Ferritic ductile cast iron was quench-hardened with the isothermal transformation in the range of temperatures 400÷ 300 0C, in the range of time 7, 5÷ 240 min. Soaking before the transformation was gradual. Nominal austenitizing temperature t= 950 0C, and then there appeared cooling to t’= 850 or 800 0C. Heat treatment was performed with the usage of specimens for the impact strength test, and after that hardness was measured and microstructure analyzed. Test results were used to elaborate mathematical equations and to make threedimensional charts with the usage of STATISTICA and EXCEL software. It was stated, that austenitizing temperature and conditions of the isothermal transformation influence on hardness and impact strength of ADI. On the base of hardness and impact strength it can be stated that as a result of gradual austenitizing of specimens there was obtained ADI corresponding to grades of ASTM 897 (850/550/10) Grade 1,(1050/700/7) Grade 2 and (1200/850/4) Grade 3.