US2242239A - Method of centrifugal casting - Google Patents

Description

Patented May 20, 1941 UNITED. STATES PATENT OFFICE METHOD or CENTRIFUGAL CASTING Walter G. Donald, Cambridge, and James'L.

Martin, Lowell, Mass.

No Drawing. Application December 19 Serial No. 246,646

5 Claims.

The invention described herein may be manufactured and used by or for the Government for (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0.6. 757) governmental purposes, without the payment to side diameter or equivalent wall thickness which class will hereinafter be referred to generally as thick-walled cylinders or castings.

Heretofore, where the need has arisen for thick walled cylinders or castings capable of at once withstanding high pressures and offering ,a wear and corrosion resistant interior surface, 'it has been necessary to either compromise on a single material not complying with the full requirements of the need, or to use concentrically bonded materials wherein trouble has been'experienced in obtaining a satisfactory bond.

This necessity has existed because of the dimculty in avoiding segregation during freezing of the large volume of metal used in thick-walled cylinders, or similar castings which causes the actual composition of the metal to widely vary in different. portions of the casting. If the molten metal or heat is poured into an ordinary stationary chill mold of a size to give a thickwalled casting of any appreciable length, both longitudinal and transverse segregation results which is either exceedingly difficult or impossible to control within economically feasible limits,

this being especially true of longitudinal segregation. On the other hand longitudinal segregation is avoided in the usual centrifugal cast iron castings as made in the art at present, but such castings are so relatively thin-walled that little if any gradient of composition of any type results in the casting, consequently the fabrication of a thick-walled cylindrical or similar member of two or more such castings concentrically or contiguously arranged does not result in a structure having a uniformgradient from the outer surface of the casting to the bore or inner side thereof.

It is an object of this invention to set forth a method for casting thick-walled cast iron cylindrical or similar castings which incorporates steps heretofore employed for chilling the surfaces of similar castings of short length; such as railway car wheels and mill rolls, with present known centrifugal casting methods, in order to eliminate longitudinal segregation, and thereby render available an. economical method of producing castings of this class in substantial lengths.

It is a further object of this invention to co trol the radial segregation of carbon in thickwalled centrifugal castings by controlling the amount of carbon in the composition of metal being cast. to effect a predetermined gradient from white iron to gray iron.

While the essential objects and steps of this invention have been outlined above the same consists particularly in the steps to be hereinafter set forth, it being understood however, that the description is for the purpose of illustration and not limitation.

If a thick-walled cast iron cylinder or similar casting be cast centrifugally in a horizontally disposed cylindrical or other appropriate chill mold spun about its longitudinal axis under conditions insuring a continuous uniform flow of molten metal throughout the full length of the mold, longitudinal segregation will be eliminated due to the radial action of centrifugal force upon the flowing metal. At the same time, however, due to the solidification process and/or centrifugal force radial segregation of the carbon and other elements or impurities will be effected, and it is the control of this radial segregation of carbon, through a control of the total carbon content of the composition being cast that we have provided for regulating the gradient from white iron to gray iron in thick-walled cylindrical or similar castings.

We have found that since segregation is not due to centrifugal force alone; but, is also effected by the solidification process through which the molten metal passes in freezing, that there are several factors in any given set of conditions affecting the solidification-process that must be maintained constant in order to obtain a desired gradient from white iron to gray iron solely through control of the total carbon in the" cornpractically all gray iron with approximately 4.0% I

carbon may be cast in accordance with a predetermined specification through control of the carbon contentof the molten metal to be cast. BY increasing the carbon in steps above 2.5%, the outer white iron ring or zone becomes less in depth and the inner gray iron ring or zone becomes correspondingly greater in depth, owing to the fact that the carbon is physically displaced inwardly by the process of solidification and centrifugal force.

The method of this invention avoids or eliminates the longitudinal or top segregation characteristic of castings made in stationary chill molds, provides castings having the desirable density characteristic of centrifugal castingaand teaches a method of coordinating the radial segregating action of centrifugal force and the solidification process for a more accurate control of the radial carbon gradient than has hitherto" been possible.

As examples of the control that may be attained of the depth of the white iron ring or zone, through variation in the total carbon content of the melt alone, when the remaining factors above enumerated are kept constant the following is given for a composition which except for carbon is otherwise constant.

It will be noted that the depth of the white iron ring or zone is changed through a control of total carbon alone and that it is not necessary to otherwise relatively change the proportions of the silicon and sulphur with respect to the carbon to regulate this ring or zone.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

We claim:

1. In the method of centrifugally chill casting thick-walled cast iron castings of the class having an outside diameter of four to six times the inside diameter and of appreciable length and uniform composition except as to carbon and predetermining the depth of white iron zone formed 2. In the method of centrifugally chill casting thick-walled cast iron castings of the class having an outside diameter of 4 to 6 times the inside diameter and of uniform composition except as to carbon and predetermining the depth of white 1 ,iron zone formed therein through regulation of the carbon content of the iron to be cast alone, the steps which comprise selecting a rotating chill mold having its cross-sectional area proportioned with respect to the cross-sectional area of the casting in a chill ratio adapted to dissipate heat therefrom at a rate effective to control radial segregation of carbon in a casting of the class recited varying from all white iron with 2.5% carbon to substantiallyall gray iron with 4.0% carbon, correlating the carbon content of an iron with the depth of whiteiron zone predetermined for the casting and the chill ratio between the limits of 2.5% and 4.0%, introducing said iron in the mold, and maintaining rotation of the mold until solidification-of the casting.

3. In the method of forming cast iron cylindrical tubular articles of the class having an outside diameter of 4 to 6 times the inside diameter, the steps which comprise providing a rotatable chill mold having its cross-sectional area proportioned with respect to the'cross sectional area of a tubular article of said diameters in a ratio between the limits of 65:1 and 1:1, selecting a cast iron, solely regulating the carbon-content of said iron to a point between the limits of 2.5% to 4.0% in accordance with said ratio and a predetermined depth of White iron zone to be formed in the article between the limits of 0% and 100%, rotating the mold, and pouring said iron in molten form into the mold at a constant temperature.

4. In the method of forming cast iron cylindrical tubular articles of the class having an outside diameter of 4 to 6 times the inside diameter, the steps which comprise providing a rotatable chill mold having its cross-sectional area proportioned with respect to the cross-sectional area of a tubular article of said diameters in a ratio of 6.5:1, selecting a cast iron, solely regulating the carbon content of said iron to a point between the limits of 2.5% to 4.0% in accordance with said ratio and a predetermined depth of white iron zone to be formed in the article between the limits of 0% and 100%, rotating the mold, and

thereinthrough regulation of the carbon content of the iron to be cast, the steps which comprise employinga horizontally disposed chillmold rotating on its longitudinal axis and having a crosssectional area with respect to the cross-sectional area of the casting proportioned in the ratio of 6.5:1 to dissipate heat therefrom at a rate effective to control radial segregationof carbon in a casting of the class recited varying from all white iron with 2.5% carbon to substantially all gray a constant temperature.

- content of theiron to a point between the limits of 2.70% to 3.65% in correlation with a predetermined depth of white iron zone to be formed in the article between the limits of 2.0 inches and 1.55 inches and sell ratio of 6.5:1, rotating the mold, and pouring said iron in molten form into the mold at a temperature of 2950 F.

WALTER G. DONALD.

JAMES L. MARTIN.