TESTING 1-2-3
New As-Cast Ductile Iron Raises Bar for Properties
Researchers are developing ductile iron that features properties similar to ADI, without heat treatment. SUSANA MENDEZ, U. DE LA TORRE, PELLO LARRANAGA, IK4-AZTERLAN, DURANGO, SPAIN; RAMON SUAREZ, VEIGALAN ESTUDIO 2010, DURANGO, SPAIN; AND DORU M. STEFANESCU, OHIO STATE UNIVERSITY, COLUMBUS, OHIO, AND UNIVERSITY OF ALABAMA, TUSCALOOSA, ALA.
and forged steel in a large number of applications due to its combination of high strength and toughness, in addi- tion to lower density. Because of this, the search for new
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ductile iron alloys with improved mechanical properties and lowered production costs is an important research field. Te ductile iron with the highest resistance/ductility rate is austempered ductile iron (ADI), which gets its superior properties from
ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.
“Processing Thickness Window for As-cast Ausferritic Castings,” Susana Men- dez, U. de la Torre, and Pello Larranaga, IK4-Azterlan, Durango, Spain; Ramon Suarez, Veigalan Estudio 2010, Durango, Spain; and Doru M. Stefanescu, Ohio State Univ., Columbus, Ohio, and Univ. of Alabama, Tuscaloosa, Alabama
Background—One of the key points to achieve as-cast ausferritic microstruc- tures is to define the minimum cooling rate. The goal of the research was to develop an experimental model able to define the thickness window where the as-cast ausferritic microstructures can be guaranteed.
Procedure—To obtain different cooling rates, castings with different thermal moduli (thicknesses) and several geometries were poured and studied to build data for the model.
Results and Conclusions—Based on the results of the experiment, a spreadsheet model was developed to establish if a specific casting, with specific thickness differences, can be produced through engineered cooling and exhibit fully ausferritic microstructures on all sections. The experimental model has been validated with different geometries in a defined range of thermal moduli (0.4-1.5 cm) and for specific range of chemical composition (3-5%Ni; 0-0.2%Mo; 0.1-1%Cu by weight).
uctile iron has a wide range of mechanical properties, depend- ing on its metallic matrix. Te mate- rial can replace cast
its microstructure. Tis microstructure, called “ausferritic,” is different than that of conventional irons. Fine grains of ferrite yield high strength, and the distribution of austenite and ferrite together make ADI more ductile and tougher than conventional irons. ADI achieves its microstructure
through a heat treatment process called austempering. Tis is a well- established method, and ADI parts have replaced hundreds of steel forg- ings and fabrications in production volume levels. As a secondary operation, austem-
pering adds cost and time to casting production. Researchers are investigat- ing a new process to achieve castings
with the same microstructure as ADI (ausferritic) without heat treatment. In this process, engineered cooling is used to coerce the metal to form the desir- able ausferritic microstructure. For end- users, it could mean lower cost, high strength parts with shorter lead times. Te process variables that must
be controlled to achieve the as-cast ausferritic microstructure include the chemical composition of the metal and the cooling rate of the different sec- tions of the casting. Te length of the solidification process and the param- eters of the transformation needed to create the ausferritic microstructure must also be included in the analysis. Researchers first developed a way to
achieve the as-cast ausferritic micro- structure for a single alloy for a specific casting (steering knuckle). However, many automotive castings that are candidates for this technology present geometries with significant thickness variations and consequently different cooling rates. Tese differences can complicate or make impossible the production of fully ausferritic as-cast parts by engineered cooling. In order to utilize engineered cool-
ing for a wider variety of parts in real- world applications, work was needed to develop an experimental model that defines the thickness window in which an ausferritic as-cast microstructure can be achieved without the use of conven- tional austempering heat treatment by chemical composition adjustments.
Question Can a simple method be developed
to determine the process parameters necessary to produce as-cast ausferritic parts with given mechanical properties?
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