Complete Casting Handbook, Second Edition: Metal Casting Processes, Metallurgy, Techniques and Design

1015 Pages·2015·82.1 MB·English

by  Campbell, John

Complete Casting Handbook
Metal Casting Processes, Metallurgy,
Techniques and Design
Second Edition
John Campbell
Emeritus Professor of Casting Technology,
Department of Metallurgy and Materials,
University of Birmingham, UK
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To Robert Puhakka
For his dedicationtothe 10 rules and the living proofthat they work.
Preface
InthisfirstupdateoftheHandbook,themajorrevisionsareprobablythoserelatingtorunningsystemdesigninwhichthe
vestigesoffillingdefectshavefinallybeeneliminatedfromcastings.
Thus, the powerful benefits of contact pouring (in which the universal conical trumpet decorating all traditional
fillingsystemsisnoweliminated)isfinallyshowntohavebeenhugelyunderestimatedbyanumberoffoundries.Contact
pouringhasprobablybeenthemostimportant(andthemostsimpleandzero-cost)initiativetorevolutionisequalityin
castings.Inaddition,theadoptionofvariousformsoftangentialfilterdesignstogateshasfinallyeliminatedtheproblem
oftheentrainmentofprimingbubbles.Theseresidualbubbleshavelongimpairedthebenefitsofpreviousfillingsystems.
GravitypouringhasnowadvancedtothepointatwhichIfindmyselfhavingtoadmitthatitstartstothreatenmy
cherishedandfavouredcastingproductionsystem:countergravity.
Thisisseentobeespeciallytrueforthoselow-pressuresystemswhichusearefractoryliningforthepressurised
furnace.Ionlyrecentlydiscoveredthehugelydamagingemissionofbubblesfromtheseliningsduringdepressurisation
ofthefurnace.Thisproblemhasclearlybeenamajorsourceofimpairedcastingsinthelow-pressurecastingindustryand
hashamperedthisindustrysinceitsbeginnings.
Theuseofmypneumaticpumpisdescribedforthefirsttime.Itwouldlowercostsandsolvemostoftheproblemsof
thisindustry.Thus,Icontinuetostandbycountergravityastheoptimumcastingsystemwhereitcanbeused.Myhopeis
thatitwillbeteamedupwithagoodmeltingandmetalhandlingsystem.Onlycarefulfoundrydesignwillminimise
bifilm populations in metals. Only when castings can be produced substantially free from bifilms will we enjoy the
fullbenefitsofcastings,andmetalsingeneral,resistanttohottearing,cracking,blisters,corrosionpittingandattack
ofgrainboundaries,plusthebenefitsofextraordinarymechanicalproperties,potentiallyeliminatingfuturefailureby
fractureorfatigue.
Theseareheadypredictions.However,earlyresultsinfoundriesarealreadyindicatingthatbeautifuldefect-freecast-
ingswithrevolutionarymetallurgicalbenefitsappeartoberoutinelyattainable.Despitechallengesfromtheundoubtedly
uniquebenefitsofsuchnewprocessesasadditivemanufacture,myhopeforthefutureforcastingsisbasedontheadop-
tion ofsimple principles whichcouldnotonlysecure thefutureof ourcastingindustry,butimprove thewelfare and
environmentofallofuswhoselivesdependonit.
JC
Ledbury,Herefordshire,England
02April2015
xxi
Introduction
CASTINGS HANDBOOK, 2ND EDITION, 2015
WhenCastingsHandbookfirstappearedin2011,Ihadnotexpectedtoreviseitsosoon.However,thelatestfindings
requirepublicisingasquickly aspossibledthereisstillalongwayfortheindustrytogo!Themessageofthebook,
summarised in the section “Bifilm-free Properties”, is that a quality improvement of astonishing scale is possible
now.WhenIfirststartedtoexperimentwithnovelfillingsystemsforcastings,therewerenaturallymanydisappoint-
ments. However, those days are long gone. The concepts of entrainment and bifilm creation laid out in the book are
now proven. Some foundries are already being designed to take advantage of a unique and easily affordable quality
revolutionandscrapreduction.Moreneedtofollow.Therisksareminimalandtherewardsaregreat.
Withregardtoimprovedcastingtechniqueswhichcanreduceoreveneliminatetheusualvastpopulationsofbifilms
inourmetals,Ihavealwaysbeenawareofthepotentialbenefitofcontactpouring,buthadcompletelyunderestimatedits
effects. It achieves miraculous improvements to castings by eliminating the 50% air mixing step. Contact pouring is
stronglyrecommendedinthisvolumeasamajorbutlow-coststepforward.
Theultimatestepforwardiscountergravitycastingwhichshouldalwaysbetargetedifpossible.However,althoughI
discussthetraditionalpumpingtechniques,Ipresenthereforthefirsttimemynewpneumaticpump.Itisanotherlow-
cost,reliabletechniquewhichenjoysuniquelylowturbulenceandmightallowarapidtakeupofthisuniquetechnology.
Turningtotheseriousnessofthecurrentpositionincasting,andinmetallurgicalengineeringasawhole,thefactthat
mostcurrentmetalscanfailbycrackingshouldalertustotheglaringinconsistencyinourmetallurgicalthinkingbecause
manyofourmetalsandalloysareductile,sofailurebycrackingshouldbeimpossible.Inaductilemetalanattemptto
propagateacrackshouldmerelyresultinthecracktipblunting,preventingpropagation.
Intheabsenceofanyotherviablealternativemechanism,thatmetalsdocrackisastrongindicationthatcrackspre-
existinmetalsintheformofbifilmsformedintheliquidstateduringpouringofthemetaltomakeacasting.Thepoor
practiceisalmostuniversallyassociatedwithpouringmethodswhichensurethatthemoltenmetalismixedandemul-
sifiedwithatleast50volume%ofairduringitsjourneyintothemould.Idefyanyonetomakearespectablecastingfrom
suchadisgracefullyinappropriateandinepttechnique.Thisbookpresentsthecasethatthisneednotbeso;metalsneed
notcontainbifilms,andthusneednotcontainthoseGriffithcrackswhichcaninitiatefailurebycracking.Toachievethis,
wesimplyhavetoimproveourcastingtechnology.
Itremainsthecasethatbifilmsarestilllamentablyresearched,sothatthisbookhastoresorttosiftingthroughincon-
clusive and fragmentary evidence from researchers who were not looking for bifilms. Unfortunately, researchers up
untilnowhavenotbeenawareoftheirpresence,andcertainlydidnotsuspecttheiroverwhelminginfluenceontheirre-
sults.Althoughawelcomestartisbeingmadebyafewworkers,Iremainimpatientformoredefinitiveresearchtobe
carriedout.
Inthemeantime,whileresearchersslowlygetaroundtoprovingthebackgroundtheory,foundersneednotwaitfor
answers.Practicallowbifilmcastingtechniqueshavealreadybeendevelopedandaredescribedhere.Theypromisethe
qualityimprovementandcostreductionthatthecastingindustrysobadlyneeds.
xxiii
Acknowledgements
It is a pleasure to acknowledge the significant help and encouragement I have received from many good friends.
John Grassi has been my close friend and associate at Alotech, the company promoting the new, exciting ablation
castingsprocess.KenHarrishasbeenaninexhaustiblesourceofknowledgeonsilicatebinders,aggregatesandrecycling.
HisassistanceisclearinChapter15.Clearly,thecastingindustryneedsmorechemistslikehim.BobPuhakkahasbeen
thefirstregularuserofmycastingrecommendationsfortheproductionoflargealuminiumandsteelcastings,whichhas
providedmewithinspirationalconfirmationofthesoundnessofthetechnologydescribedinthisbook.Inaddition,the
practical feedback and warm friendship over several years from those at the UK steel foundry Furniss and White is
a pleasure to record. Murat Tiryakioglu has been a loyal supporter and critic, and provides the elegantly written
publicationsthathaveprovidedwelcomescientificunderpinning.Hehasprovidedgenerousandinvaluablehelpwith
the important section The Statistics of Failure. Naturally, manyother acknowledgements are deserved among friends
andstudentswhoseresearchhasbeenaprivilegetosupervise.Idonottaketheseforgranted.Evenifnotlistedhere,
theyarenotforgotten.
TheAmericanFoundrySocietyisthankedfortheuseofanumberofillustrationsfromTransactions.
xxv
CHAPTER
1
THE MELT
Some liquid metals may be really pure liquid. Such metals may include pure liquid gold, possibly some carbon-
manganesesteelswhileinthemeltingfurnaceatalatestageofmelting.These,however,arerare.
Manyliquidmetalsareactuallysofullofsundrysolidphasesfloatingaboutthattheybegintomorecloselyresemble
slurriesthanliquids.Thisslurrytypenaturecanbeseenquiteoftenassomemetalsarepoured;themeltoverflowsthelip
of the melting furnace as though it were a cement mixture. In the absence of information to the contrary, this awful
conditionofaliquidmetalshouldbeassumedtobecorrect.Thusmanyofourmodelsofliquidmetalsthatareformulated
toexplaintheoccurrenceofdefectsneglecttoaddressthisfact.Astechniqueshaveimprovedoverrecentyears,therehas
beengrowingevidencefortherealinternalstructureofliquidmetals,revealingmeltstobecrammedwithdefects.Some
ofthisevidenceisdescribedinthischapter.Muchevidenceappliestoaluminiumanditsalloyswherethegreatesteffort
hasbeenfocussed,butevidenceforsomesteelsandallNi-basealloysisalreadyimpressiveandisgrowingsteadily.
Itissoberingtorealisethatmanyofthestrength-relatedpropertiesofmetalscanonlybeexplainedbyassumingthat
theoriginalmeltwasfullofdefects.Classicalphysicalmetallurgyandsolidificationsciencethathasconsideredmetals
asmerelypuremetalscurrentlycannotexplainaspectsoftheimportantpropertiesofcastmaterialssuchastheeffectof
dendritearmspacing;itcannotexplaintheexistenceofporesandtheirareadensity;itcannotexplainthereasonforthe
crackingofstrong,crack-resistantprecipitatesformedfromthemelt.Thesekeyaspectsofcastmetalswillbeseento
arisenaturallyfromtheassumptionofapopulationofaparticulartypeofdefect:thebifilm.
Anyattempttoquantifythenumberandsizedistributionofthesedefectsisanon-trivialtask.McClainandco-workers
(2001)andGodlewskiandZindel(2001)havedrawnattentiontotheunreliabilityofresultstakenfrompolishedsections
ofcastings.Atechniqueforliquidaluminiuminvolvesthecollectionofinclusionsbyforcingupto2kgofmeltthrougha
finefilter,asintheporousdiscfiltrationanalysisandPrefiltests.Themethodovercomessomeofthesamplingproblemby
concentratingtheinclusionsbyafactorofabout10,000times(EnrightandHughes,1996andSimardetal.,2001).The
layerofinclusionsremainingonthefiltercanbestudiedonapolishedsection.Thetotalquantityofinclusionsisassessed
astheareaofthelayerasseenunderthemicroscope,dividedbythequantityofmeltthathaspassedthroughthefilter.The
unitisthereforethecurious quantity mm2kg(cid:1)1.(Itistobehopedthatatsomefuturedate thisunhelpfulunitwill,by
universalagreement,beconvertedintosomemoremeaningfulquantitysuchasvolumeofinclusionspervolumeofmelt.
Inthemeantime,thestandardprovisionofthediameterofthefilterinreportedresultswouldatleastallowareaderthe
optiontodothis.)
To gain some idea of the huge range of possible inclusion contents, an impressively dirty melt might reach
10mm2kg(cid:1)1,whereasanalloydestinedforacommercialextrusionmightbeinthe0.1–1range,foilstockmightreach
0.001,andcomputerdiscs0.0001mm2kg(cid:1)1.Forafilterof30mmdiameter,thesefiguresapproximatelyencompassthe
rangeofvolumefraction10(cid:1)3(0.1%)downto10(cid:1)7(0.1partpermillionbyvolume).
Other techniques for the monitoring of inclusions in Al alloy melts include Liquid Metal Cleanness Analyser
(LiMCA)(SyvertsenandEngh2001),inwhichthemeltisdrawnthroughanarrowtube.Thevoltagedropappliedalong
thelengthofthetubeismeasured.Theentryofaninclusionofdifferentelectricalconductivityintothetubecausesthe
voltagedifferentialtorisebyanamountthatisassumedtobeproportionaltothesizeoftheinclusion.Thetechniqueis
generallythoughttobelimitedtoinclusionsapproximatelyinthe10–100mmrangeorso.
Althoughwidelyusedforthecastingofwroughtalloys,theauthorregretsthattheLiMCAtechniquehastobeviewed
withgreatreservation.Inclusionsinlightalloysareoftenoxidebifilmsuptoa10mmdiameter,aswillbecomeclear.
3
CompleteCastingHandbook.http://dx.doi.org/10.1016/B978-0-444-63509-9.00001-7
Copyright©2015JohnCampbell.PublishedbyElsevierLtd.Allrightsreserved.
4 CHAPTER 1 THE MELT
SuchinclusionsdofindtheirwayintotheLiMCAtube,wheretheytendtohang,caughtupatthemouthofthetube,and
rotateintospiralslikeaflagtiedtothemastbyonlyonecorner.Thesearetornfreefromtimetotimeandsedimentinthe
bottomofthesamplingcrucibleoftheLiMCAprobe,wheretheyhavetheappearanceofaheapofspiralItaliannoodles
(Asbjornsonn, 2001). It unfortunate that most workers using LiMCA have been unaware of these serious problems.
Becauseoftheairenfoldedintothebifilm,thedefectsintheLiMCAprobehaveoftenbeenthoughttobebubbles,which,
probably,theysometimespartlyareandsometimescompletelyare.Onecanseetheconfusion.
Ultrasonic reflections have been used from time to time to investigate the quality of melt. The early work by
MountfordandCalvert(1959)isnoteworthy,andhasbeenfollowedupbyconsiderabledevelopmenteffortsinAlalloys
(Mansfield,1984),Nialloysandsteels(Mountfordetal.,1992).Ultrasoundisefficientlyreflectedfromoxidebifilms
(almost certainly because the films are double, and the elastic wave cannot cross the intermediate layer of air, and
thereforeisefficientlyreflected).However,thereflectionsmaynotgiveanaccurateideaofthesizeofthedefectsbecause
theirirregular,crumpledformandtheirtumblingactioninthemelt.Thetinymirror-likefacetsofalarge,scrambled
defectreflectsbacktothesourceonlywhentheyhappentorotatetofacethebeam.Theresultisageneralscintillation
effect,apparentlyfrommanyminuteandseparateparticles.Itisnoteasytodiscernwhethertheimagescorrespondto
manysmallorafewlargedefects.
NeitherLiMCAnorthevariousultrasonicprobescandistinguishanyinformationonthetypesofinclusionsthatthey
detect.Incontrast,theinclusionscollectedbypressurised(forced)filtrationcanbestudiedinsomedetail,althougheven
heretheareasoffilmdefectsareoftendifficulttodiscern.Inadditiontofilms,manydifferentinclusionscanbefoundas
listedinTable1.1.
Nearlyalloftheseforeignmaterialswillbedeleterioustoproductsintendedforsuchproductsasfoilorcomputer
discs.However,forshapedcastings,thoseinclusionssuchascarbidesandboridesmaynotbeharmfulatall.Thisis
because having been precipitated from the melt, so they are usually therefore in excellent atomic contact with the
matrix.Thesewell-bondednon-metallicphasesaretherebyunabletoactasinitiatorsofotherdefectssuchasporesand
cracks.Conversely,theymayactasgrainrefiners.Furthermore,theircontinuedgoodbondingwiththesolidmatrixis
expectedtoconferonthemaminorornegligibleinfluenceonmechanicalproperties.(However,weshouldnotforget
thatitispossiblethattheymayhavesomeinfluenceonotherphysicalorchemicalpropertiessuchasmachinabilityor
corrosion.)
Generally,therefore,thisbookconcentratesonthoseinclusionsthathaveamajorinfluenceonmechanicalproperties,
and that can be the initiators of other serious problems such as pores and cracks. Thus the attention will centre on
entrainedsurfacefilmswhichexhibitunbondedinterfacesinthemeltandleadtoaspectrumofproblems.Usually,these
inclusions will be oxides. However, carbon films are also common, and occasionally nitrides, sulphides and other
compounds.Thatthesefilmsarenecessarilyentrainedintothemeltasdoublefilmsisakeyfeatureoftheirstructure.This
aspectoftheirmake-upwillbeacentralfeatureofthebook.
Table1.1 TypesofInclusionsinAlAlloys
InclusionType PossibleOrigin
Carbides Al C PotcellsfromAlsmelters
4 3
Boro-carbides Al B C Borontreatment
4 4
Titaniumboride TiB Grainrefinement
2
Graphite C Fluxingtubes,rotorwear,entrainedfilm
Chlorides NaCl,KCl,MgCl ,etc. Chlorineorfluxingtreatment
2
Alphaalumina a-Al O Entrainmentafterhigh-temperature
2 3
melting
Gammaalumina g-Al O Entrainmentduringpouring
2 3
Magnesiumoxide MgO HigherMgcontainingalloys
Spinel MgOAl O MediumMgcontainingalloys
2 3
1.1 REACTIONS OF THE MELT WITH ITS ENVIRONMENT 5
Thepressurisedfiltrationtestscanfindmanyoftheseentrainedsolids,andtheanalysisoftheinclusionspresenton
thefiltercanhelptoidentifythesourceofmanyinclusionsinameltingandcastingoperation.However,theonlyin-
clusionsthatremainundetectablebutareenormouslyimportantarethenewlyentrainedfilmsthatoccuronacleanmelt
asaresultofsurfaceturbulence.Thesefilmsarecommonlyentrainedduringthepouringofcastings.Theyaretypically
only20nmthick,andsoremaininvisibleunderanopticalmicroscope,especiallyifdrapedaroundapieceofrefractory
filterthatwhensectionedwillappearmanythousandsoftimesthicker.Theonlydetectiontechniqueforsuchinclusions
isthelowlyreducedpressuretest.Thistestopensthefilms(becausetheyarealwaysdoubleandcontainair,aswillbe
explainedindetailinChapter3)sothattheycanbeseen.Metallographicsections(orradiographs)ofthecasttestpieces
clearlyrevealthesize,shapeandnumbersofsuchimportantinclusions,ashasbeenshownbyFoxandCampbell(2000).
Thetestwillbediscussedindetailagainandagain.
1.1 REACTIONS OF THE MELT WITH ITS ENVIRONMENT
Aliquidmetalisahighlyreactivechemical.Itwillreactbothwiththegasesaboveitand,ifthereisanykindofslagor
fluxfloatingontopofthemelt,itwillprobablyreactwiththat,too.Manymeltsalsoreactwiththeircontainerssuchas
cruciblesandfurnacelinings.
Thedrivingforcefortheseprocessesisthestrivingofthemelttocomeintoequilibriumwithitssurroundings.Its
successinachievingequilibriumis,ofcourse,limitedbytherateatwhichreactionscanhappenandbythelengthoftime
available.
Thusreactionsinthecrucibleorfurnaceduringthemeltingofthemetalareclearlyseentobeseriousbecausethereis
usually plenty of time for extensive changes. Hydrogen being picked up from damp refractories is common. Similar
troublesareoftenfoundwithmetalsthataremeltedinfurnacesheatedbytheburningofhydrocarbonfuelssuchasgasor
oil.
WecandenotethechemicalcompositionofhydrocarbonsasC H andthusrepresentthestraightchaincompounds
x y
such as methane CH , ethane C H and so on, or aromatic ring compounds such as benzene C H etc. (Other more
4 2 6 6 6
complicated molecules may contain other constituents such as oxygen, nitrogen and sulphur, not counting impurities
whichmaybepresentinfueloilssuchasarsenicandvanadium.)
Forourpurposes,wewillwritetheburningoffueltakingmethaneasanexample:
CH þ2O ¼CO þ2H O (1.1)
4 2 2 2
Clearly,theproductsofcombustionofhydrocarbonscontainwater,sothehotwastegasesfromsuchfurnacesare
effectivelywet.
Evenelectricallyheatedfurnacesarenotnecessarilyfreefromtheproblemofwetenvironment:anelectricresistance
furnacethathasbeenallowedtostandcoldoveraweekendwillhavehadthechancetoabsorbconsiderablequantitiesof
moistureinitsliningmaterials.Mostfurnacerefractoriesarehygroscopicandwillabsorbwaterupto5or10%oftheir
weight.Thiswaterisreleasedintothebodyofthefurnaceoverthenextfewdaysofoperation.Ithastobeassumedthat
theusualclay/graphite cruciblematerials commonlyusedformelting nonferrous alloysare quitepermeable towater
vapourand/orhydrogenbecausetheyaredesignedtobeapproximately40%porous.Additionally,hydrogenpermeates
freely through most materials, including steel, at normal metallurgical operating temperatures of around 700(cid:3)C and
higher.
ThemoisturefromliningsoratmospherecanreactinturnwiththemetalM:
MþH O¼MOþH (1.2)
2 2
Thusalittlemetalissacrificedtoformitsoxide,andthehydrogenisreleasedtoequilibrateitselfbetweenthegasand
metalphases.Whetheritwill,onaverage,enterthemetalorthegasabovethemetalwilldependontherelativepartial
pressureofhydrogenalreadypresentinbothofthesephases.Themolecularhydrogenhastosplitintoatomichydrogen
(sometimescalled‘nascent’hydrogen)beforeitcanbetakenintosolution,asisdescribedbythesimplerelation:
H ¼2H (1.3)
2
6 CHAPTER 1 THE MELT
Theequationpredictingthepartialpressureofhydrogeninequilibriumwithagivenconcentrationofhydrogenin
solutioninthemeltis:
[H]2¼kP (1.4)
H
2
where the constant k has been the subject of many experimental determinations for a variety of gas-metal systems
(Brandes,1992;RansleyandNeufeld,1948).Itisfoundtobeaffectedbyalloyadditions(SigworthandEngh,1982)and
temperature. The relation is a statement of the famous Sievert law, which describes the squared relation between a
diatomicgasconcentrationanditspressure;forinstance,ifthegasconcentrationinsolutionisdoubled,itsequilibrium
pressureisincreasedby4times.AfurtherpointtonoteisthatwhenthepartialpressureofhydrogenP¼1atm,itis
immediatelyclearthatkisnumericallyequaltothesolubilityofhydrogeninthemetalatthattemperature.Figure1.1(a)
showshowthesolubilityofhydrogeninaluminiumincreaseswithtemperature.
Figure1.1(b)showsthatalthoughmanymetalsdissolvemorehydrogenthanaluminium,itisaluminiumthatsuffers
mostfromhydrogenporositybecauseofthehugedifferenceinsolubilitybetweentheliquidandthesolid.Thesolidcan
holdonlyabout1/20thofthegasintheliquid(correspondingtoapartitioncoefficientof0.05)sothatthereisamajor
driving force for the rejection of nearly all the hydrogen on solidification, having the potential to create significant
porosity.Thiscontrastswithmagnesiumandmanyothermetals,wherethepartitioncoefficientsarecloserto1.0,sothat
theirhigherhydrogencontentis,ingeneral,notsuchaproblemtodrivethenucleationofpores(eventhoughitmay
contributesignificantlytothegrowthofporesbecauseofitshighrateofdiffusion,drainingthehydrogencontentfrom
significantlygreatervolumesofthealloy).ThesefactorsarediscussedatlengthinSection7.2relatingtothegrowthof
gasporosity.
(a) (b)
Temperature (°C)
500 600 700 800 90010001100
100 500
mg
200
100 Ni
50
l/kg)m 10 Al Al–4Cu 20 Fe
( g
ogne Al–12Si n L/k 10 Cu
ydrofh ydroge 5
h
y
bilit ? 2 Al
u1.0
ol
S
? 1
0.5
0.2
0.1 0.1
1.3 1.2 1.1 1.0 0.9 0.8 0.7 200 400 600 800 1000 1200 1400 1600
Reciprocal absolute temperature (K–1× 103) Temperature (°C)
FIGURE1.1
(a)Hydrogensolubilityinaluminiumandtwoofitsalloys,showingtheabruptfallinsolubilityonsolidification;(b)
hydrogensolubilityforanumberofmetals.