METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, MAY 1997—1143 Influence of the Amount and Morphology of Retained Austenite on the Mechanical Properties of an Austempered Ductile Iron J. ARANZABAL, I. GUTIERREZ, J.M. RODRIGUEZ-IBABE, and J.J. URCOLA High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of . In these conditions (370 °C and 410 °C treat- ments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: ‘‘autotempered’’ plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections. I. INTRODUCTION THE austempering applied to ductile cast iron produces different bainitic structures, as a function of the heat-treat- ment conditions that can lead to an attractive combination of mechanical properties. [1–4] This renders the austempered ductile iron (ADI) useful in a relatively large number of applications [5–8] as an economical substitute for high strength steels. The bainitic transformation in steels gives a mixture of ferrite and carbides, but the presence of silicon in the duc- tile iron leads to significant austenite volume fractions be- ing retained in the microstructure. The presence of silicon inhibits the precipitation of the carbides associated with the normal bainitic transformation in steels, [9] and the austenite is progressively enriched by the carbon rejected from the growing bainitic ferrite units. The bainitic transformation is an incomplete reaction, in which the volume fraction of ferrite and the final carbon content of the remaining aus- tenite depend both on the composition and the treatment conditions. [9] When the bainite formation ceases, the aus- tenite can have a carbon content high enough to be stable on quenching to room temperature. However, this structure is not stable, mainly at high temperatures, on increasing the treatment time, when the austenite can decompose to ferrite and complex carbides. [10] This constitutes the stage II of the transformation. [11] For short treatment times, the volume fraction of bainitic ferrite being formed is small and, as a consequence, the austenite is relatively unstable and trans- J. ARANZABAL, Senior Researcher, is with INASMET, 20009 San Sebastia ´n, Spain. I. GUTIERREZ and J.M. RODRIGUEZ-IBABE, Senior Researchers, are with the Department of Materials, CEIT, 20009 San Sebastia ´n, Spain, and are also Lecturers, ESII, San Sebastia ´n, Spain. J.J. URCOLA, formerly Head, Department of Materials, CEIT, and Professor, ESII, is deceased. Manuscript submitted February 15, 1996. forms, at least partially, to martensite on quenching. This is the so-called stage I of the transformation. Changing the treatment conditions results in different distributions of bainitic ferrite and austenite. High temper- atures lead to the formation of relatively low bainite volume fractions and, in consequence, large pools of austenite are present in the microstructure. At low temperatures, when the diffusion of the carbon from the bainitic units is difficult and the driving force for the transformation is higher, a finer microstructure is formed with a higher volume fraction of bainitic ferrite. The material used in the present work has been the sub- ject of an intense study from a microstructural and me- chanical point of view, taking into account the change of the austempering conditions. The microstructures generated as a function of the treatment time and temperature and the corresponding mechanical properties, mainly the toughness, have been fully described in previous articles. [12–15] The present work is concerned with a more detailed study of the tensile properties of the duplex (ferrite + austenite) re- sultant microstructures with special emphasis on the trans- formation-induced plasticity (TRIP) effect that has been observed to occur under certain conditions. II. EXPERIMENTAL PROCEDURE The chemical composition of the ductile iron used in the present research is shown in Table I. The ductile iron was poured in a Y block mold. Specimens for metallography, tension, and short rod fracture toughness were machined from the test section of the Y block. The austempering heat treatments were performed by austenitizing for 30 minutes at 900 °C and transforming the austenite isothermally at temperatures in the range 300 °C to 410 °C for times be- tween 5 minutes and 24 hours. Standard preparation techniques were used for optical