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42
Most metals, when heated, react with the atmosphere or other nearby
metals. These reactions can be extremely detrimental to the properties of a
welded joint. Most metals, for example, rapidly oxidize when molten. A
layer of oxide can prevent proper bonding of the metal. Molten-metal
droplets coated with oxide become entrapped in the weld and make the joint
brittle. Some valuable materials added for specific properties react so quickly
on exposure to the air that the metal deposited does not have the same
composition as it had initially. These problems have led to the use of fluxes
and inert atmospheres.
In fusion welding the flux has a protective role in facilitating a
controlled reaction of the metal and then preventing oxidation by forming a
blanket over the molten material. Fluxes can be active and help in the process
or inactive and simply protect the surfaces during joining.
Inert atmospheres play a protective role similar to that of fluxes. In
gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas —
usually argon—flows from an annulus surrounding the torch in a continuous
stream, displacing the air from around the arc. The gas does not chemically
react with the metal but simply protects it from contact with the oxygen in the
air.
The metallurgy of metal joining is important to the functional
capabilities of the joint. The arc weld illustrates all the basic features of a
joint. Three zones result from the passage of a welding arc: (1) the weld
metal, or fusion zone, (2) the heat-affected zone, and (3) the unaffected zone.
The weld metal is that portion of the joint that has been melted during
welding. The heat-affected zone is a region adjacent to the weld metal that
has not been welded but has undergone a change in microstructure or
mechanical properties due to the heat of welding. The unaffected material is
that which was not heated sufficiently to alter its properties.
Weld-metal composition and the conditions under which it freezes
(solidifies) significantly affect the ability of the joint to meet service
requirements. In arc welding, the weld metal comprises filler material plus
the base metal that has melted. After the arc passes, rapid cooling of the
weld metal occurs. A one-pass weld has a cast structure with columnar
grains extending from the edge of the molten pool to the centre of the weld.
In a multipass weld, this cast structure may be modified, depending on the
particular metal that is being welded.
The base metal adjacent to the weld, or the heat-affected zone, is
subjected to a range of temperature cycles, and its change in structure is
directly related to the peak temperature at any given point, the time of
exposure, and the cooling rates. The types of base metal are too numerous to
discuss here, but they can be grouped in three classes: (1) materials
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