Effects of niobium alloying on microstructure, toughness and wear resistance of austempered ductile iron

Abstract

Austempered ductile iron (ADI) is a heat-treated nodular iron variant that has for a given strength level much higher elongation than conventional iron with spheroidized graphite. This paper considers the effect of niobium addition on the graphite microstructure, bainite microstructure, bainite transformation process and properties such as hardness and impact toughness under given heat-treatment conditions. The resulting properties are significant with regard to the wear resistance of the material. The effects of niobium addition on the graphite morphology can be understood based on a detailed precipitation analysis of the niobium precipitation behavior in the liquid phase. Niobium influences the temperature and kinetics of pearlite as well as bainite transformation. Furthermore, niobium refines the bainite microstructure generated during austempering treatment. It was found that niobium addition in the range of 0.2–0.5 wt percent allows obtaining an optimum combination of hardness, impact toughness and wear resistance.