水介质对超声空蚀纳米生成物形貌的影响

为探究水介质下超声空蚀纳米结构的生成机制,研究不同水介质条件下超声空蚀纳米生成物的形貌特征。利用超声振动空蚀实验装置,在4种不同水介质中分别对45钢样品进行超声空蚀实验,通过激光共聚焦显微镜、扫描电子显微镜对实验后样品表面空蚀纳米生成物形貌进行分析。结果表明:45钢在不同水介质中空蚀生成的纳米微结构有很大差异;在去离子水和Na Cl溶液中空蚀坑环状区域纳米结构呈现为不规则絮状结构,在自来水和Na₂SO₄溶液中生成的空蚀坑周围形成了纳米多层片状结构;在自来水中,随着超声时间的增加,纳米单层片状结构先是长度方向尺寸增大,后逐渐叠加成纳米多层片状结构,总厚度增大。45钢在自来水中超声空蚀生成的纳米多层片状结构的尺寸,与实验时间和介质中离子有关,源于空蚀-腐蚀耦合作用产生;自来水中的SO₄^2-等离子也为片状纳米层间的组装起到促进作用。     

Investigation of the Ring Area Formed Around Cavitation Erosion Pits on the Surface of Carbon Steel

The ring areas formed around micropits in cavitation erosion experiments were investigated. The corrosion behavior and vibratory cavitation erosion tests of mild carbon steel in tap and distilled water were carried out. Thus, the ring areas were densely formed in tap water and scarcely formed in distilled water in cavitation tests. The ring areas formed around micropits in cavitation and free cavitation tests have a similar shape. Moreover, SEM examinations showed that the corrosion products spread within the ring areas. Thus, the ring areas formed around micropits are the result of corrosion effect and are not the result of thermal effects due to bubble collapse.     

Cavitation damage incubation with typical fluids applied to a scroll expander system

During the operation of a scroll expander system overpressure may occur resulting in cavitation damage. Impacts due to implosion of cavitation bubbles near to suction ports can result in damage to the scroll plate in the expander. The accumulation of cavitation pits across the scroll plate leads to cavitation erosion hence efficiency drop. An experimental analysis to identify the mechanical damage of the cavitation on various steel surfaces with different liquid environments was conducted.Three liquid environments and four steel grades were utilised experimentally. The liquids used for the tests were distilled water, used as a reference liquid, and the two working fluids of the scroll expander a synthetic lubricant and a high molecular refrigerant. The steel grades were a high carbon (AISI 1085) and low carbon (AISI 1010) martensitic steel with retained austenite, a chromium martensitic steel (AISI 52100) and a martensitic scroll plate (SP) sample. An ultrasonic transducer was utilised to produce cavitation conditions using a 5 mm diameter probe. The comparison of the results revealed the most hostile liquid environment according to the morphology evaluation of the incubation pits. The cavitation mechanisms are discussed and the cavitation resistance of the steel grades is evaluated. The best performing steel material against cavitation is determined for the conditions described.     

Cavitation damage incubation with typical fluids applied to a scroll expander system

During the operation of a scroll expander system overpressure may occur resulting in cavitation damage. Impacts due to implosion of cavitation bubbles near to suction ports can result in damage to the scroll plate in the expander. The accumulation of cavitation pits across the scroll plate leads to cavitation erosion hence efficiency drop. An experimental analysis to identify the mechanical damage of the cavitation on various steel surfaces with different liquid environments was conducted.Three liquid environments and four steel grades were utilised experimentally. The liquids used for the tests were distilled water, used as a reference liquid, and the two working fluids of the scroll expander a synthetic lubricant and a high molecular refrigerant. The steel grades were a high carbon (AISI 1085) and low carbon (AISI 1010) martensitic steel with retained austenite, a chromium martensitic steel (AISI 52100) and a martensitic scroll plate (SP) sample. An ultrasonic transducer was utilised to produce cavitation conditions using a 5 mm diameter probe. The comparison of the results revealed the most hostile liquid environment according to the morphology evaluation of the incubation pits. The cavitation mechanisms are discussed and the cavitation resistance of the steel grades is evaluated. The best performing steel material against cavitation is determined for the conditions described.     

Formation and Development of Iridescent Rings Around Cavitation Erosion Pits

The erosion pits formed in an ultrasound vibration cavitation were compared to those corrosion pits formed in static water. Concentric iridescent rings around the erosion pit make it different from those corrosion pits. The distinct color and the α-FeOOH structure of the rings indicated that the iridescent rings were oxidization film formed under high temperature generated in the cavitation environment. According to the direct observation on the development of the iridescent rings under microscope, the formation of the iridescent rings was attributed to the repeated interactions between mechanical removal and high temperature oxidization during the cavitation erosion. This work also illustrated the synergistic effect of erosion-enhanced corrosion during the cavitation erosion.     

Cavitation erosion in a dilute emulsion

The cavitation erosion of mild steel, brass, and pure iron in emulsions made with distilled water and 1.0% NaCl solution was investigated using a vibratory apparatus operating at 20 kHz and 15 μm amplitude. The emulsion reduced the steady-state erosion rate of mild steel in distilled and salt water to about 80% and 30%, respectively, of the values for no emulsion, but had no effect on brass in distilled water. Changes in the microstructures of the eroded surfaces and in the pH of the solution are presented and discussed in relation to the erosion curves.     

Erosion of aluminum 6061-T6 under cavitation attack in mineral oil and water

Studies of the erosion of aluminum 6061-T6 under cavitation attack in distilled water, ordinary tap water and a viscous mineral oil are presented. The mean depth of penetration for the mineral oil was about 40% of that for water at the end of a 40 min test. The mean depth of penetration and its rate did not differ significantly for distilled and tap water. The mean depth of penetration rate for both distilled and tap water increased to a maximum and then decreased with test duration, while that for mineral oil had a maximum during the initial period. The ratio $\text{h}\text{2a}$ of the pit depth h to the pit diameter 2a varied from 0.04 to 0.13 in water and from 0.06 to 0.20 in mineral oil. Scanning electron microscopy indicates that the pits are initially formed over the grain boundaries and precipitates while the surface grains are deformed under cavitation attack.     

The erosion of heat-treated steels

The erosion behavior of a plain carbon steel (AISI-SAE 1020), an austenitic stainless steel (type 304) and a low alloy steel (AISI-SAE 4340) in various heat-treated conditions was determined. The testing was conducted at room temperature using aluminum oxide particles with an average size of 140 μm in an air stream. An attempt was made to characterize the erosion behavior as it relates to the mechanical properties obtainable in these alloys by conventional heat treatments. It was found that the ductility of the steels had a significant effect on their erosion resistance which increased with increasing ductility and that hardness, strength, fracture toughness and impact strength had little effect on erosion behavior. The platelet mechanism of erosion occurred in all the steels tested at all conditions.     

Cavitation erosion of pure iron in distilled water containing chloride and chromates

The erosion of pure iron in distilled, 1% NaCl, and 1% NaCl/chromate waters has been investigated using a vibratory cavitation apparatus and the damage examined using profilometry and metallographic techniques. The maximum erosion rate is associated with the formation of a deeply corrugated surface and transcrystalline cracks up to 50 μm deep. Salt in the water decreased only slightly the nominal incubation period, but increased by nearly 40% the maximum erosion rate and etched the eroding surface. Addition of 2000 ppm chromate eliminated the effects of chloride.     

1100-0铝和1018碳钢文氏管空蚀试验的流速指数和空化数

本研究的目的是要估价一下文氏管喉部流速在空化数不变时对试件空化侵蚀的影响。此处的空化数是根据文氏管的出口条件算出的。最近在密执安大学高速空蚀试验装置中曾用27℃的自来水对1018碳钢和1100－0铝进行了试验。目前的试验结果与密执安大学过去所得的结果很符合,表明对于流速范围为10～49米/秒的情况,流速破坏指数在±1～5的范围内变动。     

Velocity exponent and cavitation number for Venturi cavitation erosion of 1100-O aluminum and 1018 carbon steel

The purpose of the present investigation is to evaluate the effect of Venturi throat velocity on the cavitation erosion of specimens for constant cavitation number, which is here based on Venturi discharge conditions. 1018 carbon steel and 1100-O aluminum were tested in the University of Michigan high speed cavitation tunnel with tap water at 27 °C (80 °F). Results of present tests are consistent with previous work done at the University of Michigan, showing that the velocity-damage exponent varies over the range ±1–5 for the velocity range 10–49 m s^?1.     

An investigation on component and formation of tribochemical film in Si3N4-white iron sliding pair lubricated with distilled water

A tribofilm was formed during wear tests of a Si₃N₄-white iron pair lubricated with distilled water. In order to clarify the formation of the film, the wear tests for Si₃N₄-white iron pair with different sliding distances were carried out on a ring-block tester, using distilled water as lubricant. The worn surfaces of white iron specimens were observed under SEM. Furthermore, the component and structure of the film were analyzed by using AES, XPS, FTIR and XRD. From the investigation, the following results are presented. During the wear tests of Si₃N₄-white iron pair lubricated with distilled water, the oxidation and hydrolysis of Si₃N₄ occur on the wearing surfaces, and a tribochemical film, which mainly consists of silica gel, is formed on the wearing surface. The reason for the film formation is that the carbides in cast iron spall off during the wearing tests and the spalling pits are left on the wearing surface of the white iron. Then, the debris of Si₃N₄ or its oxidized product are embedded into the pits, and are further oxidized and hydrolyzed. The products of reactions are concentrated in the pits and polycondensed into silica gel, and a silica gel film is formed on the wearing surface. The film protects both Si₃N₄ and white iron, and makes the paired surfaces smooth. Therefore, the friction coefficient of the pair is down to 0·02, and the wear rates of Si₃N₄ and iron are near zero. However, because the resultants of oxidation and hydrolysis of Si₃N₄ can not be enriched on the wearing surface of carbon steel to form an effective tribofilm, both friction coefficient and wear rate of Si₃N₄-carbon steel pair lubricated with distilled water are still high in value.     

Effect of Magnetic Oxidation and Surface Carbon Distribution on Friction and Wear Characteristics of 45 Steel under the DC Magnetic Field

Abstract

The sliding dry friction behaviors and wear properties of normalized 45 steel and annealed 45 steel under different magnetic field intensities were experimentally studied using a self-made HY-100 pin–disc friction and wear tester. The influence of a magnetic field on the friction and wear of 45 steel were also investigated by analyzing the microscopic friction surface and subsurface using a 3D shape analyzer and metallographic microscope. The experimental results show that the friction coefficient of the normalized 45 steel was greater than that of the annealed 45 steel under magnetic field. In addition, the results illustrate that the external magnetic field can effectively improve the wear performance of carbon steel. The wear rate of normalized 45 steel was lower than that of annealed 45 steel. The wear performance of normalized 45 steel was more significantly improved compared to annealed 45 steel with a magnetic field. Energy spectrum analysis indicated that the effect of the magnetic field on the degree of friction and oxidative wear of the normalized 45 steel was weaker than that of the annealed 45 steel, and the ratio of oxygen to iron and the oxidation area on the friction surface of the normalized 45 steel were smaller. It is believed that as an oxidation protectant, carbon reduces the oxidation wear of 45 steel. The carbon on the normalized 45 steel surface was uniformly distributed and the protective effect of friction surface was better. Therefore, the oxidation wear and oxide shedding were reduced and the wear performance was improved.     

Characterization of Erosion of Metallic Materials Under Cavitation Attack in a Mineral Oil

Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied. The erosion experiments were conducted with a 20-kHz ultrasonic electrostrictive oscillator in a viscous mineral oil. The erosion rates of the metals and alloys varied over three orders of magnitude. The erosion rates of brittle metals, iron, and molybdenum were higher than that of the titanium alloy but lower than the rest of the soft ductile metals and alloys. Studies with scanning electron microscopy indicated that the cavitation pits were initially formed at the grain boundaries and precipitates and that the pits formed at the junction of grain boundaries grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements showed that the pits formed over the grain boundaries deepened faster than other pits. Computer analysis revealed that a geometric expression describes the normalized erosion curves during the time period 0.5 t₀ < t < 2.5 t₀, where t₀ is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period.     

Cavitation damage prediction

New results from cavitating venturi water tests were used to reinforce the concept of cavitation erosion efficiency previously developed from tests in a vibratory facility with both water and sodium. The concept emerges from a technique which allows a priori prediction of eventual cavitation erosion rates in flow machines. Bubble collapse pulse height spectra obtained from submerged microprobes are correlated with measured erosion rates in given laboratory and/or field devices to allow this prediction. Preliminary results from such correlations are presented together with other measurements of the effects of gas content, velocity and cavitation condition upon the mechanical cavitation intensity as measured by the pulse height spectra.New results from vibratory facility tests in tap water and synthetic seawater upon three materials of variable corrodability (304 stainless steel, 1018 carbon steel and 1100-0 aluminum) are presented. The ratio between maximum erosion rates for the saltwater and freshwater tests were found to increase toward unity as the mechanical cavitation intensity is increased, i.e. increased mean depth to penetration (MDPR), as expected on theoretical grounds.The relation between the incubation period and MDPR_max was examined from the vibratory test results, and was found to depend upon the material properties as well as the fluid flow conditions.     

Cavitation damage studies using plasticine

Two mechanisms, namely, the spherical pressure wave and the microjet, have been used to account for the erosion of materials resulting from the collapse of cavitation bubbles. In recent years, however, high speed photography of collapsing bubbles has added support to the microjet mechanism. Experiments have been undertaken by the authors to examine the mechanism of the erosion of materials subjected to a cavitation environment. Stationary specimens of plasticine held in close proximity to the end of an ultrasonic horn have been damaged by cavitation in distilled water. By virtue of the features of the pits formed, as shown in the photographs in the paper, it is concluded that the cavitation erosion damage results from the impingement of high velocity microjets on the material surface during bubble collapse.     

Deformation and Fracture of Single-Crystal and Sintered Polycrystalline Silicon Carbide Produced by Cavitation

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25°C using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 μm. The results of the investigation indicate that the SiC {0001} surface could be deformed, in a plastic manner during cavitation. Dislocation etch pits ware formed when the surface was chemically etched. The number of defects, including dislocations in the SiC {0001} surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated during cavitation. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.     

Experimental and numerical investigations on development of cavitation erosion pits on solid surface

Polished, grinded, and milled samples made of 40Cr stainless steel were prepared for the cavitation erosion experiment. A typical phenomenon of “pits chain”, which consisted of two contact pits and a smaller pit on the ridge between them, was found on the sample surfaces after 15-minute experiment. Numerical simulation indicated that the pressure fluctuation caused by the sequentially formed pits on the solid surface was the main reason for the formation of the “pits chain”. It proves that the early-formed pits affect the formation of the subsequent erosion pits, and the whole cavitation erosion process is not a probability event. Based on the numerical analyses, the development of erosion is divided into four stages, which describe how a pit develops vertically and horizontally under the effect of the pressure perturbation. The development was validated by the characteristics of the damaged surface observed at different experimental time.     

加工方法和材料种类对空蚀结果的影响

利用旋转圆盘空蚀试验装置,以流动的自来水为介质,对铣削和抛光加工的1Cr18Ni9Ti、40Cr、45#钢3种钢材料,进行了累计5min、20min和1h的空蚀试验,利用扫描电子显微镜对试验后试样的表面形貌进行原位观察。结果表明同种材料,铣削和抛光的试样蚀坑形态不同;同种加工方法,1Cr18Ni9Ti最难发生空蚀,40Cr的空蚀现象明显,45#钢发生了严重的空蚀破坏。结果显示不同加工方法影响了试样表面微形貌和显微组织,从而得到形态不同的空蚀坑;材料种类对空蚀破坏程度有显著影响,硬度和润湿性可能是重要影响因素。     

Cavitation erosion resistance of Cr–N coating deposited on stainless steel

Results of investigation on cavitation-erosion resistance of Cr–N coating deposited on stainless steel X6CrNiTi18-10 (1H18N9T) by means of the cathodic-arc method are presented. The evaluation of Cr–N coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The investigation was performed at variable-cavitation intensity and the estimated cavitation resistance parameters of coatings were the incubation period of damage and the instantaneous erosion rate after exposure of specified duration. It has been confirmed that the incubation period of the Cr–N coating damage is approximately 50% longer than that of the uncoated X6CrNiTi18-10 steel, and the instantaneous erosion rate after exposure of specified duration is comparable in both cases. The scanning microscope analysis indicates that the damage of Cr–N coating is due mainly to its delamination, while the erosion of deeper parts of the coating is of minor importance. The character of the coating and substrate damage in multiple locations indicates that the hard coating microparticles torn-off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness are subject to cavitation erosion and to solid particle erosion with the hard torn-off microparticles of coating. The results of the investigation and the analysis indicate that the factors mainly responsible for a long incubation period and low cavitation erosion rate of the steel substrate/hard coating systems are the gained high hardness of substrate and high level of coating adhesion.