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<p>In this investigation, an attempt has been made to enhance the mechanical properties of modified 9Cr-1Mo steel. For this purpose, two sets of modified 9Cr-1Mo steel samples, namely Set-III and Set-IV, were prepared from hot-rolled modified 9Cr-1Mo steel. Both sets were austenitized at 1000 °C for 1 h. Then, the samples were austempered for 6 h at two isothermal temperatures, 460 °C (Set-III) and 480 °C (Set-IV), respectively, followed by water-quenching. Different fractions of bainitic/martensitic microstructure phases were observed in both cases. Subsequently, all the austempered samples were tempered at 760 °C for varying time durations. A broad range of materials characterization techniques, including XRD, SEM, TEM, and mechanical testing, were carried out. The results revealed that lower austempering temperature (460 °C) promoted a finer bainitic ferrite structure with higher dislocation density and better phase distribution, leading to improved strength–ductility balance. The size and volume fraction of precipitated carbides were found to be highly sensitive to tempering time, with a secondary hardening peak observed at 10 min due to the optimal Cr/Fe ratio in <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({M}_{23}{C}_{6}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>M</mi> <mn>23</mn> </msub> <msub> <mi>C</mi> <mn>6</mn> </msub> </mrow> </math></EquationSource> </InlineEquation> carbides. EBSD analysis showed higher low-angle boundary content in Set-III samples, contributing to better strain accommodation and toughness. Overall, the sample austempered at 460 °C and tempered for 30 min exhibited substantial improvement in ductility (13.26%), tensile toughness (10.28%), and hardness (7.50%), along with increased strength (1.38%), compared to the as-received steel. The study demonstrates that controlled bainite–martensite phase tuning and carbide evolution via optimized thermal routes can significantly enhance the mechanical performance of P91 steel.</p>

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Enhanced Strength–Ductility Balance in Modified 9Cr-1Mo Steel via Optimized Austempering and Tempering Processes

  • 期刊论文

  • 发表日期：

    2025年8月20日

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  • Satish Kumar,
  • J. Bhagyaraj,
  • K. Mondal,
  • S. Sangal

摘要

In this investigation, an attempt has been made to enhance the mechanical properties of modified 9Cr-1Mo steel. For this purpose, two sets of modified 9Cr-1Mo steel samples, namely Set-III and Set-IV, were prepared from hot-rolled modified 9Cr-1Mo steel. Both sets were austenitized at 1000 °C for 1 h. Then, the samples were austempered for 6 h at two isothermal temperatures, 460 °C (Set-III) and 480 °C (Set-IV), respectively, followed by water-quenching. Different fractions of bainitic/martensitic microstructure phases were observed in both cases. Subsequently, all the austempered samples were tempered at 760 °C for varying time durations. A broad range of materials characterization techniques, including XRD, SEM, TEM, and mechanical testing, were carried out. The results revealed that lower austempering temperature (460 °C) promoted a finer bainitic ferrite structure with higher dislocation density and better phase distribution, leading to improved strength–ductility balance. The size and volume fraction of precipitated carbides were found to be highly sensitive to tempering time, with a secondary hardening peak observed at 10 min due to the optimal Cr/Fe ratio in \({M}_{23}{C}_{6}\) M 23 C 6 carbides. EBSD analysis showed higher low-angle boundary content in Set-III samples, contributing to better strain accommodation and toughness. Overall, the sample austempered at 460 °C and tempered for 30 min exhibited substantial improvement in ductility (13.26%), tensile toughness (10.28%), and hardness (7.50%), along with increased strength (1.38%), compared to the as-received steel. The study demonstrates that controlled bainite–martensite phase tuning and carbide evolution via optimized thermal routes can significantly enhance the mechanical performance of P91 steel.