High-efficiency thermal and nuclear power plants require creep-resistant steel capable of reliable service for several decades in steam pipes and other high-temperature structures. It is crucial to accurately predict long-term creep behavior and rupture in such components. The existing prediction methods are often not satisfactory. Many studies have traditionally focused on creep deformation rather than the creep damage and microstructural degradation that lead to creep rupture. Classical approaches like Continuum Damage Mechanics (CDM), usually calibrated on short-term high-stress tests, fail to predict low-stress long-term behavior. Similarly, time–temperature parameter (TTP) extrapolation methods lack a physical basis and have limited predictive power outside the range of fitted data. Thus, a new physically based creep life prediction model with robust extrapolation capability is needed to ensure safe long-term operation of high-temperature structures.To address this research gap, this study focuses on Grade 91 creep-resistant steel and aims to develop a new creep life prediction model based on grain-boundary cavitation, with the capability to extrapolate life predictions to lower stress levels. First, a quantitative analysis of the effects of three different creep damage mechanisms on creep behavior is conducted in the range of 125MPa to 80MPa. The constant Ks (solid solution depletion) and Kp (particle coarsening) are shown to have stress dependence. This provides a different explanation of a widely used assumption in CDM-based creep equation developments. Second, a modified hyperbolic sine law minimum creep rate equation is applied to Grade 91 steel across a wide stress range (1MPa-350MPa). This model achieves higher accuracy over a wide stress–temperature range than traditional creep rate equations and its predictions of creep rate show good agreement with long-term data. The extrapolation curves from high stress to low stress follow the trend of the experimental data. The Monkman–Grant relationship is used together with the modified hyperbolic sine law (MHS) equation to predict creep life (35MPa-360MPa), and the simulated results show good agreement with experimental data. It shows that the MHS equation can effectively extend and revitalize the creep life predictive capability of traditional empirical models.Third, based on the previous work of our group, a cavitation-based creep rupture model for Grade 91 steel has been further modified (formulated with and without an explicit temperature dependence). This cavitation-based model shows improved life extrapolation to lower stress compared to conventional models, addressing the stress-dependence of creep damage. The extrapolation result shows good agreement with experimental data. Additionally, the cavitation model’s parameters were calibrated for Grade 91 steel, providing a preliminary quantitative description of cavity nucleation and growth in this material. Overall, this study provides a novel cavitation-based approach for creep life prediction with extrapolation capability. This study contributes to the creep lifetime modeling and extrapolation over a wide stress range.
| Date of Award | 18 Dec 2025 |
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| Original language | English |
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| Supervisor | Qiang Xu (Main Supervisor) & Helen Miao (Co-Supervisor) |
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