Influence of the vibratory test facility type and parameters upon the cavitation erosion evolution

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1 IOP Conference Series: Earth and Environmental Science Influence of the vibratory test facility type and parameters upon the cavitation erosion evolution To cite this article: I Bordeasu et al 2010 IOP Conf. Ser.: Earth Environ. Sci View the article online for updates and enhancements. Related content - Considerations upon the cavitation erosion resistance of stainless steel with variable chromium and nickel content A Karabenciov, A D Jurchela, I Bordeasu et al. - Experimental comparison of cavitation erosion rates of different steels used in hydraulic turbines L Tôn-Thât - Cavitation erosion - corrosion behaviour of ASTM A27 runner steel in natural river water L Tôn-Thât This content was downloaded from IP address on 23/11/2018 at 10:28

2 Influence of the vibratory test facility type and parameters upon the cavitation erosion evolution 1. Introduction I Bordeasu 1, M O Popoviciu 2, V Balasoiu 1, A D Jurchela 1 and A Karabenciov 1 1 Department of Mechanical Engineering, Politehnica University of Timisoara Bd. Mihai Vitezul, No.1, Timisoara, Romania 2 Academy of Romanian Scientists, Timisoara Branch ilarica59@gmail.com Abstract. Paper analyses the configuration of area and depth for the laboratory produced cavitation erosion. The affected zones were examined using both an optic microscope and a device for obtaining the cross-sectional profile of the eroded area. The cavitation was produced with a nickel tube magnetostrictive device (vibration amplitude 94 μm, vibration frequency 7 khz, specimen diameter 14 mm and power 500 W) as well as with a standard piezoceramic crystals device (vibration amplitude 50 μm, vibration frequency 20 khz, specimen diameter 16 mm and the power 500 W). The test specimens were manufactured from two different materials (steel and bronze). We found that the vibrations amplitude has greater influence upon the erosion (we obtained increase in the erosion maximum depth, in the total eroded mass and in the erosion velocity). Regardless of the running parameters, the way in which the deformations, the cracks and the dislocations are produced, is very similar. There are encountered great difficulties in controlling the cavitation erosion because to many phenomena are simultaneously involved [3, 4]: the hydrodynamic intensity of cavitation, the structure and the material properties and the liquid used. As a result, to estimate the cavitation erosion resistance there are used laboratory tests with different parameters [1, 2], depending of the influence which must put into evidence [4-6]. Regardless of modifying the parameters, there are only a few methods to determine the material resistance: cumulative (mass, weight or volume) losses and the erosion velocities variations during the cavitation. At the present time, an increased attention is given to the manner in which occur various influences upon deformations, cracks and microstructure modifications of the material, during the cavitation attack. The final purpose is the finding of procedures (manufacturing or/and heat treatment), which can increase the lifetime of pieces subjected to moderate or intense cavitation. Because the manufacturing of new materials is in full development and the laboratories use various vibratory methods to produce cavitation erosion, we think it is usefully to study the differences occurring on the area attacked with dissimilar test facilities. Therefore, the paper analyzes the similarities and the differences of the erosions produced through vibratory cavitation in the two facilities of the Timisoara Politehnica University Cavitation Erosion Laboratory, the first with nickel tube (T1) and the second with piezoceramic crystals (T2). The study is accomplished through analyzing both the evolution of the eroded area and the cavitation mechanisms. For this purpose, there were used pictures of the eroded area, taken with performing microscopes and photo cameras. For the discussions upon cavitation resistance and the intensity of the erosion given by the used test facilities there are considered both the characteristic curves and the specific parameters. For these tests were chosen two materials: a stainless steel used for manufacturing hydraulic turbine runners and copper alloy used for manufacturing marine propellers. 2. Tested materials As test materials there were used a stainless steel having a structure with 81% ferrite and 19 % austenite (established from the chemical components with the Shäffler diagram [1] and the bronze CuNiAl III - RNR used for see boat propellers having a structure formed by γ solid solution [1]. S.C. Prod SRL Bucharest (a production unity specialized in casting stainless steels) cast the used stainless c 2010 Ltd 1

3 steel upon a genuine formula. The CuNiAl alloy was taken in 1989, directly from a see boat propeller. The chemical composition is: 1. for the stainless steel: 0,118 %C, 23,86 %Cr, 10,09 %Ni; 2,89 % Mn; 2,32 %Si; 0,038 %Mo; 0,007 %W; 0,071 %V; 0,85 %Ti; 0,041 %Nb; 58,218 % Fe; the rest accompanying elements; 2. for the sea boat propeller CuNiAl III-RNR bronze: % Cu; 4.5 % Ni; 9.0 % Al; 4.0 % Fe; 3.5 % Mn; 1.0 % Zn; 0.1% Sn; 0.03 % Pb. In table 1 there are presented the mechanical characteristics of the materials used for the tests. For the stainless steel the values were obtained in CEMS Bucharest laboratories and for the CuNiAl bronze in the Material Science laboratory of the Timisoara Polytechnic University. Table 1 Mechanical properties Material Mechanical properties R m [N/mm 2 ] R p02 [N/mm 2 ] Hardness Stainless steel HRC CuNiAl III-RNR ,5 HB 3. Research method and the test facilities used The research method is that established by ASTM G [8]. The cavitation liquid used was tap water with the temperature maintained during tests at 20 ± 2 0 C. The vibratory devices used are presented in figures 1 and 2. Fig. 1 The T1 vibratory test facility with nickel tube and the specimen geometry Fig. 2 The T2-ASTM-G32 vibratory test facility with piezoceramic crystals and the specimen geometry The T1 magnetostrictive vibratory device with nickel tube has the following running parameters: 2

4 - electric power of the ultrasound generator: 500 W; - double amplitude of vibrations: 94 μm; - vibration frequency: 7000±3 % Hz; - specimen diameter: 14 mm. The T2 vibratory device with piezoceramic crystals was realized in 2009 (financed by the exploratory research ID-34 [9]). The test facility respects all the ASTM G requirements. The running parameters are: - the electric power of the ultrasound generator: 500 W; - double amplitude of vibrations: 50 μm; - vibration frequency: 20,000±2 % Hz; - specimen diameter: 15.8 mm. During every test, the parameters were carefully checked-up and maintained at the mentioned values. The total maximum duration of the cavitation attack, accepted in the Cavitation Erosion Laboratory of the Timisoara Polytechnic University, is 165 minutes [7]. This duration is divided into the following intervals: the first one of 5 minutes, the second of 10 minutes and the rest of 15 minutes. At the beginning and after each test period, the specimen was successively washed with double distilled water, alcohol and acetone and after that carefully dried with hot air. The weighing was done with an analytical balance which allows reading five significant digits. Finally, pictures of the eroded areas were taken with a photo camera and after that the same area was analyzed with an optical microscope OPTIKA VISION. During intermissions, the specimens were maintained in special containment vessel with dry atmosphere. After the final cavitation exposure (165 minutes effective attack) the specimens were axially sectioned and the structures analyzed using SEM (scanning electronic microscope) method, at the Bucharest Polytechnic CEMS Laboratory (Laboratory for Researches and Survey of Special Materials). 4. Experimental results In Table 2 are presented pictures of the tested specimens and the diameters used to establish the mean diameter of the eroded area. This mean diameter was used for computing the value (MDP) comp, (computed mean depth penetration), see relation (1). From the pictures the differences of the eroded area can be easily observed. Table 2 The eroded area aspect (x8, after 165 minutes of attack) and the diameters used to obtain the mean one Material T1 Test facility T2 Test facility Steel D m = 8570 <μm > D m = <μm > P <%> = P <%> =

5 CuNiAl III-RNR D m = 7840 D m = P = 32,5 P = To put into evidence the similarities and differences, of the cavitation erosion obtained in the used devices, in Table 3 is given a synthesis of the structural analyze, and in Fig. 4-5, 7-8, 10-11, as well as Fig 13-14, the structure aspect obtained with a scanning electronic microscope, in the axial cross section of the specimen. There are presented only the erosions appeared after 165 minutes of attack. In Fig. 3, 6, 9 and 12 are presented the eroded microstructures and the maximum depth of the area destroyed after 165 minutes of attack. Material- Facility Steel - T2 Steel - T1 Table 3 Structural analyse Important features of the eroded zone Area with uniform aspect of fine erosions, the wave propagation is put into evidence Propagation of the fracture through very fine inter-granular cracks Extremely fine excavations 1-5 μm Excavations with great diameters, over 50 μm, on their faces can be seen intergranular cracks The fracture face has preferentially trans-crystalline propagation Fragile fractures with inter-granular propagation Bronze T2 Fine and very fine erosions in equal proportions Surface with secondary fissures separate erosions with different dimensions Fragile fracture with inter-granular propagation of fissures Bronze - T1 Surface with uniform aspect of fine erosions and very fine inter-granular cracks Extremely fine excavations, 1-5μm 4

6 Fig.3 Micro structural aspect of the stainless steel specimen, in the zone with maximum penetration, x100 (facility T1) Fig. 4 The structure of the steel specimen at scanning electronic microscope (SEM), after cavitation erosion, (x100) - facility T1 Fig. 5 Detail of the surface presented in Fig. 4, x2000 Fig.6 Micro structural aspect of the stainless steel standard specimen, in the zone with maximum penetration, x100 (facility T2) 5

7 Fig.7 The structure of the steel specimen at scanning electronic microscope (SEM), after cavitation erosion, (x100) - facility T2 Fig. 8 Detail of the surface presented in Fig. 7, x2000 Fig.9 Micro structural aspect of the bronze specimen, in the zone with maximum penetration, x100, (facility T2) Fig. 10 The structure of the bronze specimen at scanning electronic microscope (SEM), after cavitation erosion, (x100) - facility T2 Fig. 11 Detail of the surface presented in Fig. 10, x2000 6

8 Fig.12 Micro structural aspect of the bronze specimen, in the zone with maximum penetration, x100 (facility T1) Fig.13 The structure of the bronze specimen at scanning electronic microscope (SEM), after cavitation erosion, (x100) - facility T1 Fig. 14 Detail of the surface presented in Fig. 13, x2000 In Fig are presented, the cavitation erosion characteristic curves, putting in evidence the erosion intensity of each test facility, correlated with the behavior and properties of the material. In Fig. 15 and 17, are given pictures of the exposed area, for three test duration, with the purpose to evidence the time evolution of the erosion for each test facility. 7

9 Fig. 15 Time dependence of cumulative mass erosion for stainless steel specimens Fig. 16 Time dependence of the erosion rate for stainless steel specimens Fig. 17 Time dependence of cumulative mass erosion for bronze specimens 8

10 Fig. 18 Time dependence of the erosion rate for bronze specimens From Figs. 15 and 17 but especially from Figs. 16 and 18 it can be seen that the scatter of the experimental points is similar for the two test facilities used. In conclusion, the scatter is dependent primary of the specimen material behavior, the influence of the test facility type being without great significance. Both the mass losses and the erosion rate differ significantly from one test device to another. So, comparison between the results obtained with different facilities is a very difficult task. For this reason, is necessary to standardize both the test facilities and the used procedures. Table 4 Cavitation erosion parameters Parameter Stainless steel CuNiAl III Bronze Device T1 Device T2 Device T1 Device T2 v s <g/min> v max <g/min> MDP comp <μm> ,87 MDP max <μm> 94,59 47,12 49,23 26,44 MDPR max <mm/min> The values of the material resistance parameters, resulted from the erosion characteristic curves (Fig ) are given in Table 1. Analyzing these values, we obtain the cavitation intensity of each device but at the same time is confirmed the necessity to use both the computed characteristic numbers and the diagrams of the evolution in time, in order to obtain a good prediction for the material behavior to cavitation erosion. 3 m 10 MDP comp = 4 2 ρ π Dm <mm> (1) (for steel ρ = 7,81 kg/m 3 ; for bronze ρ = kg/m 3 ) 5. Discussions Analyzing the eroded area with the help of an optic microscope (x4, x8), the following differences result: 1. For both materials subjected to cavitation, with the test facility T1, after 5 minutes of attack the erosion begin in the shape of a ring with a diameter of approximately 5 mm. If the attack is continued the eroded area is extended simultaneously both toward the outside diameter and the depth of the specimen, finally reaching the values given in Table2. 2. For both materials subjected to cavitation with the test facility T2, from the first minutes of attack the area is eroded approximately reaches specimen external diameter. Continuing the attack, the depth increases but the area remains approximately unchanged (see Fig. 15, 17 and Table 2). From the observation given under the points 1 and 2, we believe that this behavior occur mainly from the frequency differences than from the amplitudes (the electric powers supply of the ultrasound generators has identical values, 500 W). 9

11 3. Because the vibration amplitude of the device T1 is approximately double in comparison with the device T2, we appreciate that the increase of amplitude determines deeper erosions. Table 5 gives the rates T1/T2. It is to be noted that the rates for computed Mean Depth Penetration have greater values (4.69 for steel and 5.41 for bronze), because the eroded area is also involved. The metallographic structure, the grain dimensions and the brittleness of the tested material influence the rate values. Regardless of the used parameter and the numerical values, it is clear that the depths of the cavitation erosion are strongly influenced by the amplitude. Table 5. Rates T1/T2 Comparison parameter Steel Bronze Frequency MDP max MDPR max Stable erosion rate We appreciate that the extension in plane and depth of the eroded area depend on the shape and dimension of the bubble cloud attached to the working face of the specimen. This cloud at his turn is depended on the frequency and amplitude of the device. 5. The structure analyze of the eroded area obtained by using the scanner electronic microscope shows that regardless of the vibrating device parameters, the erosion has the same mechanism: deformation, crack inception and propagation and finally fracture of the material. 6. Conclusions 1. The cavitation erosion of a material produced in vibratory test facilities depends on the main parameters of the device (frequency and amplitude), which influence both the eroded area and its depth. The extension of the eroded area is influenced mainly by the frequency. Mainly the amplitude influences the depth. 2. Regardless of the device type (with nickel tube or piezoceramic crystals) and the parameters used for producing the cavitation erosion, the mechanism of the material fracture is the same. 3. The scatter of the measured points, (appearing mostly in the erosion characteristic curve for the rate of mass losses), depends in principal from the specimen material behavior, the influence of the test facility type having little significance. Acknowledgments The present work has been supported from the National University Research Council Grant (CNCSIS) PNII, ID 34/77/2007 (Models Development for the Evaluation of Materials Behavior to Cavitation) MDP comp MDP max MDPR max Dm P m Nomenclature Computed Mean Depth Penetration, [μm] Measured Maximum Depth Penetration, [μm] Measured Maximum Depth Penetration Rate, [mm/min] Mean diameter of the cavitation eroded zone, [μm ] Percentage of eroded area from the toal one, [%] Eroded mass in 165 minutes of cavitation erosion, [g] R m R p02 v s v max ρ Ultimate strength, [N/mm 2 ] Yield limit, [N/mm 2 ] Stable erosion rate, [g/min] Maximum erosion rate, [g/min] Material density, [kg/m 3 ] 10

12 References [1] Bordeasu I 2006 Eroziunea cavitaţională a materialelor (Editura Politehnica Timişoara) ISBN: (10) , (13) [2] Bordeasu I, Popoviciu M, Mitelea I, Ghiban B, Balasoiu V and Tucu D 2007 Chemical and Mechanical Aspects of the Cavitation Phenomena Chem Abs RCBUAU 58(11) Revista de Chimie 58(12) [3] Bordeaşu I, Karabenciov A, Jurchela A, Badarau R, Balasoiu V, Mitelea I and Ghiban B 2009 Considerations on the Influence of Nickel on the Cavitation Damage of Stainless Steels with 0.1% Carbon Content and Constant Chromium Content Metalurgia International XIV(12) 5-8 [4] Franc J P, Michel J M 2004 Fundamentals of Cavitation (The Netherlands: Kluwer Academic Publishers, P.O.Box, 322, 3300 AH Dordrecht) [5] Garcia R, Hammitt F G and Nystrom R E 1960 Correlation of Cavitation Damage with Other Material and Fluid Properties Erosion by Cavitation or Impingement ASTM (STP 408 Atlantic City) [6] Hammitt F G., De M, He J, Okada T and Sun B H 1980 Scale Effects of Cavitation Including Damage Scale Effects Conf. Cavitation Michigan Report No. UMICH, I [7] Popoviciu, M O and Bordeasu I 1998 Considerations Regarding the Total Duration of Vibratory Cavitation Erosion Test 3 rd Int. Symp. on Cavitation (Grenoble, France) [8] 2008 Standard Method of Vibratory Cavitation Erosion Test ASTM (Standard G ) [9] Proiect de Cercetare Exploratorie (CNCSIS-PN II) ID-34/2007 Dezvoltarea de modele pentru evaluarea comportării materialelor la eroziunea prin cavitaţie 11