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<p>Ultra-high strength oil casing steels are of significant importance for applications in extreme service environments; however, the enhancement of strength often comes at the expense of low-temperature impact toughness. To address this trade-off, various pretreatments were introduced prior to the conventional quenching and tempering (QT) process in this study. The effects of four pre-treatment routes—namely, quenching pretreatment (Q-QT), normalizing pretreatment (N-QT), tempering pretreatment (T-QT), and double quenching and tempering (QT-QT)—on the microstructure and mechanical properties of a V-bearing microalloyed steel were systematically investigated, with the aim of identifying an optimized heat treatment route that balances strength and toughness. The results indicate that all pretreatment routes improved the impact toughness compared to the QT steel. Among them, the QT-QT steel exhibited the most significant enhancement, increasing the impact toughness from 50.5 to 77.2&#xa0;J, representing a 53% improvement. The Q-QT steel achieved an impact toughness of 76.2&#xa0;J, with only a 7&#xa0;MPa reduction in yield strength. Microstructural characterization revealed that the pretreatments promoted the selection of martensite variants, leading to an increased proportion of high-angle grain boundaries (approximately 65% in all cases). Furthermore, the effective grain size was refined, counteracting the coarsening effect of pretreatments on prior austenite grains and thereby enhancing the resistance to crack propagation. The refined lath width contributed to the retention of high yield strength, while the reduced internal stresses and uniformly distributed fine precipitates further improved the impact toughness. This study provides a significant theoretical basis and industrial guidance for the development of high-strength and high-toughness oil casing steels, demonstrating that pretreatment is an economically effective approach for tailoring strength—toughness balance.</p>

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Effect of pretreatment on microstructure and properties of high strength and high toughness steel

  • 期刊论文

  • 发表日期：

    2026年7月9日

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  • Yunkai Wang,
  • Guo Yuan,
  • Jian Kang,
  • Fuchang Xu,
  • Zihan Wu,
  • Mingshan Zhang,
  • Dongsheng Liu,
  • Yunjie Li,
  • Chao Wang

摘要

Ultra-high strength oil casing steels are of significant importance for applications in extreme service environments; however, the enhancement of strength often comes at the expense of low-temperature impact toughness. To address this trade-off, various pretreatments were introduced prior to the conventional quenching and tempering (QT) process in this study. The effects of four pre-treatment routes—namely, quenching pretreatment (Q-QT), normalizing pretreatment (N-QT), tempering pretreatment (T-QT), and double quenching and tempering (QT-QT)—on the microstructure and mechanical properties of a V-bearing microalloyed steel were systematically investigated, with the aim of identifying an optimized heat treatment route that balances strength and toughness. The results indicate that all pretreatment routes improved the impact toughness compared to the QT steel. Among them, the QT-QT steel exhibited the most significant enhancement, increasing the impact toughness from 50.5 to 77.2 J, representing a 53% improvement. The Q-QT steel achieved an impact toughness of 76.2 J, with only a 7 MPa reduction in yield strength. Microstructural characterization revealed that the pretreatments promoted the selection of martensite variants, leading to an increased proportion of high-angle grain boundaries (approximately 65% in all cases). Furthermore, the effective grain size was refined, counteracting the coarsening effect of pretreatments on prior austenite grains and thereby enhancing the resistance to crack propagation. The refined lath width contributed to the retention of high yield strength, while the reduced internal stresses and uniformly distributed fine precipitates further improved the impact toughness. This study provides a significant theoretical basis and industrial guidance for the development of high-strength and high-toughness oil casing steels, demonstrating that pretreatment is an economically effective approach for tailoring strength—toughness balance.