PRACTICAL 5 : WELDING AND WELDING SYMBOLS
Welding may be described as a process of uniting two pieces of metal or alloy by
raising the temperature of the surfaces to be joined so that they become plastic or
molten. This may be done with or without the application of pressure and with or
without the use of added metal.
DIFFERENT METHODS OF WELDING
There are numerous methods of welding, but they can be grouped broadly into two
Forge welding is the term covering a group of welding processes in which the parts
to be joined are heated to a plastic condition in a forge or other furnace, and are
welded together by applying pressure or impact, e.g. by rolling, pressing, or
Fusion welding is the process where the surfaces to be joined are melted with or
without the addition of filler metal. The term is generally reserved for those
processes in which welding is achieved by fusion alone, without pressure. Forge
welding will be dealt with first.
Pressure welding is the welding of metal by means of mechanical pressure whilst
the surfaces to be joined are maintained in a plastic state. The heating for this
process is usually provided by the process of resistance welding, where the pieces
of metal to be joined are pressed together and a heavy current is passed through
Projection welding is a resistance-welding process in which fusion is produced by
the heat obtained from the resistance to flow of electric current through the work
parts, which are held together under pressure by the electrodes providing the
current. The resulting welds are localized at predetermined points by the design of
the parts to be welded. The localization is usually accomplished by projections or
Spot welding is a resistance-welding process of joining two or more overlapping
parts by local fusion of a small area or ‘spot’. Two copper-alloy electrodes contact
either side of the overlapped sheets, under known loads produced by springs or air
Stitch welding is spot welding in which successive welds overlap.
Seam welding is a resistance-welding process in which the electrodes are discs.
Current is switched on and off regularly as the rims of the discs roll over the work,
with the result that a series of spot welds is at such points. If a gas-tight weld is
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required, the disc speed and time cycle are adjusted to obtain a series of
Flash-butt welding is a resistance-welding process which may be applied to rod,
bar, tube, strip, or sheet to produce a butt joint. After the current is turned on, the
two parts are brought together at a predetermined rate so that discontinuous arcing
occurs between the two parts to be joined. This arcing produces a violent expulsion
of small particles of metal (flashing), and a positive pressure in the weld area will
exclude air and minimize oxidation. When sufficient heat has been developed by
flashing, the parts are brought together under heavy pressure so that all fused and
oxidized material is extruded from the weld. Fusion-welding processes can now be
dealt with. The heat for fusion welding is provided by either gas or electricity.
Gas welding is a process in which heat for welding is obtained from a gas or gases
burning at a sufficiently high temperature produced by an admixture of oxygen.
Examples of the gases used are acetylene (oxy-acetylene welding), hydrogen (oxy
hydrogen welding), and propane (oxy-propane welding).
In air-acetylene welding, the oxygen is derived from the atmosphere by induction.
Electrical fusion welding is usually done by the process of ‘arc welding’. Metal-
arc welding is welding with a metal electrode, the melting of which provides the
Carbon arc welding is a process of arc welding with a carbon electrode (or
electrodes), in which filler metal and sometimes flux may be used.
Submerged-arc welding is a method in which a bare copper-plated steel electrode is
used. The arc is entirely submerged under a separate loose flux powder which is
continually fed into and over the groove which is machined where the edges to be
welded are placed together. Some of the flux powder reacts with the molten metal:
part fuses and forms a refining slag which solidifies on top of the weld deposit; the
remainder of the powder covers the weld and slag, shielding them from
atmospheric contamination and retarding the rate of cooling.
When welds are specified on engineering and fabrication drawings, a cryptic set of
symbols issued as a sort of shorthand for describing the type of weld, its size, and
other processing and finishing information. The purpose of this practical is to
introduce you to the common symbols and their meaning.
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