10 Tips Design in Gray Iron

8/13/2019 10 Tips Design in Gray Iron

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Home>Tips and Facts> Gray Iron Casting Design

Basics of Gray Iron Casting

Design: 10 Rules forEngineered Quality

With attention to these key dos and donts of engineering gray ironcastings, you can ensure fewer designre!ated defects and optimi"edcast geometry#In its simp!est form, casting in$o!$es using a pattern to form a mo!dca$ity into which mo!ten meta! is poured, so!idifies and %ecomes acomponent# In actua!ity, the process %egins %efore the pattern is

made, at the point when an engineer decides on the %est design forthe component# &n!y with a working know!edge of what a foundry canand cant do can an engineer design a tru!y efficient part' one thattakes ad$antage of %oth materia! and specific casting processad$antages#Gray iron is one of the most $ersati!e meta!s, %ut a know!edge of itsdistinct characteristics is integra! to an efficient design# This artic!edetai!s () ru!es of gray iron design# *+uipped with this %asicknow!edge, an engineer wi!! ha$e a good %asis for optimi"ing castcomponents#

Visualize the Casting in theMold

It can %e difficu!t to fo!!ow a!! section changes and shapes from a%!ueprint# y creating a threedimensiona! drawing or constructing asma!! mode!, an engineer can study how the meta! wi!! enter the mo!d,disco$er how so!idification proceeds and define how parts must %e fed

to assure casting soundness#- mode! to sca!e or a fu!!si"e pattern can %e used !ater to he!p thedesigner see how cores must %e designed, p!aced or omitted# It a!sowi!! he!p the foundry to determine how to mo!d the casting, detectcasting weakness .shrinks and cracks/, where to p!ace gates andrisers, and answer other +uestions affecting casting soundness, costand de!i$ery#
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1o!idification of mo!ten meta! a!ways proceeds from the mo!d face,forming un%a!anced crysta! grains that penetrate into the mass at rightang!es to the p!ane of coo!ing surface# - simp!e section presentsuniform coo!ing and greatest freedom from mechanica! weakness#When two or more sections con2oin, mechanica! weakness is induced at

the 2unction and free coo!ing is interrupted, creating a 3hot spot,4 themost common defect in casting design#In designing ad2oining sections, rep!ace sharp ang!es with radii andminimi"e heat and stress concentration# 1ome e5amp!es of impro$eddesigns are shown in Fig# 6#!ig" &: Designs that incorporate sharp corners wi!! resu!t in structura!weaknesses due to !oca!i"ed hot spots at the 2unctions, howe$er,adding radii that are too !arge a!so may resu!t in shrinkage defects.sections that are !arger wi!! feed thinner wa!!s, !ea$ing a $oid in thethick 2unction/# y incorporating sma!! fi!!et radii, hot spots are a$oided,assuring impro$ed strength#


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*ngine cy!inder parts such as steam2acketed cy!inders often are i!!designed with !oca!i"ed casting weaknesses due to sharp corners .Fig#7/# When engineering a cored part, a$oid designs that do not ha$e acoo!ing surface#

!ig" ':These schematics show how a steam2acketed cy!inder designwas impro$ed with rounded 2unctions#


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1tream!ining an e5terior may resu!t in a hea$y section at 2unctions#Thin, e!ongated cores forming the ca$ities within the component resu!tin a heat concentration accentuating the hea$y sections# The resu!tingshrinkage defect causes !eakage# - rounded 2unction wi!! offer uniformstrength properties#(e)t8 9u!e :# 0inimi"e the ;um%er of 1ections >>

Mini*ize the (u*+er ofections

- we!!designed casting %rings the minimum num%er of sectionstogether at one point# - simp!e wa!! section wi!! coo! free!y from a!!surfaces, %ut %y adding a section .forming a T/, a hot spot is created atthe 2unction, and it wi!! coo! !ike a wa!! that is


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E*%loy .nifor* ectionsThicker wa!!s wi!! so!idify more s!ow!y, so they wi!! feed thinner wa!!s,resu!ting in shrinkage $oids# The goa! is to design uniform sections thatso!idify e$en!y# If this is not possi%!e, a!! hea$y sections shou!d %eaccessi%!e to feeding from risers#!ig" /:This hydrau!ic coup!ing was origina!!y designed with a core thatcaused !oca!i"ed porosity# y redesigning the component with uniformwa!!s, the weight of the casting was reduced, !owering themanufacturing cost and remedying the shrinkage pro%!em#

Correctly ro%ortion Inneralls


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Inner sections of castings .resu!ting from comp!e5 cores/ coo! muchs!ower than outer sections and cause $ariations in strength properties#- good ru!e is to reduce inner sections to )# of the thickness of theouter wa!!#-$oid rapid section changes %ecause they wi!! resu!t in porosity

pro%!ems simi!ar to what is seen with sharp ang!es# Whene$er comp!e5cores must %e used, design for uniformity of section to a$oid !oca!hea$y masses of meta!#The inside diameter of cy!inders and %ushings shou!d e5ceed the wa!!thickness of castings# When the inside diameter of a cy!inder is !essthan the wa!! thickness, it is %etter to cast the section so!id, as ho!escan %e produced %y cheaper .and safer/ methods than with e5treme!ythin cores#!ig" 2:To remedy the shrinkage defects in the origina! design of ahydrau!ic coup!ing, e5cessi$e meta! was remo$ed, resu!ting in a !ighterweight casting with !ess strain in the !ight radia! $eins#

!illet #ll har% #ngles

Fi!!ets .rounded corners/ ha$e three functiona! purposes8 to reduce the stress concentration in a casting in service;

to eliminate cracks, tears and draws at reentry angles;

to make corners more moldable by eliminating hot spots.

The num%er of fi!!et radii in one pattern shou!d %e the minimumpossi%!e, prefera%!y on!y one# To fu!fi!! engineering stress re+uirementsand reduce stress concentration, re!ati$e!y !arge fi!!ets may %e usedwith radii e+ua!ing or e5ceeding casting section# Fi!!ets that are too!arge are undesira%!e'the radius of the fi!!et shou!d not e5ceed ha!fthe thickness of the section 2oined#


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To a$oid a section si"e that is too !arge at an ?@? 2unction, round anoutside corner to match the fi!!et on the inside wa!!# Where this is notpossi%!e, consideration must %e gi$en to which is more $ita!8 theengineered design or the possi%!e casting defect#In the case of a ?A? or ?B? sections and other angu!ar forms, a!ways

design them so that a generout radius e!iminates !oca!i"ation of heat#!ig" 3a - 3d: In a, wa!!s meet at acute ang!es, creating hot spots# The%est design is d in which honeycom%ing produces uniform coo!ingconditions, assuring impro$ed strength with minima! risk of distortionand structura! weakness#

#$oid #+ru%t ection Changes

The difference in re!ati$e thickness of ad2oining sections shou!d note5ceed a ratio of 68(# If a greater difference is una$oida%!e, consider adesign with detacha%!e parts, !ike machine too! %eds that can %e%o!ted#When a change in thickness is !ess than 68(, it may take the form of afi!!et# When the difference is greater, the recommended shift is in the

form of a wedge# Howe$er, wedgeshaped changes in wa!! thicknessshou!d not taper more than ( in :# Where a com%ination of !ight andhea$y sections is una$oida%!e, use fi!!ets and tapered sections totemper the shifts#


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Ma)i*ize Design of Ri+s andBrac4ets

9i%s ha$e two functions8 to increase stiffness and to reduce weight# Ifthey are too sha!!ow or too wide!y spaced, they can %e ineffecti$e# Thethickness of ri%s shou!d appro5imate )= of the ad2oining thicknessand shou!d %e rounded at the edge# In other words, thin ri%s shou!d %ea$oided when 2oined to a hea$y section or they may !ead to highstresses and cracking# The design preference is for the ri%s to %edeeper than they are thickE the ri%s shou!d so!idify %efore the castingsection they ad2oin# In addition, the space %etween ri%s shou!d pre$entany !oca!i"ed accumu!ation of meta!#In genera!, ri%s in compression offer a greater safety factor than ri%s intension# Howe$er, castings ha$ing thin ri%s or we%s in compression

may re+uire design changes to pro$ide necessary stiffening and a$oid%uck!ing#-$oid cross ri%s or ri%%ing on %oth sides of a casting# Cross ri%%ingcreates hot spots and makes feeding difficu!t# Instead, design crosscoup!ed ri%s in a staggered dou%!e ?T? form# -$oid comp!e5 ri%%ing,which comp!icates mo!ding, hinders uniform so!idification and createshot spots# Conse+uent!y, ri%s are on!y prefera%!e when the casting wa!!cannot %e made strong or stiff enough on its own#9i%s meeting at acute ang!es a!so may cause mo!ding difficu!ties,increase costs and aggra$ate the risk of casting defects#?Honeycom%ing? often wi!! pro$ide increased strength and stiffness

without creating hot spots .Fig# /#rackets carrying offset !oads introduce %ending moments'!oca!i"edand in the %ody of the casting# -s a remedy, taper ?@?shaped %racketsand make the !ength of contact with the main casting as amp!e aspossi%!e# -s another option, %rackets may fre+uent!y %e castseparate!y and then attached, simp!ifying mo!ding#- ri%%ed %racket wi!! offer a stiffness ad$antage, %ut a$oid heatconcentration %y pro$iding cored openings in we%s and ri%s# 1uchopenings shou!d %e as !arge as possi%!e, and consistent with strengthand stiffness# -$oid rectangu!arshaped cored ho!es in ri%s or we%sEuse o$a!shaped ho!es with the !ongest dimension in the direction of

the stresses#

10 Rules for Good Casting
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The !ast few years ha$e witnessed an unprecedented increase in ourunderstanding of the casting process# Thus, as understanding hasincreased, the !ist of re+uirements has %een steadi!y amended as theyha$e %ecome known# 1tarting from an initia! !ist of four ru!es, () ru!esha$e now %een identified that incorporate the !atest techno!ogy for

producing re!ia%!e castings# These are 2ust the start# -dditiona! ru!esmay e5ist, %ut they remain to %e further researched and c!arified#The () ru!es that fo!!ow are proposed as necessary, %ut not, of course,sufficient# They shou!d %e used in addition to e5isting con$entiona!technica! specifications such as a!!oy type, strength and tracea%i!ity $iaI1& ))), etc#, and other con$entiona! foundry contro!s such as castingtemperature, etc#-!though not yet tested on a!! cast materia!s, there are fundamenta!reasons for %e!ie$ing that the ru!es ha$e genera! $a!idity, and areapp!ica%!e for a!! types of meta!s and a!!oys, inc!uding those %ased ona!uminum, "inc, magnesium, cast irons, stee!s, air and $acuumcast

nicke!, co%a!t and titanium# ;e$erthe!ess, a!though a!! materia!s wi!!pro%a%!y %enefit from the app!ication of the ru!es, some wi!! %enefita!most out of recognition, whereas others wi!! %e !ess affected#@isted in summary form, the ru!es are intended to assist the castingindustry# -dhering to them wi!! speed up the process of producing thecasting right the first time, and shou!d contri%ute great!y to reducingscrap when the casting goes into production# In this way, the castingindustry wi!! %e a%!e to raise standards without any significant increasein costs# 1uperior +ua!ity can %e offered with confidence# &n!y in thisway wi!! castings %e accepted %y the engineering profession asre!ia%!e, engineered products, and assure the future prosperity of the

casting industry and its customers#Con$erse!y, the ru!es constitute a draft process specification, which%uyers of castings cou!d demand if they wished to %e assured that theywere %uying the %est possi%!e casting +ua!ity# If %uyers specified thattheir casting sources fo!!owed such ru!es, the +ua!ity and re!ia%i!ity ofthe castings wou!d %e higher than cou!d %e achie$ed %y any amount ofe5pensi$e +ua!ity contro! on the finished product#

Rule 1" ro$ide a Good Quality

MeltImmediate!y prior to casting, the me!t sha!! %e prepared and treated ifnecessary using the %est current practice# The aim is to pro$ide a me!tat the correct temperature, correct chemistry and with !ow residua!!e$e!s of disso!$ed gas and inc!usions# Inc!usions, particu!ar!y o5idefi!ms, are reintroduced into the !i+uid meta! %y poor hand!ing, such as


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pouring from furnaces and !ad!es prior to casting# rocedures to a$oidsuch reintroduction of damage need to %e introduced#The figure %e!ow shows the hydrogen so!u%i!ity in a!uminum and two ofits a!!oys, i!!ustrating the a%rupt fa!! in so!u%i!ity on so!idification#

The chart %e!ow shows mechanica! property regimes for an -!:#< Cu (#< 0g a!!oy in fi!tered and unfi!tered conditions Hed2a"i et a!# .(


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Home>Tips and Facts> () 9u!es for Good Casting> 9u!e 6

Rule &" #$oid 5i6uid !ront

Da*ageThis is the re+uirement that the !i+uid meta! front .the meniscus/shou!d not go too fast# 0a5imum meniscus $e!ocity is a%out )#< ms#.This ma5imum $e!ocity may %e raised to ( ms in sections on!y a fewmm thick#/ Howe$er, surface tur%u!ence damage can %e a$oided e$enat higher speeds if the casting fi!!ing system is sufficient!y narrow toconstrain the meniscus, not a!!owing room for sp!ashing or drop!etformation#The scematics %e!ow show the effect of increasing height on a fa!!ingstream of !i+uid, i!!ustrating8 .a/ the o5ide fi!m remaining intactE .%/ the

o5ide fi!m %eing detached and accumu!ating to form a dross ringE .c/the o5ide fi!m and air %eing entrained in the %u!k me!t#

The schematic $iew of a sp!ash of mo!ten a!uminum shows theformation of a fo!ded .dou%!e/ fi!m that might consist of thick o!d fi!m,or new thin fi!m, or %oth, a!! !ike!y to occ!ude air in the fo!ds#
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Rule '" #$oid #rrest of the5i6uid !ront

This is the re+uirement that the !i+uid meta! front shou!d not go toos!ow!yJ -nd, more precise!y, shou!d not stop at any point on the front#

The ad$ancing !i+uid meta! meniscus must %e kept ?a!i$e? andfree from thick o5ide fi!m that can %e incorporated into thecasting# This is achie$ed %y ensuring that the !i+uid front on!yad$ance uphi!! .in the case of gra$ity poured casting processes,from the %ase of the sprue onwards/# This imp!ies that8 on!y%ottom gating is permissi%!e, and no fa!!ing or s!iding downhi!! of!i+uid meta! is a!!owed#

The meniscus must e5perience continuous uninterrupted upward

ad$ance# That is, no stopping due to arrest of pouring, and noe5tensi$e hori"onta! sections or waterfa!! effects .a$oided %ycasting design or mo!d orientation/, which wi!! cause a genera!temporary arrest of the !i+uid front in the casting, and thus !eadto o5ide !aps, or if pro!onged, e$en co!d !aps#

This drawing shows the unsta%!e ad$ance of a fi!mforming a!!oy,showing the formation of !aps as the interface intermittent!y stops andrestarts %y %ursting through and f!ooding o$er the surface fi!m#


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Rule ," #$oid Bu++le Da*age

;o air %u%%!es shou!d %e entrained %y the running system# If they areentrained, then they shou!d not %e a!!owed to pass through the !i+uid

meta! in the mo!d ca$ity# If this happens, then a mi5ture of o5ide%u%%!e trai!s, together with residua! misshapen %u%%!es in the casting,wi!! resu!t# This is %y far the most common defect in castings, and iscommon!y mistaken for shrinkage porosity# This se$ere defect may %ea$oided %y8

Fast %ackfi!! of fi!!ing system %y $irtue of proper!y designed sprueand pouring %asin, and use of a stopper in sma!! castings,ceramic foam fi!ters p!aced c!ose to spruerunner 2unction, and%u%%!e traps# -ny so!utions must %e demonstrated as effecti$eusing some suita%!e techni+ue such as rea!time Kray

radiography# ;o interruptions to pouring#

This drawing i!!ustrates the most common defect in castings %u%%!edamage as a mi5ture of o5ide cracks and residua! entrapped %u%%!es#


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Rule /" #$oid Core Blo7s

Gases from cores or e$en occasiona!!y from parts of the mo!d shou!dnot %e a!!owed to penetrate the !i+uid meta!# Core or mo!d %!ows causea rather different type of defect than the entrained air %u%%!es#

Cores shou!d %e demonstrated to %e of sufficient!y !ow gascontent andor ade+uate!y $ented to pre$ent %u%%!es from core%!ows# .This demonstration might %e carried out %y a $ideorecording of the fi!!ing of the mo!d without a cope, for instance#/

No clay-based core or mold repair paste should be used unless demonstrated to be

resistant to the creation of blow defects.

e!ow is a ?core %!ow? a trapped %u%%!e containing core gases#1hown at the %ottom of the i!!ustration is a %u%%!e trai!, ending in ane5fo!iated dross defect as the resu!t of a passage of copious $o!umesof core gas# -fter Fraw!ey et a!# .(:/#


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I!!ustrated %e!ow is the detachment of a %u%%!e from the top of a core,%e+ueathing a %u%%!e trai! as a permanent !egacy of its 2ourney# This%u%%!e may %e ear!y enough to escape at the free surface of the risingmeta!#

Home>Tips and Facts> () 9u!es for Good Casting> 9u!e L
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Rule 2" #$oid hrin4ageDa*age

;o feeding uphi!! %ecause of unre!ia%!e pressure gradient .a!sosee 9u!e on con$ection/# Demonstrate good feeding design fo!!owing a!! se$en feeding

ru!es, %y an appro$ed computer so!idification mode!, and testcastings#

Contro! the !e$e! of f!ash at mo!d and core 2ointsE mo!d coats andmo!d temperatures# 9andom f!ashmo!d coatsmo!d temperaturescan gi$e random feeding effecti$eness %y unpredicted changesto the temperature distri%ution in the casting#

This figure shows castings with %!ind risers# F6 is correct!y $ented, %uthas mi5ed resu!ts on sections 17 and 1:# 9iser F7 is not $ented andtherefore doesnMt feed at a!!# The unfa$ora%!e pressure gradient draws!i+uid from a fortuitous skin puncture in section 1# The %ottomdrawing i!!ustrates the p!astic coffee cup ana!ogue8 the water is he!d upin the upturned cup and cannot %e re!eased unti! air is admitted $ia apuncture# The !i+uid it contains is then immediate!y re!eased#

Rule 3" #$oid Con$ectionDa*age

Thin or thick section castings automatica!!y a$oid con$ection pro%!ems#Thin section castings free"e +uick!y %efore con$ection %ui!ds up# In
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thick section castings, con$ection he!ps redistri%ute hot meta! into therisers on the top of the casting, since there is p!enty of time %eforefree"ing starts#Howe$er, intermediate sections, which represent the great ma2ority ofcastings, ha$e a free"ing time commensurate with the time taken for

con$ection to operate .in a few minutes/ and can cause damage#Nnsuita%!e temperature gradients in the casting can undermine theeffecti$eness of risers, and !ead to segregation and apparent shrinkagedamage in castings# This is a !itt!eknown and !itt!eresearched areathat most current computer simu!ations cannot tack!e# For suchintermediate sections, either8

Reduce the problem by avoiding convection loops in the geometry of the casting

and rigging.

Eliminate convection by 1!-degree roll-over after mold filling.

These i!!ustrations show the encouragement of therma! con$ection %y

.a/ side and .%/ %ottom feeding, and its e!imination %y .c/ top feeding#


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Rule 8" lan egregationDistri+ution

Change of section wi!! norma!!y in$o!$e change in distri%ution ofa!!oying e!ements in the casting# This can mean parts of the casting areout of chemica! specification# 1uch pro%!ems may need to %eaddressed if properties are threatened .such as a !oca! %ritt!eness,especia!!y in a stressed region/ and may re+uire prior negotiation withthe customer#

Rule 9" Control Residual tress


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-$oid unknown or random residua! stress %y for%idding +uenching of!ow ducti!ity, !ight a!!oy castings into water fo!!owing high temperatureso!ution treatment# oi!ing water is a!so not permitted since itrepresents a neg!igi%!e impro$ement o$er co!d water# Howe$er,po!ymer +uenchant or forced air+uench may %e accepta%!e if casting

stress is shown to %e neg!igi%!e# !anned residua! stress may %e%eneficia! if designed correct!y into the +uenching process#This chart shows the rates of coo!ing of a 6) mm diameter a!uminum%ar when +uenched %y $arious means from 76OF .


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10 Tips Design in Gray Iron