Английский для сварщиков. Гричин С.В. - 47 стр.

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47
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
unaffected by welding heat, (2) materials hardened by structural change, (3)
materials hardened by precipitation processes.
Welding produces stresses in materials. These forces are induced by
contraction of the weld metal and by expansion and then contraction of the
heat-affected zone. The unheated metal imposes a restraint on the above, and
as contraction predominates, the weld metal cannot contract freely, and a
stress is built up in the joint. This is generally known as residual stress, and
for some critical applications must be removed by heat treatment of the
whole fabrication. Residual stress is unavoidable in all welded structures, and
if it is not controlled bowing or distortion of the weldment will take place.
Control is exercised by welding technique, jigs and fixtures, fabrication
procedures, and final heat treatment.
After-reading activity
Complete the following sentences:
1. A characteristic feature of fusion welding is:
a) molten metal b) low-voltage discharge c) inert atmosphere

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