Key Takeaways & Executive Findings
- •• Increasing Si content from 1.38wt% to 2.67wt% increases graphite particle number density but decreases their average size, while Fe3C particles continuously decrease in both number density and size. • Higher tempering temperature (715°C vs 680°C) promotes graphite growth and accelerates Fe3C decomposition and refinement in steels with graphite precipitation, but only coarsens Fe3C in Si-lean steel without graphite. • Fe–0.58C–1.0Al steel with 1.89wt% Si exhibits significantly lower hardness than other Si contents, especially after tempering at 715°C, achieving nearly HV 20 lower hardness than previously reported Fe–0.55C–2.33Si steel. • The study demonstrates a silicon-saving, aluminum-containing free-cutting steel design that balances graphitization and hardness, offering improved machinability and environmental benefits over traditional S/Pb-based steels.
Abstract
In order to avoid poor machinability caused by excessive hardness under high-silicon conditions in the traditional free-cutting graphited steel, it is important to develop a suitable silicon-saving, aluminum-containing free-cutting steel. This study investigated the microstructure and graphite precipitation behavior of Fe–0.58C–1.0Al (wt%) steels with varying silicon contents (0.55wt%–2.67wt%) after tempering at different temperatures (680°C, 715°C). The tempering structure and the precipitation behavior of graphite and Fe3C in Fe–0.58C–1.0Al steels were systematically studied by optical microscopy (OM), field emission scanning electron microscopy (FESEM), and electron microprobe analyzer (EPMA). The results showed that, at both tempering temperatures, the microstructure of 0.55wt% Si steel is ferrite + granular Fe3C, and the microstructures of 1.38wt%–2.67wt% Si steels are ferrite + petaloid graphite + granular Fe3C. With increasing Si content from 1.38wt% to 2.67wt% at constant tempering temperature, the number density of graphite particles increases, though their average size decreases. Meanwhile, the number density and average size of Fe3C in experimental steels continuously decrease with the increase of Si content. For 0.55wt% Si steel without graphite precipitation, increasing tempering temperature promotes the accumulation and growth of Fe3C. For 1.38wt%–2.67wt% Si steels with graphite precipitation, higher tempering temperature promotes graphite particles growth while accelerating the decomposition and refinement of Fe3C. Furthermore, compared with the experimental steels containing 0.55wt% Si, 1.38wt% Si, and 2.67wt% Si, the 1.89wt% Si steel exhibits significantly lower hardness. Especially, when tempered at 715°C, Fe–0.58C–1.0Al steel with 1.89wt% Si exhibits enhanced graphitization behavior and reduced hardness, which is nearly HV 20 lower than previously reported Fe–0.55C–2.33Si steel.
1. Introduction
With the rapid development of the manufacturing industry and the popularity of computer numerical control (CNC) machine tools, the demand for free-cutting steel used in mechanical structures has grown substantially in recent years [1–2]. Currently, sulfur-based and lead-based free-cutting steel remains the most widely used and the largest quantity of free-cutting steel both domestically and internationally [3]. However, the toxicity of S and Pb elements raises significant environmental and health concerns. During smelting, recycling, and wet cutting processes, these elements generate harmful fumes and wastewater that pose risks to both human health and ecosystems. Therefore, exploring a new lead-free, low-sulfur type of free-cutting steel has become a primary research focus for steel companies and metallurgical scholars [4–5].
Free-cutting graphited steel has attracted widespread attention from scholars and manufacturers in recent years due to its superior machinability, cold-forming property, and fatigue property [6]. Research institutions and production units, such as the University of Leeds [7–8], Kyungpook National University [9], University of Science and Technology Beijing [10], JFE Company [11–12], and POSCO Company [13], have successively developed free-cutting graphited steel. The main research and development focus on promoting the rapid decomposition of cementite in the steel to obtain the free-cutting graphite phase.
Si is commonly used as an alloying element in ductile iron and free-cutting graphited steel due to its ability to promote graphitization and stabilize ferrite [14–15], and it typically maintains a mass fraction of at least 2.3%. However, such high Si content can lead to excessively hardness in graphited steel and compromised machining performance [16]. Therefore, under the premise of ensuring sufficient complete graphitization behavior in the steel, reasonably reducing the Si content to achieve moderate hardness has become an important direction for upgrading the machinability. Similar to the Si, Al can also promote graphitization and stabilize ferrite while having a minimal impact on the hardness of the steel [17]. Samek et al. [18] pointed out that the addition of 1wt% Al to steel can effectively promote graphitization. However, at high Al contents, the extent to which the silicon content can be decreased to achieve an optimal balance remains to be explored.
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Yong Wan, Lijie Tian, Qing Tang, Jianwei Hou, Fengyou Qi, Xingli Zhang, Jinzhong Zuo, Yonghong Wen (2025). Effect of Si content and tempering temperature on microstructure and precipitation behavior of graphite particles in Fe–0.58C–1.0Al steel. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3115-9
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Frequently Asked Questions
What is the effect of silicon content on graphite precipitation in Fe–0.58C–1.0Al steel?
Increasing silicon content from 1.38wt% to 2.67wt% increases the number density of graphite particles but decreases their average size, while the number density and size of Fe3C particles continuously decrease.
How does tempering temperature affect the microstructure of Fe–0.58C–1.0Al steel?
Higher tempering temperature (715°C vs 680°C) promotes graphite particle growth and accelerates Fe3C decomposition and refinement in steels with graphite precipitation. In Si-lean steel (0.55wt% Si) without graphite, it promotes accumulation and growth of Fe3C.
Which silicon content provides the best hardness reduction in Fe–0.58C–1.0Al steel?
The 1.89wt% Si steel exhibits significantly lower hardness compared to 0.55wt%, 1.38wt%, and 2.67wt% Si steels, especially after tempering at 715°C, achieving nearly HV 20 lower hardness than previously reported Fe–0.55C–2.33Si steel.
Why is aluminum added to free-cutting graphited steel?
Aluminum promotes graphitization and stabilizes ferrite while having minimal impact on hardness, allowing reduction of silicon content to achieve moderate hardness and improved machinability.
What are the environmental benefits of this new free-cutting steel?
The steel is lead-free and low-sulfur, reducing toxic fumes and wastewater during smelting, recycling, and wet cutting processes, thus mitigating environmental and health risks associated with traditional S/Pb-based free-cutting steels.
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