gas welding report in mechanical engineering

1. 1.
Gas Welding

2. 1. Gas Welding
7
Although the oxy-acetylene process has been introduced long time ago it is still applied for its
flexibility and mobility. Equipment for oxyacetylene welding consists of just a few elements, the energy necessary for welding can be transported in cylinders, Figure 1.1.
density in normal state [kg/m3]
7
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0
0
°C
10.3
370
8.5
330
KW
k
/cm2
natural gas
natural gas
2850
2770
propane
flame temperature with O2
flame efficiency with O2
flame velocity with O2
43 1350
acetylene
3200
510
propane
air
300
490
335
acetylene
200
oxygen
400
oxygen cylinder with pressure reducer
acetylene cylinder with pressure reducer
oxygen hose
acetylene hose
welding torch
welding rod
workpiece
welding nozzle
welding flame
cm
/s
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Properties of Fuel Gas in
Combination with Oxygen
Equipment Components
for Gas Welding
Figure 1.1
oxygen
645
645
600
2
1
2
3
4
5
6
7
8
9
1.43
0.9
ignition temperature [OC]
9
1
1.17
natural gas
8
1.29
propane
6
5
2.0
acetylene
4
2.5
2.0
1.5
1.0
0.5
0
air
3
Figure 1.2
Process energy is obtained from the exothermal chemical reaction between oxygen and a
combustible gas, Figure 1.2. Suitable combustible gases are C2H2, lighting gas, H2, C3H8 and
natural gas; here C3H8 has the highest calorific value. The highest flame intensity from point
of view of calorific value and flame propagation speed is, however, obtained with C2H2.
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3. 1. Gas Welding
8
C2H2 is produced in acetylene gas genera-
loading funnel
tors by the exothermal transformation of calcium carbide with water, Figure 1.3. Carbide
material lock
is obtained from the reaction of lime and carbon in the arc furnace.
gas exit
C2H2 tends to decompose already at a pres-
feed wheel
sure of 0.2 MPa. Nonetheless, commercial
quantities can be stored when C2H2 is disgrille
solved in acetone (1 l of acetone dissolves
sludge
approx. 24 l of C2H2 at 0.1 MPa), Figure 1.4.
to
sludge pit
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Acetylene Generator
Figure 1.3
Acetone disintegrates at a pressure of more
acetone
acetylene
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 cylinder (40 l). For gas ex-
porous mass
change (storage and drawing of quantities up
to 700 l/h) a larger surface is necessary,
N
acetylene cylinder
therefore the gas cylinders are filled with a
acetone quantity :
porous mass (diatomite). Gas consumption
acetylene quantity :
6000 l
during welding can be observed from the
cylinder pressure :
15 bar
~13 l
weight reduction of the gas cylinder.
filling quantity : up to 700 l/h
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Storage of Acetylene
Figure 1.4
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4. 1. Gas Welding
9
Oxygen is produced by
fractional distillation of
gaseous
cooling
liquid air and stored in cyl-
cylinder
nitrogen
air
inders with a filling presbundle
sure of up to 20 MPa, Fig-
oxygen
liquid
air
oxygen
ure 1.5. For higher oxygen
pipeline
liquid
consumption, storage in a
liquid state and cold gasifi-
tank car
nitrogen
vaporized
cleaning
compressor
separation
cation is more profitable.
supply
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Principle of Oxygen Extraction
Figure 1.5
The standard cylinder (40 l) contains, at a
50 l oxygen cylinder
filling pressure of 15 MPa, 6m³ of O2 (pres-
protective cap
cylinder valve
sureless state), Figure 1.6. Moreover, cylin-
gaseous
take-off connection
N
ders with contents of 10 or 20 l (15 MPa) as
p = cylinder pressure : 200 bar
V = volume of cylinder : 50 l
Q = volume of oxygen : 10 000 l
well as 50 l at 20 MPa are common. Gas
consumption can be calculated from the pres-
content control
Q=pV
sure difference by means of the general gas
foot ring
equation.
manometer
liquid
safety valve
vaporizer
filling
connection
user
still
liquid
gaseous
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Storage of Oxygen
Figure 1.6
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5. 1. Gas Welding
10
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.
old condition
DIN EN 1089
blue
white
old condition
DIN EN 1089
grey
brown
grey
blue (grey)
also of
are
different designs.
Oxygen cylinder connec-
helium
oxygen techn.
yellow
The cylinder valves
brown
red
red
tions show a
right-hand
thread union nut. Acetylene
acetylene
grey
hydrogen
dark green
grey
vivid green
grey
grey
argon-carbon-dioxide mixture
argon
darkgreen
black
grey
grey
carbon-dioxide
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valves
are
equipped with screw clamp
retentions. Cylinder valves
for
darkgreen
nitrogen
cylinder
other
combustible
gases have a left-hand
thread-connection with a
Gas Cylinder-Identification
according to DIN EN 1089
circumferential groove.
Figure 1.7
cylinder pressure
working pressure
Pressure
regulators
re-
duce the cylinder pressure
to the requested working
pressure, Figures 1.8 and
1.9.
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Single Pressure Reducing Valve
during Gas Discharge Operation
Figure 1.8
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6. 1. Gas Welding
11
At a low cylinder pressure (e.g. acetylene cylinder) and low pressure fluctuations, singlestage regulators
discharge pressure
locking pressure
are applied; at higher cylinder pressures normally
two-stage pressure regulators are used.
The requested pressure is
set by the adjusting screw.
If the pressure increases
on the low pressure side,
the throttle valve closes the
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increased
Single Pressure Reducing Valve,
Shut Down
pressure
onto
the membrane.
Figure 1.9
The injector-type torch consists of a body with valves and welding chamber with welding
nozzle, Figure 1.10. By the selection of suitable welding chambers, the flame intensity can be
adjusted for welding different plate thicknesses.
The special form of the mixing chamber guarantees highest possible safety against
flashback, Figure 1.11.
welding torch
injector or blowpipe
The high outlet speed of
the escaping O2 generates a negative pressure
mixer tube
coupling nut
mixer nozzle
oxygen valve
hose connection
for oxygen
A6x1/4" right
in the acetylene gas line,
in consequence C2H2 is
sucked
and
injector
pressure nozzle
suction nozzle
drawn-in.
C2H2 is therefore avail-
fuel gas valve
welding nozzle
hose connection
for fuel gas
A9 x R3/8” left
able with a very low
pressure of 0.02 up to
0.05 MPa
with
O2
0.3 MPa).
welding torch head
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-compared
(0.2
up
torch body
© ISF 2002
Welding Torch
to
Figure 1.10
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7. 1. Gas Welding
12
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
1st stage of combustion
2. welding zone:
2C + H2 + O2 (cylinder) -> 2CO + H2
2nd stage of combustion
3. outer flame:
4CO + 2H2 + 3O2 (air) ->
4CO2 + 2H2O
complete reaction:
2C2H2 + 5O2 ->
4CO2 + 2H2O
acetylene
oxygen
acetylene
welding torch head injector nozzle
coupling nut
pressure nozzle
torch body
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Injector-Area of Torch
Figure 1.11
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8. 1. Gas Welding
13
welding flame
ratio of mixture
welding flame
combustion
welding nozzle centre cone
welding zone
2-5
excess of
oxygen
normal
(neutral)
excess of
acetylene
outer flame
3200°C
2500°C
1800°C
1100°C
effects in welding of steel
foaming
spattering
sparking
400°C
consequences:
carburizing
hardening
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Temperature Distribution
in the Welding Flame
reducing
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oxidizing
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Effects of the Welding Flame
Depending on the Ratio of Mixture
Figure 1.12
Figure 1.13
welding flame
By changing the mixture ratio of the volumes
balanced (neutral) flame
nozzle size: for plate thickness of 2-4 mm
O2:C2H2 the weld pool can greatly be influ-
discharging velocity and weld heat-input rate: low
2
enced, Figure 1.13. At a neutral flame adjustment the mixture ratio is O2:C2H2 = 1:1. By
reason of the higher flame temperature, an
soft flame
excess oxygen flame might allow faster
discharging velocity and weld heat-input rate: middle
3
welding of steel, however, there is the risk of
oxidizing (flame cutting).
Area of application: brass
moderate flame
discharging velocity and weld head-input rate: high
4
The excess acetylene causes the carburising
of steel materials.
Area of application: cast iron
hard flame
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Effects of the Welding Flame
Depending on the Discharge Velocity
Figure 1.14
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9. 1. Gas Welding
14
By changing the gas mixture outlet speed the flame can be adjusted to the heat requirements
of the welding job, for example when welding plates (thickness: 2 to 4 mm) with the welding
chamber size 3: “2 to 4 mm”, Figure 1.14. The gas mixture outlet speed is 100 to 130 m/s
when using a medium or normal flame, applied to at, for example, a 3 mm plate. Using a
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.
Depending on the plate thickness are the working methods “leftward welding” 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 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 movement of the torch (s ≤ 3 mm).
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.
1,5
welding-rod
flame
welding bead
denotation
flange weld
1,0
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
plain butt
weld
4,0
3,0
12,0
1,0
8,0
8,0
lap seam
1,0
8,0
fillet weld
V - weld
1-2
1-2
corner weld
flame
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Flame Welding
Figure 1.15
1,0
1,0
weld-rod
symbol
s
gap
preparations
~ s+1
~
plate thickness
range s [mm]
from to
r=
Advantages:
easy to handle on thin plates
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Gap Shapes for Gas Welding
Figure 1.16
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10. 1. Gas Welding
15
In rightward welding the flame is directed
onto the molten pool; a weld keyhole is
gravity fillet welds
PA
butt-welded seams in
gravity position
formed (s ≥ 3 mm).
Flanged welds and plain butt welds can be
horizontal fillet welds
applied to a plate thickness of approx.
vertical fillet and butt welds
PB
1.5 mm without filler material, but this does
f
PF
PG
vertical-upwelding position
vertical-down position
PC
s
not apply to any other plate thickness and
horizontal on
vertical wall
weld shape, Figure 1.16.
By the specific heat input of the different
PE
welding methods all welding positions can be
overhead position
carried out using the oxyacetylene welding
PD
method, Figures 1.17 and 1.18
horizontal overhead position
© ISF 2002
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Welding Positions I
Figure 1.17
When working in tanks and confined
spaces, the welder (and all other persons
present!) have to be protected against the
welding heat, the gases produced during
welding and lack of oxygen ((1.5 % (vol.) O2
per 2 % (vol.) C2H2 are taken out from the
PA
ambient atmosphere)), Figure 1.19. The addition of pure oxygen is unsuitable (explosion
hazard!).
PB
PF
A special type of autogene method is flame-
PC
straightening, where specific locally applied
flame heating allows for shape correction of
PG
workpieces, Figure 1.20. Much experience is
PD
needed to carry out flame straightening proc-
PE
esses.
The basic principle of flame straightening de© ISF 2002
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pends on locally applied heating in connecWelding Positions II
tion with prevention of expansion. This procFigure 1.18
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11. 1. Gas Welding
16
ess causes the appearance of a heated zone. During cooling, shrinking forces are generated
in the heated zone and lead to the desired shape correction.
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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Gas Welding in Tanks and
Narrow Rooms
Figure 1.19
© ISF 2002
Flame Straightening
Figure 1.20
2005