On the Decomposition of Martensite during Bake Hardening of Thermomechanically Processed Transformation-Induced Plastici
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I.
INTRODUCTION
FOR decades, the tempering of martensite has attracted the attention of scientists and engineers from both fundamental and applied perspectives. As a result of tempering, carbides precipitate in hard, brittle martensite that is supersaturated in carbon, leading to improved ductility and fracture toughness.[1–6] It is well accepted that there are four stages of tempering of ferrous martensite: (1) formation of transition carbide, presumably e-carbide, Fe2.4C, or g-carbide, Fe2C; (2) decomposition of retained austenite into ferrite and cementite; (3) replacement of transition carbide by cementite, Fe3C; and (4) secondary hardening manifested by the development of alloy carbides in alloy steels.[1–6] However, prior to tempering of martensite, room-temperature aging or autotempering of martensite formed at high temperatures on quenching takes place.[3,4,7–10] This process is associated with carbon segregation to dislocations and twin boundaries, to and from retained austenite films, the formation of a periodic tweed structure consisting of carbon modulations in Ni-containing martensites, and carbon clustering before the precipitation of iron-carbide.[7–11] Several experimental techniques, such as electrical resistivity measurements, X-ray diffraction (XRD), transmission electron microscopy, Mo¨ssbauer spectroscopy, and atom probe field ion microscopy, have been used to understand the mechanisms and stages of aging and E.V. PERELOMA, Professor of Physical Metallurgy and Director of the BlueScope Steel Metallurgy Centre, is with the School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, NSW 2522, Australia. Contact e-mail: elenap@uow. edu.au M.K. MILLER, Distingueshed R&D Staff, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6136. I.B. TIMOKHINA, Research Academic, is with the Centre for Material and Fibre Innovation, Faculty of Science and Technology, Deakin University, Geelong, Victoria, Australia, 3217. Manuscript submitted March 16, 2008. Article published online October 21, 2008 3210—VOLUME 39A, DECEMBER 2008
tempering in low- and high-carbon martensitic steels. Previous atom probe studies of martensite decomposition have used atom probe field ion microscopes, which have a severe limitation on the volume of material analyzed.[9–17] Development in recent years of the latest generation of atom probes, namely, the local electrode atom probe, now allows for much faster data gathering from much larger volumes than was possible 5 to 7 years ago.[18,19] This development results in the direct determination of chemical composition, shape, and distribution of fine microstructural features present in the material, even in low densities, such as nanoscale precipitates, clusters, and other solute segregations. The aim of this work is to gain a more detailed insight into martensite decomposition that resulted from the bake hardening of thermomechanically processed (TMP) transformation-induced plasticity (TRIP) steels with and wi
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