MICROSEGREGATION OF SILICON IN CAST IRON

Abstract

The literature presents contradictory data on the microsegregation of silicon in cast iron. Authors of the previous works have investigated silicon microsegregation in dendritic cell consisting both of primary austenite and eutectic austenite that grows on the primary austenite dendrites. Since the silicon content in austenite affects the temperature and completeness of the phase transitions during cooling and heat treatment, the absence of data on silicon microsegregation do not allow to predict the microstructure of cast iron in the as-cast condition and after heat treatment. The silicon microsegregation in gray cast iron with composition Fe + 3.11 % С + 1.6 % Si + 0.4 % M n by the energy dispersive X-ray spectroscopy and calculation of the nonequilibrium solidification in the Thermo-Calc software were investigated. The silicon microsegregation in the primary austenite is not observed. The silicon content in the primary austenite was ≈2 % that is higher than silicon content in the eutectic austenite ≈1.5 %. Due to the calculations results the microsegregation of silicon in primary austenite are negligible. By calculation the non-equilibrium solidification of ternary alloys Fe – Si – C [Fe + 1.5 % Si + X % C], at X = 1.5; 2.8; 3.7 it was shown that the only primary austenite solidified at 1.5 % С and microsegregation of silicon are positive (Si content in the center of the dendritic cells is lower than on its boundaries). For the alloy with 2.8 % С, the microsegregation of Si in the primary austenite is also positive, however the silicon content in the eutectic austenite, growing on the primary austenite during the austenite-graphite eutectic formation, will be lower and the “rim effect” (maximum Si content at the boundary between primary and eutectic austenite) should appears. At the 3.7 % of C in alloys the primary dendrites amount should be limited, so the major part of the dendritic cell should consist of the eutectic austenite, the Si content in which should be decreasing during the solidification. In this case the so-called negative microsegregation should occur.