Tuesday, December 12, 2006

Spheroidal Graphite Ductile Iron

Ductile or spheroidal graphite irons: These irons have higher mechanical properties than a comparable grey cast iron with the same composition, because the carbon is in the shape of spheroidal graphite. This is achieved by inoculating low-sulphur molten iron having low silicon content with magnesium or cerium or both, followed by addition of silicon. Subsequent cooling can produce a variety of matrix structures with ferrite and pearlite being the most common. Compared to grey cast iron, spheroidal graphite irons have higher ductility, tensile strength, modulus of elasticity and resistance to elevated temperature oxidation. Machinability and corrosion resistance are comparable to grey cast iron, though damping capacity is lower. Fluidity is lower than grey cast iron but better than steel.

Spheroidal graphite irons are designated based on the specified minimum tensile strength in MPa and the minimum elongation (in percentage) after fracture of a test piece. This includes SG350/22, SG400/18, SG400/15, SG450/10, SG500/7, SG600/3, SG700/2, SG800/2 and SG900/2. The corresponding tensile strength varies from 350 MPa to 900 MPa, while the Brinell hardness varies from 150 to 320. Density and specific heat of the various grades remain nearly constant at about 7100 kg/m^3 and 600 J/(KgK) (between 20 and 700 C) respectively. Coefficient of thermal expansion between 20 and 400 C also remains constant at 12.5┬Ám/(m K). Thermal conductivity of the various grades falls from around 36 W/(m K) for SG350/22 to around 33 W/(m K) for SG900/2.

The SG irons are widely used in automobile and farming industry: axle housings, brake calipers, brake cylinders, camshafts, connecting rods, crankshafts, gears, pistons and yokes. They are also used to make bulldozer parts, conveyor frames, couplers, crawler sprockets, elevator buckets, railway wheels and hoist drums. Other general engineering applications include boiler segments, coal crushers, hammers, die blocks, frames and jigs, nuclear fuel containers, tank covers, tunnels segments and turret heads.

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