Key Engineering Materials Vol. 795

Abstract: In this article, a set of aging anti-corrosion epoxy coating was prepared in Xenon lamp light chamber of 0h, 72h, 144h, 216h, 288h, 360h and 432h. Meanwhile, the changes of coating thickness, adhesion properties, functional groups, and corrosion resistance properties increasing with aging time were determined. The results showed that the thickness of epoxy coating was increased more than 50% as the aging time increased to 432h, and reached 32μm. The adhesion reduced significantly, which was 0 grade in cross-cut test at 0h, but decreased to 2 grade after been aged for 432h. Meanwhile, the value of adhesive force declined more than 56% in pull-off test. The corrosion resistance properties were investigated by electrochemical impedance spectra (EIS) in both 3.5% NaCl neutral solution (pH=7) and acid solution (pH=2). The Nyquist plots in neutral solution transferred from single time constant to two time constants in neutral solution. The capacitive impedance loop became shorter, while coating resistance R_f declined more rapidly as the aging time increasing. Meanwhile, the Nyquist plots possessed three time constants in acid solutiondue to the dissolution of zinc in the coating. The FT-IR spectrum showed that the peak value of C-O-C and C-H declined with aging time increasing, and the peak of C=O becamemore significantly. The results showed that the protective properties of epoxy coating declined while the aging reaction going on, with chain broken of C-O-C and C-H bond and hydrophilicity enhancing.

Abstract: Addictive manufacturing (AM) allows for the layer-by-layer fabrication of components via sequential material deposition and it is of immediate interest in many applications, in particularly aviation field. This work is tackling the issue that the influence of the inner-defects and building orientations on the fatigue behavior of Ti-6Al-4V Alloys produced by Selective Laser Melting (SLM). Specimens were built in two orientations (horizontal and vertical to the substrate) in order to evaluate the impact of the induced anisotropy of fatigue properties. A series of fatigue tests at five stress ratios ranged from-1 to 0.8 are conducted at 400°C. Scanning Electron Microscopy (SEM) is used to examine the fracture surfaces of fatigue specimens to qualify the failure mechanism and crack initiation sites, which are most likely attached to the surface defects. The fracture surface analysis of HCF specimen tested at two temperatures reveals that near 85% of the crack initiates from the defect under subsurface. The results of this study imply that the fatigue properties at 400°C are highly dependent on the specimen orientations relative to build directions, as the defects close to surface are the mainly cause of the crack initiations.

Abstract: Cladding stainless steel layer on the inner surface of ferrite pressure vessel is a common method to improve the corrosion resistance and save the economic cost. However, the movement of heat source and temperature gradient in the process of cladded welding will lead to the anisotropy of cladded layer material. When measuring the residual stress of pressure vessel steel plate with stainless steel cladded layers by contour method, it is necessary to know the elastic mechanical properties of stainless steel cladded layers accurately. The assumption of transversely isotropy was employed, and the relationship between the material compliance matrix and the elastic modulus of transversely isotropic material was utilized. Based on the elastic modulus of each cladded layer and the whole steel plate from the longitudinal direction (0°) until the transverse direction (90°) measured by the experiment, the independent constants S11, S13, S33 and S44 in the compliance matrix of each cladded layer and the whole steel plate were obtained by regression analysis method. Furthermore, by using the relationship between the independent constants of the stiffness matrix of the transversely isotropic material and the single crystal material, the independent constants S12 in the compliance matrix of each stainless steel cladded layer and the whole steel plate were obtained. And then the independent constants of the stiffness matrix of each cladded layer and the whole steel plate were acquired. Hence, a method for calculating the anisotropic elastic constants of the stainless steel cladded layer and the whole steel plate was proposed. The results will provide material data support for measuring residual stress of pressure vessel steel plate with stainless steel cladded layers by contour method.

Abstract: To introduce a method for reliability analysis of China's large-scale natural gas pipeline system, one should first have a method to calculate the reliability of pipe segments, compressors, valves, and other factors. This article models the rules prescribed in BS7910-2013, a guide to methods for assessing the acceptability of flaws in metallic structures, and combines pipeline reliability assessments from CSA Z662-2015 to present a method (based on the Monte Carlo method) to calculate the failure probability/reliability of long-distance pipelines containing a large number of girth weld defects. The method involves the destruction of plastic material, brittle fracture failure analysis, consideration of the division of units by using pipeline area classes for calculation, and reliability index analysis. The results of a magnetic flux leakage (MFL) in-line inspection (ILI) of a defective girth weld from a section of a pipeline in China are used to demonstrate how to determine the reliability of the entire pipeline.

Abstract: Corrosion has been the most common type of defects on oil and gas pipelines. For in-service pipelines the corrosion defects are detected through the in-line inspection and evaluated by integrity assessment methods such as ASME B31G. However, the safety factors of these methods have not been carefully studied. In this paper, the Monte Carlo simulation is carried out to estimate the reliability of the corrosion defects of critical sizes with different safety factors. The results show that the reliabilities of the critical defects increase with the increase of the safety factor, but decrease with the increase of the defect length even under the same safety factor. The commonly used safety factor 1.39 can ensure the target reliability is met in the specified case in this paper. But for high consequence cases the selection of safety factor needs further research.

Abstract: This study investigated the microstructural characteristics and property degradation of service-exposed reformer tubes. The reformer tubes were graded into three damage levels based on on-site ultrasonic testing. The microstructural and mechanical experiments were carried out included optical observations, scanning electron microscope (SEM) observations, room temperature tensile tests, high temperature tensile tests, and stress rupture tests. The results showed that the coarsen of carbides occurred during service. A decrease of tensile elongation was observed. The mechanical properties and rupture behaviours of reformer tubes for different inspection levels were also obtained. The results showed that the ultrasonic inspection levels of tubes could be significantly influenced by carbide morphology of tube materials. A residual life of 8.3 years was estimated for these tubes.

Abstract: Leakage accidents often occur at the bolted flange joints, which are widely used in the petro-chemical plant. This is mainly resulted by the reduction of bolt force during operation. Therefore, bolt force is very important for sealing performance of the joints in service. Based on the leakage rate parameter, the relationship between the bolt force and gasket stress was derived. Moreover, the leakage rate model was established on the basis of bolt force. With this model, the leakage rate can be directly calculated through the bolt force, and the sealing performance can be evaluated. In order to verify this model, corresponding experiments have been performed. It shows that with the internal pressure increasing, the bolt force increases and the gasket stress reduces. There exists a monotonic relationship between the bolt force and gasket stress. The theoretical gasket stress value corresponds well with the experimental value. The calculated leakage rates for the joints also agree well with the experimental value. Then, it is feasible to evaluate the sealing performance for the bolted flange joints based on the bolt force.

Abstract: In this study, the assessment and calculation methods for the crack propagation life of steam turbine rotor shafts containing defects are presented. The analytic methods for estimating the average stress and the alternating stress amplitude of the steam turbine rotor shafts are introduced. The defects on/in the rotor shafts were regularized by the method of fracture mechanics, and the high cycle fatigue crack propagation life and low cycle fatigue crack propagation life of the rotor shafts are estimated from Paris formula. Taking the 60MW turbine rotor shafts containing an initial surface defect and an initial internal defect as the examples respectively, the crack propagation life of them were calculated. The results indicated that the assessment method for the crack propagation life can preliminarily be both used to estimate the safety-operating life and to analyze the fracture reason of a steam turbine rotor shaft containing defects. This paper can provide reference for periodic maintenance and safety evaluation of turbine rotor shafts.

Abstract: Standards GB 150.3-2011 and JB4732-1995 (Confirmed in 2005) provide design methods for the cylindrical pressure vessels with spherically dished head under internal pressure. It is available for the ratio of the internal pressure p to the allowable stress Sm, p/Sm≥0.002. Engineers desire the design curves for p/Sm<0.002. This paper presents a stress analysis method based on elastic thin shell theory for a spherically dished head jointed to the end or the middle of the cylindrical shell. The design criteria in the current standards are modified. Based on the theoretical stress solution and design criteria, the suitable range of the design curves is extended to p/Sm≥0.001. Nonlinear elastic perfectly-plastic finite element method ensures the reliability of the design curves.

Abstract: Pressurizer surge line is one of the key equipments of nuclear power plants. The thermal stratification due to the intersection of hot and cold fluids inside the pressurizer surge line may affect the safe operation of nuclear power plant. In order to investigate the stress distribution and fatigue characteristics of surge line subjected to long-term thermal stratified loadings, a mechanical model of the surge line was established. And then, according to different temperature distribution assumptions, thermal stress analysis and fatigue assessment were conducted. The results show that the maximum stress appears under the load condition with maximum temperature difference, and finer temperature distribution can obtain more accurate stress and displacement results. The maximum value of fatigue cumulative coefficient appears at the junction of straight pipe and elbow with large temperature difference.