The microstructure and mechanical properties of austempered ductile iron

This paper represents a summary of experimental results dealing with the time dependence of microstructure and mechanical properties during austempering, and with the austempering temperature dependence of microstructure and mechanical properties. Alloys with a nominal compositions of 3.7 C, 2.5 Si and various controlled amounts of manganese, molybdenum, and nickel were prepared in the MTU foundry. Austenitization at 927° C (1700°F) and 871°C (1600°F) was followed by austempering at temperatures between 230° C (446° F) and 420° C (788° F) at 10°C (18°F) intervals for one hour, and at 316°C (601°F) and 371°C (700° F) for various times from 2 min to 1440 min. Optical and electron microscopy as well as x-ray metallography were used to determine the kinetics and details of the transformations during stages I and II. It is shown that the carbon gradient within the austenite during the transformation controls the rate of Stage I and alloy content controls the rate of Stage II. Interdendritic segregation of alloying elements leads to the presence of significant quantities of untransformed austenite, especially at early austempering times. It is shown that these volumes constitute convenient crack paths, thereby reducing ductility. Minimizing the continuity of those volumes increases ductility, a job aided by a lower austenitizing temperature and a minimum alloy (especially manganese) content. A processing window concept used to optimize ductility at austempering temperatures in excess of 350° C (662° F) is defined by the times needed to avoid excessive untransformed austenite volume (UAV) (the minimum time) and to avoid excessive decomposition of austenite (the maximum time). Tensile strength and ductility are shown to be a function of austenite volume fraction, scale of the microstructure, alloy content, the presence of carbide formed during the austenite transformation, and the presence of intrinsic defects such as eutectic alloy carbides. © 1988 Springer-Verlag New York Inc

Evolution of Microstructures During Austempering of Ductile Irons Alloyed with Manganese and Copper

The influences of relatively high manganese (0.45 through 1.0 wt pct) and copper (0.56 through 1.13 wt pct) contents on microstructure development and phase transformation in three austempered ductile irons have been studied. The experimental ductile irons alloyed with copper and manganese are found to be practically free from intercellular manganese segregation. This suggests that the positive segregation of manganese is largely neutralized by the negative segregation of copper when these alloying elements are added in appropriate proportions. The drop in unreacted austenite volume (UAV) with increasing austempering temperature and time is quite significant in irons alloyed with copper and manganese. The ausferrite morphology also undergoes a transition from lenticular to feathery appearance of increasing coarseness with the increasing austempering temperature and time. SEM micrographs of the austempered samples from the base alloy containing manganese only, as well as copper plus manganese-alloyed irons, clearly reveal the presence of some martensite along with retained austenite and ferrite. X-ray diffraction analysis also confirms the presence of these phases. SEM examination further reveals the presence of twinned martensite in the copper plus manganese-alloyed samples. The possibility of strain-induced transformation of austenite to martensite during austempering heat treatment is suggested