The effect of ADI heat treatment conditions on the core microstructures and mechanical properties of GCD 45 has been investigated. In the case of $25{ss}{AE}$ in diameter, it has been found that pearlite structure is not obtained, and the matrix is consisted of graphite, bainite and retained austenite. Upon cooling isothermally from $900^{circ}C$, the core structure of 35 mm in diameter is converted to pearlite. However, the structure is transformed to bainite after slow cooling from $900^{circ}C$ to $800^{circ}C$.
