The predictable tensile strength of Gray iron, expressed in Ksi follows the empiric relation U = 10 (b - 2 x C.E.) x (F1) x (F2) x (Fx), with C.E. = %C+1/3 (%Si + %P), where b is a wall thickness factor, F1, F2, Fx are alloy factors for each of the elements added such as Cu, Mo, Cr, Ni… For a given non-alloyed Gray iron plate 1 in. thick, the traction resistance levels tend towards the following values :

25 Ksi (170Mpa) : C.E. 4.25 as cast, graphite structure with dominance of type A, with some content of type C , size 4, coarse pearlite matrix with ferrite content, hardness averaging 160 BHN;

30Ksi (210Mpa) : C.E. 4.00 as cast, essentially type A graphite structure, size 5, matrix with a dominance of pearlite, hardness averaging 170 BHN;

40Ksi (275 Mpa) : C.E. 3.50 as cast, type A graphite structure, size 5, pearlite matrix, hardness averaging 175 BHN;

50 Ksi (345 MPa) : C.E. 3.00 as cast, type A graphite structure, size 6, pearlite matrix, hardness averaging 180 BHN;

Note : For 50 Ksi class, and to a lesser extent , for 40 Ksi class, current practice avoids cast irons with C.E. that is to low and compensates with some alloying elements.