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Wear 267 (2009) 1954–1960

Contents lists available at ScienceDirect

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avitation erosion resistance of stellite alloy weld overlays

huji Hattori ∗, Norihiro Mikamiraduate School of Engineering, University of Fukui, 9-1 Bunkyo 3-chome, Fukui 910-8507, Japan

r t i c l e i n f o

rticle history:eceived 9 September 2008eceived in revised form 17 May 2009ccepted 31 May 2009vailable online 11 June 2009

a b s t r a c t

Stellite alloys have excellent cavitation erosion resistance and are often used for liquid machinery, butthe erosion properties of various stellite alloys have not been evaluated by a standard method. In thisstudy, we evaluate the erosion resistance for various stellite alloy weld overlays of ST6 and ST21 in avibrating method and in a cavitating liquid jet method. The grain size of the Co matrix affects the cavitationerosion resistance of stellite alloy weld overlays of ST6. The erosion rate of the maximum rate stage of

eywords:rosionavitation erosionibratory methodavitating liquid jet method

stellite weld overlay alloys of ST6-1, ST6-2 and ST6-3 were found to be about 1/13 to 1/7 times that ofSUS304. Moreover, we clarified the cavitation erosion mechanism of SUS304 and ST6 by scanning electronmicroscopy. Furthermore, by comparing the erosion behavior in a cavitating liquid jet method with thatin a vibratory method, it was found that the erosion rate of the cavitating jet method and the vibratorymethod have a good correlation.

telliterosion resistance

. Introduction

Cavitation is defined as the formation and subsequent collapse,ithin a liquid, of cavities or bubbles that contain vapor or gas, or

oth, in the ASTM G32-03 standard [1]. When the machine com-onents were exposed to cavitation, erosion occurs. The erosion ishe progressive loss of original material from a solid surface dueo continued exposure to cavitation [1]. Preece [2], Hammitt [3],arimi and Martin [4], and Lecoffre [5] reviewed the material effectf cavitation erosion, test methods and degradation mechanisms.ne of the present authors constructed the erosion database of car-on steels, alloy steels, aluminum alloys, copper alloys, titaniumlloys and so on, and analyzed the erosion data of carbon steels [6].

Recently, cavitation erosion of fluid machinery components hasecome more serious problem. Stellite alloys (Co-base alloys) withigh erosion resistance are often used for turbine blades, valves andhe like. Gould [7] carried out a cavitation erosion test with a vibra-ory method for five kinds of commercially available stellite alloys,nd reported that the erosion resistance of stellite 6B is relativelyndependent of hardness or grain size. But the chemical composi-ion of the five alloys was very similar, so that the comprehensive

aterial differences were not clear. Antony and Silence [8] used Co-

ase cast alloys with a carbon content widely ranging from 0.12% to.35% and carried out cavitation erosion tests. They reported thathe erosion resistance increased with the carbon content until 0.2%,nd kept constant between 0.3% and 1.4%. Heathcock et al. [9] used

∗ Corresponding author. Tel.: +81 776 27 8546; fax: +81 776 27 8546.E-mail address: hattori@mech.u-fukui.ac.jp (S. Hattori).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2009.05.007

© 2009 Elsevier B.V. All rights reserved.

six kinds of stellite cast alloys and found that the erosion resistanceof a stellite alloy with a carbon content of 2.6% increases twice asmuch as that of stellite 6 because of the fine carbide microstructurein the alloy. However, few studies have been carried out for coatingssuch as weld overlay. Moreover, the cavitation erosion mechanismof stellite alloys has not been clarified yet.

In this study, cavitation erosion tests were carried out on stel-lite alloy weld overlays which are often used for components offluid machineries. A vibratory method based on ASTM G32-03 wasemployed for the test. The cavitation erosion–time curves in thevibratory method are discussed. Three base materials (SUS304,SUS316 and S15C) were tested and the erosion characteristics ofthe test materials are compared. Simultaneously, the cavitation ero-sion process was observed with an SEM and a model of the erosionmechanism was established. The result clarifies the cavitation ero-sion mechanism of stellite. Cavitating liquid jet tests were carriedout to obtain the cavitation erosion–time curves and the erosionresistance is compared with that in the vibratory test method.

2. Material and experimental procedures

Tables 1 and 2 show the chemical composition and the mechan-ical properties of the test materials. The stellite overlays used in thisstudy are stellite 6 (ST6) and stellite 21 (ST21) of cobalt base alloys.ST6 is a hardfacing material and has resistance both to corrosion and

erosion. ST6 is often used for the components of fluid machineries.Since ST21 includes Mo, this alloy has better corrosion resistanceand better thermal shock resistance than ST6, which is used for fluidmachineries operating in seawater, or for components operated athigh temperatures. The substrate of the overlay ST6 was a carbon

S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960 1955

Table 1Chemical composition of materials (mass%).

Material Co Ni Cr W Mo Fe C Si Mn S P

ST6 Bal. ≤3 28 4 ≤3 1 – – – –ST21 Bal. 2.5 27 – 5 ≤2 0.25 – – – –S25C – – – – – Bal. ≤0.30 ≤0.60 ≤1.00 ≤0.04 ≤0.04SUS316 – 9–13 17–21 – 2–3 Bal. ≤0.03 ≤1.50 ≤1.50 ≤0.04 ≤0.04SUS304 – 8.21 18.47 – – Bal. 0.071 0.48 1.21 0.022 0.025

Table 2Mechanical properties of materials.

Material Density [g/m3] Yield point [MPa] Tensile strength [MPa] Elongation [%]

ST6 8.42 – 920 –ST21 8.30 – 800 –S25C 7.80 ≤245 ≤480 ≤19SUS316 7.87 ≤205 ≤480 ≤33SUS304 7.98 – 618 62

Fig. 1. SEM photographs of the origi

Table 3Vickers hardness of the materials.

Material HV0.2

ST6-1 451ST6-2 506ST6-3 567ST21 381S25C 166SUS316 218SUS304 209

Fig. 2. MDE curves of ST6.

nal surfaces of ST6-1, 2 and 3.

steel S25C with 0.25% carbon content, whereas the substrate of ST21was an austenitic stainless steel SUS316. For comparison, stainlesssteel SUS304 was used as highly erosion resistant material.

ST6 was overlaid with 3–5 mm in thickness on the S25C sub-strate with three methods, which were called ST6-1, ST6-2 and

ST6-3. But the detailed methods are kept unknown at present. Thehardness and the microstructure of each weld overlay are listed inTable 3 and Fig. 1(a)–(c), respectively. The hardness was measuredat a load of 200 gf and for a holding time of 15 s at room temperaturewith a Vickers microhardness tester. The hardness of ST6-1 is the

Fig. 3. MDE curve of ST21.

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owest, and it becomes higher in the order of ST6-2 and ST6-3. How-ver, all materials are almost twice as hard as SUS304. The surfacesf all overlay ST6 alloys were etched on the mirror-polished testpecimen for 30–40 s in potassium ferricyanide solution and thenbserved by scanning electron microscopy (SEM). Fig. 1 shows thathe surface of the overlay ST6 consists of carbide (black) and Co-

atrix (white). The difference in size of the Co-matrix was observedor the three materials. By comparing Fig. 1(c) with (a) and (b), theo-matrix Fig. 1(c) of was clearly smaller than those of Fig. 1(a)nd (b). By comparing Fig. 1(a) with (b), the Co matrix of Fig. 1(a)

ooks smaller than that of (b). But, the cobalt grain regions werelongated, and some of them joined together. On the other hand,arger round cobalt regions are observed for ST6-2. Thus, the Co

atrix of ST6-1 with the lowest hardness of the three is relatively

Fig. 4. SEM photographs of er

67 (2009) 1954–1960

large. The Co matrix of ST6-3 with the highest hardness is relativelysmall.

Cavitation erosion tests were carried out by using a vibratoryapparatus as specified in the ASTM standard G32-03 [1]. The testmethod was a stationary specimen method. A disk of 16 mm indiameter made of erosion resistant SUS304 steel was screwedinto the amplifying horn of an oscillator, and the test specimenwas placed in close proximity to the vibrating disk. The distancebetween disk and test specimen was 1 mm. The resonance fre-quency of the oscillator was 19.5 kHz, and the double (peak to

peak) amplitude of the disk was 50 �m. After using a vibratingdisk for 10 h, it was replaced by a new one. Deionized water wasused as test liquid and kept at 25 ± 1 ◦C with a temperature controldevice.

oded surface of SUS304.

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S. Hattori, N. Mikami / W

Cavitating liquid jet erosion tests were carried out in a test cham-er at a constant upstream pressure of 17.4 MPa (the correspondingelocity was 185 m/s), at a liquid temperature of 25 ◦C, and at a cav-tation number of 0.025 (the downstream pressure was 0.44 MPa),

hich is specified in the ASTM G134 standard [10]. The stand-ff distance between the nozzle inlet and the test specimen was0 mm.

. Experimental results and discussion

.1. Erosion of ST6 and ST21 weld overlays

Fig. 2 shows the MDE (mean depth of erosion) curves obtainedrom the cavitation erosion test with ST6 weld overlays and a ref-rence material SUS304. The key “ST6 overlay (previous)” indicateshe data points of the ST6 alloy weld overlay which have previ-usly been obtained in our laboratory. Since the material densitiesf ST6 and SUS304 are different, the test results are expressed byhe MDE which is the mass loss divided by the material density andest area (a circular area of 201 mm2 with a diameter of 16 mm).he MDE curves of all materials pass through a period where therosion rates are very low, followed by a liner increase in a max-mum rate period. The incubation period has been defined as thealue obtained from the intersection of a straight extension linef the maximum rate period with the time axis. The incubationeriods of ST6-1 to ST6-3 are about 14–17 h, which are about 4–5imes as long as that of SUS304 (which is about 3 h). The peri-ds are a little shorter than that of the ST6 overlay (about 20 h).he numbers in Fig. 2 show the slope of the straight line portion

n the maximum rate stage of the MDE curves. The slopes of theaximum rate stage are 0.28 �m/h for ST6-1, 0.22 �m/h for ST6-

, 0.17 �m/h for ST6-3, 0.16 �m/h for ST6 overlay, and 2.17 �m/hor SUS304, respectively. The slope of the maximum rate stage ofT6 overlays is about 1/13 to 1/7 times less than that of SUS304,nd it is therefore seen that ST6 overlays have a good erosionesistance. Among ST6-1, ST6-2 and ST6-3, ST6-3 has the best ero-ion resistance and the resistance is getting worse in the orderf ST6-2 and ST6-1. The slope of ST6-3 is about 0.6 times that ofT6-1.

Fig. 3 shows the MDE curves of ST21, ST6-3, the two substrateaterials (S25C and SUS316) and of a reference material (SUS304).

he incubation period of ST21 is about 18 h, which is 6 times asong as that of SUS304 with about 3 h. The period is lightly longerhan that of ST6-3 with about 18 h. The slope of the maximum rateeriod of ST21 is 0.32 �m/h, which is one-seventh that of SUS304ith 2.17 �m/h. The slope is 2 times steeper than that of ST6-3 with

.17 �m/h. By comparing the slopes of the maximum rate periodf the overlay stellites with that of the substrate materials, ST6-turns out to be about one-27th that of SUS316 with 4.72 �m/h

nd ST21 is about one-34th that of S25C with 11.1 �m/h. Thus, aT21 weld overlay has a better erosion resistance than its substrateaterial.

.2. Cavitation erosion mechanism of SUS304 and ST6

Eroded surfaces of SUS304 and ST6 were observed by scan-ing electron microscopy (SEM) to clarify the erosion mechanism.ig. 4 shows the SEM photographs of SUS304. After 45 min (Fig. 4a),lastic deformation occurred inside crystal grains by cavitation

nd accumulated at grain boundaries. After 3 h (Fig. 4b), crack-likerooves began to occur in the highly deformed area along a gainoundary near point A′. After 4 h (Fig. 4c), material removal (ero-ion) occurred in the grain. After 5 h (Fig. 4d), the erosion extendedcross a wide area inside the grain.

Fig. 5. Model of erosion process on SUS304 specimen.

Fig. 5 is a schematic model of the erosion on the cross sectionA–A′ based on the observation of the eroded surface. The origi-nal surface was repeatedly exposed to the collapses of cavitationbubbles, and therefore plastic deformation of the material surfaceoccurred by shock waves and micro jets in the bubble collapses(Fig. 5a). Since bubble collapse repeatedly acted on the deformedarea, the area gradually expanded, and plastic deformation accu-mulated at crystal grain boundaries as shown in Fig. 5(b), whichcorresponds to the SEM photograph after 45 min (Fig. 4a). Since thematerial surface was plastically deformed, swelled parts appearedat the crystal grain boundaries. These swelled parts produced astep relative to the adjacent grain with less plastic deformation andcaused a high stress concentration, resulting in crack initiation asin Fig. 5(c). Fig. 5(d) shows that the erosion easily occurred at thecrack initiation site. Fig. 5(c) and (d) corresponds to the SEM pho-tographs after 3–4 h (Fig. 4(b) and (c)), respectively. Fig. 5(e) showsthe more eroded surface after 5 h (Fig. 4d).

Similarly, the erosion mechanism of ST6 was investigated. Fig. 6shows the SEM photographs of ST6, which have already beenreported [11]. The Co matrix and the eutectic structure of carbideconsist of a virgin surface in Fig. 6(a). After 5 h (Fig. 6b), plasticdeformation occurred in Co matrix. The white part near the car-bide shows the eroded Co matrix. Eutectic carbides were removedfrom the whole area after 10 h (Fig. 6c), and then the matrix nearthe carbide was preferentially eroded (Fig. 6d).

Fig. 7 shows the erosion model of ST6 which was newly estab-lished in this study on the basis of the observation of Fig. 6. Fig. 7(a)shows the virgin surface of the ST6 eutectic structure consistingof the Co matrix and carbide. Fig. 7(b) shows that the Co matrix issofter than the carbide and plastic deformation occurs in the matrix.Thus, carbide plays the role of a grain boundary in the erosion mech-anism of SUS304. Plastic deformation occurs in the matrix, and aswelled part appears near the carbide. The swelled part near thecarbide has high stress concentration which easily initiates cracks.Fig. 7(c) shows that the erosion proceeds near the interface betweenthe Co matrix and carbide. And then the carbides falls off, which isobserved as a white portion after the removal of carbides in Fig. 6(b).

Fig. 7(d) corresponds to the SEM micrograph taken after 10–20 h(Fig. 6c and d), and shows that carbides fell off preferentially. Bycomparing the erosion result of ST6-1, 2 and 3 weld overlays, theCo matrix of ST6-3 turns out to be smaller than that of ST6-1, 2.Therefore, the matrix of ST6-3 does not easily cause plastic defor-

1958 S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960

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ation. Amongst the three kinds of weld overlays, ST6-3 has theest erosion resistance.

.3. Comparison of the erosion behavior in cavitating liquid jetethod with that in vibratory method

Fig. 8 shows the MDE curves in the cavitating liquid jet methodsing the same test materials as in the vibratory method. The MDEas defined as the mass loss divided by the material density and

roded area. The eroded area was obtained by a surface profileeter, which was 26–27 mm2. By comparing Fig. 8 and Fig. 2, the

ncreasing tendency of the erosion for all materials is relatively

imilar, but the behavior of the erosion in the incubation periods different. For example, by comparing the methods for ST6-3, the

DE curve for the cavitating liquid jet method gradually increasesntil 10 h and reaches 2 �m, while the MDE curves in the vibra-ory method reaches less than 0.2 �m at 15 h. Thus, the MDE curves

eroded surface of ST6.

in the vibratory method increase hardly, and the erosion in thecavitating liquid jet method progresses gradually in the incubationperiod.

Fig. 9(a) shows the surface profile of the eroded surface of ST6-1 tested after 30 h with the vibratory method. The peripheral areaaffected by the edge of the vibrating disk was eroded to about 5 �mdepth. The eroded area shows that the specimen was uniformlyeroded by about 3 �m in mean depth over the whole area with anunevenness of about 2 �m, except for the peripheral area. The meandepths of ST6-2 and ST6-3 tested after 30 h are different, but theeroded surfaces exhibit similar shapes. Fig. 9(b) shows the surfaceprofile of the eroded surface of ST6-1 after 30 h of testing with the

cavitating liquid jet method. The eroded surface of the cavitatingliquid jet method became W-shaped, because the center positionwith a high cavitation bubble population is weakly eroded, whilethe area between 1 and 3 mm from the center is strongly eroded.In the cavitating liquid jet method, the cavitation bubble collapse

S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960 1959

Fig. 7. Model of erosion process on ST6 specimen.

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Fig. 10. Relation between Vickers hardness and erosion resistance.

Fig. 8. MDE curves for cavitating jet method.

ressure depends on the location. The incubation period finishedn some areas while it was not yet finished in other areas, thus therosion proceeded in spite of the apparent incubation period.

Fig. 10 shows the relation between Vickers hardness and erosionesistance for all specimens. The erosion resistance was defined ashe reciprocal of the slope of an MDE curve in the maximum ratetage. Because the erosion mechanism of stellite has been clarified

n Fig. 7, and the erosion begins normally in the Co matrix, the ero-ion resistance should be evaluated in terms of the hardness of theo matrix. However, the size of the Co matrix was very small and

t was difficult to measure the hardness of the Co matrix only. Theonventional hardness is important to evaluate the erosion resis-

Fig. 9. Profiles of eroded surface.

Fig. 11. Relation between erosion resistances for vibratory method and for cavitatingliquid jet method.

tance, which is shown in Fig. 10. By comparing the differences in theweld overlay methods, the erosion resistance in both the vibratorymethod and the cavitating liquid jet method decreases in the orderof ST6-3, 2, and 1. By comparing the Vickers hardness of ST6-3 andST21, these are 567HV and 381HV, respectively. ST6-3 turns out tobe about 1.5 times harder than ST21. The harder the material is, thebetter its erosion resistance.

Fig. 11 shows a comparison of the erosion resistance in thevibratory method and the cavitating liquid jet method. The hori-zontal axis shows the erosion resistance for the cavitating liquid jetmethod, and the vertical axis shows the erosion resistance for thevibratory method. By taking the slope of the straight line, the testtime of the vibratory method is about 3.5 times that of the cavitat-ing liquid jet method. It can be seen that cavitating jet method andthe vibratory method have a good correlation.

4. Conclusions

Cavitation erosion tests were carried out on stellite alloy weldsoverlays which are often used for the components of fluid machiner-ies. Eroded surface, MDE curves and SEM observations of eachmaterial were compared with a reference material. The results ofthe vibratory method and the cavitating liquid jet method werecompared and discussed. We come to the following conclusions:

1. The harder the material surfaces of ST6 overlays due to differentweld overlay methods, the better becomes their erosion resis-

tance.

2. The erosion mechanisms of SUS304 and ST6 were clarified bySEM observations of the surface over the test time, and modelsof the erosion mechanisms were established.

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. In case of the cavitating liquid jet method, the surface profileeroded to a W-shape, and the erosion rate depends on the erodedarea. The erosion proceeds in spite of the apparent incubationperiod.

. The erosion resistance of stellite weld overlays is correlated withthe hardness of the material.

. The erosion rates in the vibratory method and the cavitationliquid jet method have a good correlation.

eferences

[1] ASTM Designation, G32-03, Standard Test Method for Cavitaion Ero-sion Using Vibratory Apparatus, Annual Book of ASTM standards, 2005,pp. 106–119.

[2] C.M. Preece, Cavitation erosion, in: C.M. Preece (Ed.), Erosion, Trea-tise on Materilas Science, vol. 16, Academic Press, New York, 1979,pp. 249–308.

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[3] F.G. Hammitt, Cavitation and Multiphase Flow Phenomena, McGraw-Hill Inc.,1980, pp. 220–299.

[4] A. Karimi, J.L. Martin, Cavitation erosion of materials, Int. Met. Rev. 31 (1986)1–26.

[5] Y. Lecoffre, Cavitation-Bubble Trackes, A.A. Balkema Publishers, 1999, pp.244–290.

[6] S. Hattori, R. Ishikura, Q. Zhang, Construction of database on cavitation erosionand analyses of carbon steel data, Wear 257 (2004) 1022–1029.

[7] G.C. Gould, Cavitation erosion of stellite and other metallic materials, in: Proc.3rd Int. Conf. Rain Erosion, 1970, pp. 881–901.

[8] K.C. Antony, W.L. Silence, The effect of composition and microstructure on cav-itation erosion resistance, in: Proc. 5th Int. Conf. on Erosion by Solid and LiquidImpact, 67-1-6.

[9] C.J. Heathcock, A. Ball, D. Yamey, B.E. Protheroe, Cavitation erosion of cobaltbased stellite alloys comented carbides and surface treated low alloy steels, in:

Proc. 3rd Int. Conf. on Wear of Materials, ASME, 1981, pp. 597–606.

10] A.S.T.M. Designation, G134-03, standard test method for erosion of solid mate-rials by a cavitating liquid jet, in: Anuual Book of ASTM standards, 2005, pp.576–588.

11] S. Hattori, A. Tainaka, Cavitation erosion of Ti–Ni base shape memory alloys,Wear 262 (2007) 191–197.