Castings Practice: The Ten Rules of Castings

219 Pages·2004·2.2 MB·English

by  John Campbell

Castings Practice
The 10 Rules of Castings
Dedication
To Merton C. Flemings of MIT for inspirational teaching and research
Castings Practice
The 10 Rules of Castings
John Campbell
Elsevier Butterworth-Heinemann
Linacre House,Jordan Hill, Oxford OX2 8DP
200Wheeler Road,Burlington, MA01803
First published 2004
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Contents
Dedication ii 3.3 Waterfallflow 104
Preface vii 3.4 Horizontalstreamflow 104
Summary xi 3.5 Hesitationandreversal 106
Rule 4. Avoid bubble damage 108
Rule 1. Achieve a good quality melt 1
4.1 Gravity-filledrunning
1.1 Background 1
systems 111
1.2 Melting 3
4.2 Pumpedandlow-pressurefilling
1.3 Holding 3
systems 112
1.4 Pouring 4
1.5 Melttreatments 5
1.5.1 Degassing 5 Rule 5. Avoid core blows 114
1.5.2 Additions 6 5.1 Background 114
1.6 Filtration 7 5.2 Prevention 117
1.6.1 Packedbeds 7
1.6.2 Alternativevarietiesof Rule 6. Avoid shrinkage damage 120
filters 8 6.1 Feedingsystemsdesign
1.6.3 Practicalaspects 8 background 120
6.1.1 Gravityfeeding 123
Rule 2. Avoid turbulent entrainment (the 6.1.2 Computermodellingof
critical velocity requirement) 9 feeding 124
6.1.3 Randomperturbationsto
2.1 Maximumvelocity
feedingpatterns 124
requirement 10
6.1.4 Dangersofsolid
2.2 The‘nofall’requirement 13
feeding 125
2.3 Fillingsystemdesign 15
6.1.5 Thenon-feedingrolesof
2.3.1 Gravitypouringof
feeders 125
open-topmoulds 15
6.2 Thesevenfeedingrules 126
2.3.2 Gravitypouringofclosed
Rule1: Donotfeed 126
moulds 16
Rule2: Heat-transfer
2.3.3 Horizontaltransfer
requirement 127
casting 68
Rule3: Mass-transfer
2.3.4 Counter-gravity 72
requirement 128
2.3.5 Surfacetensioncontrolled
Rule4: Junction
filling 75
requirement 132
2.3.6 Inclusioncontrol:filters
Rule5: Feedpath
andtraps 78
requirement 133
2.3.7 Practicalcalculationofthe
Rule6: Pressuregradient
fillingsystem 93
requirement 138
Rule7: Pressure
Rule 3. Avoid laminar entrainment of the requirement 140
surface film (the non-stopping, 6.3 Thenewfeedinglogic 142
non-reversing condition) 102 6.3.1 Background 142
3.1 Continuousexpansionofthe 6.3.2 Thenewapproach 143
meniscus 102 6.4 Activefeeding 145
3.2 Arrestofverticalprogress 103 6.5 Freezingsystemsdesign 146
vi Contents
6.5.1 Externalchills 147 10.2.1 Rectilinearsystems 177
6.5.2 Internalchills 149 10.2.2 Cylindricalsystems 178
6.5.3 Fins 150 10.2.3 Trigonalsystems 179
10.2.4 Thin-walledboxes 179
10.3 Locationjigs 180
Rule 7. Avoid convection damage 157
10.4 Clampingpoints 180
7.1 Convection:theacademic
10.5 Moulddesign:thepractical
background 157
issues 181
7.2 Convection:theengineering
10.6 Castingaccuracy 182
imperatives 157
10.7 Toolingaccuracy 183
7.3 Convectiondamageandcasting
10.8 Mouldaccuracy 183
sectionthickness 160
10.9 Summaryoffactorsaffecting
7.4 Counteringconvection 162
accuracy 186
10.10 Metrology 186
Rule 8. Reduce segregation damage 163
Appendix 188
Rule 9. Reduce residual stress (the ‘no water The1.5factor 188
quench’ requirement) 166 TheBernoulliequation 189
9.1 Introduction 166 Rateofpourofsteelcastingsfroma
9.2 Residualstressfromcasting 166 bottom-pourladle 191
9.3 Residualstressfrom Runningsystemcalculationrecord 191
quenching 167 Designmethodologyforinvestment
9.4 Distortion 172 castings 194
9.5 Heattreatmentdevelopments 173
9.6 Epilogue 174 References 195
Index 199
Rule 10. Provide location points 175
10.1 Datums 175
10.2 Locationpoints 176
Preface
Castings can be difficult to get right. Creating point of catastrophic failure, tearing it apart,
thingsneveriseasy.Butsensetheexcitementof or causing insidious thin cracks. Most trea-
this new arrival: cherous of all, the strain may not quite crack
The first moments of creation of the new the casting, leaving it apparently perfect, but
casting are an explosion of interacting events; loaded to the brink of failure by internal resi-
thereleaseofquantitiesofthermalandchemical dual stress.
energy trigger a sequence of cataclysms. These events are rapidly changing dynamic
Theliquidmetalattacksandisattackedbyits interactions. It is this rapidity, this dynamism,
environment,exchangingalloys,impurities,and that characterizes the first seconds and minutes
gas. The surging and tumbling flow of the melt ofthecasting’slife.Anunderstandingofthemis
through the running system can introduce crucial to success.
clouds of bubbles and Sargasso seas of oxide This new work is an attempt to provide a
film.Themouldshockswiththeviciousblastof frameworkofguidelinestogetherwiththeback-
heat, buckling and distending, fizzing with the ground knowledge to ensure understanding; to
volcanic release of vapours that flood through avoid the all too frequent disasters; to cultivate
theliquidmetalbydiffusion,orreachpressures the targeting of success; to encourage a profes-
to burst the liquid surface as bubbles. sional approach to the design and manufacture
During freezing, liquid surges through the of castings.
dendrite forest to feed the volume contraction The reader who learns to guide the produc-
on solidification, washing off branches, cutting tionmethodsthroughthisminefieldwillfindthe
flow paths, and polluting regions with excess rare reward of a truly creative profession. The
solute, forming segregates. In those regions cut student who has designed the casting method,
offfromtheflow,continuingcontractioncauses and who is present when the mould is opened
the pressure in the residual liquid to fall, possi- for the first time will experience the excitement
bly becoming negative (as a tensile stress in the and anxiety, and find himself asking the ques-
liquid) and sucking in the solid surface of the tion asked by all foundry workers on such
casting. This will continue until the casting is occasions: ‘Is it all there?’ The casting design
solid, or unless the increasing stress is suddenly rules in this text are intended to provide, so far
dispelled by an explosive expansion of a gas or as present knowledge will allow, enough pre-
vapour giving birth to a shrinkage cavity. dictive capability to know that the casting will
The surface sinks are halted, but the internal be not only all there, but all right!
defects now start. The clean lines of the finished engineering
Thesubsequentcoolingtoroomtemperature casting,sound,accurate,andstrong,areaplea-
isnolessdramatic.Thesolidifiedcastingstrives sure to behold. The knowledge that the casting
to contract while being resisted by the mould. containsneitherdefectsnorresidualstressisan
The mould suffers, and may crush and crack. additionalpowerfulreassurance.Itrepresents a
Thecastingalsosuffers,beingstretchedasona miraculous transformation from the original
rack. Silent, creeping strain and stress change two-dimensionalformonpaperorthescreento
anddistortthecasting,andmayintensifytothe athree-dimensionalshape,fromamobileliquid
viii Preface
to a permanently shaped, strong solid. It is an one!’.TheRulesandthisbookaretheoutcome.
achievement worthy of pride. ItwasnotperhapstheoutcomethateitherDale
The reader will need some background orIoriginallyimagined.AProcessSpecification
knowledge. The book is intended for final year has proved elusive, proving so difficult that
studentsin metallurgy orengineering, forthose I have concluded that it will need a more
researching in castings, and for casting engi- accomplished author.
neersandallassociatedwithfoundriesthathave TheRulesastheystandthereforeconstitutea
to make a living creating castings. firstdraftofaProcessSpecification;morelikea
Good luck! checklist of casting guidelines. A buyer of cast-
This new book is the second of three books ings would demand that the list were fulfilled if
dealing with castings. The three books are he wished to be assured that he was buying the
(i)Principles(thenewmetallurgyofcastmetals; best possible casting quality. If he were to spe-
the metallurgist’s book) (ii) Practice (the prac- cify the adherence to these Rules by the casting
tical founder’s book) and (iii) Processes (an producer, he would ensure that the quality and
appraisal of the various methods of making reliability of the castings was higher than could
castings; perhaps a casting buyer’s book). The be achieved by any amount of expensive
three are intended as a sequence, dealing with checking of the quality of the finished product.
thetheoryandpracticeofthecastingofmetals. Conversely,ofcourse,theRulesareintended
At the rate at which new understanding is to assist the casting manufacturer. It will speed
emerging, an additional text may also be up the process of producing the casting right
required; (iv) Properties (abook for everyone). firsttime,andshouldcontributeinamajorway
ThesecondintheseriesisdevotedtotheTen to the reduction of scrap when the casting goes
Rules.Thesearemyownchecklisttoensurethat into production. In this way the caster will be
nokeyaspectofthedesignofthemanufacturing able to raise standards, without any significant
route for the casting is forgotten. increase in costs. Quality will be raised to the
The Ten Rules listed here are proposed as point at which castings of quality equal to that
necessary, but not, of course, sufficient, for the offorgingscanbeofferedwithconfidence.Only
manufacture of reliable castings. It is proposed in this way will castings be accepted by the
that they are used in addition to existing engineering profession as reliable, engineered
necessary technical specifications such as alloy products, and assure the future prosperity of
type,strength,andtraceabilityviainternational both the casting industry and its customers.
standardqualitysystems,andotherwell-known It is recognized that many users of this book
and well-understood foundry controls such as will be students of casting technology. For
casting temperature etc. completeness therefore, the strict description of
Although not yettested on all cast materials, the Rules as intended as the caster’s checklist
therearefundamentalreasonsforbelievingthat has been relaxed a little. A small addition has
theRuleshavegeneralvalidity.Theyhavebeen been made to paragraph 10, extending the sec-
applied to many different alloy systems includ- tion describing the requirement for location
ing aluminium, zinc, magnesium, cast irons, points. This extension includes related aspects
steels, air- and vacuum-cast nickel and cobalt, not included elsewhere, such as the accuracy of
and even those based on the highly reactive thewhole mould assembly, and the many-sided
metalstitaniumandzirconium.Nevertheless,of problems of mould design.
course, although all materials will probably Afurtherfeatureoftheworkthatemergedas
benefit from the application of the Rules, some the book was being written was the dominance
will benefit almost out of recognition, whereas ofChapter2,thedesignofthefillingsystemsof
others will be less affected. castings.Itposedtheobviousquestion‘whynot
The Ten Rules are first listed in summary devotethebookcompletelytofillingsystems?’.I
form. They are then addressed in more detail decided againstthis option on the grounds that
in the following ten chapters with one chapter both caster and customer require products that
per Rule. aregoodineveryrespect.Thefailureofanyone
The Rules originated when emerging from aspect may endanger the casting. Therefore,
a foundry on a memorable sunny day. The despite the enormous disparity in length of
authorwasdiscussingwithindefatigableBoeing chapters,nonecouldbeeliminated;theywereall
enthusiastsforcastings,FredFeiertagandDale needed.
McLellan, that the casting industry had speci- Finally, it is worth making some general
fications for alloys, casting properties, and points about the whole philosophy of making
casting quality checking systems, but what did castings.
not exist but was most needed was a process Forasuccessfulcastingoperation,oneofthe
specification.Dalethrewoutachallenge:‘Write revered commercial goals is the attainment of
Preface ix
productsalesbeingatleastequaltomanufactur- Itisalsoworthnotingatthisearlystage,that
ingcosts.Therearenumerousotherrequirements we hope that meeting the customer’s specifica-
for the successful business, like management, tion will be equivalent to meeting or exceeding
plantandequipment,maintenance,accounting, servicerequirements.However,occasionallyitis
marketing, negotiating etc. All have to be ade- necessarytolivewiththeironythattheaimsof
quate, otherwise the business can suffer, and the customer and the requirements for service
even fail. are sometimes not in the harmony one would
This text deals only with the technical issues like to see.
of the quest for good castings. Without good Theseproblemsillustratethatthereareeasier
castingsitisnoteasytoseewhatfutureacasting ways of earning a living than in the casting
operation can have. The production of good industry. But few are as exciting.
castings can be highly economical and reward-
ing. The production of bad castings is usually
expensive and damaging. J.C.
The‘goodcasting’inthistextisdefinedasone West Malvern
thatmeetsorexceedsthecustomer’sspecification. 3 September 2003