Control of austenitic transformation in ductile iron aided by calculation of Fe-C-Si-X phase boundaries

Abstract

To control austenite transformation of ductile iron, thermodynamics procedures were used to calculate the Ae₃, the Gr/γ (A_cm), and the A₁ phase boundaries of high Mn and Ni-Cu-Mn alloyed iron as a function of austenitization temperature. The results of calculation show that segregation of Mn in the intergraphite regions reduces the carbon content of austenite at the Ae₃ phase boundary to the lowest value. If one ignores the effect of substitutional alloying elements on the nucleation of austenite, the austenite should first nucleate in the cell boundaries and then grow to the graphite nodules. In addition, the calculated results show that the A₁ temperature is the lowest in the intercellular region of a high Mn alloy. Therefore, if the austenitization temperature is not sufficiently high, only those parts of the matrix that have the A₁ phase boundary below the austenitization temperature transform to austenite, and dual formation of the α and γ phases will occur. By using the procedure introduced in this study, the volume fraction of each phase can be evaluated by calculating the A₁ phase boundary as a function of intergraphite distance. In the case of Ni-Cu-Mn alloy, Ni stabilizes austenite, which lowers the Ae₃ phase boundary. In this alloy, carbon content of austenite at the Ae₃ phase boundary is lower near the graphite nodule and higher in the intergraphite regions. However, the variation of carbon content of austenite at the Ae₃ phase boundary in the matrix of this alloy is much lower than in the high Mn alloy.