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.     

Effect of separation distance on cavitation erosion of vibratory and stationary specimens in a vibratory facility

Vibratory cavitation erosion with vibratory and stationary specimens is studied for three materials in tap water at room temperature. The separation distance is varied from 0.127 to 6.096 mm. Test materials were commercially pure lead, soft (1100-O) aluminum and type 316 stainless steel. The double-horn amplitude was 58.4 μm (2.3 × 10^?3in) in a 20 kHz facility. The total duration of all tests was 10 min.The weight loss of both vibratory and stationary specimens of course depends on materials. The weight loss of stationary specimens is best correlated as a function of the reciprocal of the separation distance.     

Cavitation erosion incubation period

To evaluate the “incubation period” (IP) stage of cavitation erosion, short-duration vibratory horn tests in tap water were made on soft aluminum alloy (aluminum alloy 1100-O) and also on a much more resistant alloy (316 stainless steel). Curves of weight loss versus time, and corresponding scanning electron microscopy photomicrographs taken during the IP, are presented and discussed. The effects of horn amplitude and temperature are investigated for “open-beaker” tests. The IP for 316 stainless steel is found to be about 500 times that for aluminum alloy 1100-O for the same amplitude and temperature. This ratio can be predicted almost exactly by applying an assumed relation between MDPR_max and IP, i.e. MDPR_max^?1 = k(IP)ⁿ.Fatigue cracks and individual-blow craters were found for 316 stainless steel but only individual craters were found for aluminum alloy 1100-O, although their ductilities are approximately equal. It is found that the IP based on the eroded area only, IP_erod, is much less than the conventional IP (based on the total specimen area) if IP is based on the attainment of a given mean depth of erosion MDP.Relations between the eventual erosion rate MDPR_max and the IP are considered. It is found that IP data can often be used to predict eventual MDPR_max values according to the relation MDPR_max^?1 ∝ (IP)ⁿ where n ≈ 0.93 and n ≈ 0.95 for our vibratory and Venturi data respectively. However, different values for n have been reported in the literature. By assuming a “characteristic” erosion-time curve the time of occurrence of MDPR_max can also be estimated.It is verified that only bubble collapse stresses are important in the vibratory horn test, although specimens are vibrated under very high accelerations.     

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.     

Comparison of cavitation erosion test results from venturi and vibratory facilities

A detailed comparison of cavitation erosion performance in tap water for five alloys in a vibratory (no-flow) system and a Venturi (flow) system was made. The effects of temperature variation (80 – 200 °F), Venturi throat velocity (34 – 49 m s^?1) and vibratory horn double amplitude were studied. Correlations between maximum erosion rate (maximum mean depth of penetration rate (MDPR_max)) and incubation period IP, and the material mechanical properties Brinell hardness and ultimate resilience $\text{UR =}\text{UTS}{}^2\text{2E}$. (where UTS is the ultimate tensile strength and E is the elastic modulus), were examined. Only moderate success was achieved in correlations between “erosion resistance” MDPR_max^?1 and IP and these mechanical properties. However, a good correlation was found between MDPR_max and IP, pertinent to both facilities, of the form MDPR_max^?1 = aIPⁿ, where n is near unity (0.94). The cavitation intensity, as measured by MDPR_max, was found to be 10–20 times greater in the vibratory system, depending on horn amplitude and material. This ratio varies between 5 and 30 if individual materials are considered separately, being greatest for 1018 carbon steel and least for 316 stainless steel. This indicates the important differences in form between these cavitating regimes and the imprecision of material comparisons made in both regimes.     

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.     

Slurry Erosion–Corrosion of Carburized AISI 5117 Steel

The erosion–corrosion of carburized and untreated low alloy steel (AISI 5117) has been investigated using slurry whirling-arm test rig. Erosion–corrosion tests were carried out in slurries composed of sand particles and either tap water or 3 % NaCl solution. The tests were carried out with particles concentration of 1 wt% and slurry stream impact velocity of 15 m/s. Silica sand having a nominal size range of 250–355 μm was used as an erodent. It has been shown that the erosion and erosion–corrosion resistance of AISI 5117 low alloy steel can be effectively improved by carburizing for all impact angles. However, the effectiveness of carburizing was the highest for an impact angle of 45°, where the erosion and erosion–corrosion resistance were increased by 60–40 %, respectively, compared with that of the untreated material. The results showed that the treated and untreated specimens behaved as ductile materials under erosion and erosion–corrosion tests, and the maximum mass loss occurred at an impact angle of 45°. SEM analysis showed that the erosion tracks developed on the untreated specimens were wider and deeper than that formed on the carburized specimens for erosion and erosion–corrosion tests.     

Effect of liquid metal composition and hydrodynamic parameters on cavitation erosion

Cavitation erosion testing machine for low-temperature melting alloy liquid was developed by using a vibratory apparatus. The erosion tests of SUS304 were carried out in three kinds of lead–bismuth and deionized water. We defined a relative temperature as the percentage between freezing and boiling points. At relative temperature at 14 °C, the erosion rate is 10–12 times in various lead–bismuth alloys, and 2–5 times in sodium, as compared with that in deionized water. When SUS304 was exposed to a cavitation in PbBi, the surface was work hardened 20% harder compared with original surface. In deionized water, SUS304 was work hardened by 5%. Therefore, we can conclude that larger collapse pressure can be estimated to act on the specimen surface in lead–bismuth, as compared with that in water.We discussed the effect of hydrodynamic properties on cavitation erosion in a flowing system. It is considered that the erosion rate in sodium is in proportion to 1st to 6th power of flow velocity similarly to that in mercury. The incipient cavitation number is approximately unity irrespective of test liquids. Furthermore, the relation between MDER and cavitation number is expressed as power low of function with an exponent of 2.5.     

文氏管及振动设备的气蚀腐蚀试验结果比较

本文对五种金属材料在振动（不流动）和文氏管（流动）的系统中用自来水作试验的气蚀腐蚀性能作了详尽的比较,不但研究了温度变化（27到93℃）、文氏管喉部流速（34到49米/秒）及振动器双振幅的影响,而且探讨了最大腐蚀速度MDPR max和气蚀孕育期IP与材料的布氏硬度BHN和极限变形能UR（=UTS{sup}2/2E）等机械性能之间的关系。在“腐蚀阻抗”MDPR max{sup}-1和IP与这些机械性能的关系上只获得中等地成功。但是,发现了一个适用于两种设备的MDPR max和IP间的令人满意的关系,其形式是MDPR max{sup}-1＝a×（IP）{sup}n,这里n接近于1（0.94）。由MDPR max量度的气蚀强度在振动设备中要大10到20倍,与振动器的振幅和材料有关。如果对各别的材料分别考虑的话,此数值在5到30之间变化,其中变化最大的是碳钢1018,而最小的是不锈钢316。这表明了在这些气蚀方式间形式上的重大差别,亦表明了对这两种设备中测得的数据进行的比较尚有不精确的地方。     

Comparison of erosion mechanisms in different types of cavitation

A new cavitation erosion device producing vortex cavitation has been extensively used. A comparative study between various cavitation erosion situations was carried out to verify the ability of this vortex cavitation generator to produce realistic cavitation erosion with respect to that observed in hydraulic machinery.For this purpose, specimens of indium and α + β brass were subjected to different cavitation erosion situations in a Francis turbine model, a cavitation water tunnel, a vibratory cavitation device and our vortex cavitation generator. The surface deformation and the development of damage in exposed specimens were examined using scanning electron microscopy. In all cases, except for the vibratory cavitation device, the damage starts with the formation of isolated hollows and craters of similar morphologies and sizes, produced by collapse impingements. The accumulation of isolated damage distributed statistically over the specimens results in erosion. Meanwhile, for vibratory cavitation the damage is initially scattered uniformly over the specimen surface and develops progressively. In spite of this, the topographies of severely eroded surfaces in various types of cavitation did not present noticeable differences. However, transmission electron microscopy observations of subsurface microstructures in eroded specimens indicate the same arrangements of dislocations and the appearance of largescale deformation twins. Hardened superficial layers in specimens exposed to flow cavitation are thicker than those in vibratory cavitation, which leads to higher erosion rates.     

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.     

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 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.     

Effects of materials and solution temperatures on cavitation erosion of pure titanium and titanium alloy in seawater

Cavitation erosion was studied for various pure titanium and titanium alloy samples using a rotating disk method in seawater at 303, 318, and 333 K. Their respective erosion resistances were evaluated in terms of Vickers hardness (HV). The resistance increased in order with increasing hardness: pure titanium samples of first, second, and third types, and titanium alloy (Ti–6Al–4V). The relative temperature was defined as 273 K for freezing temperature and 373 K for boiling temperature under pressurized water. The volume loss rate of test specimens increased with rising seawater temperature of 289–316 K of the relative temperature, as well as in cases using cavitating liquid jet and vibratory apparatuses.     

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.     

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.     

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.     

Corrosion fatigue of steel under cavitation erosion generated intermittently in 3% salt water (results of two-stage tests with and without cavitation erosion)

Two-stage fatigue tests with and without cavitation erosion were performed in 3% salt water. When corrosion fatigue with cavitation erosion (erosion fatigue) is changed to corrosion fatigue during a test, the fatigue life decreases remarkably and the total number of stress cycles to failure becomes smaller than that for tests involving erosion fatigue only. In the reverse case, when corrosion fatigue conditions are changed to erosion fatigue conditions, the fatigue life increases slightly compared with the life expected from the linear damage law (Miner's law). The variation in fatigue strength can be explained by the characteristics of crack propagation in each environment. From these results, it is presumed that the corrosion fatigue strength of the material exposed intermittently to cavitation erosion decreases remarkably.     

An experimental study on cavitation erosion-corrosion performance of ANSI 1020 and ANSI 4135 steel

Cavitation erosion is quite complex, containing corrosion-erosion interaction effect. High temperature oxidization may be aroused after bubble collapse, accompanied by hot gas contacting with the pump component surface. The analysis of the erosion pits can be an effective way to know the mechanism of cavitation erosion. In present paper, the cavitation erosion resistance of carbon steel (ANSI 1020) and alloy steel (ANSI 4135) were tested in an ultrasonic vibration apparatus. By using energy dispersive X-ray spectroscope and three dimensional laser microscope, the chemical composition around erosion pits and the oxidation film structure were analyzed. By using metallographic microscope and scanning electronic microscope, the metallographic structure of specimens (e.g., carbon steel and alloy steel), the nano structured iron oxide and corresponding influence on specimen’s anti-erosion performance were discussed. Based on the comparison between the different tests performed in distilled water and tap water respectively, results can be obtained that erosion rate of carbon steel and alloy steel varies with the component of water which had close correlation to the oxidation effect. Erosion rate of alloy steel 4135 was much lower in distilled water compared to tap water while the difference of carbon steel 1020 was not that large. The remarkable different responses of these two materials had close relationship with oxidation effect. The oxidation effect transferred the original structure of alloy steel surface which had high anti-erosion capability, into newly generated iron oxide structure, which was preferentially to be attacked. The pumping of slightly corrosive fluids frequently leads to erosion-corrosion damage on impellers, and corrosion can further amplify the erosion process.     

Electrochemical behaviour of stainless steel 304 alloy in erosion–corrosion conditions and synergism effect

Abstract

A study has been made of the erosion–corrosion behaviour of stainless steel (SS) 304 in tap water in the presence and the absence of solid particles. Water at ambient temperature impinged in various angles (15–90°) on specimen surfaces at different velocities (7·85–14 m s^–1) and sand concentrations (0·43–2%). In this research, potentiodynamic, electrochemical impedance spectroscopy and weight loss measurements were used to study the damage mechanism and to estimate corrosion rate. The SEM micrographs and optical microscopy images were used to study the corrosion morphology. Under test conditions, protective passive film and non-protective film formations formed on SS 304 surface. It was found that maximum corrosion–erosion rate happened at the impact angles between 60 and 75°. The synergism effect was positive in all conditions and it was greater for the lower angles, the higher velocities and the higher solid contents.