A transmission electron microscope study of 1 Pct Mn ductile iron with different austempering treatments

Abstract

A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature) and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water. The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis.