Dependence of fracture toughness of austempered ductile iron on austempering temperature
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INTRODUCTION
WHEN ductile iron is subjected to an austempering treatment, a range of microstructures is obtained depending on heat treatment parameters such as austenitizing time and temperature and austempering time and temperature.[1–5] This results in austempered ductile irons (ADIs) of different grades ranging from high-strength–low-ductility types, to low-strength–high-ductility ones, which have been found to be economical substitutes for high strength steels in several applications. The influence of heat treatment parameters on the microstructure has been extensively studied using optical microscopy, electron microscopy, and X-ray diffraction. It is now understood that the austempering reaction in ductile iron is a two-stage process. At the austempering temperature, ferrite precipitates out of, and grows into, the austenite. Simultaneously, carbon is rejected from the growing ferrite plates into the surrounding austenite. Carbide precipitation is suppressed because of the high silicon content. The enrichment of austenite with carbon inhibits the growth of ferrite and also stabilizes the austenite. This decomposition of austenite into ferrite and high carbon austenite is referred to as the stage I reaction. If now quenched from the austempering temperature, the microstructure will consist of ferrite platelets in a matrix of stabilized high carbon austenite. This is the desired microstructure of ADI. If the iron is held at the austempering temperature for too long a time, the high carbon austenite will decompose into ferrite and carbide. This is the stage II reaction and is not desired, as the carbide precipitation will embrittle the iron. If the austempering time is too short, the austenite may not be fully enriched with carbon, and some of it may transform P. PRASAD RAO, Professor, is with the Department of Metallurgical and Materials Engineering, Karnatak Regional Engineering College, Karnatak State, India 574 157. SUSIL K. PUTATUNDA, Associate Professor, is with the Department of Chemical Engineering and Materials Science, College of Engineering, Wayne State University, Detroit, MI 48202. Manuscript submitted January 14, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
to martensite on quenching, again leading to embrittlement of the iron. The optimum austempering time is, therefore, the period between the end of stage I and the beginning of stage II. This is called the processing window. The influence of microstructure on mechanical properties such as hardness, yield strength, tensile strength, and ductility has been reported by several investigators.[10–11] The important microstructural features of ADI that influence their mechanical properties are retained austenite content, carbon content of retained austenite, morphology of ferrite, precipitated carbide if any, and the presence of any unstabilized austenite that transformed to martensite. The number of graphite patches, their size, distribution, and nodularity are also important factors. However, these are unaffected by heat treatment conditions
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