An Experimental Study to Corelate Water Jet Impingement Erosion Resistance and Properties of Metallic

Wear 253 (2002) 650661

An experimental study to corelate water jet impingement erosionresistance and properties of metallic materials and coatings

B.S. Mann, Vivek AryaSurface Coatings and Treatment Laboratory, Corporate R&D Division, BHEL, Vikasnagar, Hyderabad 500093, India

Received 12 November 2001; received in revised form 10 April 2002; accepted 1 May 2002

Abstract

This paper describes the water jet impingement erosion characteristics of titanium alloy (Ti6Al4V), Hadfield steel, laser hardened, plasmanitrided and pack borided 12Cr steel along with most commonly used steels in hydro turbines. Round samples as per ASTM G73-98 weretested for water jet impingement erosion study. While testing, in the incubation period, plasma nitrided and pack borided 12Cr steelperformed much better than 12Cr and 13Cr4Ni steels. Plasma nitrided 12Cr steel performed much better than pack borided 12Cr steel.This is due to the integrity of plasma nitrided layers and their ability to absorb shocks due to jet impingement. During incubation as wellas in the long run, Hadfield steel and laser hardened 12Cr steel performed exceptionally well followed by 17Cr4Ni PH steel. Based onthis experimental study, a suitable criterion based on ultimate resilience (UR) for metallic materials and a composite modified resilience(CMR) for hard metallic coatings has been discussed. Water jet impingement erosion test results along with the mechanical properties ofmaterials and coatings, and their scanning electron microstructural details are reported in this paper. 2002 Elsevier Science B.V. All rights reserved.

Keywords:Hydromachinery; Cavitation erosion; Plasma nitriding; Boronising; Laser hardening; Jet impingement erosion

1. Introduction

Despite tremendous developments in the hydroturbinedesign and material improvements, liquid jet impingementerosion still remains an unsolved problem. This phenomenonoccurs in various cases besides hydroturbine: (i) propellers,hubs and rudders in case of ships, (ii) high speed pumpsof all types, (iii) regulators, valves and gate valves, (iv)flow-measuring equipment like orifices, venturies, (v) sud-den enlargements and bends, etc. It is essential to know themechanism of degradation of materials and identify a suit-able material or coating to combat jet impingement erosion.It has been assumed that the phenomenon of jet impingementerosion and cavitation erosion is identical. For evaluatingthe materials in cavitation erosion, ASTM G32, venturi androtating discs apparatus are used while for liquid jet impinge-ment erosion, ASTM G73 is used. ASTM G32 is less expen-sive compared to other techniques. Due to the similarities inthe erosion pattern of both techniques, liquid jet impinge-ment technique is preferred for coatings, elastomers andbrittle materials because various jet sizes can be used and

Corresponding author. Tel.:+91-40-3882332; fax:+91-40-3776320.E-mail address:[email protected] (B.S. Mann).

these are quite effective for evaluating all types of materials[1,2].

It has also been reported that the liquid jet impingementtest generally simulates the environmental conditions moreclosely[2]. In the present study, the mechanism of degrada-tion of materials due to liquid jet impingement erosion andcavitation erosion has been discussed and compared.

Material removal in water jet impingement and cavi-tation erosion processes in various materials lead to theidentification of four primary modes by which water dropimpingement or cavitation erosion can produce damage inmaterials. These are, direct deformation, stress wave prop-agation, lateral outflow jetting and hydraulic penetration.The damage produced by one or more of these loadingconditions on a material surface exposed to a single or mul-tiple water drop impact is responsible for initiating damageand subsequent material removal[311]. The evaluation ofdamage produced in target materials due to single waterdrop loading cycle is a complex dynamic process, whichinvolves a number of closely phased actions. Several im-portant properties of materials, such as material being cast,forged, rolled, either in heat treated condition or havinghard protective layers/coatings play an important role tocombat impingement erosion[1220]. Among all these, thehardness of material/protective layer of the coating plays

0043-1648/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0043-1648(02)00118-7

B.S. Mann, V. Arya / Wear 253 (2002) 650661 651

a significant role. Martensitic stainless steels possess thehighest cavitation erosion resistance followed by austeniticstainless steels while the ferritic steels have the lowestcavitation erosion resistance.

Cast martensitic stainless steels (Fe, 1118% Cr, 0.58%Ni, 29% Mn, 1% Si and 0.1% C) have high cavitationerosion resistance and are suitable for use as turbine ele-ments in hydroturbine power plants[21,22]. The applicationof martensitic stainless steels for high speed components inmarine environments against cavitation damage is feasiblebecause of its high hardenability developed by suitable heattreatment[21].

1.1. Cavitation-erosion correlation

It is reported that materials with different mechanicalproperties exhibit varying degrees of cavitation erosionresistance. Garcia and Hammitt[3] have proposed a corre-lation based on ultimate resilience (UR) which is definablefrom a stressstrain diagram. UR is a combined materialproperty given by the area of the triangle obtained whenthe yield point is raised to the level of ultimate tensilestrength (UTS), ifE is the modulus of elasticity. UR is givenbased on cavitation erosion experiments using a venturiapparatus[3,4].

UR = (UTS)2

2E

Another correlation has been proposed by Rao et al.[23]which is based primarily on the strength and hardness of thematerials. The cavitation damage rates in standard materialsaluminium, copper, brass and stainless steels, for example,were compared and test results were obtained using a ro-tating disk apparatus. The correlation proposed is based onthe modified ultimate resilience (MUR) and is given as,

MUR = UTS hardness of substrate2E

Thiruvengadam have proposed another correlation, basedon the energy absorption theory[24,25]. The cavitation dam-age rate versus the energy storage capacity of a materialwas determined for various materials, by taking into accountthe energy put into an erosion test rig minus the energy notconsumed in causing the damage (such as noise or heat). Ineffect, this is the area under the curve in the stressstrain di-agram up to the point of failure of materials. The theory ap-pears to be valuable for comparing test results from variousmaterials under different test conditions. The results wereobtained using a magnetostriction vibratory method.

In a composite structure, such as a hard coating on a softsubstrate, one attempt to increase the length of the cavitationincubation period is to combine the hardness of the coatingswith the UTS of the substrate. However, a pre-requisite isthe integrity of the coatings together with a high degree ofbonding to the substrate. A correlation based on composite

modified resilience (CMR) integrating base and coating isgiven [20,26,27].

CMR = UTS of the substrate hardness of the coating2 Youngs modulus of the substrate

Diffused coatings or nitride layers produced either by chem-ical or physical vapour deposition are the techniques ofachieving such a composite structure as reported earlier[20].

Richman and McNaughton[16,17] have proposed a the-ory to correlate the cavitation erosion resistance based onfatigue strength coefficient, 1f with an index of cyclicstress resistance measurement. Explosive cladding of anNiTi alloy with stainless steel is described as a cavita-tion erosion shield. The cavitation erosion mechanism wasconfirmed by finite element modelling rather than by arational search for materials with requisite high fatiguestrength coefficient combined with a high cycling strainhardening component. It was found that the NiTi alloyhas this property. Strong correlation was demonstratedbetween cyclic deformation property rate and cavitationerosion. The main determination of erosion resistance isthe fatigue strength coefficient ( 1f ), which is a measureof cyclic stress resistance. Material removal rate corre-lates well with the product of ( 1f , n

1), wheren1 reflectsthe cyclic strain resistance. The results are general over awide range of metal and alloys. Furthermore, this explainswhy previous attempts to correlate cavitation erosion andliquid droplet erosion behaviour with a single mechanicalor material property were unsuccessful. This is because f

1 is strongly influenced by cyclic strain hardening.Erosion behaviour is not simply related to any monotonicproperty, such as true fracture stress or ultimate tensile stress.

The cavitation erosion studies based upon four cavitationtunnels and four rotating disc apparatus, six vibratory testrigs, one liquid jet and two cavitating jet techniques werecompared[1,2]. Theoretical and experimental predictionsof different materials for liquid impingement erosion at dif-ferent velocities from different laboratories are also given.The erosion resistance of different materials with a standardmaterial (stainless type 316) is also compared. The differ-ence in the volume loss and standard deviation with regardsto their maximum erosion rate (depth of penetration) is alsoreported. However, there are still gaps in the informationwith regards to jet impingement erosion characteristics ofmaterials and coatings.

This paper presents data on water jet impingement ero-sion resistance of the various materials commonly used inpumps and hydro turbines along with boronising, plasmanitriding and surface hardening by laser. This also includeserosion characteristics during initiation of damage. Basedupon the study, a suitable correlation between impingementerosion resistance and mechanical properties was devel-oped. The results of our recent findings regarding water jetimpingement erosion resistance of pack borided, plasmanitrided and laser hardened 12Cr steel along with hydrotur-bine materials are reported.

652 B.S. Mann, V. Arya / Wear 253 (2002) 650661

1.2. Surface modifications and coatings

1.2.1. Laser surface meltingSurface modification of martensitic stainless UNS S42000

and laser claded stainless steel using a 3.5 kW continuouswave CO2 laser has been reported[28,29]. After laser sur-face hardening, the corrosion and cavitation erosion charac-teristics in 3.5% NaCl solution at 23C were studied usinga 20 kHz ultrasonic vibrators at a peak to peak amplitudeof 30m. The cavitation erosion resistance of laser meltedspecimens using a power of 1.7 kW and a scanning speedof 25 mm/s was reported 70 times that of the as received(annealed) S42000 and 1.8 times that of conventionally heattreated steel. The excellent cavitation erosion resistance wasdue to the combined effect of a high volume fraction of re-tained austenite (89%) and at moderate hardness (450 Hv).By using different processing parameters, it was foundthat the cavitation erosion resistance of the laser meltedspecimens increased with the increase in volume fraction ofretained austenite, a result attributable to the high marten-sitic transformability of the austenite in UNS 42000 steel.On the other hand, cavitation erosion resistance increasedwith the increase in hardness to a maximum value and thendropped with further increase in hardness. This indicated thatmartensitic transformability played a more important rolethan hardness in cavitation erosion. Due to pitting potentialvariation, the pits formed in the laser melted specimens wereshallower than those formed in as received and hardenedS42000 steel. The improvement in pitting corrosion resis-tance resulted from the dissolution or refinement of carbideparticles and the presence of retained austenite, as evidencedby the fact that the pitting potential increased linearly withthe amount of retained austenite[28]. Laser surface melting(LSM) for improving the cavitation erosion resistance bymeans of a continuous Nd:YAG laser has been attempted onaustenitic stainless steels UNS S31603 and UNS S30400,and super duplex stainless steel UNS S32760[29]. It is re-ported that the cavitation erosion resistance of the laser sur-face melted stainless steels was found to be highly dependenton the microstructural changes and the residual stress in thelaser melted layer. The cavitation erosion resistance of UNSS31603 was improved only by 22% after LSM. The lowcarbon content present in these stainless steels limits theirhardenability, and hence, limits the beneficial effect of LSM.

1.2.2. Plasma nitridingPlasma nitriding is a modern technique of surface hard-

ening of metallic components to improve their service life.Nitriding processes based upon solid, liquid and gas treat-ment have been traditionally used, which however, suffersfrom several drawbacks. Plasma nitriding overcomes themand moreover, it has replaced the conventional process inindustry. Basically, the plasma nitriding is a glow dischargeprocess in a mixture of nitrogen and hydrogen gases. Theapparatus consists of a vacuum chamber, a gas handlingsystem and a high voltage power supply. First, the vacuum

vessel is evacuated using a mechanical pump to a basepressure of 101 Torr then a nitrogen and hydrogen mix-ture is introduced into the chamber through a valve andfilled up to a few Torr. A high voltage is established be-tween the grounded vessel and the sample to be nitridedwhich is made negative. By adjusting the high voltage, itis possible to control the current density, which in turncontrols the nitriding temperature. The main advantages ofplasma nitriding over conventional nitriding processes are;reduced cycle time, controlled growth of the surface layer,elimination of white layer, reduced distortion, no need forfinishing (grinding, machining, etc.), pore-free surfaces andmechanical masks instead of copper plating.

Plasma nitriding is extensively covered in[3034]. Thenitrided layers consist of FeN, Fe2N, Fe3N, Fe2N3, Fe3N4diffused layers. The diffused layers range from tens of mi-crons to hundred of microns, and these are ideal for im-proving the wear resistance. By optimising the nitrogen andhydrogen ratio, it is possible to either eliminate some of thelayers or to improve the erosion/corrosion properties.

1.2.3. BoronisationBoronised steels have effectively improved their perfor-

mance in adhesive, sliding and abrasive wear. Boronising isvery effective, especially on low alloy steel, chrome-molysteel and cobalt-based alloys. An improvement in abrasivewear of boronised steels of order of 400500% has beenreported. In fact, pack borided 12% Cr steel, for steam tur-bine nozzle has performed exceptionally well and surpassedall other coatings including plasma nitriding and detona-tion sprayed coatings[35]. Not much data is available onboronised 12Cr and cast 17Cr4Ni PH steel applicable toimpingement erosion. Some data on boronised 817M40steel (equivalent to EN 24) in a cavitating environment isavailable[36]. It is reported that it has not performed well.The reasons for its adverse performance have not beeninvestigated. Some information on the adversely affectingof the mechanical properties, especially the toughness andcavitation erosion resistance has been reported[27].

2. Experimental techniques

2.1. Properties of materials and coatings

The mechanical properties viz. UTS, % elongation, URof the metallic materials and hard coatings were determinedby using a250 kN static and dynamic MTS servo hy-draulic universal testing machine. For this purpose, roundsamples were fabricated as per Annual Book of ASTMStandards 1978, Part 6 of E-8. The metallic materials andhard coatings, such as stainless steel (18:8), stainless steel(13:4), stainless steel (17:4 PH), Hadfield steels along withborided and plasma nitrided steels were studied.Table 1gives their composition. The mechanical properties werecorrelated with the jet impingement erosion resistance. The


Table 1Various materials used for testing

Materials Composition (wt.%)

12Cr AS 0.10 C, 12.1 Cr, 0.6 Si, 0.70 Mn, balance FeST AS 0.20 C, 12.0 Cr, 0.5 Si, 0.50 Mn, 0.5Ni, balance Fe13Cr4Ni 0.058 C, 12.06 Cr, 0.5 Si, 0.5.Mn, 3.85Ni, balance FeMDN AS 0.06 C, 15.67 Cr, 0.27 Si, 0.64 Mn,

4.25 Ni, 3.6 Cu, 0.19 Nb, balance FeHadfield 1.20 C, 12.0 Mn, 0.6Si, balance FeTi6Al4V 6Al, 4V, balance Ti

Table 2Mechanical properties of different coatings

Coatings Fracturetoughness(MPa m1/2)

Micro-hardness(HV)

Compositeresilience

Plasma nitrided12Cr steel

4.55.0 9421000 1.79

Pack boronised12Cr steel

4.04.5 18002000 3.16

Laser hardened12Cr steel

500650 0.945

micro-hardness of the coated steels was measured usingLeitzs micro-hardness tester by applying a load of 2.942 N.The micro-hardness values are given inTable 2.

2.2. Analysis of materials and coatings

Scanning electron micrographs (SEMs) and X-ray diffrac-tion (XRD) analysis of coated and uncoated materials werecarried out using Leica Electron Optics SEM and PhilipsExpert XRD system, respectively.

2.3. Plasma nitriding

Plasma nitriding was carried out using 15 kV plasma ni-triding reactor. This is because plasma nitriding ranks amongthe top accepted and extensively used technologies. Plasmanitriding is extensively covered in the literature[3037].The test specimens were nitrided for 48 h to obtain a layerthickness of 250m at 545C in a nitrogen and hydrogenatmosphere and then were slowly cooled in a nitrogen atmo-sphere after nitriding. This is to avoid formation of oxidesduring cooling. The following plasma nitriding parameterswere recorded while nitriding.

Voltage (V) 648700N2/H2 ratio 65/35Current density (mA/cm2) 12.25Partial pressure (Torr) 2.44.5Hydrogen flow rate (l/min) 1Temperature (C) 545575Work piece CathodeTotal time of nitriding (h) 48Holding time at 545C (h) 40

Steels used for nitriding were 17Cr4Ni PH steel of com-position 0.06% C, 15.67% Cr, 0.64% Mn, 0.27% Si, 4.25%Ni, 0.04% P, 0.03% S, 3.6% Cu, and balance Fe and 12Crsteel of composition, 0.12% C, 13.20% Cr, 0.55% Ni, 0.55%Mn, 0.30% Si and balance Fe (referTable 1).

2.4. Boronising

The boronising of the 12CR AS steel was done using apack cementation technique. The steel samples to be boro-nised were cleaned with acetone in ultrasonic equipment,dried and packed in the boronising pack mixture in a steelbox. The steel box was sealed using a copper washer be-tween the box and the cover. The packed steel box washeated in an electric furnace (250C/h), held at a tem-perature of 930 5C for 4 h and then forced air-cooled.After boronising, all the steel samples were thoroughlycleaned before testing and were tempered at 650C for6 h. All these samples were subjected to XRD analysis.This revealed the presence of FeB and Fe2B phases. Thelatter was dominant. The thickness of boronised steel wasmeasured using an optical microscope and was in the range5560m. An upper layer of 100m, including 5560mof boronised layer, was removed by grinding. XRD analysisof this sample was also carried out. The analysis did notshow the presence of metallic carbide, thus, eliminating thechances of carburising while boronising.

2.5. Liquid impingement erosion resistanceof different coatings

A liquid jet impingement erosion test facility has beendesigned and fabricated. After establishing the accuracyof results similar to those reported in ASTM G73-98, thisfacility has been used for testing of materials/coatings. Thetest facility consists of a chamber of diameter 700 mm anda round disc on which the test samples are fixed on the pe-riphery. Details of the facility are given inFig. 1. The disc isrotated at 4575 rpm to obtain the test sample tangential ve-locity of 147.0 m/s. Two water jets impinge on the cylindri-cal test samples and cause impingement erosion. The cylin-drical specimens were selected because the impingementerosion on actual turbine blades occurs at the leading edge.

A precision balance to an accuracy of 0.1 mg was usedfor measurement of weight loss occurring after a certain testduration. The test duration was selected in such a way thatsteady-state impingement erosion occurred. The accuracy ofthe results has been confirmed using a reference 12Cr steel.The standard deviation and resulting accuracy are also given.The deviation and accuracy lie within specified accuracydata available from different laboratories[8]. The resultshave been plotted as commutative erosiontime curve onco-ordinate of mean depth of erosion versus time. The depthof erosion is calculated from the weight loss divided by thedensity of the coatings and the materials. The test results aregiven inFig. 2.


Fig. 1. Water jet impingement erosion test facility.

Fig. 2. Volume loss vs. time for different materials and coatings.


Experimental test conditions:

Water jet size (mm) 4.24Water jet velocity (m/s) 20.166Test sample size, (mm) 12.70 40Number of specimens used 12Test duration (h) 7Radius of the specimen (mm) 6.375Exposed area of specimen (cm2) 0.785Jet distance (mm) 100Angle of impingement () 90Impact frequency (cycle/s) 78Experimental accuracy (%) 21.25Chamber pressure (atm) 1Water temperature (C) 30

3. Results and discussion

3.1. Scanning electron micrographs

Scanning electron microstructures of the eroded samplesare given inFigs. 310. From the micrographs, it is seen thatthe damage mechanism of 12Cr AS, ST AS, and 13Cr4NiAS steels is identical and all these materials are failing dueto formation of pits due to cleavage. The cleavage damagein 13Cr4Ni AS steel is slightly less compared to the othertwo steels. The hard-coated steels, such as 12Cr PN, and12Cr BN have failed in brittle mode. This is clear fromgrain removal in 12Cr BN and 12Cr PN. All these hardcoatings are very effective in delaying incubation period.Among these, plasma nitrided 12Cr steel is very effective.This is because the nitrided steel is more tough and ductilecompared to the boronised steel (Table 3). It is also seen fromthe micrographs that there is an oxide layer formation and

Fig. 3. Scanning electron microstructure of eroded 12Cr AS steel, 400.

this layer is removed due to erosion. In plasma nitrided andboronised steel, the percentage of chromium is reduced dueto the formation of chromium nitride and chromium boride,respectively, and so the resistance to corrosion is reduced. Itis also seen from the micrographs that 12Cr BN structure isvery fine. However, it erodes significantly more. The boridedlayers lack the UTS (Table 3). The grain morphology ofMDN AS, MDN PN, MDN HT and ST HT is identicaland similar. MDN HT is the finest among all these followedby MDN AS, MDN PN and ST HT. Because of thisMDN HT has performed much better than other steels. TheMDN series (MDN AS, MDN PN and MDN HT) erodein the ductile mode in a similar way to that of 12Cr steel.No oxide deposits were seen on ST HT, MDN AS, MDNPN and MDN HT, Ti 6Al4V and 12Cr LH (Figs. 710)confirming that corrosion does not contribute to erosion forthese materials. SEM of Ti6Al4V shows that the grains arefine, so its performance is also similar to MDN series. Themicrographs of 12Cr LH are similar to ST HT. After20 h of testing, the damages on Hadfield steel are negligible.Only water droplet marks are seen on this steel (Fig. 10).

After long exposure, deep microtunnels confirming mi-crojetting effects similar to cavitation erosion were observedin these steels, whereas in MDN series, 12Cr LH andST HT these deep microtunnels confirming microjettingeffects are not observed. Deep microtunnel confirming mi-crojetting effects may appear after a long duration. Noneof the steels have failed due to fatigue. This confirms thatthe mechanism of jet impingement erosion and cavitationdiffers on this aspect.

3.2. Properties of materials and coatings

Tables 24give the properties of all the materials andcoatings. All these properties are measured using tensile


Fig. 4. Scanning electron microstructure of eroded 12Cr PN steel, 400.

and hardness testing machines. From the properties, thebest property which has given an excellent correlation withimpingement erosion resistance is UR as well as MUR(product of UTS and hardness). As the UTS of the bulksteels varies linearly with hardness the UR and MUR rep-resent the same property of the steels. The properties ofplasma nitrided, pack borided steels are given inTable 2. Itis difficult to adopt a simple criterion based on hardness ofsurface coatings as information on other properties, such asUTS, hardness, modified resilience, fracture strength, etc.of surface coatings is not available. A suitable index, i.e.CMR as reported earlier correlates the hardness of the coat-ing and mechanical properties of the base steels[26]. TheCMR values for the coatings studied are given inTable 2. Itis also seen fromTables 24that other properties, such astoughness and yield strength of bulk materials do not play

Fig. 5. Scanning electron microstructure of eroded 12Cr BN steel, 400.

a significant role either in delaying the incubation period orimproving liquid impingement erosion resistance. However,in coated steel, the toughness and hardness have playedsignificant roles. Due to these properties, plasma nitridedsteel has performed much better than borided steel.

3.3. X-ray diffraction test results

Formation of nitride and boride phase in plasma nitridedand borided steels has also been confirmed by XRD analysis.The details of XRD analysis are reported elsewhere[27,37].

3.4. Liquid impingement erosion test results

The erosion test results of different coatings along withstainless steels are given inFig. 2. It is seen from this figure


Fig. 6. Scanning electron microstructure of eroded ST HT steel, 400.

that excellent performance is given by Hadfield steel andlaser-hardened 12CR steel. Kwok et al.[28] has reportedthat excellent resistance to cavitation erosion in case of lasertreated AISI 420 was due to the retention of austenitic phaseand complete dissolution of carbides. The austenitic phasehas an excellent characteristic of absorbing cavitation bub-ble collapse energy and later on the austenite is convertedinto martensitic phase. Conversion of austenitic phase intomartensitic phase has induced compressive stresses on thesurface, which are beneficial to cavitation erosion. Evidenceof transformation of austenitic phase into martensitic phaseafter cavitation was also observed. The potentiodynamicpolarisation studies showed that the pitting corrosion is lowin laser treated steel because laser hardening has resulted incomplete dissolution of carbide[28]. The corrosion stud-ies carried out on different volume fractions of retained

Fig. 7. Scanning electron microstructure of eroded MDN HT steel, 400.

austentic also prove that corrosion rate decreased linearlywith increased volume fraction of retained austenitic phaseduring laser hardening. Based upon Kwoks observations,the laser hardening studies were limited to a narrow powerdensity in the range of 17402400 W/(cm2 s) and after con-ducting all these tests, it is confirmed that a laser powerof 2120 W/(cm2 s) is ideal for laser hardening for thisapplication.

It is also known that mechanical properties of materialsand coatings, such as UTS, modified resilience, binding en-ergy and crystal structure play a crucial role in determiningthe cavitation erosion resistance. Feller and Kharrazi[19]have shown that the higher the yield strength and crystalbinding energy of a material, the longer is the cavitationincubation period. While criteria available for grading cav-itation erosion resistance are applicable to bulk materials


Fig. 8. Scanning electron microstructure of eroded Ti6Al4V alloy, 400.

only, there is very little phenomenological knowledge aboutsurface coatings on substrates. In composite structures, thehardness of coating combined with the ultimate strength ofthe substrate enhances the incubation period. Diffused coat-ings produced either by chemical vapour deposition or byplasma nitriding are the techniques for achieving such com-posite structures as reported by Frees[20]. FromFig. 2, it isseen that plasma nitrided 12Cr steel, 12Cr LH followed by12Cr BN have performed excellent during the incubationperiod. Later on after a long duration, plasma nitrided aswell as pack boronised steels have started behaving in a sim-ilar manner to 12Cr steel. Though the pack boronised 12Crsteel has a greater hardness and CMR than plasma nitrided12Cr steel but it shows an improved liquid impingement re-

Fig. 9. Scanning electron microstructure of eroded 12Cr LH steel, 400.

sistance in comparison to plasma nitrided steel. The jet im-pingement erosion resistance for all the coated and uncoatedsteels has been plotted with time and is given inFig. 2. Thegrain morphology of MDN AS, MDN PN, MDN HTand ST HT is identical. MDN HT is the finest amongall these followed by MDN AS, MDN PN and ST HT.Because of this MDN HT has performed much better thanthe other steels. Crystal structure plays a crucial role in de-termining the liquid impingement similar to the cavitationerosion resistance, as reported by Feller and Kharrazi[19]in the case of cavitation erosion.

The plasma nitrided steels have eroded by smooth removalof the hard layer without initiation of cracks and chipping ofthe coating. The impingement erosion resistance of diffused


Fig. 10. Scanning electron microstructure of eroded Hadfield steel, 400.

Table 3Mechanical properties of different coatings and materials

Materials/coatedmaterials 157.3

Yield strength(N mm2)

Ultimate tensile(N mm2)

Hardness(HV)

Elongation(%)

Ultimate resilience(J cm3)

Strain energy(J cm3)

Impactstrength (J)

12Cr AS 464.79 720.45 200210 26.98 1.23 157.3 15312Cr BN 462.0 666.0 18002000 18.0 1.05a 78.02 38.812Cr PN 448.0 740.00 9421000 13.87 1.30a 85.7 131ST AS 721.0 876.0 300350 23.17 1.83 164.3 93.0ST HT 1134.0 1562.7 450500 15.6 5.78 197.27 46.213/4 AS 813.2 892.20 300310 14.8 1.90 112.1 78.0MDN AS 863.49 1224.28 365380 13.44 3.577 139.68 112.0MDN PN 1162.00 1244.0 750900 12.63 3.69a 133.38 62.0MDN HT 1305.09 1448.68 450460 13.04 5.0 160.3 38Ti6Al4V 850 874 330350 13 3.17 96.0 65Hadfield 369 833.3 202450a 47.5 1.658.18 336.0 142

The 12Cr AS was in an annealed condition. The ST AS was in a forged condition and later on stress relieved at 250C for 4 h. The ST HT was inforged condition and heat treated at 950 c for 1 h followed by water quenching and later on stress relieved at 250C for 4 h. The 13/4 AS was in ascast condition and later on stress relieved at 250C for 4 h. The MDN AS was in forged condition and later on stress relieved at 250C for 4 h. TheMDN HT was in as cast condition and later on aged at 490C for 3 h. The titanium alloy was in forged condition and later on heat treated at 950Cfor 1 h followed by water quenching and aged to 535C for 6 h.

a Samples showed reduction in UTS, elongation and impact strength after coating. This has earlier been reported in case of boronising[27].

Table 4Mechanical properties of different materials

Materials Hardness(HV)

Modifiedresilience (HV)

Impactstrength (J)

12Cr AS 200210 0.369 153ST AS 300350 0.712 93.0ST HT 450500 1.86 46.213/4 AS 300310 0.68 78.0MDN AS 365380 1.13 112.0MDN HT 450460 1.68 38Ti6Al4V 330350 1.29 65Hadfield 202450a 0.4282.08 142

a Samples showed reduction in UTS, elongation and impact strengthafter coating. This has earlier been reported in case of boronising[27].

hard coatings can be explained on the basis that microjetsformed by breaking a big jet into small jets cannot pen-etrate the hard coatings easily. It is clear that while theUTS, strain energy, elongation and fracture strength of coat-ings are less than those of the substrate, the hardness ofthe coatings is much higher. The significant improvementin the incubation period of coated substrates indicate thatthe hardness of coatings has a crucial role to play in theirperformance. Catastrophic damage after the incubation pe-riod in a composite system follows the trends observed forhard and brittle materials, such as tungsten carbide andHaynes alloy 6B[5]. The incubation period and ranking ofthe coatings and materials after long duration are given inTables 5 and 6.


Table 5Incubation period of different coatings/materials

Coatings/materials Incubation period (h)

12Cr AS 1ST AS 213/4 AS 312Cr BN 4Ti6Al4V 4MDN PN 4MDN AS 4ST HT 512Cr PN 5MDN HT 712Cr LH > 7Hadfield > 7

Jet size= 4.24 mm.

It is reported that the fatigue strength coefficient and strainhardening have a strong correlation with the cavitation ero-sion behaviour of all types of materials[16,17]. However, inour microstructures, fatigue cracks were not observed evenafter long duration tests (7 h). So the theory of material re-moval based upon cyclic stress strain does not hold goodin jet impingement erosion. Due to the high frequency in-volved in cavitation erosion[38] fatigue strength coefficientand strain hardening play a significant role. It is also seenfrom the Table 3that Hadfield steel followed by ST HTand MDN series, has the highest ultimate resistance. So acorrelation of metallic materials based on UR as proposedby Hammitt and co-workers for cavitation erosion is also avalid criterion for liquid impingement erosion[4]. A metal-lic material having higher UR has excellent liquid impinge-ment erosion resistance. On the other hand the strain energytheory as proposed by Thiruvengadam and Waring[24] forcavitation erosion does not hold good for jet impingementerosion. The results from the 12Cr steel contradicts the pro-posed theory. (Table 3). CMR helps in delaying incubationperiod and later, the UR of the bulk material comes intopicture.

Table 6Volume loss and ranking of different coatings/materials after 7 h

Coating/materials Ranking Volume loss (mm3)

12Cr AS 12 43.46ST AS 11 7.5813/4AS 10 2.37ST HT 9 1.7712Cr BN 8 1.3412Cr PN 7 1.26Ti6Al4V 6 1.16MDN PN 5 0.30MDN AS 4 0.30MDN HT 3 0.0912Cr LH 2 0.01Hadfield 1 0

Jet size= 4.24 mm.

4. Conclusions

The following conclusions can be drawn from the exper-imental study.

1. To correlate water jet impingement erosion resistanceand mechanical properties of metallic materials, the URappears to be a valid criterion. Due to this criterion, Had-field steel, MDN series and ST HT and ST AS haveperformed much better than other steels. The impinge-ment erosion resistance of Hadfield steel were found tobe excellent among various stainless steels during the allstages of impingement erosion.

2. Scaling due to corrosion was visually observed in thecase of boronised and plasma nitrided steels but no sucheffect was observed in the case of other stainless steels.

3. Laser hardened 12Cr steel has also shown excellent per-formance in jet impingement erosion. This is due to theretention of higher austenitic phase and complete disso-lution of carbides as reported by Kwok et al. in cavitationerosion [28]. The austenitic phase has excellent char-acteristics of absorbing the water impact shocks. Later,it is converted into the martensitic phase. Conversionof austenitic phase into martensitic phase has inducedcompressive stresses on the surface, which are beneficialto jet impingement erosion.

4. The significant improvement in the incubation periodof plasma nitrided and borided steels indicate that thehardness of coatings has a crucial role to play in theirperformance. Catastrophic damages after the incubationperiod in hard coated steel follows the trends observedfor hard and brittle materials. The plasma nitridedsteels have eroded by smooth removal of the hard layerwithout initiation of cracks and chipping off of thecoating.

5. From the micrographs, it is seen that after long exposure,deep microtunnels confirming to microjetting effectssimilar to cavitation erosion mechanism are observedin 12Cr AS, ST AS, and 13Cr4Ni steels, whereas inMDN series, 12Cr LH and ST HT, these deep micro-tunnels confirming microjetting effects are not observed.Deep microtunnels confirming microjetting effects mayappear after a long duration.

6. The microstructures of all the metallic materials andcoatings are free from fatigue cracks. This proves that thefatigue strength coefficient and strain hardening criterionas reported by Richman and McNaughton for cavita-tion erosion does not hold good in liquid impingementerosion[16,17].

Acknowledgements

The authors are thankful to Mr. Pankaj Joshi for his helpin experimentation. Authors are also thankful to the man-agement of Corporate Research & Development Division,


Bharat Heavy Electricals Limited, for granting permissionto publish this paper.

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An experimental study to corelate water jet impingement erosion resistance and properties of metallic materials and coatingsIntroductionCavitation-erosion correlationSurface modifications and coatingsLaser surface meltingPlasma nitridingBoronisation

Experimental techniquesProperties of materials and coatingsAnalysis of materials and coatingsPlasma nitridingBoronisingLiquid impingement erosion resistance of different coatings

Results and discussionScanning electron micrographsProperties of materials and coatingsX-ray diffraction test resultsLiquid impingement erosion test results

ConclusionsAcknowledgementsReferences

Documents

An Experimental Study to Corelate Water Jet Impingement Erosion Resistance and Properties of Metallic