Cavitation resistance, microstructure and surface topography of materials used for hydraulic components

The cavitation resistance of a stainless steel and two flame thermal spray coatings was tested in laboratory according to ASTM G32 standard. The time-variation curves of cumulative volume loss, erosion rate and roughness parameters were related to the microstructure of the samples and to the wear mechanisms. Pores, unmelted particles and other microstructure defects prevented the coatings from showing an incubation period during the tests, while the stainless steel exhibited the expected incubation, acceleration and maximum rate stages. In the stainless steel, a correlation between the transition from incubation to acceleration stage and the R_sm/R_q and R_y/R_q ratios was established.     

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 erosion characteristics of nickel-based alloy-composite coatings obtained by plasma spraying

Nickel-based alloy and composite coatings were obtained on an 18-8 stainless steel substrate by a plasma spraying technique. They were sintered in a vacuum furnace (10^?2 Pa) to improve toughness and to reduce hardness. The cavitation erosion resistance of these coatings was evaluated using a rotating disc apparatus. The results of the investigation reveal that sintered coatings are several times better than “as-sprayed” plasma coatings. The sintered coatings can help in increasing the length of the incubation period.     

Experimental investigations of mechanical properties and slurry erosion behaviour of high velocity oxy fuel and plasma sprayed Cr2O3–50%Al2O3 coatings on CA6NM turbine steel under hydro accelerated conditions

Slurry erosion behaviour of HVOF (High Velocity Oxy Fuel) and plasma sprayed coatings on CA6NM hydraulic turbine steel has been investigated at different levels of various parameters. The Cr₂O₃–50%Al₂O₃ composite powder was prepared and deposited on CA6NM steel samples to get the uniform thickness coatings. The surface roughness, porosity and microhardness of as-coated samples were measured. The as-coated samples were subjected to SEM/EDS analysis to evaluate the surface microstructure of the developed coatings. Erosion tests were performed on self made erosion test rig under hydro accelerated conditions. The study reveals that the velocity, impact angle and slurry concentration were the most significant parameters, influencing the erosion rate of these coatings. The average particle size had least affect on the erosion rate. HVOF-coated samples showed better corrosion resistance as compared to plasma-coated samples due to high hardness of HVOF-coated CA6NM samples.     

A study on erosive wear behavior of HVOF sprayed nanostructured WC-CoCr coatings

WC-CoCr cermet coatings were deposited on stainless steel substrate using high-velocity oxy-fuel (HVOF) thermal spray process. The coatings were developed with two different thermal spray powders: one has WC grains of conventional micron size and the other is composed of nanosized (near-nanostructured) grains. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control the process parameters that have resulted in quality coatings. Cavitation erosion testing was performed using a vibratory test apparatus based on ASTM standard G32-98. Surface morphology of powders and coatings was examined using the FESEM images, and phase identification was performed by XRD analysis. The erosion behavior of coatings and mechanism of material removal was discussed by examining the microstructure images of worn-out surfaces. WC-CoCr cermet coating deposited with nanosized WC grains exhibited higher cavitation erosion resistance as compared to conventional coating.     

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 of Fe–Mn–Si–Cr shape memory alloys

Cavitation erosion tests of three Fe–Mn–Si–Cr shape memory alloys were carried out at speed 34 and 45 m/s using a rotating disc rig, and their cavitation damage has been investigated by comparison with a referring 13Cr–5Ni–Mo stainless steel used for hydraulic turbine vanes. The research results proved that the cavitation erosion of the Fe–Mn–Si–Cr shape memory alloys is a failure of low cycle fatigue and fracture propagates along grain boundaries. After 48 h cavitation erosion the cumulative mass losses of the studied alloys at speed 45 m/s are more than theirs at speed 34 m/s; however, the effect of velocity on cavitation damage of the Fe–Mn–Si–Cr alloys is much lower than that of 13Cr–5Ni–Mo stainless steel. The cumulative mass loss of the 13Cr–5Ni–Mo stainless steel are 26.3 mg at speed 45 m/s and 3.2 mg at speed 34 m/s, and the mass losses of the Fe–Mn–Si–Cr alloys are within the range of 3.6–7.3 mg at speed 45 m/s and 2.0–4.1 mg at speed 34 m/s. The surface elasticity of the Fe–Mn–Si–Cr shape memory alloys is better than that of the 13Cr–5Ni–Mo stainless steel, and the effect of surface elasticity on cavitation damage increases with velocity. The excellent surface elasticity of the cavitation-induced hexagonal closed-packed (h.c.p.) martensite plays a key role in contribution of phase transformation to the cavitation erosion resistance of the Fe–Mn–Si–Cr shape memory alloys. The cavitation damage of the studied alloys at speed 45 m/s mainly depends on their surface elasticity, and the variation of 48 h cumulative mass loss (Δm) as a function of the elastic depth (h_e) can be expressed as Δm=2.695+[1371.94/(4(h_e−46.83)²+12.751)] with a correlation factor of 0.99345.     

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.     

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.     

The slurry erosive wear of physically vapour deposited TiN and CrN coatings under controlled corrosion

The erosion-corrosion of mild steel (BS6323), in the presence and absence of physically vapour deposited (PVD) TiN and CrN coatings, was studied, in comparison with that of AISI 304 stainless steel, in an aqueous alkaline slurry solution containing alumina particles. The influence of applied potential and particle velocity on the total erosion-corrosion loss was examined, and the respective corrosion and erosion damage (both contributing to the overall weight loss) then assessed by means of microscopical investigation of the morphology of the damaged surface, and subsequently evaluated quantitatively. The superior erosion-corrosion resistance of both the coatings compared to that of the uncoated mild and stainless steels was shown to be due to their resistance to both wear and corrosion. According to the detailed corrosion mechanisms revealed and different responses to wear, schematic diagrams were proposed to outline the main features of the corrosion-erosion process and the individual roles of erosion and corrosion. Discrete differences, in terms of the respective erosion and corrosion processes, between the TiN and CrN coatings, and between the mild and stainless steels, were also investigated and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Comparative Study of Thermally Sprayed Coatings Under Different Types of Wear Conditions for Hard Chromium Replacement

The tribological properties of part surfaces, namely their wear resistance and friction properties, are decisive in many cases for their proper function. To improve surface properties, it is possible to create hard, wear-resistant coatings by thermal spray technologies. With these versatile coating preparation technologies, part lifetime, reliability, and safety can be improved. In this study, the tribological properties of the HVOF-sprayed coatings WC–17%Co, WC–10%Co4%Cr, WC–15% NiMoCrFeCo, Cr₃C₂–25%NiCr, (Ti,Mo)(C,N)–37%NiCo, NiCrSiB, and AISI 316L and the plasma-sprayed Cr₂O₃ coating were compared with the properties of electrolytic hard chrome and surface-hardened steel. Four different wear behavior tests were performed; the abrasive wear performance of the coatings was assessed using a dry sand/rubber wheel test according to ASTM G-65 and a wet slurry abrasion test according to ASTM G-75, the sliding wear behavior was evaluated by pin-on-disk testing according to ASTM G-99, and the erosion wear resistance was measured for three impact angles. In all tests, the HVOF-sprayed hardmetal coatings exhibited superior properties and can be recommended as a replacement for traditional surface treatments. Due to its tendency to exhibit brittle cracking, the plasma-sprayed ceramic coating Cr₂O₃ can only be recommended for purely abrasive wear conditions. The tested HVOF-sprayed metallic coatings, NiCrSiB and AISI 316L, did not have sufficient wear resistance compared with that of traditional surface treatment and should not be used under more demanding conditions. Based on the obtained data, the application possibilities and limitations of the reported coatings were determined.     

Cavitation erosion resistance of high nitrogen stainless steel in comparison with low N content CrMnN stainless steel

Abstract

The cavitation erosion (CE) of a high nitrogen stainless steel (HNS) and a low nitrogen CrMnN stainless steel in both distilled water and 3%NaCl solution at 20±1°C was investigated by using a magnetostrictive induced cavitation facility. The evolution of CE with test time was analysed by morphology observation by SEM and roughness measurement after different CE intervals. The possible phase transformation of austenite to martensite due to cavitation was analysed by XRD, and cross-sectional microhardness after cavitation was also measured to evaluate the work hardening ability. The role of corrosion was analysed by polarisation curve. The test results indicated that HNS had a relatively higher CE resistance than CrMnN steel, which was mainly attributed to its higher work hardening ability, thicker wok hardening layer and lower stacking fault energy. Different from that of the HNS, many tiny cracks could be clearly seen in the cross-section of eroded CrMnN steel especially at the ferrite zones. The pure erosion dominated the whole cavitation damage process, and the synergistic effect between corrosion and erosion was relatively small for both steels. The CE behaviour of HNS was relatively more sensitive to the corrosion media than that of CrMnN steel. Therefore, it should be a little bit careful when HNS was used in corrosive media.     

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.     

Slurry Erosion Behaviour of Detonation Gun Spray Al2O3 and Al2O3–13TiO2-Coated CF8M Steel Under Hydro Accelerated Conditions

Slurry erosion performance of detonation gun (D-gun) spray ceramic coatings (Al₂O₃ and Al₂O₃–13TiO₂) on CF8M steel has been investigated. Slurry collected from an actual hydro power plant was used as the abrasive media in a high speed erosion test rig. Attempt has been made to study the effect of concentration (ppm), average particle size and rotational speed on the slurry erosion behaviour of these ceramic-coated steels under different experimental conditions. The analysis of eroded samples was done using SEM, XRD and stylus profilometry. The slurry erosion performance of the D-gun spray Al₂O₃–13TiO₂-coated steel has been found to be superior to that of Al₂O₃-coated steel. Both the coatings showed brittle fracture mechanism of material removal during the slurry erosion exposure.     

Experimental Study on Influences of Surface Materials on Cavitation Flow Around Hydrofoils

In order to resist on the cavitation erosion, many researchers try to change the solidity and tenacity of the coatings,but ignore the influence of surface characteristics of materials on cavitation flow and the interaction with each other.In this paper, high speed visualization system is used to observe the cavitation flow patterns in di erent stage. After comparing the characteristics of cavitation flow around hydrofoils made of aluminum(Foil A), stainless steel(Foil B)and the hydrofoil painted with epoxy coating(Foil C), the study shows that material has a significant e ect on the cavitation flow. Firstly, when the incipient cavitation occurs, cavitation number of Foil A is highest among three hydrofoils, generating horseshoe vortex randomly. For Foil B and Foil C, it shows in the form of free bubbles. When the sheet cavitation occurs, Foil A has the highest cavitation number and shortest period, which is contrary to Foil C. And cavity consists of lots of small finger-like cavities. For Foil B and Foil C, it both constitutes with many bubbles. Compared with the high-density and small-scale cavities over surface of Foil C, the cavity of Foil B has larger scale and less density,which causes a minimal scope of influence of the re-entrant jet and strong randomness. When the cloud cavitation occurs, Foil C has the lowest cavitation number and shortest period. Secondly, compared with aluminum, both of stainless steel and epoxy coating restrains the occurrence and development of cavitation, and stainless steel and epoxy coating performs better than aluminum. For inception and sheet cavitation, stainless steel performs better than epoxy coating and aluminum. For cloud cavitation, epoxy coating performs better than stainless steel and aluminum.The objective of this paper is applied experimental method to investigate the e ect of surface materials on cavitation around Clark-Y hydrofoils.     

Revision of cavitation erosion database and analysis of stainless steel data

Cavitation erosion data have been accumulated in our laboratory for about 32 years since 1970. The database was constructed as electronic data in MS Excel files. The data files are able to offer quick search in terms of the test material, test method and test conditions from among 859 data. In this study, 131 data since 2003 were newly added to the database constructed in our previous study. The stainless steel data were analyzed, including various stainless steels such as ferritic, austenitic, duplex and martensitic stainless steels. Vibratory cavitation test results for different stainless steels, obtained with varying test conditions of frequency, amplitude and attachment of specimen, were converted analytically to obtain average erosion rates under assumed standardized conditions of a stationary specimen test with 1 mm standoff distance, and with frequency and amplitude as specified by ASTM G32. The average of erosion rate under the standardized condition (ASTM G32, stationary specimen method, standoff distance 1 mm) was determined for different stainless steels. The erosion resistance was defined as a reciprocal of erosion rate, and the correlation between erosion resistance and hardness of the specimen after erosion test was better than with the other mechanical properties. The erosion resistance is equal to 2.6E?07 × (HV × F_mat)^2.4 (HV; Vickers hardness, F_mat; material factor), and the correlation coefficient is 0.98. It was concluded that the erosion resistance of different stainless steels could be estimated with high reliability from the material hardness and the material factor.     

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.     

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.     

Electrochemical Analysis of the Slurry Composition for Chemical Mechanical Polishing of Flexible Stainless-Steel Substrates

The surface quality of the stainless steel affects the efficiency of flexible photovoltaics. Chemical mechanical polishing (CMP) is a finishing process that is used to prepare substrates for electronic devices. The CMP slurry composition is an important factor because additives in the slurry generally improve the polishing performance. However, it is limited to find the optimum conditions for the slurry by only experimental approaches. Thus, this study uses electrochemical analysis and friction monitoring to examine the effects of the abrasive, oxidizer, chelating agent, and pH. Electrochemical and monitoring analysis are useful for validating predictions and understanding interactions between the slurry and the stainless steel surface. Good correspondence was found between the predictions and the polishing results in more accurate. The corrosion rate (CR) obtained from the potentiodynamic polarization curve is proportional to the experimental results, as is the behavior of the curve and the coefficient of friction (COF). After only 3 min CMP, the best performing slurry (abrasive 39 wt %, oxalic acid 1 wt %, H₂O₂ 0.03 wt %, pH 1.5) improved the surface quality of 304 stainless steel by 4 nm. As a result, the proposed methods could help reduce the risks involved in stainless steel CMP slurry and these results could provide a reference for optimizing CMP slurry for flexible 304 stainless steel substrates.