Mechanism for the Role of Silicon on the Transition from Graphite to Cementite Eutectic in Cast Iron

In this work, an analytical solution is proposed to explain the influence of silicon on the transition from graphite to cementite eutectic in cast iron. It is found that this transition can be related to (1) the graphite nucleation potential (directly characterized by the cell count N and indirectly by the nucleation coefficients N _s and b), (2) the growth rate coefficient of graphite eutectic cells μ, (3) the temperature range ΔT _sc = T _s – T _c (where T _s and T _c are the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic, respectively), and (4) the liquid volume fraction f _l after preeutectic austenite solidification. Analytical equations were derived that describe the absolute and the relative chilling tendencies (CT and CT_r, respectively) as well as the critical cooling rate Q _cr and, hence, the chill w of the cast iron. Theoretical arguments are experimentally verified for castings with various silicon contents. This work also describes the methods used in the determination of N _s, b, and μ values. It is found that the main role of silicon on the transition from graphite to cementite eutectic is to raise the density of the graphite nuclei N and temperature range ΔT _sc. In addition, it is shown that increasing the silicon content of cast iron leads to an increasing value of Q _cr and decreasing values of CT and CT_r, and of the chill width w. In particular, this work shows that the chilling tendency indexes and, hence, the chill all can be estimated from a simple thermal analysis using reference castings.