Non-destructive evaluation techniques for assessment of creep and fatigue damage in materials and components

In this article, application of various non-destructive evaluation (NDE) techniques for assessment of creep and fatigue damage in materials and components has been discussed. X-ray diffraction technique has been used for finding fabrication-induced residual stresses in components. Ultrasonic and magnetic techniques have been applied for characterizing changes in microstructures due to creep and fatigue. Crack closure phenomenon and fatigue crack growth have been studied in austenitic stainless steels using acoustic emission technique. Dual-frequency eddy current technique has been used for high sensitive detection of fatigue cracks in compressor discs of aeroengines.     

Creep ductility evaluation of low alloy steels used for steam turbine components

1Cr1Mo¼V low alloy forging and casing steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 773 and 873 K. The primary stage, though present, was rather negligible at higher temperature ranges. Casting steel showed wedge type cavities which grew as cracks along the grain boundaries. In the case of forging steel, the voids were elliptical and flat which developed during the tertiary stage of creep deformation. Tertiary creep deformation and creep ductility of the two steels investigated have been analysed based on the type of voids developed during the tertiary stage.     

A probabilistic approach to evaluate creep and fatigue damage in critical components

The subject of reliability assessment of plant components is of considerable importance to maintenance and operations engineers. There exist different degradation mechanisms such as creep and fatigue in the plant components operating under conditions of high pressure and temperature. Hence, the safety issue of these components must be addressed by a realistic reliability model so that the frequency of in-service inspection activities can be optimized. In this work, a probabilistic module based on first order reliability method and Markov model is used in order to calculate different failure state probabilities of an in-service thermal power plant pipe-bend. This procedure helps in online risk assessment of critical plant components. The method of determination partial safety factors of the above component for optimum design based on a target reliability level is also discussed.     

Tensile,creep, and low-cycle fatigue behavior of a cast γ-TiAl-based alloy for gas turbine applications

The influence of chemical composition on the microstructure of the γ-titanium aluminide alloy Ti-48Al-2W-0.5Si (at. pct) and the accompanying tensile, low-cycle fatigue, and creep properties has been evaluated. The study showed that small variations in chemical composition and casting procedures resulted in considerable variations in the microstructure, yielding vastly different mechanical properties. Low contents of aluminum and tungsten led to a coarse-grained lamellar (γ/α ₂) microstructure with high creep resistance. A composition close to the nominal one produced a duplex (γ+γ/α ₂) structure with favorable strength, ductility, and low-cycle fatigue properties. By controlling the solidification and cooling rates at casting, a pseudoduplex (PS-DP) microstructure with a unique combination of high strength and high fatigue and creep resistance can be obtained. These unique properties can be explained by the diffuse boundaries between the relatively small γ grains and the neighboring lamellar colonies, combined with semicoherent interfaces between the γ and α ₂ phases. At tensile and low-cycle fatigue loading, these boundaries act like high-angle boundaries, producing a virtually fine-grained material promoting strength, whereas at creep loading, grain-boundary sliding is hindered in the semicoherent interfaces leading to high creep resistance.     

谈汽轮机叶片用钢标准

根据生产实践,对汽轮机叶片用钢标准进行了分析和探讨。     

Modeling creep and fatigue of copper alloys

This article reviews expressions to quantify the thermal creep and fatigue lifetime for four copper alloys: Cu-Ag-P, Cu-Cr-Zr, Cu-Ni-Be, and Cu-Al₂O₃. These property models are needed to simulate the mechanical behavior of structures with copper components, which are subjected to high heat-flux and fatigue loading conditions, such as molds for the continuous casting of steel and the first wall in a fusion reactor. Then, measurements of four-point bending fatigue tests were conducted on two-layered specimens of copper alloy and stainless steel, and thermal ratchetting behavior was observed at 250 °C. The test specimens were modeled with a two-dimensional elastic-plastic-creep finite-element model using the ABAQUS software. To match the measurements, a primary thermal-creep law was developed for Cu-0.28 pct Al₂O₃ for stress levels up to 500 MPa and strain rates from 10⁻⁸ to 10⁻² s⁻¹. Specifically, (s⁻¹)=1.43×10¹⁰ exp (−197,000/8.31 T(K)) (σ(MPa))^2.5 (t(s))^−0.9.     

Direct creep curve assessment and optimization of creep equations for high temperature alloys

The conventional use of a time temperature parameter method for the basic assessment of a multi-heat creep data set facilitates the establishment of a creep equation for a material type. However, the parameter can introduce some distortion to the creep data and hence the creep equation. To avoid this disadvantage, a new method for the direct creep curve assessment of multi-heat data was developed. Systematic deviations in creep behaviour of individual test materials in respect to the mean behaviour of the multi-heat data set are reduced and the creep curves for individual stresses are transformed to mid creep curves of a limited number of stress classes. With this method large multi-heat creep data sets of Alloy 100 and Alloy 738 LC could be reduced to mean creep curves. On this basis, existing parameter based creep equations were examined and optimized, if necessary. Further, the confidence limits of these equations were determined. With a stress modification the creep behaviour of a material similar in structure can be described. In finite element analyses some verification experiments which simulate typical loading conditions of components could be successfully recalculated with the optimized creep equations.     

Induced creep and creep/fatigue of a nickel-base superalloy at ambient temperatures

The stress controlled fatigue of Nimonic*115, a typical γ’-strengthened nickel-base superalloy, was studied at ambient temperature, using a trapezoidal wave form at 1 Hz, with stresses chosen to produce failure in the lO⁴ to lO⁴ cycle range. In tests with maximum stress greater than the proportional limit, most of the fatigue damage occurs within the first few test cycles. Much of this strain is accumulated under static load and is therefore identified as creep strain. Transmission electron microscopy shows that these creep strains occur in slip bands which disrupt the ordered γ’ precipitates. Strain is found to follow a logarithmic time dependence, which suggests a low activation energy mechanism.     

三菱轴流风机汽轮机调节系统的调试

本文介绍了日本三菱轴流风机汽轮机调节系统的结构及转速的调试过程，并提出了改进方案。     

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm^−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.     

采用调整抽汽汽轮机替代纯凝机组的节能效果

章简要介绍了中国铝业山东分公司热电厂采用调整抽汽机组替代纯凝机组和减温减压站组合，不仅避免了常开减温减压站的节流降温损失，而且降低了机组的热耗率，提高了整个电厂的热经济性；同时机组采用DCS系统进行控制，提高了劳动生产率，取得了较好的节能效果。     

汽轮机凝汽器黄铜管腐蚀原因分析

通过对电厂凝汽器新铜管短期发生腐蚀穿孔事例的研究和分析，确定其是由于管内表面存在残碳膜而引起。探讨了有关机理，并提出了消除和避免管内表面存在残碳膜的措施。     

30Cr1Mo1V汽轮机转子钢的精炼渣优化

摘要：为有效控制30Cr1Mo1V汽轮机转子钢中非金属夹杂物和有害杂质元素含量,利用热力学软件FactSage 8.1,计算了1873K下CaO-SiO2-Al2O3-5%MgO系精炼渣与30Cr1Mo1V钢液平衡时的等［O］线、等［S］线,以获得最优精炼渣成分范围。研究了不同精炼渣对钢中氧、硫含量,夹杂物特性的影响,继而揭示了钢中典型MgO·Al2O3夹杂物的热力学形成机制以及夹杂物与精炼渣之间的成分关系,并构建了“钢 渣”界面MgO·Al2O3夹杂物运动模型。实验和模型结果表明,优化渣系50.4%CaO-40-3%Al2O3-4.3%SiO2-5%MgO对钢液脱氧、脱硫和非金属夹杂物控制的效果明显,模型预测结果与夹杂物去除率对应关系良好。     

Microstructure,creep properties,and rejuvenation of service-exposed alloy 713C turbine blades

A study was carried out on the microstructure and creep properties of aero engine first-stage turbine blades made from Alloy 713C nickel-base superalloy. Results are reported for new blades, blades in two service-exposed conditions, and service-exposed blades subjected to one of three rejuvenation treatments: a recoating heat treatment, a hot isostatic pressing (HIP) + recoating heat treatment, and a HIP + controlled cooling + recoating heat treatment. The blade microstructure undergoes significant change during service, and this leads to a loss in creep properties exhibited by specimens machined from the blade airfoils. Good correlations were observed between the rupture time and the amount of blade airfoil untwist and between the minimum creep rate and the amount of untwist. The recoating heat treatment and the HIP + controlled cooling + recoating treatment were moderately successful in restoring the microstructure and creep properties of the service-exposed blades. In comparison, the HIP + recoating treatment was very successful in rejuvenating creep properties but only for blades having a chemical composition with a lower propensity to form σ phase. For the blades with an unfavorable composition, σ phase was found to form preferentially near the grain boundaries during creep testing, and this had a detrimental effect on the creep properties. Nonetheless, the degree of rejuvenation for these blades was always at least as good as that obtained through the recoating heat treatment alone. Formerly National Aeronautical Establishment     

A perspective on creep and fatigue issues in sodium cooled fast reactors

Development of advanced materials alongwith improved high temperature mechanical properties, particularly creep and fatigue are important and play a major role for the successful development of robust, safe and economical sodium cooled fast reactor (SFR) technology. The components of SFRs operate in demanding environments at high temperatures under complex creep, fatigue and creep-fatigue loading conditions. Based upon the service requirements in terms of different environments, temperature and loading conditions, different materials are chosen for different components. Ti modified 15Cr-15Ni austenitic stainless steel is chosen for clad and wrapper tubes in the reactor core, which experience high fast neutron flux of ～ 10¹⁵ ncm^?2s^?1 along with high temperatures. Type 316L(N) SS is used for out-of-core structural components like main and inner vessels, and sodium pipelines. For steam generators, modified 9Cr-1Mo steel is chosen for all the components, where liquid sodium and steam/water coexist. Some of the important experiences and exciting achievements in the areas of in-house materials development and its characterization in terms of creep, low cycle fatigue and creep-fatigue properties important to design of reactor components for core, out-of-core and steam generator applications are described in the paper. Future directions for materials research and development activities involving critical issues like radiation damage resistance along with improved mechanical properties for advanced clad and wrapper materials necessary for achieving high fuel burnup and design life up to 60 years for out-of-core structural components leading to economical nuclear energy have been highlighted.     

HNK50/80汽轮机轴位移超标的原因分析及处理

分析AV-80轴流压缩机拖动原动机-HNK50/80汽轮机在运行过程中多次出现轴位移超标的原因,以及消除这一故障的具体措施.     

Creep-fatigue life prediction in terms of nucleation and growth of fatigue crack and creep cavities

In low cycle fatigue at elevated temperature, the interaction between fatigue crack and creep damages is known to be responsible for the significant reduction of the fatigue life. In this investigation, a model for the life prediction for low cycle fatigue with hold time at tensile peak strain is suggested for the temperature range of 0.5T _m. This model is formulated on the basis of the assumptions that the creep cavities are formed due to the vacancies generated during fatigue, and are grown during the hold period. The fatigue crack nucleated at the surface due to fatigue loading is affected by the creep damages for its propagation. The model is checked by experimental results with various hold time periods. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 304 stainless steel and 13CrMo44 steel. Formerly Graduat3e student, Department of Materials Science and Engeneering ,KAIST, Seoul, Korea.     

Thermomechanical fatigue,oxidation, and creep: Part i. Damage mechanisms

Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20 ‡C to 700 ‡C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2 to 12 times greater in helium than in air. Interrupted tests were performed to characterize the pro-gression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermome-chanical fatigue tests. In these tests, transgranular cracking is observed. However, creep-induced intergranular cracking is the dominant damage mechanism during in-phase thermomechanical fatigue tests.     

Interactions between creep,fatigue, and strain-aging in two refractory metal alloys

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-lll) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first stage creep strain and the second stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency andA ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strainaging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain-aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials.) By combining two analytical expressions which are normally used to describe creep and strain aging behavior, an expression was developed which correctly described the influence of temperature, frequency, andA ratio on the TZC creep rate acceleration.     

宣钢8~#高炉1~#汽轮机计算机监控系统设计

在宣钢8 #高炉1 #汽轮机改造中 ,监控系统采用大众586工控机 ,实现了防喘振和防逆流自动控制 ,并能在线检测60点热工参数 ,显示各种参数和工艺流程画面。自投运以来 ,运行状况良好