ELECTRICAL WELDING
• Resistance Welding – The two parts are
pressed together and an alternating current
(A/C) is passed through the contact zone.
• Spot welding – used extensively on sheet
metals (holds handles on pots, car body
together)
• Ribbon welding rollers. - parts to be welded
are drawn between electrodes rollers while
electricity is applied.
IT 208
Chapter 14
1
Arc Welding
A sustained arc generates the heat for melting
the work piece and filler material.
– Consumable electrodes
– Non-consumable electrodes
IT 208
Chapter 14
2
Consumable electrodes
• Flux Core Arc Welding (FCAW) developed in the early 1950s as an
adaptation to SMAW to overcome limitation imposed by the use of a
stick electrodes. Uses a spool of filler wire fed through the hand-piece.
A core of flux is inside the wire
• Two versions
– Self-shielded flux-cored arc welding – includes not only fluxes but
also ingredients that generate shielding gases for protecting the arc.
– Gas-shielded flux-cored arc welding – developed primarily for
welding steels, obtains a shielding from externally supplied gases,
similar to GMAW
IT 208
Chapter 14
3
Consumable electrodes
• Submerged Arc Welding (SAW) – uses a continuous, consumable
bare wire electrode, and arc shielding is provided by a cover of
granular flux. Low-carbon, low alloy, and stainless steels can be
readily welded by SAW.
• Electrogas Welding (EGW) – uses a continuous consumable
electrode (either flux-cored wire or bare wire with externally supplied
shielding gases) and molding shoes to contain the molten metal.
• Shielded Metal Arc Welding (SMAW) (stick) – arc is struck between
the rod (shielded metal covered by flux) and the work pieces to be
joined, the impurities rise to the top of the weld in the form of slag (1819a, handout pg. 40)
IT 208
Chapter 14
4
A sustained arc, shielded by molten slag, is maintained in consumable-electrode welding by the
(a) shielded metal-arc, (b) submerged arc, and (c) electrogas methods.
IT 208
Chapter 14
5
Oxyfuel Gas Welding
• Welding process that uses fuel gas combined with oxygen to
produce flame
• This flame heat melts the metals at the joint
• Acetylene fuel is used in gas welding process
Primary combustion process
C2H2 + O2
2CO + H2 + heat
• This reaction dissociates into carbon monoxide and hydrogen.
Secondary combustion process
2CO + H2 + 1.5 O2
2CO2 + H2O + heat
Types of flames
• Neutral flame
• Oxidising flame
• Carburising flame
Filler Metals :
•
•
•
•
Additional material to weld the weld zone
Available as rod or wire
They can be used bare or coated with flux
The purpose of the flux is to retard the
Torch used in Oxyacetylene Welding
Fig : (a) General view of and
(b) cross-section of a
torch used in
oxyacetylene welding.
The acetylene valve is
opened first; the gas is
lit with a park lighter or
a pilot light; then the
oxygen valve is opened
and the flame adjusted.
(c) Basic equipment
used in oxyfuel-gas
welding. To ensure
correct connections, all
threads on acetylene
fittings are left-handed,
whereas those for
oxygen are righthanded. Oxygen
regulators are usually
painted green, acetylene
regulators red.
Pure Acetylene and Carburizing
Flame profiles
Neutral and Oxidizing Flame
Profiles
Flame definition
• The neutral flame (Fig. 4-1) is produced when the ratio of oxygen to
acetylene, in the mixture leaving the torch, is almost exactly one-toone. It’s termed ”neutral” because it will usually have no chemical
effect on the metal being welded. It will not oxidize the weld metal; it
will not cause an increase in the carbon content of the weld metal.
• The excess acetylene flame (Fig. 4-2), as its name implies, is created
when the proportion of acetylene in the mixture is higher than that
required to produce the neutral flame. Used on steel, it will cause an
increase in the carbon content of the weld metal.
• The oxidizing flame (Fig. 4-3) results from burning a mixture which
contains more oxygen than required for a neutral flame. It will oxidize
or ”burn” some of the metal being welded.