Centrifugal Casting Of A Composite Roller

Abstract

A highly viscous flux at high temperature added to a mass of outer layer-forming metal during centrifugal casting to produce and maintain a flux coat of the outer layer subsequent to its solidification and prior to adding the molten mass of inner layer-forming metal with the mold at rest.

Description

The present invention relates to improvements in centrifugal casting processes for casting two or more metal alloys into a unitary body, for instance, a composite roller. More particularly, the invention relates to a centrifugal casting process which uses a special flux composition to prevent the formation of an oxide film on the boundary between the separately cast alloys.

A primary object of this invention is, therefore, to eliminate the casting defects, such as insufficiency in the melt-bond which exists between the alloy layers and to eliminate thickness variation of one or more of the cast alloy layers due to one-sided pervading of a metal alloy, both of these problems being common in the centrifugal casting of multiple layer metal bodies employing conventional centrifugal casting methods and fluxes.

In cases where cast articles are required to have mechanical properties which differ for the various metals forming the cast article, such as between the outside and inside layers, it is common to cast two or more different metal alloys in a single cast which is laminar in form having multiple layers, since a casting of only one metal alloy normally cannot satisfy the requirement for the cast article.

A composite roller for metal-rolling machines normally comprises an inner layer made of ductile cast iron of high toughness known as high-duty cast iron and an outer layer formed of a chilled alloy cast iron characterized by high hardness, Adamite, alloy grain, etc. Such rollers have been manufactured by casting processes involving the steps of forming the outer layer by centrifugal casting techniques and forming the inner layer by pouring a molten mass of tough metal inside the outer layer after the mold and the layer are positioned vertically erect. In order to prevent the formation of an oxide film on the inside surface of the outer layer during the solidification of that layer, it has been conventional to cover this inner surface with a coating of flux since the presence of the oxide film causes the melt-bond between the outer layer and the inner layer to be extremely poor. The flux coating has been achieved by either sprinkling a flux, such as soda ash, lime, etc., on the inner surface of the outer layer after solidification while maintaining the mold in rotation, such teaching being present in Japanese Pat. No. 202,616, or by adding the flux to a molten mass of metal which forms the outer layer when pouring the same into the centrifugal mold, as taught in Japanese Pat. application No. 3,906,065.

Since the flux materials employed in accordance with these methods, namely, soda ash (Na.sub.2CO.sub.3), calcium fluoride (CaF.sub.2), common salt (NaC1) and limestone (CaCO.sub.3) have a certain melting point and exhibit a low viscosity in the molten state, any one of the foregoing fluxes flows down the inner surface of the outer metal layer after rotation of the metallic mold ceases, and with the mold horizontally oriented, the flux forms a molten pool F, as seen in FIG. 2 of the drawing, this pool keeping a portion of the outer metal layer in contact therewith at a relatively high temperature. If the inside of the outer metal layer is maintained at a temperature which is just below its melting temperature, upon erection of the mold for subsequent pouring of the inner metal layer, the molten pool F flows down along the inner wall of the outer layer, as best seen in FIG. 3. If the molten metal forming the inner layer is then poured into the outer layer which due to the flux concentration is maintained at this relatively high temperature and condition, the outer layer O is then thermally injured at that surface portion which is in contact with the molten pool F of flux. The result is the production of a cast article having a variation in layer thickness of the outer layer, as shown in FIG. 4. On the contrary, if the inside surface of the outer layer is kept at a temperature lower than the solidification point of the flux, the flux solidifies on the internal surface of the outer layer to the point where it becomes difficult for the flux to rise to the surface of the molten metal which is then poured inside the outer layer to form the inner metal layer of the laminate structure. This results in the formation of a poor melt-bond between the outer and inner metal layers of the composite casting. There is tendency in the prior art casting processes to cause these deficiencies to be especially present at the ends of cast articles which are cylindrical in form and it has been very difficult to obtain roller blanks free of such defects.

According to the present invention, such casting defects are eliminated by forming a coating film of flux on the internal surface of the outer layer and pouring an inner layer forming metal onto the coating film.

It is a primary object of this invention, therefore, to provide an improved casting process for the production of a cast article of metal laminar construction having neither variation in layer thickness nor insufficiency of melt-bond between the outer and inner metal layers through the use of a flux composition that exhibits proper viscosity sufficient to resist the tendency of flux coat sagging even at elevated temperatures.

Another object of this invention is to provide such a casting process which may be easily practiced, and in which the step of forming the inner layer by casting after the centrifugal cast formation of the outer layer is achieved with relative ease due to the fact that the viscosity of the flux composition employed in accordance with the present invention suffers no material change over a wide range of working temperatures.

A still further object of this invention is to provide an economical centrifugal casting process for the production of castings which does not adversely affect the properties of the metal layers forming the casting while allowing the employment of a flux composition consisting of compounds which form conventional fluxes and in which silica and borax form the glass-forming ingredient.

Other objects of this invention will be pointed out in the following detailed description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode which has been contemplated of applying that principle.

In the drawings:

FIG. 1 is a sectional view of a hollow metallic mold carrying a centrifugal cast outer layer of a roller blank and a uniform flux coating under the method of the present invention.

FIG. 2 is a sectional view of a hollow metallic mold carrying a centrifugal cast outer layer and a flux coating in accordance with a prior art method of centrifugal casting.

FIG. 3 is a sectional view of the metallic mold and centrifugal cast product of FIG. 2 subsequent to vertical erection of the same.

FIG. 4 is a sectional view of the outer cast layer after vertical erection in the manner of FIG. 3 and prior to introduction of the inner metal layer material.

FIG. 5 is a graph showing the relationship between the temperature and viscosity of the flux composition of the present invention as contrasted to conventional flux used in prior art centrifugal casting methods.

In general, the centrifugal casting process of this invention is characterized by the employment of a flux composition comprising 50 to 90 percent by weight of a mixture composed of silica and borax and 10 to 50 percent by weight of at least one compound selected from either the group consisting of sodium compounds, such as soda ash, sodium fluoride, common salt, etc., or the group consisting of calcium compounds, such as limestone, calcium chloride, calcium fluoride, etc. The flux composition is added to a molten mass of outer layer-forming metal to be cast in a metal mold rotated on rollers having a horizontal axis, or is poured together with the foregoing molten mass in the metallic mold to produce an outer layer having an inside surface which is coated with a thin, uniform film of flux, in either case, as a result of centrifugal casting. In lieu of this, flux may be sprinkled on the inside of the outer layer which has been produced by centrifugal casting to produce a thin, uniform film of flux on the inside surface. Rotation of the metallic mold ceases after solidification of the molten mass forming the outer metal layer and the metallic mold is erected vertically after removal from the rotating means. A separate molten mass of metal which forms the inner metal layer is then poured inside the outer layer, while the coating film is maintained at a temperature ranging from 800.degree. C. to 1,300.degree. C. to produce a unitary body comprising inner and outer metal layers which are melt-bonded to each other.

Since the flux composition used in accordance with this invention is composed mainly of silica (SiO.sub.2) and borax (Na.sub.2B.sub.4O.sub.7) of a glass-forming nature, it is capable of producing, solely or together with sodium compounds or calcium compounds added thereto, a vitrified mass of the ternary system: SiO.sub.2-B.sub.2O.sub.3-Na.sub.2 O (CaO) having a melting point far lower than any of the melting points of the foregoing ingredients. In addition to this, at an elevated temperature, the vitrified mass thus produced exhibits a viscosity higher than the sodium or calcium compounds heretofore used with the degree of change in viscosity as the temperature lowers being extremely slight.

Regarding the flux composition of this invention, there is a tendency that the characteristics inherent to such amorphous substance will be unsatisfactory if the proportion of borax is lowered below 50 percent and further, the viscosity at elevated temperatures becomes too large when the proportion of borax is greater than 90 percent. The sodium and calcium compounds serve to control the fluidity of a mixture composed of silica and borax, that is, to convert it from a nonsolidified nature into solidifying nature and to lower the viscosity thereof at elevated temperatures. With proportions of these materials below 10 percent, there is no noticeable result in lowering of the viscosity and where the proportion is increased to above 50 percent, the characteristics inherent to amorphous substances which are desirable may not be present. Therefore, these ingredients should be employed in the range of proportions given to ensure that the flux composition exhibits the superior characteristics over those of conventional fluxes.

Referring to the graph of FIG. 5, the excellent characteristics of the flux composition of the present invention may be readily seen in contrast to conventional fluxes of the prior art methods. The graph shows the relationship between the temperature and viscosity of the sodium or calcium compounds as represented by the thick line curve b, while the same relationship is shown by curve a for the flux composition of the present invention, which is composed of 50 to 90 percent by weight of a mixture of silica and borax and 10 to 50 percent by weight of soda ash. The hatched region represents the proper range of viscosity. Viscosity higher than the one falling within this range causes a poor melt-bond between the layers and the casting may be thermally injured if the viscosity is lower than that shown by the hatched region. As clearly understood from FIG. 5, the curve a covers a very wide range of temperatures, from 800.degree. C. to 1,300.degree. C., while still falling within the crosshatched region and thus providing flux of suitable viscosity. This region R.sub.1 is to be contrasted to the region R.sub.2 for curve b, the region R.sub.2 being very narrow. In other words, the rate of change in viscosity of the flux composition in accordance with this invention is very slow in terms of temperature, as compared with that of conventional flux material. Furthermore, it may be seen from FIG. 5 that the flux composition in accordance with this invention has a low solidification point and exhibits a high viscosity even at a relatively low temperature, while conventional flux has a high solidification point.

The manner in which the flux composition may be applied to the outer metal layer in accordance with the present invention may vary, as indicated by three examples. First, a molten mass of metal which forms the outer metal layer of the laminate casting including the flux composition mixed therein is poured into the metallic mold which is then rotated at high speed. Since the flux composition which is carried by the molten mass, has a specific gravity lower than that of the metal, it moves to the inside surface of the outer metal layer by means of the centrifugal force created during rotation of the metallic mold M to create a coating film of flux composition F on the outer metal layer O. A second method involves the pouring of the flux composition into the metallic mold separately but at the same time that the molten mass of metal which forms the outer layer is poured, for instance, during rotation to achieve the result identical to the first method. Thirdly, the flux composition may be sprinkled on the internal surface of the outer layer after centrifugal casting and solidification of the same.

In the case where a casting of large dimensions is to be achieved, the rotation of the metallic mold M ceases after solidification of the outer metal layer, and the mold is then erected vertically to prepare it for the pouring of a molten mass of metal which forms the inner layer. When casting relatively small sized articles, the inner layer-forming metal is poured while the metallic mold is maintained in rotation.

Since the flux composition, in accordance with the present invention, exhibits a high viscosity over a relatively large temperature range, the flux composition does not flow down to form a molten pool of flux on the lower side of the outer layer and a section of the mold at this point showing the uniformity of flux coating may be readily seen by viewing FIG. 1 in contrast to FIG. 2. This even layer of flux is maintained even if rotation of the metallic mold is stopped and further, it does not stream downwardly in the manner of FIG. 3 even if the metallic mold is erected vertically. Therefore, there is formed a coating of flux of approximately uniform thickness over the entire inner surface of the outer metal layer. Before pouring molten metal to form an inner layer in a metallic mold which carries an outer layer produced by centrifugal casting, rotation of the metallic mold may be stopped and some time may occur prior to pouring of the inner layer material. However, since there is a wide range of acceptable temperatures, from 800.degree. C. to 1,300.degree. C. to maintain the proper viscosity of the flux, there is no need to immediately pour this inner layer material.

Since the flux composition, which is capable of forming a noncrystalline or amorphous mass upon fusion, has a low solidifying point, bubbles of gas, which are generated from the outer layer-forming metal as it solidifies, are driven onto the inside surface of the outer layer from portions interior of this layer. These bubbles rise to the inside surface of the coating film of the flux composition after penetrating through the coating film and even if the bubbles of gas are broken on the inside surface of the outer layer, since the gas delivered from the broken bubbles is reenveloped in the coating film to again form bubbles within the coating film, the inside surface of the metal layer is protected against oxidation and is also kept suitably warm by a coating film of uniform thickness which covers the same.

Since borax has the tendency to form a eutectic mixture with metal oxides, it causes any iron oxide or any grains of sand which may be carried to the inside surface of the outer layer to rise to the surface of the molten mass of metal resulting in in the production of a casting free of foreign matter, such as nonmetallic inclusions, and free of casting defects, such as insufficiency in the melt-bond between the layers. The coating film of flux aids in maintaining the outer metal layer surfaces in contact therewith uniformly warm without the formation of oxide film. No flux composition is left between the outer and inner layers, that is, at their interface due to the fact that the flux composition rises to the upper surface of the molten metal pool forming the inner layer, as in the case of pouring the same into a metallic mold which is vertically erected in the manner of FIG. 3.

Alternatively, if the inner layer is centrifugally cast, this flux material will move to the inside surface of the inner layer, for instance, when the metallic mold is rotated about a horizontal axis. Hence, the inner metal layer of the cast metal laminate or composite structure closely contacts and bonds with the outer layer without the formation of any intermediate layer between the two metal layers over the entire interface between the layers. Thus, there is produced a casting without the normal casting defects, such as insufficiency in melt-bond between the metal layers or nonuniformity in wall thickness of the outer layer due to local maintenance of heat, such as that produced in the prior art method wherein the flux tends to concentrate in the manner of FIG. 2. The flux composition of the present invention is composed mainly of inexpensive material, such as silica and borax. Moreover, the silicon and boron forming the flux does not injure the casting if they should diffuse into the cast metal, this statement likewise applying to sodium and calcium. A specific example of a cast product employing the process of the present invention is as follows:

EXAMPLE

To produce a composite roll for mill use which is 695 mm. in diameter and 2,184 mm. in length, cast iron comprising 3.4 percent C, 0.8 percent Si, 0.6 percent Mn, 4.3 percent Ni, 1.7 percent Cr, 0.4 percent Mo and the balance (iron) Fe was poured at 1,380.degree. C. in a metallic mold for centrifugal casting use, the mold being rotated on rollers driven about a horizontal axis to form an outer metal layer, and simultaneously, 45 kg. of a flux composition comprising 50 percent silica, 30 percent borax and 20 percent soda ash was added to the cast iron. The metallic mold was maintained in rotation until the outer layer forming metal solidified and was then erected vertically, after which high-duty cast iron was poured at 1,390.degree. C. to form the inner layer. Of the 24 composite rolls manufactured by this process, none were found having defects involving insufficiency in melt-bond between the outer and inner layers or variations in wall thickness of the outer layer.

Control

The same procedure was repeated with the exception that soda ash was used alone as a flux, in which case, 67 composite rolls were produced, among which 39 rolls proved to have the referred-to defects.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that the foregoing and other changes in the form and details thereof, may be made therein without departing from the spirit and scope of the invention.

Claims

We claim:

1. A centrifugal casting process for manufacturing composite metal bodies comprising the steps of:

supplying a first ferrous alloy in molten form to a horizontal metallic mold;

rotating said mold to cause said first ferrous alloy to contact said mold surface and form an outer layer;

cooling said first ferrous alloy to a temperature of 800.degree. C. to 1,300.degree. C. at its inner surface;

adding enough flux composition of 10 percent to 50 percent by weight of at least one compound selected from the group consisting of sodium compounds and calcium compounds and 90 percent to 50 percent by weight of a mixture of silica and borax in which mixture the proportion of borax varies between 50 percent and 90 percent by weight and the proportion of silica varies between 50 percent and 10 percent to coat said inner surface; and

erecting said mold to a vertical position, and adding a second molten ferrous alloy to said mold to produce a unitary cast metal body having a uniform outer layer which is uniformly melt-bonded to an inner layer.

2. The process as claimed in claim 1 wherein said flux is added to said mold by sprinkling the same on the inner surface of the outer metal layer subsequent to centrifugal casting and solidification of the same.

3. The process as claimed in claim 1 wherein said flux is added simultaneously to said mold as a separate mass during the supplying to said mold of said molten mass of outer layer-forming metal.

4. The process as claimed in claim 1 wherein said flux composition is supplied to said mold as a mixture with the molten mass of outer layer-forming metal.