COULEE CENTRIFUGE

CENTRIFUGAL CASTING

Abrégé anglais

IGOR Y. KHANDROS
For: CENTRIFUGAL CASTING
Abstract of the Disclosure
A centrifugal casting high in an easily oxidized element
achieved by casting a high melting point metal on to an easily
oxidized metal of lower melting point. The desirable distribution
of the oxidizable element through the cross section of a casting
is achieved relying on the basic effect of centrifugal separation.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of centrifugally casting a metal tube
comprising:
pouring an easily oxidized metal onto the inside
diameter of a rotating centrifugal mold and allowing the
easily oxidized metal to solidify in the mold so that it
has a thick section at one end of the mold, tapering to a
thinner section;
pouring a denser metal initially onto the thick
section of the solidified metal while rotating the mold,
the denser metal having a higher melting point than the
easily oxidized metal thereby gradually remelting the easily
oxidized metal and progressing down the length thereof as
a protective blanket for preventing oxidation of the easily
oxidized metal at the pouring temperature of the denser
metal;
continuing rotation of the centrifugal mold until
the easily oxidized metal is entirely covered by the blanket
and also until the easily oxidized metal has moved radially
inward through the denser metal to reach the inside surface
of the tube, whereby the easily oxidized metal has been
prevented from undergoing objectionable oxidation; and
allowing the metals to solidify to complete a
centrifugally cast tube of both metals characterized by a
centrifugal casting haying at and near the outside surface
a first zone having a high concentration of the denser metal
alloyed with a low concentration of the easily oxidized
metal; a second zone having a higher concentration of the
easily oxidized metal, higher than in the first zone, alloyed
with a lower concentration of the denser metal, lower than
in the first zone, located at and near the inside surface;
and a transition zone between the other two zones where the
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concentration of the easily oxidized metal is continuously
increasing radially in the direction of the inside surface.
2. A method according to claim 1 in which the
easily oxidized metal is aluminum or an aluminum alloy.
3. A method according to claim 1 in which the
easily oxidized metal is principally aluminum and in which
the denser metal is a heat-resistant alloy consisting in
percent by weight essentially of:
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4. A method according to claim 1 in which the
denser metal is selected from the group consisting of steel,
cobalt base alloys, nickel base alloys, and heat-resistant
alloys containing both nickel and chromium.
5, A method according to claim 1 including the
step between pouring the easily oxidized metal and pouring
the denser metal of displacing air from the interior of the
mold with a nonoxidizing gas, confining the body of non-
oxidizing gas to the interior of the mold, and providing
for escape of the body of nonoxidizing gas when pouring
the denser metal.
6. A method according to claim 5 in which the
mold has end caps of which at least one is provided with a
vent route for the escape of nonoxidizing gas and including
the step of sealing that vent route to confine the body of
nonoxidizing gas until the denser metal is poured.
7. A centrifugal casting having at and near the
outside surface a first zone having a high concentration of
a dense metal alloyed with a low concentration of a less
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dense metal; a second zone having A higher concentration of
the less dense metal, higher than in the first zone, alloyed
with a lower concentration of the dense metal, lower than
in the first zone, at and near the inside surface; and a
transition zone between the other two zones where the con-
centration of the less dense metal is continuously increas-
ing radially in the direction of the inside surface.
8. A centrifugal casting according to claim 7
of tubular form in which the less dense metal is principally
aluminum and in which the dense metal consists essentially
of:
carbon 0.25 to 0.8
nickel 8 to 62 balance iron
except for
chromium 12 to 32 ? impurities and
tramp elements.
silicon up to 3.5
manganese up to 3
9. A casting according to claim 7 achieved by
pouring the less dense metal into a rotating centrifugal
mold and allowing it to solidify while rotating the mold,
and afterwards pouring the dense metal into the mold atop
the less dense metal to melt it while rotating the mold until
the melted less dense metal has attained the inside of the
casting.
10. A tubular centrifugal casting having a low
concentration of a less dense metal and a high concentration
of a dense metal combined in a zone at and near the outside;
having a lower concentration of the dense metal, compared
to the outside zone, and a higher concentration of the less
dense metal, also compared to the outside zones, combined
in a zone near the inside of the casting; and having a
transition zone between the other two zones where the less
dense metal exhibits a continuously increasing concentration
radially inward in the direction of the inside zone.
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11. A centrifugal casting according to claim
10 in which the less dense metal is principally aluminum and
in which the dense metal consists essentially of:
carbon 0.25 to 0.8
nickel 8 to 62 balance iron
except for
chromium 12 to 32 ? impurities and
tramp elements.
silicon up to 3.5
manganese up to 3
12. A centrifugal casting according to claim 10
or 11 achieved by pouring the less dense metal into a
rotating centrifugal mold and allowing it to solidify while
rotating the mold, and afterwards pouring the dense metal
into the mold atop the less dense metal to melt it while
rotating the mold until the melted less dense metal combined
with the dense metal has attained the inside of the casting.
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Description

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This invention relates to centrifugal casting and
in particular casting centrifugally alloys containing a sub-
stantial amount of a light, easily oxidized element, either
as a pure metal or a light alloy itself.
Castings employed under oxidation, carburization
or corrosion conditions at elevated temperatures are
usually cast from an ailoy containing a high percentage of
chromium. In view of the price and the potential shortage
o~ chromium as a strategic metal, -the problems of chromium
substitution, lower chromium content or increase oE the
service life of a chromium-containing alloy are of great
importance. One of the.main alternatives for chromium as
an element providing oxidation-corrosion resistance is
aluminum, but unfortunately high aluminum steels case in
air are generally unacceptable due to poor castability and
the large amounts of dross and oxides present in the metal.
. . .
According to a method aspect of this invention
there is provided a method of centrifugally casting a metal
tube comprising: pouring an easily oxidized metal onto~
the inside diameter of a rota-ting centrifugal mold and
allowing the easily oxidized metal to solidify in the mold
so that it has a thick section at one end of the mold !
tapering to a thinner section; pouring a denser metal ini-
tially onto the thick section of the solidi~ied metal while
rotating the mold, the denser metal having a higher melting
point than the easily oxidized metal thereby gradually re-
melting the easily oxidized metal and progressing down the
length thereoE as a protectiye blanket :Eor preventing oxi-
dation of the easily oxidized metal at the pouring tempera-
. ture of the denser metal; continuing rotation oE the centri-
Eugal mold until the easily oxiclized metal is en-tirely
covered by the blanket and also until the easily oxidized
.. ~
mab/1-

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metal has moved radially inward through the denser metal -to reach the
inside surface of the tube, whereby -the easily oxidized metal has been
prevented from undergoing objectionable oxidation; an~ allowing the
metals to solidify to complete a centrifugally cast. tube of both metals
characterized b~ a centrifugal casting having at and near the outside
surface a first zone having a high concentra-tion of the denser metal
alloyed with a l~w concentration of the easily oxidized metal; a
second zone having a h.igher concentration of the easily oxidized metal,
higher than in the first zone, alloyed wi-th a lower concentration of
the denser metal, lawer than in the first zone, located at and near the
inside surfacei and a -transition zone between the other two zones where
the concentration of the easily oxidized metal is continuously increasing
radially in the direction of the inside surface.
According to a product aspect of this invention there is
provided a cen-trifugal casting having at and near the outside surface
a first zone having a high concentration of a dense ~etal alloyed with
a low concentration of a less dense metal; a second zone having a
higher concentration of the less dense ~etal, higher than in the first
- zone! alloyed with a lower concentration of the dense me-tal! lower than
in the first zone, at and near the inside surface, and a transition
zone between the other two zones where the concentration of the less
dense metal is continuously increasing radially in the d.irection of the
inside sur~ace.
According to a further product aspect of this .invention there
is provided a tubular centrifugal casting having a low concentration of
a less dense metal and a high concentration of a dense metal combined in
a zone at and near the outside; having a lower concentration oE the dense
metal, compared to the outside zone, and a higher concentration of the
less dense metal, also compared to the outside zone, combi.ned in a
zone near the inside of the castiny; and having a transitlon zone be-tween
the other two zones where the less dense metal exhibits a continuosly in-
creasing concentration rad:ially inward in the direction of the inside zone.
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mab/.

In the drawing:
Figs. 1, lA and lB are sectional views, partly sche-
matic, of a centrifugal mold in several stages of producing a
casting and wherein, for convenience, the ordinary end caps for
the mold are omitted;
Fig. 2 is a photomicrograph (magnification 6X) of a
cross section of a tube cast centrifugally in accordance with
the present invention;
Fig. 2A is a graph showing distributions of elements
in the casting
Fig. 3 is a photomicrograph of the casting of Fig. 2
at a magnification of 40X; and
Figs. 4 and 4A are views similar to Fig. 1 showing
several stages of casting centrifugally under another embodiment
of the invention.

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Typical centrifugal mold apparatus is shown in Fig. 1
comprising a centrifugal mold 10. The molten metal for the cast-
ing pours from the end of a spout 13A which is part of a pouring
vessel 13. Because of the rotating mold the entrant metal, what-
ever its kind, spirals down the ID of the mold, as the molten
metal will act like any other free body of liquid seeking its
own level, especially with the force of the reservoir (vessel 13)
behind it.
Earlier in the process, a light, low melting point metal
12 was deposited in the same way on the ID of the mold, having
solidiied, and as shown in Fig. 1 a heavier metal 14 having a
much higher melting point is being deposited on the previous layer
of lighter metal 12.
The first portions of heavier metal 14, therefore, will
remelt outer layers of the lighter metal 12 and will spirally
slip across the partially remelted substrate of the lighter metal
like a skate on ice. The oncoming streams of the high melting
point metal gradually remelt remaining light metal and the rest
of the heavier metal eventually slips over the molten alloy con-
taining both heavier and lighter metal. At these moments thelighter metal is dissolved only in the O.D. adjustment zone of
the molten tube and, therefore, this zone is lighter than the
rest of the metal. Because of centrifugal force, the heavier
metal 14 will gravitate in the direction of the outside tOD) dia-
meter of the centrifugal mold, or stated in other words, the
lighter metal will be at the ID of the resultant cast tube T.
Essentially, there are four stages in principle although
in actual practice they may by no means exhibit the distinctive-
ness shown in the drawing. The first stage is solidification
of the light metal followed next by the occurrence of the heavier,
high melting point spiralling across the earlier deposited light
metal, Fig. 1. The taper shown for the lighter metal in Fig. 1

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is actual, and is desirable in some cases for the achieving of
a uniform ID alloyed layer t especially when a lower rotating
speed of the mold is employed. In the third stage the melted
metals attain uniform wall thickness with the heavier metal at
the ID, but because the mold continues to rotate the heavier metal
moves to the OD/ Fig. lB, where it remains while the casting cools
to the solid state during the last stage.
More specifically, a No. 356 aluminum alloy (6.5 to
7.0~ silicon) was poured at 1450F into the rotating mold which
had been preheated to 400F. Afterwards, a heat-resistant alloy
(HRA alloy) of 35% nickel, 19% chromium, 0.42% carbon, 1.2~ silicon
and 1.2~ manganese (balance iron except for impurities) is poured
at 2900F onto the earlier formed, thin aluminum "tube" 12 from
the same end of the mold.
The resultant centrifugally cast tube is found to con-
tain three zones of metal:
(1) an ordinary HRA zone at the outside diameter with
some residual Al dissolved in it,
(2) a transition zone, and
(3) an aluminum-rich alloy zone at the inside diameter,
all zones being shown in Figs. 2 and 3, as will be ex~
plained in more detail below.
Aluminum oxide clusters were observed only near the
inside diameter (ID) surface of the tube, and in surprisingly
small quantities for an air-melted heat containing so much aluminum.
The three zones (1), (2), and (3) are designated in
Figs. 2 and 3. The OD for the most part is the HRA alloy identi-
fied above but containing evenly distributed aluminum nitrides
while the aluminum-rich alloy at the ID contains Fe-Ni-~l with
some chromium carbides precipated in intermetallic phases pre-
cipitated in lnterdendritic areas.

Clearly, when the heavier metal 14, Fig. 1, was poured
the standard HR~ melt covered and remelted the aluminum alloy
which was then shifted toward the inside diameter durin~ continued
rotation of the mold. However, some aluminum is dissolved in
the HRA alloy during the shift, lowering the melting point of
the alloy at the OD. The greater alloying with aluminum occurs
at the ID, lowering the melting point of that alloy sti]l further.
The ID may be covered by an aluminum-rich oxide film providing
protection against further oxidation. Those light oxide inclu-
sions which get underneath the film do not propagate deeply intothe metal owing to their light weight and the centrifugal force.
Because of the increase in aluminum content a tube cast
centrifugally in the manner of the present invention will exhibit
higher corrosion, oxidation and carburization resistance compared
to the corresponding HRA alloy having no aluminum. Also, the
aluminum-rich layer at the ~D, having heavy precipitation of
intermetallic phases and carbides will be harder and will exhibit
improved abrasion resistance for those applications where hardness
is a controlling factor. The hardness measured at the ID surface
~0 of several tubular products produced according to the present
invention was up to 430 BHN.
~ n any event, the process of the present invention may
permit reduction in chromium content relying on aluminum substi-
tution, especially for those applications where high temperature
corrosion and oxidation resistance are most needed.
The HR~ alloy specified above is only one o a whole
host to which the invention may be applied. A family of HRA alloys
to which the present invention may be applied is given in U.S.
Patent No. 4,077,801, issued March 7, 1978 to applicant:

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Carbon 0.25 to 0.8 )
Nickel 8 to 62 ) balance iron
Chromium 12 to 32 ) except for
Silicon Up to 3.5 ) impurities and
Manganese Up to 3 ~ tramp elements
Most of aluminum alloys may be employed without diffi-
culty, depending on the final composition of metal required.
Additions of other easily oxidi~ed elements, such as titanium or
boron, can be placed into the metal 12 in the form of a coarse
powder of their low melting temperature alloys.
When additions of surface active elements such as boron
are employed the time of solidification of the casting is apparently
reduced due to lowering of the surface tension between the solid
state nuclei and liquid phase. As a consequence, less centrifugal
separation was observed and almost uniform distribution of aluminum
through the wall of the casting resulted.
The principles of the invention would be equally appli-
cable when replacing the HRA alloy with any steel such as a stain-
less steel, any other HRA alloy, or a nickel or cobalt base alloy;
indeed the replacement can be any alloy meltlng appreciably higher
and which is appreciably heavier than the light weight alloy and
which is advantaged or improved by having the light weight, low
melting point metal move therethrough while both are in the molten
state.
Preferably the mold will be preheated at 350F-400F
to avoid premature solidification when the lower melting point
metal is first introduced to the mold cavity. Since the mold
in most instances will have a mold wash lining (e.g. one sixteenth
of an inch thick) on the inside diameter derived from a mixture
of silica and watier, h~eating the mold to drive off the water will
also aford all, if not the major part of the preheat

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For any given amount of lighter, low melting point metal
initially poured the distribution of the lighter element through
the cross section of the casting will be proportional to the
following major influences:
(1) The rotational speed of the mold over the time
period required for solidification to be attained
because a higher speed means higher degree of
centrifugal separation and more of the heavier
metal moving radially outside; higher rotational
speed will also result in higher longitudinal
velocity of the heavier metal, so that less heat
is lost during this period of the process and,
therefore, more time is available for the centri-
fugal separation;
(2) The pouring temperature of the heavier metal,
because when the metal is poured "hotter, 7r the
total time of solidification is increased and more
centrifugal separation occurs; and because metal
possesses higher fluidity at higher temperatures
it will move more quickly in the longitudinal
direction in the first moments of the process;
(3) The thickness of the mold wash, because it also
influences the total time of metal solidifications.
It will be recognized that alloying between the light
and heavy metals takes place inside the mold. At all times the
light metal~ if easily oxidi2ed, is prevented from doing so to
any objectionable degree. The objectionable oxidation is that
which ordinarily occurs when an HRA metal, combined with aluminum,
is poured into the mold from a vessel as 13, at or above the melt-
ing point of the HRA-aluminum alloy. Objectionable oxidation
does not occur when merely pouring the aluminum alloy at its melt-
ing point into a preheated mold, say when pouring at 1400F into
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23
a mold at 400F. Now then, when the HRA metal 14, not yet alloyed
with aluminum 12, is poured at say 2900F, the aluminum, though
melting on contact, Fig. 1, is covered by the molten HRA metal
which induces the melting, and hence the easily oxidized metal
is blanketed against oxidation. In comparison, an HRA-aluminum
alloy of the proportion specified above, when poured all at once,
will exhibit a drossyl porous, heavily oxidized ID which can be
rendered acceptable only at an exhorbitant machining cost to
reduce the wall thickness to a radius of sound metal; the loss
in yield is prohibitive in most instances.
A further advantage is the ability to pour the HRA metal
14 at a temperature lower than hereto~ore. Thus, the HRA metal
or the high melting point metal is usually poured at a temperature
considerably above the liquidus so it will not be solidified too
quickly by the much cooler mold. Such is not necessary under the
present invention, especially when the lighter metal is aluminum
because in that case the aluminum not only melts, becoming a "lubri-
cant~" it is dissolved in the HRA molten metal at the same time
and heat or solution is generated, meaning the HRA metal need not
be poured at the higher temperature to assure sustained 1uidity.
The lower temperature results in a finer grain size
which usually means (and in the case of HR~-aluminum) does mean
a stronger casting.
In accordance with the broader objective of the inven-
tion it is possible to reduce further the ~ormation of nonmetallic
inclusions and improve the surface quality of the castings even
at the ID. This is made possible by displacing air from the mold,
after the light metal has solidified, with a confined body of
non-oxidizing gas which itself is afterwards displaced as an inci-
dent to casting the heavy metal or alloy. Thus, referring to
Fig. 4, a centrifugal mold 20 is provided with the usual end caps,
but in this instance one end cap 22 is provided with one or more

~8~2;~3
vent openings 24 and the other end cap 26 has a central aperture
26A of a size to admit a lance 28 which feeds a non-oxidizing
gas such as argon into the mold interior after the light metal
has solidified. Argon displaces air out the vent hole, which
is continued until the body of gas inside the mold is the non-
oxidizing gas. The lance is withdrawn and the openings in the
end caps are temporarily sealed with a displaceable plug or ruptur-
able diaphragm (not shown) which may be nothing more than a piece
of plastic film.
When the casting is to be completed, the pouring spout
30 of a pouring vessel 32 is positioned in aperture 26~ incidental
to allowing molten metal 34 (heavy metal) to pour onto the pre-
viously poured light alloy at the inside diameter of the mold,
which is being rotated.
The molten metal expands the gas (NG) which is forced
from the mold at the vent 24 and at the annular venting space
presented by aperture 26A~
The non-oxidizing gas continues to be displaced as the
molten metal spirals down the mold, seeking its own level as any
other fluid body.
Since the mold was and remains air-free ~rom the incep-
tion of pouring the heavier metal there can be no appreciable
oxidation of the molten metal, nor formation of nonmetallic inclu-
sions at the ID.
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