US2689990A - Centrifugally cast product and method of making same - Google Patents

Description

Sept. 28, 1954 M. SAMUELS CENTRIFUGALLY CAST PRODUCT AND METHOD OF MAKING SAME Fi led March 5, 1951 F /'g. PRIOR ART As Cast (Outside Diameter) Band of Segregated Cementite Pearlite Cementite Segregated Cementite Fig. 4

As Cast (Outside Diameter) Fig, 2 PRIOR ART As Cast (Outside Diameter) Fig. 3

As Cast (Outside Diameter) 3mm MARTIN L. SAMUELS FAM Gkowa,

Patented Sept. 28, 1954 CENTRIFUGALLY CAST PRODUCT AND METHOD OF MAKING SAME Martin L. Samuels, Mount Holly, N. 1., assignor to United States Pipe and Foundry Company, a corporation of New Jersey Application March 3, 1951, Serial No. 213,745

6 Claims.

The present invention relates generally to centrifugal casting and pertains particularly to a novel method of centrifugallycasting chilled iron (white cast iron) tubular bodies, and to the improved castings obtained thereby. For the sake of simplicity the chilled iron tubular products to which the invention relates will be referred to hereinafter as rolls since they are admirably suited for use as grinding or milling rolls and are widely used in industry for that purpose.

When chilled iron rolls having wall thicknesses exceeding about one-half inch are produced by conventional centrifugal casting methods they usually do not have a homogeneous crystalline structure throughout the wall thickness but, instead, are characterized by the segregation or banding of eutectic cementite, roughly concentrically of the roll, resulting in products which, for reasons appearing hereinafter, are not commercially acceptable for some purposes. This difficulty is generally not encountered in chilled iron rolls having wall thicknesses of less than one-half inch since such rolls freeze so quickly after they are poured that the mechanism of segregation does not have any appreciable opportunity to take place. The invention, therefore, is limited to rolls whose wall thicknesses exceed one-half inch as cast.

In some cases, for example when the chilled roll is used in its true cylindrical form, the lack of uniformity due to segregation of eutectic cementite may pass unnoticed when the roll is first placed in use, and may be discovered only after it has caused the roll to wear unevenly after considerable use. However, when such rolls are corrugated prior to use, the segregated cementite, being considerably harder than the adjacent layers of metal which form the bulk of the roll, quickly manifests itself by dulling the corrugating tool, and by greatly resisting the passage of the tool, making the corrugating procedure more difiicult and time consuming.

Whether or not such rolls are to be used with smooth surfaces or in the corrugated form, it would be highly desirable to eliminate this segregation, for by doing so would permit the rolls to wear uniformly in use and prolong their effective life.

Accordingly, it is an object of the present invention to provide a centrifugally cast chilled iron roll characterized by a uniform distribution of its pearlite constituent in a matrix of cementite.

A further object is to provide chilled iron rolls of the character described above wherein the carbon constituent, calculated as total carbon, is distributed throughout the Wall thickness of the roll with greater uniformity than has been possible by prior art centrifugal casting methods.

An additional object is to provide castings having the foregoing properties and in which the primary crystals are disposed substantially perpendicular to the wall of the casting and extend throughout the thickness thereof.

A further object is to provide a centrifugal casting method capable of producing the foregoing cast products.

With the above objects in view, and others which will become apparent from the following specification, the invention will now be described in detail, reference being made to the i, accompanying drawing in which:

Fig. 1 is a full cross-Wall section, actual size, etched, of a chilled iron centrifugal casting manufactured according to the prior art;

Fig. 2 is the same section shown in Fig. 1, but at 20 diameters magnification;

Fig. 3 is a full cross-wall section, actual size, etched, of a chilled iron centrifugal casting made in accordance with the present casting method; and

Fig. 4 is the same section shown in Fig. 3, but at 20 diameters magnification.

As used herein, except Where the context indicates a broader meaning, the terms segregation, cementite segregation and expressions of similar import, will be used to denote the condition of gross, banded segregation of eutectic cementite which constitutes a serious deficiency of relatively thick walled chilled iron tubular castings produced by conventional centrifugal casting methods, as noted above.

In my experimentation directed to the elimination of cementite segregation, I have discovered that this desirable end may be achieved by the employment of mold rotation speeds considerably in excess of those conventionally employed in centrifugal casting.

That cementite segregation could be eliminated by increasing mold rotation speeds considerably above conventional centrifugal casting speeds is quite unexpected since the mechanism of segregation has been thought to have been linked to centrifugal action. statically cast chilled iron castings, for example, do not exhibit the type of segregation referred to above. Accordingly, it would be expected that the greater the centrifugal force imposed upon the molten metal during casting, the greater would be the tendency toward segregation.

One possible explanation for the segregation of cementite in chilled iron centrifugal castings may be based upon the relative movement between the molten metal and the mold during part of the freezing interval. According to that theory, after the molten cast iron which is subject to segregation has been poured into the rotating metal mold, and during the freezing interval or mushy range of the iron, primary crystals (skeletal dendrites) which are relatively poor in carbon form at the mold wall and grow substantially radially inwardly thereof. These dendrites are very weak structurally and are easily broken off the mold by the slip and flow of the molten iron relative to the mold resulting from the necessary lag of the inner regions of the molten iron in attaining the speed of the mold. Thus, these broken dendrites float freely for a brief time in the molten mass and, being more dense than the liquid metal, are concentrated at the outer surface of the melt by centrifugal force. The carbon-rich interdentritic liquid, which remains behind to form the next inner layer of metal after the dendrites have collected on the outside, is next to freeze, and, since freezing takes place rapidly, allowing only very little time for diffusion between the interdendritic liquid and the inner layers of molten metal, the composition of this layer remains substantially that of the interdendritic liquid.

The remaining innermost layer of liquid metal by this time has attained the rotational speed of the mold and therefore freezing thereof takes place without any further breaking off of the dendrites growing inwardly from the previously frozen layer. The result is that the centrifuging action outlined above may not now take place and the innermost layer of the casting is therefore substantially free from cementite segregation.

It may be said here that the several layers described immediately above donot arrange themselves, truly concentrically of the casting so that when a chilled iron roll exhibiting segregation is machined or otherwise finished, segregated cementite is normally uncovered in patches rather than continuous layers, which accounts for the undesirable properties of such rolls mentioned above.

According to the foregoing explanation, it should be possible to avoid segregation of cementite in centrifugally cast chilled iron castings by bringing substantially the entire molten metal charge up to the rotational speed of the mold before freezing commences and maintaining that kinetic equilibrium between mold and molten metal for substantially the entire time that the molten metal is freezing.

When. extremely high mold speeds are employed in accordance with the invention, this equilibrium condition is realized, the molten metal being quickly picked up and brought to the speed of the mold before freezing on the outer casting surface has progressed to any considerable depth. solidification then takes place in a quiet liquid much as in the case of a static casting, with the primary crystals growing substantially perpendicular to the wall of the casting and extending throughout the entire thickness thereof. Consequently, the high centrifugal forces operating will produce no separating effect because there will be no solid crystals floating freely in the liquid metal.

Whether or not the rationale proposed above is correct, the fact remains that I have overcome the undesirable property of cementite segregation in centrifugally cast chilled iron castings, and I have accomplished this result simply by using higher mold rotation speeds than were customarily used in the past, and maintaining such higher mold speeds throughout. substan-' tially the entire time that the molten iron is were obtained as follows.

freezing in the mold. By this means I have been able to produce centrifugally cast rolls having a uniformity of composition comparable to statically cast chilled iron products, with the additional advantage that my castings are more dense and freer from non-metallic inclusions than the corresponding statically cast product.

The mold speeds that I prefer to use in the practice of my invention are those which will impose a centrifugal force of 175 to 200 times gravity on the molten metal, measured at the outer wall surface of the tubular body being cast (1. e., the inner wall surface of the mold), although I have found that somewhat lower and considerably higher mold speeds may be used advantageously. For example, I have produced my improved chilled iron. castings free from cementite segregation at mold speeds imposing a centrifugal force of 125 times gravity on the outer wall of the cast product. As regards the upper limit of mold speeds utilizable in the invention, it may be said that any higher mold speed may be used which is consistent with safe operation of the spinning apparatus.

The conventional basic formula for determining the centrifugal force on a particle moving in a circular path is:

where F is the centrifugal force exerted by the rotating body, expressed in pounds; W is the weight of the rotating body in pounds; V is the velocity of the rotating body in linear feet per second; g isthe acceleration due to gravity, usually taken as 32.17 feet per second per second; and r is the radius of the circular path of the body, in feet.

From Formula 1, it follows that the angular velocity V1, expressed in linear feet per second, which is just sufficient to overcome gravitational force, may be calculated as follows:

2) VFW and that any new angular velocity V2 which will exert a force K times that of the mass of the body affected is related to V1 as follows:

Thus, it is a simple matter to calculate the rotational speed of a mold of any given diameter either in linear feet per minute or revolutions per minute, which would be necessary in order to produce any desired centrifugal force upon molten metal in the mold.

The following examples (Table I) will serve to illustrate the present casting method and distinguish it from casting methods of the prior art. In all the trials represented by the examples a steel mold was used having an inside diameter of 9% inches, an outside diameter of 16 /2 inches and a length of 33 /2 inches, before each casting was made the mold was provided on its inner surface with a thin refractory coating for the purpose of protecting it against direct contact with the molten metal. The mold coating had no appreciable effect on the cooling rate of the molten iron cast therein. In all of the e2 amples' the quantity of metal poured was the same, viz., 250 pounds, which yielded castings having wall thicknesses of approximately 1 inch. The mold in each case was disposed horizontally and rotated about its longitudinal axis.

Data on carbon distribution, in. the examples One ring one inch in points of carbon for a percentage segregation of 13.1 and 14.4, respectively (Examples 1 and 2). On the other hand, the spread in carbon content occurring in my improved chilled iron castings is less than pointsspecifically, 10 and 11 points in Examples 3 and 4, for percentage segregations respectively of 2.8 and 3.1. As used herein, a point of carbon equals 0.01% of carbon, based on the total composition of the cast iron.

The superiority of my chilled iron castings as determined by the chemical analyses given above is borne out by a comparison of the crystalline TableI EXAMPLES 1 AND 2 (PRIOR ART) Rota Cross Wall Carbon Analyses-Dis- Pouring Pouring gg tional Full tance of g gggg From Percent Example HeatNo. Temp., Time 3 Speed Wall Spread Segrega- F. Seconds (Times 0 tion (Apprx) Gravity) 3 I/ A 5A 7A 154 li l EXAMPLES 3 AND 4 The compositions of cast iron used in the structure of the prior art castings (Figs. 1 and 2) above examples were as follows: with the structure characteristic of castings Table H made by the present casting method (Figs. 3

and 4). Example 0 Mn P S or Both of these types of castings, as well as conventional statically cast chill rolls of ordinarily 1 M5 .025 016 employed compositions, consist of two major 2.. 3.53 53 .025 .0 19 33 71 microstructural constituents, (a) dendrites or i m5 primary crystals which originate first as the temperaturs drops below the liquidus, and (b) a eutectic which is composed, when first formed, of iron carbide (cementite) and austenite. The primary or tree-like crystals increase in size and number as the temperature falls to the solidus, or end of freezing, and transform upon cooling through the critical range into pearlite with the rejection of some free carbide. These dendritesare also designated as the pro-eutectic phase,

0 'Mn P S Si Or V 3. 30-4. 00% 0. 20 1. 25% 0. Max. 0. 20% Max.

mold temperature and pouring time shown in.

Examples 3 and 4 are only illustrative, and that these values may be varied to suit the conditions of a particular casting operation. In general, however, I prefer to use a pouring temperature for the cast iron of at least 2400 F., a mold temperature at pour of at least 200 F., and a pouring rate of at least 8 pounds of molten cast iron per second.

It will be immediatelyapparent from examination of Table I that the chilled iron tubular castings made according to the present invention are superior in several ways to those made by conventional centrifugal casting methods. For example, considering uniformity of carbon distribution (calculated as total carbon) throughout the wall thickness of the casting, prior art castings showed a spread of 46 and 51 and are the dense black areas easily visible in Figs. 2 and 4. The dendritic areas contain theoretically about 1.40% carbon and, after slow cooling, are comparatively soft and ductile-being similar to a high carbon steel rather than chilled cast iron. The eutectic material or ledeburite consists, when first formed at the solidus, of free carbide and austenite. A portion of the eutectic austenite joins on to the primary dendrites and another portion separates as islands within the cementite, both portions transforming into pearlite upon cooling through the critiron carbide which bands are separated by a band very high in iron carbide (eutectic cementite) the latter being non-columnar in structure and therefore destroying the continuity of the columnar primary crystal structure of the dendritic bands across the Wall of the casting. The :ands exhibiting low and high carbon content are not always exactly concentric with the outside circumference of the roll. The finishing machine cut, therefore, can be in a low carbon or soft zone in one place and in a high carbide band in another, so irregularities exist as regards surface luster, hardness (and therefore wear resistance) and machinability where fine longitudinal grooves or corrugations have to be cut in the roll. In contrast, as shown in Figs. 3 and 4, chilled iron centrifugal castings made by the present casting method are characterized by a remarkably uniform distribution of the primary dendrites (pearlite) in the matrix of eutectic carbide material, throughout all of the dimensions of the castings. This structural uniformity is reflected by a very regular surface luster, uniform hardness (and therefore uniform wearing qualities), and much easier machinability. A further characteristic of the present castings is that the primary dendrites, i. e. primary crystals, when viewed macrostructurally, are disposed radially of the casting, this macrostructure extending throughout the entire wall thickness thereof, producing the columnar structure visible in Fig. 3.

What I desire to claim is:

l. The method of casting a white cast iron tubular body comprising rotating a cylindrical metal mold supported substantially horizontally for rotation about its longitudinal axis, at a speed which will impose a centrifugal force measured. at the inner wall surface of said mold of at least 125 times gravity on molten iron introduced into said mold, introducing into said rotating mold a charge of molten cast iron at such a rate that substantially said entire charge is introduced into said mold before any appreciable freezing of charge takes place, the charge of molten cast iron being of a quantity suificient to yield a tubular casting having a wall thickness of at least one-half inch as cast and of a composition which will freeze in said mold entirely as white iron, and maintaining said rotational speed of the mold throughout substantially the entire time that said molten iron is freezing therein.

2. The method according to claim. 1 wherein said mold is rotated at a speed to impose a centrifugal force of between 175 and 290 times gravity on said molten iron measured at the inner wall surface of said mold.

3. The method according to claim 2 where-in the composition of said cast iron falls within the following specification:

C BBQ-4.00% Si (HO-1.00% Mn 0.20-1.25% P 0.50% max. S 0.20% max. Cr 3.00%

l. The method of casting a white cast iron tubular body comprising introducing into a rotating cylindrical metal mold supported substantially horizontally for rotation about its longitudinal axis a charge of molten cast iron suincient to yield a casting having a wall thickness of at least one-half inch, the temperature of said mold being greater than 200 F., the pouring temperature of said cast iron being at least 2400 F.,. the pouring rate of said cast iron being such that substantially said entire charge is intro-- duced irrto said mold before any appreciable freezing of said charge takes place, and the composition of said cast iron falling within the following specification:

C 330-40070 Si (MO-1.00% Mn 0.201.25% P 0.50% max. S 0.20% Cr 0-3.00%

the rotational speed of said mold being such as to impose a centrifugal force of between and 200 times gravity on said molten iron, which force is measured at the inner wall surface of said mold, and maintaining said rotational speed throughout substantially the entire time that said molten iron is freezing.

5. A centrifugally cast white cast iron tubular product containing about 3.31) to 4.00% carbon and having a wall thickness of at least one-half inch as cast, wherein, as cast, the pearlite component is uniformly distributed in a matrix of cementite throughout the dimensions of product, the variation in carbon contei t across the wall thickness of the product, calculated as total carbon, is no greater than about 15 points, and the primary crystals of said product, viewed macrostructurally, are disposed substantially ra dially thereof, said macrostructure extending throughout the entire wall thickness of said product.

6. A white cast iron tubular product centrifugally cast from iron of a composition falling within the following specification:

C 3.30-4.007 Si 0.10--1.00% Mn 0.20-1.25% P 0.50% max. S 0.20% max, Cr 03.00%

having a wall thickness of at least one-half inch cast, wherein, as cast, the pearlite component is uniformly distributed in a matrix of cementite throughout the dimensions of the product, the variation in carbon content across the wall thickness of the product, calculated as total carbon, is no greater than about 15 points, and the primary crystals of said product, viewed niacrostructurally, are disposed substantially radially thereof, said macrostructure extending throughout the entire wall thickness of said product.

References i'Jited in the file of this patent UNITED STATES PATENTS dumber Name Date 1,277,543 Carney Sept. 3, 1913 1,707,117 Foster Mar. 26, 1929 1,882,516 Naugle et a1. Oct. 11, 1932 2,420,298 Breakefield et al. May 13, 1947

Claims (1)

1. THE METHOD OF CASTING A WHITE CAST IRON TUBULAR BODY COMPRISING ROTATING A CYLINDRICAL METAL MOLD SUPPORTED SUBSTANTIALLY HORIZONTALLY FOR ROTATION ABOUT ITS LONGITUDINAL AXIS, AT A SPEED WHICH WILL IMPOSE A CENTRIFUGAL FORCE MEASURED AT THE INNER WALL SURFACE OF SAID MOLD OF AT LEAST 125 TIMES GRAVITY ON MOLTEN IRON INTRODUCED INTO SAID MOLD, INTRODUCING INTO SAID ROTATING MOLD A CHARGE OF MOLTEN CAST IRON AT SUCH A RATE THAT SUBSTANTIALLY SAID ENTIRE CHARGE IS INTRODUCED INTO SAID MOLD BEFORE ANY APPRECIABLE FREEZING OF SAID CHARGE TAKES PLACE, THE CHARGE OF MOLTEN CAST IRON BEING OF A QUANTITY SUFFICIENT TO YIELD A TUBULAR CASTING HAVING A WALL THICKNESS OF AT LEAST ONE-HALF INCH AS CAST AND OF A COMPOSITION WHICH WILL FREEZE IN SAID MOLD ENTIRELY AS WHITE IRON, AND MAINTAINING SAID ROTATIONAL SPEED OF THE MOLD THROUGHOUT SUBSTANTIALLY THE ENTIRE TIME THAT SAID MOLTEN IRON IS FREEZING THEREIN.

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