Microstructural Evolution of a C-Mn Steel During Hot Compression Above the Ae 3
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ynamic transformation (DT) of austenite to ferrite has been shown to take place over the entire single phase austenite region.[1–3] This unusual phenomenon has been accounted for thermodynamically using the concept of transformation softening.[4] In this approach, the driving force for transformation is the flow stress difference between the work-hardened austenite and the yield stress of the fresh Widmansta¨tten ferrite that takes its place. The obstacle energies opposing the transformation consist of the free energy difference between the austenite and ferrite and the work required to accommodate the shear displacements and dilatation associated with the transformation. Once the driving force is greater than the total barrier energy, the transformation can take place. More recently, the method described above was employed to calculate the metastable Fe-C phase diagram applicable to a particular steel.[5] In that work, the shear displacement and dilatation energies were added to the Gibbs energy of the ferrite and the driving force to CLODUALDO M. ARANAS, Jr., Postdoctoral Researcher, YUJACK SHEN, Undergraduate Student, SAMUEL F. RODRIGUES, Ph.D Student, and JOHN J. JONAS, Professor, are with the Materials Engineering Department, McGill University, Montreal, Canada. Contact e-mail: [email protected] Manuscript submitted May 2, 2016. Article published online July 14, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
that of the austenite. Although such ‘dynamic’ phase diagrams can be readily calculated, less is known about the kinetics of DT. The aim of the present work was to determine the ‘kinetics’ of the transformation applicable to the steel of Reference 5. For this purpose, hot compression tests were performed above the Ae3 to establish the dependence of the volume fraction of the DT ferrite formed in this way on strain and temperature. By using this information, an isothermal strain–temperature–transformation (STT) diagram was generated. The results obtained are described below. Hot compression tests were carried out under an argon atmosphere on the C-Mn steel of Reference 5 using a 100 kN MTS machine equipped with a radiation furnace and a temperature controller. The samples were machined into cylindrical specimen, 6 mm in diameter and 9 mm in length. The chemical composition of the steel is presented in Table I, which includes the paraequilibrium and orthoequilibrium Ae3 temperatures.[6] The thermomechanical schedule employed in the tests is displayed in Figure 1. The samples were heated at the rate of 1 K/s up to test temperatures of 1173 K, 1203 K, and 1233 K (900 °C, 930 °C and 960 °C). These were then held for 10 minutes before deforming them to strains in the range 0.15 to 0.75 applied at the rate of 1 s 1. After straining, samples were immediately quenched and prepared for examination using optical microscopy. Finally, the phase fractions were measured with the aid of the MagniSci software. The compression flow curves determined in this way are illustrated in Figure 2. The solid circles identi
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