1.
Gas Welding
2003
1. Gas Welding
3
Although the oxy-acetylene process
has been introduced long time ago it
3
is still applied for its flexibility and mo4
6
5
8
7
9
1
2
bility. Equipment for oxyacetylene
welding consists of just a few elements, the energy necessary for welding can be transported in cylinders,
Figure 1.1.
1
2
3
4
5
6
7
8
9
oxygen cylinder with pressure reducer
acetylene cylinder with pressure reducer
oxygen hose
acetylene hose
welding torch
welding rod
workpiece
welding nozzle
welding flame
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Figure 1.1
3
density in normal state [kg/m ]
oxygen
propane
1.43
0.9
ignition temperature [OC]
600
ral gas; here C3H8 has the highest
400
calorific value. The highest flame in-
200
tensity from point of view of calorific
0
645
645
value and flame propagation speed is,
3200
flame temperature with O2
flame efficiency with O 2
flame velocity with O2
43 1350
2850
2770
0
300
490
335
510
natural gas
C2H2, lighting gas, H2, C3H8 and natu-
however, obtained with C2H2.
1.17
propane
1.2. Suitable combustible gases are
1.29
air
oxygen and a combustible gas, Figure
2.0
air
exothermal chemical reaction between
2.5
2.0
1.5
1.0
0.5
0
oxygen
Process energy is obtained from the
°C
10.3
370
8.5
330
KW
k
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Figure 1.2
/cm2
cm
/s
© ISF 2002
1. Gas Welding
4
C2H2 is produced in acetylene gas
loading funnel
generators by the exothermal transformation of calcium carbide with wa-
material lock
ter, Figure 1.3. Carbide is obtained
from the reaction of lime and carbon
in the arc furnace.
gas exit
feed wheel
C2H2 tends to decompose already at
a pressure of 0.2 MPa. Nonetheless,
grille
commercial quantities can be stored
sludge
when C2H2 is dissolved in acetone
(1 l of acetone dissolves approx. 24 l
of C2H2 at 0.1 MPa), Figure 1.4.
to
sludge pit
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© ISF 2002
Acetylene Generator
Figure 1.3
Acetone disintegrates at a pressure of
acetone
acetylene
more than 1.8 MPa, i.e., with a filling
pressure of 1.5 MPa the storage of 6m³
of C2H2 is possible in a standard cylinporous mass
der (40 l). For gas exchange (storage
and drawing of quantities up to 700 l/h)
N
a larger surface is necessary, therefore
acetylene cylinder
acetone quantity :
~13 l
the gas cylinders are filled with a po-
acetylene quantity :
6000 l
rous mass (diatomite). Gas consump-
cylinder pressure :
15 bar
tion during welding can be observed
from the weight reduction of the gas
filling quantity : up to 700 l/h
cylinder.
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© ISF 2002
Storage of Acetylene
Figure 1.4
1. Gas Welding
5
Oxygen
duced
gaseous
is
by
profrac-
cooling
tional distillation
cylinder
nitrogen
air
of liquid air and
bundle
stored in cylinders
oxygen
liquid
air
with a filling pres-
pipeline
liquid
oxygen
sure of up to 20
MPa, Figure 1.5.
tank car
nitrogen
vaporized
cleaning
compressor
For higher oxygen
separation
consumption, stor-
supply
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age in a liquid state
© ISF 2002
Principle of Oxygen Extraction
and cold gasification is more profit-
Figure 1.5
able.
The standard cylinder (40 l) contains,
50 l oxygen cylinder
at a filling pressure of 15 MPa, 6m³ of
protective cap
cylinder valve
O2 (pressureless state), Figure 1.6.
gaseous
take-off connection
N
Moreover, cylinders with contents of
p = cylinder pressure : 200 bar
10 or 20 l (15 MPa) as well as 50 l at
V = volume of cylinder : 50 l
Q = volume of oxygen : 10 000 l
20 MPa are common. Gas consumpcontent control
tion can be calculated from the pres-
Q=pV
sure difference by means of the gen-
foot ring
eral gas equation.
manometer
liquid
safety valve
vaporizer
filling
connection
user
still
liquid
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Storage of Oxygen
Figure 1.6
gaseous
1. Gas Welding
6
In order to prevent mistakes, the gas cylinders are colour-coded. Figure 1.7 shows a
survey of the present colour code and the future colour code which is in accordance
with DIN EN 1089.
The cylinder valves are also of
show a
thread
right-hand
union
Acetylene
different designs. Oxygen cylinder connections
actual condition
nut.
DIN EN 1089
blue
actual condition
white
DIN EN 1089
grey
cylinder
helium
oxygen techn.
valves are equipped
yellow
brown
grey
blue (grey)
brown
red
dark green
grey
with screw clamp
acetylene
retentions. Cylinder
valves
for
grey
other
argon
a
darkgreen
left-hand
vivid green
grey
grey
combustible gases
have
hydrogen
argon-carbon-dioxide mixture
black
grey
grey
darkgreen
thread-connection
nitrogen
carbon-dioxide
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with a circumferen-
© ISF 2002
Gas Cylinder-Identification
according to DIN EN 1089
tial groove.
Figure 1.7
Pressure regulators reduce the cylinder pressure to the requested working pressure, Figures 1.8 and 1.9.
cylinder pressure
working pressure
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© ISF 2002
Single Pressure Reducing Valve
during Gas Discharge Operation
Figure 1.8
1. Gas Welding
7
At a low cylinder pressure (e.g. acetylene cylinder) and low pressure fluctuations,
single-stage regulators
are applied; at higher cylinder pressures normally two-stage pressure regulators are
discharge pressure
locking pressure
used.
The
requested
pressure is set by
the
adjusting
screw. If the pressure increases on
the low pressure
side,
the
throttle
valve
closes
the
increased pressure
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© ISF 2002
onto
Single Pressure Reducing Valve,
Shut Down
the
brane.
Figure 1.9
The
injector-type
welding torch
injector or blowpipe
torch consists of a
body
with
valves
and welding chamber
with
mixer tube
coupling nut
mixer nozzle
oxygen valve
hose connection
for oxygen
A6x1/4" right
welding
nozzle, Figure 1.10.
injector
pressure nozzle
suction nozzle
By the selection of
suitable
welding
chambers,
fuel gas valve
welding nozzle
the
welding torch head
flame intensity can
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be
adjusted
for
different
plate thicknesses.
Figure 1.10
torch body
© ISF 2002
Welding Torch
welding
hose connection
for fuel gas
A9 x R3/8” left
mem-
1. Gas Welding
8
The special form of the mixing chamber guarantees highest possible safety against
flashback, Figure 1.11. The high outlet speed of the escaping O2 generates a negative pressure in the acetylene gas line, in consequence C2H2 is sucked and drawn-in.
C2H2 is therefore available with a very low pressure of 0.02 up to 0.05 MPa compared with O2 (0.2 up to 0.3 MPa).
acetylene
oxygen
acetylene
welding torch head injector nozzle
coupling nut
pressure nozzle
torch body
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© ISF 2002
Injector-Area of Torch
Figure 1.11
A neutral flame adjustment allows the differentiation of three zones of a chemical
reaction, Figure 1.12:
0. dark core:
escaping gas mixture
1. brightly shining centre cone:
acetylene decomposition
C2H2 -> 2C+H2
2. welding zone:
1st stage of combustion
2C + H2 + O2 (cylinder) -> 2CO + H2
3. outer flame:
2nd stage of combustion
4CO + 2H2 + 3O2 (air) ->
4CO2 + 2H2O
complete reaction:
2C2H2 + 5O2 ->
4CO2 + 2H2O
1. Gas Welding
9
welding flame
combustion
welding nozzle centre cone
welding zone
2-5
outer flame
3200°C
2500°C
1800°C
1100°C
400°C
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© ISF 2002
Figure 1.12
welding flame
ratio of mixture
By changing the mixture ratio of the
excess of
oxygen
normal
(neutral)
excess of
acetylene
volumes O2:C2H2 the weld pool can
greatly be influenced, Figure 1.13. At a
neutral flame adjustment the mixture
ratio is O2:C2H2 = 1:1. By reason of the
higher flame temperature, an excess
oxygen flame might allow faster welding of steel, however, there is the risk
of oxidizing (flame cutting).
effects in welding of steel
Area of application: brass
foaming
spattering
sparking
The excess acetylene causes the
carburising of steel materials.
consequences:
carburizing
hardening
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reducing
oxidizing
© ISF 2002
Area of application: cast iron
Effects of the Welding Flame
Depending on the Ratio of Mixture
Figure 1.13
1. Gas Welding
10
By changing the gas mixture outlet
welding flame
speed the flame can be adjusted to
balanced (neutral) flame
nozzle size: for plate thickness of 2-4 mm
the heat requirements of the welding
discharging velocity and weld heat-input rate: low
2
job, for example when welding plates
(thickness: 2 to 4 mm) with the welding chamber size 3: “2 to 4 mm”, Fig-
soft flame
discharging velocity and weld heat-input rate: middle
3
ure 1.14. The gas mixture outlet
speed is 100 to 130 m/s when using a
medium or normal flame, applied to
moderate flame
at, for example, a 3 mm plate. Using a
discharging velocity and weld head-input rate: high
4
soft flame, the gas outlet speed is
lower (80 to 100 m/s) for the 2 mm
plate, with a hard flame it is higher
(130 to 160 m/s) for the 4 mm plate.
hard flame
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© ISF 2002
Effects of the Welding Flame
Depending on the Discharge Velocity
Figure 1.14
Depending on the plate thickness are
the working methods “leftward weld-
Leftward welding is applied to a plate thickness of up to 3 mm.
The weld-rod dips into the molten pool from time to time,
but remains calm otherwise. The torch swings a little.
Advantages:
easy to handle on thin plates
ing” and “rightward welding” applied,
Figure 1.15. A decisive factor for the
designation of the working method is
the sequence of flame and welding rod
as well as the manipulation of flame
and welding rod. The welding direction
itself is of no importance. In leftward
welding-rod
flame
welding bead
Rightward welding ist applied to a plate thickness of 3mm
upwards. The wire circles, the torch remains calm.
Advantages:
- the molten pool and the weld keyhole are easy to observe
- good root fusion
- the bath and the melting weld-rod are permanently protected
from the air
- narrow welding seam
- low gas consumption
welding the flame is pointed at the
open gap and “wets” the molten pool;
the heat input to the molten pool can
be well controlled by a slight move-
weld-rod
flame
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ment of the torch (s = 3 mm).
© ISF 2002
Flame Welding
Figure 1.15
1. Gas Welding
11
In rightward welding the flame is directed onto the molten pool; a weld
can be applied to a plate thickness of
1,5
approx. 1.5 mm without filler material,
symbol
flange weld
1,0
but this does not apply to any other
plain butt
weld
1,0
4,0
3,0
12,0
1,0
8,0
1,0
8,0
lap seam
1,0
8,0
fillet weld
plate thickness and weld shape, Figure 1.16.
denotation
s
gap
preparations
r=
Flanged welds and plain butt welds
plate thickness
range s [mm]
from to
~
~ s+1
keyhole is formed (s = 3 mm).
V - weld
1-2
1-2
corner weld
By the specific heat input of the different welding methods all welding positions can be carried out using the
oxyacetylene welding method, Figures
1.17 and 1.18
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© ISF 2002
Gap Shapes for Gas Welding
Figure 1.16
butt-welded seams in
gravity position
When working in tanks and confined
PA
spaces, the welder (and all other per-
gravity fillet welds
sons present!) have to be protected
against the welding heat, the gases
PB
produced during welding and lack of
horizontal fillet welds
vertical fillet and butt welds
oxygen ((1.5 % (vol.) O2 per 2 % (vol.)
s
f
C2H2 are taken out from the ambient
atmosphere)), Figure 1.19. The addi-
PF
PG
vertical-upwelding position
vertical-down position
PC
horizontal on
vertical wall
PE
overhead position
PD
horizontal overhead position
tion of pure oxygen is unsuitable (explosion hazard!).
© ISF 2002
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Welding Positions I
Figure 1.17
1. Gas Welding
12
A special type of autogene method is
flame-straightening, where specific locally applied flame heating allows for
shape correction of workpieces, Figure
PA
1.20. Much experience is needed to
PB
PF
carry out flame straightening processes.
The basic principle of flame straightening
depends on locally applied heating in
PC
connection with prevention of expansion.
This process causes the appearance of a
PG
PD
heated zone. During cooling, shrinking
forces are generated in the heated zone
PE
and lead to the desired shape correction.
© ISF 2002
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Welding Positions II
Figure 1.18
Safety in welding and cutting inside of
tanks and narrow rooms
Flame straightening
welded parts
first warm up both
lateral plates, then belt
Hazards through gas, fumes, explosive mixtures,
electric current
protective measures / safety precautions
1. requirement for a permission to enter
2. extraction unit, ventilation
butt weld
3 to 5 heat sources
close to the weld-seam
3. second person for safety reasons
4. illumination and electric machines: max 42volt
double fillet weld
1,3 or 5 heat sources
5. after welding: Removing the equipment from the tank
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© ISF 2002
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Gas Welding in Tanks and
Narrow Rooms
Figure 1.19
© ISF 2002
Flame Straightening
Figure 1.20