NIST recommended practice guide : Rockwell …

1 ; |ocS? ~ , , , ,equipment, ,norisitintendedtoimplythattheentities,m aterials, (January2001) :(202)512-1800 Fax:(202)512-2250 Mail:StopSSOP,Washington, "goodpractice" ,muchoftheinformationgivenisalsoapplicab leforusersthatonlyrequiretestresultstobe withinwidetolerancebands, ; * ,ConsultantRichardAntonik, ' , , , ,NewageTestingInstruments, ,GilmoreDiamondTools, , , , , , ,Jr., , ,andJohnSongfortheircontributionstowards theRockwellhardnessstandardizationprogra matNIST.* ,Error, :AppliedForceEffect71 (a) (b) (FigureA) (FigureB)demonstratingtheeffectofanincre aseinthepreliminary-forceforaRockwellHRA test16 Figure3 (FigureA) (FigureB)demonstratingtheeffectofanincre aseinthetotal-forceforaRockwellHRAtest17 Figure4:Diagramofcross-sectionalviewofsp heroconicaldiamondindentertip24 Figure5:EightstepsoftheRockwelltestcycle 28 Figure6:Fourexamplesofthehardnessprofile acrossthetestsurfaceof25 HRCtestblocks, ,andthelocationsofthecertifiedvaluesforu ntestedlocationsareindicatedbytheopencir cles40 Figure8:Alternatepatternforrepeatability measurements48 Figure9:Illustrationofthethreebiaspoints correctedbyalinearfitcorrectioncurve50 Figure10:Illustrationofreproducibilityda tatakenovertendays51 Figure11:NISTR ockwellhardnessstandardizingmachine57 TableofContentsFigure12 (forhardnesslevelsasindicatedonthebottom axis).

2 ChangeintheRockwellhardnessvalueduetoach angeinthepreliminaryforcefordiamondinden terscales(FigureA)andselectedballscales( FigureB) :ChangeintheRockwellhardnessvalueduetoac hangeinthetotalforcefordiamondindentersc ales(FigureA)andselectedballscales(Figur eB) :ThepossibleoffsetinRockwellhardnessmeas urementvaluesthatcouldbeobtainedforthedi amondindenterscalesbyvaryingtheappliedpr eliminaryforcesandtotalforceswithintheAS TM tolerances(FigureA)andtheISOtolerances(F igureB) :ChangeinapparentHRChardnessduetochanges intheadditionalforceapplicationrate(inde ntervelocity) # (lettersAthroughK) :Rockwellhardnessscaleswiththecorrespond ingindentertype,appliedforcesandtypicala pplications5 Table2 :Specifiedtestforceswithtolerances72 TableO1 ,y=isattheblockcenter(NIST indentation4), :Inversematrixr~1withelementsg^91 TableO7 :Thecoordinatesforthelocationsusedinthec alculationsofExample2100xii1 INTRODUCTIONW orkinginaball-bearingmanufacturingplanti n1919, ^.

3 Soonafter, 'sinvention, , , , "goodpractice" ,suchastheAmericanSocietyforTestingandMa terials(ASTM)andtheInternationalStandard sOrganization(ISO), ,muchoftheinformationgivenisalsoapplicab leforusersthatonlyrequiretestresultstobe withinwidetolerancebands, ;however, ;however, ,inexpensive,andrelativelynon-destructiv e, ,theRockwellhardnesstestcanprovideimport antinformationaboutmetallicmaterials,suc hasthetensilestrength,wearresistance, ,forprocessandqualitycontrol, ,intoday'smanufacturingfacilities,Rockwe llhardnessmachinescanbefoundinuseinalmos teverytestingenvironment,fromthehot,oily surroundingsofsomemanufacturingfacilitie s, , ,twolevelsofforceareappliedtotheindenter atspecifiedratesandwithspecifieddwelltim es,asillustratedfortheRockwellCscale(HRC ) ,wherethesizeoftheindentationismeasuredf ollowingtheindentationprocess,theRockwel lhardnessofthematerialisbasedonthediffer enceinthedepthoftheindenterattwospecific timesduringthetestingcycle, ,therearenoalternativemeasurementsystems todirectlyorindependentlymeasureRockwell hardness, ,andapreliminaryforce(formallyreferredto astheminorload) (dwelltime)

4 , ,anadditionalamountofforceis218001600140 01200^ | ^ *HPreliminaryForceDwellTimeRecoveryDwell TimeFirstDepthMeasurementFinalDepthMeasu rementIncreasingTimeFirstDepthMeasuremen tFinalDepthMeasurement180160140120100 80 6040200) (a) (b) (formallyreferredtoasthemajorload).Theto talforceisheldconstantforasettimeperiod, afterwhichtheadditionalforceisremoved, ,thedepthofindentationismeasuredforaseco ndtime, ,h,(seeFigure1) , ' ,size, ,severaldifferentindentertypes( ) (2)definesthirtydifferentRockwellscales, , (10kgf), (60kgf), (100kgf)or1471N(150kgf).ThesuperficialRo ckwellscalesemploylighterforcelevels, , (3kgf), (15kgf), (30kgf) (45kgf).Table1providestypicalapplication sforthedifferentRockwellscalesasrecommen dedbyASTM(2), , , ,appliedforcesandtypicalapplicationsScal eSymbolIndenterType(Balldimensionsindica tediameter.)PreliminaryForceTOTotalForce N(kgf) (10) (60)Cementedcarbides,thinsteel, (1/16in.) (10) (100)Copperalloys,softsteels,aluminumall oys,malleableiron, (10)1471(150)Steel,hardcastirons,pearlit icmalleableiron,titanium,deepcasehardene dsteel, (10) (100)Thinsteelandmediumcasehardenedsteel , (1/8in.

5 (10) (100)Castiron,aluminumandmagnesiumalloys , (1/16in.) (10) (60)Annealedcopperalloys, (1/16in.) (10)1471(150)Malleableirons,copper-nicke l-zincandcupro-nickelalloys. ,furthertestingshouldnotcontinuewiththed amagedanvil. IftheobjectiveoftheRockwelltestistomeasu rethehardnessofasurfacefeaturesuchasacas e-hardenedsurface, (orWidth)Inthesamewaythatthedeformatione xtendsbelowanindentation,thuslimitingthe minimummaterialthickness, ,thedeformationsurroundingtheindentation mayextendtotheedgeandpushoutthematerial, (3) ,incaseswhereRockwellhardnesstestingisto bemadeonnarrowwidthmaterialormaterialhav ingasmallareasize, , (seeabovediscussions).Ifthedeformationzo nesurroundingaRockwellindentationextends intoadjacentregionsofadifferinghardness, suchastheheataffectedzoneofaweld, , ^ ,theoretically,theentirescalecanbeusedfo rhardnesstesting, ,theselimitsresultfromtheindenterpenetra tingtoodeeplyintothematerial, , , ,thesensitivityofthetestdiminishes, ^ * *GoodPracticeRecommendations WhenseveralRockwellscalesareacceptablefo rtestingamaterial,generally, , , , Incircumstanceswheretheuserwantstocompar emeasurementswithpreviouslyobtainedRockw ellhardnessdata, Iftheapproximatehardnessofamaterialisnot known, , , ,asageneralrule,thebetteratestsurfaceisp repared, ,thetestsurfaceandthesurfaceincontactwit hthesupportanvilshouldbesmooth,flat,andf reeofoxides,foreignmatter, ,surfaceeffects,suchascarburizationordec arburization,shouldberemovedpriortotesti ng, ,othertypesofcoatings,suchaspaint,galvan izing,etc.

6 , (100kgf).Ingeneral, (15kgf), "Rockwellindentationhardnesstest." , , ,suchasstrikerreboundoreddy-current, ,suchasportability, ,themannerinwhichtheindentationdepthisme asuredandthehardnessvaluedisplayed,andin theautomationofthetestingmachine' ,manyoftheolderdesignsofRockwellmachines arestillinuse,sothatabriefdiscussionofth edifferencesmaybebeneficial.* , ,considerfactorssuchas:theaccuracyandmea surementrepeatabilitythatisrequired;whet herversatilityinthetestingcyclemayberequ ired;therequiredspeedoftesting;theRockwe llscalesthatwillbeused;therequiredresolu tionofthehardnessnumber;thesizeofmateria lnormallytested;andtheaccessoriesthatmay beneeded. TestingPrecautionsWhenusingdevicesthatem ploymeasurementmethodsotherthantheRockwe llindentationhardnessprinciple, , ,sometimesreferredtoas"twintesters"or"co mbinationtesters."Thesemachinesusuallyca ntestallofthedifferentRockwellscales,and ,insomecases, ,themostcommondesignsofRockwellmachinesh aveappliedthepreliminarytestforcebycompr essionofahelicalspring, , , ,anderrorsassociatedwithaleverarmorpreli minaryforcespringareeliminated;however, , , ,Figure2B,Figure3A,andFigure3B,whicharep lotsofRockwellAscale(HRA)testdatameasure datNIST(14\Figure2 Aillustratesthesequenceofhowthetestforce sareappliedduringtheHRAtest, ; (10kgf), ( ).)

7 Thetesthavingthehigherpreliminaryforce(d ashedline) , ,15 TestProcedureh, , */CDf2[o/i_l_f easing!cj11 IncreasingTime-> ( )( )^PreliminaryPreliminaryForceForceIncrea singTime-> (FigureA) (FigureB) (dashedline) ,astheadditionalforceisremovedreturningt othepreliminaryforcelevel, ,h,and,thus, (showninFigure2) ,therefore, (Vi6in) , >IncreasingTime-> (FigureA) (FigureB) ; ,itcontinuestobeusedinsomeoftoday' ,thedialdivisionshaverepresentedwholeRoc kwellnumbers, ,dialgagesandleversystemsoftenbecomeworn ormisalignedinmanymachines, ,whichisdisplayeddigitally, ,thesenewdisplacement-measuringinstrumen tshaveagreateraccuracythanmostdialgage/l eversystems,butasoftenhappenswithdigital displays, , , ,anerrorinthedepthmeasurementrelatestoan errorinthehardnessmeasurementresultas: (2)andISO(4) , ;however, , , , ,itretainedconsistencyfromoperatortooper ator;however,thetestingcyclewasusuallyse tbythemanufacturertocompleteatestinarela tivelyshorttime, , ,aswellascontrolofthetestingcycleduetova ryingmaterialplasticity, 'sRockwellmachinescanbesettoa"standard"t estcycle,whilealsoallowingthetestingcycl etobeadjustedtobetterfittheusers' (Anvils) , ,anerrorinthedepthmeasurementofone-hundr edthofamillimeterwillproduceanerrorof10 Rockwellpoints( ).]

8 , ,sheetmetal,orpartsthatdonothaveflatunde r-surfacesshouldbetestedonaspotanvilhavi ngasmall,elevated, GoodPracticeRecommendations ,suchasscratchesorindents, Whentestinglargesamplesoftestmaterialorm aterialwithalongshapethatsignificantlyov erhangsthehardnessmachine'sanvilsupport, ,theoverhangmaycauseacantileverorlateral forcetobeappliedtotheindenter, ,althoughforcriticalapplications,itmaybe beneficialtorigidlyaffixtheanviltothetes tingmachine. Eachtimeananvilisinstalled,regardlessofi tsdesign,itmustbeadequatelyseatedtothete stingmachinebymakingrepeatedhardnesstest sonauniformpieceofmaterial, Whentestingcurvedparts, ,then,afterrotatingtheanvil90cwithoutmov ingthetestpiece, , ,themeasurementwillbeadverselyaffected, , , , , Excessivehysteresismayindicateproblemswi ththeRockwellmachinecausedbywornordirtyp arts,suchasinthedepthmeasurementsystem, (includingtheoperator) , ,itwouldbefoundthatthemeasurementresults werelikelynotidentical, ,nomatterhowmucheffortismadetoeliminatet hesourcesofthisrandomvariability, , ,errorsinforce,depth,andhysteresisaretyp icallysystematicerrorsthatcontributetoab iasinthehardnessmeasurement, ,whenexcessivefrictionisoccurringduringa test, (2)andISO(4)standardsspecifyamethodforas sessingthelackofrepeatabilityofaRockwell machine,whichinvolvesmakinghardnessmeasu rementsacrossthesurfaceofreferencetestbl ocks( ).

9 , , , ' ,anoften-usedproceduretocertifyRockwelli ndenterswastomakehardnesstestsonreferenc etestblocks, ,iftheindenterperformancedidnotagreewith theblockvalue,itwasdifficulttodeterminew hetherthesourceoftheerrorwasduetotheinde nter,thestandardizingmachine,thereferenc eblockvalues, (2) , (4) "penetrator"or"stylus."Thereareseveraldi fferentdesignscurrentlyusedforthebase(op positeendoftheindentationtip)ofRockwelli ndentersbecauseofthevaryingstylesofinden terholdersfoundondifferentmanufacturer' ,threadedfixtures, ,itisimperativethatthereisnomovementofth eindenterinitsholderduringatest. GoodPracticeRecommendations Indentersshouldbeusedthatarecertifiedtob ewithintolerancesforbothshape(geometry) ,itwascommonfordiamondindenterstobecerti fiedforperformanceonly. Onlyindentersshouldbeusedthathavebeenver ifiedforusewiththeparticularRockwellmach ine,suchasduringanindirectverification( ).Incasesthatotherindentersmustbeused, Periodically,indentersshouldbevisuallyin spectedfordamagewiththeaidofadequatemagn ification(20 Xorhigher).

10 Everyeffortshouldbemadetokeepindenterscl ean, Eachtimeanindenterisinstalled,regardless ofitsdesign,itsseatingsurfacemustbeadequ atelyseatedagainsttheindenterholderbymak ingrepeatedhardnesstestsonauniformpieceo fmaterial, TestingPrecaution Ifanindenterisdroppedorhitwiththetestpie ceoranvil,itisimperativethatbeforeusingi tfurther, ,particularlyadiamondindenter, ,HRC,HRD,HR15N,HR30N, ,tungsten, , , ,thiscommonlyoccurredbecausediamondisver ydifficulttomachineintothespheroconicalg eometry,and,untilrecently, , ,today' ,essentiallythesamevolumeofmaterialwillb edisplaced,butthedepthofindentationwillv ary,and,thus, ,thenasthematerialhardnessincreases, ,dependingonthehardnessofthetestmaterial , ,thetestmaterialisprimarilyincontactwith theradialtip, , ,thesurfaceroughnessofthediamond,thealig nmentoftheindenteraxiswithrespecttothese atingsurfaceoftheindentertothetestmachin e,apoorlymachinedseatingsurface,andhyste resisintheindenteritselfasitisloadedandu nloaded, ,thehardnesslevelofthetestmaterial, ,Rockwelldiamondindentersaresometimescer tifiedforspecificRockwellscales.