CA1186128A - Method of casting steel grinding balls - Google Patents

Abstract

METHOD OF CASTING STEEL GRINDING BALLS
Abstract of the Disclosure Molds for use in casting steel grinding balls are con-tinuously recycled for repeated use and are continuously main-tained at a temperature above the boiling point of water between successive uses thereof during recycling.

Description

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This invention relates to the art of metal casting and, more particularly, to casting of metal in molds which are carried at the periphery of a rotatable wheel. The invention is particularly applicable to casting of steel grinding balls and will be described with specific reference thereof. However, it will be appreciated that certain aspects of the invention may be used for casting other items.
Metal casting apparatus of a known type includes a large wheel mounted for rotation about a generally vertical axis and carrying a plurality of molds adjacent its outer periphery. The wheel rotates for carrying the molds successively past a pouring station where the molds are filled with molten metal. The filled molds are then cooled, stripped and finally renewed for another pour as the wheel continues to rotate. Apparatus of this type is disclosed in U.S. Patent Nos. 1,923,553 issued August 22, 1933, to Payne and 2,326,164 issued August 10, 1943, to Payne, and in Philippines Patent No.
13,520 issued June 9, 1980, to Navarro. The molds carried by the wheel for casting steel grinding balls are specifically ~o disclosed in U.S. Patent No. 2,837,797 issued June 10, 1958, to Norton et al. These molds include opposite outer metal chill portions mounted for movement toward and away from one another, and between which sand cores are positioned. The sand cores cooperate with the chill sections to define a plurality of spherical cavities in which steel grinding balls are formed. The sand cores cooperate to define an upwardly ~.
sb/`~--~æ6~s ope~.lng trough in which metal is poured. Passages lead down-wardlv from the trough through the sand cores and the passages are connected with the spherical cavities bv sprue openings.
In previous apparatus of the type described, the molds are cooled by a water sprav during travel of each mold over an arc of approxima-tely ninetv degrees shortlv after pourinq of metal into the mold. The mold is then opened for discharging the solidified balls and sand cores from between the outer chill sections. The chills are then spraved with water for cleaning same and for cooling same to substantially ambient temPerature while thechillsections travel over an arc of approximately fiftv degrees. ~he chill sections are then dried ancl further cleaned bv blowing air against them. New cores are then positioned be-tween the chill sections so the mold is readv for another pour as it again approaches the pouring station. This operation qoes Oll continuously during continuous rotation of the wheel.
r~ith casting apparatus of the type described, it is de-sirable to rotate the wheel at as high a speed as possible in order to achieve maximum production. The slower the wheel ro-tates, the lonqer it takes to pour a complete heat of metal.
Taking a long time to pour a heat of metal results in a large vari-ation in the metal temperature over the length of the pour, and this results in a high number of x-rav defects and skulling of ladles. Taking a long time to pour a heat of metal also wastes energv required -to superheat the metal over the long period of time. Taking a long time to pour a complete heat of metal also results in shorter life of the chill sections because the chill sections are eroded bv superheated metal. r~ith a relatively .slow rotational speed of the wheel and limited production rlt/

~L~L86~8 output~ -the time for furnace meltdown of an additional heat of metal is prolonged.
It would be desirable to alleviate these undesirable characteris-tics of current apparatus of the -type described by increasing the production output of the wheel.
According to the present invention there is provided a method of continuous casting, the method including the steps of providing a wheel rotatable about a vertical axis and a plurality of separable molds with outer chills positioned at fixed intervals around the periphery of the wheel, and inserting sand cores having article forming cavities and sprue openings therein, into separated molds and thereafter closing the molds around the sand cores. The wheel is continuously rotated at a constant speed for moving the molds in a Eixed circle, and molten steel is continuously poured from a ladle through a spout to successively fill the molds with liquid steel at a fixed point on the circle during the step of rotating while maintaining the mold temperature greater than 250F. Each mold is continuously cooled and the steel cast therein is solidified by continuously spraying water against the molds during travel of each mold over a cooling arc of at least 160 of the circle downstream of the fixed poin~ after the molds have passed the spout while maintaining a mold temperature of at least 275F., during the step of rotating. The molds are continuously opened and the molds are shaken out to remove the solidified metal and sand therefrom along a shake-out arc of the circle downstream of the cooling arc while maintaining the molds at a temperature greater than 250F. during the step of rotating. Water is ~;
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sprayed and air is blown continuously on the molds to clean -the molds over a cleaning arc downstream of ~he shake-out arc while maintaining the mold temperature greater than 250 F. during the step of rotating. New sand cores which have article forming cavities and sprue openings therein are inserted into the molds, and the molds are closed and the molds are moved beneath the pouring spout and along a closing arc of the circle downstream of the cleaning arc while maintaining the temperature of the molds above 250F.
during the step of rotating. For all of the steps, the lengths of the arcs are provided so that the length of the cooling arc is at a maximum and the length of the remaining arcs are at a minimum to operatively perform the steps therein while the step of rotating rotates the wheel at the maximum speed permissible to obtain a satisfactory article, thus variations of the metal temperatures are minimized within the ladle and the spout that would produce defects, and minimize energy required to heat the molten steel.
Instead of continuously spraying the chill sections after discharging the sand cores and cast metals therefrom for cooling the chill sections to substantially ambient temperature, it has been found highly advantageous to simply spray the chills briefly for cleaning same while allowing the temperature to remain above the boiling point of water.
In the previous arrangement, there was sometimes moisture remaining in the chill sections when a new pour took place and this crea~es an explosive hazard. In addition, the thermal shock is very high when the molten metal is poured - 3a -sb/,~1 into chill sections which are at approximately ambient temperature. It has been found that maintaining the chill sections at a temperature above the boiling point of water completely vaporizes any moisture therein before the next pour.
In addition, maintaining the chill sections at an elevated temperature reduces the amount of thermal shock to which the chill sections are subjected when a pour is made.
Increasing the producting and rotational speed of the wheel - 3b -i,~,~ i sb/;'

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resul-ts in improved quality because the pourinq temperature of the metal is more even over an entire heat because the entire heat is poured in a much shorter period of time. The improved ar~angement also improves chill life because superhea-ted metal is eliminated and the cast metal is cold to simplify discharae and shake-out of same from the molds. The improved arrangement also provides signiflcant cost reductions because there is a area-t increase in production along withthe use of less power for melting and holding the heat of metal. There is also im-proved refrac-tory life because the metal is colder and is in contact with the refractory for a shorter period of time. The added coolinq section also allows production of substantially larger castings.
I-t is a principal object of the present invention to pro-vide an improved method for casting grinding ballsA
It is also an object of the invention to provide such a method which makes it possible to substantially increase the production capacity of known apparatus.
Brief Description of the Drawings Figure 1 is a schematic plan view of the prior art cast-ing apparatus; and Fiaure 2 is a schematic plan view of the apparatus in Fiaure 1, and with the improvements of the present application incorporated therein.
Description of a Preferred Embodiment Fiqure 1 shows a prior art arrangement wherein a large w;~eel ~ rotates counterclockwise as indicated by arrow 12 about a generally vertical axis 14. A plurality of individual molds 18 are mounted on wheel A adjacent the outer circumference thereof. Molds 18 are mounted in close side-by-side rlt/Cl~ 4 relationship around -the outer periphery of wheel A. The outer metal chill portions of each mold 18 are spaced from one another generally radially of axis 14, and at least one of such chill sections ls movable qenerally radially of axis 14 toward and away from the other chill sections. This allows discharge of solidified grinding balls and the inner sand core, and also al-lows positioning of new sand cores between the outer chill sec-tions.
An elevated platform B surrounds wheel A for providing access to the molds bv ~orkmen. Stairs generallv indicated at 22,24 provide access to platform B from the floor surface. An enlared portion 26 of platform B defines a pouring platform where-at a ladle 28 is supported for holding molten metal to be used in filling molds 18. ~ne workman 30 on pouring platform 26 oper-ates ladle 28 for Eeeding molten metal therefrom to a pouring box and pourinq spout to fill molds 13. Another ~,~arkman generallv indicated at 32 occupies pouring platform 26 and chllls the metal in -the pouring box to a desireable temperature so that the molds will not be damaged bv liauid metal at too high a temperature.
llheel A rotates continuously at a substantially constant velocity during filling of the moldS with molten metal and during recvclinq of the molds.
The gap between adjacent molds at the upwardly opening troughs in the sand cores thereoE is closed by mud so that there is a continuous upwardlv opening trough during pouring of metal.
Thus, a continuous string of metal normallv extends from one mold to another, and a string cutter generally indicated at 3~ is provided for cutting this string of metal between adjacent molds.

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Shortlv after pouring of the molds and operation of st~ing cutter 34, a water sprav is initia-ted as generally in-dicated at 38 and continues over an angle slightly less than nine-ty degrees to end at a point generally indicated at 40. The molds are continuouslv sprayed with cooling water between points 38,40 to solidify the metal in the molds. Immediately after termination of the cooling water sprav indicated generally at 40, the outer metal chill sections of each mold are separated from one another to discharge the inner sand cores and solidified balls.
This occurs at a shake-out station generallv indicated at 42 where -the molds are shaken and stripped. A shake-out man generallv indicated at 44 operate.s the shake-out station. Immediatelv after the shake-out station, a water sray is started generally indicated at 46 and continues over to a point generallv indlcated at 48. The water spray taking place continuously between points 46,48 is directed against the internal surfaces of the metal chill sections for cleaning same. In this prior art arrangement, the water sprav between points 46,48 cooled the outer metal chill sections substantially to ambient temperature. Subsequent to ~n stopping of the cooling water sprav at point 48, the outer metal chill sections were simply allowed to air drv up to point 50 where-at a blast of air was directed against the internal surfacesof the chill sections for further drying and cleaning of same. A work-man mechanic generally indicated at 52 occupies platform B for making any adjustments or repairs which may be required.
Subsequent to deliverv of the air blast at point 50, new internal sand cores are supplied to a workman core setter generallv indicated at 56 bY sand core feed belts 58, 60. ~ork-man 56 places new sand cores between the separated metal chill sections which are -then closed to complete a new mold rlt/ ~ - 6 -6~
iemblv. Shortly after -the core setting station, a workman known as a mudder ~enerally indicated at 64 places mud in the gap between adjacent upwardly openina troughs so this trough will be continuous as metal is poured and this prevents metal from ~lowing down ~etween any gap between adjacent molds.
In -this previous arrangement described with respect to Fiaure 1, it has been found that some of the metal chill sections may ~ot be completely dry when they again reach the pouring station.
The presence of moisture in the cavities of the metal chill sec-tions creates an explosive hazard and can also result in damage to the mold. In addition, pourin~ of the molten metal into molds havina chill sections at substantially ambient temperature pro-duces a significant thermal shock which tends to crack and warp the metal chill sections.
With the cooling water spray takina place between points 38,~0 over an angle slightly less than ninety deqrees, wheel A
can rotate at a certain maximum velocity which will still permit solidification of the metal as each mo]d travels through an arc slightlv less than ninety degrees. This limitation on the max-imum velocity of wheel A limits the production capacity of the apparatus. As previously explained, this means that it requires a significant length of time to pour a complete heat of metal from ladle 28 and this makes it difficult to maintain an optimum temperature for the molten metal. In addition, sianificant energy is required for maintaining the metal in the ladle at a high temperature over a significant period of time.
Figure 2 shows the grinding ball casting arrangement with the improved features of the present application.
Characteristics and features of the Figure 2 arrangement which rlt/~ ~ 7 ~

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are the same as -those of Figure 1 are given the same reference numerals. In the improved arrangement of Figure 2, the end of -the cooling water spray is located at point 40a which is slightly less _han one hundred eighty degrees from the starting point 38 of the cooling water spray. Thus, point 40a where the cooling water spray is stopped in Figure 2 is displaced approximately eighty degrees in the di.rection of wheel rotation from point 40 of Fi~ure 1. This means that each mold 18 is continuoulsy sprayed with cooling water as it travels over an arc of approximately one hundred seventy degrees between points 38 and 40a. This makes it possible to substantially increase the rotational speed of wheel A because cooling each mold over a greaterarc of wheel rota-tion subjects each mold to cooling water spray over the same period of time as in the arrangement of Figure 1. The production capacity of the apparatus is greatly increased and it is also possible to cast qrinding balls of substantially largerdiameters.
Shake-out station 42a in Figure 2 is also displaced approximately ninety degrees from shake-out station 42 of Figure l. Obviously, shake-out man 44a also occupies a new position.
It was previously believed that it would not be possible to gre.~tly increase the arc over which the molds were subjected to cooling water spray before stripping thereof because this would leave insufficient arc for providing cleaning and cooling water sprav to the stripped chill sections as between points 46,48 oE
Figure 1. ~owever, it has surprisingly been found that it is unnecessary and undesireable to direct a cleaning and cooling spray of wa-ter against the inner surfaces of the chill sections over a significant arc. Thus, in the arrangement of Figure 2, a spray wash is provided at 70 for spraying rlt/r~

.~ter against the 1nner surfaces of the outer metal chill sections to clean same and somewhat reduce the temperature ther_of. The water spray at point 70 is insufficient to reduce the remperature of the chill sections below the boiling point of water. In fact, the temperature of the metal chill sections remains substantially above the boiling point of water. Thus, as the wheel continues to rotate, any water left in the cavities of the chill sections will be evaporated. The air blast directed against the inner surfaces of the chill sections at point 50 further helps to clean those surfaces. The temperature of the metal chill sections is maintained at approximately at least 275F. as they are recycled for another pour. Thus, when a reassembled mold is again poured at pouring platform 26, the metal chill sections are still at a temperature substantially above the boiling point of water. This elevated temperature in the metal chill sections substantially reduces stress fractures therein when the super heated metal contacts the chill sections.
In the improved arrangement of the present application the metal chill sections of the mold are always maintained at a temperature of at least 250F. during recycling thereof to insure evaporation of all of the water from the metal chill sections before the ~.old is again filled with molten metal. In the arrangement of the application, the molds also travel over an arc of at least appxoximately one hundred sixty degrees during continuous cooling thereof by water sprayed before the molds are opened for stripping the solidified balls and sand core, therefrom. The arrangement of the present applica-tion provides for continuous recycling of the molds for successive pours of metals in each mold while maintaining the metal chill cw/ - 9 ;`,: .

~36~2~3 sections above the boiling point of water to insure that the sections are completely dry. Filling the molds when the metal chill sections are at an elevated temperature also reduces the thermal shock to these sections when contacted by the high temperature molten metal.
A1-though the invention has been shown and described with respect to a preferred embodiment, it is obvious that equiva~
lent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specifica-tion. The present invention includes all such equivalent al-terations and modifications, and is limited only by the scope of the claims.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of continuous casting, comprising the steps of, in order:
(1) providing a wheel rotatable about a vertical axis and a plurality of separable molds with outer chills positioned at fixed intervals around the periphery of said wheel;
(2) inserting sand cores having article forming cavities and sprue openings therein, into separated molds and thereafter closing the molds around the sand cores;
(3) rotating continuously said wheel at a constant speed for moving said molds in a fixed circle;
(4) continuously pouring molten steel from a ladle through a spout to successively fill said molds with liquid steel at a fixed point on said circle during said step of rotating while maintaining the mold temperature greater than 250°F.;
(5) continuously cooling each mold and solidifying the steel cast therein by continuously spraying water against said molds during travel of each mold over a cooling arc of at least 160° of said circle downstream of said fixed point after said molds have passed said spout while maintaining a mold temperature of at least 275°F., during said step of rotating;
(6) continuously opening said molds and shaking said molds out to remove the solidified metal and sand therefrom along a shake-out arc of the circle downstream of said cooling arc while maintaining the molds at a temperature greater than 250°F., during said step of rotating;

(7) continuously spraying water and blowing air on said molds to clean said molds over a cleaning arc downstream of said shake-out arc while maintaining the mold temperature greater than 250°F., during said step of rotating;
(8) inserting new sand cores having article forming cavities and sprue openings therein into said molds;
(9) closing said molds and moving said molds beneath the pouring spout and along a closing arc of said circle downstream of said cleaning arc while maintaining the temperature of said molds above 250°F. during said step of rotating; and (10) for all of said steps, providing the lengths of said arcs so that the length of said cooling arc is at a maximum and the length of the remaining arcs are at a minimum to operatively perform the steps therein while said step of rotating rotates said wheel at the maximum speed permissible to obtain a satisfactory article, to thereby minimize variations of metal temperature within the ladle and spout that would produce defects, and minimize energy required to heat the molten steel.

2. A method of continuously casting according to claim 1, further including conducting said step of cooling and solidifying over a cooling arc of at least 170° of said circle.

3. A method of continuously casting according to claim 2, wherein said step of pouring forms a connecting string of steel between adjacent molds; said method casts steel grinding balls as said articles and further includes the step of cutting the steel string between the adjacent molds immediately downstream of said pouring fixed point and immediately before said cooling arc.