thin sheet

carbon steel

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Plasma arc modes

used for cutting and

joining non-conductive

workpiece

workpiece is not in thearc circuit

heat is obtained from

plasma jet only

low energyconcentration

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Plasma cutting

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Arc Air Gouging

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Arc-air gouging features

Operate ONLY on DCEP

Special gougingcoppercoated carbon electrode

Can be used on carbonand low alloy steels,austenitic stainless steels

and non-ferrous materials Requires CLEAN/DRY

compressed air supply

Provides fast rate of metal removal

Can remove complex shape defects

After gouging, grinding of carbured layer is mandatory

Gouging doesnt require a qualified welder!