Beside the new materials introduced for the fabrication
of the dies,Table 2, a new bonding method between the
insert and the housing was employed, namely hot isostatic
pressing (HIP). It was believed that this configurationwould prevent the insert from micromoving during extrusion. This means that the profile would have better support
from the housing, especially to the weak parts of the
design. In order to explore the new possibilities two dies
were fabricated, D02 and D03. The only difference between
the two dies was that in the first one the insert, from Stellite
4B material, was in the as cast condition while in the second one the same type of alloy was produced from gas
atomized powder. The latter possessed an increased hardness of 64HRc vs 48HRc of the former. The very fine
microstructure of the Stellite 4B P/M was assumed to prolong die life, by abating pronounced aging phenomena.
Unfortunately, both dies were cracked during the heat
treatment cycle of the housing or early in the extrusion process,Fig. 4a.
The cracks appeared circumferential to the insert in the
vicinity of the insert/housing interface. Metallographic
studies of the dies showed that the cracks were mainly
intergranular and situated inside the insert rather than at
the insert/housing interface, although some small cracks
and cavities were also observed at the interface on the
housing side,Fig. 4b. The major cracks were attributed
to the difference in thermal expansion between steel and
stellite which caused high stresses during the heat treatment. The formation of small cracks at the interface should
be assisted by the presence of hard carbides and martensite
in this area. Chemical analysis obtained by EDS of the
SEM revealed that reciprocal diffusion of elements betweenthe insert and the housing occurred during the HIP treatment. The diffusion zone was about 30lm thick on each
side of the interface. The stellite diffusion zone presented
a high iron content, from 0.5% to 5.0%w/w, while the steel
diffusion zone presented a high chromium content, from
5.0% to 10.0%w/w and a high cobalt content from 0% to
6.0%w/w. It was also assumed that carbon was diffused
from stellite to steel. Both problems could be probably
overcome if a soft nickel interlayer was placed at the interface, which would absorb the stresses and block the diffusion of carbide former elements.
To overcome the fatigue problem two new designs were
introduced, designs B and C of Fig. 1. The materials used
for the dies are presented in Table 2, respectively D04
and D05. These designs were based on the modification
of the die lands in order to balance the flow of the material
inside the die [15–18]. In addition, design C had a prechamber for guiding the brass to the entry side of the die
and optimizing the flow in the entrance [7–10]. Both
designs were free of sharp corners and all the weak regions
of the old design A were better supported. During the
extrusion trials with the design B, the extrudate went out
of the press with a wavy motion, especially during the first
meters of the extrusion, afterwards the product became less
wavy, however its quality was not acceptable from a commercial point of view,Fig. 5a. The profile was longer at its
circular section, indicating that at this section it went out of
the die with a higher flow rate than at its thinner section. The bearing at the circular section was probably too short
and needed to be increased, but with the actual configuration of the die, this was not possible, since the insert was
only 20 mm thick.
The design C failed to produce a smooth profile, too, as
seen from Fig. 5b. The product was curved and the thin
section of the hinge was rippled. Although the use of a
pre-chamber had increased the velocity of the extrudate
at its thinner section, the bearing length at the circular section of the hinge was severely reduced, resulting in uneven
flow.
In view of the above results, it was decided to respect
most possible the main geometrical dimensions which
determine the flow characteristics of the original design
A, since they were proved to provide correct extrusion conditions in industrial practice. Modifications of the die
design were restricted only in the mass distribution around
the bearing, so as to stiffen the die for preventing initiation
of fatigue cracks and to strengthen the weakest regions of
the die in order to avoid local overloading fracture. As a
result, the new design, marked A(M) inTable 2, was materialized using an EDM wire cutting machine capable of
inclined cutting. This offered the possibility to smoothen
the sharp corners and to shape the bearing with a taper
in the back side of the die, for better controlling the shape
of the die lands. The advantage of this profile was that the
bearing length in the circular section of the insert was prolonged within the steel housing, offering further support to
the weak areas of the insert,Fig. 1. Actually the new design
A(M) was a combination of designs A and B.
Two new dies were fabricated according to the above
cited design using a Stellite 4B P/M insert, as shown in
Table 2under designation D06. Furthermore, another die
alternative was fabricated (D07) by which the insert length
was the same as the die thickness and it was bonded to the
housing by the HIP process, using a nickel interlayer.
Metallographic study revealed that the nickel interlayer
blocked the diffusion between dissimilar materials and no
transition zone was detected. Furthermore, there was a
good bond between the materials with no cracks or voids,
Fig. 6, and no cracks were observed after the heat treatment of the dies.
The operation life of the first two dies, D06, in the industrial tests was 7 and 17 extrusions respectively, after what
the extruded product presented increased roughness andline defects. Chipping and spalling on the front face of
the die and on the die land were responsible for the defective extruded product. The poor performance of these dies
was attributed to the brittleness of the Stellite 4B in the
powder form using the HIP process and this is justified
by the increased hardness of 65HRc compared to 48HRc
in the as cast condition. Furthermore, cracks were formed
in the stellite region and were arrested by the steel body of
the housing, as illustrated inFig. 7on a cross section of the
used dies.
The die with the long insert was destroyed immediately
during industrial testing, since the insert was detached from
the steel housing. In the front face of the used die brass
material has penetrated into the cracks formed,Fig. 8a.
In the back face the insert was detached and pushed out
of the H13 steel housing. Failure has occurred due to plastic deformation of the
nickel interlayer between Stellite 4B and H13 material; this
caused tearing of the nickel interlayer, as shown inFig. 8b
and local distortions, which promoted the initiation of
cracks in stellite. The fracture of stellite has proceeded from
the back to the front face of the die, as shown by river patterns in Fig. 8c. It is clear that the die failed because of
absence of support from the backer (supporting piece
assembled in the back of the die during extrusion). Since
the backer is a piece that is used for several die shapes, it
was difficult to change its design for the particular case
and the idea of a full length insert was abandoned for the
time being.
Based on the failure mechanism of the aforementioned
dies two new materials were selected [4–6] Stellite 100
and Stellite 12, both materials in powder form,Table 1.
Two new dies were manufactured, D08 and D09 ofTable
2, according to the design A(M) with HIP as fitting
method. Unfortunately, the hardness of these stellites after
the bonding was over 70HRc. This must be attributed to
the high carbon content of the materials and to the very
fine dispersion of complex hard carbides in their microstructure. As it was expected, the dies cracked after a small
number of extrusions.
This investigation showed that hardness and toughness
are key elements for a successive brass extrusion die material. It was therefore decided to use for the insert a high
speed steel whose hardness could be adapted by choosing
a convenient heat treatment cycle of the die. For this, Vanadis 30, a cobalt alloyed PM high speed steel was used,
Table 1. The cobalt addition has a positive influence on
the hot strength/hot hardness, in particular compressive
strength, temper resistance and modulus of elasticity. On
the other hand it reduces slightly the toughness and hardenability and has little influence on the wear resistance[26].
Three dies were fabricated, one monolithic using the HIP
process (D10) with a hardness of 61HRc and two with
Vanadis 30 as an insert (D11) with a hardness of 52HRc,
shrink fitted into the housing. The monolithic HIP processed die lasted for five extrusions after that it was cracked
underlying once more the crucial role of the hardness. The
other two dies, D11, lasted 200 and 230 extrusions respectively, which is 2.5–2.8 times better than the original die
life. What is more important to empasize is that the two
dies were rejected from the industrial procedure because
the product was out of dimensional tolerance and not from
cracks caused by fatigue or promoted by aging phenomena.
From an economical point of view, taking into consideration the cost of the materials, the machining and the
subsequent heat treatment, the new die is estimated to be
2.8–3.0 times more cost effective in price per ton of
product.