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Back Mechanical Mechanical Engineering Dictionary Machine Design Welding - Processes, Welded Joints, Symbols, Stress in Welds

Welding - Processes, Welded Joints, Symbols, Stress in Welds

  • It is a process of joining parts by means of heat & pressure, causing fusion of parts (Resistance Welding) OR heating metal to fusion temperature with or without addition of weld metals (Fusion Welding).
  • Code & specification AWS(American Welding Society)
  • Fusion welding employs either
  • Electric arc (used for most structural welding) OR Oxyacetylene flame

Welding processes

  • Welding is a process of joining metal parts by means of heat and pressure,which causes fusion of parts(resistance welding), or by heating the metal to fusion temperature,with or without addition of weld metal (fusion welding)
  • Forge welding
  • Resistance welding
  • Fusion welding
    • Oxyacetylene welding
    • Electric arc welding

Forge welding

It consists of simply heating the pieces above certain temperature and hammering them together

Resistance welding

Heat is generated by electrical resistance to a current of high amperage & low voltage passing through small area of contact between parts to be connected.

Fusion welding

Method of connecting pieces by molten metal

  • In Metal-arc welding, arc is a sustained spark between a metallic electrode and work to be welded.
  • At the instant arc is formed the temperature of work and tip of electrode are brought to melting point.
  • As the tip of electrode melts, tiny globules of molten metal form.

  • The molten metal, when exposed to air combines chemically with oxygen & nitrogen forming oxides & nitrides, which tend to embrittle it & less corrosive resistant.
  • Tough, ductile weld are produced if molten pool is shielded by an inert gas, which envelops molten metal & tip of electrode.

Shielded Metal Arc Welding (SMAW)

  • When an arc is struck between the metal rod (electrode) and the work piece, both the rod and work piece surface melt to form a weld pool.
  • Simultaneous melting of the flux coating on the rod will form gas and slag which protects the weld pool from the surrounding atmosphere.

Submerged Arc Welding (SAW)

  • A bare wire is fed through welding head at a rate to maintain constant arc length.
  • Welding is shielded by blanket of granular fusible material fed onto the work area by gravity, in an amount sufficient to submerge the arc completely.
  • In addition to protecting weld from atmosphere, the covering aids in controlling rate of cooling of weld.

Flux Cored Arc Welding (FCAW)

The FCAW process utilizes the heat of an arc between a continuously fed consumable flux cored electrode and the work. The heat of the arc melts the surface of the base metal and the end of the electrode. The metal melted off the electrode is transferred across the arc to the work piece, where it becomes the deposited weld metal. Shielding is obtained from the disintegration of ingredients contained within the flux cored electrode.

Metal-arc Inert Gas (MIG) Welding

The "Metal-arc Inert Gas" (Mig) Welding refers to the wire that is used to start the arc. It is shielded by inert gas and the feeding wire also acts as the filler rod.

Tungsten-arc Inert Gas (TIG) Welding

The arc is started with a tungsten electrode shielded by inert gas and filler rod is fed into the weld puddle separately. The gas shielding that is required to protect the molten metal from contamination is supplied through the torch..

Welded Joints

They are classified as:

Butt Joint is groove-welded

Lap Joint is fillet-welded

Tee Joint can be fillet-welded or groove-welded

Corner Joint

Nomenclature of Welds

  • The part of weld assumed to be effective in transferring stress is Throat.
  • The faces of weld in contact with the parts joined is called its Legs.
  • For equal-legged fillet weld throat is 0.707s, where s is leg size.

  • Large fillet welds made manually require two or more passes.
  • Each pass must cool, and slag must be removed before next pass.
  • Most efficient fillet welds are those which can be made in one pass.
  • Largest size can be made in one pass depends upon welding position & should not exceed the following.
    • 5/16
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