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.     

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.     

Resistance to wear and cavitation erosion of bearing alloys

Sliding wear and vibratory cavitation erosion tests in paraffin oil were carried out on bearing alloys, i.e. tin-based and lead-based white metals, Cu-Pb alloy and leaded bronze. In lubricated wear under mild conditions the surface is worn smooth and a slight difference exists between the wear resistances of the four alloys. In cavitation erosion an eroded surface which is much rougher than the worn surface is formed. Cavitation erosion is affected strongly by the composition and crystal structure of the alloy and thus the erosion resistances of the four alloys differ greatly, the ranking of resistance being lead-based white metal < Cu-Pb alloy < tin-based white metal < leaded bronze. The surface damage, which is caused by the joint action of cavitation erosion and wear, was also investigated by rubbing the eroded surfaces which had been exposed to cavitation erosion for various times. This damage becomes larger with increasing cavitation damage. The resistance to this damage differs much more in the four alloys tested and tends to correlate with the results of the erosion tests rather than those of the wear tests. Therefore, it is clear that the cavitation erosion resistance should be considered in the selection of bearing materials.     

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.     

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.     

Time dependant measurements of cavitation damage

Due to the extremely long length of experiments, in most studies of cavitation erosion only damage in the incubation period is measured and the final damage (mass loss rate) is then predicted by extrapolation. The methods of extrapolation are usually very basic since there were almost no in depth time dependant measurements of cavitation erosion performed in the past. A rotating disc test rig that generates a very aggressive cavitation and pure copper specimens, as erosion sensors, were used to investigate the correlation between the damage within the incubation period and final mass loss. The damage was measured optically three times during the incubation period and by weighing the specimen during the rest of the experiment.The results confirmed that the same clear relationship between the damage in the incubation period and the final mass loss rate exists, what means, that the mass loss rate can indeed be qualitatively predicted on the basis of measurements performed within the incubation period. This is a good basis for developing laws of extrapolation from a short time scale (laboratory measurement within the incubation period) to the real time scale (machine operation).     

Predictive capability of long-term cavitation and liquid impingement erosion models

A brief overview is presented of long-term cavitation and liquid impingement erosion and modeling methods proposed by different investigators, including the curve-fitting approach and a power law relationship recently suggested from this laboratory. A table is prepared to highlight the number of variables necessary for each model in order to compute the erosion versus time curves. An attempt was made to investigate the importance of the different models using data from both cavitation and liquid impingement devices. Detailed analysis of data sets indicates that the normalized curve-fitting approach suggested from this laboratory affords a better solution than other models for certain sets of data with scatter bands to cover a wide range of experimental conditions. The study also indicates that the methods proposed by Heymann and Thiruvengadam are good for the prediction of individual materials following the peak erosion rate. A power law relation proposed between cavitation erosion rate and cumulative erosion may solve several modeling problems.     

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。     

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.     

冲击形式产生的冲蚀、空蚀和腐蚀联合磨损试验研究

舰船等海洋运输装备在海水中行驶时,船体表面受到含沙海水的冲蚀磨损,舰船行驶速度变化引起的空蚀磨损,以及海水的腐蚀磨损,因此舰船表面材料的破坏形式是冲击式的冲蚀、空蚀和腐蚀联合磨损。为了研究联合磨损的影响因素,研制冲蚀、空蚀和腐蚀联合磨损试验台,并实验研究沙粒浓度、沙粒尺寸、含盐浓度以及冲蚀速度对45钢试件的联合磨损影响规律。实验结果表明:冲蚀、空蚀和腐蚀联合磨损程度强于冲蚀和空蚀交互磨损或单纯腐蚀磨损,在较低的沙粒浓度和较低的含盐浓度条件下,金属材料联合磨损质量损失与沙粒浓度、沙粒尺寸、含盐浓度和冲蚀速度均成正比关系。磨损面形貌分析表明:随着沙粒浓度、沙粒尺寸、含盐浓度、冲蚀速度的增加,试件表面的冲蚀沟和空蚀孔的磨痕深度和面积增大,而在人造海水中,试件表面不仅存在冲蚀沟和空蚀孔,还产生了腐蚀坑。     

Cavitation Erosion and Sliding Wear to Assess Carbide Integrity in AISI D2 Tool Steel

Ultrasonic cavitation and ball-on-disk wear tests were conducted in order to assess the carbide integrity in AISI D2 steel specimens with different heat treatments and carbide orientations on the exposed surface. The response of the tested materials under cavitation erosion was clearly distinguishable for each carbide orientation and heat treatment. Particularly, carbide predominant orientation can be successfully associated with the incubation period of cavitation damage. Validity of the cavitation erosion tests as a useful characterization technique in materials with reinforcement phases is discussed based on the small contact areas associated with the microjets generated in the ultrasonic cavitation test. On the other hand, ball-on-disk tests in conjunction with conventional wear analysis (coefficient of friction graph, wear volume) did not provide a clear relationship between research variables. However, by using discrete Fourier analysis from friction coefficient data, it was possible to establish a correlation between integrity of primary carbide, its orientation and matrix hardness.     

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.     

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.     

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。     

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.     

Effect of nozzle geometry on a standard cavitation erosion test using a cavitating jet

In order to accurately and reliably evaluate the cavitation erosion resistance of materials using cavitating jet apparatus according to ASTM G134, the effect of various types of nozzle geometries on the erosion rate was investigated. As the erosion rate depends on the erosion time and the distance from the nozzle to the specimen, i.e., the standoff distance, the mass loss as a function of erosion time at the optimum standoff distance was measured. It was shown that the erosion rate depended on the nozzle geometry. In fact, the aggressive intensity of the cavitating jet I_J depends on the nozzle geometry. When a cavitating jet of low I_J was used in the erosion test, it took some time to reach the maximum cumulative erosion rate E_Rmax, which is recommended in ASTM G134 as a parameter for determining the cavitation erosion resistance of materials. In the present experiment, the difference in E_Rmax was more than 600%, and the time required to reach E_Rmax was also scattered over 600%, for the different nozzles used. It was also revealed that E_Rmax could be obtained from the product of I_J and the reciprocal of the relative cavitation erosion resistance of the material, R_ER.     

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 behaviour of Zr-based bulk metallic glasses

The cavitation erosion (CE) behaviour of four Zr-based bulk metallic glasses (BMGs) of general composition Zr–Al–CuNi(Co) was investigated by means of an ultrasonic vibration device and compared with that of S30431 austenitic stainless steel. The results show that the CE resistance of Zr-based BMGs, in terms of incubation time and maximum rate of erosion, is approximately 10 times higher than that of S30431 austenitic stainless steel. It was also found that a small addition of Co (3 at.%) decreases significantly the maximum rate of erosion of the Zr-based BMG investigated. Initial hardness of the Zr-based BMGs was found to be in a good correlation with their maximum cavitation erosion rates.     

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.