Development of nondestructive technique for measuring carburization thickness and of A new carburization-resistant alloy

Nondestructive measurement of carburization of cracking tube and selecting carburization-resistant alloys have been of much importance in design and maintenance of such equipment as ethylene cracking heater. In this paper, the authors firstly describe the newly developed manner of measuring carburization and then report a new carburization resistant alloy which is also proved to retard coke deposition from hydrocarbon stream on tube surface. The authors studied the effects of tube metallurgy on coke deposition under the conditions simulated to ethylene cracking tube with the aim of preventing carburization and concluded that the coke deposition amount increases consistantly with increase of nickel content of alloys. Based on the above results, the authors developed a new cracking tube designated as CORET tube which has inner layer of Fe-25Cr-9Mn-3Ni alloy clad to HP tubes by centrifugal double layer casting. Those tubes were installed in commercial ethylene cracking heaters and tested for their carburization resistance, mechanical stability and process performances. The results of the field test lasting for more than three years are as follows: 1. The CORET tube proved to be carburization resistant and mechanically reliable enough for actual use. 2. Preliminary process performance data suggest some degree of coking retardation in the CORET tubes when compared with HP tubes; both operating at approximately the same conditions. 3. These data also indicate some degree of enhancement in the olefins yield, when operating at or near high severity conditions.     

Detection of fatigue in structural steels by magnetic property measurements

The magnetic properties of ferromagnetic materials are sensitive to the mechanical and microstructural condition of the material. Fatigue can affect the magnetic properties due to microstructural changes, primarily dislocation production. Magnetic hysteresis measurements have been used to monitor the changes in the parameters due to low cycle fatigue, with the overall objective of developing a new tool to enhance the present NDE techniques for detecting failure. The magnetic measurements were performed using the Magnescope, a portable magnetic inspection system. Materials for fatiguing included plain low carbon steel and samples of quenched and tempered AISI 4340. The coercivity and remanence of the low carbon steel samples increased during the early stages of fatigue, reflecting strain hardening. As cycling progressed, the magnetic parameters leveled off and dropped sharply shortly before failure. The coercivity and remanence of the 4340 samples decreased during the initial stages of cycling, reflecting fatigue softening. The parameters plateaued, then decreased shortly before failure. The amount of change in the magnetic parameters was found to depend on the strain amplitude of the cycling.     

Changes in electrochemical impedance with microstructural development in TBCs

Thermal barrier coatings (TBCs) are improving the performance and efficiency of advanced gas turbine engines by allowing higher inlet temperature and insulation of critical hot-section components. Monitoring the integrity of TBCs prior to failure is critical to the overall performance of gas turbine en gines and requires a robust nondestructive evaluation (NDE) technique. In this paper, changes in electrochemical impedance with microstructural degradation of critical constituents in TBCs are summarized for the development of electrochemical impedance spectroscopy as an NDE technique for TBCs.     

The effect of stresses approaching and exceeding the yield point on the magnetic properties of high strength pearlitic steels

The results of a study of the effects of stresses that approach and exceed the yield point on the magnetic properties of a sample of 50D pearlitic steel are reported. Unlike previous work which has only examined residual stress behaviour, measurements were made in-situ while sample remained under stress. Hysteresis loops, permeability curves and magnetostriction loops are presented and a variety of magnetic parameters analysed. The implications of this analysis for the application of non-destructive evaluation techniques such as magnetic flux leakage, magnetic Barkhausen noise and magnetoacoustic emission are discussed. Key changes in magnetic behaviour occur well before yield and this raises the possibility of developing magnetic NDE methods of predicting when a sample is approaching the yield point.     

低碳低硅合金钢盘条氧化铁皮微观结构的电子显微学研究

采用EDS和XRD等技术研究了吐丝温度为950 、980 ℃时低碳低硅钢盘条氧化铁皮的显微结构特征及组成相。采用EBSD技术研究了低碳低硅合金钢盘条氧化铁皮的微观结构。实验结果表明：盘条的氧化铁皮结构主要有3层,最内层为FeO,中间层为Fe3O4,而最外层为Fe2O3,且FeO层最厚。吐丝温度对氧化皮厚度有显著影响,吐丝温度较高的情况下氧化皮较厚且结构疏松,有利于机械剥离。盘条氧化铁皮FeO层晶粒尺寸小于Fe基体的晶粒尺寸。盘条氧化铁皮中的FeO相比例最高,相比例与氧化铁皮各层的厚度关系一致,其中吐丝温度较低的实验钢中FeO比例较小,Fe3O4和Fe2O3比例较高。     

Confidence metric for signal classification in Non Destructive Evaluation

Quantitative assessment of the reliability of defect classification is critical in non-destructive evaluation (NDE) applications. Particularly in automated data analysis systems, such a measure enables the system to monitor its own performance and automatically flag indications where operator intervention is required. Apart from inherent ambiguity of non-discriminative features and inadequate training samples, noisy measurement is a primary reason underlying the classifier's unreliable decisions. In this paper, we have developed a framework to incorporate the major sources of classification errors into a single quantitative measure. By bootstrapping and weighting Bayes posterior probability with estimated noise distribution, effect of noise in NDE measurements is embedded in the resultant confidence measure. The effectiveness of the proposed method is first demonstrated on synthetic dataset from an eddy current simulation model. It is then used to analyze confidence of classifying experimental data from eddy current inspection of defects in steam generator tubes.     

Three-dimensional microstructural changes in the Ni–YSZ solid oxide fuel cell anode during operation

Microstructural evolution in solid oxide fuel cell (SOFC) cermet anodes has been investigated using X-ray nanotomography along with differential absorption imaging. SOFC anode supports composed of Ni and yttria-stabilized zirconia (YSZ) were subjected to extended operation and selected regions were imaged using a transmission X-ray microscope. X-ray nanotomography provides unique insight into microstructure changes of all three phases (Ni, YSZ, pore) in three spatial dimensions, and its relation to performance degradation. Statistically significant 3D microstructural changes were observed in the anode Ni phase over a range of operational times, including phase size growth and changes in connectivity, interfacial contact area and contiguous triple-phase boundary length. These observations support microstructural evolution correlated to SOFC performance. We find that Ni coarsening is driven by particle curvature as indicated by the dihedral angles between the Ni, YSZ and pore phases, and hypothesize that growth occurs primarily by means of diffusion and particle agglomeration constrained by a pinning mechanism related to the YSZ phase. The decrease in Ni phase size after extended periods of time may be the result of a second process connected to a mobility-induced decrease in the YSZ phase size or non-uniform curvature resulting in a net decrease in Ni phase size.     

Local electrochemical properties of laser beam‐welded high‐strength Al–Zn–Mg–Cu alloys

Butt welds of two high‐strength Al–Zn–Mg–Cu alloys with different zinc contents were welded by a laser beam welding technique. Due to the high energy density of the laser beam, the microstructural changes are confined to very thin regions. Electrochemical properties of the weld heat‐affected zones are investigated by local electrochemical measurement techniques and correlated with microhardness measurements, macroscopic corrosion behaviour and metallographic sections. It turned out that microelectrochemical techniques, especially the EC‐pen is a versatile and easy to handle tool for the resolution of changes in the electrochemical properties across a weld bead. It unveils modifications, which cannot be resolved by hardness measurements. By microcell measurements, local corrosion kinetics can be estimated.     

Prediction of rheological behaviour and segregation susceptibility of semi-solid aluminium–silicon alloys by a simple back extrusion test

This paper studied the segregation susceptibility of a number of samples based on the Al–Si system and quenched from the semi-solid state. The work was performed on two compositions (A356 and A356 + 0.5Sn), characterized by spheroidal microstructures obtained by three different conditioning techniques. Viscosities were determined by a simple back-extrusion test and in order to obtain meaningful data a quantitative microstructural mapping of the extruded cups was carried out. From these measurements, a precise assessment of the segregation extent of the samples was compiled and the data was rationalized in terms of the D’Arcy law. Results showed a positive correlation associating segregation extent, microstructural permeability, liquid phase percolation velocity during extrusion and microstructural features such as the Al- particle size and shape.     

内壁喷丸处理对TP304H耐热钢锅炉管抗水蒸汽氧化性能的影响

将经表面喷丸处理和未处理的TP304H钢管安装于锅炉再热器的高温段运行7474h后,对内壁氧化膜进行形貌观察、能谱分析和X射线衍射分析。结果表明,喷丸处理可大幅度提高TP304H钢的抗水蒸汽氧化性能,氧化膜的生长速率显著降低且氧化膜的粘附性提高。显微分析表明,氧化膜由双层结构转变为单一高Cr氧化膜层,外层Fe的氧化层消失。分析认为喷丸在管子内壁产生的剧烈形变引起晶粒细化和马氏体相变,在高温下促进了Cr向表面扩散及生成富Cr的氧化层。     

Investigations on the effect of chloride on the general corrosion behavior of low‐alloy steels in oxygenated high‐temperature water

This project focused on investigations on the effect of chloride contaminations on the general corrosion and crack initiation behavior of low‐alloy steel (German reactor pressure vessel steel 22NiMoCr3 7) in oxygenated high‐temperature water (HTW). Therefore, tests were performed in oxygenated HTW without chloride and at different chloride contamination levels up to 50 ppb. Chloride was added either permanently or temporarily to simulate a chloride transient during plant operation. During these tests, electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) measurements were performed to monitor the electrochemical behavior depending on the adjusted environment conditions, especially the effect of chloride on the degradation of low‐alloy steel. After the tests, the specimens were examined macroscopically and microscopically. In addition, the oxide layer thickness was determined using the focused ion beam (FIB) technique and different surface analysis techniques as, e.g., TOF‐SIMS were performed to analyze the composition of the oxide layer. A change of the corrosion behavior of the tested specimens was revealed by the applied electrochemical methods EN and EIS during high‐temperature testing. In addition, the applied post‐test investigations showed a decrease in the oxide layer thickness due to permanently increased chloride concentrations in the HTW. Temporary transients, however, did not cause a long‐term memory effect as shown by both, the electrochemical and the metallographic post‐test investigations.     

The non-destructive and nano-microstructural characterization of thermal-barrier coatings

The durability of thermal barrier coatings (TBCs) plays an important role in the service reliability and maintainability of hot-section components in advanced turbine engines for aerospace and utility applications. Photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) are being concurrently developed as complimentary nondestructive evaluation (NDE) techniques for quality control and liferemain assessment of TBCs. This paper discusses recent achievements in understanding the residual stress, phase constituents, and electrochemical resistance (or capacitance) of TBC constituents—with an emphasis on the thermally grown oxide. Results from NDE by PSLS and EIS are correlated to the nano- and microstructural development of TBCs. Authors’ Note: More information on the authors’ research and education activities can be obtained from mmae.ucf.edu/∼ysohn and me.udel.edu/karlsson. For more information, contact Y.H. Sohn, University of Central Florida, Advanced Materials Processing and Analysis Center (AMPAC) and Department of Mechanical, Materials and Aerospace Engineering, Orlando, FL 32816-2455, USA; (407) 882-1181; fax (407) 882-1462; e-mail ysohn@mail.ucf.edu, and A.M. Karlsson, Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140; (302) 831-6437; fax (302) 831-3619; e-mail karlsson@mde.udel.edu.     

New insight into crack formation during corrosion of zirconium-based metal-oxide systems

Zirconium alloys are typically used in nuclear pressurized water reactors (PWR) as fuel cladding tubes due to their chemical stability and their mechanical strength at operating temperatures (≈300 °C). However, the corrosion of Zr-based cladding tubes is one of the factors limiting the burn-off rate in PWRs. It is commonly accepted that the corrosion kinetics involves a periodic succession of growth where the oxide thickness varies parabolically with time. As the oxide thickens, a cracking structure forms. The oxide appears striated with periodic layers of cracks running parallel to the metal/oxide interface. This cracking structure has been experimentally related to the periodicity of the oxide growth. In the present work, a finite-element study is used to investigate the development of stresses in the oxide under the combined influence of molar volume expansion during oxide formation, metal/oxide interface geometry and metallic substrate creep. The generation of tensile stresses capable of initiating the cracks that are observed experimentally is explored.     

Gas phase deposition of diffusion barriers for metal substrates in solid oxide fuel cells

One way to improve the mechanical properties of solid oxide fuel cells is the development of metal supported designs. This type of SOFC offers improved thermal shock resistance, reduced temperature gradients due to the greater thermal conductivity of the metal, and lower operating temperatures. Switching from ceramic supports to metal supports also allows the uses of conventional metal joining and forming techniques and could significantly reduce the material and manufacture costs. However, one persistent problem needs to be solved: inter-diffusion of chemical elements contained in the metal substrates and in the anodes of SOFC leads to degradation, which is to be prevented by protective coatings. In order to address the issues of sintering and delamination for metal supported SOFC, the deposition of gadolinia doped ceria on metal substrates made of Crofer 22 APU has been done by electron beam evaporation and reactive spray deposition technique, as two direct deposition techniques that will not require a sintering step, respectively. Additionally, the effect of ion-assistance on layers made by electron beam evaporation was studied. Because metal supported fuel cells aim at low/intermediate operating temperatures, reducing the thickness of these protective coatings is crucial, since layer thickness is directly correlated to its ohmic resistance. A layer of nickel was applied by magnetron sputtering to prove the effectiveness of the deposited diffusion barrier layers.     

化学镀镍换热器管束腐蚀破裂失效分析

针对化学镀镍换热器管束腐蚀破裂进行分析,采用XRD物相分析、EDS成分分析、力学性能测试、金相显微组织分析、SEM微观形貌分析、电化学测试等手段,分析了管束破裂的原因。结果表明,管柬外表面镍磷镀层局部发生破坏后对碳钢管束基体的加速电偶腐蚀是管束发生破裂的主要原因。     

The concept and development of emission spectroscopy for the non-destructive evaluation (NDE) of the failure of thermal barrier coatings

An emission spectroscopy non-destructive evaluation (NDE) method was proposed and developed for in situ monitoring of the degradation of plasma-sprayed thermal barrier coatings (TBCs). Lithium oxide (Li₂O) was chosen as the emission spectroscopic marker material doped in the yttria-stabilized zirconia (YSZ) layer in TBC system. The lithium spectral response was examined in flame in terms of lithium oxide concentration, temperature, exposed area of the doped inner layer and distance from collection port to lithium excitation source. The possible viability of emission spectroscopy as a NDE method for the TBC failure inspection was demonstrated. The Li₂O concentration in YSZ was optimized based on the thermal cycling test for the Li₂O-doped TBCs with different structural configurations. And the durable Li₂O-doped plasma-sprayed TBC structure was generated with the optimized Li₂O concentration and make it potentially applicable in industry.     

Oxidation of a Mn-Cr austenitic steel during creep testing and its interaction with the fracture

The interaction between oxidation and creep rupture was studied in a 17 Mn-10 Cr austenitic steel, of interest as structural material for the internal components of fusion reactors. The observation of the creep specimens tested in air at temperatures ranging from 773 to 973 K revealed the presence of an adherent oxide scale and of a ferritic phase underneath, which forms as a consequence of the Mn depletion of the austenitic matrix. The microstructure of the two layers was investigated by optical microscopy, SEM, X-ray diffraction, EDS and magnetic permeability measurements. The scale has a complex structure, being composed mainly of manganese oxide. The ferritic layer is completely recrystallized and does not present grain boundary precipitates as the austenitic phase does. The effect of the surface modification on the creep rupture process is discussed in the light of a recent model of deformation-oxidation interaction.     

金属镁在腐蚀介质中界面结构特征与负差数据效应关系研究

复扫描隧道显微镜,在微观尺度观察腐蚀界面形貌特征,发现ＰＨ＝１１的１ｍｏｌ／ＬＮａＣｌ水溶液中,金属镁表面形成块状氧化物,但微观尺度上氧化厚度不均,阳极化后块状氧化物尺度变小,保护作 用降低,导致镁金属溶解出现负差数效应,金属镁在０．５ｍｏｌ／ＬＮａ２ＳＯ４和１ｍｏｌ／ＬＮａＣｌ两种介质中都产生负差数效应,但氧化膜结构不同,负差数效应强弱有别。     

Implementation of an effective time-saving two-stage methodology for microstructural characterization of cemented carbides

Linear intercept on scanning electron microscopy micrographs is the most commonly used measurement method to determine carbide grain size and contiguity in WC–Co cemented carbides (hardmetals). However, it involves manual time-consuming measurements and is critically dependent on the quality of the micrographs as well as on the identification and definition of grain boundaries. In this study a two-stage methodology for microstructural characterization of hardmetals is presented. First, a digital semi-automatic image analysis procedure for grain size determination of the carbide phase is presented. It involves an experimental assessment of grain size on processed images corresponding to a series of WC–Co and WC–Ni cemented carbide grades with different microstructural characteristics. Obtained results are then compared to the values obtained by means of the linear intercept technique. A good correlation between the mean grain sizes determined following both measurement techniques was attained. Based on experimental findings, a series of empirical relations were found to correlate grain size distributions obtained following both methods. Second, an empirical relation for estimating carbide contiguity in WC–Co cemented carbides is proposed. This relation considers simultaneously the influence of the binder content and the experimentally determined mean grain size on contiguity. The proposed equation for contiguity estimation is based on extensive data collection from open literature. An excellent agreement was attained between contiguity values estimated from such equation and those obtained using the linear intercept technique. This validates the two-stage procedure as an effective time-saving methodology for microstructural characterization of WC–Co cemented carbides.