New Equation for Prediction of Martensite Start Temperature in High Carbon Ferrous Alloys
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odern ferrous alloys properly designed with martensitic microstructure provide favorable property combinations of strength and durability. As martensite start (MS) temperature gives important guidance for compositional and microstructural design, empirical equations for the prediction of MS temperature, which will be simply referred to as MS equation hereafter, have been suggested.[1–18] Classical MS equations are usually expressed as a linear function of the composition of alloying elements,[1–18] meanwhile they have been modified to account for interactions between elements,[8,9] lattice distortion strain,[10] thermodynamic parameters,[11,12] and prior austenite grain size.[12–14] Along with artificial neural network methods,[15,16] a machine learning
JIHYE PARK and JAE-HYEOK SHIM are with the High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea. SEOK-JAE LEE is with the Division of Advanced Materials Engineering, Chonbuk National University, Jeonju, 54896, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted September 4, 2017. Jihye Park and Jae-Hyeok Shim have equally contributed to this work.
METALLURGICAL AND MATERIALS TRANSACTIONS A
technique[17] was recently applied to improve the efficiency for the MS prediction. Although these MS equations cover a wide spectrum of alloying elements throughout compositions, C concentration is usually limited up to 2 wt pct. For high carbon ferrous alloys containing more than 2 wt pct C, a substantial amount of carbide coexists with austenite at temperatures between Ae1 and Ae3 temperatures so that actual C concentration in austenite is much lower than the C composition of a sample during the austenitization heat treatment. Therefore, the MS temperature of high carbon ferrous alloys would be seriously misestimated when the C content of the composition table is simply used in the MS equations. Eiselstein et al.[19] measured the C concentration in austenite by an X-ray diffraction method to predict the MS temperature of high carbon ferrous alloys. As Fe-2.4 C and Fe-3.0 C-1.5 Cr alloys contained 1.67 and 1.43 wt pct C in austenite, respectively, the MS temperatures calculated by the Andrews equation[6] were 77 K ( 196 °C) for the former and 171 K ( 102 °C) for the later. However, the measured MS temperatures are in a range between 403 K and 573 K (130 °C and 300 °C) for ferrous alloys containing 2 to 4 wt pct C,[20] which show a serious discrepancy between measured and calculated MS temperatures. Therefore, the previous MS equations are not suitable for high carbon ferrous alloys, even though the exact C concentration in austenite is taken into account. The objective of this paper is to establish a new equation for the prediction of MS temperature specialized for high carbon (> 2 wt pct C) ferrous alloys. Equilibrium fraction of austenite and carbide phases at austenitization temperature as well as C concentration in austenite are calculated to construct a simple equation for estim
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