Martensitic Transformation Behavior and Ultra-high Toughness Mechanism of Mf Point Quenching Isothermal in the Medium Carbon Steel Containing Si-Al

HU Feng; WANG Tongliang; WANG Kun; QIU Baowen

doi:10.3969/j.issn.1674-6457.2026.04.014

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Journal of Netshape Forming Engineering ›› 2026, Vol. 18 ›› Issue (4) : 136-146. DOI: 10.3969/j.issn.1674-6457.2026.04.014

Iron and Steel Forming

Martensitic Transformation Behavior and Ultra-high Toughness Mechanism of M_f Point Quenching Isothermal in the Medium Carbon Steel Containing Si-Al

HU Feng^1,2,3, WANG Tongliang^1,2,4,*, WANG Kun³, QIU Baowen^1,2,4

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Abstract

In response to the bottleneck problem of difficult synergistic improvement of high strength and ultra-high toughness in traditional medium carbon steel, the work aims to take the medium carbon steel containing Si-Al as the object, to systematically compare the traditional quenching and tempering (QT) with the new M_f point quenching isothermal (QI) process, and reveal the effect mechanism of M_f point (200 ℃) isothermal phase transformation behavior on material strength and toughness, so as to develop structural steels that combine high strength and ultra-high toughness by adjusting the isothermal time to optimize the carbon distribution effect and retained austenite stability. The microstructure was observed through OM and SEM, the width of lath and the morphology of retained austenite were analyzed by TEM, the carbon content of retained austenite was calculated by XRD, and the mechanical properties were evaluated by tensile and impact tests. The microstructure under QT process consisted of tempered martensite and ε-carbides. However, the QI process formed a lamellar structure of isothermal martensite/high carbon retained austenite (1.46%) through carbon partition effect, the volume fraction of retained austenite was stabilized at 5.5%, and the average width of isothermal martensite lath was reduced to (216±95) nm (42.7% finer than QT). The impact absorption energy of QI200-32 reached >130 J, which was 2.7 times that of QT200-32, while a tensile strength of 1 436 MPa was maintained, achieving a synergistic improvement of high strength and ultra-high toughness. The study breaks through the inversion limit of strength and toughness of the traditional QT process, and the M_f point quenching isothermal process regulates the dual phase structure (martensite and retained austenite) through the transformation mechanism dominated by carbon distribution. The ultra-high toughness is attributed to the refinement of isothermal martensite lath, which significantly improves the dislocation blocking effect, and the high carbon retained austenite film absorbs plastic deformation work, deflects and passivates cracks through the TRIP effect. This study provides a new approach for the development of high-strength and ultra-high toughness medium carbon steel. By regulating the isothermal phase transformation behavior of M_f point quenching and optimizing the layer structure and carbon distribution design, the fracture resistance of the material can be significantly improved.

Key words

medium carbon steel containing Si-Al / M_f point quenching isothermal / martensitic transformation / retained austenite / toughness mechanism

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HU Feng, WANG Tongliang, WANG Kun, QIU Baowen. Martensitic Transformation Behavior and Ultra-high Toughness Mechanism of M_f Point Quenching Isothermal in the Medium Carbon Steel Containing Si-Al[J]. Journal of Netshape Forming Engineering. 2026, 18(4): 136-146 https://doi.org/10.3969/j.issn.1674-6457.2026.04.014

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