Low-carbon strip steels 101shapes. A further advantage is that these components may be satisfactorily joined
together to form complete assemblies using a wide variety of methods. Some
of these methods involve joining at discrete points or larger areas at intervals
along the join depending on the load-bearing requirements of the component.
Some methods involve continuous joining along edges between components
whereas other techniques, including the use of adhesives, involve joining over an
area. Some of the methods involve melting of the steel (welding) or the use of
additional materials including the use of adhesives, brazing or soldering. Some
fastening techniques involve deformation or cutting of the steel whereas others
involve the use of special joining components such as screws or rivets.
Each joining method has its own particular advantages and disadvantages,
including the ease of joining, whether fume is produced, the time required,
whether the method may be adapted to mass-production methods and whether
access to both sides of the strip is necessary. It is clear, however, that the
most important consideration is that any joint must provide the rigidity, strength
(including fatigue strength) and resilience (ability to absorb energy) necessary
for the application. It is also essential that any joint should not downgrade too
seriously as a result of contact with environmental conditions. Some joining
methods may be used in combination to complement each other's characteristics
and others may be chosen because of the need to be able to undo a join and
reassemble it later.
In service, a joint could in principle be subjected to a wide range of different
modes of stress. It has been considered, 151 however, that there are three modes
that can normally occur. These are two modes of tensile shear, parallel to or
across the joint, and cross-tension normal to the joint.
Welding
A wide variety of welding processes may be used for the joining of strip steels
which may be categorized into resistance and fusion types. Some of the more
commonly used processes are resistance spot welding, resistance seam welding,
projection welding, laser welding and MIG/TIG welding.Resistance spot welding
In this method, heat is generated by passing an electric current through two or
more sheets to be welded together by means of two electrodes which clamp on
opposite sides of the sheets.
Spot welding is generally used for joining steel strip up to 3 mm thick, though
thicknesses up to 10 mm can be welded by this process. The size and shape
of the welds are controlled primarily by the size and shape of the water-cooled
electrodes. In general, the weld nugget should be oval in cross-section but should
not extend to the outer surfaces. The electrodes exert a significant clamping force
on the strip materials and the contacting surfaces are generally heated by the
passage of a high current (5000-20000 A) using a low voltage (5-20 V). When
sufficient melting is achieved, the current is switched off but the clamping force
of the electrodes is maintained until the weld pool has completely solidified. The