【作者】 李煜佳；

【导师】 涂善东； 轩福贞；

【作者基本信息】 华东理工大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 汽轮机末级钛合金长叶片技术是新型大容量、高效率汽轮机组开发的关键,然而由于复杂应力、湿蒸汽和腐蚀环境等问题,其疲劳寿命与可靠性是长叶片设计面临的难题。本文围绕超超临界汽轮机末级Ti-6A1-4V长叶片的疲劳设计问题,系统研究了应力比、湿蒸汽和盐溶液区环境、显微组织和表面加工性能对其疲劳强度和断裂机理的影响,建立了叶片疲劳寿命设计的Goodman曲线。主要研究内容和结论如下：(1)基于升降法,进行了5个应力比下(R=-1、-0.6、-0.3、0.1和0.6)钛合金Ti-6A1-4V在室温空气环境下的疲劳极限的测试(对应于107循环次数),得到了末级叶片疲劳寿命设计的Goodman曲线。研究了不同加工方法(车削加工和轴向表面抛光及消应力热处理)对Goodman曲线的影响。结果表明,随着应力比的提高,残余压应力对Goodman曲线的影响逐步减小并消失；与消除残余应力后试样疲劳破坏主要起源于表面相比,表面残余压应力使得低应力比下疲劳源于内部；Ti-6A1-4V疲劳裂纹的萌生是自由表面滑移和内部双相组织非均匀循环塑性损伤累积两种机制相互竞争的结果。(2)研制了低氧饱和蒸汽、氯化钠溶液环境与高频疲劳试验机相结合的环境疲劳试验系统。系统开展了末级钛合金Ti-6A1-4V叶片在模拟服役环境下(低氧饱和蒸汽、氯离子环境)对应于107循环次数的疲劳强度实验,获得其在模拟服役环境下的Goodman曲线。研究发现,模拟服役环境下Ti-6A1-4V的Goodman曲线低于室温空气环境下的结果；当应力比为R=0.1时,服役环境导致的疲劳极限降低最显著,低氧饱和湿蒸汽环境下下降约12.3%,而80℃、3.5%氯化钠溶液环境下导致疲劳极限降低约9.6%；与空气环境下的疲劳不同,环境疲劳导致的疲劳裂纹均源于表面,且未见点蚀坑损伤。(3)随动硬化和各向同性硬化Chaboche模型与晶体有限元相结合,分析了钛合金Ti-6A1-4V中α相与p相的细观循环响应。结果表明,α相晶粒比β相更易产生较大的塑性应变,二者变形不协调导致的界面应变集中是疲劳裂纹萌生的根源；α相的体积分数越低,局部塑性变形不协调降低,使得钛合金Ti-6A1-4V的抗循环塑性变形能力增加；β相的晶粒尺寸减小,可有效延迟疲劳裂纹萌生。(4)相比于消应力后的试样,车削加工导致表面存在显著的残余压应力,进而导致钛合金Ti-6A1-4V的Goodman曲线显著提高,且影响随着应力比的增加而逐渐减小。XRD残余应力测量与循环塑性有限元模拟表明,Ti-6A1-4V车削残余应力的大小和分布以及随循环次数松弛程度与施加的载荷水平、应力比有关。当应力比R=-1时,疲劳循环导致表面残余压应力松弛且保持稳定,其产生的表面裂纹闭合是疲劳强度提高的主要机制；当R=-0.6～0.1时,未见疲劳循环导致表面残余应力松弛,由于表面层的平均应力减小使得疲劳破坏主要为内部缺陷和局部应力集中控制。(5)把概率统计方法引入钛合金叶片Goodman曲线的构建,建立了基于可靠度的模拟环境下(空气室温环境、100℃低氧饱和蒸汽环境)以及加工残余应力影响的钛合金叶片疲劳设计Goodman曲线；进一步,建立了钛合金叶片用双参数Haigh修正疲劳等寿命曲线,相比于Goodman图及Gerber拟合方法,所提出的等寿命曲线与实验数据更吻合。更多还原

【Abstract】 Development of long last-stage blades is the one of the core technologies for modern steam turbines with large capacity and high efficiency. However, fatigue life and reliability are the critical problems for long blades design due to complicated stress state, wet steam and aggressive environment. The scope of this dissertation focuses on fatigue design issues of Ti-6A1-4V alloy blades, which has been used for long last stage in an ultra super critical steam turbine. The effects of stress ratio, steam and sodium-chloride (NaCl) aqueous environments, material microstructure and surface properties on the fatigue strength and fracture mechanisms of Ti-6A1-4V alloy have been studied. A Goodman diagram for fatigue design of the blade is presented. The main contents and conclusions are as follows:(1) Axial force-controlled fatigue tests were conducted at stress ratios of-1,-0.6,-0.3,0.1, and0.6on Ti-6A1-4V specimens in air at room temperature. The staircase method was employed to obtain accurate values of fatigue limit at each stress ratios and the Goodman diagrams corresponding to101cycles. The effect of mechanical processing method on Goodman diagrams was investigated, with particular emphasis on the hard turning plus polishing with and without vacuum stress relieve anneal carried out after polishing. Results indicate that beneficial effect of residual compressive stress decreases and vanishes finally with increasing of stress ratio. Compared to fatigue crack originating from surface for annealed specimens, the fatigue crack initiation sites are located in the interior of the specimen due to the effect of residual stress when low stress ratios are present. The mechanism of fatigue crack initiation of Ti-6A1-4V owes to the competition of slipping at free specimen surface and cyclic plasticity damage accumulation inside the specimen.(2) An environment assisting fatigue testing system was developed for investigating the fatigue properties in saturated steam with low oxygen concentration and sodium-chloride (NaCl) aqueous. In order to investigate the influence of simulated environments on Goodman diagrams, fatigue experiments were performed in saturated steam with low oxygen concentration at100℃and NaCl aqueous at80℃. For0.1stress ratio loading conditions, steam environment demonstrated the most serious effect on the endurance limit, and the reduction of fatigue strength was12.3%. The environment of NaCl aqueous also produced a9.6%drop in fatigue strength. For all corrosion environments, cracks originated from surface and no corrosion pits were observed.(3) By combining crystal finite element method with Chaboche model which is simultaneously contained kinematic hardening and isotropic hardening rules, the mechanical properties of α/β grains undergoing cyclic loadings were analysed. The simulation results indicate that the plastic strain accumulated more in a grains than that in β grains. The incompatible deformation induces stress concentration at α-β interfaces, thereby promoting fatigue crack initiation at these sites. As the volume fraction of a grains decreases, the area containing incompatible deformation shifts down, so as to increase cyclic deformation resistance of Ti-6A1-4V. The probability of fatigue crack initiation falls off rapidly asβ grain size decreases.(4) Compared to annealed specimens, the surface compressive residual stresses induced by the turning could improve the fatigue performance defined by Goodman diagram of Ti-6A1-4V. The improvement in fatigue strengths seems to decrease with stress ratio increasing. Based on X-ray measurement and finite element simulation, it is manifested that the relaxation of residual stresses at surface and in depth is associated with number of cycles, stress amplitude and stress ratio investigated. At R=-1, the residual stresses were relaxed after several cycles and then maintained stable. The improvement on fatigue strength at this stress ratio is attributed to surface crack closure induced by compressive residual stress. For R=-0.6-0.1, however, the relaxation of residual stress is very limited. The fatigue failure thus occurs as a result of internal defect and local stress concentration due to the decrease of the mean stresses by the larger compressive stress.(5) Considering the effects of environments (in air at room temperature, saturated steam low oxygen concentration at100℃and NaCl aqueous at80℃) and machining residual stresses, Goodman diagram for Ti-6A1-4V is developed for fatigue design of turbine blades by using statistics method. Haigh parameter was proposed to modify fatigue curves for titanium blades design. A comparison of different methods shows that the proposed isochronous curve approach agrees better with experimental results than Goodman and Gerber models.更多还原