An arc is generated between two conductor of
electricity cathode and anode when they are touched to establish the flow of
current and then separated by a small distance. Electrons liberated from the
cathode more towards the anode and are accreted in their movement. When they
strike the anode at thigh velocity, large amount of D is generated. In
order to produce the arc potential difference between the two electrodes should
be sufficient to allow them to move across the air gap.
If air gap become too large the arc will be extinguish
the temp across the arc would be uniform for a DC arc rather than an Arc of AC
as cathode and anode would change continuously.
Requirement of Arc welding equipment
a) AC
= transformer
ii) Motor or engine
driven alternator
b) DC
= transformer with DC rectifier
ii) Motor or engine
driven alternator
Welding machine =
constant current type which vary the voltage.
=Constant voltages are
welding
OXYACETYLENE GAS CUTTING
OXY- Acetylene welding equipment
Acetylene cylinder is
filled with (80-85%) porous material such as calcium silicate and then filled
with acetone. This can absorb large volume of acetylene.
Acetylene would be
released from acetone at a slow rate and thus would not form any high pockets
of high-pressure acetylene.
FLAME ADJUSTMENT FOR
ORY-ACETYLENE WELDING
MILD STEEL - NUETRAL
ALUMINIUM – SLIGHTLY CARBURISING
HIGH CARBON STEEL- REDUCING
BRASS- SLIGHTLY OXIDISING
GREY CAST IRON – NUETRAL
COPPER, BRONZE – NUETRAL, SLIGHTLY OX)
ALLOY STEEL – NUETRAL
NICKERL ALLOYS- SLIGHTLY CARBURISING
LIGHTING THE TORCH FLAME
1) Check
the recommended C2H2 and O2 pressures for size of welding blow pipe (0.3) BAR.
Pr cutting nozzle
selected
2) 
C2H2 0.3 – 0.5
C2H2 0.3 – 0.5 
O2 SB 2-4
DB 5-6
3)
Purge line especially acetylene to remove
any entrapped air. Stop purging when odor of C2H2 is
noted
4)
Open the O2 V/V slightly to
obtain low flow O2. This is to minimize amount of soot produced
when flame is first lighted.
5)
Open the acetylene V/V approx ½ turn.
6)
Light the flame with gas igniter adjust
flame to the type required.
Type of flame
(A)
Carburizing
This flame consists of (3) elements
(1) Primary
cone which is a distinct blue cone extending a short distance from ori fice,
length to
diameter ratio of 5:1
(2) Secondary
cone- This appears to surround sand extend beyond primary cone. This is much
lighter intensity and white in case of
highly carbonizing flame.
(3) Flame
envelope- this is darker, less intense blue.
(b) Neutral flame- this is obtained by
increasing the O2 flow until secondary cone just
disappears. This flame is metallurgical
neutral and is mot commonly used for mild steel.
(c) Oxidizing flame- this is obtained by
adding more O2. The primary cone further darkness and
primary and secondary cone get
shortened.
Shutting
down
1) Close the C2H2 V/V
flame will go out with a pop.
2) Close the O2 V/V
3) Close the cyl. Top V/Vs or shut off V/Vs
at outlet station.
4) Open the C2H2 V/V
on welding torch and exhaust C2H2.
5) Open O2 V/V and exhaust O2
form the line
6) Remove and store equipment in proper
place.
Chemical reaction
C2H2
+ 2(1/2) O2 2
CO2 + H2O + E
Flame temp as
high as – 3480deg C
Combustion
process = first phase
2) Se con phase
First phase-
At edge of
primary cone
C2H2
+ O2 2 CO + H2
+ E
Equal volume
Producing a
neutral flame
Second phase
2CO
+ O2 2 CO2
+E
H2
+ ½ O2 h2O
+E
O2 is taken
from surrounding area
Reactions take
place in the flame envelope.
Flame stability
Flame is
started by raising temperature of mixture to ignition temp. The reactions occur
and thermal energy is released. This energy raises the adjacent mixt to
ignition temp and this reaction continues.
The thermal
energy release is far more than is necessary to sustain the reaction. Resulting
in as stable flame over a wide range of C2H2 + O2 (MIXT)
Flash back –
Back fire (back flow check V/Vs)
Flame velocity
+ gas mixt velocity
Initiation of
this reaction is possible only if metal has achieved the temp of 870 deg C or
above for steel.
Fe3O4
has lower MP than the MP of steel.
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