Wednesday, March 31, 2010

Composition Of Grey Cast Iron

Elements are added to the molten iron at the foundry.

Iron and its alloys are widely used in almost all industry and their products reach almost every household. Mixing iron with different elements is varying proportions results in alloys with different properties. Each element added to iron gives it some special characteristic. Grey cast iron, so-called because of its fracture face, has some unique properties due to its composition. The most widely used cast iron, it is brittle with low tensile strength and is used in the manufacturing of engine cylinder blocks, flywheels, gears and many machine-tool bases.

Composition of Grey Cast Iron

Grey cast iron is composed of Carbon, 2.7 to 4.0 percent; Manganese, 0.8 percent; Silicon, 1.8 to 3.0 percent; Sulfur, 0.07 percent; and Phosphorus, 0.2 percent. These are the maximum percentages of these elements in grey cast iron. It has low wear resistance, corrosion resistance and welding ability. Other elements present in varying amounts in grey cast iron are molybdenum, copper, nickel, vanadium, titanium, tin, antimony and chromium. Grey cast iron can also have nitrogen in the range of 20 to 92 ppm.


Carbon adds to the hardness and strength of the cast iron by heat treatment. It adds to machinability, or the ease with which a metal can be machined to an acceptable surface finish, at higher temperatures, dimensional stability, or the ability of a substance or part to retain its shape when subjected to varying degrees of temperature, moisture, pressure, or other stress, and damping of high vibration forces.


Manganese being an austenite forming element (austenite is a metallic non-magnetic allotrope of iron or a solid solution of iron with an alloying element, stable at high temperatures) is used as a substitute for nickel and improves the hot working properties and increase strength of the grey cast iron. It adds to toughness and hardenability of the cast iron.


Silicon is primarily used for deoxidising -- a chemical used in a reaction or process to remove oxygen --of the cast iron during melting process and therefore becomes a small part of the grey cast iron. It adds to the hardening and strength of the cast iron.


Sulfur improves machinability of grey iron but does not cause hot shortness in the cast iron. It combines with manganese to form manganese sulfide that actually reduces hot shortness, or brittleness at high temperatures.


Phosphorus also improves the machinability of grey cast iron. It is added in small amount to increase corrosion resistance in the cast iron. Low phosphorus levels (up to 0.8 percent) also improve fluidity (ability to flow) and shock-resistance of the grey cast iron. If added in large amounts, it can also reduce the strength of the cast iron.


Chromium is usually found below 0.10 percent and Chromium is used to improve hardness and strength of grey cast iron. The percentage of chromium is can be raised to 0.35 percent. Chromium also increases elevated temperature range of grey cast iron.


It is most widely used element for increasing the strength of grey cast iron and is added in percentages 0.20 to 0.75 percent. Since the modulus of elasticity of molybdenum is quite high, it increases the modulus of elasticity, or the ability of a substance to be stretched without breaking, of grey cast iron.


Vanadium also affects the grey cast iron as molybdenum, but must be added in smaller quantities limiting to less than 0.15 percent.

Tags: cast iron, cast iron, grey cast, grey cast iron, grey cast iron, cast iron added