不锈钢制品表观缺陷的SEM分析

利用扫描电子显微镜(SEM)及其附带的X射线能谱仪(EDS)、X射线波谱仪(WDS)对某些不锈钢制品的表观缺陷进行了较深入的分析。从中可以看到:SEM分析手段及其结论对于不锈钢制品、尤其是进、出口产品的表观质量鉴定和商业纠纷仲裁等方面能起到非常重要的作用。     

镍基单晶铸造气膜孔低周疲劳断裂特征研究

气膜孔结构作为一种重要的热防护技术广泛应用于涡轮叶片中。气膜孔的存在会引入制造缺陷、应力集中等不利因素,成为疲劳失效的主要诱因之一。本研究开展不同温度下镍基单晶铸造气膜孔低周疲劳试验,分析熔模铸造直接成型的气膜孔疲劳断裂特征。结果表明:不同温度下疲劳寿命取决于应力幅值;随温度降低,疲劳断裂路径由Mode-I型转换为晶体学平面断裂,断口形貌呈现出从类解理至韧性断裂特征。气膜孔周围微结构分析表明,在1000 ℃及循环应力作用下,氧化膜发生破裂形成氧化裂纹;在700 ℃条件下,疲劳裂纹形核主要由滑移累积导致;在850 ℃条件下,疲劳损伤由氧化损伤和滑移累积共同作用。晶体塑性有限元分析揭示铸造气膜孔疲劳断裂特征主要受气膜孔附近应力分布的影响。     

冷轧带钢电镀后表面起泡原因的分析

冷轧带钢在氰化物碱性介质中电镀锌后,基材表面出现间断的呈直线排列小鼓泡,经扫描电子显微镜观察和能量色散谱检测,发现气泡内有一大小约10μm以Mg的氧化物为主的块状物.在电镀过程中,由于氢原子的渗入引起基材表层起泡.     

Magnesium matrix nanocomposites fabricated by ultrasonic assisted casting

Abstract

Magnesium matrix composites reinforced with nano-sized SiC particles (n-SiC_p/AZ91D) were fabricated by high intensity ultrasonic assisted casting. The microstructure of the nanocomposites was investigated by optical microscopy, scanning electronic microscopy (SEM), high resolution transmission electronic microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) methods. The results showed that the dispersion and distribution of n-SiC_p in magnesium alloy melts were significantly improved by ultrasonic processing. Compared to the unreinforced AZ91D matrix, mechanical properties of the nanocomposites including tensile and yield strengths were remarkably improved and the yield strength increased by 117% after gravity permanent mould casting.     

Oxide film characteristics of Al–7Si–Mg alloy in dynamic conditions in casting

Abstract

'New' oxide film which forms in a very short time in the casting process was studied. Samples for the study were prepared based on a technique in which an oxide–metal 'sandwich' could be made. Alloy A356 (Al–7Si–0.4Mg) was selected for the study. Features such as thickness of the oxide film, its morphology, rigidity and presence of eutectic phase have been examined and shown by SEM study. Possible consequences of the morphology of the oxide film are discussed.     

The investigation of the floatation of double oxide film defect in liquid aluminium alloys by a four-point bend test

A mould was designed to produce considerable splashing during filling, resulting in the entrainment of freshly produced double oxide film defects. Two moulds were cast to produce 60 testbars in total, suitable for four-point bend testing, to determine their Weibull moduli. Different faces of the cast test-bars, top and bottom, would have different distributions of double oxide film defects, depending on whether the defects were expected to float or to sink during casting. The faces of the test-bars placed downwards in the four-point bend-tests would experience the maximum tensile load, meaning that the test-bar faces associated with the greatest defect densities (top or bottom) would be associated with the lowest Weibull Modulus. The experiment confirmed that double oxide film defects are positively buoyant, initially, and would float towards the surface of the casting during pouring and solidification.     

Influence of casting defects on fatigue behaviour of A356 aluminium alloy

Abstract

The fatigue behaviour of A356 cast aluminium alloy under 10⁴ cycles was investigated by a servo-hydraulic machine at the temperature of 150°C. The specimens for the fully reversible tension compression tests were cut from cylinder head castings of automobile engines. The testing data were treated by S–N curve and the staircase method and the mean values of fatigue property coincided with each other by these two methods, though the data scattered with different casting defects. The fracture surfaces of fatigue specimens were examined by OM, SEM and EDS. It was found that in most of the specimens two or more fatigue crack initiations appeared at a fracture surface. Compared with casting defects such as oxide film, inclusion and blowhole, shrinkage porosity is more detrimental to the fatigue behaviour of aluminium alloy castings.     

Oxidation tuning in AlCrN coatings

In this work, we have studied the influence of the coating design and composition on the oxidation behavior of Al_xCr_1−xN (x = 0.70) coatings. In particular, we have studied the effect brought about by the deposition of an additional subsurface titanium nitride barrier layer as well as by the doping of the AlCrN-based coatings by tungsten, boron and silicon. The coatings studied have been deposited using the cathodic arc vacuum (CAV) technique. The multilayered AlCrN/TiN coatings with TiN sublayer were oxidized in air at 900 °C over 3 h and then analyzed by Glow Discharge Optical Emission Spectroscopy (GDOES) and X-ray photoelectron spectroscopy (XPS). Oxidation tests were performed in air at 900 and 1100 °C for the AlCrN and AlCrWN, AlCrSiN, and AlCrBN coatings. In each case weight gain was measured and the surface morphology of the oxidized samples were studied using Secondary Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The results obtained showed that the oxidation behavior of the aluminum rich AlCrN-based coatings could be improved in two ways: (1) by controlling the chromium outward diffusion rate in multi-layered coatings and (2) by alloying the AlCrN-based coatings with Si. Both improvements are related to the enhancement of the protective oxide film formation.     

Corrosion of NiTi Wires with Cracked Oxide Layer

Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO₂ surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.     

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利用金相显微镜和扫描电镜手段研究了几种典型的热轧钢板表面翘皮缺陷,结果表明,热轧板表面翘皮缺陷皮下成分主要有氧化铁,二次氧化颗粒,夹渣等3种组成情况。根据翘皮缺陷皮处的能谱分析结果可以判断引起热轧板表面翘皮缺陷的原因主要有表面氧化铁皮的轧入、铸坯中的气泡、铸坯表面或边部开裂、侧压定宽机参数的调整不当、结晶器保护渣的混入等,详细讨论了热轧钢板表面翘皮缺陷产生的原因。     

保护渣对430不锈钢铸坯表面质量的影响

针对太钢(TISCO)430不锈钢连铸坯表面存在结疤、凹陷等质量问题,采用半球点熔点仪、黏度仪、扫描电镜等方法,系统研究了保护渣理化性能、连铸结晶器振动参数以及冷却强度对表面缺陷的影响。结果表明,保护渣黏度低、结晶性能弱是造成铸坯表面缺陷的主要原因。为此,通过优化保护渣的化学成分,将保护渣的黏度由0.20提高至0.33 Pa·s,改善了渣膜的均匀性;碱度由1.00提高至1.16,提高了保护渣控制传热的能力。从而消除了铸坯结疤、凹陷等缺陷,实现了铸坯无修磨。同时,受保护渣中氟含量和黏度的影响,浸入式水口的寿命明显提升,连浇炉数由10炉提高到12炉。     

铸造充型过程数值模拟及实验研究的进展

铸造充型过程与铸件多种缺陷密切相关,对铸件力学性能有重要影响.对铸造充型过程的数值模拟研究进行了回顾与分析,着重探讨了近年来该方面的研究热点——气体和氧化膜卷入模拟的研究现状,及该方面研究所面临的问题;分析了铸造充型过程实验方法的现状,并展望了铸造充型过程气体和氧化膜卷入模拟的发展.     

I级涡轮叶片线性缺陷分析

发动机在工厂试车考核后分解检查时，发现其中一件I级涡轮叶片表面有线性荧光显示。通过金相、扫描电镜等手段，对失效叶片进行断口形貌观察、组织及成分分析，并与同种材料金相光面、冲击断面的表面氧化形貌进行对比，确定线性缺陷的性质及产生原因。结果表明:线性缺陷是由富含Ti、Mo碳化物的膜状缺陷导致的开裂，起因是在浇铸过程中合金液表面发生翻动或飞溅挂壁时形成氧化物薄膜，在后续时效过程中沿氧化物薄膜析出该膜状碳化物；膜状缺陷开裂后经过长时间高温氧化，在开裂面形成由Ni、Co的氧化物为主的外氧化层和由Cr、Al、Ti的氧化物为主的内氧化层组成的双层复合氧化膜。在生产过程中，可采用过滤网、提高真空度、保持平稳浇注来避免此类缺陷的产生。     

Evaluation of Silicone Rubber Insulators used in High-Voltage Transmission Lines

Three long-rod silicone rubber composite insulators used in 230 kV power transmission lines were evaluated for aging affects. The insulators were obtained from various outdoor desert locations within Saudi Arabia where they had been in service for 6 years. Surface degradation associated with the aging process was analyzed by using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infra-Red Spectroscopy (FTIR), and Electron Spectroscopy for Chemical Analysis (ESCA). Electrical performance of insulators was evaluated by Rapid Flashover Voltage Tests (RFVT) and Pollution Severity Measurement Tests. Depending on their locations of service, the samples exhibited various types of pollutants at their surfaces. The analysis indicated that the insulator material showed localized surface degradation while there was no evidence of micro-cracking. Furthermore, it was noted that the electrical performance of the insulators was not significantly affected by exposure during the in-service period.     

Effect of dissolved hydrogen on removal of double oxide film defects from Al melt

Abstract

Effect of increasing the hydrogen content of the melt on the removal of double oxide (bifilm) defects in commercial purity Al melt was investigated using Weibull statistical analysis of mechanical properties of solidified castings and a semiempirical mathematical model that had been developed previously. The results of the Weibull analysis revealed that increasing the H content of the melt from 0·2 to 0·4 mL/100 g and holding the melt in furnace for 30 min caused the Weibull modulus of the casting to increase by ～73%. Studies (SEM and EDX) of fracture surfaces of the test bars indicated that the number of oxide defects in the melt decreased due to this treatment. The mathematical model suggested that increasing the H content of the melt to ～0·8 mL/100 g would force all the oxide defects to expand rapidly and float out of the liquid metal in a very short time.     

Diagnosis of oxide films in cast aluminum alloys

Aluminum (Al) alloy castings often contain many defects, such as micro-shrinkage, gas pores, shrinkage pores, hot tearing, and oxide film entrapment. When oxide films are entrapped in Al alloy castings, they are very difficult to identify simply by optical observation. In this study we used an auto-scanning eddy current testing method to detect oxide films entrapped in Al alloy castings. The detection signal and the measurement data were confirmed by scanning electron microscope (SEM) observation and partly backed-up by the ultrasonic-vibration method.     

Wear behavior of adaptive nano-multilayered TiAlCrN/NbN coatings under dry high performance machining conditions

Application of quaternary nitride nano-multilayered coatings results in significant improvements in tool life as well as wear behavior of ball nose end mills under severe conditions of dry high speed machining of hardened H13 steel (HRC 55-57). Tool life of different nano-multilayered TiAlCrN-based coatings with addition of transitional metals based (of V and VI groups) nitride layers has been compared. Tool life of TiAlCrN/NbN coating was found to be higher than compared to the other nano-multilayered coatings. Investigation of surface structure characteristics of the TiAlCrN/NbN coating using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS) has been performed. The properties of the coatings such as microhardness, modulus of elasticity, coefficient of friction and oxidation stability at elevated temperatures were also studied. Cutting forces at the tool/workpiece interface have been measured in-situ. Temperatures on the surface of cutting tools were evaluated. The features of friction and wear behavior as well as mechanisms of tribo-adaptation of TiAlCrN/NbN nano-multilayered coatings were outlined.     

高钨镍基高温合金K416B富W相的析出行为

研究了K416B合金中富W相的析出行为与合金浇注温度和凝固速率的关系。结果表明,在相同冷却速率下,合金的浇注温度由1500℃降低到1450℃时,晶粒尺寸明显减小。在不同浇注温度下,合金中均有块状α-W相在残余共晶中析出,α-W相形貌差别不大。合金的残余共晶中存在大尺寸的M6C相,而残余共晶的边缘处有小尺寸的M6C相。高凝固速率时,合金中富W相数量减少、尺寸减小,表明富W相析出受到明显抑制。对于铸造高钨镍基高温合金,选择合适的浇注温度以及保温体系加快凝固初期的冷却速率,可以控制富W相的析出和转变,从而优化合金性能。     

Indentation size effect on mechanical properties of HVOF sprayed WC based cermet coatings for a roller cylinder

The present paper concerns the determination of mechanical properties such as hardness, elastic modulus and yield strength of WC-based cermet coatings for a roller cylinder. With this regard, Co and Ni containing WC-based coatings were sprayed on Ni-Al deposited 316 L stainless steel substrates by using High Velocity Oxygen Fuel (HVOF) technique. These HVOF sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) with an Energy Dispersive Spectroscopy (EDS) system attachment. Mechanical properties of the coatings were examined by Shimadzu Dynamic Ultra-micro hardness test machine in order to determine the Young's modulus through load-unload sensing analysis. In addition to mechanical investigation, hardness-depth and hardness-force curves of WC-based coatings were investigated. It was found that both of these characteristics exhibit significant peak load dependency. Experimental indentation studies were carried out to determine load-unload curves of WC-Co and WC-Ni based coatings under 300 mN, 350 mN, 400 mN and 450 mN applied peak loads. Hardness and Young's modulus of WC-based coatings were calculated from experimental indentation test data of samples. It has been observed that the hardness and Young's modulus of the coating depends on the contact area and indentation size. The originality of this study is to determine the indentation size effect and contact area variations on mechanical properties of HVOF sprayed WC-based coatings.     

感应重熔Ni60/WC涂层的界面组织与耐磨性研究

用冷涂工艺在45钢表面制备了Ni基WC覆层,并采用高频感应加热的方法获得重熔涂层.用金相显微镜、扫描电镜和X射线能谱仪分析涂层的成分、显微组织和界面特征,分析了不同重熔温度对涂层组织和硬度分布的影响.进行了磨损试验.比较Ni基WC涂层与Ni60、T10钢的磨损性能.结果表明,冷涂结合高频感应重熔可以获得WC颗粒弥散分布的致密的Ni基涂层,涂层与钢基体间由于原子扩散形成一条冶金结合的白亮带;涂层的磨损机理是磨粒磨损和微犁削,Ni基WC涂层的磨损性能优于Ni60涂层,远高于淬火T10钢.