3 Fundamentals of press design

458 Solid forming (Forging)
indicate an insufficient tool clearance. With increasing shearing velocity,
the deformation zone reduces, the hardness distribution becomes
more uniform and the hardness increase in the sheared surface
becomes less pronounced, i.e. the material characteristics become more
“brittle” (exception: austenitic chrome-nickel and chrome-manganese
steels).
Open shears and closed shearing guides are used as tools (Fig. 6.3.2).
Shearing guides produce very high quality blanks, but involve a number
of drawbacks: Sheared billets are more difficult to eject, stops have
to be used and demands on the diameter tolerance of the starting material
are more stringent. For round materials, the relative shearing clearance
for each billet thickness to be sheared can be kept constant by
grinding an elliptical relief cut on the guides. The parts sheared in this
way demonstrate improved angularity and reduced burr formation at
the edge of the fractured surface. Moreover, the sheared billet quality is
less dependent on variations in material properties, which in turn
reduces variations in volume.
The shearing force F S and shearing work W S can be calculated
approximately when seperating round material with the diameter d
using the following formula:
F S = A S · k S
W S = x · F S · s,
whereby A S is the sectional surface to be sheared, k S the shearing resistance
of the billet material and s approx. 20% (hard, brittle materials)
to 40% (soft, tenacious materials) of the shearing stroke, i.e. the diameter
d. The correction factor x indicates the extent to which the increase
in force deviates from a rectangular force-stroke curve. In general, x is
taken to be between 0.4 and 0.7. The shearing resistance k S amounts to
approximately 0.7 to 0.8 · R m.
Heat treatment of steel
Soft-annealing (spheroidized annealing) plays a particularly important
role in the heat treatment of steel materials used for billet production.
Recrystallization annealing, normalization and under certain circumstances
also recovery annealing are typical heat treatments used for
intermediate annealing of already formed workpieces whose formability
has been exhausted.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998