Effects of Mo on Carbon Enrichment During Proeutectoid Ferrite Transformation in Hypoeutectoid Fe-C-Mn Alloys
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e local condition at the moving ferrite/austenite interface is of vital importance to understand the nature of proeutectoid ferrite transformation in low carbon alloy steels, as well as the effects of alloying elements. However, although many investigations[1–15] have been performed, this classical problem is still not fully understood. It has long been recognized that proeutectoid ferrite could have the same contents of substitutional elements as the austenite matrix.[1,2] Hultgren[1] proposed the para-equilibrium (PE) model to explain this phenomenon, in which local equilibrium is only achieved for carbon at the interface, while there is completely no redistribution for substitutional elements. In contrast, Hillert[2] then pointed out that ferrite could inherit the bulk alloy contents while still maintaining full local equilibrium (LE) with austenite if a thin alloy spike YUAN XIA, formerly Student with the Department of Metallurgy, Tohoku University, Aoba-ku, Sendai 980-8577, Japan, is now Ph.D. Student with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, People’s Republic of China. GORO MIYAMOTO, Associate Professor, and TADASHI FURUHARA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai, Japan. Contact e-mail: miyamoto@imr. tohoku.ac.jp ZHI-GANG YANG, Professor, and CHI ZHANG, Associate Professor, are with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University. Manuscript submitted January 5, 2015. Article published online March 31, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
builds up in austenite at the interface. This was termed negligible partition local equilibrium (NPLE). In both PE and NPLE models, no bulk redistribution of substitutional alloying elements happens thus transformation kinetics is controlled by carbon diffusion. However, a kinetic transition may occur when carbon enrichment in austenite accompanying the growth of ferrite reaches a certain limit, beyond which long range diffusion of substitutional atoms is expected.[3–5] The transition boundaries are the Para-Ae3 and PLE/NPLE transition lines for the PE and LE models, respectively. Since the diffusivities of substitutional atoms are generally several orders of magnitude smaller than that of carbon, the rate of transformation becomes so sluggish once the kinetic transition takes place. The slow growth of ferrite controlled by diffusion of substitutional atoms can be reasonably neglected for industrially relevant timescales. From this standpoint, the local carbon condition determines an effective maximum extent to which ferrite transformation can proceed, and a better understanding of it is also meaningful for correctly predicting the factions and compositions of transformation products and consequently the mechanical properties of steels as well. A number of previous studies using ferrite precipitation[6–12] or decarburization experime
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