Research On The Key Technology Problems Of Low-cycle Fatigue Life Assessment Of Steam Turbine Rotors

The research background to this paper is a project named’life assessment technique of ultra-supercritical steam turbine rotors based on temperature direct detection and coupling analysis of damage-mechanics’ which was supported by the National High-tech Research and Development Program (863). Experimental and theoretical researches on the key technology problems of low-cycle fatigue life assessment of steam turbine rotors are carried out. And then, based on the analysis of assessment of the metal structure integrity standards coming from different countrys, a normal procedure for low-cycle fatigue life assessment of steam turbine rotors is established. The special situations in engineering application are considered in the following two simplified procedures.Firstly, the rotors surface temperature is a critical parameter for rotors thermal-stress analysis and can’t be obtained easily in the tradition way. We propose to detect the rotors surface temperature using infrared radiation (IR) method. The detection values can be used as the first boundary condition for rotors thermal-stress analysis. The effect of many factors on IR temperature measurement in steam turbine were investigated. We are sure that the surface emissivity of rotors is the key factor. Therefore, the experimental apparatus were build up to detect the surface emissivity of rotor steel specimens, which were oxidized in supercritical water vapour. The influence of specimen surface emissivity on IR temperature measurement was studied in detail. The results show that the rotor surface temperature can be detected using IR method by setting fixed emissivity value. An optic viewport must be set in steam turbine to protect the IR thermometer from being destroyed by high parameter water vapour. The results of material experiment show that the fused silica glass can’t be used as optic viewport in steam turbine for the reason of formation of hydroxyl on the glass surface in high parameter water vapour circumstance. The hydroxyl will reduce the accuracy of IR temperature measurement. Sapphire is a suitbale kind of material to be used as optic viewport. At last, the fusion welding technology was proposed to connect sapphire and metal. The numerical computation results show that the thermal-stress between connecting pieces can be tolerated. The probes used in IR temperature measurement are designed.Secondly, cylinder-vapour-rotors is an integrated thermodynamic system in steam turbine. It is difficult to calculate the temperature field of this system for its complex structure and multi-field coupling characteristics. Based on this, cylinder-vapour-rotors was treated as an isolated system in this paper. It is indicated that business software can be used to analyse the thermal-fluid-solid coupling problems of cylinder-vapour-rotors system according to calculation of a simple geometry model. The method of cascade construction simplification was presented for cylinder-vapour-rotors model. The temperature field of this model was calculated in variable working conditions and the value can be use in thermal-stress analysis.Thirdly, life assessment of steam turbine rotors is based on material tests. In this paper, mototonic tensile tests and total strain-controlled low-cycle fatigue tests were carried out to five supercritical turbine rotor steels (two30CrlMol V and three refined12Cr) respectively at540℃and593℃, after which corresponding mechanical properties and strain-life formulas were obtained based on the tests data. Results show that the Vickers hardness of the five rotor steels has reduced by about15at540℃and20at593℃, indicating an obvious cyclic softening behavior. The rules of material property parameters, which are named cyclic strain hardening exponent (n), cyclic strength coefficient (K) and cycle modulus of elasticity (E*) respectively, are analyzed. The great effect of modulus of elasticity on life estimation is discussed at last. It suggests that the estimation accuracy of fatigue life can be improved using modified modulus of elasticity in comparison with the traditional method at540癈.But, this improvement is indistinct at593℃.Fourthly, fatigue properties of materials are essential for the strength design and life assessment of mechanical structures and components. Fatigue properties, such as strain-life, can be estimated using tensile data. Nine methods currently available for fatigue properties from simple tensile data are discussed in detail. The estimation accuracies of these methods were compared according to the tests data of nine types of steam turbine rotor steels. The results show that the values calculated using the nine methods resulted in unacceptable values. In double logarithmic coodinates, the nine method functions were linearly modified, where the predicted lives were all less than quadruple the observed lives, except for the mixed method. A new method, named temperature modified method, that takes the temperature influence into account is presented. The method is suitable for fatigue life estimation of steam turbine rotor steels at high temperature. The estimation results show that the new method can provide acceptable accuracy. On the other side, the S-N curves of stuctures can be obtained by revising that of materials in engineering application. The influence of parameters, such as tensile strength (σb),size factor (CD), surface machining factor (Cs), theoretical stress concentration factor (Kt), notch sensitivity factor (q) and so on, disturbance on fatigue strength when there are no tests data is investigated using parameter sensitivity method. The results show that the tensile strength, size factor and surface machining factor have same sensitivities to the fatigue property index, of which the specific values are all1and must be choosen carefully. The notch sensitivity factor effects minimum on the fatigue strength and can be selected from technical manuals. The theoretical stress concentration factor should be calculated by numerical analysis and elastic-plastic theory. The results can provide selection corrected parameters with theoretical reference.Finaly, a fit-for-engineering procedure for low-cycle fatigue life assessment of steam turbine rotors was established based on above investigation, in which some other assessment procedures for metal structure integrity (such as R6) were used for reference and the national guide of fatigue life assessment of steam turbine is taken into account. This criterion consists mainly of preparation of rotors information, condition evaluation of inservice rotors, setting load spectrum, criterion of life assessment, calculation methods of crack initiation life and expansion life, life assessment reports, and so on. Furthermore, two simplified fatigue life assessment procedures for steam turbine rotors are presented, in which the special conditions in engineering application are take in account.