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REVIEW ON PRODUCTION PROCESSES AND MECHANICAL PROPERTIES OF DUAL PHASE AUSTEMPERED DUCTILE IRON


A. Basso and J. Sikora National University of Mar del Plata, Mar del Plata, Argentina Copyright © 2012 American Foundry Society Abstract


During the last few years, researchers have sought new ductile iron (DI) applications focusing on the develop- ment of mixed microstructures, such as ferritic-bainitic or ferritic-martensitic. These kinds of structures result in DI with a good combination of mechanical properties, compared to other conventional DI. The combination of properties offered by the mixed structure DI, particu- larly in “dual phase austempered ductile iron” (ADI) has opened new horizons for cast iron to replace steel castings and forgings in many engineering applications. This new DI has a particular microstructure composed


Introduction


The engineering community is constantly pushed towards lighter, stronger and stiffer metallic parts, which is why the worldwide production of ductile iron (DI) has increased dur- ing the last few decades. Ductile iron’s advantage is that parts can be obtained by casting technologies, making it possible to produce high resistant parts of complex shape at relatively low cost. For this reason, DI has been successfully used to replace cast and forged steels in many applications.1


If the graphite phase is adequate, the mechanical properties of cast DI parts depend largely on the matrix microstructure (type, amount and distribution of microconstituents). On the other hand most casting defects (inclusions, micro-shrink- age, and carbides) are present mainly in the last-to-freeze regions. The final microstructure of DI can be conveniently modified by using a wide range of heat treatments, such as ferritizing, normalizing, quenching and tempering and austempering, to obtain ferritic, pearlitic, martensitic and ausferritic matrices.2,3


Recently, researchers and producers


have left no stone unturned in their search for new DI ap- plications, focusing on the development of DI with mixed microstructure: ferritic-bainitic or ferritic-martensitic. These kinds of microstructures make DI a good combination of mechanical properties and physical characteristics, proving advantageous over other conventional DI.4-8


Ductile iron researchers are currently working to enhance DI properties, searching for new applications in the criti- cal parts market where high strength, elongation until fail-


International Journal of Metalcasting/Winter 2012


of different amounts and morphologies of ausferrite and free ferrite.


Dual phase ADI microstructures have afforded more op- portunities for DI applications, acquiring better combina- tions of strength, ductility and toughness. This review de- scribes the methodologies used to obtain this new kind of DI and the principal mechanical properties.


Keywords: ductile iron, dual phase ADI, microstructure, thermal cycles, mechanical properties


ure and toughness are pressing requirements. In this regard, a new type of DI, called “dual phase austempered ductile iron” (ADI) or “dual matrix ductile iron” has become an ac- tive field of research and development. The dual phase ADI matrix is composed of different amounts and morphologies of ausferrite (regular ADI microstructure) and free (or allot- riomorphic) ferrite, which are obtained by subjecting DI to special heat treatments.


The technological interest awoken by this new approach has motivated the execution of several studies to determine the production process and mechanical properties of dual phase ADI. This article summarizes some of the extensive efforts made over the last few years in this respect.


Methodologies to Obtain Dual Phase ADI Intercritical Interval


To understand the different methodologies used to obtain dual phase ADI, it is necessary to review some concepts regarding the “intercritical interval”. Figure 1-a shows a sketch of pseudo binary diagrams (which are cuts from the F-C-Si ternary diagram) where the phases in thermodynam- ic equilibrium of free graphite cast irons are represented.9 At temperatures close to the eutectoid, Figure 1-a depicts a region where ferrite, graphite and austenite coexist. This region is called intercritical interval, and it is delimited by the upper and lower critical temperatures. Such tempera- tures define the starting point at which ferrite transforms into austenite and austenite into ferrite in heating and cool-


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