复杂应力下电站部件蠕变损伤及微观组织演变的研究

发布时间：2018-08-02 14:04

【摘要】：随着全球范围内能源紧张、环境恶化、气候变暖等问题的日益突出,发展低能耗、低排放、低污染的低碳科技已经成为人们的共识。超(超)临界发电机组发电效率高、污染物排放低,是我国电力行业发展低碳科技的重要组成之一。然而,大容量、高参数的机组意味着电站部件的服役条件更为恶劣,由于几何尺寸、载荷类型、所处环境等影响,实际构件大多受到复杂应力状态作用。据统计,蠕变是过热器、再热器及其联箱等部件发生失效的主要原因之一。而P92钢以其优良的焊接性、高温强度和蠕变性能等,成为超(超)临界燃煤发电机组高温受热面的常用钢之一。研究复杂应力状态下P92钢的蠕变损伤扩展和微观组织演变规律是对燃煤发电机组高温部件进行寿命预测的关键部分,对发电机组安全经济运行具有重要的意义。本文以高温蠕变试验、蠕变本构模型和有限元数值模拟互为验证为技术路线,开展对P92钢复杂应力状态下高温蠕变损伤扩展及微观组织演变的研究。论文的具体工作如下:首先,完成不同温度、不同应力水平下标准光滑试样的单轴蠕变试验,从微观角度上分析光滑试样的蠕变断裂机制;建立Norton-Bailey and Kachanov-Robotnov蠕变本构模型,明确模型的参数确定方法,并对其有效性进行验证,为后续研究提供基础数据库。其次,进行多轴蠕变试验及模型的研究。完成不同应力、不同缺口锐度下的双缺口蠕变试验,从宏观和微观两个尺度上对蠕变结果进行分析;采用修正后的Kachanov-Robotnov模型预测P92钢缺口试样蠕变损伤的发展过程,得到试样内部应力与损伤的分布及演变规律;基于应变耗竭理论,引入延性耗竭的概念,将连续损伤力学与孔洞长大理论相结合,建立可以描述P92钢蠕变行为的延性耗竭模型,并完成对延性耗竭模型的验证;结合蠕变试验微观结果与有限元数值模拟结果,对比分析了多轴度对P92钢断裂韧性、微观孔洞和硬度分布的影响。再次,基于P92钢高温蠕变的损伤机理,引入具有实际物理意义的损伤变量,建立基于微观机制的多损伤变量蠕变本构模型,确定出模型参数,并对模型的有效性加以验证;进行P92钢光滑试样和双缺口试样的蠕变中断试验,对蠕变过程中的析出相、孔洞进行定量化统计;采用该模型对P92钢缺口试样的蠕变损伤演变进行预测,损伤值的预测结果与P92钢中断试验结果吻合良好,进一步证明了该模型的有效性。最后,设计全尺寸的管道弯头蠕变试验,现已完成大部分试验开始前的准备工作,如应变、温度的测量等;采用有限元数值计算软件探究不等壁厚、初始椭圆度对管道弯头蠕变损伤扩展的影响规律,以期为后续的全尺寸管道弯头的蠕变试验提供数据基础。
[Abstract]:With the global energy tension, environmental degradation, climate warming and other issues increasingly prominent, the development of low energy consumption, low emissions, low pollution low-carbon technology has become a consensus. Supercritical generating units have high generation efficiency and low pollutant emission, which is one of the important components in the development of low-carbon technology in China's power industry. However, the large capacity and high parameter units mean that the service conditions of the components of the power station are worse. Due to the influence of geometry, load type and environment, the actual components are mostly affected by the complex stress state. According to statistics, creep is one of the main reasons for the failure of superheater, reheater and its header. For its excellent weldability, high temperature strength and creep properties, P92 steel has become one of the most common steels for supercritical coal-fired generating sets. The study of creep damage propagation and microstructure evolution of P92 steel under complex stress is a key part in predicting the life of high temperature components of coal-fired generating units, which is of great significance to the safe and economical operation of generating units. In this paper, the creep damage propagation and microstructure evolution of P92 steel under complex stress state are studied by means of creep test at high temperature, creep constitutive model and finite element numerical simulation. The specific work of this paper is as follows: firstly, uniaxial creep tests of standard smooth specimens at different temperatures and different stress levels are completed, and creep fracture mechanism of smooth specimens is analyzed from a microscopic point of view, and Norton-Bailey and Kachanov-Robotnov creep constitutive model is established. The method of determining the parameters of the model is defined, and its validity is verified, which provides the basic database for further research. Secondly, the multiaxial creep test and model are studied. The creep test of two notches under different stress and notch acuity was completed, and the creep results were analyzed from macroscopic and microscopic scales, and the creep damage process of notched specimens of P92 steel was predicted by modified Kachanov-Robotnov model, and the creep damage of P92 steel notched specimens was predicted by using the modified Kachanov-Robotnov model. Based on strain exhaustion theory, the concept of ductility depletion is introduced to combine the theory of continuous damage mechanics with the theory of pore growth to establish a ductile depletion model which can describe the creep behavior of P92 steel. The effects of multiaxiality on fracture toughness, micropore and hardness distribution of P92 steel were compared and analyzed by combining the microscopic results of creep test and the results of finite element numerical simulation. Thirdly, based on the damage mechanism of high temperature creep of P92 steel, the creep constitutive model of multiple damage variables based on microscopic mechanism is established, and the validity of the model is verified. The creep interruption tests of smooth and double notched specimens of P92 steel were carried out, the precipitated phases and voids in creep process were quantitatively counted, and the creep damage evolution of notched specimens of P92 steel was predicted by the model. The predicted damage value is in good agreement with the test results of P92 steel, which further proves the validity of the model. Finally, the design of full-scale pipe bend creep test, has now completed most of the preparatory work before the start of the test, such as strain, temperature measurement, using finite element numerical calculation software to explore the unequal wall thickness, The influence of initial ellipticity on creep damage propagation of pipe elbows is expected to provide a data basis for subsequent creep tests of full-scale pipe bends.
【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM621

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