Austenitization and formation of ausferrite structure in austempered ductile iron with dual matrix

The effect of austenitizing conditions on the microstructure and mechanical properties of an austempered ductile iron (ADI) with a dual matrix was investigated. Carbon diffusion plays a critical role in the phase transformation of austenitization. In initially pearlitic structures, the carbon diffusion distances involved during austenitization are smaller compared to those in ferritic structures. The study was carried out to examine the influence of temperature and times of the austenitization process on the maximum carbon content in austenite and then its effect on the processing window of ADI with the dual matrix. An alloyed ductile iron (3,6%C; 2,44%Si; 0,36%Mn; 0,9%Ni; 0,61%Cu; 0,11%Cr; 0,036%Mg; 0,015%S và 0,006%P) was fully austenitic at various temperatures 870 °C–930 °C. The minimum hardness reaches the value of 270 HB. An increase in austenitization temperature increases the carbon content dissolved in the austenite, which in turn, decreases the free energy controlling the transformation of austenite to bainite ferrite and high carbon austenite. Raising the austenitization temperature makes the process window shift toward the extending time. The carbon atoms must diffuse out of the ferrite needle with a diffusion distance equal to the ferrite layer thickness. At 360 °C austempered temperature, diffusion coefficient D = 4.60.10 –17 (m 2 /s). With a ferrite needle thickness of 1 μ m = 10 –6 m, the diffusion time would be t = 5330 (s).