Microstructure and properties of austempered ductile iron subjected to single and two step processing

Abstract

The predicted processing window has been used to select temperature–time parameters for austempering of a 3·39C–2·28Si–0·48Mn–0·13Mo–0·23Cu–0·05Ni–0·034Mg (wt-%) ductile iron. Comparison of a single step austempering with two step high–low temperature regime has been carried out. The results have shown that with increasing austempering temperature from 300 to 380°C, the morphology of ausferritic ferrite changes from lenticular or lath to feathery, accompanied by an increase in the volume fraction of high carbon retained austenite and carbon concentration in it. This microstructural evolution leads to an increase in ductility and impact energy, but a decrease in strength and hardness after high temperature treatment. All samples have satisfied the mechanical properties of ASTM 897M:1990. Two step high–low (380–340°C) temperature austempering produced microsturctures similar to those seen in high temperature single step processes but with significant time savings. The improvement in the mechanical properties was also achieved after 380–340°C two step processing.