Key Takeaways & Executive Findings
- •• Increasing cooling rate from 1.0 to 7.0°C/s reduces ferrite proportion from 52vol% to 22vol% at 400°C and refines pearlite lamellae spacing from 1.01 to 0.67 μm at 660°C. • Decreasing prior austenite grain size from 25.23 to 8.92 μm enhances phase transformation driving force, leading to finer pearlite clusters and proeutectoid ferrite. • Raising carbon content from 0.22wt% to 0.37wt% lowers phase transition temperature from 795 to 750°C, increases pearlite proportion from 27vol% to 61vol% at 500°C, and refines pearlite spacing from 1.25 to 0.87 μm at 600°C. • Multiphase field modeling provides a comprehensive understanding of austenite decomposition kinetics and elemental diffusion, aiding in optimizing processing conditions for hypoeutectoid steels.
Abstract
Hypoeutectoid steel, a crucial metal structural material, is characterized by the coexisting microstructure of ferrite and pearlite. Driven by multiphase competition and multicomponent characteristics, the intricate interplay among its composition, processing conditions, and microstructure substantially complicates the understanding of austenite decomposition kinetics and elemental diffusion mechanisms during phase transformations. The present study explores the effects of cooling rate, prior austenite grain size, and C content on the component distribution and microstructure evolution during the austenite decomposition of hypoeutectoid steels to address the aforementioned complexities. Results of a multiphase field model reveal that an increase in the cooling rate from 1.0 to 7.0°C/s leads to a reduction in the ferrite proportion and fine pearlite lamellae spacing from 52vol% to 22vol% at 400°C and from 1.01 to 0.67 μm at 660°C, respectively. Concurrently, a decreased prior austenite grain size from 25.23 to 8.92 μm enhances the phase transformation driving force, resulting in small average grain sizes of pearlite clusters and proeutectoid ferrite. Moreover, increasing the C content from 0.22wt% to 0.37wt% decreases the phase transition temperature from 795 to 750°C and enhances the proportion of pearlite phases from 27vol% to 61vol% at 500°C, concurrently refining the spacing of pearlite layers from 1.25 to 0.87 μm at 600°C. Overall, this work aims to elucidate the complex dynamics governing the microstructural transformations of hypoeutectoid steels, thereby facilitating their wide application across different industrial scenes.
1. Introduction
Hypoeutectoid steels, characterized by their pearlite–ferrite microstructure, are extensively utilized in various industrial applications [1–2], including oil and gas pipelines, heavy-duty trucks, automotive components, bridges, and ships. The cooling phase transformation of hypoeutectoid steel involves the sequential decomposition of austenite: the formation of low-carbon proeutectoid ferrite is initially observed, followed by the decomposition of austenite into a mechanical mixture of alternating lamellar ferrite (F) and lamellar cementite (Fe3C) [3]. The mechanical properties of hypoeutectoid steel essentially originate from the microstructural features of pearlite and ferrite, in which the phase volume fraction of pearlite–proeutectoid ferrite and the lamella spacing within pearlite strongly affect the strength and formability of the products [4–9]. Consequently, a comprehensive understanding of the details of the temporal microstructure during the phase transformation of hypoeutectoid steels is particularly important.
Extensive experiments on the cooling phase transformation of hypoeutectoid steels have been reported [10–22]. For example, Liu et al. [23] utilized an isothermal salt bath to investigate the influence of prior austenite grain size (PAGS) on the volume fraction of proeutectoid ferrite–pearlitic steels and achieve different PAGS values in Fe–2Mn–0.3C hypoeutectoid steels by adjusting the austenitizing temperature. A correlation was observed in which an increase in PAGS from 14 to 77 μm resulted in a corresponding reduction from 39vol% to 8vol% in the proportion of proeutectoid ferrite. Similarly, Hu et al. [24] conducted thermal simulation experiments on 60Si2MnA spring steel to examine the effect of cooling rate on the microstructure. The results indicated that an increase in the cooling rate from 0.5 to 5°C/s resulted in a decrease in the percentage of proeutectoid ferrite from 24vol% to 2vol% while concurrently reducing the spacing of pearlitic clusters and layers. Tomota et al. [25] selected three medium carbon steels with different carbon contents and analyzed their pearlite lamellae spacing and tensile behavior to study the effect of C content on the microstructure and mechanical properties of medium carbon steels. The increase in carbon content from 0.39wt% to 0.77wt% would refine the pearlite lamellae spacing from 0.17 to 0.10 μm and thus enhance the yield strength (0.2% proof stress).
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Kaiyang Wang, Honghui Wu, Shaojie Lv, Linshuo Dong, Chaolei Zhang, Shuize Wang, Guilin Wu, Junheng Gao, Jiaming Zhu, Xinping Mao (2025). Multiphase field modeling of austenite to pearlite–ferrite transformation in hypoeutectoid steel. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-2993-6
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Frequently Asked Questions
What is the effect of cooling rate on the microstructure of hypoeutectoid steel?
Increasing the cooling rate from 1.0 to 7.0°C/s reduces the ferrite proportion from 52vol% to 22vol% at 400°C and refines pearlite lamellae spacing from 1.01 to 0.67 μm at 660°C, as revealed by multiphase field modeling.
How does prior austenite grain size influence phase transformation in hypoeutectoid steel?
Decreasing prior austenite grain size from 25.23 to 8.92 μm enhances the phase transformation driving force, resulting in smaller average grain sizes of pearlite clusters and proeutectoid ferrite.
What is the role of carbon content in the phase transformation of hypoeutectoid steel?
Increasing carbon content from 0.22wt% to 0.37wt% decreases the phase transition temperature from 795 to 750°C, increases the proportion of pearlite from 27vol% to 61vol% at 500°C, and refines pearlite layer spacing from 1.25 to 0.87 μm at 600°C.
What method was used to study the austenite decomposition in hypoeutectoid steel?
The study employed a multiphase field model to simulate the austenite decomposition, capturing the effects of cooling rate, prior austenite grain size, and carbon content on microstructure evolution.
Why is understanding austenite decomposition important for hypoeutectoid steels?
Understanding austenite decomposition is crucial because it determines the final microstructure (ferrite and pearlite) and thus the mechanical properties such as strength and formability, which are essential for industrial applications.
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