Production of nanostructured ausferrite in alloyed and non-alloyed ductile iron

Abstract

Austempered ductile cast iron (ADI) is a material with exceptional mechanical properties with potential applications in the automotive, agricultural and mining industries. The microstructure of ADI consists on well formed graphite nodules in a matrix of ausferrite (bainitic ferrite + high carbon austenite). In this ductile iron, the ausferritic matrix is obtained by austenitising followed by austempering heat treatment carried out under specific conditions of time and temperature. In this work, the effect of the austempering parameters were studied in order to produce nanostructured bainitic ferrite. The study was conducted in two ductile irons with chemical compositions 3.45C-2.72Si-0.80Ni-0.6Cu (wt%) and 3.53C-2.66 Si-0.12Ni-0.01Cu (wt%). The austenitization conditions were 900 °C for 2 hours. The austempering temperatures were 200 °C and 230 °C for the first alloy (high Cu- high Ni) and 215 °C and 245 °C for the second alloy (low Cu-low Ni). The austempering times were 0.5, 1, 2, 4, 8, and 10 hours for both alloy systems. The chemical and microstructural characterization were conducted by using optical emission spectrometry, optical microscopy, scanning electron microscopy and X-ray diffraction techniques. The results indicate that the microstructure produced by the austempering process depends strongly on temperature and transformation time. The higher volumetric fraction of bainitic ferrite was reached at the longest transformation times. The time to obtain the highest fraction of high carbon austenite decreases with temperature. Cu and Ni do not have a clear influence on the formation of ausferrite. The bainitic ferrite produced is between 7-70 nanometers thick.