The Strain-Hardening Behavior of Partially Austenitized and the Austempered Ductile Irons with Dual Matrix Structures

Article

Abstract

In the current study, an unalloyed ductile iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.%, and 0.047 Mg wt.% was intercritically austenitized (partially austenitized) in two-phase regions (α + γ) at different temperatures for 20 min and then was quenched into salt bath held at austempering temperature of 365 °C for various times to obtain different ausferrite plus proeutectoid ferrite volume fractions. Fine and coarse dual matrix structures (DMS) were obtained from two different starting conditions. Some specimens were also conventionally austempered from 900 °C for comparison. The results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high-carbon austenite (retained or stabilized austenite)) has been developed. Both of the specimens with ∼75% ausferrite volume fraction (coarse structure) and the specimen with ∼82% ausferrite volume fraction (fine structure) exhibited the best combination of high strength and ductility compared to the pearlitic grades, but their ductility is slightly lower than the ferritic grades. These materials also satisfy the requirements for the strength of the quenched and tempered grades and their ductility is superior to this grade. The correlation between the strain-hardening rates of the various austempered ductile iron (ADI) with DMS and conventionally heat-treated ADI microstructures as a function of strain was conducted by inspection of the respective tensile curves. For this purpose, the Crussard-Jaoul (C-J) analysis was employed. The test results also indicate that strain-hardening behavior of ADI with dual matrix is influenced by the variations in the volume fractions of the phases, and their morphologies, the degree of ausferrite connectivity and the interaction intensities between the carbon atoms and the dislocations in the matrix. The ADI with DMS generally exhibited low strain-hardening rates compared to the conventionally ADI.

Keywords

ausferrite volume fraction austempered ductile iron (ADI) dual matrix structure (DMS) new ferrite volume fraction partial austenitizing proeutectoid ferrite volume fraction strain-hardening 

Notes

Acknowledgments

The authors wish to acknowledge the financial supports of Gazi University Scientific Research Fund (Project Code 07/2003-41 and 07/2005-25). The author is also indebted to ALFA Casting Company (Ankara, Turkey) for castings and to Ankara Nuclear Research and Education Center (TAEK-ANAEM) for providing X-Ray facilities. My acknowledgements are also extended to the UMIST Manchester Materials Science Centre for the provision of its laboratory facilities and to its staff who have assisted me throughout our study.

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Copyright information

© ASM International 2007

Authors and Affiliations

  1. 1.Materials Division, Metallurgy Education Department, Faculty of Technical EducationGazi UniversityBesevler-AnkaraTurkey

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