Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to methods for manufacturing
compacted graphite cast iron with a uniform distribution
of vermicular or compacted graphite in thin walled
shaped castings involving a range of cross sectional
sizes. More particularly, this invention relates to
continuous methods for manufacturing shaped castings,
which have a uniformly distributed vermicular or compacted
graphite morphology throughout the casting. Still more
particularly, this invention relates to continuous,
automated, high productivity methods for producing
shaped castings of compacted graphite iron. The methods
can be readily adopted by foundries presently equipped
with high productivity, automated lines for production
of ductile iron castings by nodularizing treatment
inside the mold. The invention can be used for producing
compacted graphite iron castings intermittently in a
production line that is otherwise automated to produce
ductile iron castings.
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High productivity automotive lines have adopted a
nodularizing treatment inside the mold, referred to
hereafter as the "in-mold process", because the in-mold
process better lends itself to automation than previous
processes that involved batch treatment (i.e., sandwich
or plunge processes). In the in-mold process, measured
quantities of a graded magnesium ferrosilicon treatment
alloy are introduced into a reaction chamber in the
mold and the liquid iron to be treated is directed to
flow over the treatment alloy in the reaction chamber.
The incoming metal, having a well-controlled chemistry
reacts with the magnesium ferro-silicon alloy in the
mold resulting in a spheroidal graphite structure which
imparts ductilitY to the cast iron. Descriptions of
the in-mold technique can be found in U.S. Patents
3,703,922, 3,819,365, 4,004,630 and 4,134,757 and British
Patents 743,121, 1,132,055 and 1,132,056.
During the in-mold process, the turbulence generated
by magnesium vapor is used to advantage to ensure good
mixing of the alloy with the melt. Also, the ferrosilicon
serves not only as a convenient carrier of magnesium,
but in addition inoculates the melt, producing the
required number of graphite nuclei and thereby suppressing
eutectic carbide formation. By carrying out the magnesium
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treatment inside the mold, good recovery of magnesium
is ensured with minimal pollution problems. Further,
the degeneration of graphite spherulitic structure
caused by reoxidation is minimized. ~lso, a high nodule
count is ensured, as there is no holding time involved
for fading to occur. In comparison, in a batch treatment,
the holding duration increases from the first casting
poured till the last one, and consequently the quality
of the castings within a batch varies. With the continuous
in-mold process, on the other hand, this inherent variation
of the batch process is eliminated.
An automated ductile line in a high productivity
foundry typically consists of:
(i) A dispensing system for introducing measured
quantities of the magnesium ferro-silicon treatment
alloy into a pocket in the mold prior to the
closing of the top half of the mold (i.e., cope).
(ii) A pouring ladle system in which a measured
quantity of liquid metal is received into a ladle
attached to the end of a radial arm, which swings
into position and locks onto a traversing mold on
a conveyor system. Liquid metal from the ladle
is dispensed into the pouring basin of the mold,
once the ladle locks into position.
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(iii) After dispensing the liquid metal into the
pouring basin of the mold, the arm swings through
an idle cycle, before it locks into position at a
metal receiving station. Typically, a number of
radial arms are mounted on a central pivot, and
the rate of pouring is varied to match the rate
of production of the mold.
The ease of operation of the high productivity
automated line for the manufacture of ductile iron
castings having a spheroidal graphite structure has led
to attempts to develop an in-mold alloy suitable for
the production of iron castings having a compacted
i graphite structure. Since the turbulence created by
magnesium vapor has been considered essential for the
homogeneous mixing of the treatment alloy, the efforts
to produce compacted graphite iron heretofore have
focused on the development of an in-mold alloy based on
magnesium
Because of the narrow critical range (i.e., narrow
window) within which magnesium is effective to provide
compacted graphite morphology, the control of compacted
graphite iron technology based on magnesium-containing
in-mold alloys has proved formidable. Too little magnesium
does not produce full compacted graphite structure;
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while over-treatment produces nodular graphite. The
difference between under- and over-treatment can be as
little as 0.005% by weight of magnesium.
Wider latitude or tolerance for magnesium has
heen obtained by using magnesium in conjunction with
titanium to suppress the formation of nodular graphite.
In such cases, however, a further complication is encountered
in the formation of additional inclusions of titanium
carbo-nitrides. See, for example, Schelleng, U.S.
3,421,886. Moreover, residual titanium in the recycled
scrap will prove detrimental to the development of
fully nodular graphite and thus impair the physical
properties of ductile iron castings produced from the
melt contaminated with titanium. In practice, it is
not possible in an automated foundry to segregate scrap
containing residual titanium arising from compacted
graphite iron castings produced by magnesi~m-titanium
technology, from those of ductile iron return scrap.
Therefore, the process route for compacted graphite
iron based on magnesium-titanium alloys is not favored
in foundries that predominantly produce ductile iron
castings lest there should be contamination oE ductile
iron castings by titanium.
Efforts to design treatment alloys based on a
fixed ratio of magnesium to cerium or rare earths, in
combination with inoculants, by empirical methods have
~;~7~9~3
not, a~ yet, yielded consistent results because of the
difficulty in designing an optimal alloy to achieve the
narrow window of magnesium required by compacted graphite,
under the operating conditions in the field.
A recent patent, U.S. 4,396,428, is directed to
the development of a low silicon, but magnesium-bearing
iron alloy in order to establish a ready supply of
treated molten iron in the holding vessels with a selected
composition at a given temperature. However, the control
of compacted graphite morphology warrants that the
residual magnesium should be controlled within a narrow
window and therefore the technology based on magnesium
is inherently difficult to control. For the same reason,
efforts to design treatment processes based on the
injection of molten additives containing magnesium
~nder high kinetic energy as in U.S. Patent 4,227,924
are rendered difficult, in the case of compacted graphite
iron.
Subramanian et al have identified the broader
window offered by rare earths to control the impurity
concentrations in the cast iron melt for the consistent
production of compacted graphite morphology in thick
sections of tonnage castings, see U.S. Patent 4,227,924.
~;~789~8
For instance the model test blocks used in examples 2-6
cited in the patent are of size 15"x15"x8". At slow
cooling rates characteristic of such thicX sections,
there is negligible kinetic undercooling, and therefore
the freezing occurs at near equilibrium conditions. At
such low undercoolings typical of thick sections, the
competitive growth of the carbide (cementite) phase
does not occur. Accordingly, inoculation is not cited
as an essential step to control the compacted graphite
morphology. Further, at such low undercoolings involvin~
smaller driving forces for graphite growth, the impurity
dependent crystal growth mechanism that favors compacted
graphite growth dominates over spherulitic graphite
growth, thereby minimizing the nodularity problem in
compacted graphite structure. Thus, in the absence of
both the carbide problem and the nodularity problem at
the small ~ndercoolings operating in thick sections,
the production of compacted graphite morphology is
determined by controlling the impurity concentrations
in the melt, t~ correspond to a Henrian sulfur activity
window between 0.004 and 0.035, the upper and lower
bounds of the Henrian oxygen activity window being 10-4
and lo-6
In a thin walled shaped casting involving a range
of cross sectional sizes, a range of cooling rates and
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therefore, a range of kinetic undercoolings are involvec~;
the thinner the section size, the larger the kinetic
undercooling or the deviation from equilibrium freezing.
Consequently, the structure varies as a function of
cross sectional size. Thus, thinner sections Ereeze
with large kinetic undercooling, leading to significant
deviation from equilibrium freezing, in marked contrast
to thicker sections that freeze under near equilibrium
conditions.
Under the conditions of large kinetic undercoolings
that characterize the freezing of thinner cross sections,
competitive growth of cementite domlnates over graphite
growth, resulting in the formation of hard and brittle
carbide eutectic structure. Further, under conditions
lS of large kinetic undercoolings, the spiral growth on
the basal face of graphite that promotes spherulitic
morphology dominates over impurity dependent crystal
growth mechanisms that promote prism flake growth also
described hereinbelow. Consequently the tendency for
nodularity increases as the cross section siæe decreases.
The graphite morphological variation as a function of
section size in a casting is referred to as section
sensitivity of the casting. In Fig. 1 described hereinbelow,
the design of a finned casting used in the section
sensitivity test is illustrated; Fitgure 2, also described
hereinbelow, shows a typical increase in the degree of
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nodularity as the section size decreases, in a melt
treated with cerium to produce compacted graphite morphology
in thicker sections and inoculated with ferrosilicon
just prior to casting. On holding the melt after inocu~ation
prior to casting, carbides develop in thinner sections
of the fins. Thus, compacted graphite morphology control
in thinner sections of a shaped casting is inherently
more difficult than in thicker sections because the
thinner sections freeze with a large kinetic undercooling
that deviates significantly from equilibrium freezing
conditions, and are therefore prone to a greater tendency
toward carbide formation and an increased desree of
nodularity.
In view of the many problems associated with the
use of magnesium and magnesium-containing alloys to
produce compacted graphite structure, it is an object
of the present invention to provide a reliable process
for rapidly producing compacted graphite iron which is
not based on magnesium.
, It is another object of the present invention to
provide a reliable process for obtaining compacted
graphite morphology in a thin walled shaped casting
involving a range of cross sectional sizes.
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It is a further object of the present invention
to eliminate the formation of carbides in thin sections.
It is still another object of the present invention
to promote the required degree of interconnected vermicular
graphite growth in thinn'er sections with a minimal
degxee of nodularity.
It i5 a still further object of the present invention
to reliably and rapidly produce compacted graphite iron
using a continuous process of the type currently practiced
on automated high productivity lines in which the nodularizing
treatment is carried out inside the mold.
It is another object of the present invention to
avoid the use of titanium in the alloy design.
It is still another object to provide compacted
graphite iron with residuals that are not harmful to
the development of spherulitic morphology and, thus, clo
not present any problems to ductile iron production
through charge contamination.
It is another object of the process to provide
processes that are compatible with high productivity
automated ductile lines.
It is another object of the present invention to
provide a process for the production of compacted graphite
iron thin walled shaped castings intermittently in a
production line that otherwise is designed to produce
ductile iron using nodularizing treatment in the mold.
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.
According to the present invention, a method
for producing compacted graphite cast iron exhibiting
a uniform distribution of compacted graphite in
shaped castings of varying cross-sectional dimensions
within the range of 1/8 inch to 3/4 inch comprises
the steps of:
(i) orming a near eutectic melt of cast
iron having the chemistry of a ductile
base metal for use in the in-mold
treatment and having a low sulfur
content, for example, about 0.01~ by
weight,
(ii) adding to -the melt sufficient graphite
stabilizing agent, e.g. silicon, to
suppress the carbide eutectic formation
at large undercoolings characteristic of
thinner sections of the cas-ting;
(iii) admixing at least one rare earth
containing additive with said melt, e.g.
by introducing a measured quantity of
the additive into the pouring ladle just
~5 prior to tapping of the metal, the
quantity of the additive being computed
on the basis of reducing and maintaining
Henrian activity of residual o~ygen
within the range of about 10-5 and 10-7
or more preferably within the range of
about 10-5-5 and 10 6.5, or most
preferably to about 10 6 ;
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(iv) tapping a predetermined quantity of liquid
metal into the pouring ladle containing the
adaitive; and
~v) inoculating the melt and thereby provide the
required degree of interconnected vermicular
graphite growth. As the control of the required
degree of nucleation is a critical step, this
process step is preferably conducted by inoculating
each mold individually just prior to casting, just
as the molten metal enters the mold or inside the
mold, for e~cample in the molten metal path at the
pouring basin, sprue or at a location between the
sprue and ingate such as the nodularizing cavity or
in-mold reaction chamber.
The present invention is discussed in detail below
with reference to the drawings, wherein:
Fig. 1 illustrates the design of a five fin pattern
used in the section sensitivity test. The section thickness
of the fins ranges from 3/4" to 1/8";
In Fig. 2, 45 kgs of a cast iron melt of analysis
3.6% C, 1.4% si, 0.6% Mn and 0.009% S was raised in
temperature to 1510C and treated with 25 grams of
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cerium. Each 5 kg of melt was ta~en out at intervals
of 3 minutes in a pouring ladle and inoculated with
0.9% ferrosilico~ (75~ Si grade) and top poured into an
oil-bonded sand mold. The section sensitivity at the
end o~ 3 minutes of holding in the furnace is shown in
Fig. 2 in the microstructural comparison that compares
the graphite morphology as a function of fin section
sizes. Note the increase in nodularity as the section
size decreases to 1/8".
Figure 3 is a graphical representation of the
Henrian oxygen activity in equilibrium with the Henrian
sulfur activity in an iron melt having an effective
carbon concentration of 3.~ wt ~ and silicon concentration
of 2.0 wt % at 1500C. The graph illustrates regions
wherein various rare earth compounds exist as stable
phases. In particular, the shaded region illustrates
the Henrian sulfur and oxy~en activity eguilibrium
levels in the cast iron melt, which upon solidification
gives rise to the vermicular graphite morphology in
thin walled shaped castings, over the entire range of
section sizes, provided there is an optimum nucleation
of graphite. The horizontal dotted line represents the
equilibrium oxygen level attributable to the presence
of 3.5% carbon in the melt at a carbon monoxide partial
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pressure of one atmosphere at 1500C.
Figure 4 shows a flow diagram of a typical automated
high productivity line for producing ductile iron castings
using nodularizing treatment in the mold. The notable
features are that a series of molds are carried by a
conveyor system; the pouring ladles carried by a ra~ial
arm lock into position and receive a predetermined
quantity of liquid metal at a receiving station; the
ladle carrying radial arm swings and locks into position
over the mold and dispenses the melt into the pouring
basin of the mold.
Figure 5 shows a schematic diagram indicating the
location of a reaction chamber in the mold with respect
to the pouring basin sprue in a typical ~ating design
for nodularizing treatment in the mold.
Figure 6 shows a flow diagram of the process
steps in accordance with the present invention. In
addition to the stations shown in Figure 4, there is
the additional station labelled "alloy dispensing station"
for dispensing a rare earth containing alloy into the
ladle, which is returning empty after dispensing the
melt into the mold. After the alloy dispensing station,
the pouring ladle swings to the liquid metal receiving
station.
Figure 7 shows three possible locations or the
addition of the inoculant - i.e.,
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(i) Stream inoculation as the liquid stream
falls into the pouring basin.
(ii) Sprue inoculation as the liquid enters the
sprue and flows over the sprue basin.
(iii) inoculation in a reaction chamber located
in the molten metal path between the sprue basin
and the ingate.
Fig. ~ shows a photograph of two sections of a
casting made in accordance with the present invention.
The thinner section is 4 mm. thick, and the thic]cer
section is 22 mm. thick.
Fig. 9 is an optical micrograph (200x) of a polished
section representative of the thinnest region of the
casting, i.e., the 4 mm. thick section. The compacted
graphite (dark) is shown uniformly distributed throughout
a ferrite-pearlite matrix. It should be noted that the
compacted graphite iron is further characterized by the
absence of eutectic carbides.
~ig. 10 is an optical micrograph (200x) of a
polished section taken from the thickest section of the
casting, iOe., the 22mm thick section. The compacted
graphite (dark) is again uniformly distributed. It
should be noted that the nodularity of the thickest
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section is clearly less than that of the thinnest section
but the section sensitivity is clearly less than that
in Fig. 2. Also, the structure is characterized by the
absence of eutectic carbides.
Fig. 11 shows optical micrographs of specimens
taken from a casting that was produced in accordance
with this invention, using a commerical grade rare
earth silicide, containing 31% rare earths. The upper
micrographs (a) and ~b) taken at magnification lOOx and
200x respectively, of a thin section (6mm) of the casting
show compacted graphite dispersed in a ferrite pearlite
matrix. Apart from graphite, the other dark etchin~
constituent is pearlite. The lower micrograph, (c)
taken at a magnification of 200x of the thick section
of the casting (18 mm) exhibits compacted graphite in a
ferrite pearlite matrix. The thin section does not
; exhibit the carbide phase; the nodularity in the thin
section is not pronounced.
Figure 12 shows the microstructural results of a
casting that was held for 20 minutes in the pouring
ladle after rare earth treatment, on account of a hold
up of the line, before the casting was poured. The
microstructures of thin (6 mm) and thick (18 mm) sections
seen at magnifications lOOx and 200x clearly show compacted
graphite morphology without any significant fading.
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At large undercoolings characteristic of thin
sections that deviate significantly from equilibrium,
it has now been discovered that in ultra low sulfur
iron containing sulfur impurity at concentrations less
than 10 parts per million, (i.e., a Henrian sulfur
activity of about 10-3), compacted graphite morphology
can be consistently obtained if the Henrian oxygen
activity is lowered and maintained at about 10-6. Further,
it has been discovered that once the melt picks up
oxygen impurity and the dissolved oxygen concentrat~on
increases, the compacted graphite morphology changes to
flake morphology. As the Henrian oxygen activity approaches
10-4, the eguilibrium oxygen level attributable to the
presence of 3.5~ carbon in the melt at a carbon monoxide
partial pressure of one atmosphere at 1500C., as represented
by the horizontal line in Fig. 3, flake graphite morphology
is obtained, irrespective of the control of sulfur to
rather low levels. These observations have led to the
discovery that compacted graphite morphology in thinner
sections of the castings is critically dependent upon
the dissolved oxygen concentration in the melt.
The Henrian activity, hi Of any component i in
solution in iron is the effective concentration of that
component in the iron melt and is given by
hi = fi X [w 96 i]
1 ~ 7 ~
where Iw~ i] is the weight percent of component i and
fi is the Henrian activity coefficient of component i.
The activity coefficient fi can be calculated from the
relationship
n 2
g i j-2 e, [w% j] + r~ [w~ j]
where e~ and rl are the first order and second order
interaction parameters which are determined for the
system of interest by conventional thermodynamic techniques,
such as those set forth in Thermodynamics of Alloys,
Carl Wagner, Addison-Wesley Publishing Company, ~eading,
Massachusetts (1952).
The processes of the present invention are based
on the discovery that graphite morphology in the melt
is determined by impurity dependent crystal growth
mechanisms, oxygen being identified as a key impurity
that influences growth morphology of graphite. The
processes of the present invention are further based on
the discovery that the control of the dissolved oxygen
impurity concentration in the cast iron melt, corresponding
~ to a Henr.ian oxygen activity of about 10-6 is essential
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for the consistent production of compacted graphite in
thin walled shaped castings. According to the instant
invention, the mere removal of sulfur is an inadequate
condition for compacted morphology control. In the
absence of sulfur, oxygen as an impurity, because of
its strong bond energy with carbon can promote flake
growth even at low concentrations. In this re~ard, it
should be noted that in cast iron, which is carbon
saturated, soluble oxygen concentration is extremely
low, typically on the order of 5 parts per million by
weight. Because of this low level of oxygen, the critical
role of oxygen impurity in influencing graphite growth
morphology, especially in the context of graphite morphology
control, heretofore has not been recognized. Rather,
the emphasis has been placed on other impurities, in
particular sulfur. Although the removal of sulfur is
an essential condition for graphite morphology control,
it is not a suf~icient condition. Thus, according to
the present invention, compacted graphite morphology
can be established in a melt with a Henrian sulfur
activity of 10-3, only if the Henrian oxygen activity
is preferably maintained at about 10-6, but within the
range fro~ 10-5 5 to 10-6-5. Thus the present invention
is based on the discovery that compacted graphite morphology
in a thin walled shaped casting can be quantitatively
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1~78gO~
related to soluble oxygen concentration in the melt,
corresponding to a Henrian oxygen activity level of
about 10-6, the window of the compacted graphite morphology
being rather narrow in terms of Henrian oxygen activity
but extends to the ultra sulfur range, as shown by the
cross-hatched area in Figure 3.
Rare earths offer distinct advantage over magnesium
to reduce and maintain the Henrian oxygen activity in
the range required to obtain compacted graphite morphology,
because rare earths have rather low vapor pressure at
the treatment temperatures involved compared with magnesium.
Further, rare earths have extended solubility in iron
compared with calcium.
The amount of rare earth alloys added depends
upon the rare earth concentration in the alloy, the
melt chemistry, the amount of tramp elements carried by
the melt, the percentage recovery, and the extent of
reoxidation. The rare earth addition is based on reducing
the Henrian oxygen activity in the melt to within the
range of about 10-7 to 10-5, more preferably to within
the ranges of about 10-6-5 to 10-5-5 and most preferably
to about 10-6. The rare earths for use in the present
invention are the elements of the lanthanide series of
the Periodic Table Of the Elements, plus yttrium. Thus,
rare earths such as cerium, praseodymium, neodymium,
promethium, samarium, europium, gadolinium, terbium,
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1.;~789U~
dysprosium, holmium, erbium, thulium, lutetium and
mixtures thereof can be suitably employed. Similarly,
ores, compounds or metals containing a mixture of rare
earths such as rare earth fluorides, rare earth fluoro
carbonates, misch metals, rare earth silicides, rare
earth aluminum silicide alloys, nickel-cerium alloys
and the like, can be suitably employed. Cerium rich
rare earth contained in ferro-silicon is found to give
consistent results. The amount of rare earth added is
based on the reaction products produced, which include,
rare earth oxysulfide (Re202S) and rare earth sulfides
of the type Re3S4 and ReS. Depending upon the sulfur
level and the recovery rate, suitable addition rates
for rare earths in accordance with the present invention
have been found to range from about 0.05 wt. ~ to 0.15
wt % of the melt weight.
Other surface active elements such as selenium
and tellurium and tramp elements such as tin, lead,
bismuth, and antimony, if present, can also be rendered
innocuous by the addition of suitable quantities of
rare earths. The addition rates for tramp elements are
usually small, as compared with amounts needed to produce
compacted graphite and are as practiced in current
nodular iron technology.
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1~89()8
In order to maintain the melt at the reduced
Henrian oxygen activity level of about 10-6, it is
essential to add a calculated excess of rare earths to
buf~er the melt against reoxidation, appropriate to the
plant practice. In an automated high productivity
system, the excess rare earth required to counteract
reoxidation can be readily established.
It is an essential aspect of the processes of the
present invention to prevent carbide (cementite) formation.
Once the cementite phase is allowed to nucleat~, the
competitive growth of cementite dominates over that of
graphite. In order to overcome the competitive growth
of cementite over graphite, ternary addition of graphite
stabilizing agents such as silicon to the melt is made,
such that the driving force for graphite growth is
selectively increased, and yet the driving force for
the nucleation and growth of cementite is suppressed.
Ternary silicon addition raises the graphite eutectic
temperature and depresses the temperature of carbide
eutectic in an iron-carbon-silicon ternary equilibrium
diagram, thus providing the driving force for graphite
growth selectively. It is essential to control the
concentration of carbide stabilizing elements in the
base iron. For example, in the case of Cr or Mn addition,
the silicon concentration should be adjusted to suppress
the carblde formation. A typical chemistry of the base
metal consists of carbon in the range of 3.5 to 4~0~ by
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.
~78~3~8
weight, Mn up to 1.2% Si in the range of 1.0 to 2.5%
and sulfur around 0.01%. If desired, additional alloying
elements such as nickel, molybdenum, copper and chromium
can be used for special purposes. Convention~l desl~lf~lri~ation
procedures should precede the use of the base metal in
the processes of the present invention to ensure consistently
low sulfur levels in the melt.
The analysis of the nodularity problem in thinner
sections is one of kinetics of crystal growth. Impurity
dependent crystal growth mechanisms operate at smaller
kinetic undercooling and promote prism flake growth.
In the absence of impurities, prism flake growth is
suppressed and basal spiral growth operates at large
kinetic undercooling (large driving force) resulting in
spherulitic morphology. Compacted graphite morphology
involves growth of the prism and basal faces of graphite,
and therefore impurity control in the melt i5 a critical
step. However, at large kinetic undercooling characteristic
of thin sections, impurity dependent crystal growth
mechanisms are dominated ~y spiral growth, leading to
increased nodularity. In order to reduce the nodularity
problem in thinner sections, it is essential to reduce
the degree of growth undercooling. An essential aspect
of this invention is the discovery that the growth
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39~8
undercooling of graphite can be reduced by increasing
the degree of nucleation, but that there exists an
optimum degree of nucleation to promote the required
degree of interconnected growth of compacted graphite
under a reduced degree of growth undercooling. Thus,
the processes of the instant invention emphasi~e not
only exerting control over the impurity concentration
in the melt, notably oxygen, but also controlling the
degree of nucleation of graphite in the melt entering
the mold. An essential aspect of this invention is the
optimization of the number of nuclei in the melt, such
that the required degree of interconnected vermicular
graphite growth is promoted. Far too many nuclei tend
to promote nodularity and suppress the interconnected
vermicular graphite growth. On the other hand, far too
few nuclei can promote carbide formation. Thus, according
to the instant invention, there exists an optimum number
of graphite nuclei to promote the required degree of
coupled or interconnected growth of graphite.
The processes of the present invention are based
on the discovery that optimum nucleation is a crucial
step for obtaining compacted graphite morphology in
thin walled shaped castings, and therefore, this process
step is recited as an integral part of the processes of
the present invention. Even though it is not possible
to control the number and periodicity of dispersion of
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1~789~
graphite nuclei per se, the processes of the present
invention are based on exerting control over the nucleation
step by carrying out inoculation for each individual
mold, just as the melt enters the mold cavity. This
process step eliminates the holding and handling of the
melt after inoculation and, therefore, eliminates the
variables associated with the coalescence and flotation
of nucleants and any possible impairment of nucleating
substrates on account of reoxidation. Since the deoxidation
and desulfurization reaction products are potential
sites for the heterogeneous nucleation of graphite, the
processes of the present invention stress the importance
of the control of base metal sulfur to consistently low
levels for better reproducibility of results. It should
be pointed out that the processes of the instant invention
lay emphasis on the principle of optimum degree of
nucleation of graphite in order to achieve the required
degree of interconnected growth of graphite, irrespective
of the type of inoculant chosen or the technique of
inoculation adopted. For instance, according to the
processes of the present invention, finely graded 75%
Si foundry grade inoculant added to the melt in one of
the following ways have been found to give carbide free
compacted graphite structure:
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(i) inoculation into the pouring ladle just
before pourin~
(ii) stream inoculation as the melt is discharged
into the pouring basin
(iii) inoculation into the pouring basin
(iv) inoculation in the path of the molten metal
flow inside the mold, in particular in the reaction
chamber designed for nodularizing treatment inside
the mold.
Each of these techniques is compatible with a high
productivity automated casting line. It has been found
that graded ferrosilicon (75% Si) gives adequate nucleation
of graphite to eliminate the carbide~ in thin sections,
but the quantity of treatment required optimization to
promote the required degree of interconnected vermicular
graphite growth. Other commercial grade inoculants,
such as compacts or inserts made out of inoculants are
compatible with the processes of the instant invention.
It is essential to design the treatment such that the
graphite nuclei are uniformly dispersed in the melt.
Thus, for example, a controlled dissolution of t~le
insert in the path of the molten metal can be ensured
throughout the entire flow of molten metal, thereby
promoting a uniform distribution of the inoculant, as
described in U.S. Patent 3,658,115.
In another variation of the processes of the
present invention, the rare earth-containing additive
and the inoculant can be added ~o the cast iron me1t in
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'
,.:
~7~9~
the ladle by injection metallurgy techniques or wire
feeding techniques, just prior to pouring the molten
metal into the pouring basin.
In accordance with the foregoing, it has been
found that the melt containing adequate silicon and low
sulfur, on rare earth treatment to lower and maintain
the Henrian oxygen activity at about 10-6 and optimum
nucleation by inoculation, upon pouring into a sand
mold solidifies to give a uniformly distributed compacted
graphite morphology over a wide ranges of section sizes.
The following examples further illustrate the
present invention. The examples are included solely
for purposes of illustration and are not to be construed
in limitation of the present invention. ~nless otherwise
specified, all percentages and parts are by weight.
EXAMPLE 1
Compacted graphite castings were produced by the
high productivity, automated process of the present
invention using the following process steps:
(i) ~are earth carried in a ferro-silicon alloy
was dispensed into returning empty ladles at an
alloy dispensing station.
1~78~
(ii) Liquid metal was tapped onto the alloy in
the ladle.
(iii) Optimum amount of inoculant was placed
into an in-mold reaction chamber before closing
mold.
(iv) The rare earth treated liquid melt from the
pouring ladle was poured into the mold carrying
the inoculant in the in-mold reaction chamber.
The quantity of the liquid metal dispensed by
each ladle was 230 lbs. The chemistry of the desulfurized
hot metal was as follows:
Carbon 3.6 - 3.7 wt %
Sulfur 0.008 wt %
Mn 0.53 wt
Si 1.6 - 1.8 Wt ~
Rare earth was added in the form of a rare earth
containing ferro-silicon alloy containing 11% rare
earths. The alloy had a size grading between -4 mesh
and ~8 mesh. The amount of alloy added corresponded to
a rare earth addition of 0.05 wt ~ of the melt weight.
The melt was poured at 2570F.
The inoculant used was 75% Si-containing ferro-
silicon. The grading of the inoculant used ranged in
size between -20 mesh and ~100 mesh. The quantity of
the inoculant used was 0.4~ of the melt weight.
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.
1;~78908
The variations in section thickness of each castinc3
ranged from 22 mm. for the thickest section to 4 mm.
for the thinnest section. Fig. ~ shows photoyraphs of
the thick and thin casting sections which are used for
metallographic investigation.
Fig. 9 illustrates the typical microstructure of
compacted graphite obtained in a ferrite-pearlite matrix,
essentially free of eutectic carbides, which was obtained
in the thinnest section (4 mm. thick). The pearlite
content ranged from 25 - 35%.
Fig. 10 illustrates the typical miçrostructure of
compacted graphite in a ferrite-pearlitic matrix, essentially
free of eutectic carbides which was obtained in the
thickest section (22 mm. thick). The amount o~ pearlitic
content ranged from 20 ~ 30%.
Six sections were taken from each automotive
casting to confirm the uniformity of structure. In
general, the process ensures total absence of eutectic
carbides even in sections as thin as 4 mm. The nodularity
in the thinnest section, though present, was significantly
reduced, which can be clearly seen by comparison with
Fig. 2.
EXAMPLE 2
The section sizes of the casting ranged from 3/4"
to 1/4"~ A commerical grade rare earth silicide was
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~l~7890~
used in this Example, with the following analysis:
31.2~ rare earths, 35% Si, 0.16~ C and 33.5% Fe. The
treatment procedure was identical to that used in Example
I. The microstructural results are shown in Fig. 11,
which clearly show that the section sensitivity can be
minimized by programmed rare earth treatment and optimum
nucleation.
The melt was held in the pouring ladle for 20
minutes after rare earth treatment, but before pouring
into the mold. This would simulate an extreme condition
of line break-down, in a continous flow operation. The
microstructual results pertaining to thin and thick
sections of a typical casting are shown in Fig. 12.
The remarkable persistence of compacted graphite rnorphology
in thin and thick sections of the casting can be clearly
seen.
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