Effect of fast cooling rate on acicular ferrite kinetics in high-strength low-alloy steel

Abstract

This work investigates acicular ferrite (AF) transformation through morphological evolution analysis, kinetic studies, and cooling rate effects using laser scanning confocal microscopy, thermal simulation, and electron backscattered diffraction experiments. The results demonstrate that AF transformation follows a continuous nucleation model and one-dimensional growth model, leading to the development of a new AF kinetic equation. The study quantitatively analyzes cooling rate effects (6.3–19.4 K/s) on the kinetic constant and qualitatively examines its impact on nucleation constant, nucleation number density, and overall AF growth rate. Key findings include: (i) kinetic constant shows quadratic dependence on cooling rate, (ii) nucleation constant and nucleation number density increase with cooling rate, and (iii) overall AF grain growth rate shows no pronounced dependence on cooling rate. These results confirm that higher cooling rates promote finer AF grain formation. Building on this kinetic understanding, the work predicts and verifies prior austenite grain size (PAGS) effect on AF transformation that decreasing PAGS is beneficial to obtain fine AF grains. The comprehensive findings significantly advance fundamental knowledge of AF transformation kinetics.

Keywords

acicular ferrite / kinetic equation / nucleation model / growth model / phase transformation rate

References

[1] Shao Y, Liu C-x, Yan Z-s, et al. . Formation mechanism and control methods of acicular ferrite in HSLA steels: A review [J]. Journal of Materials Science & Technology, 2018, 34(5): 737-744

[2] Loder D, Michelic S K, Bernhard C. Acicular ferrite formation and its influencing factors-a review [J]. Journal of Materials Science Research, 2016, 6(1): 24

[3] Lv S-j, Wu H-h, Wang K-y, et al. . The microstructure evolution and influence factors of acicular ferrite in low alloy steels [J]. Computational Materials Science, 2023, 218: 111989

[4] Babu S S. The mechanism of acicular ferrite in weld deposits [J]. Current Opinion in Solid State and Materials Science, 2004, 8(3-4): 267-278

[5] Xuan C-j, Mu W-zhong. New insights of heterogeneous nucleation and anisotropic growth of acicular ferrite on non-metallic inclusion [J]. Materials & Design, 2022, 221: 110892

[6] Blázquez J S, Romero F J, Conde C F, et al. . A review of different models derived from classical Kolmogorov, Johnson and mehl, and avrami (KJMA) theory to recover physical meaning in solid-state transformations [J]. Physica Status Solidi (b), 2022, 259(6): 2100524

[7] Lei X-w, Huang J-h, Jin X, et al. . Application of Johnson-Mehl-Avrami-Kolmogorov type equation in non-isothermal phase process: re-discussion [J]. Materials Letters, 2016, 181: 240-243

[8] Liu F, Sommer F, Bos C, et al. . Analysis of solid state phase transformation kinetics: Models and recipes [J]. International Materials Reviews, 2007, 52(4): 193-212

[9] Mu W-z, Hedström P, Shibata H, et al. . High-temperature confocal laser scanning microscopy studies of ferrite formation in inclusion-engineered steels: A review [J]. JOM, 2018, 70(10): 2283-2295

[10] Wang L-b, Wan X-l, Hu C-y, et al. . High-temperature confocal laser scanning microscopy analysis of phase transformation of steels: A review [J]. Journal of Iron and Steel Research International, 2025, 32(2): 353-363

[11] Lei X-w, Yang R-b, Xiao L-l, et al. . The expanded lever rule on the phase transformation rate curve for phase volume fraction calculation in thermal simulations [J]. Metallurgical and Materials Transactions A, 2020, 51(10): 5049-5055

[12] Lei X-w, Huang J-h, Chen S-h, et al. . Expanded lever rule for phase volume fraction calculation of high-strength low-alloy steel in thermal simulation [J]. Metallurgical and Materials Transactions A, 2016, 47(6): 2795-2803

[13] Lei X-w, Huang J-h, Chen S-h, et al. . Method for assessing grain boundary density in high-strength, high-toughness ferritic weld metal [J]. Metallurgical and Materials Transactions A, 2017, 48(1): 198-207

[14] Liu Y-n, Hou T-p, Yan Z, et al. . The effect of element characteristics on bainite transformation start temperature using a machine learning approach [J]. Journal of Materials Science, 2023, 58(1): 443-456

[15] Wan X L, Wei R, Wu K M. Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone [J]. Materials Characterization, 2010, 61(7): 726-731

[16] Jiang C-l, Lei X-w, Lai C-bin. Effects of prior austenite grain size and fast cooling rate on lower bainite kinetics of high-strength low-alloy steel [J]. Journal of Central South University, 2026, 33(3): 1028-1049

[17] Luzginova N V, Zhao L, Sietsma J. Bainite formation kinetics in high carbon alloyed steel [J]. Materials Science and Engineering: A, 2008, 481–482: 766-769

[18] Quidort D, Brechet Y J M. A model of isothermal and non isothermal transformation kinetics of bainite in 0.5% C steels [J]. ISIJ International, 2002, 42(9): 1010-1017

[19] Blázquez J S, Borrego J M, Conde C F, et al. . Extension of the classical theory of crystallization to non-isothermal regimes: Application to nanocrystallization processes [J]. Journal of Alloys and Compounds, 2012, 544: 73-81

[20] Blázquez J S, Conde C F, Conde A. On the use of classical JMAK crystallization kinetic theory to describe simultaneous processes leading to the formation of different phases in metals [J]. International Journal of Thermal Sciences, 2015, 88: 1-6

[21] Díaz-Fuentes M, Iza-Mendia A, Gutiérrez I. Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior [J]. Metallurgical and Materials Transactions A, 2003, 34(11): 2505-2516

[22] Flower H M, Lindley T C. Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures [J]. Materials Science and Technology, 2000, 16(1): 26-40

[23] Qiu W-j, Lei X-w, Zhang Y, et al. . Perimeter procedure to produce average equivalent area grain size [J]. Ultramicroscopy, 2023, 253: 113802

[24] Rees G I, Bhadeshia H K D H. Thermodynamics of acicular ferrite nucleation [J]. Materials Science and Technology, 1994, 10(5): 353-358

[25] Wang T-t, Yang S-f, Li J-s, et al. . Effect ofin situobservation of cooling rates on acicular ferrite nucleation [J]. High Temperature Materials and Processes, 2022, 41(1): 181-190

[26] Shi L, Yan Z-s, Liu Y-c, et al. . Effect of acicular ferrite on banded structures in low-carbon microalloyed steel [J]. International Journal of Minerals, Metallurgy, and Materials, 2014, 21(12): 1167-1174

[27] Yang Y-k, Zhan D-p, Lei H, et al. . In situ observation of acicular ferrite nucleation and growth at different cooling rate in Ti-Zr deoxidized steel [J]. Metallurgical and Materials Transactions B, 2019, 50(6): 2536-2546

[28] Liu F-c, Wang Q, Li J-w, et al. . Systematic study on orientation relationships between acicular ferrite and Ti-Mg oxide at different cooling rates in low-carbon steel [J]. Materials Characterization, 2021, 181: 111503

[29] Zhao H-t, Palmiere E J. Effect of austenite grain size on acicular ferrite transformation in a HSLA steel [J]. Materials Characterization, 2018, 145: 479-489

[30] Cai C, Mu W-zhong. Utilization of non-metallic inclusion to induce intragranular acicular ferrite formation contributing to “oxide metallurgy”: Effect of Ti/Al content on the microstructure evolution [J]. Steel Research International, 2025, 96(8): 2400360

[31] Blázquez J S, Caballero-Flores R, Manchón-Gordón A F, et al. . A practical analysis for decelerated growth processes to get physically meaningful kinetic parameters from classical nucleation and growth theory despite of overgrowth [J]. Journal of Non-Crystalline Solids, 2023, 610: 122305

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