Grey Iron Investment Casting Bearing Housing

Custom grey iron bearing housing by investment casting and CNC machining. 

Gray iron, or grey cast iron, is a type of cast iron that has a graphite microstructure. Thanks to its ease of machining, high compressive strength, better thermal conductivity and excellent vibration absorption, the gray cast iron is one of the most widely used casting metal for a very long time, even in current industry production. Carbon content is by far the most important element for gray iron (usually 2% to 4%) to create tensile strength and machinability. When molten cast iron solidifies some of the carbon precipitates as graphite, forming tiny, irregular flakes within the crystal structure of the metal which enhances the desirable properties of cast iron, the flakes disrupt the crystal structure leading to cast iron's characteristic brittleness.

Cast iron, mainly including the gray cast iron and ductile (nodular) cast iron are mainly used for casting by processes of sand casting, shell moulding casting, coated sand casting or lost foam casting. However, for some special situation, the lost wax investment casting process is also used due to their fine surface and higher accuracy. At RMC, we also have tha ability of casting gray iron and ductile with investment casting using silica sol and water glass for shell building. 

When cast iron is slowly cooled, the cementite decomposes into iron and carbon in the form of graphite which is called graphitization. Cast irons where a large percentage of cementite is decomposed by graphitization are called grey cast irons. Cast iron in which graphitization has not taken place, i. e, all the carbon is in the combined form, is called white cast iron. The graphitization process requires time and therefore, when liquid cast iron is cooled rapidly, white cast iron would result. White cast iron is comparable in properties to that of high carbon steels. However, it is highly brittle and as such is not used for structural parts. It is useful for parts where abrasive wear is present. Tensile strength varies between 170 to 345 MPa and is usually about 240 MPa. The hardness ranges from 350 to 500 BHN. In view of the very high hardness, the machinability is poor and is commonly finished by grinding.

Investment casting (or lost wax casting) refers to the formation of ceramic around the wax patterns to create a multi or single part mold to receive molten metal. This process utilizes an expendable injection molded wax pattern process to achieve complex forms with exceptional surface qualities. Precision castings can achieve exceptional accuracy for both small and large casting parts in a wide range of materials.

To create a mold, a wax pattern, or cluster of patterns, is dipped into ceramic material several times to build a thick shell. De-wax process is then followed by the shell dry process. The wax-less ceramic shell is then produced. Molten metal is then poured into the ceramic shell cavities or cluster, and once solid and cooled, the ceramic shell is broken off to reveal the final cast metal object.