designing with ductile iron

8/11/2019 Designing With Ductile Iron

1/4

Designing with Ductile Iron toAchieve Strength and Economy

The inherently strong structure of this relatively new and versatile metal makesit an ideal choice for automotive and other high-performance castings.

excerpted from material provided by theDuctile Iron Marketing Group

I

n 1948, ductile iron was first unveiled tothe engineering world. Treatment of

molten iron with a small but adequateamount of magnesium causes the graphiteto precipitate as spheroids, rather than flakesas in gray cast iron, giving the material aninherently stronger structure. The materialis 4-12 times tougher than gray cast iron andso ductile that it can be bent and twistedwithout breaking.

The new ductile iron family bridgedthe gap between gray iron and steel, of-fering the processing advantages of grayironlow melting point, good fluidity andcastability, and ready machinability

combined with many of the engineeringadvantages of steelhigh strength, duc-tility and wear resistance.

Strength at Less ExpenseWhether in an automobile component

or a water pipe, ductile iron has mademajor inroads in every industrially devel-oped country. Today, 16 million tons ofductile iron castings are produced each

year worldwide, representing nearly one-third of all U.S. casting shipments.

The motivating factor for using thismetal is the opportunity for greater strength

at less expense. Other reasons to select duc-tile iron include its reliability in service, vi-bration damping and surface hardenability.

In addition, since the modulus of elas-ticity of ductile iron is somewhat lower thanthat of steel, the stresses due to unavoidablemisalignment in parts such as gears will re-sult in lower bending fatigue strength.

While not all foundries produce allductile iron grades, virtually all castings orcast shapes can be made from an appro-priate grade of ductile iron.

The Ductile Iron FamilyThe majority of ductile iron castingsare produced in one of the following three

types, which do not require heat treat-ment. The parenthetical information rep-resents the common notation for grades,

and the three numbers express tensilestrength in pounds per square inch, yield

strength and percent elongation (mini-mum 2 in.), respectively:

ferritic ductile iron (60-40-18)Composed of graphite spheroids in aferrite matrix (basically pure iron),this grade exhibits high impact resis-tance, relatively good thermal conduc-tivity, high magnetic permeability, lowhysteresis loss, and good machinabil-ity and corrosion resistance;

pearlitic-ferritic ductile iron (80-55-06)Composed of graphite sphe-roids in a mixed matrix of ferrite and

pearlite, this grade is probably themost common and least expensivegrade of ductile iron, exhibitingproperties between those with fullyferritic or fully pearlitic structures,including good machinability;

pearlitic ductile iron (100-70-03)Composed of graphite spheroids in amatrix of pearlite (a fine aggregate offerrite and cementite), this grade isrelatively hard, displaying moderateductility, high strength, good wear

Metals Focus

Because the graphite in ductile iron pre-cipitates as spheroids rather than flakes,as in gray iron, they act as crack arrest-ers, allowing the metal to bend withoutbreaking. Shown above is a ductile barmachined flat from a casting and a simi-lar bar that has been twisted, illustrating

the irons exceptional ductility.

Ductile iron is versatile enough to be used in a variety of applications from this52-ton turbine casting used in hydro power generation (l) to this assortment offairly intricate truck parts.

167


2/4

This graph shows the tensile and hardness properties of ductile iron con-forming to different grades of ASTM Specification A536.

resistance, moderateimpact resistance,somewhat reducedthermal conductiv-

ity, low magneticpermeability, highhysteresis loss andgood machinability.

The designer also mayencounter special situationsthat call for the specialgrades of ductile iron alloys:

austempered ductileiron (ADI)This re-cent addition to theductile iron family of-fers a remarkablecombination of

strength, toughnessand wear resistance.ADI is almost twice asstrong as the regularASTM grades of duc-tile iron while still re-taining high elonga-tion and toughnesscharacteristics. In addition, ADI of-fers exceptional fatigue strength,enabling designers to reduce com-ponent weight and costs. A closelycontrolled heat treatment operation

(austempering) develops a uniquematrix structure of bainitic ferrite(60%) and retained (high carbon)austenite. The retained austenite isthermally stable to extremely low tem-peratures but is work hardenable andwill locally transform to martensiteunder suitable conditions of stress;

martensitic ductile iron (quenchedand tempered)In the as-cast con-dition the alloy is hard and brittle andseldom used, however, tempered mar-tensite has very high strength and wear

resistance. A 930F (500C) temperingresults in 300 Bhn and 1110F (600C)tempering results in 250 Bhn;

austenitic ductile iron (also known asductile Ni-Resist)The outstandingfeatures of this grade are good cor-rosion and oxidation resistance,magnetic properties, strength and di-

mensional stability at el-evated temperatures.

Production

Familiarizing oneselfwith ductile iron designadvantages helps not onlyto achieve engineeringelegance (uniform stress-flow and optimumeconomy) but also helpsthe designer to decidewhen the use of ductile ironis preferable to an alterna-tive material. The volumechanges that occur duringthe cooling and solidifica-tion of ductile iron are un-

like those in any other al-loy. The volume of the liq-uid decreases with decreas-ing temperature untilslightly above the solidifica-tion temperature. Uponfurther cooling, the con-traction stops and a definite

volumetric expansion starts. Unfortunately,the expansion phase prevails through onlypart of the solidification process. The ex-pansion gives way to another contractionphase, secondary shrinkage, which contin-

ues until all of the liquid metal solidifies.The liquid iron temperature should behigh enough to provide for complete fusionof the separate streams and to avoid the en-trapment of small gas bubbles. Each sectionthickness has its optimum pouring tem-perature range. Ductile iron castings with 3mm-thick walls may need to be poured as

Table 1. Short Summary of Ductile Iron Specifications

Specifying Spec. Class or Min. Min. % Heat Other Uses TypicalBody No. Grade Tensile psi Yield psi Elongation Treatment Requirements Applications

ASTM A536-80 60-40-18 60,000 40,000 18 May be For maximum Pressure-Annealed shock resistant containing

parts to be used castings suchat sub-zero as valve &temperatures. pump bodies.

ASTM A536-80 65-45-12 65,000 45,000 12 - Most widely used Machinerygrade for normal castingsservice. subject to

shock andfatigue loading.

ASTM A536-80 80-50-06 80,000 55,000 6 - Suitable for flame Crankshafts,and induction gears andhardening. rollers.

ASTM A536-80 100-70-03 100,000 70,000 3 Usually Best combination High strengthNormalized of strength, wear gears,

resistance and automotiveresponse to surface and machinehardening. components.

ASTM A536-80 120-90-02 120,000 90,000 2 Quenched Maximum strength Pinions, gears,and and wear rollers andTempered resistance. slides.

In all gradesin this

specification,chemicalcomposition issubordinate tomechanicalproperties.However, thecontent of anyelement maybe specifiedby mutualagreement.

1200

1000

800

600

400

200

30

20

10

0180 200 220 240 260 280 300160

Tensile strength

Yield strength

Streng

th,

MPa

Elongation,

%

Hardness, HB

160

120

80

40

100-70-03, air quenched

Grace 120-90-02, oil quenched

80-55-06, as-cast

65-45-12, annealed

60-40-18, annealed

Grade 60-40-18, annealed

65-45-12, annealed

80-55-06, as-cast

100-70-03, air quenched

120-90-02, oil quenched

168


3/4

hot as 2640F (1450C), while 100 mm thickcastings can be poured at between 2300-2400F (1260-1320C). Production complex-ity may increase when large differences in

section thickness exist in one casting. There-fore, it is best to design with as little differ-ence in wall thicknesses as possible.

Strength RequiredThe grade of ductile iron selected will

have a considerable effect on cost. Grade65-45-12 is one of the least expensive, es-pecially when considering machiningcosts. Pearlitic-ferritic costs approximatelythe same to cast but is somewhat less ma-chinable. Pearlitic is still relatively inexpen-sive and should be selected where its highstrength and good wear resistance are

needed. Ferritic and martensitic grades canbe the most expensive to cast, but they maybe the most economical choice if the par-ticular material characteristics are impor-tant. The evaluation of the higher cost aus-tenitic grades must be considered indi-vidually on the basis of their excellent cor-rosion, erosion and oxidation resistance,performance at elevated temperatures,magnetic properties, low thermalexpansivity and other unique features.

Machining Cost

Strength, weight, service life andother considerations may be overruledon occasion to minimize machiningcost. The grades easiest to machine arepearlitic and martensitic, followed byaustenitic and pearlitic-ferritic. The ma-chinability of ferritic is superior to steelwith the same hardness, and the machin-ability of austenitic ductileiron is generally superior tothat of stainless steels.

Wall ThicknessIn terms of wall thickness,

6 mm-or-heavier sections arerelatively easy to produce.Thinner walls are increasinglydifficult to produce without de-terioration in the as-cast con-dition. Brittleness and highhardness can be eliminatedthrough heat treatment, butthis is expensive and results indistortion to various degrees.Whenever practical, cast wallthickness should be at least 6mm to facilitate as-cast deliv-

ery. Otherwise, heat treatmentmay be required, increasingcasting cost by 10-30%.

This graph shows the relationships between endurance ratio, tensile

strength and matrix microstructure for ductile iron. The fatiguestrength of a material is related to its tensile strength by an endur-ance ratio, or the ratio of a fatigue limit to tensile strength.

Casting SoundnessUnlike most other alloys, the designer

should aim at simultaneous solidification ofthe whole casting. This minimizes andsometimes eliminates the need for riserswhile improving casting yield. Conversely,parts of a ductile iron casting that cool muchslower may require additional feeding. Someexamples of such isolated hot spots are:

cast-on heavy bosses; cast-on heavy test coupons; sharp internal corners; joints between equally thick walls; multiple joints (2 is better than 3, 3

is better than 4, etc.); joints at acute angles (90is best); isolated heavy sections.

Dimensional AccuracyWith the exception of dimensional prob-

lems caused by patterns and coreboxes, gen-erally the larger the casting the wider the tol-

erances that can be expected. To minimizecosts, designers should strive to specify nomore dimensional accuracy than what isabsolutely needed.

Dimensional inaccuracies from molddeformation during pattern withdrawal areminimized through proper tapering (draft)of the pattern. A minimum draft of 1:100suffices only for very shallow patterns. Deeppatterns may require as much as 5:100 formaximum accuracy.

Mechanical QualitiesThe effect of section size on proper-

ties is the result of changes in micro-scopic structure, which is influenced bycooling rate. Three prominent effects ofcooling rate are:

very high cooling rates may not per-mit all the insoluble carbon to pre-cipitate in the form of spheroidalgraphite. Instead, various amounts ofa hard and brittle component, ironcarbide (Fe

3C), may form;

very slow cooling can result inlarge-diameter, irregularly shapedspheroids of graphite up to 1.5 mmin diameter;

varying the cooling rate in the1560-480F (850-250C) tempera-ture range from very fast to very

slow produces different structuresfrom martensite (very fast cooling)through pearlite, pearlite-ferrite toall ferrite (slow cooling).

For design calculations, the engi-neer must request test castings thatwill cool at approximately the samerate as the final product. Mechanical

tests on these castings willy ie ld the most accurateprediction of properties.

The presence or absence ofcarbides and the type of matrixobtained in any given section

can be controlled by alloying orheat treatment. The size of thespheroidal graphite, on theother hand, can be influencedby, among other things, thecooling rate of the casting,which in turn can be deter-mined by the shape or designof the casting.

Safety-CriticalApplications

Both automobile steering

knuckles and plow-shares areoften made in ductile iron,even though these castings

0.5

0.4

0.3

Enduranceratio

60 100 140 160

Tensile strength, 1000 psi

PearliticFerritic

300 500 700 900 1100 1300

Tensile strength, MPa

Tempered Martensite

This ductile iron roller bracket, used on alarge cement truck, replaced a nine-piecesteel weldment. The 12 x 12 x 8-in., 27-lb casting was made by The Dotson Co.Mankato, Minnesota, in grade 80-55-06ductile iron using the green sand pro-cess. Per part savings were approxi-mately $15, and with an annual usageof 8000 components, the customerachieved an annual savings of $120,000.

169


4/4

Ductile Iron Data for DesignEngineers, Rio Tinto Iron & Titanium,Chicago, IL (1990).

Design of Ferrous Castings,American Foundrymens Society, Des

Plaines, IL (1984).A Design Engineers Digest ofDuctile Iron, 7th Edition, Rio TintoIron & Titanium, Montreal, Quebec,Canada (1990).A Tribute to Keith Millis andthe Unveiling of Ductile Iron 50

Years Ago, p. 33-66, moderncasting, Des Plaines, IL (Oct. 1998).

have different safety requirements. An en-gineer must design with a degree of re-quired safety in mind, and a foundry mustcast a quality component to meet those re-quirements. Safety requirements, however,must be applied judiciously, because in-creasing part safety invariably increasescost. The available controls include:

approximating the ultimate load car-rying capacity of the material to vari-ous predetermined degreesUnderstatic loads, the maximum permissiblestress equals the yield strength. Usualdesign stresses vary from 50-75% ofthe yield strength (0.2% offset proofstress). Parts exposed to frequentlyvarying loads should be designed soas not to exceed the maximum endur-

ance limit for a particular stress level.Design stresses of 50-100% of the en-durance limit are customary, corre-sponding to a very high and low mar-gin of safety, respectively;

estimating the effect of potential emer-gency overloads involves the determi-nation of the most likely failuremechanismExcessive static or dy-

namic loads may cause failure eitherthrough deformation of the part or byfracture. Depending on the use, onefailure mechanism will prove to besafer than the other. For example, in apressure-tight container a major in-crease in the internal pressure mayburst or deform the casting. A perma-nent deformation will probably be thesafer failure mechanism. Dont indis-criminately use the high-ductilitygrade (ferritic). Pearlitic-ferritic canwithstand 70-80% higher loads;Grade 120-90-02 can withstand 2.5times more load, either static or dy-namic, than ferritic. Beyond these lim-its, grades 120-90-02, 100-70-03 and80-55-06 will fail through fracture

while grade 65-45-12 and ferritic willfail through permanent deformationunder much lighter loads.

Trust the FoundryFoundries producing ductile iron are

well aware of choices in manufacturing pro-cesses, the effects of chemical compositionand heat treatment, and the in-plant con-

trols necessary to produce a casting. Deci-sions regarding fine details affectingeconomy and performance require a betterunderstanding of the casting process and thefoundry involved. Its best to meet with thefoundryman during the design of the rawor ideal part. Engineers may even be ableto see some good examples of castings simi-lar to what they require.

For a free copy of this article circle No. 339 on theReader Action Card.

For More Information

170

designing with ductile iron

Documents