Conversion Model for the Martensitic Transformation of Banded Austenite in a Ferrite Matrix
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etry is commonly used to investigate microstructural changes in Fe-based alloys. The dilation data provide essential information about the solid-state phase transformations during heat treatments and allow for the control and minimization of distortions in heat treated parts. Based on the calculations of the unit cell structures and the lattice parameters of the relevant phases, several models have been developed to convert the measured transformation strain to the volume fraction of phases transformed during austenite formation or austenite decomposition.[1–21] These models provide better predictions of the volume fraction of transformed phases compared to those obtained by means of the lever rule. Most of the available models focus on the austenite decomposition during cooling.[1–15,17,20,21] These models have been applied to the decomposition of austenite during cooling from a high austenitizing temperature where austenite was the only stable phase. Recently, Mola et al.[22] analyzed the dilation curve of a hot-rolled stainless steel containing 16 mass pct Cr with a SEOK-JAE LEE, formerly Research Assistant Professor with Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea, is now Assistant Professor, with Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea. Contact e-mail: seokjaelee@ jbnu.ac.kr JAVAD MOLA, formerly Postdoctoral Researcher with Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, is now Scientific Assistant with Institute for Iron and Steel Technology, Freiberg University of Mining and Technology, Freiberg, Germany. BRUNO C. De COOMAN, Professor, is with Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology. Manuscript submitted January 5, 2012. Article published online October 23, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
banded microstructure. The coefficient of thermal expansion (CTE) and the strain associated with the formation of martensite clearly indicated a pronounced anisotropic behavior because of the presence of a banded microstructure composed of austenite and ferrite after annealing at different intercritical temperatures. The previous studies about the dilation anisotropy in steels with a banded microstructure focused on the decomposition of the austenite phase cooled from high austenitizing temperature.[14,15,23,24] While the CTE remains constant during cooling from the single austenite stability range, stress partitioning due to differences in CTE takes place when austenite coexists with ferrite during cooling from an intercritical annealing temperature.[22] This makes it impossible to apply existing models to steel with multiple initial phases. The analysis of dilatometry data is further complicated when the martensite transformation leads to an additional strain. In the present study, a new conversion mod
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