【作者】 吕亚玲；

【导师】 张胜寒；

【作者基本信息】 华北电力大学(北京) ， 化工过程机械， 2016， 博士

【摘要】 全世界发电总量的80%左右是由汽轮发电机组发出的,汽轮机是现代化国家中极为重要的动力机械设备,按照国际通用标准,运行超过30年即属于老旧机组,如何对老旧机组的安全性进行合理评判,预测甚至延长在役汽轮发电机组的残余寿命具有重要的理论价值和现实意义。转子是汽轮机的关键部件,长期在高温下高速运行,所受应力复杂,在运行一段时间后不可避免地会出现热脆化破坏,若不能及时检测并修改工艺参数,将严重影响机组运行的安全性。本文采用韧脆转变温度FATT50作为表征转子钢热脆化程度的指标。利用实验室制备的磷含量不同的试样进行研究,分析其化学成分、机械性能等参数认为影响材料FATT50的元素包括碳、硫、磷、锰、钼、硅、铌、钛等,其中影响最大的是磷元素；材料的化学成分、组织和热处理状态的影响是相互关联的；能否发生韧性-脆性转变在很大程度上决定于金属的晶格类型,体心立方晶格的金属的热脆性倾向最大,而面心立方晶格的金属则不具有热脆性；粗晶粒的材料其韧脆转变温度较细晶粒的为高；分析表明步冷热处理对FATT50的提高是有影响的。在研究汽轮机转子热脆化形成机理及特点的基础上,本文提出了用交流阻抗谱方法来评判转子钢材料劣化程度的非破坏性检测方法。交流阻抗谱方法测量的前提是必须满足因果性、线性和稳定性三个基本条件。本文在测量过程中采取屏蔽箱等有效措施排除外界噪音等干扰,使测量尽量满足因果性条件；施加扰动信号幅值为5 mV,满足线性条件：施加的扰动电压很小,在扰动消失后体系恢复到热力学平衡的状态,未过多考虑稳定性问题。本文用钼酸钠溶液做电解液,采用正交实验法确定最佳实验条件为温度25℃、电解液浓度0.01 mol/L、酸碱值pH=5.5。在此条件下对工作电极施加不同的极化电压,研究其界面结构的变化,结果表明随着电极电位的正移,材料在前活化区、过渡区、活化区、钝化区和过钝化区等区域具有不同的电极／溶液界面结构特点。在一系列温度下对试样进行阻抗测量,结果表明在极化电压不变的前提下,温度的变化不会从根本上改变电极/溶液界面结构和性质。在开路电压下极化,研究各试样的阻抗谱,结果表明只要测量频率足够高,界面阻抗就会与材料的FATT50呈现出较强的相关性,该阻抗的变化基本可以反映材料热脆化程度。本文采用SPSS软件进行全回归分析,取测量频率为103Hz时测得的界面阻抗、元素P、S、Cr的百分含量、脆化指数J、相关系数R、晶粒度N、硬度Hv为自变量,FATT50值为因变量,建立转子钢FATT50预测模型,结果表明预测误差在15℃之内,误差较小,该模型合理有效,证实这一技术具有一定的实用性和可靠性。更多还原

【Abstract】 Steam turbine is a very important power machinery equipment in modern country because about 80% of the world’s total electricity is issued by the turbine generator. In accordance with international standard, these units will be aging units after running for more than three decades.The issues how to make a reasonable judgment on their security, prediction even prolong the residual life of the steam turbine in service have important theoretical and practical significance.As a key component of the turbine, the rotor works in a complex environment with high temperatures, high-speed rotation and complex stress. Thermal embrittlement damage will inevitably occur after running for a period of time, which will seriously effect the safe operation of the unit if not change process parameters in time.In this dissertation, the FATT50, fracture appearance transition temperature is adopted as an indicator to characterize the degree of rotor steel. The samples used in the experiments come from Lab existing material, and be analyzed for their chemical composition, mechanical properties and other parameters. As a result of investigation, it is believed that some elements including carbon, sulfur, phosphorus, manganese, molybdenum, silicon, niobium, titanium, etc. would effect FATT50, in which the phosphorus has the greatest influence; microstructure and heat treatment will affect the FATT50 too; ductile-brittle transition largely depends on the type of the metal lattice:body-centered cubic lattice of metal has an embrittlement tendency, face-centered cubic lattice of metal does none of such tendency; FATT50 of the material having a coarse grains is higher than which have finer grains, stress fracture in ductile-brittle transition temperature is also low; step cooling heat treatment can affect the improved of FATT50.On the basis of research of formation mechanism and characteristics of turbine rotor thermal embrittlement, the paper proposes a non-destructive testing electrochemical method to evaluate the degree of deterioration of the rotor steel based on EIS.Premise of AC impedance measurement is causality, linearity and stability, these three basic conditions must be met. In this dissertation, three-electrode is used to measure the impedance spectroscopy of each sample. and some effective measures such as shield box and so are taken to ensure exclude outside noise interference, etc. as far as possible to satisfy the causality condition in the measurement process; the applied perturbation signal amplitude up to 5 mV to ensure the test procedure under linear conditions; small polarizing voltage is applied, and system can be restored to the state of thermodynamic equilibrium after the disappearance of the disturbance, then it did not give much thought to the stability issue.In this dissertation, all experiments were performed using ammonium molybdate solution as electrolyte, the optimum condition is determined with orthogonal experimental method to be the temperature as 25 ℃, the electrolyte concentration as 0.01 mol/L, and electrolyte as pH= 5.5. Under these conditions, different voltages are applied to the working electrode polarization, the results show that as the positive electrode potential shift the material will experience active area, passivation regions and over passive region in the process of polarization, and the electrode/solution interface exhibit different structural characteristics in the three regions.At a series of temperatures, impedance measurements show that under the premise of constant polarization voltage, temperature changes could not underlying alter the electrode/solution interface structure and properties fundamentally.Analysis of the impedance spectroscopies with the open circuit voltage as polarization potential show that the interfacial resistance (ie, the total impedance-solution resistance) and FATT50 have a strong correlation when measured at enough high frequency, and the change in impedance can basically reflect the degree of thermal embrittlement.In this dissertation, SPSS software is used for full regression analysis, take the interfacial resistance measured at a frequency of 103 Hz, impurity elements P, S, percentage of Cr, embrittlement index J, the correlation coefficient R, grain size N, hardness Hv as independent variables, and take the FATT50 value of the dependent variable, a prediction model for FATT50 of the rotor steel is established, the prediction error of the model is confirmed in a range of ±15 ℃, the error is small enough to proof the rationality and Effectiveness. It confirmed that the techniques described In this dissertation has a certain practicality and reliability更多还原