3 Fundamentals of press design

26 Basic principles of metal forming
The value of the flow stress depends on the material, the temperature,
the deformation or strain, �, and the speed at which deformation or
.
strain rate is carried out, �. Below the recrystallisation temperature, the
flow stress generally rises with increasing deformation, while the temperature
and deformation rate exert only a minimal influence. Exceptions
to this rule are forming techniques such as rolling and forging, in
which extremely high deformation rates are used. Above the recrystallisation
temperature, the flow stress is generally subject to the temperature
and deformation rate, while a previous deformation history
has only minimal influence. The flow stress generally drops with increasing
temperature and decreasing deformation rate.
Accordingly, DIN 8582 differentiates between metal forming processes
involving a lasting change in strength properties and those involving
no appreciable change in strength properties, previously designated as
cold and hot forming. In the temperature range between, deformation
involves only a temporary change in the strength properties of the
material. In this case, the deformation speed is higher than the recovery
or recrystallisation rate. Recrystallisation starts only after completion
of the forming process. The rules of metal forming with lasting
change in the strength properties apply in this case.
The DIN 8582 standard also breaks down the process according to
forming without heating (cold forming) and forming after the application
of heat (hot forming). These terms simply specify whether heating
devices are necessary. Unlike their former meaning, these terms are not
physically related to the material concerned. The flow stress of the individual
materials is determined by experiments in function of deformation
(or strain) and deformation rate (or strain rate) at the various temperature
ranges, and described in flow curves. One of the uses of flow
curves is to aid the calculation of possible deformation, force, energy
and performance.
2.2.2 Deformation and material flow
Actual deformation �, also called logarithmic or true strain, is given by:
1
ϕ1 = ∫ =
dh h
ln
h h
h 0
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
h
1
0