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 1173Kfor 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 (×10{sup}(-8)m{sup}6hr℃{sup}2/kcal{sup}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 (τs) for the start of ausferrite transformation was also optimized by using a regression model. Time ofτs 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 τs critical cooling rates associated with critical section moduli or values of(V/A) {sup}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.