15. thermal cutting processes
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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!
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