Guidelines to Hardness Testing

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  • 10/28/2015 Guidelinestohardnesstesting

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    Contents:

    1.Generalnotesonhardnesstesting2.TheRockwellprocedure2.1StandardRockwell2.2SuperficialRockwell2.3FieldsofapplicationwithdifferentRockwellscales2.4Testsoncylindricandsphericalsurfaces2.5ProsandconsoftheRockwellprocedure2.6VariationsoftheRockwellprocedure

    3.TheBrinellmethod3.1ThelabellingofBrinelltests3.2DifferentapplicationsofBrinelltests3.3ProsandconsoftheBrinellprocedure3.4RockwelltestswithBrinellloadsandBrinellpenetrators

    4.TheVickersmethod4.1ApplicationsofdifferentVickerstestloads4.2ProsandconsoftheVickersprocedure

    5.Otherhardnesstestingprocedures5.1TheShoreprocedure(formetals)5.2TheKnoopmethod

    6.Revaluationtablesandhardnesscomparisonplates6.1Theuseofrevaluationtables6.2Theuseofhardnesscomparisonplates

    1.GeneralNotesonHardnessTesting

    Amongthedifferentkindsofmeasurementsthatarecarriedoutinalaboratory,hardnesstestingisoneofthemostcomplexones.Ontheonehand,therearedifferentmeasurementproceduresontheotherhand,itisnecessarytomeasurelarge,small,hard,soft,thinorthickmetalparts.Consideringthedifferentproceduresandthelargenumberofscales,itisunderstandablethatevenveryexperiencedpersonscanbechallengedbyhardnesstestingtasks.

    Asinsomanyotherareasofapplication,electronicdevelopmenthasledtoasignificantsimplificationofhardnesstesting.Withcomputeraidedhardnesstesters,ahigherprecisionduringresultreadout,datastorageandthepossibilityofdataprocessingtostatistics,graphicrepresentations,documentation,etc.havebecomeamatterofcourse.

    However,electronicsarestillonlyusedforresultsreadout(and,ifnecessary,automationofthemeasuring/drive),whilethedifferentmechanictestingresultsarestillapplicable.

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    Althoughlaterwewillbetalkingaboutdefinitions,advantagesanddisadvantagesoftheRockwell,Brinell,andVickersprocedures,itisstillbeneficialtodealbrieflywiththemostimportantfeaturesthatshouldbeconsideredbeforebuyingsuchadevice,hereintheintroduction:1)thetotaltestload2)thehardnessrange3)accuracy,4)theflexibilityofthedevicewithregardtoformsanddimensionsofspecimens,and5)economicaspects.

    1. Totaltestload

    Ontheonehand,thereisthegeneralruletouseatestloadashighaspossible.Thisallowsforhigheraccuracy(becausethemeasurementislesssensitivetothesurfacetexturewithahighertestload).

    Ontheotherhand,theindentationshouldnotbedeeperthan1/10ofthethicknessofthespecimenorthehardenedsurface.

    Thedegreeofhomogeneityofthematerialisalsoanimportantcriterion:atypicalexampleiscastiron,whichisusuallyonlytestedwithahightotalload,exceptintherangeswhereithasbeeninductionhardened,e.g.machinetoolbases.

    2. HardnessrangeAboveahardnessofapproximately650HB/30,adiamondpenetratorshouldbeusedbelowthatvalueitisalsopossibletouseapenetratormadeofsteelorhardmetal.

    TheBrinellmethod,whichdoesnotallowdiamondpenetrators,cannotbeusedforhardenedsteel.

    TheRockwellmethodismoreuniversal,becauseitallowsfortheusedofdiamondconeandsteelballpenetrators.

    TheVickersmethod,whichonlyallowsforadiamondpyramidpenetrator,canbeemployedintheentirehardnessrange.However,itismostsuitablefortestsinlaboratoriescomparedtotestsinworkshops.

    3. AccuracyTheprecisionofthemeasurementisheavilydependentontheaccuracyemployedbytheoperator.Thisalsoincludeswellgroundsurfaces,sufficientmeasurementperiodsandfrequentrevisionsofthetestingdevicewithreliabletestplates.Ifpossible,theuseofstaticsystemsshouldbepreferredtodynamicsystems.Usingverylowtestingloadsisaparticularrestrictiontotheprecisionofthemeasurements

    4. FlexibilityofthedevicewithregardtoformsanddimensionsofspecimensThespecimencanbeputonthedevice,orthedevicecanbeputonthespecimen.Thefirstcasedescribesstationarydevices,whichhaveenoughcapacitytoholdthespecimen.Stationarydevicesare,thus,primarilysuitablefortestsonsmallandmediumsizedspecimens.

    Portabledevicescanbeclampedtothespecimens(clampingjaw,chain,etc)orwhentestinglargeorbulkyspecimensjustputonthespecimens.

    Portabledevicescanonlybedynamicwhenusinghightestingloads.Whenthetestingloadsaresmaller,theycanalsobestatic.Itispossibletofindcustomerspecificsolutionsforspecialcases.

    5. EconomicaspectsThisincludesthefollowingelements:thepurchasepriceofthedevice,theuniversalismoftheapplication,themeasurementperiod,andthequalificationneededtooperatethedevice.

    Thefirsttwoaspectsareimportantwhenspecimensofdifferentformsandwithdifferentsurfacetreatmentsare

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    tested.Thisisusuallythecaseintechnicalcompaniesandinsmallscaleindustries.

    Incompaniesthatdotheirtestsseriallythequicknessofthemeasurementsandthepossibilitytoemployunskilledstaffareveryimportant.Here,suchdevicesarepreferredthatdonotneedspecialclampingequipment.

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    2.TheRockwellmethod

    Forabetterunderstanding,thesequenceoftheRockwellmethod,whichisdescribedbelow,isalsoshowninthenumberedimagesbelow(figure1).Themeter,whichisconnectedtothepenetratoranddisplaysthepenetratorsshiftsonalargerscale,isalsoincludedinthefigure.

    1. ThetestedsurfaceisexposedtothepenetratorandthefirsttestloadFo(preload)isapplied.Asmallindentation

    appears.Atthispoint,themeterissettozero.

    2. SlowlyandwithoutshockstheloadF1isappliedadditionally.TogetherwiththepreloadthisisdefinedastotaltestloadF.Withthisloadthepenetratorentersthematerialmoreorlessdeep,dependingonthehardnessofthematerial.Thispositionneedstobekepttoreachthefinalpenetration(whentestinghardmaterialsthepenetrationisalmostimmediatewithsoftmaterialsitisnecessarytowaitforanumberofseconds).Thepenetrationprocedurecanalsobeobservedontheindicatorofthemeter.

    3. Whentheindicatorofthemeterfinallystopsmoving,theadditionalloadF1isremoveduntilthepreloadisapplied

    respectively.Thisway,thepenetratorremainsintheimprintandallelasticdeformations,whichwerecausedbytheapplicationofthetotaltestload,areeliminatedthus,themeteronlyshowstheremainingpenetrationdepth(asdifferencebetweenpreloadandtotaltestload).

    Thepenetrator,preloads,testloads,andtheunitsarestandardisedintheRockwellmethodandcanbedividedintotwogroups:standardRockwell(methodN)andsuperficialRockwell(methodT).

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  • 10/28/2015 Guidelinestohardnesstesting

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    2.1StandardRockwell

    ThestandardRockwellprocedureisintendedfortheuseofonesinglediamondconepenetratorof120witharoundedoffpeakof0.2mmradius(seefigure2),ordifferentballpenetratorsmadefromhardmetalwithdiametersof1/16"1/8"1/4"1/2"(inch).

    Figure2profileoftheRockwellpenetratorwithdiamondcone

    Thepreloadisunchanging:98.07N.Thetotaltestloadsare(preload+additionalload):588.4N980.7N1,471N.ThemeasuringunitinstandardRockwellcorrespondsto0.002mmpenetration.Thehardnessvalueincreaseswiththehardnessofthematerial,butatthesametimethepenetrationdifferencebetweenthepreloadandthetotaltestingloaddecreasestheharderthematerialis.Thus,Rockwellhardnessvaluesarecalculatedbysubtractingthepenetrationdepth(per0.002mm)from100(whenusingthediamondpenetrator)orfrom130(whenusinganyballpenetrator).

    Example:withadiamondpenetratorandapenetrationdepthof0.082mmthismakes1000.082/0.002=59Rockwellthesamepenetrationdepthmeasuredwithaballpenetratormakes1300.082/0.002=89Rockwell

    Whenusinganaloguedeviceswithdialgauges,whichusuallyhave100partitions(onerotation=0.2mm),theRockwellvaluescanbereaddirectlyfromthedial.Thedialthenusuallyhas2seriesofnumbers:theblackonesarefordiamondpenetratorsandtherednumbersaremadeforballpenetrators.Forthezeropositionalwaysusetheblack0(orthered30).Whenusingadigitaldevice,thedataaredisplayedafterthecompletemeasuringcyclewasrunthrough.Duetothedifferentcombinationsofpenetratorsandtestloads,thereisagreatnumberofscales,whicha