Influence of austempering on microstructural characteristics in thin-walled ductile cast iron
Austempered ductile iron (ADI) is a well established material in many engineering components. Due to its remarkable combination of high strength, ductility and toughness, it exhibits good resistance to wear and fatigue which makes it suitable for a wide variety of applications in the automotive, rail or heavy engineering industry. To gain these properties, ADI is conventionally produced by a successive heat treatment consisting of an austenitization and austempering treatment of cast iron with spheroidal graphite (EN-GJS). In the present research, the effect of the austempering treatment on the microstructural evolution and its global hardness is investigated in a commercial cast iron EN-GJS-500-7 (ASTM A536) that has been produced by high-pressure die casting to a comparatively small diameter of 4 mm. To determine the optimum processing window, a TTT-diagram was recorded via quenching dilatometry combined with microstructural analysis using optical light and scanning electron microscopy. It is shown that after austenitizing at 1100 °C for 30 min, the ledeburitic as-cast microstructure approaches the austenite-graphite equilibrium. Upon controlled quenching, bainitic ferrite forms at austempering temperatures above the martensite-start-temperature (MS) of 200 °C whereby a high cooling rate of 5 s (t8/5) is required to suppress an excessive formation of pearlite during cooling. Low-temperature austempering at 260 °C generates a high hardness (550 HV10) but requires a long austempering time of 40 h. Increasing the temperature to 320 °C shorten the time needed to achieve a comparatively high hardness of 420 HV10 to 40 min. In this condition, the microstructure consists of uniformly distributed fine graphite nodules embedded in a mixed matrix of primarily bainitic ferrite and thermally-stable retained austenite without carbide precipitates. Therefore, optimum austempering parameters could be defined that ensure an ADI microstructure with improved mechanical properties even in thin-walled ductile cast iron.