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Steel Designers' Manual - 6th Edition (2003)
240 Applied metallurgy of steel
alternatively, if necessary, any hardened edge can be removed by subsequent
machining. In recent years laser cutting has become a valuable additional cutting
method for thin material, in that intricate shapes and patterns can be cut out rapidly
by steering a laser beam around the required shape.
Drilling of holes presents little problem and there are now available numerical/computer
controlled systems which will drill multiple groups of holes to the
required size and spacing. For thinner material hole punching is commonly used and
although this, like shearing, can produce a hardened edge, provided the punch is
sharp no serious detrimental effects occur in thinner material.
It is sometimes necessary to bend, form or draw steel into different shapes. Reinforcing
steel for reinforced concrete structures commonly has to be bent into the
form of hooks and stirrups. The curved sections of tubular members of offshore
structures or cylindrical parts of pressure vessels are often rolled from flat plate to
the required curvature. In these cases yielding and plastic strain take place as the
material is deformed beyond its elastic limit. This straining moves the material condition
along its basic stress–strain curve, and it is therefore important to limit the
amount of plastic strain used up in the fabrication process so that that availability
for subsequent service is not diminished to an unacceptable extent. The important
variable in limiting the amount of plastic strain which occurs during cold forming
is usually the ratio of the radius of any bend to the thickness or diameter of the
material. Provided this ratio is kept high the amount of strain will be limited. Where
the amount of cold work which has been introduced during fabrication is excessive,
it may be necessary to carry out a reheat treatment in order to restore the condition
of the material to give its required properties.
In the manufacture of wire, the steel is drawn through a series of dies gradually
reducing its diameter and increasing the length from the initial rod sample.This cold
drawing is equivalent to plastic straining and has the effect of both increasing the
strength of the material and reducing its remaining ductility as the material moves
along its stress/strain curve. In certain types of wire manufacture, intermediate heat
treatments are necessary in order to remove damaging effects of cold work and
enhance and improve the final mechanical properties.
6.6.2 Welding
One of the most important fabrication processes for use with steel is welding. There
are many different types of welding and this subject is itself a fascinating multi-disciplinary
world involving combined studies in physics, chemistry, electronics, metallurgy,
and mechanical, electrical and structural engineering. Most welding processes
involve fusion of the material being joined, by raising the temperature to the melting
point of the material, either with or without the addition of separate filler metal.
Although there is a huge variety of different welding processes, probably the most
common and most important ones for general applications are the group of arc
welding processes and the group of resistance welding processes. Among the newer