GB2096928A - Method of casting grey iron - Google Patents

Abstract

A method of casting grey iron by using a mold comprised at least in part of hollow fly ash spheres and a binder. The hollow fly ash spheres in the mold control the solidification rate of the metal to prevent the formation of white iron even in thin sections of the casting. The binder may be bentonite or a resin.

Description

SPECIFICATION Method of casting grey iron The present invention relates to the casting of gray iron and, more particularly, to a method of forming gray iron castings which have extremely thin sections without forming white iron in such sections.

Gray iron is a form of cast iron which contains a relatively large percentage of its carbon in the form of graphite, and substantially all the remainder of its carbon in the form of eutectoid carbide. White iron is a form of cast iron in which substantially all the carbon is in the form of iron carbide.

When attempting to cast articles consisting entirely of gray iron, problems are encountered in thin sections of the casting where the cooling rate is very high. More specifically, white iron may form in these thin sections if there is insufficient time for graphitization to take place and/or if the silicon content is at low or medium levels. This formation of white iron, within a casting intended to be gray iron, is highly undesirable because the properties of white iron differ significantly from those of gray iron. To prevent the formation of white iron, foundries tend to use higher silicon contents and higher casting temperatures than are theoretically necessary. Costly innoculants are also used to promote the formation of gray iron in the thin sections of the casting.In some cases, the casting is even made thicker than necessary, to avoid the formation of white iron, and then machined to the desired final thickness in the thin areas.

It is a primary object of the present invention to provide a method of casting gray iron which avoids the formation of white iron even in extremely thin sections of the casting.

It is another object of this invention to provide an improved method of casting gray iron which ensures the formation of gray iron even in thin sections of the casting without use of excess silicon, innoculants, or machining operations. In this connection, a related object of the invention is to provide a more economical and reliable method of casting gray iron.

Othel objects and advantages of the invention will be apparent from the following detailed description.

In accordance with the present invention, there is provided a method of casting gray iron comprising casting the molten metal in a mold comprised at least in part of hollow fly ash spheres and a binder, and allowing the molten metal to solidify in the mold whereby the hollow fly ash spheres in the mold control the solidification rate of the metal to prevent the formation of white iron in the casting.

It has been found that the use of the hollow fly ash spheres in the mold permits the thermal properties of the mold to be tailored to ensure the formation of gray iron even in extremely thin sections of a casting, e.g., in sections of 0.25 inch and less. Because of its low mass the fly ash has a minimal chilling effect on the molten metal as it first enters successive regions of the mold; and because of its low thermal conductivity the fly ash is capable of providing the high levels of thermal insulation needed to achieve cooling rates that are sufficiently slow to prevent the formation of white iron in thin casting sections. Thus, gray iron can be reliably and consistently produced in virtually any desired shape.

The fly ash particles that are used in this invention are not only spherical but also hollow and, therefore, have a low density. These spheres are a common constituent of fly ash as obtained from most sources, and are separated from the heavier solid constituents by well known and commercially practiced processes using selective flotation. For example, fly ash spheres which are commercially available under the designation "Fillite" have a specific gravity of less than 0.7 and a particle diameter in the range of 5 to 300 microns. As is well known, fly ash is composed primarily of silica and alumina with small amounts of iron oxide and alkali metal oxides, and has a high melting point, e.g., 12000 C.

If desired, conventional foundry sands and other particulate and/or fibrous materials can be mixed with the fly ash spheres, depending on the solidification rate desired in a given application.

Also, different percentages of the fly ash spheres may be used in different portions of the mold, e.g., varying with the thickness of the casting to be molded.

The binder for the fly ash particles may be bentonite, which is commonly used in green molding sand in foundries, or a resin. With a bentonite binder, the mold can be formed by the same techniques used to form conventional green sand molds.

When resin is used as the binder, it is preferably in liquid form with a viscosity of less than about 10 Stokes, as described in my copending U.S.

patent application Serial No. 032,270 filed April 23, 1979, for "Shaped Rigid Articles Containing Fly Ash and Resin." It is preferred that the liquid resin be stable at room temperature so that the fly ash and the resin can be mixed without the use of heaters, and also to permit room temperature storage of the liquid resin. The liquid resin can be formed by dissolving solid resin in a solvent, by dispersing extremly fine particles of solid resin in a liquid carrier, or by the use of a resin which is normally in a liquid state before it is set. The setting of the resin is preferably effected by the use of warm air which drives off the liquid solvent or carrier and/or raises the temperature of the resin to its setting point, or at least accelerates the setting. With certain resins, a catalyst is mixed with the resin in its liquid form to promote the setting of the resin.

To facilitate the forming of the fly ash-resin mixture into the desired mold shape, the liquid resin and the fly ash are preferably mixed in proportions that produce a doughlike mass that is still moldable but will not run. This mass is then formed into the desired mold shape by using a pattern, and finally the resin is set and dried to form the desired mold. With certain resins, such as phenol formaldehyde dissolved in alcohol, the resin-coated fly ash can be dried as a particulate material, e.g., by subjecting the fly ash to heated air to a temperature or by sustained mixing at room temperature, before it is formed into the desired shape. The coated fly ash is subsequently formed into the desired mold shape and heated to a temperature in the range of 400 to 5000F to soften and cure the resin and thereby bond the fly ash particles together.

Examples of liquid resins that are suitable for use in this invention are phenol formaldehyde resin dissolved in ethyl alcohol and mixed with hexamethylenetetramine as a catalyst (e.g., "Plenco 212" resin made by Plastic Engineering Company): solutions of alkyl base resins (e.g., "Linocure" resin made by Ashland Chemical Co. of Columbus, Ohio); solutions of phenolic urthane resins (e.g., "6100/6300" resin made by Ashland Chemical Co.); and furan resins which are normally in a liquid state at room temperature and thus do not require a solvent (e.g., "Chem Rez 290" resin made by Ashland Chemical Co.).

As can be seen from the foregoing detailed description, this invention provides a method of casting gray iron which avoids the formation of white iron even in extremly thin sections of the cast, without the use of excess silicon, innoculants, or machining operations. Thus, the invention provides a more economical and reliable method of casting gray iron.

Claims (5)

1. A method of casting gray iron comprising casting the molten metal in a mold comprised at least in part of hollow fly ash spheres and a binder, and allowing the molten metal to solidify in said mold whereby the hollow fly ash spheres in the mold control the solidification rate of the metal to prevent the formation of white iron in the casting.

2. The method of claim 1 wherein a portion of said mold forms a casting section having a thickness of less than about 0.25 inch.

3. The method of claim 1 wherein the entire mold is made of hollow fly ash spheres and a binder.

4. The method of claim 1 wherein said binder is bentonite.

5. A method of casting grey iron substantially as hereinbefore described.