This paper presents the research into behaviour of HSLA steel affected by cavitation. The parent material (PM) and simulated heat-affected zone (SHAZ) were studied. Ultrasonic vibratory cavitation test was performed in laboratory conditions (by stationary specimen method). Scanning electron microscopy (SEM) was used to observe the variations in the surface morphology that occurred within different time intervals

I. Samardžić

Lj. Milović

M. Dojčinović

V. Aleksić

English

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423METALURGIJA 55 (2016) 3, 423-425V. ALEKSIĆ, M. DOJČINOVIĆ, LJ. MILOVIĆ, I. SAMARDŽIĆCAVITATION DAMAGES MORPHOLOGY OF HIGH-STRENGTH LOW-ALLOY STEELReceived – Primljeno: 2015-05-27Accepted – Prihvaćeno: 2015-12-20Preliminary Note – Prethodno priopćenjeV. Aleksić, Institute for Testing Materials-IMS Institute , Belgrade, Serbia, M. Dojčinović, Lj. Milović, University of Belgrade, Faculty of Technology and Metallurgy, Belgrade, Serbia, I. Samardžić, University of Osijek, Mechanical Engineering Faculty in Slavonski Brod, Croatia.This paper presents the research into behaviour of HSLA steel affected by cavitation. The parent material (PM) and simulated heat-affected zone (SHAZ) were studied. Ultrasonic vibratory cavitation test was performed in laboratory conditions (by stationary specimen method). Scanning electron microscopy (SEM) was used to observe the varia-tions in the surface morphology that occurred within different time intervals. Key words: HSLA steel, cavitation damage, SHAZ, surface, time of testingINTRODUCTIONCavitation is a phenomenon that comprises forma-tion, growth and implosion (collapse) of bubbles in a liquid flow. During cavitation, when the bubbles col-lapse, high temperatures and pressures develop locally in a very short time interval, [1, 2]. Implosion of bub-bles creates impact-shock waves and micro jets, which energy dissipates within the liquid or becomes absorbed by a solid surface, which the liquid is in contact with, [3]. The energy of imploded bubbles absorbed by the material induces the elastic or plastic deformation or failure, thus representing the destruction affected by cavitation. Previous studies showed that cavitation re-sistance of materials commonly used for manufacturing of hydraulic elements depends on the mechanical prop-erties and microstructure, [4 - 6]. Low-alloy steels are alloyed with one or more elements wherein total con-tent of all alloying elements does not exceed 5 %. Such steels are classified according to the level of achieved strength, i.e. yield limit. The benefits of high-strength steels in relation to other structural steels are the follow-ing: reduced structure weight, increased carrying capac-ity, increased service life and lower costs of manufac-ture, [7, 8]. HSLA steels are, among others, used in shipbuilding. Due to cavitation occurring in exploita-tion of hydraulic machinery, HSLA steel NIONIKRAL 70 (NN70) was selected for testing of its resistance to the effects of cavitation. The aim of the presented re-search was to demonstrate surface damage morphology of HSLA steel affected by cavitation.EXPERIMENTSMaterialWithin this research, there were two types of speci-mens tested: (1) one specimen made of NIONIKRAL 70 PM specimen and (2) one specimen taken from the simu-lated HAZ of the same steel-SHAZ specimen. Chemical composition of tested material is presented in the Table 1. Table 2 overviews the tensile properties of tested materi-al. Microstructure of the parent material (PM) comprises tempered martensite and bainite (Figure 1a).Table 1 Composition of NN 70 / wt. % C Si Mn P S Cr Ni Mo0,1 0,21 0,22 0,01 0,02 1,3 2,4 0,3Table 2 Tensile properties of NN 70Specimen Rp0,2 / MPaRm / MPaPM 780 820HAZ 750 800In addition to the above specified micro constitu-ents, the microstructure of SHAZ includes a small quantity of ferrite with carbides (Figure 1b). It was de- (a)   (b)  Figure 1  Microstructure of a) parent material PM (microstructure of tempered martensite and bainite with carbides), b) simulated heat affected zoneISSN 0543-5846METABK 55(3) 423-425 (2016)UDC – UDK 669.15:539.21.004.64:621.9.015.620.4.18=111 424V. ALEKSIĆ et al.: CAVITATION DAMAGES MORPHOLOGY OF HIGH-STRENGTH LOW-ALLOY STEEL METALURGIJA 55 (2016) 3, 423-425termined that the material was characterized by a high level of porosity. Value of measured PM hardness was 269 HV1 and the value of measured SHAZ hardness was 276 HV1.Test methodsUltrasonic vibratory cavitation test was carried out in laboratory conditions to test the resistance of material to cavitation (by stationary specimen method), [9,10]. Testing was performed within the recommended stand-ard values of the following parameters [11]:•  The frequency of mechanical vibrations: 20 ± 0,2 kHz;•  The amplitude of vibrations at the top of the trans-former: 50 μm;•  Gap between the test specimen and the transformer: 0,5 mm;•  Water flow rate of 5 - 10 ml/s;•  The water temperature in the bath: 25 ± 1 °C.Morphology of damaged surfaces of the specimens was analyzed by scanning electron microscopy (SEM).RESULTS AND DISCUSSIONPM specimenIn the initial stages of exposing the specimen to cav-itation, there were undulations appearing on the surface, which indicated that there was a strain hardening of the surface. There were also determined pits of local char-acter, which were formed either as a result of separating inclusions from the matrix, or as a result of porosity pre-sent in the steel (Figure 2).After 120 minutes of testing, higher level of strain hardening occurred due to more pronounced undula-tions on the material surface layer, Figure 3. Further-more, an increase occurred in the size of pits that were in some places connected to each other. Those pits in the surface layer induced focus of energy of the impact waves created by the implosion of cavitation bubbles, which intensified the cavitation effects.After 180 minutes of exposure to cavitation effects, it was observed that the surface layer of the PM specimen developed two zones with different morphology of dam-age (zones A and B, Fig. 4). Changes in morphology cre-ated in the zone A did not differ from those created in the previous test intervals. It can be concluded that the zone A did not exhibit changes in the morphology of the sur-face even after an additional 60 minutes of exposure to cavitation effects, Figure 4a. In the zone B, there were groups of pits formed as a result of material porosity. The degree of damage in this interval was more pronounced than in the previous one, as the surface had cracks prob-ably initiated because of an increase of pits and their in-terconnecting, Figure 4b. After 240 minutes of exposing the PM specimen surface to the cavitation effects, there were numerous pits developed, which was most probably caused by the porosity of the initial PM. Both the number and size of pits were increased. Morphology of the entire specimen surface after testing indicated porosity of the material (Figure 5).SHAZ SpecimenAfter 90 minutes of exposing the SHAZ specimen to cavitation effects, there were initial grooves appearing, Figure 2  Morphology of the PM specimen surface test time 90 minutesFigure 3  Morphology of the PM specimen surface test time 120 minutes (a)  (b) Figure 4  Different morphology a) zone A and b) zone B of the same PM specimen after 180 minutes of testing.Figure 5  The morphology of the PM specimen test time 240 minutes425V. ALEKSIĆ et al.: CAVITATION DAMAGES MORPHOLOGY OF HIGH-STRENGTH LOW-ALLOY STEELMETALURGIJA 55 (2016) 3, 423-425around which the material deformed, Figure 6a. Such morphology is characteristic to tempered martensite. The Figure 6b shows the defect caused by the porosity of material.These two types of morphology were repeating until the end of experiment, where only the degree of damage differed, as depending on duration of exposure to cavi-tation effects. Prolonged exposure of 120 and 180 min-utes of testing to cavitation effects, affected in the simi-lar manner the increase in number and depth of pits from which the craters were formed and around which the material deformed. Because of similar surface dam-age mechanism during testing time, in Figure 7 is shown the morphology of the specimen surface after 180 min-utes of testing. As shown in the Figure 8a, after 240 minutes of testing, craters with a high degree of plastic deformation were observed. The Figure 8b shows a high degree of damage, which morphology indicates porosity of material.CONCLUSIONTesting the resistance of low-alloy high-strength steel NIONIKRAL 70 to cavitation effects by the ultra-sonic vibration method with stationary specimen in laboratory conditions provided the data used for analy-sis of morphology of damages, which was performed by the scanning electron microscopy (SEM). As of the ob-tained experiment results, the following was concluded:•  Morphology of PM surface is characterized by pro-nounced strain hardening; however, the morphol-ogy of damages in the subsequent stages only indi-cated the PM porosity, which cannot be correlated to the microstructure of PM represented by tem-pered martensite and bainite.•  During testing of SHAZ, two types of morphology were observed: morphology characteristic to tem-pered martensite and morphology of damage cre-ated as a result of material porosity.•  In the case of SHAZ, it was assumed that the pres-ence of bainite led to the reduction of negative ef-fects of ferrite, which has low resistance to cavita-tion effects.AcknowledgmentThe research is supported by the Project TR 35011, funded by the Ministry of Education and Science of the Republic of Serbia.REFERENCES[1] Brennen, C.E.,Cavitation and Bubble Dynamics, Universi-ty Press, Oxford, 1995.[2] Ahmed, S.M., K. Hokkirigawa, K., Oba, R., Fatigue failu-re of SUS 304 caused by vibratory cavitation erosion, Wear. 177 (1994), 129-137.[3] Hammitt, F.G. Cavitation and Multiphase Flow Phenome-na, McGraw-Hill, New York, 1980.[4] Karimi, A., Martin, J.L., Cavitation erosion of materials, Int. Met. Rev. 31 (1986), 1-26.[5] Heyman, F.J., Characterization and Determination of Ero-sion Resistance, ASTM STP474, 1970, pp. 212-222.[6] Knapp RT, Daily JW, Hammit FG. Cavitation. New York: McGraw-Hill; 1970.[7] Grabulov V, Blačić I, Radović A, Sedmak S., Toughness and ductility of High Strength Steels Welded Joints, Inte-gritet i vek konstrukcija, 8 (2008) 3, 181-190.[8] Gubeljak N, Predan J, Rak I, Kozak D., Integrity Asses-sment of HSLA Steel Welded Joint with Mis-Matched Strength. Integritet i vek konstrukcija, 9 (2009) 3, 157-164.[9] Dojčinović, M., Volkov-Husović, T., Cavitation damage of the medium carbon steel: Implementation of image analysis, Mat. Lett., 62 (2008), 963-958.[10] Dojčinović,M., Roughness measurement as an alternative method in evaluation of cavitation resistance of steels, Hem. ind., 67 (2013) 2,  323-330.[11] Standard Method of Vibratory Cavitation Erosion Test, G32-92. Annual Book of ASTM Standards, Vol. 03.02. Philadelphia: ASTM; 1992.Note:  Responsible person for English translation is prof. Martina Šuto, University of Osijek and prof. Marina Karšić, Croatia (a)  (b) Figure 6  The morphology of the SHAZ specimen test time 90 minutes (a)  (b) Figure 7  Morphology of the SHAZ specimen test time 180 minutes (a)  (b) Figure 8  The morphology of the SHAZ specimen test time 240 minutes

Cavitation damages morphology of high-strength low-alloy steel

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