A study of cavitation bubble collapse pressures and erosion part 3: the results in a venturi facility

Cavitation erosion generated in a venturi facility was studied by comparing the erosion loss with the distributions of cavitation bubble collapse pressures (impact loads). The erosion process in the venturi tests is similar to that in the vibratory tests, although its progression is very slow. That is, the surface first deforms and fractures as a result of fatigue with repeated bubble collapse pressures below the threshold pressure needed to form a pit impulsively. By comparing the distributions of impact loads measured using our method with the hypothetical stress-number of cycles curves for fatigue, it is found that the incubation period and the volume loss rate during the stable period follow Miner's law regardless of the venturi, vibratory and cavitation conditions and materials. Therefore we found that we are able to estimate cavitation damage in a flowing system in the same way as damage in the vibratory tests from Miner's law although the distributions of cavitation bubble collapse pressures are markedly different.     

Estimation of Incubation Time of Cavitation Erosion for Various Cavitating Conditions

Estimations have been made, resulting in a general method for the prediction of the incubation time for cavitation erosion using various cavitating conditions and materials. From a single erosion test, the incubation time can be estimated for various conditions and materials by plotting the mass loss as a function of exposure time to cavitation on a log–log scale.     

Improvement of cavitation erosion resistance of a duplex stainless steel through friction stir processing (FSP)

The cavitation erosion (CE) resistance of an UNS S32205 duplex stainless steel (DSS) was improved through microstructural modification using friction stir processing (FSP). As-received material was processed using 200 rpm and 100 mm/min spindle and travel speeds, respectively. The cavitation erosion tests were performed in a vibratory apparatus according to ASTM G32 standard. The incubation period, the maximum erosion rate and the variation of surface roughness during the tests are reported and the results are compared with those obtained for the base metal samples (BMS). The worn surfaces were characterized using roughness measurements and scanning electron microscopy (SEM). After a CE testing time of 10 h, FSP samples showed a 70% diminution of the mass loss when compared to the BMS. Moreover, a 200% enhancement of incubation time and 100% reduction in the erosion rate were achieved after FPS. The improvement of CE performance is related to the recrystallized and refined microstructure, as well as to the modification of the elongated α/γ interfaces.     

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.     

40Cr冲蚀与空蚀交互磨损试验研究

对40Cr进行模拟化工泵叶轮流道内工况条件下的冲蚀与空蚀交互磨损试验,分析40Cr材料的抗交互磨损性能和冲蚀与空蚀交互磨损试件表面磨痕。结果表明:40Cr的冲蚀与空蚀交互磨损失重曲线分为孕育期、上升期、衰减期和稳定期。在孕育期中,材料失重量很小;在上升期中,材料的冲蚀与空蚀交互磨损失重量增加。40Cr材料的交互磨损特性是以微切削、犁削和脆性剥落为主,同时存在针眼和剥落坑。40Cr试样表面的冲蚀沟槽和空蚀坑主要沿水流线速度矢量的方向,但也存在一些与线速度成一定夹角的摩擦痕迹或沟槽,说明影响冲蚀与空蚀交互磨损的因素不仅是水流速度,还与沙粒运动轨迹、水流方向及冲蚀角度等有关。     

Scaling study of cavitation pitting from cavitating jets and ultrasonic horns

Cavitation erosion prediction and characterization of cavitation field strength are of interest to industries suffering from cavitation erosion detrimental effects. One means to evaluate cavitation fields and materials is to examine pitting rates during the incubation period, where the test sample undergoes localized permanent deformations shaped as individual pits. In this study, samples from three metallic materials, an Aluminum alloy (Al 7075), a Nickel Aluminum Bronze (NAB) and a Duplex Stainless Steel (SS A2205) were subjected to a vast range of cavitation intensities generated by cavitating jets at different driving pressures and by an ultrasonic horn. The resulting pitted sample surfaces were examined and characterized with a non-contact 3D optical scanner and the resulting damage computer-analyzed. A statistical analysis of the pit population and its characteristics was then carried out. It was found that the various cavitation field strengths can be correlated to the measured pit distributions and that two characteristic quantities: a characteristic number of pits per unit surface area and unit time, and a characteristic pit diameter or a characteristic pit depth can be attributed to a given “cavitation intensity level”. This characterization concept can be used in the future to study the cavitation intensity of the full scale and to develop methods of full scale predictions based on model scale erosion data.     

钢的气蚀规律的研究

在旋转圆盘仪上对五种钢的气蚀破坏规律进行了研究。结果表明：钢的气蚀累积失重量与工作时间符合指数关系，且不同性能的钢有不同的气蚀孕育期。     

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.     

空蚀初生期破坏程度的表征方法

空蚀破坏实验中,传统表征方法不能准确表征空蚀初生期的破坏程度.提出一种蚀坑法和面积法相结合,利用SEM空蚀图像和数字图像处理来表征空蚀初生期破坏程度的方法.采用抽样方法获取空蚀试样表面SEM图像,对图像进行蚀坑轮廓识别、蚀坑填充和分割等处理,最后进行蚀坑信息统计,获得样本总体的单位面积蚀坑数量、蚀坑面积百分、蚀坑平均直径3个参数,从而实现了对试样整体空蚀初生期破坏程度的准确衡量.     

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.     

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.     

Inelastic damages by stress wave on steel surface at the incubation stage of vibration cavitation erosion

In cavitation erosion, stress waves will be generated and propagated in the solid when a collapse impingement is acted on it. The cavitation damages on the solid surface are considered to be under the effect of the stress waves. An ultrasonic vibration cavitation erosion experiment was performed on a polished steel specimen, and not only the plastic deformations but also the brittle fractures appeared on the surface at the incubation stage of cavitation erosion. Some characteristics such as the hemispherical shape of the crater, intergranular fractures and subsurface comminution make the damages distinguishable from the common plastics deformations, and they are thought to be the results of shear waves. Thus, stress waves are proved to take part in the cavitation erosion, and they bring some special damage styles depending on the conditions of the impaction and mechanical properties of the specimen.     

Cavitation erosion of NiAl-bronze layers generated by friction surfacing

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material.     

Cavitation erosion behaviour of niobium

Cavitation erosion behaviour of niobium was investigated by means of a 20 kHz ultrasonic vibrator at peak-to-peak amplitude of 50 μm, aiming to determine the niobium potential as a material for the manufacturing of hydraulic machine components. The study was emphasized for the three first cavitation stages of the cumulative erosion–time curve. The modification of the niobium surface morphology as a function of the testing time in the incubation, acceleration, and maximum erosion rate stages was verified by SEM analysis. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bar. The study was performed for niobium samples in both the cold-worked and annealed conditions. Samples of CA-6NM martensitic stainless steel, a typical material utilized for hydraulic turbines manufacturing, were also analysed for comparison purpose. Annealing treatment of niobium decreases its hardness and increases its ductility, leading to an increase of the incubation period when compared with the cold-worked niobium. Cavitation erosion failure mechanism in niobium occurs in a sequence of events comprising the work-hardening effect and the fracture of debris allied to the effect of fatigue and microcracks formation. Finally, annealed niobium presents similar incubation period but worse behaviour in the maximum erosion rate stage than CA-6NM steel.     

Characterizing cavitation erosion particles by analysis of SEM images

Wear particles produced by vibratory cavitation erosion on stationary-aluminum Al-99.999 were analyzed using scanning electron microscope (SEM), forming a database for further analysis. The particle morphology features were first clarified based on the characteristic stages of the vibratory erosion rate-time pattern. Next, size, area, perimeter and shape factors (elongation and roundness) were determined for eroded particles, using image analysis software. In incubation period, the particles have distinctive characteristics which differed from that for the subsequent periods. These characteristics include the value of longitudinal ratio and roundness factor, limit size range and morphological features such as lamella shape, folding, curving with one of the particle surfaces as the virgin surface. In acceleration, steady-state and attenuation periods, the particles have a wide size range and larger thickness compared with that for the incubation period. The maximum particle size reached about 360 μm in acceleration and steady-state period. For all the cavitation erosion rate periods, the particles were out of sphericity and they have a roundness factor larger than 2. Detailed surface characteristics of the particles produced during cavitation erosion is significant and can open ways for monitoring the cavitation erosion progress.     

Cavitation erosion characteristics of poly(methyl methacrylate) in a rotating disk device

Experimental results pertaining to the initiation, dynamics and mechanism of cavitation erosion on poly(methyl methacrylate) specimens tested in a rotating disk device are described in detail. Erosion normally starts at the location nearest to the center of rotation (CR). As the exposure time to cavitation increases, additional erosion areas or sites appear away from the CR and secondary erosion (induced by eroded pits) spreads upstream and merges with the main pit. The microcracks increase in density towards the end of the incubation period and transform into macrocracks in most cases. A study of light optical photographs and scanning electron micrographs of the eroded area shows that material particles are removed from the network of cracks because of crack joining and pits indicate particle debris. Optical degradation (loss of transmittance) is observed to be greater on the back of the specimen than on the front.     

Cavitation resistance of Cr–N coatings deposited on austenitic stainless steel at various temperatures

Cr–N coatings were deposited on austenitic stainless steel, X6CrNiTi18-10, by means of the cathodic arc evaporation method at three substrate temperatures: 200 °C, 350 °C and 500 °C. All coatings were found to have a composition of Cr(N), CrN and Cr₂N. The substrate temperature was found to have an influence on the hardness and Young's modulus of the Cr–N coatings. The investigation of nanocrystalline Cr–N coatings resistance to cavitation was performed in a cavitation tunnel with a slot cavitator and tap water as the medium. The estimated cavitation resistance parameters of the coatings were the incubation period of damage and total mass loss. It was found that the optimal coating cavitation resistance was deposited at 500 °C. The incubation period for the 500 °C deposition coating was the same as that of the uncoated X6CrNiTi18-10 steel, but the total mass loss was significantly lower than on the uncoated specimen. The scanning electron microscope analysis indicated that the damage process of the Cr–N coating mainly originates from the plastic deformation of the steel substrate–hard coating system, which appears by “micro-folding” of the surface. An increase of tensile stresses at the top of micro-folds initiates micro-cracks and delamination of Cr–N coating. The results of the investigation and the analysis indicate that the factors mainly responsible for cavitation resistance of the steel substrate/hard coating system are resistant to plastic deformation of the total system and coating adhesion.     

Erosion behaviour of ceramic bulk and coating materials caused by water droplet impingement

Turbine blades in the low-steam environment of energy generation systems suffer severe erosion due to the impingement of water droplets. Erosion resistance of metallic substrates to droplet impingement could possibly be improved with ceramic coating. However, the erosion resistance of ceramic material has not been sufficiently evaluated with respect to selection and maintenance of component materials in power plant systems. A water-jet apparatus, for which impingement velocity and number of water droplets have been well characterized, was used in this study. Erosion tests were conducted on various ceramic bulk and coating materials and on the metallic substrates to investigate erosion behaviour and resistance. Erosion behaviour was characterized by the incubation period and the subsequent damage depth rate. Some ceramic bulk materials had short incubation periods and significant damage depth rates. Zirconia normally had the longest incubation periods and the lowest damage depths. The erosion rate was calculated from the relationship between logarithms of damage depth and impact velocity. The incubation period was also correlated with the logarithm of impact velocity. Both velocity constants in erosion damage and incubation periods of these ceramic materials were strongly correlated with fracture toughness, but not with hardness of the materials. Damage depth rates calculated from the relationship with impact velocity and fracture toughness were comparable to experimentally measured damage depth rates for various ceramic materials.     

Cavitation erosion of aluminum considering bubble collapse,pulse height spectra and cavitation erosion efficiency

Water erosion data on 1100-0 aluminum specimens obtained using a cavitating venturi are compared with bubble collapse pulse height spectra measured using a microtransducer. The data are resolved into erosion power and acoustic power. The former is defined in terms of the power applied to the eroded material to cause the observed pitting and volume loss. The ratio between these power quantities is termed the cavitation erosion efficiency η incav and is found to be essentially constant for the range of tests, being approximately 1.4 × 10^?6. The acoustic power which is easily measured can then be used to estimate the eventual material volume erosion rates, i.e. the mean depth of penetration (MDPR), with much greater accuracy than is otherwise possible. The MDPR is measured directly from the weight loss and is calculated from individual pit counts on damaged surfaces. The effects of the degree of cavitation (the extent of the cavitation cloud or the cavitation number) and the throat velocity on the MDPR is examined. An overall velocity damage exponent of n = 4.75 is found.     

Investigation of cavitation erosion using X-ray residual stress analysis

In this paper it will be shown that by X-ray residual stress analysis it is possible to obtain information on the cavitation process after only short cavitation times. To clarify the correlation between cavitation erosion and changes in residual stress, samples of various steels were tested in a flow cavitation device. The residual stress measurements showed that cavitation leads to the development of compressive residual stresses in the surface of the samples. The rate at which these residual stresses develop depends on the intensity of cavitation. Cavitation-induced changes in residual stress may also be observed in samples with pre-existing residual stresses due to handling, e. g. mechanical working, heat treatment etc. After very short cavitation times the local variation in cavitation intensity may be very clearly seen in the residual stress distribution in the surface. After a longer period of cavitation, compressive residual stresses reach a limiting value which is not exceeded even at points of highest cavitation intensity.