CPT International 04/2016

Figure 6: Meshed geometry, red:
melt, blue: die
Figure 7: Calculated temperature distribution after the end of solidification,
left: 1.2343, right: D185
mation of a solid shell, leading to an
additional heat resistance.
For the heat transported through the
die material the result is
Q˙ = 4 kW.
It can be seen immediately that the
amount of heat transferred to the die
surface is much greater than the amount
of heat which can be removed by the die
material. The same situation is given
when the die surface is quenched rapidly
by die spraying. In the case of die
spraying the heat amount induced by
water eva poration is also much greater
than the heat which can be transported
to the die surface. These circumstances
lead to the formation of temperature
peaks and subsequently to peaks
in the thermal stresses especially at the
surface of the die. The abrupt change
of compressive and tensile stresses at
the die surface is a well-known mechanism
leading to fire cracks. A second
effect of the limited heat conduction is
that the cooling rates that are achievable
by the implementation of cooling
channels are limited by the die material.
Higher thermal conductivity of the
die material would lower the stress and
temperature peaks and also potentially
lower cycle times. Therefore benefits
can be expected for both die life time
and productivity.
Comparison of heat conductivities
As already mentioned materials with
higher heat conductivities than the
most commonly used die material, the
hot working steel 1.2343, have been developed
in the recent past. One example
is the material HTCS 130 which
was developed by Rovalma S.A., Barcelona,
Spain. The heat conductivity of
this steel at room temperature is close
to that of pure iron so that it almost
reaches the theoretical limit for iron
based materials. The thermo-physical
properties at room temperature and
elevated temperatures have been published
in [2]. If the heat conductivity
potentials of iron based materials become
insufficient for a certain purpose
one has to switch to other material
families. One possibility is the use
of tungsten or molybdenum-based alloys.
Their heat conductivities are way
higher than that ones of iron based
materials. Figure 1 shows a comparison
of the heat conductivities of the
materials 1.2343, HTCS 130 and D185.
(All data in figure 1 were obtained at
the Austrian Foundry Institute). It is
remarkable that the heat conductivity
of D185 is almost constant over the
whole temperature range. This is quite
advantageous for the hpdc-process.
For that reason D185 was tested more
comprehensively in technological test
facilities representing industrial hpdc-conditions.
Experimental – Trials at the
test facility
In order to quantify the heat removal
capacities of different cooling concepts
in hpdc with respect to cooling
media (water, oil), flow properties and
geometries as well as die materials a
test facility was developed at the Austrian
Foundry Institute. The trials with
D185 were part of a larger investigation
programme with different die materials.
The results of this series of trials
with several iron based materials have
been published in [3] and [4]. The testing
concept is similar to that presented
here.
Each testing device consists of an axially
symmetrical body which is fabricated
from the material which is requested
to be tested. This body is heated
up to a pre-set temperature of 260 °C
by a conventional oil tempering device.
The heat input of the liquid metal
is simulated by six electrical cartridge
heaters. In the center of the body is a
bore hole where the cooling medium is
passing through. The cooling media are
directed by different standardized flow
geometries. Figure 2 shows a schematic
view of the construction. The body
has 18 positions for thermocouples logging
temperature values at six different
distances from the surface of the bore
hole and at three different length positions
of the bore hole. A more detailed
description is given in [3].
The trial itself is divided into 5 stages:
» Preheating of the body via oil tempering
up to 260 °C,
» Heating up to 350 °C via oil tempering
and cartridge heaters,
» Activating the cooling at active electrical
heating with cartridges until
an equilibrium temperature T equ.
is
reached,
Casting Plant & Technology 4 / 2016 27