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
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.
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.
Fig.1 Schematic of incomplete bainite transformation in which bainite transformation stops growing before volume fraction of bainite reaching equilibrium value (ICT—incomplete bainite transformation, BF—bainitic ferrite, fBF—fraction of bainitic ferrite, t—time)
Alloy
Mass fraction of element / %
Temperature / ℃
C
Si
Fe
Para-Ae3
NPLE/PLE
T0
Ms
1.5Si
0.375
1.48
Bal.
835
823
674
380
3.0Si
0.379
3.09
Bal.
902
879
688
379
Table 1 Nominal compositions and characteristic temperatures of Fe-1.5(3.0)%Si-0.4%C alloys
Fig.2 SEM images of 3.0Si alloy transformed at 450 ℃ for 30 s (a) and 120 s (b), bright field (c) and dark field (d) TEM images as well as SAED pattern with the key diagram taken from BF region in Fig.2a (e) (θ—cementite, M/A—martensite/austenite constituent) [17]
Fig.3 SEM images of 1.5Si alloy transformed at 400 ℃ for 30 s (a) and 120 s (b) [17], bright field (c) and dark field (d) TEM images as well as SAED pattern with the key diagram taken from BF region in Fig.3a (e)
Fig.4 3DAP measured distributions of C with iso-concentration surfaces at 10%C (a1, b1), 15%C (a2, b2), 20%C (a3, b3) in 3.0Si alloy transformed at 450 ℃ for 30 s (a1~a3) and 120 s (b1~b3)
Alloy
Temperature
Atomic fraction of C
Ratio / %
℃
30 s
120 s
3.0Si
450
>10%
1.23
1.06
>15%
0.57
0.51
>20%
0.37
0.32
3.0Si
400
>10%
1.65
1.64
>15%
0.60
0.63
>20%
0.17
0.17
1.5Si
400
>10%
2.45
1.82
>15%
1.51
1.21
>20%
0.69
0.44
Table 2 Ratios between number of substitutional atoms trapped in iso-concentration surfaces of 10%C, 15%C, 20%C and number of substitutional atoms in bainitic ferrite during incomplete bainite transformation stage (denoted as ratio in table) for 3.0Si alloy transformed at 450 ℃, 3.0Si alloy transformed at 400 ℃ and 1.5Si alloy at 400 ℃
Alloy
Temperature / ℃
Transformation time / s
rγ / %
rα / %
rγ+rα / %
3.0Si
450
30
82.3
6.7
89.0
120
85.8
5.0
90.8
3.0Si
400
30
80.5
7.0
87.5
120
74.1
7.0
81.1
1.5Si
400
30
45.3
5.1
50.4
120
47.7
4.9
52.6
Fig.5 Isothermal phase diagram sections of Fe-Si-C system at 450℃ (a) and 400 ℃ (b) for discussion on cementite precipitation from austenite (Blue points represent 3DAP measured C contents at the beginning of incomplete bainite transformation stage[17] and red arrows represent supersaturation for cementite precipitation; Ortho-Acm and Para-Acm represent the austenite composition when cementite is formed from austenite following full equilibrium condition and para-equilibrium condition[18], respectively; Ortho-Ae3 and Para-Ae3 represent the austenite composition when ferrite is formed from austenite following full equilibrium condition and para-equilibrium condition[18], respectively; NPLE/PLE Ae3 represent NPLE/PLE transition line during austenite to ferrite transformation[19]; Tie-line connects compositions of two phases in full equilibrium)
Fig.6 Enlarged isothermal phase diagram sections of Fe-Si-C system at 450 ℃ (a) and 400 ℃ (b) at low carbon side for discussion on cementite precipitation from bainitic ferrite (Red arrows represent supersaturation for cementite precipitation. Blue lines denoted as CCE represent calculated C content of bainitic ferrite from experimentally measured C content in austenite and assuming same chemical potential of C between austenite and ferrite; Ortho-α/α+θ and Para-α/α+θ represent the ferrite composition when cementite is formed from ferrite following full equilibrium condition and para-equilibrium condition[18], respectively; Para-α/α+γ represent the ferrite composition when ferrite is formed from austenite following para-equilibrium condition[18])
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