Effect of Carbon Equivalent and Alloying Elements on the Tensile Properties of Superfine Interdendritic Graphite Irons

Recently, it was demonstrated that the tensile strength of gray iron of average 4% Carbon Equivalent can be increased to 300-345 MPa, without a significant increase in hardness through 0.3% titanium addition to low sulfur (<0.01%) iron. This effect is the result of a higher primary austenite-to-eutectic ratio combined with superfine interdendritic graphite precipitation. This paper explores the possibilities of further improving the strength through traditional methods such as decreasing the carbon equivalent and using alloying elements. In addition, the optimum limits for the titanium content and the fading of titanium during holding of the iron in the melting furnace were investigated. The optimum titanium level was established to be at 0.25-0.4%. Higher titanium increased the amount of TiC to the point that they agglomerated into clusters and dramatically decreased the mechanical properties. It was also found that titanium fades through oxidation and combination with nitrogen upon holding in the melting furnace. Surprisingly, it was found that lower carbon equivalent does not improve the strength of superfine interdendritic graphite cast iron. The optimum strength was obtained for CE in the range of 3.9 to 4%. Increasing the Mn content from 0.56 to 0.85% had a significant effect on strength and hardness, with Ultimate Tensile Strength (UTS) as high as 346MPa with a moderate hardness of 204 Hardness Brinell (HB). Chromium (0.23%) or chromium - tin additions (0.23%Cr, 0.078%Sn) were also beneficial to strength, which was increased to 350MPa, the highest strength in these experiments. The hardness was also increased to 230HB with a corresponding reduction of the ferrite content of the matrix. Copper did not produce the expected effect of lowering the ferrite. It increased the amount of ferrite and did not help achieve higher strength.