长时间高温服役下 T91 钢微观组织演变与力学性能退化机制

Microstructural evolution and mechanical property degradation mechanism of T91 steel under long-term high-temperature service

摘要

[目的] 揭示长期高温服役对 T91 耐热钢微观组织与力学性能的影响，明确长时间服役后其性能退化的核心机制。[方法] 取未服役(记为 T91-0)、服役 50 000 h(记为 T91-5)和服役 100 000 h(记为 T91-10)的 T91 钢管材，采用光学显微镜(OM)、X 射线衍射仪(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)表征其微观组织，并结合室温冲击、高温拉伸及持久强度试验，评价其力学性能退化行为。[结果] 随着服役时间延长，T91 钢原奥氏体晶界逐渐模糊直至消失，马氏体板条合并粗化，位错密度显著降低，碳化物(M ₂₃C ₆ 和 MX 型)粗化且分布趋于均匀，T91-10 试样中还检测到可观数量的残余奥氏体。长时间服役导致常温下 T91 钢的冲击韧性持续下降，T91-10 较 T91-0 降幅达 62%。在 640 ~ 700 °C 范围内，上述 3 种服役状态的钢的屈服强度和抗拉强度均随温度升高而降低，但 T91-10 的塑性随着温度下降而明显恶化，其 640 °C 下的延伸率与 700 °C 时相比下降约 1/3。[结论] 马氏体板条粗化、位错密度降低及碳化物析出与粗化是导致 T91 钢强度与韧性退化的主要原因。本研究为 T91 钢在高温服役条件下的安全评估与寿命预测提供了理论依据。

Abstract

[Objective] To reveal the effect of long-term service at high temperatures on the microstructure and mechanical properties of T91 heat-resistant steel, and to clarify the core mechanism of its performance degradation after long-term service. [Method] T91 steel pipes that had been in service for 0 hour (i.e. as-received and designated as T91-0), 50 000 hours (designated as T91-5), and 100 000 hours (designated as T91-10) were collected. Their microstructures were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The degradation of mechanical properties was evaluated through room-temperature impact testing, high-temperature tensile testing, and stress-rupture testing. [Result] With the extending of service time, the prior austenite grain boundaries of T91 steel gradually blurred and eventually disappeared, martensite laths merged and coarsened, dislocation density decreased significantly, and carbides (M ₂₃C ₆ and MX types) coarsened and became more uniformly distributed. A considerable amount of retained austenite was detected in the T91-10 specimen. Long-term service led to a continuous decline in the impact toughness of T91 steel at room temperature, with a reduction of 62% for T91-10 compared with T91-0. In the temperature range of 640 to 700 °C, both the yield strength and tensile strength of the steels in all three service states decreased with the increasing of temperature. However, the ductility of T91-10 steel deteriorated markedly with the decreasing of temperature, showing a reduction in elongation at 640 °C of approximately one-third compared to that at 700 °C. [Conclusion] The coarsening of martensite laths, the reduction of dislocation density, and the precipitation and coarsening of carbides are the primary causes of strength and toughness degradation in T91 steel. This study provides a theoretical basis for safety assessment and service life prediction of T91 steel under high-temperature service conditions.

关键词

耐热钢 / 长期服役 / 微观组织演变 / 残余奥氏体 / 力学性能退化 / 高温塑性

Key words

heat-resistant steel / long-term service / microstructural evolution / retained austenite / mechanical property degradation / high-temperature plasticity

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基金资助

河南省科技攻关支持项目(242102320220)

河南省基本科研业务费支持项目(2023KY36)

河南省基本科研业务费支持项目(2023KY19)

河北省自然科学基金(E2025402093)

河北省高等学校科学技术研究项目(CXY2024040)