Availability of two specific austempering treatments were tested for the improvement of drilling machinability of austempered ductile cast iron (ADI). The one is called successive austempering process, in which the upper bainite transformation was immediately followed by the lower bainite transformation by a successive isothermal heat treatment at two temperature levels. The other is the austempering with pre-heat treatment. The samples were firstly heated just below Al temperature and then austempered. With the successive austempering the volume of untransformed austenite (UAV) was reduced to 0 - 2.3% from 8.3% of ordinary austempered iron, and the borehole number was increased by 20 - 30 times in drilling test with high speed steel tool drills of 6mm dia. The second treatment increased the borehole number by about 6 times of ordinary austempered iron. The decrease in UAV raised the elongation and almost unchanged the strength of ADI.
