铸造轧辊检测和维护.docx
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铸造轧辊检测和维护
TheDavyRollCompanyLimited
ExcellenceinForgedandCastRollTechnology
InspectionandMaintenanceofCastRolls.
Thefollowinginformationisintendedtoaidrollshoppersonnelwiththeeverydaymaintenanceandinspectionofstaticandrocastworkrolls.
Byoptimizinggrindingoperationsandintroducingtargetedinspectionproceduressignificantimprovementsinrollperformanceandreliabilitycanbeachieved.
RollShopInspectionEquipment.
Automatededdycurrentsystemsareusedinalargeproportionofrollshopsacrosstheworld.Theyprovidearapidandefficientmeansoftestinglargeareasofarollssurfaceallowingtheoperatortomanageandwherenecessary,removerollinggenerateddefects.
Thesesystemsarehoweverlimitedtotestingnearsurfacedefects.Manyrollshopsfeelitisnecessarytointroducecomplementary,deepinspectionsystemsintheformofultrasonicequipment.Thiscanbeintheformofanautomatedsystemusedincooperationwiththeeddycurrentunitsormanualhandheldunits.
Inrollshopswereeddycurrentequipmentisnotavailablemanualultrasonicequipmentaswellasvisualcrackdetectionprocedures,suchasmagneticparticleordiepenetrantinspectionbecomeessential.
UltrasonicInspection
TheDavyRollCompanyinspectiondepartmentsuseportableultrasonicunitsfortheday-to-dayinspectionrequirementsofbothworkandbackuprollsduringmanufactureandatthecustomer’ssite.Ahighlevelofoperatorskillandspecifictrainingisrequiredtofullyutilisetheseunitsaswellasarangeoftailoredultrasonicprobes.TheDavyRollCompanyisabletoofferguidanceandtrainingtosuitablyexperiencedpersonnelinrollinspectionpractices.
UltrasonicProbes
Thefollowingprobesarerecommendedforrollinspectiondutiesandavailablefrommostmanufacturers.
CompressionProbes.
Type
Angle(degrees)
Crystals
Frequency(Mhz)
Diameter(mm)
Comments
Compression
0
Twin
2.5
25
Thisisageneral-purposeprobesuitedtotheinspectionofcentrifugallycastworkrollsespeciallyinterfacebondzonesandmid-deepshelldefects.Althougharelativelylowfrequencyprobethisisidealforuseonalloyindefinitechillmaterialswerethereisalargeamountoffreegraphiteinthemicrostructure.Useofhigherfrequenciesforthesematerialswillgeneratelargeamountsof‘grass’orscattermakingresolvingofthebondzonedifficult.
Compression
0
Twin
5
15
Agoodprobeforresolvingdefectsclosetothesurfaceofworkandbackuprolls.Thiswillpenetratetothebondzoneofhighchromiumandhigh-speedsteelmaterials.Goodforevaluatingandpositioningsmalldefects.
Compression
0
Single
0.5
50
Rollmakersusetheseprobesfortestinglongitudinallyaswellastransverselythroughcastings,forexamplefromonesideofthebarreltotheotherorfromdriveneckendfacetooperatorendface.Wemainlylookformanufacturingproblemsandfatigueproblemsduringservice.Theseprobescanbeexpensiveandyourusagewouldbeverylow.
TwinShearwaveProbes.
Type
Angle(degrees)
Crystals
Frequency(Mhz)
Size(mm)
Comments
Shearwave
45
Twin
2
30x15
Goodfordeepcracksinbackuprollsorexaminingdefectsatworkrollbondzones.
Shearwave
70
Twin
2
30x15
Goodforshallowcracks<10mmindepth,especiallyfirecracking.
Shearwave
90
Single
2
30x15
Alsoknownassurfacewave,isgoodforinspectingthefullbarrelofbackups,forinstanceinthecaseofeddycurrentequipmentfailure.Isverysensitivetosurfaceconditionandcleanliness.
Calibration.
Accuratecalibrationoftestequipmentisessentialtoensureminimumstockremovalandensurethecompleteremovalofdefects,optimizingrollperformanceandreliability.Toensureaccurateresultscompressionprobesshouldbecalibratedwithtestblocksmanufacturedtothesamematerialspecificationasthatoftherolltobetested.
Forshearwaveprobesasteelanglecalibrationblock,forexampleaV2typeisrequiredtoensureprecisetrigonometriccalibrationisachieved.
Thefollowingvaluesmaybeusedforthecalibrationofcompressionprobeswhenspecifictestblocksarenotavailable.Thesevaluesareapproximateonlyasvariationsbetweenmaterialgrades,forexampleenhancedApexgradeandstandardirongradeaswellasbetweenmanufacturerswillexist.
Material
Velocity
AlloyIndefiniteChill(alsoApex)
5700m/s
HighChromiumIron(alsoSTELLA)
6350m/s
HighSpeedSteel
6250m/s
AlloyCastSteel(alsoVChrome)
5918m/s
Angleprobes
3353m/s
DyePenetrantInspection.
Diepenetrantsystemsareusefulindeterminingthepresenceofgrosscrackinginbackupandworkrolls.Thesystemisrelativelyinexpensiveandrequiresalimiteddegreeofoperatortraining.
Thereishoweversomelimitationtothismethodforrollinspection.Theprocessreliesonthedyebeingdrawnintoopensurfacecracksbycapillaryactionandassuchanythingthatimpedesthisprocesscanreduceoreveneradicatetheeffectivenessofcrackdetection.Forexample;
∙Whencrackscontainlargeamountsofoilordebrisoutofthemill.
∙Aftergrindingthecracksmaybesealedbymechanicaldeformationofthesurfaceorbygrindingdebris.
Assuchadditionalinspectionmethodsareadvisedtocomplimentthisproceduretoensurecompleteremovalofdefects.
MagneticParticleInspection.
Magneticparticleinspectionisaneffectiveandreliablesystemforthedetectionofcracksinrollingmillrolls.Thissystemispreferredforthedetectionofsmallercracksespeciallyongroundorturnedsurfaces.
Thesystemworksbyattractionofaferrousbaseddyeontherollsurfacetowardsthemagnetizedcrackfaces.Thisattractionoccursacrossthefullface,atthesurfaceandatdepth.Assuchthesystemdoesnotsufferthelimitationsofthedyepenetrantsystemwhenthecracksareclosedatthesurfaceorfilledwithdebris.
Manysystemsareavailableonthemarketconsistingofeithermainspoweredelectro-magnetsorpermanenthandheldmagnets.
HardnessTesting.
HardnesstestingofcastrollsisprimarilycarriedoutwithEQUOTIPtypeinstruments.Thesedevicesdeterminehardnessbymeasuringthelevelofreboundenergyfromthesurfaceofthetestpieceorroll.
Oneofthemanyusesofharnesstestingisdeterminingifworkhardenedmaterialhasbeenremovedfromthebarrelsurfaceduringgrinding,especiallyofbackuprolls.
Thesedevicesareverysensitivetosurfacecleanlinessandfinish.Hardnessmeasurementstakenonarolledorroughgroundsurfacewillingeneralgivealowerresultthanonapolishedgroundsurface.Accuratemeasurementscanonlybeobtainedona400-gradegritequivalentorbettersurfacefinish.
UltrasonicInspectionProcedures.
MeasuringCrackDepths.
Whencracksaregeneratedinthebarrelofarolltheycanbeparalleltoorperpendiculartothesurface.Forexample,firecrackingwillinitiallyformatrightanglestothesurfacebutastheyaresubjectedtotheforcesofrollingwillpropagatecircumferentiallyandturnparalleltothesurfaceasshownbelow.
Crackingmaynotalwaysbreakthesurface,forexamplearollwhichhasbeensubjectedtoanimpactorlocalizedoverloadmayinitiatecracksbelowandalignedparalleltothebarrelsurface.
Itisthereforealwaysadvisabletoinspecttheareasurroundingasurfacecrackorbruisewithacompressionandangleprobetoensuredetectionofthedeepestcomponent.
Careshouldalwaysbetakenwhenassessingcrackdepthsoncurvedsurfaces.Withshearwaveprobesanangleshouldbechosenthatwillallowyoutoworkasclosetothesurfacecrackaspossible.
Workingwiththeprobeawayfromthecrackonacurvedsurfacecanresultinthebeammovingbackupthecrackfacetowardsthebarrelsurfaceresultingindeeperthanactualcrackdepths.
Whenusingashearwaveprobecrackdepthsshouldbecheckedfrombothedgestoensurethemaximumdepthisobtained.Crackswillnotalwaysbealignedexactlyperpendiculartothesurfacebutmaybeenteringatanangle.Iftheprobeangleandcrackanglearesimilarthecrackmaynotbedetectedwhentestedfromonlyoneside.
InspectionofCentrifugallyCastShelltoCoreBondZone(Interface).
Centrifugallycastworkrollsareinspectedpriortodeliverytoensureaninterfacebondstrengthsufficientforstandardmilloperatingconditionstodiscardsize.
Howeverifrollsaresubjectedtoabuseduringrollingitisadvisabletoinspectthebondzoneorinterfaceareatoensuretherehasbeennodetrimentaleffectthatcouldultimatelyresultintherollsfailure.
Thetestproceduredescribedbelowcomparestheamountofenergyreturnedtotheprobefromtheinterfacerelatedtotheamounttransmittedintoanddispersedwithinthecorematerial.Alowenergyreturnindicatesapoorreflectororwell-bondedinterfacewhileahigh-energyreturnindicatesagoodreflectorandhencepoorerbonding.
Theprincipleisshownbelowwithechoesforawellandpoorbondedinterfaceatthesamedecibellevel.
GoodInterfacePoorInterface
Testprocedure
ThisprocedurecanbeusedforalltypesofROCASTcentrifugallycastrolls.
Equipment
KrautkramerUSN50orsimilarcalibratedtoBS44331part2orequivalent.
A2.5MHztwincrystalcompressionprobe.
Testblock(20x40x50mm)manufacturedfromthesamegradematerialastherolltobeexamined.
Procedure
Calibratethetimebaseat100mmusingtheabovetestblock.Measureandrecordthegainindecibels(db)requiredtoobtainafullscreenheight(100%)backwallechofromthetestblock.Thisvalue(A)isthenusedastheminimumortheoreticalvaluefortotalinterfaceseparation.
Testtheroll