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Incomplete Bainite Transformation Accompanied with Cementite Precipitation in Fe-1.5(3.0)%Si-0.4%C Alloys |
Huidong WU1,2, Goro MIYAMOTO3, Zhigang YANG1,2(), Chi ZHANG1,2, Hao CHEN1,2, Tadashi FURUHARA3 |
1 Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2 Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing 100084, China 3 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan |
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Cite this article:
Huidong WU, Goro MIYAMOTO, Zhigang YANG, Chi ZHANG, Hao CHEN, Tadashi FURUHARA. Incomplete Bainite Transformation Accompanied with Cementite Precipitation in Fe-1.5(3.0)%Si-0.4%C Alloys. Acta Metall Sin, 2018, 54(3): 367-376.
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Abstract Steels containing bainite microstructure are widely applied in various industrial areas. Incomplete bainite transformation is frequently used to control volume fraction of retained austenite as well as its stability and is also closely related to bainite growth mechanism. It is generally accepted that incomplete bainite transformation could occur when carbide precipitation is absent. On the other hand, some new studies revealed that carbide with very fine size were observed in “carbide-free” bainite in Si added steels. Our previous study on bainite isothermal transformation kinetics together with its microstructural evolution with Fe-1.5(3.0)%Si-0.4%C alloys (mass fraction) at 400~500 ℃ found incomplete bainite transformation phenomenon for the 3.0Si alloy at 450 ℃ and for two alloys at 400 ℃. In contrast with the generally accepted view, cementite precipitation with Si depletion was observed at the beginning of incomplete transformation stage. Further analysis on three dimension atom probe results revealed that the carbide volume fraction as well as amount of C atoms in carbide hardly changes during incomplete transformation stage. Thermodynamic analysis revealed that small chemical driving force for cementite precipitation and/or the necessity of Si partition are two factors accounting for the extremely slow cementite precipitation kinetics. It is thus proposed that incomplete bainite transformation and carbide precipitation could co-exist. Conditions for incomplete bainite transformation are modified as follows. Firstly, bainitic ferrite growth is stopped before reaching equilibrium fraction. In addition, carbide precipitation should be absent or its kinetics should be slow enough.
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Received: 03 July 2017
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Fund: Supported by National Natural Science Foundation of China (No.51471094) |
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