<p>Tremendous efforts have been made to accelerate bainite transformation. In this study, the synergistic effect of prior ferrite and Mn heterogeneity on bainite and martensite transformation has been investigated using a two-step intercritical annealing process followed by subsequent austempering or quenching. The classical Koistinen-Marburger (K-M) equation for martensite and a nucleation-controlled kinetic model for bainite are employed to facilitate an in-depth discussion on the contribution of various factors to transformation kinetics, including the K-S orientated ferrite/austenite (<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11661_2025_7782_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="32" /> </InlineMediaObject> <EquationSource Format="TEX">$\alpha /\gamma$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>α</mi> <mo stretchy="false">/</mo> <mi>γ</mi> </mrow> </math></EquationSource> </InlineEquation>) interface, grain size, and alloy content of reverted austenite. The aforementioned synergistic effect is well displayed by the specimen annealed at 770 °C, which exhibits an intermediate prior ferrite fraction but large <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11661_2025_7782_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="32" /> </InlineMediaObject> <EquationSource Format="TEX">$\alpha /\gamma$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>α</mi> <mo stretchy="false">/</mo> <mi>γ</mi> </mrow> </math></EquationSource> </InlineEquation> interfaces per percent ferrite among those intercritically annealed samples. The lamellar and K-S oriented <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11661_2025_7782_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="32" /> </InlineMediaObject> <EquationSource Format="TEX">$\alpha /\gamma$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>α</mi> <mo stretchy="false">/</mo> <mi>γ</mi> </mrow> </math></EquationSource> </InlineEquation> interface effectively increases the number density of nucleation sites, thereby enhancing both martensite transformation and the initial stage of bainite transformation. Besides, when compared to the specimen fully austenized at 850 °C, the promotion of grain boundary and autocatalytic nucleation by Mn heterogeneity, specifically the Mn-depleted regions in austenite where previous prior ferrite was present, can be evidenced by the slower decline in transformation rate during the later stages of transformation process.</p>

The Synergistic Effect of Prior Ferrite and Mn Heterogeneity on Bainite and Martensite Transformation Kinetics

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发表日期：

2025年5月11日

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Naiyou Xiao,
Yuqi Liu,
Tao Jia

摘要

Tremendous efforts have been made to accelerate bainite transformation. In this study, the synergistic effect of prior ferrite and Mn heterogeneity on bainite and martensite transformation has been investigated using a two-step intercritical annealing process followed by subsequent austempering or quenching. The classical Koistinen-Marburger (K-M) equation for martensite and a nucleation-controlled kinetic model for bainite are employed to facilitate an in-depth discussion on the contribution of various factors to transformation kinetics, including the K-S orientated ferrite/austenite ( $\alpha /\gamma$ $α / γ$ ) interface, grain size, and alloy content of reverted austenite. The aforementioned synergistic effect is well displayed by the specimen annealed at 770 °C, which exhibits an intermediate prior ferrite fraction but large $\alpha /\gamma$ $α / γ$ interfaces per percent ferrite among those intercritically annealed samples. The lamellar and K-S oriented $\alpha /\gamma$ $α / γ$ interface effectively increases the number density of nucleation sites, thereby enhancing both martensite transformation and the initial stage of bainite transformation. Besides, when compared to the specimen fully austenized at 850 °C, the promotion of grain boundary and autocatalytic nucleation by Mn heterogeneity, specifically the Mn-depleted regions in austenite where previous prior ferrite was present, can be evidenced by the slower decline in transformation rate during the later stages of transformation process.