|Wire drawing in brief|
In drawing wire,
the required deformation is accomplished by
drawing the wire through the conical
bore section of the die,
hereby reducing the diameter through plastic
deformation. During deformation, a thin film of lubricant
between wire surface and die surface is essential
to minimise friction, to reduce die wear, and
to keep the die cool.
For good wire deformation, it is necessary to select a drawing die tool with the appropriate profile, designed for either ferrous or non-ferrous materials.
Entry Lubrication is introduced, and the material is guided to the deformation zone (reduction/bearing) of the die.
Reduction Generally speaking, harder (ferrous) materials require smaller reduction angles; softer (non-ferrous) materials require larger reduction angles. Included angles (2-alpha) may vary within the range 8°-30°, zone length (0.5...1)d.
Bearing In this cilindrical part the deformed wire is calibrated to the desired size. The bearing length depends on the materials drawn and is specified as a percent of the bore hole diameter (0.3...0.6)d. As a general rule, harder materials demand longer bearings than softer ones. Wear on the bearing only occurs when heavy wear in the reduction zone was not remedied in timely fashion.
Exit This zone, in which the deformed wire leaves the die tool, must provide sufficient support for the axial mechanical wire drawing stress which occurs.
Some purposes for which the drawing die tools were designed require that transitions among the various zones are perfectly blended, with smaller radii for harder materials, larger radii for softer. Back relief, so-called, is often provided as a high-polish transition phase to allow the wire to exit smoothly from the bearing of the die.
Die wear takes place mainly in the reduction
zone, at first by abrasion at the point
at which the incoming wire contacts the
die. Initially the mirror-polished die surface
there will show only small signs of wear.
Then rapid wear sets in, characterised by
development of a "wear or drawing ring."
Such a die tool must be withdrawn for refurbishment
at the onset of wear at the impact point.
|Wire area reduction|
products where subsequent draws are needed to reach the desired finish
diameter an average area reduction per die of
about 20-30% is usual.
Steel wire work hardens during plastic deformation and
the ductility (the degree of elasticity) is reduced while the tensile
strength increases. The degree of total area
reduction possible without intermediate annealing depends on the
composition of the steel, i.e. the work
hardening characteristic of the steel quality (=grade).
In general it is possible through subsequent or sequential passes
through ever smaller dies to reduce the cross section area of a wire
between 85-95%. Further area reduction will
require an intermediate anneal to restore ductility.
|Die Materials Overview|
Synthetic Single Crystal: