Tensile properties of a thermomechanically processed ductile iron
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INTRODUCTION
DUCTILE iron is a well-recognized engineering structural material, primarily because of its castability at relatively low temperature and attractive mechanical properties. A typical composition is about 3.5 pct C, 2.5 pct Si, 0.4 pct Mn, 0.04 pct Mg, and balance iron.[1] The graphite is typically in nodular form, and therefore, promising mechanical properties can be obtained. Its low tensile strength has been a limiting factor in its use, and austempered ductile iron has been developed.[1,2] The latter material consists of graphite nodules in a matrix of ferrite plus carbide in the form of pearlite or bainite. The structural use of these materials is somewhat limited by unfavorable strength-ductility relations. Generally, when the strength is increased, there is a loss of tensile ductility. Thermomechanical processing of ductile irons is a potential method for enhancing their properties. Some work has been done on ductile irons by researchers who have studied the influence of mechanical working on properties.[3–7] Most of the work has been done at the austenitizing temperature (900 7C) or lower. Recent thermomechanical processing routes have been shown to be effective methods for improving the properties of hypereutectoid steels.[8,9] These techniques have significantly improved the strength and ductility of these materials, normally considered brittle, to become ductile and strong. The present work explores a wider range of thermomechanical processing temperatures (700 7C to 1100 7C) and uses procedures established previously for developing unique microstructure in ultrahigh carbon (hypereutectoid) steels. The rationale for selecting appropriate thermomechanical working steps requires appreciating the phases that exist in C.K. SYN, Engineer/Materials Scientist, and D.R. LESUER, Engineer/Materials Scientist and Group Leader, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. O.D. SHERBY, Professor Emeritus, is with the Department of Materials Science and Engineering, Stanford University, CA 94305. Manuscript submitted September 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ductile iron as a function of temperature. Accordingly, the Fe-graphite and Fe-Fe3C phase diagrams for an Fe-2.5 pct Si-C material are shown in Figure 1. The phase diagram for Fe-graphite (in dotted lines) is from Reference 10, whereas the diagram for Fe-Fe3C (in solid lines) was constructed by interpolation of the diagrams for the Fe-2 pct Si-C[11] and Fe-3 pct Si-C[12] systems. There are several observations from Figure 1 that are relevant to the current study on ductile iron: (1) it defines the amount of C in solution in austenite as a function of temperature (since it is about the same line for both Fe-C and Fe-Fe3C); (2) it defines the boundary above which the ferrite (a) phase disappears (about 900 7C for our 3.5 pct ductile iron); (3) it indicates that above 900 7C, austenite is likely in metastable equilibrium with graphite and iron carbide just as, at low temperature, ferrite is in metasta
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