超超临界汽轮机转子钢高温超高周疲劳破坏机理与寿命预测

Rotor steel, which is used for ultra-supercritical steam turbine, are subjected to the high temperature(600℃) environment and long term cyclic loading during service life. Several papers indicate that fatigue-related mechanisms are responsible for the most part of mechanical failures of rotor steel. Therefore, in order to ensure its security and reliability, the rotor steel should be tested to evaluate the influence of the high temperature environment on its long-term performence. In this project, taking the initiation and development of failure, damage cumulated effect and its evolution rule of rotor steel in at elevated temperature under cyclic loading as the route, the long life fatigue performance including fatigue strength and fatigue crack propagation of rotor steel at elevated temperature will be studied systematically. The relationship between stress and fatigue life (S-N curve) will be quantitatively illustrated. A reliable prediction model of fatigue life will be proposed based on full understanding of high temperature fatigue crack initiation and propagation mechanisms and damage evolution rules. The effect of environment (temperature, loading frequency and stress ratio) on the fatigue behaviors of rotor steel will be investigated. Furthermore, from the viewpoint of material science and mechanics, the relationship between fatigue damage and variation of microstructure will be analysed. In the end, model for fatigue life prediction will be analyzed to explore an optimum method.

超超临界汽轮机转子钢在服役期间要承受高温（600℃）环境与疲劳载荷的长期作用，为了保证汽轮机转子的可靠性和安全性，需要对其在服役高温环境中的长期服役力学行为进行评估和寿命预测。本项目以交变载荷作用下转子钢材料在高温环境中失效破坏的成因和发展、损伤累积效应以及演化规律为主线，通过对汽轮机转子钢材料在高温环境下的超长寿命疲劳性能和疲劳微裂纹扩展行为进行系统试验研究，定量描述应力寿命(S-N)关系，阐明高温条件下疲劳裂纹萌生和扩展机理、损伤演变规律，建立合理的寿命预测模型。并研究环境（温度、载荷频率、加载方式）变化对超超临界汽轮机转子钢超高周疲劳性能的影响规律。另外，从材料和力学角度，深入研究高温疲劳加载后，汽轮机转子钢的疲劳损伤与微观结构变化之间的关系和规律。最后寻求更好的转子钢高温超高周疲劳寿命预测模型。

超超临界汽轮机转子在40年的服役期内，在高温环境下承受疲劳载荷高达10^10-10^11周次，为了评估汽轮机转子的安全性和可靠性，项目针对转子在高温环境下的超高周疲劳强度评估和寿命预测进行深入研究。. 项目成功研制了高温超声疲劳实验系统,可以实现高温(>1000℃)环境条件下超高周次（>10^10周次）的疲劳加载。采用超声振动(20k Hz)和旋转弯曲(50 Hz)两种实验方法，研究了两种具有代表性的新型的9%-12%Cr汽轮机转子钢CrMoW和CrMoCo，在高温（600℃、620℃）环境下的超高周疲劳（10^6-10^10周次）破坏行为。研究发现超高周疲劳裂纹萌生于材料内部的夹杂物，并详细分析了夹杂物对超高周疲劳行为的影响。计算得到常温与高温条件下疲劳裂纹扩展门槛值分别为：3.4MPm1/2和1.15 MPm1/2；采用有限元方法对汽轮机转子进行全尺寸三维建模，详细分析温度场、热应力、旋转离心力和重力引起的超高周疲劳载荷。在材料疲劳数据的基础上，综合考虑转子尺寸、表面加工情况和平均应力等因素，进行修正后得到转子构件的超高周疲劳S-N曲线。 最后，给出了汽轮机转子超高周疲劳强度和寿命计算方法。计算结果表明，转子超高周疲劳载荷对应的疲劳寿命损耗约为0.1。超高周疲劳载荷对汽轮机转子裂纹扩展的日历寿命有非常大的影响，不能忽略不计。因此，在汽轮机转子设计阶段，需要评估超高周疲劳载荷对转子疲劳强度和寿命的影响。. 研究成果对汽轮机转子钢高温超高周疲劳破坏行为、高温条件下的短裂纹扩展的具有科学理论意义，对超超临界汽轮机转子构件的设计和寿命评估具有重要工程意义。