Austempered ductile iron - Process control and quality assurance

Recent interest in the development of austempered ductile cast irons has resulted in considerable study of the physical metallurgy and mechanical properties of these high strength, high toughness cast irons. Equally important is the identification of process control and quality assurance factors to achieve the desired properties successfully and consistently. In this study, aspects of austempered ductile iron quality control are reviewed including the production of quality ductile iron that will respond to austempering heat treatments, heat treatment process control variables to achieve the desired properties, and non-destructive techniques for quality assurance. A distinction is made between austempered ductile irons transformed at high temperatures and austempered/bainitic ductile irons transformed at low temperatures. Austempering response is a complex interaction between all heat treatment variables and chemical composition on a macroscopic and microscopic level. Alloying elements are essential to provide sufficient hardenability (or austemperability) for heavy section heat treatment. Austenitizing temperature as well as austempering time and temperature affect the transformation response for a given alloy. In addition, segregation causes a non-uniform transformation response within the material on a microscopic level. In many cases the final properties of austempered ductile iron can be directly related to the amount of stabilized (retained) austenite present in the final structure. Non-destructive techniques to measure stabilized austenite are discussed and evaluated. Problems associated with dimensional control for critical tolerance components are highlighted.