Casting

A casting may be defined as a "metal object obtained by allowing molten metal to solidify in a mold," the shape of the object being determined by the shape of the mold cavity. Founding, or casting, is the process of forming metal objects by melting metal and pouring it into molds. A foundry is a commercial establishment for founding, or producing castings.

WHAT IS CAST IRON
The term cast iron is a generic one, referring to a family of materials differing widely in their properties. In general, a cast iron is an alloy of iron, carbon up to about 4.0 percent), and silicon which ordinarily is not usefully malleable as cast. GREY-IRON An iron having a chemical composition such that after solidification, a large portion of its carbon is distributed throughout the casting as free or graphic carbon in "flake form". Grey cast iron always presents a grey sooty surface when fractured. Because of a fortunate combination of engineering properties, availability, excellent casting characteristics, and favorable cost, grey irons are produced in tonnages exceeding all other casting. The basic foundry operations of pattern making, moldings, coring, sand conditioning, melting, clearing, etc, have been highly developed because of the tremendous amount of material and work involved in grey-iron-castings production. In fact, it is undoubtedly true that much of the engineering and mechanization which has developed in the foundry industry came through the needs for these improvements in order to meet the production demand of manufacturers for grey-iron castings. The production of grey-iron castings is, in fact such a large industry that twice as much grey iron is annually as all other cast metals combined.
WHAT IS DUCTILE IRON

Nodular Cast Iron is also known as ductile cast iron, or spheroidal graphite cast iron. A specially prepared iron treated in the molten condition with a small percentage of magnesium , cerium, or other agent that will cause of large proportion of its carbon to occur as spheroids of graphite rather than as flakes. Ductility is obtained in the iron as a result of the spherical type of graphite formed. This type of cast iron presents a bright steely surface when fractured. Ductile cast iron was first announced to the foundry industry as a new engineering material at the 1948 annual meeting of the American Foundrymen's Society. This revolutionary material, discovered independently by the British Cast Iron Research Association and the International Nickel Company is also referred to as nodular, or spherical, graphite cast iron. Essentially, ductile cast iron consists of graphite spheroids dispersed in a matrix similar to that of steel. The only significant difference between grey cast iron and ductile cast iron is in the shape of the graphite phase; the matrices can be similar. Ductile iron requires foundry operations which are similar to those for other cast metals. Process control is critical, however, and the development of graphite as spheroids is of principal concern in this material.
DIFFERENCES BETWEEN DUCTILE IRON & GREY IRON
1. Tensile Strength - While normal grey iron has a tensile strength of 30000 P.S.I, that of ductile iron is 50000 P.S.I, or more.
2. Elongation - While normal grey iron has no elongation at all, the elongation of ductile iron can be from 2% to as high as 18% but the normal elongation is around 7% . Because of this unique property, i.e. ductility, it is called ductile iron.
3. Microstructure - While in normal grey iron, graphite or carbon recipitates as longitudinal flakes whereas in case of ductile iron, the graphite precipitates as spheroids. That is why, ductile iron is also called spheroidal graphite iron. 4. Production of ductile iron is tricky because it needs very low sulphur and also phosphorus which are detrimental in metal and the magnesium treatment is very vigorous. For this, raw material selection , furnace selection, process selection are very important