Effect of Carbon Equivalent and Alloying Elements on the Tensile Properties of Superfine Interdendritic Graphite Irons E. Aguado, J. Sertucha, P. Larrañaga Área Ingeniería, I+D y Procesos Metalúrgicos, IK4-Azterlan, Durango, Spain D.M. Stefanescu Ohio State University, Columbus, Ohio and University of Alabama, Tuscaloosa, Alabama R. Suárez Veigalan Estudio, Durango, Spain Copyright 2014 American Foundry Society ABSTRACT 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. Keywords: gray iron, high strength, superfine inter-dendritic graphite, titanium fading, primary austenite. INTRODUCTION The usual microstructure of gray iron consists of flake (lamellar) graphite dispersed throughout a ferrite/pearlite matrix. The mechanical and physical properties depend on the length and distribution of the graphite flakes and on the ferrite-pearlite ratio. The typical tensile strength of a gray iron of 4% CE ranges from 230 to 300MPa, with an average of 260 MPa. 1 Their average hardness is 215 HB. The amount of primary austenite of irons of this composition ranges from 10 to 25%, the rest being eutectic. The strength increases with lower carbon equivalent and with the amount of austenite, although the correlation strength-austenite is rather poor. 2 Fine type D lamellar graphite is typically obtained in irons of hypoeutectic composition at relatively high cooling rates. Very fine type D graphite can be obtained for example in irons with normal sulfur content of 0.03-0.08wt%, high titanium (0.5-1%) and high cooling rates. 2 3 While the effect of titanium in gray iron has been studied extensively 4-9 and some improvement in the tensile strength with Ti addition documented, the sulfur content was within the usual range of this element for gray iron production, i.e., 0.065 to 0.11%. Concerns on the negative effect of Ti on machinability have been expressed. 10 In general, sulfur promotes type A graphite 11 by increasing the sulfur content, sulfide particles are formed which act as nucleation sites for graphite precipitation. 12, 13 However, decreasing its contents below 0.02 wt% produces type D interdendritic graphite. 14 Coral graphite is a highly branched fibrous type of graphite that is different from either type D lamellar graphite or compacted graphite. It has been obtained in pure Fe-C-Si alloys containing very small amounts of sulfur, typically around 0.001%, at high cooling rates. 15 Such compositions have no practical applications because of the unreasonable high costs associated with the Paper 14-009, Page 1 of 10 AFS Proceedings 2014 © American Foundry Society, Schaumburg, IL USA