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<p>Conventional single-step austempering of ductile iron typically produces coarse ausferrite with little or no proeutectoid ferrite, thereby limiting toughness and ductility. To overcome these limitations, this study introduces a novel two-step austenitizing heat treatment to develop dual-phase austempered ductile iron (ADI) with enhanced mechanical performance. The process combines intercritical austenitizing at 775–795&#xa0;°C with subsequent full austenitization at 925&#xa0;°C, followed by austempering at 350&#xa0;°C. The core innovation lies in the sequential control of phase transformations, which enables precise microstructural engineering by preserving proeutectoid ferrite in the intercritical stage and stabilizing carbon-enriched retained austenite during austempering, while suppressing the formation of martensite. The mechanism underlying the observed property enhancement is rooted in controlled diffusion dynamics, where partial austenitization maintains ferrite, while subsequent heating promotes carbon partitioning and stabilizes the retained austenite. The resulting dual-phase microstructure, composed of ferrite, ausferrite, and retained austenite, provides a strength–ductility balance rarely attainable through conventional ADI processing. Samples treated at 795&#xa0;°C with 60 min of full austenitization exhibited 10% retained austenite, achieving a tensile strength of 1084&#xa0;MPa and 10% elongation. In contrast, prolonged soaking (&gt;60 min) caused grain coarsening and reduced toughness, underscoring the importance of strict time–temperature coordination. Overall, this methodology expands the design space of ADI and offers a scalable strategy to surpass the conventional strength–ductility trade-off, with direct implications for automotive and heavy machinery applications.</p>

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Tailoring Strength and Ductility in Austempered Ductile Iron via a Novel Two-Step Austenitizing: Microstructural and Mechanistic Insights

  • 期刊论文

  • 发表日期：

    2025年10月29日

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  • Mohammad Alamati,
  • Hassan Jafari,
  • Amir Abedi,
  • Ahmad Sadeghzadeh

摘要

Conventional single-step austempering of ductile iron typically produces coarse ausferrite with little or no proeutectoid ferrite, thereby limiting toughness and ductility. To overcome these limitations, this study introduces a novel two-step austenitizing heat treatment to develop dual-phase austempered ductile iron (ADI) with enhanced mechanical performance. The process combines intercritical austenitizing at 775–795 °C with subsequent full austenitization at 925 °C, followed by austempering at 350 °C. The core innovation lies in the sequential control of phase transformations, which enables precise microstructural engineering by preserving proeutectoid ferrite in the intercritical stage and stabilizing carbon-enriched retained austenite during austempering, while suppressing the formation of martensite. The mechanism underlying the observed property enhancement is rooted in controlled diffusion dynamics, where partial austenitization maintains ferrite, while subsequent heating promotes carbon partitioning and stabilizes the retained austenite. The resulting dual-phase microstructure, composed of ferrite, ausferrite, and retained austenite, provides a strength–ductility balance rarely attainable through conventional ADI processing. Samples treated at 795 °C with 60 min of full austenitization exhibited 10% retained austenite, achieving a tensile strength of 1084 MPa and 10% elongation. In contrast, prolonged soaking (>60 min) caused grain coarsening and reduced toughness, underscoring the importance of strict time–temperature coordination. Overall, this methodology expands the design space of ADI and offers a scalable strategy to surpass the conventional strength–ductility trade-off, with direct implications for automotive and heavy machinery applications.