Effect of two-step austempering on fatigue behavior of austempered ductile iron (ADI)

Abstract

The effect of two-step austempering on the fatigue properties in alloyed ductile iron was studied. The ductile irons were subjected to conventional and two-step austempering processes. The austempering process was performed by making three batches. In the first batch, samples after austenitizing at 900 °C were conventionally austempered separately at two temperatures, namely 400 °C and 320 °C. In the second batch, samples were austempered first at 400 °C followed by austempering at 320 °C. In the third batch, samples were austempered first at 320 °C followed by austempering at 400 °C. Austempered samples were characterized for morphology and distribution of phases by optical microscope and scanning electron microscope. It was found that the properties increased with the increase in amount of lower bainitic ferrite and retained austenite of high carbon content. The retained austenite with low carbon content was found to transform into martensite during loading and led to decrease the properties.