| 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... |
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