Note: Descriptions are shown in the official language in which they were submitted.
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Description
Method of .~aking and Using
~ Ceramic Shell Mold
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Techn_cal Fl d
This invention relates to the preparation of a
ceramic shell mold useful for inves-tment cas-ting pur-
poses, and particularly to a method of making a shell
mold that will effectively reduce the amount of surface
decarburization of a ferrous article formed in the shell
mold.
Background Art
Investment casting, also referred to as the
"Iost wax" process, typically involves alternate appli-
cations of a ceramic coating composition and a stucco
composition to an expendable pattern in order to provide
a multi-layered shell mold. The pattern is usually made
of wax, plastic, or similar material which is melted out
to leave a correspondingly shaped internal cavity into
which molten metal is poured.
Unfortunately, there have been many attempts
to control the surface finish and the amount of decar-
burization oE steel investment castings. The problem of
a metal-mold-atmosphere reaction at the time of pouring
and initial stages of solidification of the molten metal
has continued to cause an undesirable carbon-free zone
a~ljacent the surface of the article as well as surface
blemishes. The methods of minimizing this phenomenon
have included casting in a vacuum, use of inert gas
shrouding, the addition of reducing agents into the mold
cavity prior to pouring, preheating the mold in a
carbonaceous atmosphexe prior to casting, etc. ~ll of
these production steps are costly, time-consuming or
raise issues of safety to foundry personnel such as by
producing noxious vapors.
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U.S. Patent No. 3,184,813 issued to P. J. O'Shea
on May 25, 1~65 and U.S. Patent No. 3,296,666 issued to M.
G. Lirones on January 10~ 1967 are representative of the
large number of ceramic dip coat compositions used in
building up multi-layered shell molds. Frequently, the
compositions of the shell mold layers are tailored for the
specific metal.
In the past, for example, graphite has been added
to the usual coating composition of a ceramic powder and a
binder in order to improve surface finish and to minimi2e
the amount of decarburization of steel articles. ~ut
while the use of a relatively uniform amount of graphite
throughout the full cross section of the shell mold wal]
has resulted in some improvement in the ~uality of the
castings, surface irregularities and localized
carburization have been observed because of the
undesirable contact of the molten metal directly with the
graphite particles. ~orever, the strength oE the
individually applied layers is reduced by graphite
addition and the shell mold is more costly than desired.
The present invention is directed to overcoming -
one or more of the problems as set ~orth above.
Disclosure of Invention
In accordance with one aspect of the present
invention, a ceramic shell mold is made by alternately
applying a coating composition including a ceramic powder
and a binder, and then a stucco composition including
granular refractory material to an expendable pattern a
preselected number of times, drying the coatin~ between
applicationsS and forming a resultant multi-layered mo]d,
said multi-layered mold having less than 0.5 Wt~%
graphite; heating the multi-layered mold,
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removing the pattern, and forming a resultant hardened
mold; and applying a barrler coating to the exterior
surface of the multi-layered mold while the hardenecl mold
is at a preselected temperature above ambient, said
barrier coating including a mixture of a ceramic powder, a
binder, and a preselected amount of particulate graphite
within a range of about 4 to 20 Wt.~ of the solid portion
of the barrier coating.
In another aspect of the invention, a method of
investment casting of a ferrous article in a shell mold
ineludes the steps of applying a coating composition
including a ceramic powder and binder to an expendable
pattern, said ceramic powder being selected from the group
consisting essentially of fused silica, vitreous silica,
crystalline silica, alumina silicate, alumina, magnesium
silicate, zircon, zirconium silicate, and clay, and the
binder being selected from the group consisting
essentially of colloidal silica sol, ethyl silicate,
aluminum phosphate, and aqueous alkali metal silicate;
applying a stucco composition including a granular
refractory material; alternately repeating the above steps
a preselected number of times and forming a multi-layered
mold, said multi-layered mold having less than 0.5 Wt. % ~;
graphite; heating the multi-layered mold and forming a
hardened mold having an internal cavity; applying a
barrier coating to the exterior surface of the hardened
mold at a location spaced from the internal cavity and
while the hardened mold is at a preselected temperature,
said barrier coatin~ having a solid portlon and being a
mixture of a ceramic powder, a binder, and a preselected
amount of finely divided graphite, said preselectecl amount
of graphite being within a range of about ~ to 20 Wt. ~ of
the solid portion of the barrier coating; heating the
hardened mold and barrier coating and forming a hot shell
mold; and
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pouring a ferrous molten metal into the internal cavity of
the hot shell moldO
The above-described multi-layered mold and
barrier coating are heated and a ferrous molten metal
poured into the cavity, whereupon after cooling and
removal of the article from the mold the article will be
noted to have minimal surface carbon depletion and a
relatively smooth surface.
~rief Description of Drawing
The sole figure is a diagrammatic and enlarged,
fragmentary cross sectional view through a multi-layered
shell mold having a barrier coating thereon.
Best Mode for Carryin~ out the Invention
A preferred method of making a ceramic shell mold
6 comprises the steps of alternately applying a ceramic
coating composition 8 and a stucco composition 10 to an
expendable or thermally meltable pattern a preselected
number of times, firing such multi-layered mold to remove
the pattern and provide a hardened mold 12 having an
internal casting cavity 14, and applying a barrier coating
16 including a ceramic power, a binder and a preselected
amount of graphite as is generally illustrated in the
dra~ing. The presence of any significant amount of
graphite is preferably avoided in the multi-layered mold,
particularly adjacent the casting cavity 14, and is
preferably controlled to a range o~ about 13 to 17 Wt.%
graphite of the total amount of the solid portion of the
barrier coating 16.
The aforementioned ceramic coating composition 8
basically includes a ceramic powder and a binder.
Typically, the ceramic powder is selected from the group
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consistlng essentially of fused silica, vitreous silica,
crystalline silica, alumina silicate, alumina, magnesium
silicate, zircon, zirconium silicate, and clay treated
to remove impurities, and can be mixtures thereof. The
binder is sclected from the group consisting essentially
of colloidal silica sol, ethyl silicate, aluminum phos-
phate, and aqueous alkali metal silicate.
The stucco composition 10 basically includes
conventional granular refractory materials such as
zircon.
The multi-layered mold made by alternately
applying the ceramic coating composition 8 and -the stucco
composition lO a preselected number of times to -the pattern
is desirably substan-tially free of graphite. By this term
it is meant that less than 0.5 Wt.% graphite is present
in the multi-layered mold before the barrier coating 16
is applied.
More particularly, a preferred method of
making the ceramic shell mold 6 includes the following
steps:
Step (a) Forming an expendable or mel-table
pattern of wax, plastic or similar material of a con-
struction having the desired shape;
Step (b) Applying a pr-ime or first ceramic
coating composition 8 including fused silica flour,
finely divided zircon, a limited amount of nitrile poly-
mer latex for low temperature strength, for example
2 Wt.%, and colloidal silica sol including wate~ in the
form of a slurry to the pattern by dipping the pattern
in-to an agitated thixotropic slurry thereof, removing
the coated pattern therefrom and allowing a preselected
amount of draining and initial stages of setting thereof;
Step (c) ~pplying a coarser or stucco coating
composition lO including granular refractory material
such as zircon to the still wet first coating co~posi-
tion 8 by sprinkling same thereon from a con~entional
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rainfall sander, or alternatively by immersing it in a conventional fluidized
bed, and with the AFS grain size of the stucco coating composition being
generally limited to a range of from about 35 mesh to 20 mesh ~about 0.5mm
to 0.8mm~;
Step ~d) Drying the coated and stuccoed pattern for a preselected
time period, for example 30 minutes to 6 hours, to a waterproof or gelled shape
and providing a first layer 18;
Step (e) Alternately repeating Steps ~b), ~c), and ~d) a pre-
selected number of times while preferably increasing the relative coarseness
of the solid particles therein, for example for nine cycles, and providing a
multi-layered "green" mold having a plurality of the layers 18, each layer
being about lmm (.040") thick and intimately associated with each other as
i.s representatively indicated in the drawing;
Step ~f) Heating the multi-layered "green~' mold in an autoclave at
a preselected first temperature of about 180 to 200 C ~350 to 400 P) for
about 5 to 25 minu-tes, melting out and removing the pattern, and providing
some strength to the mold;
Step (g) Firing the multi-layered mold in a furnace at a preselected
second temperature of about 800 to 1400 C (1500 to 2500 F), and preferably
about 1000 C (1800 F) for about one hour to provide a hardened mold 12
having an exterior surface 20, and an interior surface 22 facing the casti~g
~avity 1~ as shown in the drawing;
Step ~h) ~pplying a barrier coa~ing layer 24 to the exterior surface
20 of the hardened mold 12 while it is at a preselected third temperature of
about 200 C (40Q F), the barrier coating layer including a mixture of zircon,
fused silica, finely divided graphite, and colloidal silica sol, the AFS grain
size of the
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grapllite particles beinc3 preferably limitecl in size to
passing through a 200 mesh sieve (less than about
0.075mm or 0.003"), and being most desirably li~ited -to
a ranc~e of about 600 mesh to 325 mesh (abou-t .Olmm to
.05mm) r and limiting the amount of graphite to a range
of about 4 to 20 Wt.% of the solid or dry portion of
the mixture;
Step (i) Drying the barrier coating layer for
a preselected period of time;
~ Step (j) Repeating Steps (h) and (i) a plural-
ity of times, for example three times, to provide a
plurality of the graphite containing barrier coating
layers 24 to define the multi-layered barrier coating
16 as shown in the drawing; and
Step (k) ~leating the hardened mold 12 and the
barrier coating 16 in a furnace of the like to a pre-
selected third temperature of about 900 to 1~00 C
(1650 to 2550 F), and preferably about 1050 C (1920
F) to make the ceramic shell mold 6.
Subsequently, a ferrous molten metal such as
steel is poured into the casting cavity 1~ o~ the ceramic
she]1 mold 6. Most desirably, the mold is main-tained
at a temperature of about I000 C (1830 F), or slightly
below, since the molten metal poured therein is about
1350 to 1700 C (2~60 to 3100 F) and this minimizes
the temperature differential therebetween.
Various modifications of Steps (a) through
(k) set forth above can be visualized without departing
from the spirit of the present invention. For example,
drying Step (d) can be achieved under ambient air
conditions for a period of about one half to one hour~
or alternatively the drying can be achieved in an oven
or ~urnace at a temperature slightly above ambient
temperature to reduce the holding time. Of course, the
temperature cannot be elevated too much because -the
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pattern either can melt or can expand to the point of
unduly stressing the relatively weak walls of the
partially complete mold.
One of the advantages of this met}lod of
investment casting is that it is easier to melt out and
remove the pattern from the multi-layered mold because
lt has a thinner section during intermedia-te Step (f)
than the equivalent strength prior art shell mold has
at the time of pattern removal. I have also noted a
consistently higher quality of the hardened molds 12
when compared with the thicker prior art molds. Further-
more, S-tep (g) can be achieved without the need for a
reducing atmosphere because the multi-layered mold is
substantially free of graphite at that stage.
Moreover, in Step (h) zircon can be replaced
by an equivalent amount of alumina silicate. The
barrier coating is preferably about 78 Wt~ of dry
materials including the aforementioned zircon or alumina
silicate, fused silica, and graphite, and the remaining
22 Wt.% is substantially liquid binder including the
colloidal silica sol. Specifically, the preEerred pro-
portions of the dry materials in the barrier coating 16
are about 75 parts 2ircon, 25 parts fused silica, and
11 to 25 parts graphite by weight.
In actuality, Steps (h), (i), and (j) were
achieved by repetitively dipping the hardened mold 12
while hot into an agitated thixotropic solution oE the
a~or~mentioll~d ceramic and graphite materials for about
four or five seconds and removing the mold to permit
substantial gelling of the ceramic materials during
periods of about 30 seconds therebetween in ambient
air. The fact that the mold is hot accelerates the
gelling and tends -to bridge the ceramic materials over
any minor imperfections. Such dipping was automatically
accomplished by a known mechanical dipping apparatus
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provided with a suitable timing and counting con-trol
system, no-t shown.
Industrial Applicability
In order to determine the optimum range of
graphite in the barrier coating 16, various weight
percentages oE graphite were added to the zircon and
fused silica por-tions thereof. Steel articles were
made by pouring steel of about 0.3 Wt.~ carbon into the
heated ceramic shell molds 10 as mentioned above, and
the carbon free depth (CFD) and maximum affec-ted depth
(MAD) from the surface of the article measured af-ter
sectioning of the article. The carbon free depth (CFD)
is a measure of the thickness of the surface zone that
has experienced substantially total decarburization~
The maximum affected depth (MAD) is a measure of the
thickness of a thicker surface zone that has experienced
at least partial decarburization or a substantive
deviation from the carbon level of the central body
portion of the article. The test results were as
followS:
Prior ~r-t 4.8 Wt.% 9.:L Wt.~ 13.1 &
16.7 Wt.
CFD 0.3mm0.13mm 0 lOmm 0.05mm
(0.012") (0.005") (0.004") (0.002")
25 MAD 0.9mm0.64mm 0.51mm 0.3mm
(0.035") (0.025") (0.020") (0.012")
Thus, the test data indicates that the prior
art ceramic shell mold with substantially no graphite
therein exhibited an undesirably hi~h level of decarbur-
ization, and the articles prepared in accordance with
one aspect of the present invention exhibited a decreasingde~ree of decarburization as the proportion of graphite
in the barrier coa-tin~ 16 increased up to about 17 Wt.~.
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In addition to such decarburization measure-
ments, which typically reflect the amount of sur~ace
material that must be removed so that any subsequent
heat treatment effect of the carbon will be uniform
throughout the steel article, the surface smoothness of
the test articles was noted. For example, the relatively
frequent va]leys of about 1.5mm (0.060") maximum depth
in the prior art articles were proportionately reduced
to minimal blemishes of less than about 0.4mm (0.015")
with the addition of graphite toward 15 ~t.~ in the
barrier coating 16. I found out also that at about 3.4
Wt% graphite the effect on decarburization was minimal,
whereas at the other end of the range at about 20 Wt.~
graphite, -the graphite was difficult to keep in suspen-
sion, tended to agglomerate and thereby weaken thelayers, and did not appear to result in any significant
change in the results from that of about 15 Wt.~ graphite
proportion.
In view of such beneficial results, the broad
range of graphite in the barrier coating 16 is about 4
to 20 Wt.~, the preferred range is about 13 to 17 Wt.~,
and the most desirable amount is about 15 Wt.%.
It is of note to appreciate that the problems
of decarburization and surface blemishes of investment
cast articles is more severe when the amount of carbon
in the ferrous molten metal is reduced toward 0.1 Wt.
carbon. Thus, the method oE the present invention is
particularly useful for minimizing decarburization of
steel articles with less than 1.5 ~1t.~ carbon.
~loreover, although the preferred method of
making the shell mold 6 includes the step o~ heating
the multi-layered mold prior to applying the barrier
coating 16 -thereto, I contemplate that the multi~
layered mold substantially free of graphite and the
barrier coating can be sequentially built-up and then
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the resultant structure heated to remove the wax pattern and to orm
hardened shell mold 6. In either method, graphite is reactive to oxygen,
and the reac*ion is accelerated as the temperature increases. In a crystal-
line material such as the shell mold, graphite will travel in the porous
interstices thereof during heating. I theorize that during pouring of molten
metal into the shell mold a portion of t~e graphite in the barrier coating
16 diffuses inwardly toward the casting cavity 14 while at the same time a
portion of the carbon in the molten metal tends to diffuse into the shell
mold where oxygen is available. Under any theory, however, carbon depletion
is greatly minimized by the method of the present invention.
Other objects, aspects and advantages of this invention can be
obtained from a study of the drawings, the disclosure and the appended claims.
