Kesri Steels Limited - SG Iron Casting

An abbreviation for Spheroidal Graphite (SG) Cast Iron. As the name implies, graphite is present in spheroidal form instead of flakes and compared with Grey Cast Iron it has higher mechanical strength, ductility and increased shock resistance.

Cast iron usually is made into specified shapes, called castings, for direct use or for processing by machining, heat treating, or assembly. In special cases it may be forged or rolled moderately.

The production of spheroidal graphite as-cast state is an outstanding development of a new iron, initially due to the use of cerium by Morrogh (BCIRA, 1946 BP 645862) and later, magnesium by the International Nickel Co. (1947 BP 630.070). The use of magnesium, to give 0,04-0,06% residual content proved to be the more adaptable and economic of the two processes. The production of spheroidal structure is prevented, however, by certain trace elements, e.g. 0,1 Ti, 0,009 Pb, 0,003 Bi, 0,004% Sb, but their effect can be eliminated by 0,005-0,01% cerium.

SG iron casting can, therefore, compete successfully with malleable iron for thick sections, cast steel and alloy flake graphite cast iron. SG cast irons are not so section sensitive as normal iron, e.g. a variation of 25-150 mm section causes grey iron to change from 278 to 154 MPa whereas a SG iron would change from 664 to 587 MPa.

A new iron contains fine vermicular graphite similar but finer than undercooled graphite. SG cast iron has a worm-like form which enables high strengths to be obtained with 2-3% El. Very precise production control is necessary and this limits commercial production at the moment. The sulphur content must be below 0,002% and casting must be cooled rapidly.

The SG iron can be used with a pearlite matrix or ferrite after a short annealing or with an acicular or austenitic matrix when suitably alloyed. The ferrite grade of SG iron has a strength of 370 MPa with 17% El whereas a normalized pearlitic SG iron has a strength of 700 MPa with a minimum of 2% El. The strength can be increased to 925 MPa by special heat treatment or by the addition of alloying elements. Damping capacity is lower but shock, heat and growth resistance and weldability are higher than for flake graphite iron.

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