3 Fundamentals of press design

522 Solid forming (Forging)
eral, aluminium and aluminium alloys are also heated by induction to
temperatures between 420 and 480 °C, depending on the alloy. The
required inductor output is approx. 0.35 to 0.4 kW/kg with a frequency
of 0.5 to 1 kHz.
Thyristor-controlled systems are used as converters. Considerable
quantities of cooling water are required to cool the inductors. Recooling
is performed in the plant’s own recooling systems or in separate
installations.
Billet preparation
Warm forming (680 to 800 °C) of shaft-shaped parts made of steel frequently
calls for coating (pre-graphitizing) of blanks. This measure
serves to increase die service life and is particularly beneficial for first
pressing operations (generally forward extrusion or reducing operations).
The graphite can either be tumbled on prior to heating, or sprayed on
during the heating process. This entails inductive pre-heating of the
blanks (approx. 180 °C), followed by spray application of the graphitewater
mixture. The water evaporates over a relatively short travel, during
transport to the main induction heater. Thus, the billets are heated with
a dried-on coating of graphite. Although a proportion of the applied
graphite burns, a relatively high percentage (approx. 60 %) still permits
lubrication and, together with a (very thin) scale layer, creates the
desired lubricating and separating effect in the die.
6.8.6 Sizing and coining presses
Sizing of forged and sintered parts
The dimensions, tolerances and surface quality of forged or hot formed
parts frequently fail to meet the standards expected from finished products.
Consequently, often costly finish processing operations are necessary.
By introducing a sizing operation, substantially narrower tolerances
can be achieved and subsequent machining processes can be
reduced.
Many forged parts are finish machined on machining centers. These
call for precise clamping and contact surfaces in order to assure a reproducible
and well defined workpiece clamping. It is frequently not possible
to achieve, in forged parts, a surface quality that is required for
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998