Optimization of Compositions and Dimensions in Producing Austempered Ductile Irons

摘要

Different ductile irons with square, T- and L-shape block castings were austenitized at 1173K for two hours and then quenched in a 473-673K salt bath. Cooling curves of quenched irons were measured. These measured data were compared with the simulated cooling curves. Heat transfer coefficients of heat-treated and quenched irons were optimized for section moduli of castings ranging from 183 to 1333 (x10~(-8)m~6hr~0C~2/kcal~2). Increasing the section moduli of treated irons decreased their heat transfer coefficients when irons were quenched from 1173K to the designated holding temperature. Time (#tau#_s) for the start of ausferrite transformation was also optimized by using a regression model. Time of #tau#_s was a function of alloying elements, nodule counts and a radius of nodular graphite. FIDAP (a commercial software) was used to determine the cooling curves of various shapes of castings. Comparing these cooling curves with time of #tau#_s critical cooling rates associated with critical section moduli or values of (V/A)~2 were defined. This paper presented some examples for determining critical section modulus. If a critical section modulus was desired, a combination of alloying elements could be optimized for developing a critical cooling rate.