parametric study of slurry-erosion of hydroturbine steels with and without detonation gun spray...

Parametric Study of Slurry-Erosion of HydroturbineSteels with and without Detonation Gun Spray Coatingsusing Taguchi Technique

HARPREET SINGH GREWAL, SANJEEV BHANDARI, and HARPREET SINGH

WC-Co-Cr coatings were deposited on some hydroturbine 13Cr4Ni and 16Cr5Ni steels by thedetonation-gun spray process. An in-depth characterization of the as-sprayed coating was doneusing X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy-dispersiveX-ray spectroscopy (EDS) techniques. Microhardness and porosity measurements were alsomade. The coating was found to have a typical splat-like morphology with some indications ofunmelted carbide particles. The XRD results showed the presence of WC as the primary phasealong with W2C and Co6W6C as secondary phases. Furthermore, the slurry erosion behavior ofthe coatings was investigated to ascertain the usefulness of the coatings to reduce the slurryerosion of the steels. The effect of four operating factors viz. the velocity, impact angle, con-centration, and particle size on the slurry erosion of coated and bare steels has been studiedusing a high-speed jet-type test rig. The sand used as an erodent was collected from a powerplant to replicate the actual turbine conditions. It has been observed that the given cermetcoating can enhance the erosion resistance of the steel. Velocity was found to be the mostsignificant factor affecting the erosion behavior of the coating, whereas it was the erodentparticle size in the case of uncoated steel. As evidenced from the SEM images, the plateletmechanism of erosion seemed to be the prominent one, causing the removal of material from thesurface of the steel, whereas for the coating, the formation and interlinking of cracks resulted inthe removal of material.

DOI: 10.1007/s11661-012-1148-y� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

SLURRY erosion is the degradation caused by theimpact of the solid particles entrained in a liquid on thesurface of the components. Problems from slurry erosionhave been observed in many industries such as mining,marine, offshore, utility, and hydropower plants.[1–7]

Most of the fluid handling equipments and machines inthese sectors face the problem of reduced life andmalfunctioning because of the damage of either thecontacting surface or/and the whole of the component.In hydroturbines, the sand particles mixed in the riverwater cause the erosion of turbine components. Fur-thermore, it could cause the change in the surfacemorphology of the components. The change in thesurface pattern can also produce flow irregularitiescausing the initiation of the cavitation, which aggravatesthe problems. The synergistic effect of cavitation stress-ing on the abrasive action of solid particles may enhanceerosion damage and complicate the erosion mecha-nism.[8] Furthermore, erosion is a complex phenomenon,

which involves several simultaneous and interactingprocess parameters.[9,10]

Various hydroturbine components under the effect ofthe slurry erosion are runners, guide vanes, nozzles,needles, labyrinth seal, etc. It has been learned that thehydroturbines located in the Himalayan regions ofIndian subcontinent are affected severely by slurryerosion because of the high content of sand particlesin the flowing water have a hardness in excess of7 mohs.[11–13] For instance, the Naptha Jhakri hydro-power plant (1500 MW), located on river Sutlej is one ofthe largest power plant in India, faces the deleteriouseffects of slurry erosion from the sand particles.[14,15]

According to the government reports, the Naptha Jhakripower plant suffered a loss of $77 million on account ofthe closure of the station pursuant to high silt levels inriver Sutlej during the period 2003 to 2005. It isworthwhile to mention that the financial losses incurredfrom the replacement and maintenance of the slurry-affected parts is excluded from the estimate. Theconcentration of sand in the river Sutlej normally variesfrom few ppm to 12,000 ppm. During extreme eventssuch as landslides, blockades, and flash floods, theconcentration can be as high as 30,000 ppm, and insome cases, even it crosses 1,000,000 ppm (10 wt pct).To prevent the turbines from damage, the maximumlimit for the plant has been limited to 5000 ppm.Whenever the concentration of sand in river Sutlejincreases beyond this value, the plant is generally shut

HARPREET SINGH GREWAL, Research Scholar, andHARPREET SINGH, Assistant Professor, are with the IndianInstitute of Technology Ropar, Rupnagar, Punjab 140001, India.Contact e-mail: [email protected] SANJEEV BHANDARI,Assistant Professor, is with the Department of Mechanical Engineer-ing, Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib140407, India.

Manuscript submitted August 31, 2011.Article published online March 28, 2012

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, SEPTEMBER 2012—3387

down to control the damage. The petrographic analysisof the sediment has indicated a high content of quartz,garnet, zircon, and other hard minerals.[15]

The various factors contributing toward the slurryerosion of hydroturbines can be categorized into one ofthe following three types[16–18]:

� Factors corresponding to the operating conditionssuch as velocity, impact angle, type of flow, particlesize, and concentration of the erodents

� Factors corresponding to the target materials suchas composition, microstructure, hardness, toughness,fatigue strength, yield, and ultimate strength

� Factors related to the erodent’s such as composition,hardness, shape, and size of the particles

To resist the wear and improve the working life of thecomponents, the surface coatings of hard materials areoften suggested.[14,19,20] Of the various methods avail-able in the literature for applying coatings, thermalspray methods are often used because of their ability tospray almost all types of materials, high productionrates, economical use, and environment friendly naturecompared with hard chrome plating, physical vapordeposition (PVD), and chemical vapor deposition(CVD).[19,21,22] The high porosity, decomposition ofpowder, and lesser bond strength were some of theissues of concern in plasma and flame spray coatingtechniques, which have been controlled to a large extentwith the use of high velocity oxy-fuel (HVOF) anddetonation-gun (D-Gun) thermal spray processes. Thehigh spraying velocities of an order of 1000 m/semployed in these processes have reduced the porositylevels to approximately 1 pct to 2 pct and greatlyimproved the bonding strengths.[23–25] Some researchershave studied the slurry erosion performance of 13Cr4Nisteel, which is the most widely used hydroturbinematerial[1,14]; however, it has been found from the openliterature that the erosion performance of 16Cr5Ni steelhas not been evaluated so far. In this article, the slurryerosion of the 16Cr5Ni steel has been studied andcompared with that of the 13Cr4Ni steel. In addition,the effect of the WC-Co-Cr coatings applied on bothsteels using the D-gun spray process has also beeninvestigated with regard to their slurry-erosion behavior.Many studies have shown that the application of carbidecoatings, WC in particular, can provide shieldingagainst the slurry erosive environment and prolong thelife of the components.[24–26] However, the decomposi-tion of the carbide grains (WC) during coating processhas to be minimized to improve the service life of the

coatings. It has been found that the addition ofchromium to WC–Co inhibits the decomposition ofWC and prevents the formation of the metallic tungsten.The chromium addition also improves the binding ofcarbide grains to the matrix, thereby improving theerosion resistance as compared with the WC–Co coat-ings.[26,27] Taguchi’s methodology has been used forstudying the effect of the various factors related tooperating conditions.

II. EXPERIMENTATION

A. Materials

Hydroturbine steels of two grades, 13Cr4Ni (13/4) and16Cr5Ni (16/5) were used in the present research work.These steels received in as cast form were machined andsamples of size 40 9 45 9 5 mm were prepared. Theactual chemical compositions of both the materials,determined by an emission spectrometer along with theirother properties have been presented in Table I.The samples to be tested in uncoated condition were

ground on a surface grinding machine and polished usingSiC emery papers down to 1000 grit. The roughnessvalues (Ra) of these samples measured using the surfaceroughness tester (Mitutoyo 301P; Mitutoyo AmericaCorporation, Aurora, IL) is also given in Table I.The samples of both, 13/4 and 16/5 grade steels were

coated with WC-10Co-4Cr powder (H. C. Starck,Munich, Germany) with a particle size range of +15 to45 lm. A scanning electron microscope (SEM) image ofthe coating powder shown in Figure 1 was taken usingJEOL Scanning Electron Microscope (Model JSM-840;JEOLLtd.,Tokyo, Japan) equippedwith energy-dispersiveX-ray spectroscopy (EDS) facility. It is obvious from theSEM image that the coating powder has a sphericalmorphology in general. The EDS results of coatingpowder have been presented in Table II.

B. Coating Deposition

The WC-10Co-4Cr powder was deposited ontohydroturbine steels (13/4 and 16/5) using the commer-cially available detonation gun facility at SVX PowderM surface Engineering Private Limited, Noida, India.Before the coating deposition, the polished steel sampleswere grit blasted using alumina particles of grit size 80.The grit blasting provides the sites for mechanicalanchorage, which in turn improves the adherence of the

Table I. Various Properties of Test Materials

Test Material/Coating Substrate Composition*

Density(kg/m3)

Microhardness(HV)

Roughness,Ra (lm) Porosity (pct)

WC-Co-Cr 13Cr4Ni 8.5-11.5Co, 3-5 Cr, 5-6C, 0.8 Fe max, W bal. 12500 1401 1.05 1.71 ± 3 pctWC-Co-Cr 16Cr5Ni 8.5-11.5Co, 3-5 Cr, 5-6C, 0.8 Fe max, W bal. 12500 1422 0.9 1.48 ± 3 pct13Cr4Ni — 0.2C 0.9Si 0.72Mn 3.72Ni 13.5Cr 0.73Mo 0.11W 7800 336 0.7 —16Cr5Ni — 0.1C 0.9Si 0.73Mn 4.16Ni 15.5Cr 1Mo 0.08W 7800 352 0.75 —

*Composition of coating powder as given by supplier.

3388—VOLUME 43A, SEPTEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

coating. The standard spraying parameters used forapplying the coatings have been reported in Table III.

The surface roughness of as-sprayed coating has beenreported in Table I.

C. Coating Characterization

The surface and cross-sectional characterizationof the as-sprayed WC-Co-Cr coatings was done usingX-ray diffraction (XRD), SEM/EDS and microhardnesstesting techniques. The SEM/EDS analysis was carriedout using a JEOL scanning electron microscope (ModelJSM-6610LV). For a cross-sectional examination, aslow speed diamond saw was used for sectioning thesamples. The sections were mounted in the epoxy resin

using a hot mounting press. These mounted sampleswere polished using emery papers down to 3000 grit.This was followed by cloth wheel polishing with 1 lm-Al2O3 particle suspension so as to achieve a mirrorfinish. The average thickness of the coatings was alsomeasured from the cross-sectional SEM images and hasbeen reported in Table III. The XRD analysis of theas-sprayed coating was done using PANalytical (X’pertPro; PANalytical BV, Almelo, the Netherlands) in thescanning angle range of 10 deg to 120 deg with a stepsize of 0.0670 deg using CuKa radiation. The apparentsurface porosity of the coating was evaluated from theoptical micrographs using a Leica Optical Microscope(DFC 295) fitted with Metallurgical software (LMWV 3.6.6; Leica Microsystems, Wetzlar, Germany). Themicrographs taken at 10 different locations couldrepresent a wide range of variability of the coatingstructure. Each reported value of porosity (Table I) is anaverage value of these 10 readings. The variation in thereadings was of the order of ±3 pct. The maximumporosity obtained for the coating deposited on 16Cr5Nisteel was 1.52 pct and the lowest value for 13Cr4Ni steelwas 1.65 pct. Looking into this context of values it couldbe said that although the difference in not much;however, the values seem to be differing. These resultsare yet to be confirmed by some chemical or electro-chemical technique before a conclusive statement isproduced.The microhardness was measured using a micro-

hardness tester (402 MVD; Wilson Hardness, Norwood,MA) at a load of 50 g with dwell period of 10 seconds.The microhardness values for the steels reported inTable I are the average of 10 readings taken at randomlocations. For the coatings, the variation of the hardnessalong the cross section was also studied. The hardnesswas measured at 10 locations along the coating thicknesswith 3 readings at each location along the transversedirection, resulting in 30 readings all together for eachsample forming a matrix of 3 9 10 hardness readings.The elemental mappings of the sectioned coatings weretaken using an SEM/EDS system to study the distribu-tion of various elements in the coating.

D. Erodent

For bringing the testing conditions as close as possibleto the actual hydropower plants, the sand particles usedas erodents were collected from Naptha Jhakri HydroPower Plant, Himachal Pradesh, India. The sandcollected was dried and sieved into eight different sizefractions: <53 lm, 53 to 75 lm, 75 to 95 lm, 95 to152 lm, 152 to 251 lm, 251 to 354 lm, and 354 to500 lm using vibratory sieving machine. The particlesize distribution of the collected sand has been shown inFigure 2, from which it could be analyzed that almost50 pct of the particles lie in the range of 95 to 152 lm.The underground desilting basin complex beingemployed at Naptha Jhakri is one of the largest plantsof its kind used to remove the sand particles from riverwater. This plant has been designed for 90 pct settlementof 0.2 mm size sediment and above with a maximuminflow sediment concentration of 5000 ppm, above which

Table II. EDS Results of Coating Powder

Element Wt Pct

C 07.81W 83.75Cr 02.55Co 05.90

Table III. D-Gun Spray Process Parameters Usedfor Deposition of the Coatings

Spray Parameters Value

Spray distance 200 mmSpray angle 90 degOxygen flow rate 2960 slphAcetylene flow rate 2400 slphNitrogen flow rate 1200 slphThickness of coating 80-100lmNozzle length 1 m

Fig. 1—SEM image of WC-Co-Cr coating powder used in thecurrent investigation.


the plant is shut down. The analysis of Figure 2 revealsthat 15 pct of particles larger than 0.2 mm are availablein the water flowing through the turbines. Thus, the85 pct efficiency of desilting plant is in confirmation withthe results reported by Singh et al.[15] in their researchwork.

In the current work, three different size distributionsof the sand particles were used as shown in Figure 3.The SEM and EDS analysis of the different sand typeshas been presented in Figures 4 and 5, respectively.From the SEM micrographs, it could be observed thatsand is composed of a mixture of irregular and roundedparticles. The variation in the percentage weight of thevarious elements in the three types of sands could beanalyzed from the EDS results, which illustrates thepresence of different minerals in each type of sanddistribution. The EDS results show the difference in the

Fig. 2—Particle size distribution of the sand collected from thehydropower plant.

Fig. 3—Particle size distributions of the sand used in the erosiontesting.

Fig. 4—SEM images of the sand particles used in the erosion testing:(a) type I sand, (b) type II sand, and (c) type III sand.


amounts of Si, Al, and Fe elements present in these threetypes of sands. This finding indicates the variation in theamounts of Al2O3, SiO2, and Fe2O3 in three differentdistributions of sand used. This difference in composi-tion could affect the mechanical properties of the sand.

E. Slurry-Erosion Testing

To conduct the slurry-erosion testing, a jet-type slurryerosion test rig was designed and fabricated. It is arecirculating-type rig as shown in Figure 6. The mea-sured quantities of water and sand are mixed in the tankto obtain the required concentrations of the slurry. Thepyramidal shape of the tank prevents the sedimentationof the sand compared with the one with a rectangularbase. The pyramidal-shaped tank does not provide anyspace for the sand particles to sediment as shown inFigure 6, whereas with the rectangular-based tank, thereare more chances of sedimentation, and a stirrer isrequired to keep them suspended. In pyramidal shapetanks, the particles are easily facilitated to flow undergravity. The centrifugal pump used for slurry transpor-tation is driven by a 7.5 H.P./1440 rpm electric motor.Slurry from the tank, sucked through a 100-mm G.I.pipe with the help of a pump, is delivered to the nozzlethrough 25-mm pipe with a set of control valves. Usingthese valves, the amount of slurry to pass through the8-mm nozzle can be controlled easily. An electromag-netic-type flow meter is placed upstream to the nozzle tomeasure the amount of slurry passing through thenozzle. By using continuity expression, the relationbetween the velocity of the jet and the dischargeindicated by flow meter was established. The holderused to hold the sample has been designed in such amanner to facilitate the variation of impact anglebetween the slurry jet and sample surface within anaccuracy of ±1 deg. Using this specimen holder, it ispossible to mount the sample at a desired impact angleranging from 15 deg to 90 deg. The standoff distancebetween the nozzle and the specimen was kept fixed at90 mm for all tests. The procedure adopted for exper-imentation was in accordance to ASTM standard G-76.To measure the amount of erosion, the weight loss of thesamples was measured using a precision weighingbalance with 0.1 mg accuracy and converted to equiv-alent volume loss using the densities given in Table I. Allthe samples were cleaned carefully with acetone prior toeach weight measurement.

Fig. 6—Slurry erosion test rig developed for the current experimentation.

Fig. 5—EDS of the sand used in the erosion testing: (a) type I,(b) type II, and (c) type III sand.


F. Design of Experiment

The design of the experiment is a powerful tool thathelps in interpreting more meaningful results with theminimum number of experiments using statisticalmethods for the analysis of data. It also helps inmodeling and analyzing the influence of control factorson the performance output. The most important stagein the design of experiment lies in the selection of thecontrol factors. The literature on the slurry erosionbehavior of different materials reveals that operatingparameters, viz., impact velocity, impingement angle,particle size, and slurry concentration, influence theerosion rate of materials largely.[16–18] The effect ofthese four parameters was studied using the L9 (34)orthogonal array of Taguchi’s methodology. In theconventional full factorial experimental design, itwould have required 34 = 81 runs to study the effectof four factors each at three levels, whereas Taguchi’sexperiment approach reduces it to only 9 runs. Thisprovides a greater edge in terms of the experimentaltime and cost. The operating conditions under whicherosion tests were carried out have been presented inTable IV.

Two samples were tested at each combination offactors given by the orthogonal array L9 illustrated inTable V.

Each test was run for 240 minutes with weightmeasurements taken at an interval of every 15 minutes.The analysis of variance (ANOVA) of the results wasconducted using ‘‘smaller is better’’ quality characteris-tics. Randomization of the runs was ensured.

III. RESULTS AND DISCUSSION

A. Coating Characterization

1. SEM/EDS analysis

a. Surface analysis. The SEM morphology of theas-sprayed coating on 13/4 and 16/5 steels is shown inFigures 7(a) and (b), respectively. The microstructure ingeneral consists of well-formed splats in the case of 16/5steel compared with the 13/4 steel case. The coatingstructures consist of both the discontinuities (microp-ores and voids) and a partially bounded or unboundedstructure, which is more prominent in the 13/4 steel case.The difference between the morphologies of the twocoatings could be the difference in the coefficients ofthermal expansions of two substrates. The coefficientsof thermal expansion for 13Cr4Ni steels is in the rangeof 16.2 lm/m �C, whereas that of the 16Cr5Ni steels isin the range of 11.4 lm/m �C.[28] During coating pro-cess, because of the higher working temperatures, someexpansion of the substrate could take place, which aftersubsequent cooling would try to return to its initialform. The coating that by now has well deposited on tothe steel would resist this activity of steel because of itslower expansion coefficient (around 7.2 lm/m �C in caseof WC-12Co coating[29]). The effect of this mismatch ofthe expansion coefficients would lead to the generationof residual stresses in the coating; the after effect ofwhich would be formation of crack and voids.[30] Thelarger the mismatch, the more profound would be theeffect of expansion coefficient.[30] In view of the previousdiscussion, it could be visualized easily why the identicalcoatings prepared using similar process parametersresulted in a different morphologies of coatings fortwo different substrates. These nonhomogeneities pres-ent in coatings can form the preferential sites for thefailure of the coatings during the high-energy impactscaused by the sand particles during the slurry erosionprocess. The SEM/EDS results of the bulk surfacearea and at some selected points are also shown inFigures 7(a) and (b) for both coated steel cases. Fromthese results, it can be concluded that in general, thesplats geometry shows a nearly homogeneous distribu-tion of the various elements present in coating, where Wand C are the major constituents. For 16/5 steel, aminute inclusion of Fe was observed at two locationsshown in Figure 7(b). The point EDS analysis at one ofthe observed point marked in Figure 7(b) shows that thewt pct of W is nearly thrice that of C, which might becaused by the presence of W2C phase at that particularlocation. The presence of W2C has also been indicatedby the XRD analysis of the as-sprayed coatings, as hasbeen presented in subsequent sections.

b. Cross-sectional analysis. The cross-sectional SEMexamination of the coated samples presented inFigures 8(a) and (b) indicates a well-bounded coatingstructure for both steel substrates, with a laminar splat-like morphology. Only a marginal presence of microv-oids is found in the coating sections. Cross-sectionalelemental mappings of the coated steels are presented in

Table IV. Level of Different Factors Used for SlurryErosion Experimentation

Control Factor

Levels

UnitsI II III

Velocity 20 40 60 m/sImpact angle 30 60 90 degConcentration 5000 10,000 15,000 ppmAverage particlesize

155 68 252 lm

Table V. L9 Orthogonal Array Used for Conductof Experimentation

Run No. VelocityImpactAngle Concentration PSD

1 1 1 1 12 1 2 2 23 1 3 3 34 2 1 2 35 2 2 3 16 2 3 1 27 3 1 3 28 3 2 1 39 3 3 2 1


Figures 9(a) and (b), respectively, for 13/4 and 16/5 steelcases, which are similar irrespective of the substratesteel. From these figures, it could be observed that thepositions of W and C do complement each other, whichindicates the presence of WC grains as single phase. Thishas also been confirmed from the XRD results. Acareful examination of Figure 9 also reveals the presenceof W in its free form at some locations. This could affectthe hardness at such location, which could be observedin form of variations of microhardness values inFigure 10. Fe was present in the coating powder as animpurity (shown in Table I), but its relative amountgives an indication that there might have been somediffusion from the base material toward coating. This

diffusion seems to have taken place along the splatboundaries; however, the extent of the diffusion is onlymarginal. At some locations, the coexistence of Co withthat of W is also found, which indicates the formation ofsome compounds among them. This is also supportedfrom the results of XRD of the as-sprayed coating givenin Figure 11, which confirms the formation of Co6W6Cphase. Traces of Ni can be observed along the splatboundaries, which could have been caused by diffusionfrom the base metal.

2. MicrohardnessThe microhardness measurements presented in Table I

show that 16/5 steel has higher hardness than 13/4 steel,

(a)

(b)

Fig. 7—SEM/EDS images of the surface morphology of the detonation gun as-sprayed WC-Co-Cr coating on the (a) 13Cr4Ni and (b) 16Cr5Nisteels.


although the difference is not significant. The averagehardness of the coatings is approximately 4 to 5 times thatof the substrate materials for both the steels. The averagemicrohardness measurements taken in amatrix of 3 9 10for the coated samples are compiled in Figure 10. It isobserved that as we move toward the coating interface, alayer of approximately 40 microns adjacent to the coating–steel interface showed almost 1.5 times hardness com-pared with the bulk hardness for both steels. This perhapsrepresents the work hardening of the steels because of thepeening effect of the grit blasting, as well as powderparticles during coating deposition process. A similarobservation has been reported by Hidalgo et al.,[31] Singhet al.,[32] and Sidhu and Prakash.[33] In Figure 10, thevariation of the hardness along the coating thickness canalso be observed. The small variations could have beencaused by the nonhomogeneity of the carbide phase in thecoating microstructure. Moreover, the microhardnessand other properties of the thermal spray coatings areanisotropic because of their typical splat structure anddirectional solidification.[32,34]

3. XRD analysisThe XRD results presented in Figure 11 show that the

as-sprayed coating has the presence of WC as a primaryphase, along with W2C and Co6W6C as secondary

phases. The presence of W2C perhaps indicates thedecomposition of the coating powder during sprayingprocess; however, the extent of the same is not signif-icant. The decomposition of powder might have takenplace because of overheating of some smaller sizedparticles during the coating process. Some improvementmay be expected by optimizing the coating parameters.

B. Slurry Erosion

The ANOVA analysis of the experimental resultsfor the uncoated and coated steel samples has beendisplayed in Table VIa–d.The results are reported in terms of the F-values and

the percentage contribution (P) of the individual factors.At a 95 pct confidence level, a comparison of theF-values calculated for both the coated and bare steelswith the corresponding tabulated F-value (4.25) indicatesthat all the factors selected in the study are significant.This outcome has also been confirmed by severalresearchers in their respective studies.[5,11,14,17,18,21] Ofthe various operating factors studied, velocity seems to bethe most significant in the case of coatings that affect thetotal volume loss with the percentage contribution ofaround 50 pct. For the uncoated steels, the particle sizedistribution seems to be the major contributor with>50 pct contribution toward material loss. The velocityfor steels is the second most effective factor. The volumeloss of bare steels has been reported 3 to 4 times morecompared with coated ones. A high erosion resistance ofcoatings canbe explainedon thebasis of the high hardnessof coating. The relative hardness of the target materialwith respect to that of an erodent plays a significantrole.[35,36] In a similarmanner to abrasivewear, it has beenshown that the erosion rate decreases considerably whenthe ratio of hardness of impacting particle with that oftarget materials, Hp/Ht is less than 1.2.[37] In the currentcase, the ratioHp/Ht was<1 for the coating, whereas forbare steels, it was approximately 3, which might haveresulted in higher volume losses for the steels comparedwith that for the coatings. For the cases whereHp/Ht<1,erodent particles cannot penetrate deep into the target,and thus, the extent of the damage zone is affectedgreatly.[38] The effect of hardness is more significant atlower impingement angles compared with higher or nearto the normal attacking angles. At lower angles of attack,scratch resistance determines the extent of damage, whichis controlled mainly by hardness.[39] At higher angles ofattack, the proportional amount of energy transferred tothe target material is higher (tangential component ofvelocity reduces) and the damage would be controlled byother mechanical properties such as fatigue strength,toughness, and ultimate and yield strength. WhenHp/Ht

>1.2, the level of damage is accelerated and is controlledmainly by the factors related to the particle.

1. Effect of velocityThe effect of the impact velocity on the slurry-erosion

volume loss is shown in Figure 12. The trends show thatwith the increase in velocity, the loss of materialincreases for both the coated and uncoated steels. Theseresults are obvious, as doubling the velocity is causing

Fig. 8—Cross-sectional SEM image of the WC-Co-Cr coated(a) 13Cr4Ni and (b) 16Cr5Ni steels.


Fig. 9—Elemental mapping results of Fig. 8 for (a) 13Cr4Ni and (b) 16Cr5Ni coated steels.


the kinetic energy to increase by fourfold. Similar resultshave been reported by other researchers.[17,18,26,35] Everytime the velocity is doubled, the kinetic energy of theparticles increases fourfold, and thus more energy isavailable with the erodent for the deformation andremoval of the material. Clark et al.[40] introduced theconcept of specific energies (energy required to removeunit volume of material), where the authors showed thatwith the increase in velocity beyond 22 m/s, the specificenergy remained almost constant for WC-Co-Cr coating(plasma sprayed), whereas for 316L steel it was constantbeyond 14 m/s velocity. A constant specific energy withrespect to velocity indicates that the volume loss ofmaterial increases correspondingly with the increase in

velocity or in other terms relation between velocity andvolume loss becomes linear. In the current study, almost alinear trend between the volume loss and the velocity hasbeen observed for both the coatings and steels withmaximum erosion occurring at a velocity of 60 m/s. The13/4 steel showed least resistance to erosion with amaximum volume loss of 3.5 mm3. The WC-Co-Crcoating, however, showed the least volume loss whenapplied on 16/5 steel. Compared with the 13/4 steel, thecoatings showed an improvement in the range of 3 to 3.5times over the whole range of velocity. The higher erosionresistance of coatings may be attributed to their highhardness with respect to that of erodent as explainedpreviously. For steels, the value ofHp/Ht lies in the range

Fig. 11—Results of the XRD for the detonation gun as-sprayed WC-Co-Cr samples of the (a) 13Cr4Ni and (b) 16Cr5Ni steels.

Mic

roh

ard

nes

s (H

V)

Distance along coating Thickness ( m)

Fig. 10—Variation of microhardness of WC-Co-Cr coated 13Cr4Ni and 16Cr5Ni steels along coating thickness.


of 3, whereas for coatings, this value is<1. Among thebare steels, the performance of the 16/5 steel was found tobe better, which could again be because of its higherhardness compared with the 13/4 steel.

2. Effect of impact angleThe impact angle of the erodents plays a significant

role in the erosion process, which can affect themechanism of erosion significantly. For ductile materi-als, the maximum erosion is likely to occur at approx-imately 20 deg to 30 deg before declining at higherangles.[17,41] This finding contrasts with brittle materialswhere the loss of material from erosion increasessteadily with the angle, reaching a peak at 90 deg. Thisdifference in the angles where maximum erosion occursdepends on the difference in the erosion mechanismsoperating in these materials. The ductile materialsundergo mass loss by a process of direct microcuttingor plastic deformation, followed by cutting. In brittlematerials, the energy transfer associated with therepeated particle impact results in a fatigue process.Lateral and radial cracks, both surface and subsurface,are formed, and the material is assumed to be removedby the intersection of these cracks with each other, aswell as the surface.[26,35] In the case of cermet coatings,the removal of the matrix material by microcutting atshallow angles can cause the spalling out of the hardphase (carbide particles) by the impacting erodent.However, at higher angles, the loosening of the carbide

grains can take place by cracks developed at fluctuatingstresses. The interlinking of these cracks can cause theremoval of a chunk of coating.[26] It is generally believedthat the impingement angle of particles in the hydraulicmachinery is small, even though the actual case is notknown. However, the type of flow (turbulence) couldcause the particles to impinge at higher angles. Inexperimentation with the designed rig, the variation inthe impacting angle could not be avoided because of thediverging effect of the jet. Although the results havebeen reported in this article for a specific set of impactangles only, but it should be kept in mind that theseresults are the averages of the spectrum of anglesexisting in the diverged jet. The effect of the impactangles on to the coated and bare samples is shown inFigure 13. It is obvious that the maximum erosion isoccurring at the 90 deg angle for all the samples.However, it is to be noticed that with the change inangle from 30 deg to 90 deg, the volume loss of thecoatings seems to remain roughly constant. This resultindicates that the impact angle has only a marginal effecton the erosion mechanism of the cermet coatings.Experiments on the D-gun sprayed WC-Co-Cr coatingby Wood et al.[26] have also shown similar results, whichreflects that even at low angles, the fluctuating stressesdeveloped are large enough to initiate the cracks. Theerosion response shown by this type of coating has beenclassified as mixed, ductile and brittle behavior. Themaximum resistance to erosion was shown by coating

Table VI. ANOVA Analysis of the Experimental Results

Factor DOF Sum of Squares (SS) Variance F-Ratios Percentage Contribution (pct)

(a) 13Cr4Ni bare steelVelocity 2 3.56 1.78 1781 22Impact angle 2 1.59 0.79 795 10Concentration 2 2.54 1.27 1270 16PSD 2 8.28 4.14 4141 52Error 9 0.001 0.001Total 15.977 100

(b) 16Cr5Ni bare steelVelocity 2 2.00 1.00 1003 21Impact angle 2 0.97 0.48 484 10Concentration 2 1.18 0.59 588 12PSD 2 5.39 2.70 2697 57Error 9 0.001 0.001Total 9.547 100

(c) D-Gun sprayed WC-Co-Cr coating on 13Cr4Ni steelVelocity 2 0.34 0.17 172 44Impact angle 2 0.15 0.08 75 19Concentration 2 0.02 0.01 10.5 16PSD 2 0.27 0.13 136 21Error 9 0.001 0.001Total 0.790 100

(d) D-Gun sprayed WC-Co-Cr coating on 16Cr5Ni steelVelocity 2 0.34 0.17 168 50Impact angle 2 0.09 0.04 43.5 16Concentration 2 0.09 0.05 45.8 15PSD 2 0.04 0.02 18.8 18Error 9 0.001 0.001Total 0.553 100


applied on 16/5 steel. Here also the improvement in therange of 3-3.5 times was shown by the coatings incomparison with the bare steels.

The volume loss of coating applied on differentsubstrates is also visible in Figure 13. The possible causeof this kind of behavior can be related to the differentmechanical properties associated with the base materials.The coefficient of expansion, ductility, yields strength,work hardening, etc. could affect the bonding between thecoating and the substrate. The compressive residualstresses induced in the coatings during cooling to roomtemperature could also affect the erosion performance ofcoatings. To study the effect of grinding on the erosion ofcoatings, a study was conducted by Murthy et al.,[25] inwhich they concluded that the compressive residual stressinduced into the coating as a result of grinding hasdecreased the erosion. The mitigation of the cracksbecause of the presence of compressive stress wasconsidered to be the possible reason. Not much is knownabout the effect of the previously mentioned mechanicalproperties on the erosion performance of coatings, butthey can affect the performance of coatings.

3. Effect of slurry concentrationTo study the effect of concentration of the slurry, the

accelerated test conditions were selected as given inTable IV. The effect of concentration on the volume lossof coatings and the bare steels has been presented inFigure 14. A linear response of the volume loss withconcentration was noticed and as has also been reportedby investigators[11,17] in the literature. It may be caused bythe increase in the number of particles per unit targetsurface area with the increase in concentration. Whenthe concentration in increased from 5000 ppm to10,000 ppm, the number of particles impacting the targetsurface would get almost doubled. Among all the coatedand uncoated steels, the erosion of the uncoated steelswas higher with 13/4 steel showing almost four timesmore volume loss compared with the coated 16/5. Amongthe coatings, the volume loss of 13/4 coated steel washigher than that of 16/5 coated steel, although not muchdifference was observed. The slope of the lines represent-

ing the erosion of the coatings is varying with the increasein concentration. The slope of lines when the concentra-tion is changed from 10,000 ppm to 15,000 ppm is nearlytwice as compared to the slope when the concentration isvaried from 5000 ppm to 10,000 ppm. This increase inthe volume loss of the coatings with the increase in theconcentration might be caused by the increase in thefrequency of the fluctuating stresses. The change in slopealso suggests that there might be a threshold value for acoating in respect to the frequency of particles striking it.Beyond the increase in this threshold value, the volumeloss could be increased because of enhancement in thefrequency of loading cycle. As discussed, the erosion ofthe cermet coatings is a result of the cracks formed fromthe variable stresses. The increase in the frequency ofthese stresses will enhance the level of fatigue stresses inthe coating and would certainly cause more cracks todevelop and grow. The formation and the linking of thesecracks could result in the disintegration of the coatingstructure in form of small chunks.[5,26]

0

1

2

3

4

Vo

lum

e lo

ss (

mm

3 )

20

16Cr5Ni

13Cr4Ni

40

Velocity (m/s)

WC-Co-Cr(1

WC-Co-Cr(1

60

16Cr5Ni)

13Cr4Ni)

Fig. 12—Effect of velocity on the volume loss of the 13Cr4Ni and16Cr5Ni steel samples, eroded in bare and D-gun spray WC-10Co-4Cr coated conditions.

0

1

2

3

4

30 60 90

Vo

lum

e lo

ss (

mm

3)

Impact angle (deg)

16Cr5Ni WC-Co-Cr(16Cr5Ni)


Fig. 13—Effect of the angle of impact on the volume loss of the13Cr4Ni and 16Cr5Ni steel samples, eroded in bare and D-gunspray WC-10Co-4Cr coated conditions.

0

1

2

3

4

5000 10000 15000

Vo

lum

e lo

ss (

mm

3)

Concentration (ppm)



Fig. 14—Effect of the concentration of sand on the volume loss ofthe 13Cr4Ni and 16Cr5Ni steel samples, eroded in bare and D-gunspray WC-10Co-4Cr coated conditions.


4. Effect of particle size distributionWide range of particle sizes of sand are found in

water passing through turbines as the desilting plantsare not fully efficient in arresting the particles for whatthey are designed. This could be analyzed from Figure 2,which shows the particle size distribution (PSD) of thesand particles collected from the water passing throughthe turbine. Singh et al.[15] has also reported the PSD ofthe Sutlej river showing the presence of 58 pct particles<75 lm and 25 pct particles in the range of 75 lm to200 lm. Thus, three types of distribution of sandparticles were prepared for the investigation. The effectof particle size has been studied by the number ofresearchers, and it is considered to be a significantfactor, which affects the amount of erosion.[5,17,18] Theeffects of three types of sand distributions tested on thevolume loss of the coated and bare steels have beenshown in Figure 15. The AFS number was calculatedfor each PSD and result is reported in terms of averageparticle size calculated from corresponding AFS num-ber. From Figure 15, it can be concluded that theparticle size shows a nonlinear behavior. The maximumerosion is taking place corresponding to an averageparticle size of 155 lm, whereas with either the increaseor decrease of the particle size from this threshold value,a reduction in the volumetric loss takes place. It isgenerally observed that as the size of particles decreases,the amount of erosion also decreases. The similar trendhas also been observed by other researchers,[5,17] whichcan be explained using the concept of kinetic energy.Even so, the similar reasoning does not seem to holdwell when explaining the lower erosion with larger sizeparticles. Kingwell et al.[19] showed that with theincrease in particles size from 160 lm to 260,m theerosion rate decreases. Finnie[17] suggested that the vol-ume removed by a given mass of abrasive grains isindependent of particle size for particles larger thanapproximately 100 lm. For particles below this size, theerosion process becomes less and less efficient as theparticle size is decreased. According to Yashimaet al.,[42] as the velocity increases, it has been observedthat the critical value, at which the fragmentation of theparticle could occur, decreases with the increase in size.Because of fragmentation, the average size of theparticles decreases and so does the kinetic energyassociated with it. Lathabai and Pender[5] reported thatthe average size of the particles decreases after theerosion experiments. It is only possible if the fragmen-tation of the particles takes place in the process oferosion. At the size >0.15 mm, the behavior of thebrittle erodent becomes unpredictable[43] because ofincrease in the number of defects in the particle itselfwith the increase in size. Thus, the decrease in volumeloss of the target material with the increase in particlessizes may have been caused by intensive fragmentationof the particle itself. Another possible reason could bethe hike in the shielding effect with the increase in theparticle size. With the increase of particle sizes, thechance of collisions between the attacking and rebound-ing particles also increases. These interparticle collisionsmight be causing the change in the path line of theattacking particles, deviating them to such an extent

that particles might go out without even striking thesurface.

5. SEM analysis of the eroded surfaceThe SEM micrographs depicting the surface mor-

phology of the eroded samples of the coated anduncoated steels are shown in Figure 16, indicating thepossible locations of failure for the 90 deg impact angle.Figures 16(a) and (b) show the eroded surface of 13/4and 16/5 bare steels, respectively, whereas Figures 16(c)and (d) are for the corresponding coated samples. Thecareful examination of Figures 16(a) and (b) reveal thesignificant presence of craters formed from the impact ofthe sand particles. The extrusion of the material aroundthe crater in the form of lips indicates the immenseplastic deformation caused because of the impact. Assuggested by Levy and Bellman,[44] these lips might bethinned out on further impacts and can finally bedetached from the surface. This mechanism as named bythe Levy platelet mechanism seems to be causing theerosion of the 13/4 and 16/5 bare steel at normalimpacts. For the execution of this mechanism, first, thepresence of ductility in the material is the foremostrequirement. Second, the significant amount of kineticenergy must also be available with the erodent to causethe plastic deformation. From the ANOVA analysis ofbare steels given in Tables V(a) and (b), the velocity andthe particle size are the two most contributing factors.The kinetic energy of the erodents also depends on thesefactors only; thus, these SEM micrographs give an ideato why these two factors, namely velocity and particlesize, were the two most significant contributing factors.According to Pugsley and Allen,[45] the erosion behav-

ior of cermet is complicated because of the presence ofboth the brittle and ductile phases. Erosion in this kind ofceramics does take place by several competing mecha-nisms that involve the deformation, cracking, and crush-ing of hard phase (WC, W2C in present coating);extrusion and deformation of binder phase; and thedisplacement and loss of single WC grains. Thesedifferent erosion modes depend on the erosive conditions

0

1

2

3

4

5

68 155 252

Vo

lum

e lo

ss (

mm

3)

Average Particle Size ( m)



Fig. 15—Effect of the particle size distribution of the sand on thevolume loss of the 13Cr4Ni and 16Cr5Ni steel samples, eroded inbare and D-gun spray WC-10Co-4Cr coated conditions.


as well as the composition and microstructuraleffect.[5,26,35] In the case of the cermet coatings, theerosion mechanism is complicated even more because ofthe inhomogeneous microstructure and phase transfor-mation of starting material. These changes cause a widevariation in the mechanical properties. The carefulexamination of Figures 16(c) and (d) shows the locationsof possible failure. The insight analysis of these locationswill show that the removal of coating has taken place insmall chunks. The mechanism of erosion of coatings assuggested by Lathabai and Pender[5] and Wood et al.[26]

could possibly explain as to how the erosion took place.The fluctuating stresses originated by impacting sandparticles cause the propagation of cracks by the fatiguemechanism leading to a microcracked coating. Thecracks can also initiate at defects in the coatings asshown in Figure 7 and when these cracks interlink, largeareas of coatings are removed. Thus themicrostructure of

the coating plays an important role in the crack propa-gation mechanism.

IV. CONCLUSIONS

� Coatings of WC-10Co-4Cr powder could be depos-ited successfully on 13/4 and 16/5 steels using thedetonation gun spray process. The microstructure ofthe coating was found to be effected by the substratesteels. The coating deposited on the 13/4 steelshowed a higher porosity along with a lower micro-hardness compared with that on the 16/5 steel. Theformation of secondary phases in the microstructureof the coating was not significant. WC was observedas the primary phase in the coating.

Fig. 16—SEM images of the samples eroded at 90 deg impact angle: (a) uncoated 13Cr4Ni, (b) uncoated 16Cr5Ni, (c) D-gun spray WC-Co-Crcoated 13Cr4Ni, and (d) D-gun spray WC-Co-Cr coated 16Cr5Ni.


� The parametric study conducted on the bare andcoated samples showed the particle size and velocityof the erodents to be significant parameters affectingthe erosion response of bare steels. In the case of thecoating, it was found that the particle size was notthe major factor to affect the erosion. In this case,erosion was affected predominately by velocityalone. Moreover, the independency of coatings forimpact angle below 90 deg was also observed.

� Bare 16/5 steel exhibited a better slurry erosion resis-tance compared with the 13/4 steel. The coating wasfound to be useful to enhance the slurry erosionresistance of the substrate steels by a factor 3 to 3.5.Higher erosion resistances of the coating could beexplained based on the favorable Hp/Ht factor,which is a ratio of the particle hardness to the targethardness. These results could help in improving thematerial design for enhancing the working life ofhydroturbines.

� The erosion mechanism, as evident from the SEMimages, was found to be predominantly plasticdeformation for the bare steels. For the coatings, themechanism consisted of fracture resulting from theformation and interlinking of cracks.

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parametric study of slurry-erosion of hydroturbine steels with and without detonation gun spray...

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