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Experimental study of abrasive wear of structural materialsunder the high hydrostatic pressureStanislav N. Vericheva,n, Vasily V. Mishakinb, Dmitry A. Nuzhdinb, Evgeny N. RazovbaMTI Holland B.V., Smitweg 6, 2961AW Kinderdijk, The NetherlandsbInstitute for Problems in Mechanical Engineering, Russian Academy of Sciences, Belinskogo 85, 603024 Nizhny Novgorod, Russiaa rti cle in foArticle history:Received 26 February 2014Accepted 3 March 2015Keywords:Abrasive wearHyperbaric effectsDeep sea miningFriction pairabstractThis paper outlines results of theexperimental studyof abrasivewear of materials at hyperbaricpressures. Two materials have been treated: steel and tungsten carbide. The tests have been performedin a custom made pressure vessel. SEM microscopy has been used to reveal the physical mechanisms ofwear. Obtained results might be interesting for offshore mining and oil & gas industries.& 2015 Elsevier Ltd. All rights reserved.1. IntroductionThe knowledge on actual wear rates of structural materials underthe high hydrostatic pressure is critical for the design of wear life timeof structural elements of the deep water offshore systems includingthe deep sea mining systems.Theservicelifeofwearingpartsofsubseatoolsdeterminestheprotability of expensive subsea structures and systems. Unexpectedshutdownstoreplacethebrokenpartswithnewonesleadtoasignicant reduction in productivity, reduce general product quality,risenon-productioncostsetc. Generally, suchinterruptionsintheoperations make it very difcult, and sometimes completely eliminatethepossibilityof productionautomation. All thisenormouslyrisesoperational expenditures.An example of a system working offshore under the high hydro-static pressure is the deepsea mining system, whichare beingnowadays developed in the different parts of the World (Fig. 1). Thesesystems are envisaged to mine deep sea deposits such as manganesenodules, seaoormassivesuldesetc., whichcanbefoundat theseabottomat depthsrangingfrom1500upto6000 m. Themostprobabledeepseaminingsystemwouldconsistof thethreemaincomponents:(1)subseacrawler, alsocalledasseaoorminingtool,whichmovesalongtheseabottomandperformsminingby cuttingor suction of the material and its further transportation as a dilutedsolid-water mixture, a slurry; (2) vertical hydraulic transport systemcomprising a systemof (bundled) risers and a number of subsea slurrypumps; (3) mining support vessel.Obviously, themostsubjectedtowearpartsof thedeepseamining systemare the cutting tool, slurry pumps and risers.Another exampleof highlyabrasiveprocess that occurs underthe high pressure is drilling in oil and gas industry.Pioneering studies related to the friction of materials underhyperbaric pressures have been performed by Bridgman (1952). Basedon these studies, Kragelsky et al. (1982) has proposed and developedhypothesis of binomial formof the specic friction force. There exists anumber of studiesaimedto simulate the conditions ofhigh hydro-static pressure using, for example the uniaxial compression method(Deryagin et al., 1973) and thin lmmethod (Bartenev and Lavrentyev,1972). Most of the studies have been carried out for polymer materialsand have conrmed the binominal dependence of the specic frictionforce. However, those methods have the following weak points:difcultiestoaccountforthelubricationmaterials, nopossibilitytoseparately vary the value of compression pressure as well as the con-tact pressure, distribution of stresses in the material subjected to thefrictiondoesnotalwayscorrespondtothosethatreallytakeplaceunder the high hydrostatic pressure (Strelnikov, 2010).Results of studies of friction in simulating the downhole conditionshavebeensummarizedinStrelnikov(2010). Ithasbeenconcludedthat most of existing studies are the applied studies for certain specicconditions andit is hardly possible tosystematize themandtoestablishanylawsrevealingtheeffect of hyperbaricpressuresonfriction and wear.Normally, wear of metal alloys is characterized by two processes:delamination and chipping (Sosnovskiy, 2005). Domination of one oftheseprocessesdependsontheloadingconditionsandstructuralContents lists available at ScienceDirectjournalhomepage:www.elsevier.com/locate/oceanengOcean Engineeringhttp://dx.doi.org/10.1016/j.oceaneng.2015.03.0010029-8018/& 2015 Elsevier Ltd. All rights reserved.nCorresponding author.E-mail addresses: [email protected] (S.N. Verichev),[email protected] (V.V. Mishakin).Ocean Engineering 99 (2015) 913condition of material. Also, a medium, in which wear occurs can havesignicant effect, whichcanbeexempliedby corrosion. Thecycliccomponent of the load leads to wear-fatigue. The speed of accumula-tionof micro-damagesuchasmicroporesandmicrocracksintheareas of contact depends on cyclic component of stress, which mag-nitude is determined by the magnitude of the hydrostatic pressure inthe liquid medium under consideration. Note that effect of hydrostaticpressure ofthe process of wear is studied poorly (at least only fewresults are publicly available).It is worthmentioningthat relationof thewear rates tothemagnitudeofthehydrostaticpressure, underwhichtheprocessofwear takes place has a nonlinear character. First of all, this is due tothe effect of the hydrostatic pressure on strength and plastic proper-ties of metals. For some materials there exists a threshold pressure,for whichhardness andplasticityof ametal changesignicantly(Sosnovskiy, 2005). For example, for steel withconcentrationofCo46%, depending on regimes of thermal treatment, such a thresholdpressure is about 130190 MPa.Until now, thereisnouniversalmodel thatwoulddescribethemechanisms of wear andcouldforecast the wear rates at highhydrostatic pressures.An important role in the understanding of processes of wear ofhigh-strength alloys belongs to the accumulation of experimentaldata using the physical methods of investigation such as scanning-electron microscopy (SEM).Summarizingtheaforesaid, onecanseethat analysisof theexisting literature clearly shows that studies of friction and wearunder thehighhydrostaticpressures arelimitedandobtainedresults are yet insufcient for a general description of the frictionprocess.The main objective of this experimental study was to answer thequestion: how does high hydrostatic pressure affect abrasive wear ofstructural materials contactingwitha substantiallyhardmaterial(whosehardness is muchhigher thanhardness of thesamples)?The study focuses entirely on pure abrasion wear within the frictionpair placed in a highly pressurized tap water so that any other sourcesofwear(suchasdifferentwaterchemical compositions, hightem-perature, different angles of attack (erosion) etc.) or any other type ofmaterial deterioration (such as corrosion) do not occur. Two differentmaterials have been tested: (1) steel DIN 1.4028 (martensitic, stainless,heat treatable chromium steel), which is normally used for the teethofthecuttingtoolsworkinginlowaggressivemedia;(2)tungstencarbide alloy (90% WC710% Co), which is a common material forthe drill bits in oil and gas industry.2. Description of the experimental setupAn experimental setup that allows studying abrasive wear at highhydrostatic pressures was designed and built by the authors, see Fig. 2.Hydrostatic pressures of up to 250 atm can be generated, which corr-esponds to the water depth of approximately 2.5 km. A roomtemperature was maintained throughout all the tests. Samples had acylindricalshape withthe diameter of 3 mm andheight of12 mm.Inside the pressure vessel, samples werexed at the rotating disk. Aconstant frequency of rotation of 0.8 Hz has been maintained.Thetopsideof thesampleswasincontactwithimmovablecylindrical polycrystalline diamond head (PCD),xed in the centerof aweight andplacedontopof thesamples. Thereasonofchoosingthistypeof material asafrictioncounterpart canbeexplained as follows. Before choosing the PCD, a number of rockswith the different harnesses has been tested. It has been discov-ered that even after longer times of testing weight losses for thetungsten carbide were too low to be properly measured. Using thematerial, whose tribological properties are,rst, higher than thoseof the samples and, second, which almost does not wear out itselfmake the experiment moreclean since only the samples in thiscase would be subject to wear.Thefollowingtypeof thediamondheadhasbeenused:AW20n20n80n8AC4 80/63M2-01(100%14.1), whichstands for:20n20n80n8dimensions(diameterofthehead, heightandborediameter), AC4 is a mark of the diamond powder (syntheticFig. 1. Deep sea mining system.casingweightsamples holderpolycrystalline diamond headsealingsupportbearingclutchelectric motorto the high-pressure pumpguidesworm-and-worm pairshaftsample1 23Fig. 2. Experimental setup: Schematic representation (topgure) and picture fromthe lab (bottomgure). 1 is the high-pressure pump, 2 is the pressure vessel, 3 isthe electric motor and gearbox.S.N. Verichev et al. / Ocean Engineering 99 (2015) 913 10diamonds with increased fragility, whose grains represent aggre-gates withdevelopedsurface), 80/63is the grainsizes range,(63/80 mmare thesmallest/largest sizes of grains of themainfraction), 2-01 is the type of the binder (0.1% copper, 20% tin).The total mass of the weight placed above the PCDheadincluding its own weight was 432 g.Ascheme of the frictional contact betweensamples andthediamond head is shown in Fig. 3.Each test was carried out during 3 h. Four different values of thehydrostatic pressure were used: 1, 100, 150, and 200 atm. To estimatethe repeatability of results, each experiment was repeated three times.3. ResultsWeight losses for the steel and for the tungsten carbide alloy asfunctionsofthe hydrostatic pressure are shown inFigs.4and 5,respectively.As one can see, weight losses increase at the pressure of approxi-mately 150 atm. For 200 atm, weight loss for the steel samplesincreases 1.6 times,while for the tungsten carbide alloy it increasestwice.It isknownfromtheliteraturethat intheprocessof friction,mechanical and geometrical properties of the surface layers arechanging as a result of their heating, mechanical destruction,accumulation of fatigue, changes in the microstructure, which, in turn,affects the stress state and the nature of the wear (Goryacheva, 2001).To analyze actual mechanisms of the wear for both materials, ascanningelectronmicroscopy(SEM) wasused. SEMshowsthatmainly loss of the material occurs due to the peeling of layers andchippingofhardparticles. Fig. 6showsSEMimagesofthesteelsurfaceafterthetestsperformedat200 atm. Onecaseseelongstrips with traces on the surface peeling, which is due, apparently,to a large percentage of viscous binder (Fe). There are alsochippingareas. Inplaces, wherechippingoccurredonecanseebrittle fractures (see Fig. 6c and d).This material corrodes in the water. Fig. 6c and d shows parts ofthe surface covered by the iron oxide.It is known that iron oxide ( form) has increased hardness andwearresistance. Thus, protectivepropertiesofthethinlmofironoxide can lead to the reduction of the wear.Fig. 7 shows SEM image of the surface of the tungsten carbidealloy after a test performed at 200 atm.Resultsof theexperimentsshowthat thewearof materialsincreases for the increased values of the hydrostatic pressure.To describe the wear process in the absence of hydrostatic pressurethe most widely used theory is the theory of fatigue wear. The basis ofthistheoryistheconceptof fatiguefailureinslidingnear-surfacematerial layers. According to this mechanismof failure, embedded andattenedprotrusions of roughcontactingsurfaces areexposedtoconstantly repeated stresses and strains. The intensity of wear (wearrate) depends on the type of contact (elastic, plastic), frictionalstrength and elastic properties of the material, surface micro-geome-try, temperature, etc. Sosnovskiy (2005). To calculate wear rates basedon these factors, fatigue models consider specic protrusions is in theformof a hemisphere, cylinder, etc. However, the real protrusions haveanarbitraryshape andare distributedrandomly. The process offatigue wear depends on many random factors affecting the separa-tionof wear particles. Therefore, the theoryof the fatiguewearprovides mostly only qualitative description of the process of frictionaldestruction.Duringtheconsiderationof factors affectingthefeatures ofwear under the high hydrostatic pressure,rst of all, it is necessaryto account for the increase of the contact pressure caused by thenormal load FN, which consists of external load F0(in our case it isdetermined by the magnitude of a static weight applied above thefrictionpair) andadditional loadFn, whichoriginatesfromthehydrostatic pressure P acting on the actual contact area SF.FNF0FnF0PUSFIncrease of FNleads to the increase of the actual contact area,friction force and amplitude of cyclic stresses.Also, itisnecessarytoconsiderthatincreaseof thehydrostaticpressureleadstothechangeoftheratioofsphericalpartofstresstensor to the deviatoric one, which also affects the speed of accumula-tionof microdamage. AccordingtoLemaitre (1985), the rate ofdamage accumulation (micropores and microcracks) depends on thestrainenergyreleaserateof thedamagedbody. Thedeformationenergy is calculated as a sum of the shear strain energy and energy ofthe volumetric expansion. The ratio of the spherical to the deviatoricpartsofthestresstensorisveryimportantforthedevelopmentofdamages.Changeof thehydrostaticpressureupto200 atmleadstoanoticeable change of the spherical part of the stress tensor, whichshould be revealed in the rate of wear.Inageneral case, thespeedofaccumulationoffatiguedamagedependsnonlinearlyontheamplitudeof appliedstresses(Collins,1993). Apart from that, there exists a threshold hydrostatic pressure,for which hardness and plasticity of materials change signicantly.contact zone of the frictional pairsampleweightdiamond headrotating discFig. 3. A scheme of the frictional contact between samples and the diamond head.Fig. 4. Weight loss for the steel samples as the function of the hydrostatic pressure.Fig. 5. Weight loss for the tungsten carbide alloy samples as the functionof the hydrostatic pressure.S.N. Verichev et al. / Ocean Engineering 99 (2015) 913 11Thesefactsmakeit important toaccount fortheeffect of thehydrostatic pressure to properly estimate the life time of the structuralcomponents of systems and tools, which are subjected to a frictionalcontact.For amoredetaileddescriptionof theabrasivewear of suchmaterials, it is necessary to study in more details the frictional contactinteraction of the surfaces, dissipation of the mechanical energy in thecontact zone, features of the state of the surface layer of the material,especially in the local contact zones as well as features of the processesofinteractionofunevensurfacesduringtheirrelativedisplacementagainst each other. Also to be mentioned, effects of the inuence of amedium on a physical-chemical state of the surface layer and contactinteraction of hard materials (Ibatullin, 2008).Generally, to account for the effect of the hydrostatic pressureon wear rates, it is necessary to account at least for the followingtwofactors:(1)changesof theinteractionforcesinthecontactzone; (2) variation of the spherical component of the stress tensorin the contact zone(s).4. ConclusionsSEM analysis of the surfaces of samples showed that the mainmechanisms of wear are peeling of the layers and chipping of hardparticles. Thewearrateincreases1.62timesasthehydrostaticpressure increases up to 200 atm.Generally, higherwear(largerweightlosses)athighhydrostaticpressures mainly occur to the brittle-to-ductile transition. Whenmaterial becomesmoreductile, thentheinteractionforcesinthecontact areachangecomparedtothoseat atmosphericconditionsand/or small hydrostatic pressures due to the smearing and punch-ing. Due to the increased vertical force as well as due to theFig. 6. SEM images of the steel surface.S.N. Verichev et al. / Ocean Engineering 99 (2015) 913 12(increased)ductilityofsamplesthetotalcontactareabetweentwo(irregular) surfaces increases. Thus, more material is subjected to theactual friction so the weight loss increases. Apart from that, talking onamicrolevel, moreductilecarryingmaterial becomesallowshardparticles to chip out easier from the matrix.Clearly, hydrostatic pressure must be always taken into account forrealistic estimations of the actual wear rates of the frictional surfacesof structural components during the design phase of subsea structuresand tools working at large depths.AcknowledgmentsAuthors would like to thank IHC Merwede for funding andsupporting this research. This work has been also partly supp-orted by Russian Scientic Fund Support and development(grant 14-19-01637).ReferencesBridgman, P.W., 1952. Studies inLargePlastic FlowandFracturewithSpecialEmphasis on the Effects of Hydrostatic Pressure. McGraw.Kragelsky, I.V., Dobychin, M.N., Kombalov, V.S., 1982. Friction and Wear: CalculationMethods. Elsevier Science & Technology.Deryagin, B.V., Krotova, N.A., Smilga, V.P., 1973. Adhesion of Solids. Science,Moscow, in Russian.Bartenev, G.M., Lavrentyev, V.V.,1972.Friction and Wear of Polymers.Chemistry,Leningrad, in Russian.Strelnikov, Y.U.A., 2010. Friction during the contact interaction of surfaces under thehydrostatic pressure Ph.D. thesis. .Goryacheva, I.G., 2001. Mechanics of Frictional Interaction. Nauka, Moscow p. 478,in Russian.Ibatullin, I.D., 2008. Kinetics of Fatigue Damage and Destruction Surface Layers of:Monograph/I.D. Samara: Samara state technical Univ., Ibatullin p. 387, inRussian.Lemaitre, J., 1985. Acontinuous damagemechanics model for ductilefracture.J. Eng. Mater. Technol. 107 (1), 8389.Sosnovskiy, L., 2005. TRIBO-FATIGUE: Wear-FatigueDamageandits Prediction.Springer.Collins, J.A.,1993. Failure of Materials inMechanical Design:Analysis, Prediction,Prevention. John Wiley and sons.Fig. 7. SEM image of the WCCo surface.S.N. Verichev et al. / Ocean Engineering 99 (2015) 913 13