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Vol.35,No.2(2013)123‐127

TribologyinIndustry

www.tribology.fink.rs

AbrasiveWearResistanceoftheIron‐andWC‐basedHardfacedCoatingsEvaluatedwith

ScratchTestMethodA.Vencla,B.Gligorijevićb,B.Katavićb,B.Nedićc,D.DžunićcaUniversityofBelgrade,FacultyofMechanicalEngineering,Belgrade,Serbia,bInstituteGoša,Belgrade,Serbia,cFacultyofEngineering,UniversityofKragujevac,Kragujevac,Serbia.

Keywords:AbrasivewearScratchtestHhardfacingIron‐basedandWC‐basedmaterialsSEM‐EDS

ABSTRACTAbrasivewear is one of themost common types ofwear,whichmakesabrasive wear resistance very important in many industries. Thehardfacing is considered as useful and economicalway to improve theperformanceofcomponentssubmittedtosevereabrasivewearconditions,with wide range of applicable filler materials. The abrasive wearresistanceof the threedifferenthardfacedcoatings (two iron‐basedandoneWC‐based), which were intended to be used for reparation of theimpact plates of the ventilation mill, was investigated and compared.Abrasiveweartestswerecarried‐outbyusingthescratchtesterunderthedryconditions.Threenormalloadsof10,50and100Nandtheconstantslidingspeedof4mm/swereused.Scratchtestwaschosenasarelativelyeasyandquicktestmethod.Wearmechanismanalysisshowedsignificantinfluenceofthehardfacedcoatingsstructure,which,alongwithhardness,hasdeterminedcoatingsabrasivewearresistance.

©2013PublishedbyFacultyofEngineering

Correspondingauthor:A.VenclTribologyLaboratory,UniversityofBelgrade,FacultyofMechanicalEngineering,KraljiceMarije16,11120Belgrade35,SerbiaE‐mail:avencl@mas.bg.ac.rs

1. INTRODUCTIONMore than 50 % of all wear‐related failures ofindustrial equipment are caused by abrasivewear [1]. The estimated costs of abrasive wearare between 1 and 4 % of the gross nationalproductofanindustrializednation[2].Forthesereasons,theabrasivewearresistanceisasubjectof great importance inmany industries, such asagriculture,mining,mineralprocessingetc.Hardfacing could be defined as “coatingdeposition process in which a wear resistant,

usually harder, material is deposited on thesurfaceofa componentbysomeof theweldingtechniques”.Inmostcases,hardfacingisusedforcontrolling abrasive and erosive wear, like inmining, crushing and grinding, and agricultureindustries(buckets,bucketteeth,millhammers,ball mills, digging tools, conveyer screws, etc.[3,4]). Hardfacing is also used to controlcombinations of wear and corrosion, asencounteredbymud seals, plows, knives in thefood processing industry, pumps handlingcorrosiveliquids,orslurries[5].Thehardfacingisconsideredaseconomicalwaytoimprovethe

RESEAR

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performanceofcomponentssubmittedtoseverewear conditions, with wide range of applicablefillermaterials[6,7].Theiron‐basedfillermaterialshavedrawnmuchattention due to their low cost and goodresistance to abrasion in the hardfacedcondition. However, their use is limited inapplications where high impact loading ispresent, i.e.high‐stressorgougingabrasion [8].Forthisreason,effortsarebeingmadetowardsthe improvement of their impact and otherproperties [9]. Theprogress is achievedmostlybymodifying the hardfaced coating’s structure.Taking into account their low price andimprovedproperties, the resistance to abrasivewear of the iron‐based hardfaced coatings isnormallytestedagainsttheresistanceofproven,butmoreexpensivematerials,suchasWC‐basedhardfacedcoatings.Abrasive wear has been defined as “wear bydisplacement of material from surfaces inrelativemotion causedby thepresenceof hardparticles either between the surfaces orembeddedinoneofthem,orbythepresenceofhard protuberances on one or both of thesurfaces”[10].Thesecondpartofthisdefinitioncorresponds to pure two‐body abrasion,wheretested material slides against harder androugher counter face material, while the firstpart corresponds to the three‐ and two‐bodyabrasion, respectively. Another interestingexample of two‐body abrasion is the abrasiveerosion, which is the special case of erosivewear. Abrasive erosion has been defined as“erosivewearinwhichthelossofmaterialfromasolidsurfaceisduetorelativemotionofsolidparticleswhichareentrained ina fluid,movingnearly parallel to a solid surface” [10]. Scratchtest offers a possibility for comparison ofdifferent materials relatively easy and in shortperiodoftime,withgoodreproducibility[11].Insingle‐pass scratch test a stylus (which tip ismade of hard material) slide over the testsampleproducingasinglescratch,whichseemsto be appropriate simulation of the two‐bodyabrasion.Inthisstudy,theabrasivewearresistanceofthethree different hardfaced coatings (two iron‐basedandoneWC‐based)was investigatedandcompared.

2. EXPERIMENTALDETAILS2.1 MaterialsThe filler materials (coating materials) weremanufacturedbyCastolinEutecticCo.Ltd,Vienna.Their nominal chemical composition is shown inTable 1. The iron‐based filler materials (basiccovered electrodes)were deposited by using theshieldedmetal arcwelding (SMAW)process.TheWC‐basedfillermaterialwasdepositedbyoxy‐fuelgas welding (OFW) process. The substratematerialwasthehot‐rolledS355J2G3steel.Table1.Coatingscomposition,processandhardness.Coating Nominalchemicalcomposition

Hardfacingprocess

HardnessHV5

4541 Fe‐Cr‐C‐Si SMAW 7395006 Fe‐Cr‐C‐Si SMAW 7817888T WC‐Ni‐Cr‐Si‐B OFW 677

All coatings were deposited by hardfacing in asingle pass (one layer). The substratepreparation and hardfacing procedures(deposition parameters) are describedelsewhere [9,12]. The measurements of near‐surface hardness are performed on the cross‐section of hardfaced samples by Vickersindenter(HV5),andpresentedin(Table1).The samples for structure characterization areobtained by cutting the hardfaced materialsperpendicular to coatings surface. The obtainedcross‐sectionsaregroundwithSiCabrasivepapersdown to P1200 and polished with aluminasuspensions down to1μm.Thepolished surfacesare analyzed by using the scanning electronmicroscope(SEM)equippedwithenergydispersivesystem (EDS). The SEM‐EDS analysis wasperformed at University of Belgrade, Faculty ofMiningandGeologybyusingtheJEOLJSM–6610LVSEM connected with the INCA350 energydispersion X‐ray analysis unit. The electronacceleration voltage of 20 kV and the tungstenfilamentwereused.Before SEM‐EDS analysiswasperformed, polished surfaces were 20 nm goldcoated in a vacuum chamber by use of a sputtercoaterdevice.TheFig.1ashowsthenear‐surfacestructureofthe4541 iron‐based hardfaced coating. The primaryaustenite phase occupies more than a half ofvolume (50.7 vol.%) and the rest is the lamellareutecticmixture of austenite andCr‐carbides [9].

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Fig.1.Thestructures(SEM)of:(a)4541,(b)5006and(c)7888Thardfacedcoating;back‐scatteredelectronimages.The5006materialduringsolidificationachievesnear‐eutectic structure (Fig. 1b). A smallsphericalprimaryCr‐carbidesareobserved(9.1vol.%)intheeutecticmatrix.Basedonelectronmicroprobeanalysis(EMPA),bothcoatings4541and 5006 contain (Cr,Fe)7C3 primary andeutectic carbides. The Figure 1c shows a largerWC grains (60 vol.%),which are embedded intheNi‐Crbasedmatrix.2.2 ScratchabrasiontestingAbrasive wear tests are carried out on thescratch tester under the dry conditions, inambient air at room temperature (≈ 25 °C). Aschematicdiagramofscratchtestingispresentedin Figure 2. Stylus (indenter) was pressedwithselectednormalload(10,50and100N)againstsurface of the test sample and moved withconstantspeed(4mm/s),producingthescratchofcertainwidthandlength(10mm)onthetestsample. Indenter had Rockwell shape and theconewasdiamondwithradiusof0.2mm.

Fig.2.Schematicdiagramofscratchtesting.On surfaceof eachmaterialunder investigationat least three scratches are made with a gapbetweenscratchesofat least1mm.Beforeandafter testing, both the indenter and the testsamples are degreased and cleaned withbenzene.ThewearscarwidthsonthesurfaceofthetestsamplesaremeasuredfromSEMimagesat theendof testing.Thewearscarwidthsandthe known indenter geometry are used to

calculate the volume loss. After testing, themorphologyofthetestsampleswornsurfacesisexaminedwithSEM.3. RESULTSANDDISCUSSIONThe results of the wear tests are presented inFigures3,4and5.Takingintoaccountsignificantdifferences in structure homogeneity of thehardfaced coatings (Fig. 1), the repeatability ofthe results, in terms of standard deviations, issatisfactory (within 16 %). Wear rate of thetestedmaterials(volumelossdividedbyscratchlength) increases with normal loading, asexpected.Thehighestwearexhibitscoating7888T. Nevertheless, wear rates for all coatings arehigh,evenforabrasivewear.Thereasonforthisisprimarilyduetotheexperimentalconditions.Thetestconditionswerespecific,i.e.thespeedswere very low (4 mm/s) and the contactstressesveryhigh.Attheendoftest,thenormalstresses were between 2 and 5 GPa, whichdepends on thematerial, i.e. scratchwidth andapplied normal load. With these conditions, ahigh‐stress or even gouging abrasion can beexpected. With high‐stress abrasion, the wornsurface may exhibit varying degrees ofscratching with plastic flow of sufficientlyductile phases or fracture of brittle phases. Ingouging abrasion, the stresses are higher thanthose in high‐stress abrasion, and they areaccompaniedbylargeparticlesremovalfromthesurface,leavingdeepgrovesand/orpits[8].The relation between the wear rate and thehardnessof testedhardfaced coatings is showninFigure6.Thefirstfeatureisthattheabrasivewear rate decreases as the hardness increases,i.e. thehardestmaterial (coating5006) showedthehighestabrasivewearresistance.

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Coating 4541(v  = 4 mm/s)

0.75

1.88

0.10

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

10 50 100Normal load, N

Wear rate, m

m3 /m

Fig. 3. Wear rates of coating 4541 for differentnormalloads.

Coating 5006(v  = 4 mm/s)

1.67

0.68

0.09

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

10 50 100Normal load, N

Wear rate, m

m3 /m

Fig. 4. Wear rates of coating 5006 for differentnormalloads.

Coating 7888 T(v  = 4 mm/s)

1.24

2.88

0.21

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

10 50 100Normal load, N

Wear rate, m

m3 /m

Fig. 5. Wear rates of coating 7888 T for differentnormalloads.

0.10 0.09

1.24

0.75 0.68

1.67

0.21

1.88

2.88

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

670 690 710 730 750 770 790Hardness HV5

Wear rate, mm

3 /m 10 N

50 N100 N

Fig.6.Wearratevs.hardnessoftestedmaterialsfordifferentnormalloads.For all applied loads, the relation betweenhardness and wear rate is non‐linear. It is morecurvedforhigherloads(Fig.6).Thisisconnectedwith the coatings structure and exhibited wearmechanism. Coatings 4541 and 5006 exhibitmainly ploughing abrasive wear (Fig. 7a), whilecoating7888Tdominanttypeofabrasivewearisfracture(cracking)abrasivewear(Fig.7b).

Fig.7.Thewearscarappearance(SEM)of:(a)4541and(b)7888Thardfacedcoating;50Nnormalload;back‐scatteredelectronimages.

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4. CONCLUSIONScratch test offers relatively easy and quickcomparisonofdifferentmaterialsonabrasivewear.Structureoftestedcoatingsshowedinfluenceonthe dominant type of abrasive wear, whichtogether with coatings hardness determinedcoatingsabrasivewearresistance.Coatings with lower hardness showed lowerabrasive wear resistance, but the dependence(hardnessvs.wearrate)wasnon‐linear.In the case of iron‐based coatings, dominanttypeofabrasivewearwasploughingand in thecase of WC‐based coatings, it was fracture(cracking)abrasivewear.AcknowledgementThis work has been performed as a part ofactivities within the projects TR 34028, TR35021 and TR 35034. These projects aresupportedbytheRepublicofSerbia,MinistryofEducation, Science and TechnologicalDevelopment,whose financial help is gratefullyacknowledged.REFERENCES[1] A. Rac: Osnovi tribologije, Mašinski fakultet

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