Corrosion Failures in Gas Turbine Hot Components

At elevated temperatures hot corrosion often takes place in gas turbines and other combustion turbomachinery. The main corrosion issues are high-temperature environmental attack in the form of high-temperature oxidation and hot corrosion. Two case histories of gas turbine hot components are presented. The mechanisms for different types of hot corrosion are discussed. The influence of temperature, composition, and microstructure of materials on hot corrosion is described. A variety of practical approaches to minimize hot corrosion are considered.     

Stress-Rupture Characterization in Nickel-Based Superalloy Gas Turbine Engine Components

Today’s gas turbine engines utilize high volume fraction gamma prime (γ′) strengthened alloys for turbine airfoils, which typically operate at temperatures greater than ∼0.5T _m of the alloy. At these temperatures and at stresses below yield, time-dependent deformation (creep) of the airfoil can occur and, if left unabated, can result in complete separation of the airfoil. This process is commonly referred to as stress rupture. Insufficient cooling air, unintentional interruptions of cooling air as well as abnormal engine operating conditions are typical causes of stress-rupture failures in gas turbine blade components. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries. Other signs of overtemperature include partial resolutioning of the γ′ strengthening precipitates, with the remaining volume fraction of γ′ commonly used to estimate blade metal temperatures. This article highlights the visual, fractographic, and metallographic characteristics typically encountered when analyzing stress rupture of turbine airfoils. An erratum to this article can be found at     

A Novel Methodology to Estimate Life of Gas Turbine Components Under Multiaxial Variable Amplitude Loading

A novel and highly effective methodology is presented in this study to estimate the stresses and strains and also the life of gas turbine components operating in multiaxial variable amplitude loading conditions. The methodology uses a cyclic plasticity model based on multilinear kinematic hardening (MLKH) for estimation of stress-strain response and Wang and Brown algorithm for counting the reversals in the loading cycle. The stress and strain response extracted for each reversal using the MLKH model where then integrated with multiaxial fatigue damage model based on critical planes (Wang and Brown model) suitable for LCF applications, to predict the fatigue life. The proposed methodology was initially compared with experimental test results of 42CrMo4 low alloy steel specimen, under different loading conditions like proportional, non-proportional and sequential loadings available in the literature. To reinforce the life prediction capability of the methodology, an application level study was undertaken. An air-cooled high-pressure turbine disk of an aero gas turbine engine was used as the model for this study. The fatigue life obtained from the multiaxial fatigue damage model was then compared with the experimental LCF life of the disk obtained from the field data. In order to be conservative in approach, lower bound of the 95% confidence limit of the fatigue data fitted using Weibull probability distribution function was used to compare the numerical life. The study shows a good correlation between the fatigue life arrived experimentally and the predicted life using the proposed methodology.     

GW93镁合金点蚀过程的原位监测及点蚀机制

镁合金作为最轻的金属结构材料有很多优异性能,但镁自身化学性质活泼,耐蚀性差,尤其易发生点蚀,破坏性和隐患性非常大。若想降低点蚀对镁合金部件安全服役性能的影响,就需要对镁合金点蚀机制有清楚的认识。然而,适用于其他金属材料的经典的点蚀机制是以形成氧浓差电池为基础,阴极发生的是氧还原反应,而镁合金阴极发生的是析氢反应,因此镁合金的点蚀形成过程尚需深入研究。采用扫描振动电极技术(SVET)原位监测了铸态GW93镁合金在3. 5%NaCl(质量分数)溶液中的点蚀过程,采用SEM观察了腐蚀过程镁合金微观形貌变化,采用电流-时间曲线对比了阴阳极电位对点蚀发展的影响。研究结果表明,点蚀坑外是微阴极,发生析氢反应,点蚀坑内是微阳极,发生镁的溶解反应,随着时间增加,点蚀发展过程是动态变化的。镁合金中第二相所导致的微电偶腐蚀加速效应及氯离子在腐蚀坑内的聚集,两者的协同作用驱动了点蚀不断向基体内部生长。     

N80钢点蚀行为的研究

采用电化学噪声(ECN)法,原子力显微镜(AFM)原位观察,研究了N80钢在以0.5 mol/LNaHCO3为基础液加入NaCl溶液中的点蚀行为.结果表明,点蚀的噪声随Cl-浓度增大,单位时间内点蚀特征峰增多,峰值变大,寿命变长;N80钢表面状态的变化与电化学噪声谱的特征有明显相关性.     

Analysis of Low-Pressure Turbine Nozzle Guide Vane Failure in an Aero Gas Turbine Engine: A Computational Approach

Journal of Failure Analysis and Prevention - Failure of low-pressure turbine nozzle guide vane (NGV) in an aero gas turbine engine is analyzed using computational approach. Breakage in the cooling...     

Ni-Cu-P钢耐点蚀性能的机理研究

选择Ni-Cu-P钢和碳钢各两种,在pH=10的3%(质量分数)NaCl溶液中进行极化实验,比较钢的点蚀诱发敏感性;在3%海盐水中进行间浸挂片实验,评价钢的点蚀扩展速率;利用扫描电镜(SEM)、电子探针(EPMA)、X射线衍射(XRD)分析钢中夹杂物、腐蚀形貌和锈层的特征。结果表明:Ni-Cu-P钢比碳钢表现出更弱的点蚀诱发敏感性和更小的点蚀扩展速率。较弱的脱氧可降低碳钢的耐点蚀性能,但对Ni-Cu-P钢不产生明显影响。锈层分析结果发现,Ni-Cu-P钢和碳钢内锈层的主要成分接近,但Ni-Cu-P钢的内锈层明显比碳钢致密。Ni-Cu-P钢中Ni和P能有效降低酸化蚀坑内钢基体的腐蚀速率;Cu则有助于致密锈层的形成。     

A General Approach to the Estimation of Variance Components

A general method of estimation of variance components in random-effects models of the nested and/or classification type is considered. If a given parameter is estimable with respect to some particular experimental design (i.e., an unbiased estimate of the parameter may be obtained from the experiment), then the suggested estimator may be readily computed with only the aid of a desk calculator. The estimates are always unbiased and consistent (with respect to the structure of the experimental design); in the case of balanced experiments, they coincide with those obtained from the analysis of variance.

Secondly, the problem of designing experiments to estimate variance components is briefly discussed from the point-of-view of the suggested estimation procedure. As a result, certain non-balanced designs are seen to yield more efficient estimators of particular parameters in specified situations than the corresponding balanced design using the same number of observations.

Finally, the method of estimation is shown to be applicable to models more general than the variance component one. Again it is readily computed and is unbiased and consistent.     

Finite Element Approach for Failure Analysis of a Gas Turbine Blade

Journal of Failure Analysis and Prevention - In the present work, a case study on the gas turbine blade is undertaken for failure analysis through finite element approach. The analysis is carried...     

Thermo-Mechanical Fatigue Life Assessment of a Gas Turbine Rotor Through Reliability Approach

Turbine rotor is a critical and life-limiting component in gas turbine engines. The thermo-mechanical fatigue (TMF) life of a turbine rotor was studied using reliability method. The fatigue life was estimated using (a) Marrow’s model and (b) Smith–Watson–Topper model. The creep life was estimated based on Larson Miller equations and finite element analysis. The cumulative fatigue–creep damage was estimated, and the turbine rotor TMF life was estimated against the data variation. The reliability approach takes care of material property variations, load variations and geometrical variations. These variations bring out the scatter in component stress–strain and further into life. The scattered life spells out the component reliability. The TMF life was modeled as Weibull distribution, and the reliability was estimated. The component was tested for structural integrity through hot cyclic spin test, and the results were compared with the predictions. The blade growth and strain estimations using Marrow and SWT–creep methods were found in good agreement with the test values.     

铝镁合金孔蚀形貌图像增强与分割方法研究

对基于图像信息的腐蚀诊断与预测实现过程中腐蚀图像的增强、边缘提取和图像分割的小波分析等技术和算法进行了研究,并以铝镁合金在工业循环水中的孔蚀图像为例,说明了这些方法的应用。结果表明：基于灰度值的图像增强技术可增加腐蚀区域与材料基体的对比,用小波变换可对腐蚀图像进行分解,将腐蚀图像分解为包含主要特征的低频予图像和包含细节特征的高频予图像,相应的小波系数可反映出孔蚀特征,有利于用计算机图像技术实现腐蚀评价和预测。     

高氮奥氏体不锈钢耐点蚀性能研究

目前产于高氮不锈钢的研究多集中于理论基础、制造工艺和力学性能等方面,有关耐蚀性方面的研究有限。通过循环极化、Mott-Schottky曲线以及电化学阻抗(EIS)等方法,研究了Cr23Mo1N奥氏体不锈钢(高氮钢,HNSS)和316L不锈钢在Cl-溶液中的耐点蚀性能。结果表明:与316L不锈钢相比,高氮钢具有更正的自腐蚀电位,更小的维钝电流密度。阻抗谱表明高氮钢的钝化膜比316L更加稳定,且电荷转移电阻更大。Mott-Schottky曲线表明高氮钢的点缺陷施主浓度比316L不锈钢低一个数量级,钝化膜的绝缘性更好。循环极化曲线表明高氮钢的点蚀敏感性更小,钝化膜的自修复能力更强,耐蚀性能更加优越。     

Millimeter-Wave Differential Probe for Nondestructive Detection of Corrosion Precursor Pitting

Critical aircraft structural components, such as wings and fuselages, are exposed to harsh environments that vary considerably in temperature and moisture content. In most cases, the corrosion is hidden under paint and primer and cannot be visually detected. The initiation of corrosion is preceded by the presence of corrosion precursor pitting. Near-field millimeter-wave nondestructive testing (NDT) methods have been successfully used for detecting corrosion precursor pitting in exposed as well as painted aluminum substrates. However, near-field millimeter-wave measurements are susceptible to clutter that may mask indications of small defects such as pitting. Standoff distance variation produces an unwanted intensity gradient on an image and may be considered the most undesired clutter-producing effect. This paper presents a differential millimeter-wave probe consisting of a pair of radiating apertures. It is shown that the differential nature of this probe tends to significantly reduce the undesired effect of standoff distance variation, thereby enhancing probe detection sensitivity. Furthermore, when this probe is used for the purpose of millimeter-wave imaging, it produces defect indications with unique features that help in distinguishing the defect from noise. This dual differential probe was used for detecting corrosion precursor pitting. The design of the probe and the results of detecting various pittings are presented in this paper.     

316L奥氏体不锈钢点蚀的原位声发射监测

为了研究316L不锈钢自然点蚀的生长规律,发展基于声发射技术的小孔腐蚀监测方法,利用声发射技术原位研究了316L奥氏体不锈钢在pH=1.0和中性(pH=6.7)的3.5%NaCl溶液中的自然点蚀状况,同时监测了开路电位E_(OCP);通过扫描电镜对试验后样品的表面形貌进行表征,并通过Matlab平台对声发射信号进行聚类分析。结果表明:316L奥氏体不锈钢的自然点蚀具有不同的孕育周期,声发射信号的获取跟E_(OCP)的变化具有较好的对应关系。在点蚀快速发展阶段,声发射信号具有较高的绝对能量。在Matlab平台上建立了一套信号处理程序,并对声发射信号进行重新定义,对处理后的信号进行声发射信号参数分析,得到了3类较好的声发射信号聚类,来源于不同腐蚀现象或阶段。其中,在点蚀的快速发展阶段主要以高持续时间高计数和高持续时间低计数2类谐振信号为主。     

铸造双相不锈钢点腐蚀行为研究

采用化学浸泡腐蚀试验及微观组织和化学成分分析研究了5种铸造双相不锈钢在6%Fe Cl3溶液中的点腐蚀行为,并与316L奥氏体不锈钢进行了对比。结果表明,铸造双相不锈钢的抗点腐蚀性能均优于316L的,腐蚀速率和点腐蚀深度均小于316L奥氏体不锈钢的;双相不锈钢主要耐点蚀能力合金元素在奥氏体和铁素体相内分布不均匀,铬、钼更多地分配于铁素体相内,而镍、氮则更多地分配于奥氏体相内,铁素体相的耐点蚀指数PRE(Cr%+3.3Mo%+16N%)大于奥氏体相;双相不锈钢的耐点腐蚀性能与化学成分有关,随着PRE的增加,双相不锈钢的耐点腐蚀性能提高,铜元素在铁素体内析出的富铜相导致点蚀优先在铁素体内发生和发展。     

评价计测仪器精度的一种新方法

本文运用田口方法中的马田系统对改进后的计测仪器进行评价,结果表明对这种多指标的计测仪器精度的评价不失为一种好方法,这也是马氏距离与田口方法相结合在判别分析中的一次成功运用。