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Experimental comparison of cavitation erosion rates of different steels used in hydraulic

turbines

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2010 IOP Conf. Ser.: Earth Environ. Sci. 12 012052

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Experimental comparison of cavitation erosion rates ofdifferent steels used in hydraulic turbines

L Tn-Tht1

1Institut de Recherche dHydro-Qubec, Expertise Mcanique, mtallurgie et

hydroolien 1800 boul. Lionel-Boulet, Varennes (Qubec), J3X-1S1, Canada

E-mail : [email protected]

Abstract. The prediction of cavitation erosion rates has an important role in order to evaluate

the exact life of components in fluid machineries. Hydro-Qubec has studied this phenomenonfor several years, in particular in hydraulic turbine runners, to try to understand the differentdegradation mechanisms related to this phenomenon. This paper presents part of this work. Inthis study, we carried out experimental erosion tests to compare different steels used in actualhydraulic turbine runners (carbon steels, austenitic and martensitic stainless steels) to highstrength steels in terms of cavitation erosion resistance. The results for these different classes ofsteels are presented. The tests have been performed in a cavitating liquid jet apparatusaccording to the ASTM G134-95 standard to simulate the flow conditions. The mass loss has

been followed during the exposure time. The maximum depth of erosion, the mean depth oferosion, and the mean depth erosion rate are determined. As a result we found that ASTM-A514 high strength steels present excellent cavitation erosion resistance properties. Thecavitation eroded surface is followed by optical profilometry technique. Determination ofmechanical properties and examinations of the eroded surfaces of the samples have also been

carried out in order to identify the erosion mechanisms involved in the degradation of thesekinds of materials.

1.IntroductionCavitation phenomenon often occurs in fluid machineries, turbo-machineries, ship propellers and in many

other applications [1]. According to ASTM G32-06 standard [2], cavitation is the formation and the subsequentcollapse, within a liquid, of cavities or bubbles that contain vapor or a mixture of vapor and gas. When these phenomena occur near a solid surface like a turbine blade, they may generate serious erosion damages andconsequent mass losses. These losses may induce perturbations of fluid flow and decrease the efficiency of

hydraulic machines. To protect these installations from cavitation, different materials in plate or welded may beused. Among them, we should cite the stellite alloys [3-5] or high strength cobalt austenitic stainless steels which

have demonstrated excellent properties to fight cavitation erosion [6;7]. Due to their high costs and despite thegood properties they demonstrate, they are not systematically used in hydraulic turbines runners. Morefrequently, austenitic stainless steels such as, SS308L or SS309L are used to repair cavitation damages inturbines. Actually, these kinds of materials have been studied several times because of their good combination ofcavitation erosion resistance and weldability at a competitive cost [8-11]. However, these last years a new kindof materials emerges in several field of the energy industry: quenched and tempered high strength steels.Primarily these steels were applied for building large structure like bridges because of their lightness, theirexcellent mechanical properties and their cost. Rapidly they were evaluated for other industrial equipment likepipelines or hydraulic turbines. In the present paper we propose to compare the cavitation erosion resistance of

quenched and tempered high strength steels to the one of austenitic and martensitic stainless steels. Erosion testshave been carried out in a cavitating liquid jet apparatus according to ASTM G134-95 standard [12], on basematerials and on weld overlays. The mass loss has been followed by weighting frequently during the exposuretime. The mean depth of erosion (MDE), the mean depth erosion rate (MDER) and the maximum depth oferosion (MaxDE) are determined and compared for these materials. Simultaneously, the evolution of the eroded

surface is followed by optical profilometry technique to give clues about the erosion mechanisms.

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2.Materials and experimental procedures2.1 Material properties

In the present study, six different materials have been tested: one martensitic stainless steel (S41500), twoquenched and tempered high strength steels (S690QL and S550QL), one welded austenitic stainless steel(EC308LSi) and two welded high strength steels (DS T-115 and DS 110). These kinds of materials have beenchosen because they are or they will be used for hydraulic turbines runners. The chemical composition and themechanical properties of the different tested materials are presented in Table 1 and Table 2.

Table 1 Chemical composition of tested materials (wt %)

C Si Mn P S Cr Ni Mo V Ti Cu Al Nb B N

S41500 0.01 0.46 0.84 0.021 0.004 12.2 4.6 0.61 - - - - - - -

S690QL 0.176 0.25 1.3 0.012 0.001 0.67 0.21 0.3 0.001 0.003 0.029 0.078 0.023 0.002 0.005

S550QL 0.169 0.28 1.28 0.012 0.0007 0.31 0.06 0.2 0.001 0.001 0.023 0.078 0.03 0.0018 0.004

EC308LSi 0.02 0.085 1.5 0.02 0.01 20.5 10.1 0.25 - - 0.2 - - - -

DS T-115 0.063 0.45 1.6 0.03 0.01 0.16 1.2 0.27 0.01 0.01 0.04 0.01 0.01 0.0006 -

DS 110 0.065 0.48 1.7 0.04 0.01 0.04 1.3 0.31 0.02 0.07 0.06 0.01 0.02 0.0008 -

Table 2 Mechanical properties of tested materials

Yield strength (MPa) Ultimate strength (MPa) Elongation (%) Vickers hardness

S41500 705 840 23 286

S690QL 830 890 16 295

S550QL 720 780 18 247

EC308LSi 420 550 42 192

DS T-115 761 875 20 302

DS 110 760 830 19 285

(a) S41500 (b) S690QL (c) S550QL

(d) EC308LSi (e) DS T-115 (f) DS 110

Fig. 1 Optical micrograph (500x) of the materials.

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Figure 1(a) represents the microstructure of S41500, 13% chromium - 4% nickel martensitic stainless steelwith improved toughness and good corrosion resistance to fresh water. It has been quenched after a 950 C heattreatment. This steel cannot be used with the brittle martensitic structure. A tempering treatment has beenperformed at 600 C. The resulting microstructure consists in a single martensitic phase with a small amount of

retained austenite for a Vickers hardness of 286. Figure 1(b) and 1(c) present the micrographs of S690QL andS550QL, low alloyed steels quenched and tempered at 550C for 1h. The difference between the two grades is a

larger amount of alloying elements in the case of S690QL, which improves the hardenability and brings the yieldstrength to 830 MPa. For both, grain size is sensibly the same and the microstructure consists mainly oftempered martensite. Vickers hardness is of 247 and 295, respectively for S550QL and S690QL. In Fig.1(d) wecan observe the microstructure of EC308LSi, a high chromium - nickel austenitic stainless steel commonly usedto repair cavitation damages occurring in hydraulic turbines. It is provided in solid wire electrode. Themicrostructure consists on an austenite solidification structure with some delta ferrite. The alloy content allowsan increase resistance to corrosion and heat. DS T-115 and DS 110 micrographs are presented in Fig. 1 (e) and1(f). These materials are flux cored electrodes (AWS A5.29 E110 classification) designed for welding high

strength steels. Although the compositions are relatively the same, the microstructure is a bit different. The

solidification structure is finer in the case of DS T-115. They consist both of a fine mixture of martensite andbainite. We must notice the high (285 - 302) Vickers hardness for both.

2.2 Experimental procedure

To prepare the welded samples, gas metal arc welding (GMAW) technique was employed. For convenience,EC308LSi was deposited on SUS304 base material whereas DS T-115 and DS 110 were deposited on ASTMA36 25 mm steel plates. As welded materials were overlaid with 10 mm in thickness, the mechanical properties

of base materials are here neglected. Cavitation samples of 25.4 mm x 25.4 mm x 19 mm were machined foreach material with a milling finished. Cavitation liquid jet erosion tests were carried out in a test chamber filled

with tap water at a constant upstream pressure of 23.8 MPa (the corresponding velocity was 218 m/s), at a liquidtemperature of 22 C, and at a cavitation number of 0.013 (the downstream pressure was 0.21 MPa). It must benoted that these parameters were optimized to get a 1000 mg / h mass loss rate on Al6061T6 reference sample.The stand-off distance between the nozzle outlet and the test specimen was 15 mm. The experimental apparatusis shown in Fig. 2 and Fig. 3.

Fig. 2 Schematic representation of the cavitating liquid jetapparatus.

Fig. 3 3D representation of the jet nozzle.

Each test was performed for a total of 150 minutes exposure time (5 periods of 30 min). After each cavitationperiod, the sample is cleaned in a methanol ultrasonic bath, dried and weighted. The topography of the surface isdetermined by optical profilometry technique. A series of 3 tests for each sample is done. The mean values arecalculated.

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3.Experimental results and discussion3.1 Erosion resistance

Fig. 4and Fig. 5show the mean cumulative depth of erosion (MDE) and mean cumulative depth of erosionrate (MDER) curves obtained from cavitation erosion tests for S41500, S690QL and S550QL compared to a

classical low carbon steel A516. Since densities of the materials are slightly different, it is more accurate toexpress the tests results by MDE, which is the mass loss divided by the material density and the eroded area(evaluated by image analysis at each step). The MDE curves of all materials pass through a period where theerosion rates are low, followed by an increase to a maximum erosion rate at 60 min for high strength steels and90 min for martensitic stainless steels and a decrease to lower erosion rate. The first period, called the incubationperiod, has been defined as the value obtained from the intersection of a straight extension line of the maximumrate period, the maximum slope, with the time axis on the MDE curves. These periods for S690QL and S41500are 16-19 min, which are about 4 to 5 times longer than for S550QL (which is 4 min). The maximum erosion

rates of the MDER curves are 123 and 142 m/h for, respectively S690QL and S550QL whereas it is 87 m/hfor S41500. Moreover the maximum erosion rates for high strength steels are between 28 to 38 % lower than the

one of classical carbon steel (197 m/h) and the maximum erosion rate of S41500 is 66 % lower than the one ofA516. If we compare the MDE, after 150 min test S550QL, S690QL and S41500 reach respectively 259 m, 246

m and 201 m values which are nearly half of A516 value. These results are in accordance with the highmechanical properties of such steels. It is therefore seen and confirmed that martensite microstructure gives totheses materials a good cavitation erosion resistance [11]. Fig. 6 and Fig. 7 show the MDE and MDER curves forweld overlays materials.

Fig. 4 MDE curves for base materials. Fig. 5 MDER curves for base materials.

Fig. 6 MDE curves for welding overlaymaterials. Fig. 7 MDER curves for welding overlaymaterials.

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The curves show incubation periods about 14-17 min for all tested materials. The maximum erosion rate isreached in 60 min for DS 110 whereas it is reached in 90 min for DS T-115 and EC308LSi. The maximum

erosion rate of the MDER curves is 110 m/h for DS 110 whereas it is 98 m/h for DS T-115 and EC308LSi.Hence, the maximum erosion rates for weld overlays materials are from 45 to 51% lower than the one ofclassical carbon steels. If we compare the MDE, after 150 min of test DS 110, DS T-115 and EC308LSi reach

respectively 224 m, 213 m and 211 m values which are nearly the half of A516 MDE value. This behavior isalso in accordance with the mechanical properties of materials. The higher cavitation erosion resistance of softeraustenitic stainless steel EC308LSi is related to high ductility and strain hardening and to induced martensitictransformation of this alloy [13]. Moreover, we can notice that for high strength steels cavitation erosionresistance of weld overlays is higher than for base materials. Fig. 8 shows the comparison of cavitation erosion

resistance for all tested materials. The cavitation erosion resistance is defined as the inverse of the maximumvalue of MDER.

Fig. 8 Comparative table of cavitation erosion resistance.

3.2 Surface evolution and of quenched and tempered high strength steels

The surface profiles were determined at several time periods of the test. The measures were made with an

optical profilometer with a lateral resolution (x ; y) of 2 m and a Z-resolution of 280 nm. Figure 7 gives anexample of the surface evolution of S550QL during the erosion periods.

a) 0 min b) 30 min

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c) 60 min d) 90 min

e) 120 min f) 150 min

Fig. 9 Evolution with time of the S550QL eroded surface.

At t=30 min we can observe a swelled part near the eroded ring. This deformation corresponds to the

cumulated plastic deformation on the material surface due to shock waves and micro-jets produced by the bubblecollapses. At t = 60 min, erosion depths reaching 700 m start to appear. In these figures we can observe an

heterogeneity of the erosion depth. At the end of the test, whereas the calculated MDE is of 259 m, the

observed MaxDE (maximum depth of erosion) is around 1270 m. For S550QL the evolution of MDE andMaxDE with time is shown Fig. 10. We can observe that after the incubation period, there is a good correlationbetween MDE and MaxDE. In this case, MaxDE is 5 times higher than MDE.

Correlation factors between MaxDE and MeanDE are presented in Table 3 for each material after 150 min ofcavitation erosion. As S415, S690QL and S550QL are laminated steels, the erosion process is supposed behomogenous on the surface. As the position of the most eroded areas are nearly the same for all cases, theobserved heterogeneity can be attributed to a deviation of the cavitating jet resulting in more aggressive

cavitation areas in the jet. However, this kind of aggressiveness variation can also be observed in hydraulicturbines.

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MDE150min

(m)

MaxDE150min

(m)

Correlation

factor

S41500 201 1080 5.4

S690QL 246 1350 5.5

S550QL 259 1270 4.9

EC308LSi 211 1160 5.5

DS T-115 213 1320 6.2

DS 110 224 1340 6

Fig. 10 Relation between MaxDE and MDE forS550QL.

Table 3 Correlation factor between MaxDEand MDE after 150 min of cavitation erosiontest.

3.3 Morphologies of damages surfaces

Fig. 11 shows the micrographs of cavitated surface for each kind of materials after 150 min. Whereas the

materials microstructures and the cavitation erosion resistance are different, on these pictures it seems that thereare no obvious differences on damaged surfaces. We can observe that the effect of the impacts induced by

bubbles collapses took the form of crater and hollows on grains. This characteristic suggests that the erosion wasmainly controlled by ductile fracture. We also see the presence of deep cracks between the grains. It seems to bemore pronounced on S550QL and less on EC308LSi.

(a) S41500 (b) S690QL (c) S550QL

(d) EC308LSi (e) DS T-115 (f) DS 110

Fig. 11 SEM micrographs of eroded surface after 150 min of cavitation erosion.

Fig. 12 shows the cross section of eroded surface after 150 min of cavitation erosion. Cracks initiated fromthe surface could be observed. For both tested samples, the fracture mainly begins from surface. However, wecan notice that some cracks are detectable under the surface, particularly for S690QL and S550QL. We can alsosee that for martensitic stainless steel and for quenched and tempered high strength steel the cracks propagate

perpendicularly to the cavitated surface along the former austenite grain boundaries. These boundaries representthe weak point of martensitic materials. In Fig. 10(c) we can observe a significative deformation of the surface

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before the crack propagates. Contrarily, for DS T-115, cracks propagate perpendicularly to the surface buttransgranularly. Thus, the homogeneity of the microstructure and the absence of weak austenite grain boundariesallow this fracture mechanism.

Fig. 12 SEM micrographs of cavitation sample cross section after 150 min of test.

4.Conclusions(1) The cavitation erosion resistance of one austenitic stainless steel, one martensitic stainless steel and four

quenched and tempered high strength steels has been measured in cavitation liquid jet tests. The rankingof tested steels in term of erosion resistance is S41500 > S690QL > S550QL for base materials andEC308LSi DS T-115> DS 110 for welding materials.

(2) The maximum erosion rates of these steels are between 28% (S550QL) and 65% (S41500) lower than for

carbon steel (ASTM A516).(3) For all these materials, the degradation is mainly controlled by a ductile fracture mechanism. Some

evidence of formation of wider cracks during cavitation erosion tests has been enlightened for S550QLand DS T- 115.

(4) The cracks initiated from the surface propagate along the former austenite grain boundaries in S41500,S690QL and S550QL. In DS T-115 the cracks propagates according a transgranular mechanism.

Acknowledgments

The author wishes to express his gratitude to Alexandre Lapointe, technologist at Hydro-Qubec, for hismeticulous experimental work and Raynald Simoneau, former scientist of Hydro-Qubec, for his help and hisnumerous advices.

(a) S41500 (b) S690QL

(c) S550QL (d) DS T-115

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[6] Simoneau R and Mossoba Y 1986 Field experience with ultra high cavitation alloys in Francis turbinesIAHR Symp. (Trondheim, Norway)

[7] R. Simoneau 1990, US Patent 4,751,046[8] Ahmed S M, Hokkirigawa K and Oba R 1994 Fatigue failure of SUS 304 caused by vibratory cavitation

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(Deauville, France)

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