Microstructural and Mechanical Characteristics of Low Alloyed Ni–Mo–Cu Austempered Ductile Iron

Abstract

The present study investigated the effect of austempering temperature and austempering time on the microstructure and mechanical properties of low alloyed Ni–Mo–Cu ductile iron. The effect of austempering parameters and alloying additions on the austemperability of treated ductile iron has been estimated, too.
Specimens were austenitised at 900°C for 120 min, then austempered for 10, 30, 60, 120, 240 and 360 min at 300, 350 and 400°C respectively, and examined by light and scanning electron microscopy. The structure consisted of bainitic ferrite containing retained austenite, the amount of which increased, and the carbon content of which decreased, with increasing austempering temperature. The carbon content of austenite has been evaluated by measuring the lattice parameter by X-ray diffraction.
After short periods of austempering time in iron, the carbon content of the retained austenite decreases and on subsequent cooling to room temperature it transforms to martensite.
The volume fractions of retained austenite, bainitic ferrite, martensite and austenite carbon content was correlated with microstructural changes and mechanical properties. Optimum properties are obtained at intermediate austempering periods (120–240 min) when both the amount of retained austenite and austenite carbon content are maximum.