Method and apparatus for manufacturing of a thick-walled hollow casting of cast iron

A method of manufacturing a thick-walled container casting of a cast iron with a spheroidal graphite comprises forming an inflexible mold spaced between an inner cylindrical dead mold core and an outer mold arranged over the dead mold core, filling the mold space through a gate from an end thereof while cooling particularly the inner mold surface of the mold ajacent the dead mold over the amount of a cooling of a usual sand casting, and dimensioning the gate so that the cast iron solidifies in the gate before the eutectic solidification of the casting sets in. The device comprises an inner cylindrical dead bolt core which has a bottom end and a closed top end and an outer cylindrical mold positioned over the inner core with its top spaced from the top end of the inner dead mold core. The bottom open ends of the inner and outer cylindrical molds are closed by means which define a mold filling closure having a gate dimensioned so that the filling of the mold will effect first the solidification in the gates.

FIELD AND BACKGROUND OF THE INVENTION 
The invention relates, in general, to the construction of large vessels 
and, in particular, to a new and useful method and apparatus for 
feederless manufacture of thick-walled, container type castings from cast 
iron with spheroidal graphite. 
Such castings are needed, for example, as transport containers for used 
fuel elements from nuclear power plants. The quality of the castings must 
meet especially high requirements for such use. It must have a 
fine-trained and tough structure free of volume deficiency faults, in 
particular free of micropores. 
Thick-walled sand casting has long solidification times, as the 
considerable amounts of heat being released can be removed only through 
the insulating mold material. In the case of heat iron with spheroidal 
graphite this may result in a coarse globulitic structure. Besides, under 
these conditions flat temperature gradients between the residual melt and 
the solidifying peripheral shell occur, which favor the occurrence of 
volume deficiency faults, in particular micropores. If the cellular 
structure is coarse, the volume expansion, which predominates in graphite 
eutectic crystallization and exerts a pressure saturating for feeding, 
cannot completely keep the micropores shut. The harmful consequences are 
microcavities which lead to readings in non-destructive testing methods 
and microliquations, or, in extreme cases, even to carbide segregations at 
the eutectic grain (cell) boundaries, which impair the toughness of the 
material. 
German patent application DE-AS 21 23 267 discloses that for electro-slag 
remelting in the production of thick-walled hollow bodies to use as cores, 
a monolithic support body with cooling, which, after switching the cooling 
off and, hence, expansion and subsequent switching on again, can be pulled 
out of the melted ingot. The problem of the manufacture of castings from 
cast iron with spheroidal graphite without micropores is not dealt with in 
this German patent application, but rather a method for drawing of the 
core. 
German patent application DE-AS 19 52 009 solves the same problem with a 
water-cooled core in electro-slag remelting by withdrawing wedge-shaped 
parts of the core by means of a spindle drive, the core diameter being 
reduced for drawing. German patent application DE-OS 28 27 091 teaches 
that in conventional casting of steel to billets or ingots, one should 
construct a chill mold of single walls from water-cooled cooling boxes. 
The problem of the invention of pore-free casting of cast iron with 
spheroidal graphite is not dealt with by this document either. 
SUMMARY OF THE INVENTION 
The invention obtains in a thick-walled container-type casting and by 
effecting a steeper temperature gradient favoring shell-type 
solidification in connection with a shortened solidification time, a 
fine-grain, low-liquation and pore-free structure, which otherwise could 
be achieved only in thin-walled castings. 
In the inventive method, the entire mold comprising the mold core and an 
outer mold is constructed to be inflexible, and so that an improved heat 
removal as compared with sand casting is effected. The gates for passing 
melt to a mold cavity in the mold are dimensioned so that the cast iron 
solidifies in the gates before the eutectic solidification of the casting 
sets in in the mold cavity. Through the accelerated solidification of the 
cast iron in particular in the core region and the corresponding 
development of a steep temperature gradient in the mold cavity, the 
formation of a fine cellular structure is promoted and the solidification 
which, for sand casting, is largely globulitic, is shifted toward a 
solidifying peripheral shell. Both factors reduce the danger of micropore 
formation. The inflexible construction of the entire mold ensures that the 
cooling casting shrinks onto the core and thereby a gap formation is 
avoided and thus good heat transfer is preserved. By dimensioning the 
gates so that the cast iron solidifies in them before the eutectic 
solidification of the casting sets in, and by having an inflexible 
construction of the entire mold, the expansion of the metal during the 
graphitic eutectic solidification can become fully effective as pressure 
increases in the mold cavity. The result of this is that in the 
solidifying casting the formation of micropores is avoided. 
Specifically the invention can be realized to advantage as follows: 
Good heat removal can be promoted in that the castings are cooled in a 
metered and regulated manner at their inner surface by cooling the 
interior of the mold core by coolants, notably non-combustible liquid 
coolants which evaporate in the system. Suitable for this is liquid 
nitrogen or water atomized in an air stream. 
The effective pressure increase in the mold cavity during the eutectic 
solidification can be promoted, besides by the sturdy design of the mold, 
by the fact that the castings are cooled at their outer surfaces. In 
castings, such as container for fuel elements, where the outer surfaces 
must be provided with cooling fins for subsequent use in practice, 
sufficient cooling at the outer surface can be achieved simply by forming 
the castings with large-size cooling fins on their outer surface. Then the 
outer mold can be constructed, e.g. of form-stable, cold resin-bonded 
quartz sand. 
The cooling of the outer surfaces of the casting can be improved, e.g. in 
the case of relatively small cooling fins or a smooth outer surface, by a 
metallic outer mold. The metallic outer mold improves the removal of heat 
to the outside by its greater thermal conductance as compared with a 
ceramic mold, and thereby, as a result of higher temperature, it also 
improves the convection cooling by the surrounding air. This can be 
further improved by cooling fins on the outer mold. Besides, one can cool 
the metallic outer mold in a metered and regulated manner by coolants, 
notably by non-combustible liquid coolants evaporating in the system. 
The measures for improving the external cooling of the castings promote a 
shell type solidification and thereby increase the pressure increase in 
the residual melt during the eutectic solidification, which improves the 
density of the casting. 
In a casting mold, especially suitable for the method, comprising a mold 
core and an outer mold, the external contour of the mold core is formed by 
a dead mold of sheet steel, on the inner surface of which cooling elements 
traversed by coolant are arranged. The space inside the core mold which is 
not occupied by the cooling elements (i.e. the free space) is filled with 
moldable, fine-grained substances. For the dead mold, sheet steel of a 
thickness of 10 to 20 mm is suitable. The fine-grained substances serve 
for shape stabilization of the core and also promotes the heat transport 
between the dead chill mold and the cooling elements, in which the cooling 
is brought about by the through-flowing coolants. The outer surface of the 
core is generally provided with a coating as used in foundaries, to avoid 
welding on. 
The cooling elements may be formed as cooling boxes, in which inflow and 
outflow tubes are arranged side by side, to permit uniform heat removal. 
The cooling boxes are advantageously held by metallic elements such as 
wedges and are pressed against the dead mold. Instead of cooling boxes, 
cooling cells may be used. 
The fine-grained substance with which the free space within the mold core 
is filled may be ceramic molding substances as customary in foundries. To 
increase the thermal conductance, there may be used also fine-grained 
metallic substances, preferably steel shot, or metallic substances may be 
added to the molding material. 
Advantageously, the outer mold comprises sheet steel and is provided with 
cooling elements. The cooling elements may be either cooling boxes or 
cooling coils. Additional cooling fins improve the heat removal. 
Accordingly, it is an object of the invention to provide a method of 
manufacturing a thick-walled container casting of cast iron with 
spheroidal graphite which comprises forming an inflexible mold using an 
inner cylindrical dead mold core arranged within an outer mold, filling 
the space between the inner cylindrical dead mold core and the outer mold 
from an end thereof and cooling particularly the inner mold core over an 
amount of cooling which would be usual for a sand casting and 
dimensionsing the gate so that the cast iron solidifies in the gate before 
the eutectic solidification of the casting sets in. 
A further object of the invention is to provide a device for manufacturing 
a thick-walled container casting of cast iron with spheroidal graphite 
which comprises an inner cylindrical dead mold core having a bottom end 
and a closed top end spaced inwardly of an outer top end of an outer 
clindrical mold and with means defining a mold filling closure closing the 
bottom ends of the outer and inner molds so that at least one gate which 
is of a size to cause solidification in the gate before the eutectic 
solidification of the casting takes place in the mold. 
A further object of the invention is to provide a device for manufacturing 
a thick-walled container which is simple in design, rugged in construction 
and economical to manufacture. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, in particular, the invention embodied therein 
comprises a method of manufacturing a thick-walled container casting 1 
made of a cast iron with a spheroidal graphite which comprises forming an 
inflexible mold using an inner cylindrical dead mold core 6 arranged 
within an outer mold part 3, and filling the mold space formed between the 
inner and outer parts through a gate or port while cooling particularly 
the mold adjacent the dead mold core over an amount which would be cooled 
with the usual sand casting and dimensioning one or more of the gates or 
ports 2 so that the cast iron solidifies in the gate before the eutectic 
solidification of the casting sets in. 
For the feederless production of a thick-walled, container type casting of 
cast iron with spheroidal graphite, namely a container for fuel elements 
1, the entire mold was constructed to be inflexible and an improved heat 
removal as compared with the sand casting was arranged for, in particular 
at the inner surface. The gates 2 were dimensioned so that the cast iron 
solidified in them before the eutectic solidification of the casting 1 set 
in. The outer closed mold 3 is made of form-stable, cold resin-bonded 
quartz sand. For the production of the outer mold 3, cores or fins 4 for 
forming the cooling fins 5 to be produced on the exterior of the casting 
1, were inserted into the mold. The closed dead mold core 6 comprises 
externally a cylindrical sheet iron jacket 7 about 6 m long having a wall 
thickness of 15 mm with a 30 mm thick cover 8 welded on. Before the 
welding on of the cover 8, the cooling boxes 8 and 10 are placed in two 
planes and pressed against the sheet jacket 7 with steel wedges 11. Good 
cooling of the cooling boxes 9 and 10 was achieved by a system of 
parallel, vertical cooling tubes, a lower feed line and an upper discharge 
line, distributed over the total circumference, having been installed 
alternately and connected to a feed and discharge ring conduit for each. 
Also, the cover 8 is provided with a cooling box 12. Pipes in cooling 
boxes 9, 10 and 12 are in a conventional serpentine form and are supplied 
with coolant over supply ring 16 and pipes 17. The coolant returns by 
pipes 18 and discharge ring 19. The core 6 is arranged upright. The 
casting is cast uphill. The casting temperature was 1,320.degree. C., the 
quantity of magnesium-treated and seeded iron was 155 tons. The 
composition of the melt corresponded to a GGG-40.3, DIN 1693. This 
corresponds approximately to ASTM A 536, grade 60-40-18. The approximate 
dimensions of the casting 1 were 6,400 mm, the outside diameter with fins 
2,500 mm, the inside diameter 12,00 mm, the bottom thickness was 400 mm. 
Steel of grade GGG-40.3, DIN 1963 is a ferritic modular cast iron grade. 
According to the ASTM standard, its yield point is somewhat higher than 
according to the DIN standard. The somewhat higher yield points are due to 
the fact that in the U.S. annealing is in principle ferritizing. This is 
not the case with the method according to the present invention. Another 
U.S.A. steel grade similar to GGG-40.3, DIN 1693 is ASTMA A 395, grade 
60-40-18. This U.S. steel has a somewhat lower silicon content than the 
steel according to the DIN standard. 
For a casting according to the invention, the product analysis 
specification in percent by weight was: C 3.2 to 3.9, Si 1.7 to 2.3, Mn 
less than 0.3, P less than 0.03, S less than 0.015, Mg more than 0.03. 
An FeSi-Mg or similar alloy, e.g. FeSi-Mg 30, was added to the melt in 
quantities such that the added magnesium content is above 0.6%, a 
magnesium content in the finished casting of over 0.03, e.g. 0.04%, being 
adjusted. 
After casting, the core 6 was cooled with liquid nitrogen in such a way 
that upon flowing into the cooling elements 9, 10, 12 vaporization 
occurred. The gates 2 were dimensioned so that they froze shut when the 
melt in the mold had reached a temperature of 1160.degree. to 1200.degree. 
C. The cooling was maintained during the entire solidification time. Only 
just above the gamma-alpha transformation the coolant gas supply was 
turned off, in order not to disturb the ferrite formation. In all, the 
solidification time was shortened by the use of the cooling by 56% as 
compared with pure sand casting. 
After completed solidification and cooling in the mold, the casting was 
drawn and the fine-grain tampings in the core were removed, the cooling 
elements disposed in several planes were taken out, and finally the 
formwork i.e. the jacket 7 with the cover 8, was removed by cutting open 
and pulling. The remaining casting was scoured in the usual manner. 
The ultrasonic testing of the scoured and internally treated casting with 
various angle-testing heads and with frequencies of from 1 to 2 MHz gave 
no readings at a detectability of an equivalent magnitude of error of 3 
mm. 
Cast iron undergoes a two-phase solidification process as illustrated in 
FIG. 3. The cast iron contracts during the first dendritic or PRIM phase. 
According to FIG. 3, this phase continues until the melt reaches about 
1,160.degree. to 1,200.degree. C. During all this time the gates are 
dimensioned so that the cast iron remains fluid within the gates. As the 
melt shrinks during the PRIM Phase, additional melt is provided through 
the gates into the cavity. As soon as the eutectic or EUT Phase begins, at 
the stated temperature, the gates are dimensioned so that their contents 
solidifies. The gates thus are effectively plugged (cross-hatched area in 
FIG. 3) during the expanding eutectic phase of solidification. Expansion 
of the cast iron melt within the closed space produces a pressure which 
avoids the formation of any cavities within the completed cast. 
As shown in FIG. 2 and 2A, gates 2 include eight separate gates 2a and 2b, 
distributed roughly in a circle on the floor of the mold cavity around the 
core 6. 
Of the four gates in the mold cavity floor, six of the gates labelled 2a 
have a diameter of 50 mm while two of the gates labelled 2b, which are 
diametrically opposed to each other and each connected to a separate 
feeding pipe 18, have a diameter of 60 mm. Each of the pipes 18 is 
connected to 350 mm gates 2a and 160 mm, gate 2b. 
About half way up the mold cavity, four additional ports 2c are provided. 
As shown in FIG. 2A, these also are evenly distributed around the 
circumference of the mold. Each has a height of 60 mm and a width of 20 
mm. These rectangular ports are disposed between two adjacent ribs 4. Two 
of the rectangular gates 2c are connected to one supply pipe 32, there 
being a supply pipe on each side of the mold. 
Further up, the mold cavity for each additional gates or ports 2d are 
provided which also have a 60mm .times.20mm dimension. Pipes 34 on each 
side of the mold each supply two gates 2d. 
Melt is supplied to a basin 20 on each side of the mold. Each basin has 
three openings in its floor. As shown in FIG. 2A, opening 20a is connected 
to pipe 18, opening 20b is connected to pipe 32 and opening 20c is 
connected to pipe 34. 
Each opening has a conical entrance which can be plugged by a stopper 22, 
three stoppers being provided for each basin 20. 
The stoppers are manually movable in the direction of double arrows 36 for 
selectively opening and closing each of the holes 20a through 20b. This 
permits filling of the mold cavity at three distinct levels using the 
gates or ports 2a through 2d. 
The melt is charged into basins 20 and the stoppers 32 are selectively 
raised to charge the melt into the mold cavity through the gates. The 
specific gate dimensions which are noted here were found to cause 
solidification of the melt in the gates at appropriate time to achieve the 
inventive purpose. The port dimensions were used with the old structure 
having the dimension and wall thicknesses set forth above and with the 
melt composition which is also set forth above. 
An actual casting operation in accordance with the invention is now 
described: 
The charge of metal was first melted in three furnaces (not shown) and a 
total of 180 metric tons were eventually charged into the mold cavity at 
three levels of the three sets of ports 2a to 2d. 
At the start, pouring was effected only through pipes 18 and ports 2a and 
2d at the bottom of the mold cavity. This was initiated by lifting the 
stopper 22 covering holes 20a in basins 20, after the basins had been 
filled with melt. 
When the melt was initially poured into the basin, it had a temperature of 
1,480.degree. C. It was permitted to cool down to a temperature of between 
1,320.degree. to 1,350.degree. C. At this point, the first stopper 22 for 
holes 20a was manually lifted to allow melt to pour through pipe 18 and 
into the lower ports 2a and 2b. 
After 90 seconds, the second stoppers 22 covering holes 20b were manually 
raised, allowing metal to pass through pipes 32 and intermediate gates 2c. 
After 165 seconds, the stoppers 22 covering holes 20c were raised allowing 
melt to pass through pipes 34 and the upper gates 2d (which supplied metal 
that ultimately formed the bottom of the casting, the casting being upside 
down in FIG. 2). 
Pouring continued for about 198 seconds in total after which melt in all of 
the gates solidified allowing no additional melt to pass through the pipes 
18, 32 or 34. 
It was found that the 50 mm diameter gate 2a and the rectangular gate had 
melt which froze up initially, presumably because of their smaller 
dimensions. The round 60 mm diameter gate 2b, at the bottom of the mold 
cavity, which are diametricaly opposed from each other, froze up only 
immediately before the start of the eutectic solidification of the 
casting. Until that point, cast iron melt continued to flow through the 
lower large diameter gate 2d, this compensating the volume contraction 
during the primary or dendritic solidification of the casing. The use of a 
large number of gates, 16 in total, permit a fast filling of the mold so 
that the temperature losses during casting remain so small that during the 
time of dendritic solidification, the mold is sufficiently filled with 
cast iron. The temperature in the still liquid casting can become 
equalized and eutectic solidification begins evenly and simultaneously 
through the entire casting. 
It is noted that the stoppers 22 can be raised manually in a simple manner. 
For example, as shown at the right in FIG. 2, the stopper may be mounted 
for linear up and down movement in a slide 40. Teeth 42 are provided on 
the stopper 22 and engage teeth of pivotally mounted sector-shaped pinion 
44. A manually rotatable arm 46 is connected to pinion 44 and can be 
lowered to pivot pinion 44 and thereby raise stopper 22. Similar 
structures can be used to raise and lower the other stoppers at 
appropriate times during the casting process. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.