A heat treatment method of austempered ductile iron which can secure uniform product surface and matrix structure and excellent mechanical properties and can prevent the generation of water contamination by performing two steps of the quenching process, and simultaneously spraying wastewater generated in the washing process onto a workpiece and evaporating the sprayed wastewater, thereby cooling the workpiece early is provided. A method for heat treatment of austempered ductile iron comprises the processes of: heating(austenizing) a workpiece made from ductile cast iron at a temperature of 850 to 950 deg.C; and quenching the heated workpiece in two steps, wherein the quenching process comprises a first quenching process of quenching the heated workpiece at 430 to 250 deg.C, a second quenching process of quenching the first quenched workpiece at a temperature of 380 to 190 deg.C, a process of washing the quenched workpiece using water, a process of recovering wastewater containing neutral salts generated in the washing process, and a process of injecting the recovered wastewater into the first and second quenching processes, thereby washing the quenched workpiece with the recovered wastewater.
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