EFFECT OF GRAPHITE INCLUSIONS ON MECHANICAL PROPERTIES OF AUSTEMPERED DUCTILE IRON F. Serban 1a , A. Baczmanski 2 , E. Labbe 1 , K. Wierzbanowski 2 and A. Lodini 1 1 LACM, University of Reims Champagne Ardennes, 7 Bd. Jean Delautre 08000 Charleville Mézières, France 2 Faculty of Physics and Nuclear Techniques, AGH-University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland a) florin.serban@univ-reims.fr , b) baczman@novell.ftj.agh.edu.pl Abstract Recently, austempered ductile iron (ADI) has emerged as a new class of ferrous materials and represents a major achievement in cast iron technology [1]. The mechanical strength and impact toughness of nodular iron are provided by the precipitation of the graphite phase as spheroids surrounded by ferrite (bull’s-eye structure) in a continuous pearlite matrix. The quality of ductile iron increases with the number of the graphite spheroids. A high spheroids volume fraction, which is mainly controlled by the inoculation process, limits the chemical segregation during solidification and ensures the structural homogeneity of the component. In this work, a lower value of Young modulus was obtained when the graphite phase was taken into account in the self-consistent modelling. For 12% of graphite the theoretical Young modulus agrees with the measured one (mechanical tensile test). The volume fraction of graphite was confirmed independently by micrographic observation (14%). It can be concluded that the macroscopic behaviour of ADI steel can be modelled by the self-consistent approach in which the austeno-ferritic aggregate is represented by an effective matrix, while instead of the graphite spherical empty spaces are introduced. Using such an approach it was shown that in the elasto-plastic range of deformation, presence of graphite phase caused stress relaxation. Keywords: self-consistent model, austempered ductile iron, graphite influence 1. AUSTEMPERED DUCTILE IRON The development of Austempered Ductile Iron (ADI) is a major achievement in cast iron technology [1]. Austempered Ductile Iron has emerged as a major engineering material in recent years because of its excellent mechanical properties as high strength with good ductility, good wear resistance and resistance to fatigue [2]. In some cases, the properties of ADI are superior to the forged steels [3]. The ADI is obtained by a classic treatment of the nodular iron. The mechanical strength and impact toughness of nodular iron are provided by the precipitation of the graphite phase as spheroids surrounded by ferrite (bull’s-eye structure) in a continuous pearlite matrix. The quality of ductile iron increases with the number of the graphite spheroids. The higher the number of spheroids (or nodule count), the better quality of castings: a high spheroids count, which is mainly controlled by the inoculation process, limits the chemical segregation during solidification and ensures the structural homogeneity of the component [4]. The process of austempering for ductile cast iron is similar to steel and requires an adequate period of time to be maintained, first at the austempering temperature allowing matrix of ductile iron to be austenitized completely, and then rapidly Materials Science Forum Vols. 490-491 (2005) pp. 73-78 online at http://www.scientific.net © 2005 Trans Tech Publications, Switzerland Licensed to Serban (florin.serban@univ-reims.fr) - University of Reims Champagne Ardennes - France All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 194.254.193.77-17/03/05,12:47:26)