welding can be applied to processes in which metals are heated
to the temperature at which they melt and are then joined
without hammering or the application of pressure. The joint
can be formed without the use of a filler metal, but usually
a filler metal in the form of a wire or rod is employed to
fill the joint.
filler metal has the same composition as the parent metal,
but may contain alloying metals to improve its fluidity in
the molten condition or to produce a fine grained weld structure.
The wire or rod of filler metal may be sheathed in a special
coating. Such coatings perform one or more of various functions
: serve as a flux, remove oxides or other disturbing substances
that may be present, improve the wettability of the material
surface, protect the weld against external influences, prevent
excessively rapid cooling, and stabilize the arc.
composition of the coating depends more particularly on the
material to be welded and on the welding method. Mixtures
of oxides of iron, manganese and titanium, alkaline earth
carbonates, fluorite, and organic compounds are used for coatings.
Sources of heat employed in fusion welding are gas, electricity,
chemical reactions, etc. Gas welding (Fig.1) uses a flame
produced by the burning in oxygen of acetylene (oxyacetylene
welding) or sometimes another fuel gas (e.g., propane, butane,
hydrogen) to heat and liquefy the metal at the joint to be
welded. This is a very widely employed method of welding iron,
steel, cast iron, and copper. The flame is applied to the
edges of the joint and to a wire of the appropriate filler
metal, which is melted and runs into the joint.
A fairly recent development is the electroslag
process (Fig.2) in which the metal at the joint is melted
in an electrically conducting (ionized) molten-slag bath whose
temperature is above the melting temperature of the metal.
The welds are executed as vertical welds; with this method
it is, for instance, possible to form butt welds in very thick
plates quickly and economically.
current is supplied to the slag bath through bare metallic
electrodes, which melt away and provide the filler metal.
The molten filler metal sinks in the slag, fills the gap of
the joint and slowly solidifies in it, from the bottom upwards.
The gap is bridged by water-cooled copper shoes which, together
with the faces of the joint, form a mold for the molten metal.
The shoes move upwards along the joint during welding.
most important and most widely used fusion-welding technique
is arc welding, which employs an electric arc to melt the
parent metal and the filler metal. The latter may be provided
in the form of an electrode which melts away or it may be
melted thermally i.e., without carrying the welding current.
general technique can be subdivided into three categories
: open-arc welding, covered arc welding, and gas-shielded-arc
welding. Open-arc welding by Benardo’s method (Fig.3a)
employs direct current, the arc being formed between the parent
metal and a carbon electrode.
Zerener’s method (Fig 3b) the arc is formed between
two carbon electrodes; the heat of the arc is concentrated
on the workpiece by the action of a magnetic coil. The method
now most widely used was originated by Slavjanov (Fig.3c):
the arc is formed between a metallic electrode, which gradually
melts away to supply the filler metal, and the workpiece.
process known as firecracker welding (Fig.4) is an example
of a covered-arc method. A heavily coated electrode is laid
horizontally on the joint to be welded and is covered with
an insulating layer of paper and a covering bar of copper
or some other metal. The workpiece is connected to one pole
and the electrode is connected to the other pole of a current
source. An arc is struck between the end of the electrode
and the joint, and burns along the length of the electrode
form of covered-arc welding is submerged-arc welding (Fig.5).
The flux is supplied separately in the form of powder which
blankets the arc. The powder melts and protects the molten
filler metal from atmospheric contamination. Any powder not
melted is recovered by suction and reused. When cool, the
fused powder forms a slag, which peels off the weld.
welding is based on the principle of protecting the molten
filler metal by an envelope of chemically inert gas, which
may be helium (heliarc process), argon (argonarc process)
or carbon dioxide. In atomic-hydrogen welding (Fig.6a) the
heat liberated by monatomic hydrogen when recombining into
molecules is used to fuse the metal.
An alternating-current arc is maintained across two tungsten
electrodes. A stream of hydrogen gas is passed through the
arc, in which the hydrogen molecules are split up into atoms.
Outside the actual arc these atoms recombine into molecules.
This produces great heat, which melts the parts to be welded
and unites them, with or without the addition of a filler
metal. The inert-gas tungsten-arc process (Fig.6b) and the
inert-gas metal-arc process (Fig.6c) are two shielded-arc
welding processes that are used both for manual techniques
and for automatic welding by mechanized equipment.
welding (Fig.7) has already been referred to in connection
with pressure welding. It is also used as a fusion-welding
process, more particularly for iron and steel castings and
forgings. The source of heat is not electricity or gas but
a chemical reaction that produces intense heat (3000oC): the
combustion of a mixture of aluminum powder and iron oxide
by which the aluminum is converted into aluminum oxide and
the iron oxide is reduced to molten iron (or steel). The parts
to be joined are surrounded by a sand-lined mold. The powder
mixture is packed in a conical crucible and ignited. The molten
iron flows in and around the joint, where it fuses with the
preheated parent metal.
o CONTACT US |