Whereas the graphite appears in Grey Irons in the form of flakes, which gives minute fracture lines within the structure of the iron, in SG irons the graphite is modified by small additions of magnesium. The magnesium causes the graphite to 'ball' whilst still in molten solution. On solidifying the graphite remains in this spheroidal shape and creates a matrix with no built in fracture lines.
          This modified structure gives SG Iron a greater ability to bend before fracture, an increase in ductility and resistance to shock allowing suitable alternatives to some carbon steels and malleable irons.

           Before the advent of European Material numbers the descriptive code represented the following physical properties.

1st. Number : gives tensile strength in N/mm.sq.
2nd Number : denotes % elongation before fracture.

i.e. 420/12 = 420N/mm.sq. tensile and 12% elongation