NIST recommended practice guide : Rockwell hardness ...

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.

2 NISTR ockwellhardnessstandardizingmachine57 TableofContentsFigure12 (forhardnesslevelsasindicatedonthebottom axis) :ChangeintheRockwellhardnessvalueduetoac hangeinthepreliminaryforcefordiamondinde nterscales(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 )

4 ,wherethesizeoftheindentationismeasuredf ollowingtheindentationprocess,theRockwel lhardnessofthematerialisbasedonthediffer enceinthedepthoftheindenterattwospecific timesduringthetestingcycle, ,therearenoalternativemeasurementsystems todirectlyorindependentlymeasureRockwell hardness , ,andapreliminaryforce(formallyreferredto astheminorload) (dwelltime), ,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).

5 ThesuperficialRockwellscalesemploylighte rforcelevels, , (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.) (10) (100)Castiron,aluminumandmagnesiumalloys , (1/16in.) (10) (60)Annealedcopperalloys, (1/16in.) (10)1471(150)Malleableirons,copper-nicke l-zincandcupro-nickelalloys. ,furthertestingshouldnotcontinuewiththed amagedanvil.

6 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.

7 Suchascarburizationordecarburization,sho uldberemovedpriortotesting, ,othertypesofcoatings,suchaspaint,galvan izing,etc., (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.

8 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), ( ).Thetesthavingthehigherpreliminaryforce (dashedline) , ,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).)]

9 However, , , , ,itretainedconsistencyfromoperatortooper ator;however,thetestingcyclewasusuallyse tbythemanufacturertocompleteatestinarela tivelyshorttime, , ,aswellascontrolofthetestingcycleduetova ryingmaterialplasticity, 'sRockwellmachinescanbesettoa"standard"t estcycle,whilealsoallowingthetestingcycl etobeadjustedtobetterfittheusers' (Anvils) , ,anerrorinthedepthmeasurementofone-hundr edthofamillimeterwillproduceanerrorof10 Rockwellpoints( ). , ,sheetmetal,orpartsthatdonothaveflatunde r-surfacesshouldbetestedonaspotanvilhavi ngasmall,elevated, GoodPracticeRecommendations ,suchasscratchesorindents, Whentestinglargesamplesoftestmaterialorm aterialwithalongshapethatsignificantlyov erhangsthehardnessmachine'sanvilsupport, ,theoverhangmaycauseacantileverorlateral forcetobeappliedtotheindenter, ,althoughforcriticalapplications,itmaybe beneficialtorigidlyaffixtheanviltothetes tingmachine.

10 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( ).