Effect of Si content and tempering temperature on microstructure and precipitation behavior of graphite particles in Fe–0.58C–1.0Al steel

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.

Keywords

free-cutting steel / silicon content / machinability / tempering temperature / graphite particle / cementite

References

[1] ZhangRH, YangJ. State of the art in applications of machine learning in steelmaking process modeling. Int. J. Miner. Metall. Mater., 2023, 30112055

[2] SunK, LongHS, HaoFX, ZhouJ. Process development and quality control of chalcogenide free-cutting steel. Shanxi Metall., 2024, 4711199

[3] XuXY, ZhangL, WangZF, TianQR, FuJX, WangXM. Critical precipitation behavior of MnTe inclusions in resulfurized steels during solidification. Int. J. Miner. Metall. Mater., 2024, 3181849

[4] ChenBM, SunW. Typical case study on carbon emission reduction of lead-containing waste recycling enterprises in Hunan province. J. Environ. Eng. Technol., 2025, 152495

[5] TianZH, ShangCL, ZhangCL, et al.. Review of precipitation strengthening in ultrahigh-strength martensitic steel. Int. J. Miner. Metall. Mater., 2025, 322256

[6] AlonsoG, StefanescuDM, GonzalezR, SuarezR. Effect of magnesium on the solid-state nucleation and growth of graphite during annealing of white iron. Int. J. Metalcast., 2020, 143728

[7] HeK, BrownA, BrydsonR, EdmondsDV. An EFTEM study of the dissolution of cementite during the graphitisation annealing of a quenched medium carbon steel. J. Phys. Conf. Ser., 2006, 26111

[8] InamA, BrydsonR, EdmondsDV. Effect of starting microstructure upon the nucleation sites and distribution of graphite particles during a graphitising anneal of an experimental medium-carbon machining steel. Mater. Charact., 2015, 10686

[9] KimYJ, ParkSH. Effect of initial microstructure on graphitization behavior of Fe–0.55C–2.3Si steel. J. Mater. Res. Technol., 2021, 154529

[10] ZhangYJ, HanJT. A study of graphitization of free-cutting steel. Met. Sci. Heat Treat., 2019, 6011–12817

[11] IwamotoT, HoshinoT, MatsuzakiA, AmanoK, KawaberiM. A newly developed unleaded free cutting steel which has both of high fatigue strength and excellent machinability using graphitization of carbon in the steel. Materia Jpn., 2003, 422163

[12] T. Iwamoto and T. Murakami, Bar and wire steels for gears and valves of automobiles: Eco-friendly free cutting steel without lead addition, JFE Tech. Rep., (2004), No. 4, p. 74.

[13] YeJK, SangWB, KimSH, LimNS, ParkSH. Effects of B and Ti addition and heat treatment temperature on graphitization behavior of Fe–0.55C–2.3Si steel. J. Mater. Res. Technol., 2020, 9511189

[14] Y.J. Kim, S.W. Bae, N.S. Lim, and S.H. Park, Graphitization behavior of medium-carbon high-silicon steel and its dependence on temperature and grain size, Mater. Sci. Eng. A, 785(2020), art. No. 139392.

[15] HeK, DanielsHR, BrownA, BrydsonR, EdmondsDV. An electron microscopic study of spheroidal graphite nodules formed in a medium-carbon steel by annealing. Acta Mater., 2007, 5592919

[16] XuH, ZhouLJ, WangWL, YiY. A simple route for preparation of TRIP-assisted Si–Mn steel with excellent performance using direct strip casting. Int. J. Miner. Metall. Mater., 2024, 31102173

[17] EdmondsDV, InamA. An investigation of accelerated graphitisation kinetics to improve the machinability of an experimental medium-carbon free-machining steel. Int. J. Manuf. Process, 2021, 69181

[18] SamekL, De MoorE, PenningJ, De CoomanBC. Influence of alloying elements on the kinetics of strain-induced martensitic nucleation in low-alloy, multiphase high-strength steels. Metall. Mater. Trans. A, 2006, 371109

[19] ZhuR, YangYG, ZhangBZ, et al.. Improving mechanical properties and high-temperature oxidation of press hardened steel by adding Cr and Si. Int. J. Miner. Metall. Mater., 2024, 3181865

[20] AminiS, AbbaschianR. Nucleation and growth kinetics of graphene layers from a molten phase. Carbon, 2013, 51110

[21] SkalandT, GrongØ, GrongT. A model for the graphite formation in ductile cast iron: Part II. Solid state transformation reactions. Metall. Trans. A, 1993, 24102347

[22] RounaghiSA, Kiani-RashidAR. A study on graphitisation acceleration during annealing of martensitic hypereutectoid steel. Phase Transitions, 2011, 8411–12981

[23] YongQLSecond Phase in Steel Materials, 2006, Beijing. Metallurgical Industry Press. 32

[24] GhoshS, RakhaK, RezaS, SomaniM, KömiJ. Atomic scale characterization of carbon partitioning and transition carbide precipitation in a medium carbon steel during quenching and partitioning process. Mater. Today Proc., 2022, 627570

[25] J.N. Huang and D.H. Zhang, Effect of heat treatment on precipitation behavior of second phase and property evolution of martensitic stainless steel, Mater. Today Commun., 37(2023), art. No. 107267.

[26] KingeryWD. Plausible concepts necessary and sufficient for interpretation of ceramic grain-boundary phenomena: II, solute segregation, grain-boundary diffusion, and general discussion. J. Am. Ceram. Soc., 1974, 57274

[27] WangZW, LuWJ, ZhaoH, et al.. Formation mechanism of κ-carbides and deformation behavior in Si-alloyed FeMnAlC lightweight steels. Acta Mater., 2020, 198258

[28] BartlettLN, van AkenDC, MedvedevaJ, IsheimD, MedvedevaNI, SongK. An atom probe study of kappa carbide precipitation and the effect of silicon addition. Metall. Mater. Trans. A, 2014, 4552421

[29] LiDZ, ZhaoXM, ZhangHL, LiJ, HanHB. The effect of network cementite dissolution on the nucleation and growth of prior austenite grains in high carbon low alloy steels. J. Mater. Res. Technol., 2024, 30565

[30] ChipmanJ. Thermodynamics and phase diagram of the Fe–C system. Metall. Trans., 1972, 3155

[31] OikawaK, MitsuiH, EbataT, et al.. A new Pb-free machinable ferritic stainless steel. ISIJ Int., 2002, 427806

[32] LangAHigh Quality Cast Iron and Projectile Body Casting, 1958, Beijing. National Defense Industry Press. 4J. Yuan trans.

[33] T.X. Xu, Z.J. He, X. Han, X. Yang, and X.M. Hou, Exploring inhibition mechanism of Si on cementite nucleation in hypereutectoid steel: Experiments and first-principles calculations, Materials, 17(2023), No. 1, art. No. 223.

RIGHTS & PERMISSIONS

University of Science and Technology Beijing