Note: Descriptions are shown in the official language in which they were submitted.
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'~he present invention refers to a process for applying
a coating to a centrifugal casting mold for casting copper
or alloys thereof, wherein, prior to introducing the molten
metal into ~he mold, a suspension of powdery material is
being introduced in form of a thin layer.
~ ubular bodies produced by a centrifugal casting process
and consisting of copper or alloys thereof, particularl~ of
bronze, and being used as a raw material for the production
of bushings, rings and other shaped ~odies suffer from the
~0 draw-back that the outer zone and the inner ysne are different
in structure from the remaining zone of the castin~. ~he
outer zone has gas inclusions over a considerable thickness
to such an extent that this outer zone is unsuitable and must
be removed on a lathe. ~he inner layer is porous and contains
oxides and the thickness of the inner layer primaril~ depends
on the cooling speed of the meit and the rapidit~, reæpectivel~,
of transition from the liquid state into the solid ~tate. Up
till now, in centrifugal oasting processe~ for copper and
copper alloys, particularl~ bro~ze, the centrifugal casting
mold additionally had to be relatively slowly fill~a and ~ome-
time~ also ~on-uniforml~ filled for the purpose of avoiding
so-callea pin-holes, but this has as a consequence inexact
dimensions at the inner side of the casting and non-uniform
temperature stress of the mold. A further consequence is
increased material consumption and a rapid mold wear. Further-
more, the structure and the techni¢al properties of the casting
fre~uently are strongl~ different, what in most cases cannot
be tolerated. Furthermore, the mold must be kept at a relatively
high temperature for limiting gas evolution at the area of the
~0 inner wall of the mold. -
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It is an object of the invention to avoid the mentioned
draw-backs. It is a further object of the invention to improve a
process of ~he kind described above such that the casting opera-
tion can be accelerated with low expenditure. It is another
object of the in~ention to substantially improve the quality
of the tubular bodies obtained. It is still another object
of the invention that the mold used has an ex*ended lifetime.
In a process of the kind described above, these objects
of the inve~tion are essentially achieved by using, for the
purpose of providing a heat-conductive and a gas-permeable
la~er, as powdery materials for the suspension the constitutin~
components zirconium oxide (ZrO2) and an inorganic binder
material. ~he inorganic binder material preferably consists
of non-sintered aluminium-oxide (Al203). ~his layer forms a
protective layer which does, even under the heat influence of
the molten metal, not produce gases in a substantial amount
but which is capable of venting the gases in longitudinal
direction of the mold wall, which are set ~ree on cooling down
the melt, and which i8 ~ in view of its good heat conductivity,
capable to rapidly cool the melt for attaining a fine structure
of the casting~ It has been found that such rapid heat removal
provides the advantage that during the castin~ operation a solid
metal layer, restricting any gas evolution at the outer zone of
the casting, is rapidly formed. On further cooling of the
casting, any evolved gases are thus prevented from escaping
through the metal in inward direction and from disturbing the
formation of a uniform structure o~ the casting, so that the
so-called pin-holes can be avoided in an efficient manner.
In view of the ~uality of the casting being substantially
3o improved when working accordin~ to the inventive process, par-
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3l~fciV~ticularly in view of achieving a substantially uniform
structure of the casting, the oversize of the casting required
for machining the cas-ting can be reduced to a minimum, for
example to only 1 mm at the outer side of the casting. In
view of the ~entioned rapid heat removal from the melt, the
cooling time for centrifugal castings having a wall thickness
below 30 mm is reduced to such an extent that, in contrast to
processes known up till now, no substantial oxide formation can
take place a-t the inner surface of the casting. ~his allows
to restrict the oversize needed for machining purposes at the
inner surface of the casting to 1 mm for castings having a wall
thickness up to 15 mm and to 2 to 3 mm for castings having a wall
thickness up to 30 mm. In view of the small oversizes required
at the outer surface and at the inner surface it is now possible
to produce in an economic manner and with a high dimensional
accuracy raw centrifugal castings with a wall thickness of ~t
least 9 mm. ~his outstanding dimensional accuracy of the raw
centrifugal castings, which in the inventive process is + 0.2 mm
at the outer surface and + 0.5 mm at the inner surface, results,
based on work on lathes as well as on waste in the form of
- turnin~s, in a much more simple and thus also much cheaper
production of final proaucts.
Also the technical properties o~ the raw castings are sub-
stantially improved over those of raw castings produced according
to known processes. By using a suspension of zirconium oxide
as a dressing for the mold, the temperature o~ the mold can be
kept relatively low in view of the properties o~ the suspension.
Because there are no dif~iculties whatsoever to intensely cool
the mold during and after the casting operation b~ mea~s of water,
also the temperature of the inner surface of the mold can be
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kept relatively lo~. Under consideration o~ the low heat
insulating effect o~ the mold dressing formed of the zirconium
oxide layex, this results in a particularly rapid solidification
of the melt, and, as a conse~uence o~ these parametres, the
technological properties o~ the raw castings, above all the
tensile strength, the elongation on rupture and the Brinell
hardness, are increased, noting that the improvement 9~ these
properties amounts up to 20 % over the presently valid prescrip`
tions for centri~ugal castings (DIN ~705, DIN 1709).
A fu~ther substantial advantage of the procèss according
to the invention resides in a substantial increase of the life-
time of the centrifugal mold. ~his statement particularly
applies if the suspension is applied to the preheated mold and
if, according to the invention, the melt is, a~ter application
f the suspension, introduced into the mold over its whole
length simultaneously. ~his mode of operation does not onl~
allow to more rapidly fill the mold but also allows to uni~ormly
~ill the mod, so that the heat influence is essentially uniform
over the whole length of the mold and, therewith, heat tensions
and thus also premature ageing and damaging of the mola can be
avoided.
It is known (DT 0~ 2 343 174) to use ~or containers for
molten metal, refractory materials whose chemical constituents
consist of SiO2, A1203, ZrO2 and sedium silicate, noting that
the content in sodium ~ilicate i~ kept within the range of
approximately 3 and approximately 10 percent by weight.
~hereby it is intended to obtain thix~tropic properties, noting
that the refractory material is brought into a liquid state
under the influance o~ vibrations in spite o~ the low water
3o co~tent o~ the refractory material. Irrespective of the diffi-
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culties which are encountered if one tries to apply such a
material in form of a layer to the wall of a container, a heat
insulating layer shall be obtained because it is intended to
keep hot the melt within the container. The low water ¢ontent
and the relativel~ great particle ~ize of the material used
make it impoQsible to produce a suspension within the spirit
of the present invention, particularly when considering that
the material used according to this known process immediately
solidifies if it is not subjected to vibrations.
It is also known (FR PS ~ 027 534) to clad a re~ractory
centrifugal casting mold used for casting iron or steel, with
metal oxides, ~or example with A120~, which are mixed with non-
metallic constituents. However, there are considerable dif~er-
ences between centrifugal casting of cast iron or steel and
centri~ugal casting of copper and allo~s thereof. ~xperience
has shown that mold claddings known for casting cast iron and
steel fail in processes for casting copper and copper al~oys,
particularly bronze, which might be attributable to the require-
ments mentioned above. ~hus, it is not possible to derive from
the above-mentioned known claddings, irrespective of such
claddings serving as transport containers or for centrifu~al
casting molds for casting cast iron or steel, anything which
might be suitable for centrifugal casting of copper and copper
alloys.
Aocording to a pre~erred embodiment of the invention, the
; mold is, during introducing the suspension 9 rotated around its
axis with a lower speed of revolution than during introducing
the melt. ~he reason there~or being that the suspension can be
most easi~ly distributed over the inner surface o~ the mold at
a relatively low speed o~ revolution of the mold, whereas it is
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desirable to work in the m~dium range of speeds o~ revolution
when charging the mold ~ tn the mol-ten metal. Vntil the moment
of complete dis-tribution of ~the melt within the mold, a non-
uniform distribution of centrifugal ~orces can be observed ~rrhich
can be kep~ within a tolerable range at medium speeds of revolu-
tion.
As alread~ mentioned, the heat conductivity of the 1ayer
used in a process according to the inven~ion can be further
increased by selecting the thickness of this layer correspondingly
small, particularly within the range between approximately 0.1
and 0.3 mm, which is substantially smaller than in known pro-
cesses. It is convenient to aim at a thicl~ess of the layer
within quite narrow limits. For this reason it is no~ to re-
commend to apply the layer to the mold by means o~ a brush, as
is frequentl~ the case in known processes, but it is to recommend
to rotate the mold during the introduction of the suspension as
has been describea above.
The suspension used is preferrably an aequous suspension,
and the preferred inorganic binder material used is extremely
finely elutriated alumina (Al203)~
Example~.
~ his example illustrates the production of a raw casting
in a centrifugal casting process having an outer diameter of
162 mm ~nd an inner diameter o~ 138 mm and a length of 660 mm
and from which a friction bearing is to be produced which has
an outer diameter of 160 mm and an inner diameter o~ 140 mm.
~he bronze used meets the standards according to DIN 1705, melt
composition Gz-Rg 7. ~he steel mold, which is supported in
horizontal position, is preheated to approximately 150G and
3~ covered at its inner surface~ while being slowly rotated with
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300 revolutions er minu-te, by means of an aequous suspension
containing zirconium oxide and extremely ~inel~ elutriated
alumina as a binder, until a la~er o~ a thick~ess of approxim-
ately 0.2 mm is obtained. After closing the mold, the means
for introducing the molten metal is being centrally applied.
1`he spout of the casting funnel is dimensioned in correspondence
with the composition o~ the molten metal and has a diameter of
28 mm. A casting channel extending within the mold over t~o
thirds of the length of the mold in an approximately horizontal
position is connected to the casting funnel. With this means
for introducing the molten metal, a kno~m weight of the molten
metal heated to a temperature above 1150C iS9 with acceleration
of the circumferential speed of the inner surface of the mold
to 7 m/sec, introduced such that the casting funnel remains
filled with molten metal up to a level of approximately 200 mm,
whereby a constant flow of the molten metal and an even distri-
bution of the molten metal within the mold by means of the
casting channel can be reliably obtained. ~he cas~ing time
lasts only about 4 seconds. ~ubsequently, the means for casting
molten metal is being removed and the mold is being coo1ed with
water, whereupon, after solidi~ication of the raw casting, the
raw casting is being removed from the mold.
In view of the sm~ll oversize of only 1 mm of the raw
casting at its outer side and as well at its inner side, the
friction bearing can be finished in one sin~le turning operation.
~ he following table illustrates the improvement of the
technical-properties as compared with the properties required
according to DIN.
.. ...
i~ccordin~ According to
to DIN 1705 the inventive
ior Gz-~g 7 process
tensile strength (kp/mm2) 30 32
elongation on
rupture (~) 20 25
Brinell hardness (kp/mm2) ~5 95
~ he invention is particularly applicable for ca~ting in
molds, i.e. repeatedly useable molds and permanent molds,
respectively, copper and such copper-containing alloys in
which copper is a substantial constituen-t and maaor constituent,
respectively.
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