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<p>In this study, austempering experiments were performed on a 300M medium-carbon high-silicon low-alloy steel at temperatures near the martensite-starting temperature (<i>M</i><sub><i>S</i></sub>). The phase-transformation kinetics was tracked in real time by a high-precision dilatometer and combined with multi-scale characterization using color metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic mechanism of the pre-formed martensite on bainite transformation was systematically revealed. It is found that the pre-formed martensite interface provides preferential nucleation sites for bainite at a sub-<i>M</i><sub><i>S</i></sub> isothermal temperature, which increases the initial nucleation rate to 4.3 × 10<sup>−3</sup>/s (at 280&#xa0;°C) and shortens the incubation period. Due to the presence of martensite, the final volume fraction of bainite decreases significantly with the decrease in temperature. The improved Ravi-kinetic model based on the displacement mechanism shows that the pre-formed martensite has no effect on the grain-boundary nucleation. However, it can reduce the activation energy of autocatalytic nucleation, thereby accelerating autocatalytic nucleation. This catalytic effect decreased after about 3 min. The difference between the grain-boundary nucleation-activation energy and autocatalytic-activation energy is due to the energy constraint of α/γ incoherent and α/α coherent interfaces. The results show that the high content of Si (1.54&#xa0;wt.%) can effectively inhibit the precipitation of carbides. By establishing a quantitative relationship model between the bainite carbon content and phase-transformation kinetics, the accurate decoupling of the grain-boundary nucleation and autocatalytic nucleation rate is realized. It provides key theoretical support for the development of shorter heat treatment processes of aviation steels and opens up a new dimension of the strengthening and toughening design of multiphase structures.</p>

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Bainite Transformation Kinetics in Medium-Carbon Steel: Sub-MS and Supra-MS Austempering

  • 期刊论文

  • 发表日期：

    2026年4月17日

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  • Jinhao Zhao,
  • Zheng Zhang,
  • Xi Jin,
  • Peter K. Liaw,
  • Junwei Qiao

摘要

In this study, austempering experiments were performed on a 300M medium-carbon high-silicon low-alloy steel at temperatures near the martensite-starting temperature (MS). The phase-transformation kinetics was tracked in real time by a high-precision dilatometer and combined with multi-scale characterization using color metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic mechanism of the pre-formed martensite on bainite transformation was systematically revealed. It is found that the pre-formed martensite interface provides preferential nucleation sites for bainite at a sub-MS isothermal temperature, which increases the initial nucleation rate to 4.3 × 10−3/s (at 280 °C) and shortens the incubation period. Due to the presence of martensite, the final volume fraction of bainite decreases significantly with the decrease in temperature. The improved Ravi-kinetic model based on the displacement mechanism shows that the pre-formed martensite has no effect on the grain-boundary nucleation. However, it can reduce the activation energy of autocatalytic nucleation, thereby accelerating autocatalytic nucleation. This catalytic effect decreased after about 3 min. The difference between the grain-boundary nucleation-activation energy and autocatalytic-activation energy is due to the energy constraint of α/γ incoherent and α/α coherent interfaces. The results show that the high content of Si (1.54 wt.%) can effectively inhibit the precipitation of carbides. By establishing a quantitative relationship model between the bainite carbon content and phase-transformation kinetics, the accurate decoupling of the grain-boundary nucleation and autocatalytic nucleation rate is realized. It provides key theoretical support for the development of shorter heat treatment processes of aviation steels and opens up a new dimension of the strengthening and toughening design of multiphase structures.