Pearlite in Multicomponent Steels: Phenomenological Steady-State Modeling
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, JOHN A˚GREN
, and JOHAN JEPPSSON
A steady-state model for austenite-to-pearlite transformation in multicomponent steel is presented, including Fe, C, and eight more elements. The model considers not only classic ingredients (formation of ferrite–cementite interface, volume diffusion, boundary diffusion, and optimization of lamellar spacing) but also finite austenite–pearlite interfacial mobility that resolves some previous difficulties. A non-Arrhenius behavior of interfacial mobility is revealed from growth rate and lamellar spacing data. A smooth and physical transition between orthopearlite and parapearlite is realized by optimizing the partitioning of substitutional alloying elements between ferrite and cementite to maximize growth rate or dissipation rate while keeping carbon at equilibrium. Solute drag effect is included, which accounts for the bay in growth rate curves. Grain boundary nucleation rate is modeled as a function of chemical composition, driving force, and temperature, with consideration of grain boundary equilibrium segregation. Overall transformation kinetics is obtained from growth rate and grain boundary nucleation rate, assuming pearlite colonies only nucleate on austenite grain boundaries. Further theoretical and experimental work are suggested for generalization and improvements. https://doi.org/10.1007/s11661-020-05679-3 Ó The Author(s) 2020
I.
INTRODUCTION
PEARLITE is a common product of austenite decomposition in steels, typically consisting of alternating lamellae of ferrite and cementite. It is known as a product combining good strength and ductility obtained from relatively simple heat treatment of carbon or low-alloy steels. Pearlitic steels are widely used for steel wire and rail.[1] It is thus of great practical interest to accurately model the overall transformation kinetics of austenite decomposition to pearlite. Quantitative experimental characterizations of pearlite date back to the 1930s.[2] Since then, there has been a compilation of information on the morphology, growth rate, lamellar spacing, partitioning of alloying elements, nucleation, and overall transformation kinetics of pearlite formation (see References 3 through 6 for example). The theory of pearlite formation has its stage set by Zener[7] and has been further developed by Hillert,[8–10] Cahn,[11,12] and others.[13–18] Industrially relevant calculation tools for pearlite include DICTRA as part of Thermo-Calc[19] based on the work of Jo¨nsson,[20] and JMatPro based on the formulation by
JIA-YI YAN, JOHN A˚GREN, and JOHAN JEPPSSON, are with the Thermo-Calc Software AB, Ra˚sundava¨gen 18, 169 67 Solna, Sweden. Contact e-mail: [email protected] Manuscript submitted August 8, 2019. Article published online February 26, 2020 1978—VOLUME 51A, MAY 2020
Kirkaldy et al.,[21] with various levels of predictability and user-friendliness. In this work, we build a new model for pearlite formation, incorporating all major theoretical ingredients previously scattered in literature, with model parameters calibrated to best ava
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