Failure of a steam turbine rotor due to circumferential crack growth influenced by temperature and steady torsion

High cycle fatigue as a final mechanism of failure in rotating systems, especially steam turbines, is a serious phenomenon. Incomplete information of its effective parameters could lead into catastrophic failure of the rotor. In high cycle fatigue, one of the most effective factors in crack growth is high temperature. In fact, a high-temperature change alters mechanical characteristics of material including their fracture mechanic. Therefore, the investigation of crack growth of fatigue and crack front development in various steam temperatures requires a careful consideration of the different parts of the rotor along the steam turbine rotor. This study investigated the propagation of a circumferential crack at three points of the turbine rotor in 24,149 and 260 ^∘ C under the influence of rotor weight loading. In addition, the study addressed torque as another important parameter in crack growth which is caused by steam pressure in the turbine. Through the obtained results, a crack front shape is achieved which can be used in rotor vibration analysis. Reviewing the related literature indicated that crack growth rate associated with alternative mode I loading in the presence of steady mode III loading is reduced. To investigation this phenomenon, the study considered roughness induced crack closure due to steady torsion and its corresponding equations by analytical method were derived. Finally, the study investigated the influence of roughness on crack growth rate in abovementioned three points of the rotor.     

In-situ scanning electron microscopy studies of small fatigue crack growth in recrystallized layer of a directionally solidified superalloy

The small fatigue crack behavior in the surface recrystallized layer of a nickel-based directionally solidified superalloy DZ4 was studied by in-situ scanning electron microscopy. Transgranular cracking manner was identified at 350 °C, distinct from the intergranular fracture of recrystallized grains reported formerly. The transition of fracture mode is attributed to the effect of temperature, which should be particularly concerned in the safety of turbine components with recrystallization. The discontinuous crack growth in recrystallized layer is associated with the local microstructure. Carbide can act as crack source, and barrier to the crack growth as well. Effect of twin boundary on impeding the fatigue crack is evident.     

Multilevel approach to lifetime assessment of steam turbines

This paper presents a multilevel methodology for a steam turbine lifetime assessment based on the damage calculation, probabilistic analysis and fracture mechanics considerations. Creep-fatigue damage calculations serve as a basis for evaluating the current lifetime expenditure and for defining additional steps of analysis. The need for the use of probabilistic analysis results from the inherent uncertainty in estimating the lifetime expenditure primarily caused by scatter in material properties. Fracture mechanics considerations are helpful in determining additional safety margins for components containing cracks. This methodology has been illustrated using an example of the lifetime calculations of a high-temperature steam turbine rotor. The calculations were based on the results of 2D numerical simulations performed for steady state and transient operating conditions.     

Spannungsrißkorrosion in hochreinem Wasser von 3½ % NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen

Stress Corrosion Cracking in High Purity Water of 3½ % NiCrMoV – Quenched and Tempered Steel for Steam Turbine Discs and Shafts In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S, or NaCl, which initiate SCC, could be found. The stress corrosion cracking behaviour of the turbine disc steel 26 NiCrMoV 14 5 with a yield strength of approx. 850 N/mm² was examined under special corrosion conditions. Gaseous and other impurities of the water, which lead to higher conductivity can initiate stress corosion cracks and increase the stress corrosion crack velocity insignificant. Stress corrosion crack initiation can be prevented by shifting the pH-value and the free corrosion potential in the region of passivity. Unfavourable crevice conditions must be avoided. Solutions are shown, how to prevent stress corrosion cracking of steam turbine discs.     

Critical stress intensity factors in steel cracked wires

In the conceptual framework of fracture mechanics analyses, the study of cracked wires axially loaded has the highest interest since numerous structural elements (e.g. wires, cables, cordons or tendons) work under such a type of loading during their service lives. So, a method that allows the determination of stress states at the crack front should be welcome as a useful way for ensuring the structural integrity of those components for different environmental conditions (air, stress corrosion cracking, hydrogen embrittlement,…). To fill this gap, an engineering estimation of the critical stress intensity factor (SIF) is proposed in this paper for eutectoid steel cracked wires under axial loading. The critical SIF is calculated by considering, apart from the fatigue precrack, the subcritical crack propagation before final fracture. Such a subcritical crack propagation is the process zone (by micro-void coalescence MVC) in the case of fracture in air, the subcritical cracking by localized anodic dissolution (LAD) in stress corrosion cracking (SCC) and the tearing topography surface (TTS) in hydrogen assisted cracking (HAC). In addition, different SIF solutions are used in the analysis so as to have a more complete picture of the different phenomena leading to failure and to provide the designer with sound scientific tools. This method allows the engineer to design in the framework of structural integrity and damage tolerance.     

Method for assessment of the residual life of turbine disks

A method for assessment the durability of turbine disks at the stages of damage removing and development subject to structural and technological changes is developed on the basis of the nonlinear mechanics of deformation and fracture. The proposed method includes numerical analysis of the stressdeformed condition of disks in the initial and damaged states, determination of stress and strain fields in the zones of cutting of the damaged material volume, and calculation of low-cycle fatigue and the duration of crack growth from initial to critical sizes. The theory of the developed method is the concept of a fracture process zone. The method is used for comparative evaluation of stress-deformed conditions and durability parameters for options of changing the geometry of a steam turbine disk by removing the damaged material around slot fillet of key.     

CRITICAL CRACK ASSESSMENT PROCEDURE FOR HIGH PRESSURE STEAM TURBINE ROTORS

Abstract A critical crack assessment procedure for high pressure steam turbine rotors is introduce and applied. The processes relating to low-cycle thermal fatigue (LCTF), high-cycle fatigue (HCF) and creep are considered and the critical crack length is determined in accordance with its shape and position, based on a linear elastic fracture mechanics criterion. Taking this critical crack length as the final value, two mechanisms of crack growth are analysed, LCTF and creep, with the aim of defining the initial value of crack length. Alternatively, LCTF and creep are analysed as crack initiation processes with the aim of defining the appropriate time and number of cycles which can be used in relation to crack growth. The worst-case materials data are used in order to obtain a conservative estimation of the critical crack length. The procedure is also applicable, directly or modified, to other power plant components, e.g. intermediate and low pressure rotors, steamlines and castings.     

Spannungsrißkorrosion in hochreinem Wasser von 3–3 ½% NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen. Teil I

SCC in High Parity Water In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S or Nacl, which initiate SCC, could be found. To clarify the SCC-behaviour experiments on turbine disc steels with different chemical compositions an yield strength were performed in high purity water. The results show, that chemical composition has no effect on the crack initiation. Under high purity water conditions no crack initiation due to stress corrosion cracking is observed on the steel with a yield strength of 850 N/mm². On the steel with a yield strength of 1250 N/mm² which is not used in service, crack initiation occurs in pure water. But if sharp cracks already exist, crack propagation occurs in both cases. The investigations showed, that stress corrosion cracking of turbine discs can be prevented by a good water chemistry with a cation conductivity less than 0.2 μS/cm (μmho/cm).     

Apparent fracture toughness of low-carbon steel CSN 411353 as related to stress corrosion cracks

The occurrence of cracks in structural components indicates a certain threat to their reliable operation, because these cracks can grow during operation and reach critical sizes, leading to fracture. The fracture resistance of a structural component is given by the fracture toughness of the material, determined on standardized specimens with a precycled fatigue crack, and the constraint. The fracture toughness itself depends also on the environment. There is enough evidence that in the conditions of the environment assisted cracking the fracture toughness can be significantly reduced by hydrogen mechanism. Our research results have confirmed this and have demonstrated a considerable reduction in the stress corrosion fracture toughness as compared to that related to fatigue cracks. This should be taken into account when assessing the integrity of structural components with stress corrosion cracks. This paper presents experimental results concerned with the stress corrosion fracture toughness of specimens from a DN150 gas line pipe made of low-C steel CSN 411353.     

Estimation of useful life in turbines blades with cracks in corrosive environment

Geothermal turbines of 110 MW were installed in the Federal Electricity Commission in Cerro Prieto Mexico, which operating time exceeds 150,000 h. Therefore, the critical components which determine the useful life of the turbine should be evaluated to determine the rehabilitation or replacement of them. The critical components are the blades of the last stage in the steam turbine. It has been observed that different blades of the turbine of 110 MW with cracks presented corrosion products, which resulted in a failure for corrosion fatigue mechanisms. In this paper, it was studied the effect of crack propagation produced in a geothermal turbine blade of the last stage, L-0, which is made of stainless steel AISI 410 exposed to corrosion under a sea water solution. The corrosion phenomena including localized corrosion suffered by the cracking sample were studied through the electrochemical noise technique in current and potential and polarization curves. The tests were conducted on pieces of blades subjected to fatigue. The results indicated that the exposure to the corrosion solution modified the width and the length of the cracks. Using a scanning electron microscopy (SEM), the surface of the crack was observed, showing that the corrosion mechanism produced a significant increment of the velocity of crack propagation and therefore, a decrement of the useful life of the material. This research will allow us to understand the corrosion process in addition to estimate the useful life of the blades when they are subjected to load cycles.     

结构钢开裂准则及断裂试验分析

为探寻结构钢开裂机理及抗断设防,给出了结构钢在应力三轴空间的广义屈服轨迹方程。结合古典强度理论和现代损伤力学对金属屈服和断裂解释的力学原理,基于三向等拉伸应力状态下结构钢屈服和宏观脆断重合性假设,导出了结构钢在应力三轴空间的开裂准则。在结构钢开孔板的单向等速拉伸断裂试验中,较为精确地测量了初始开裂时的加载位移及全程载荷-位移曲线。试验结果显示,初始裂纹位于孔边且扩展迅速。通过对结构钢开孔板断裂试验的数值模拟分析,对比验证了该开裂准则的普适性及精度。最后,给出了结构钢广义屈服和开裂模型的物理解释及抗断设防。     

奥氏体不锈钢波纹膨胀节失效分析

采用化学成分分析、硬度测试和金相检验等方法对某电厂多次发生泄漏的膨胀节进行了检测,对其形成原因进行分析。结果表明：膨胀节在服役环境下,其内侧蒸汽中携带的氯化钠,在膨胀节的波峰内侧部位发生富集和浓缩,并在工作条件下,腐蚀裂纹从膨胀节内侧启裂,以沿晶状特征逐渐向外壁延伸穿透,最终形成穿透裂纹和点蚀坑,使波峰处材质发生应力腐蚀开裂而导致泄漏。     

Crack evaluation and subsequent solution of the last stage blade in a low-pressure steam turbine

A crack of the last stage blade in a low-pressure steam turbine is studied. From non-destructive inspection, the crack was found at the trailing edge fillet of the last stage blade. On further examination using the replication of the crack and scanning electron microscopy, the crack was induced by stress corrosion cracking. The blade integrity is evaluated for several crack-removal conditions to repair the cracked blade. Finite element analysis is utilized to determine the stresses and dynamic characteristic of the blade for these conditions. The assessment of high cycle fatigue is performed by modified Goodman diagram. From the results, it is found that the crack-removal blade satisfies the design criteria for the fatigue life and has the sufficient margin from resonant condition of vibration. Repair for the cracked blade would be recommended on the basis of the careful analysis. For the repaired blade, periodic inspection is needed and replacement of the blade will be necessary when an indication is found.     

Failure analysis of the final stage blade in steam turbine

A failure case of the low pressure blades of steam turbine is presented in this paper. The suction side of blades has been quenched to improve the erosion resistance. Cracks with different lengths were found in the quenched region of final stage blades after about 13,200 h service. The failure analysis of blades was performed in terms of composition analysis, microstructure and mechanical tests, etc. The yield strength and tensile strength conform to the corresponding standard, whereas the elongation, area reduction and impact toughness are lower than the criteria. From the crack morphology, fractography and composition analysis on the fracture surface, it was found that the failure mechanism of blades is the environment-assisted fatigue fracture. The location of fatigue crack initiation is related with the salient of blades due to the stress concentration. In order to decrease the blade cracking susceptibility, the increment of tempered temperature in both modified treatment and high-frequency hardening was recommended.     

Stress corrosion cracking of a 60 MW steam turbine rotor

The paper presents a root cause analysis of a steam turbine rotor blade groove cracking. The scope of analyses included material testing and mechanical integrity calculations. In scope of material testing, fracture microstructure was assessed and basic mechanical property characteristics of the rotor discs were determined. In scope of integrity analyses, the stress fields in the blade grooves were calculated and the possibility of cracking due to different mechanisms was assessed. Both calculations and material tests confirmed the stress corrosion cracking to be the root cause of the rotor failure. This was a basis for proposing the rotor discs repair by overlay welding with a lower strength material and modifications to the groove geometry.     

Fracturing behaviors of FRP-strengthened concrete structures

In this paper, we focus on the study of concrete cracking behavior and interfacial debonding fracture in fiber reinforced polymer (FRP)-strengthened concrete beams. An experimental program is systematically reviewed according to the observed failure modes, in which it is found that the interfacial debonding may propagate either within the adhesive layer or through concrete layer in the vicinity of bond interface. A finite element analysis is performed to investigate the different types of debonding propagation along FRP-concrete interface and crack distribution in concrete. For the numerical fracture models, interfacial debonding that initiates and propagates in adhesive layer is modeled by fictitious interfacial crack model. And concrete cracking, including the debonding fracture through interfacial concrete, is modeled by smeared crack model. Properties of the interfacial adhesive layer and concrete are considered to significantly influence the debonding propagation types and crack distribution. The interactions between interfacial bond strength, interfacial fracture energy of bond adhesive layer and tensile strength, fracture energy of concrete are discussed in detail through a parametric study. According to the results, the effects of these properties on different types of interfacial debonding, concrete cracking behavior and structural load-carrying capacity are clearly understood.     

基于SINTAP-FAD方法的含裂纹缺陷钢结构构件安全性评定研究

在施工和运营过程中,钢结构构件上出现的裂纹缺陷可能会扩展而导致其发生断裂破坏,因此对含裂纹缺陷钢结构构件的安全性进行评估具有重要意义。首先,引入断裂力学K_IC断裂判据和失效评定图（FAD）概念,并介绍了结构整体性评估程序（SINTAP）的原理;给出了裂纹缺陷评定所需材料参数的确定方法,以及含裂纹缺陷构件的断裂力学参数和塑性屈服荷载的计算方法;然后,提出了基于SINTAP-FAD的含裂纹缺陷钢结构构件的安全性评估流程。最后,以华南和华东两个电厂锅炉钢结构大板梁的焊接缺陷为例,进行了基于K_IC断裂判据和SINTAP-FAD的裂纹缺陷安全性评定研究。结果表明,基于SINTAP-FAD的方法为含裂纹缺陷工作的钢结构构件的安全性评估提供了一种行之有效的方法,考虑多裂纹耦合作用的安全性评定方法有待进一步研究。     

燃汽轮机发电机铸造风扇开裂分析

某意大利进口的燃汽轮机发电机铸造风扇在国内安装使用2 a后发生早期开裂事故。从低倍组织、断口形貌、显微组织和力学性能等方面对风扇开裂原因进行了分析,并利用有限元软件对风扇的小环应力分布进行了计算。结果表明:该风扇开裂的性质为低应力累积损伤所致的疲劳开裂,开裂的主要原因是风扇裂纹源部位存在较多的裂纹、显微疏松孔洞等补焊缺陷以及在补焊后未能进行适当的热处理。     

Über den Einfluß der Probengeometrie auf die Rißausbreitungsgeschwindigkeit bei Spannungsrißkorrosionsversuchen

The influence of specimen geometry on stress corrosion cracking rates The fracture mechanics characterization of stress corrosion cracking phenomena established the quantitative relation between the stress field around the crack front and the stress corrosion cracking rates. In order to apply the results successfully in structural design, the influence of specimen geometry and test conditions have to be eliminated or minimized. Great experimental efforts are applied worldwide to develop such a testing procedure. The following work is directed to some of the effects resulting from specimen geometry, such as the numerical equations used to obtain the stress-intensity factors, the specimen thickness and crack length.