To Promote the production and application of ductile iron castings Issue 2, 2002

Shrinkages in Iron Castings

Some facts and hypotheses related to thermal analysis with ATAS.

Author: Rudolf SillénTechnical Director, NovaCast AB

Shrinkages are one of the most common casting defects. The main cause ofshrinkage cavities is the fact that all commercial alloys contract when acasting cools from the pouring temperature to solidus. The contraction isusually between 1-5 % depending on the type of alloy and the pouringtemperature. The main contraction occurs between the pouring temperatureand the liquidus temperature. If the contraction is not compensated for byfeed metal, either by supplying feed metal or by forming a depression ofthe outer surface, a shrinkage cavity will occur. Cast iron alloys, whichsolidify with a precipitation of graphite, represent a more complexbehavior, the reason being that the dissolved carbon partly precipitates asgraphite with a lower density than the base iron. The precipitation istherefore associated with an increase in volume, which in some cases partlymight offset the contraction of the liquid and the austenite.

In the following we have classified shrinkages in four basic types. We willdiscuss how they occur and how they can be avoided. The four basic typesare Outer sunks (pull downs), Macro shrinkages, Micro shrinkages andPorosities.

A. Outer Sunks

A1 - Definition and location

Outer sunks (pull downs, sinks) can be seen from theoutside of the casting, usually as a smooth depressionin the casting surface. They are normally located onthick sections of the casting and on surfaces located on the top of thecasting during pouring. Outer sunks are also referred to as "pull downs"and they can occur not only on horizontal top faces of the casting butsometimes also on vertical surfaces.

A2 - The basic mechanism behind outer sunks

The metal starts to solidify at the surfaces of the mould cavity and a thinskin is formed. The temperature drops further, which causes the liquid andthe semi-liquid metal inside the casting cavity to contract further. If no feedmetal is available either from the gating system or from a feeder thencontraction will cause a negative pressure inside the cavity. In order toequalize the pressure difference between the atmosphere and the interiorthe solid outer skin will be "pulled" inwards. The effect is that thecontraction is compensated for by a reduction of the volume of the casting.Thus outer sunks develop at an early stage of the solidification processbefore the massive eutectic freezing has commenced.

A3 - Variables that influence the creation of outer sunks

The major variables that influence the likelihood for development of outersunks and their effect are as follows:

1. Too high pouring temperature. The volumetric contraction is about1.4% per 100 C for cast iron. Thus an increase in pouring

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temperature of 50 C will increase the need for feed metal with 0.7%.When pouring castings with small modulus in ductile iron, a higherpouring temperature might decrease shrinkage tendency especially inhard moulds. The reason is a better temperature distribution in themould and less fast initial cooling.

2. Insufficient amount of feed metal available at early stages of thesolidification. Normally no feeder is needed for castings with smallmodulus (less than 0.4 cm). Feed metal can be supplied from thepouring cup through the gating system. However, if the modulus ofthe ingate is too small then feed metal might be lacking.

3. Deformation (expansion) of the casting cavity. This might happen ingreen sand moulds and cause mould wall movement, which increasesthe casting cavity.

4. Too high liquidus temperature. A liquidus temperature higher thanwhat the system has been designed for will cause the surface skin toform earlier and thereby increase the risk for outer sunks.

5. Too high amount of primary austenite (S1) cause by too low activecarbon equivalent. A higher amount of primary austenite than usualfor the alloy means that the contraction increases at an early stage.

6. Too low eutectic minimum temperature (TElow). A eutectictemperature that is lower than the normal minimum level means thatthe contraction of austenite is higher than normal as the distancebetween liquidus (TL) and the eutectic temperature (TElow)increases. Iron undercooling below the graphitic eutectic due to lackof nucleation - too little or wrong type of inoculation or fadedinoculation.

7. A solidification that favors skin freezing (Also see number 5).8. Insufficient weighting or clamping of the mould. This might have the

same effect as a deformable mould especially if the casting has someparts with lower modulus than solidify quickly and cause a pressurethat tends to separate the mould halves.

9. Riser does not pipe due to too low height, too low modulus or toolarge modulus of the ingates.

A4 - Which variables can be

monitored by ATAS?

Thermal analysis can be used tomonitor the metallurgicalvariables that influence thedevelopment of outer sunks.ATAS is especially designed tomonitor and interpret coolingcurve data and to predict thepotential risk for shrinkages andalso other problems. Themetallurgical variables used topredict outer sunks are liquidus(TL), liquidus plateau, primaryaustenite (S1) and the low eutectic temperature (TElow). (Parametersnumber 4,5,6 and 7). The illustration shows a typical cooling curve for ahypoeutectic iron and the "time-position" for various types of shrinkages.

ATAS makes it possible to monitor and control these metallurgicalvariables. However, as can be understood from chapter A3, several othervariables do influence the outcome. Even if the metallurgical variables arewithin their acceptable limits outer sunks might occur, e.g. if the pouringtemperature is higher than normal or the mould is softer. Also the reversemight be true - the metallurgical variables might be slightly outside theirnormal limits but no shrinkage will occur because the mould was muchharder than normal or the pouring temperature was lower than normal.

The important fact is that ATAS allows the foundry man to ensure that 4out of 8 variables are within the specification.

A5 - How can out sunks (pull downs) be avoided?

The pouring temperature must be chosen individually for the casting - e.g.castings with small modulus and/or long flow length must use a highpouring temperature and/or be poured very fast to avoid casting defectssuch as cold shuts. Castings with a high modulus and/or short flow lengthcan be poured with a low temperature. To minimize the volume contractionin liquid state, the general advice is to use as low a pouring temperature aspossible without getting any casting defect. The feeding system can bedesigned and dimensioned once the optimal pouring temperature has beenestablished for a casting. Mould hardness must also be considered whendimensioning the feeding system. It should be as high as possible.

Feeding from the gating system is normally sufficient if the castingmodulus is less than 0.3 cm. However, make sure that the ingates do notfreeze off too early and thereby block the flow of feed metal from thepouring cup.

In some cases where a casting has been successfully made without a feeder,that is using ingate feeding, shrinkage occurs when a ceramic filter isplaced in the gating system. A filter reduces the modulus locally and mightblock the feeding path.

If the modulus is larger than 0.3 cm then it is recommended to use a feeder.A feeder, which only has the purpose to eliminate outer sunks, can be madesmaller than if it also has to assist in eliminating macro shrinkages (as theyare developed at a later stage).

From a metallurgical standpoint the main thing is to reduce variations in theprocess variables TL, S1 and TElow. If ATAS is used to monitor thesevariables it will soon become evident how to melt, hold and treat the metalto reduce variations.

B. Macro shrinkages

B1 - Definition and location

Macro shrinkages are usually found inside thecasting and close to heat centers. They appear aslarger holes, usually with rough surfaces, oftendendritic and are often larger than 5 mm. Macro shrinkages are usually notrevealed unless the casting is machined or deliberately cut through sectionsof heat centers. Can also appear in or close to ingates.

B2 - The basic mechanism behind macro shrinkages

Macro shrinkages develop after the initial solidification on the surfacewhen a shell that can not be deformed by the pressure difference to theatmosphere. If no more feed metal is available at that point in time then thecontraction of the liquid and semi liquid metal as well as the alreadysolidified parts results in a cavity.

B3 - Variables that influence the creation of macro shrinkages

Shrinkages are influenced by the behaviour of both the alloy and the mould.The dominating variables for macro shrinkages are:

1. Insufficient supply of feed metal - too small feeder modulus or feederneck or feeder does not pipe due to too large ingate modulus or heightposition.

2. Feeding path closed too early - wrong position of feeder. More dendritesthan usual at an early stage (CLF factor in NovaSolid). Too low activecarbon equivalent (ACEL=the true carbon equivalent measured withATAS). For ductile iron, ACEL should be eutectic and carbon minimum3.6%.

3. Mould hardness during solidification. Soft moulds which favor mouldwall movement. A green sand mould, hard at room temp can be soft duringsolidification due to the high water level in the condensation zone.

4. Mould weighting or clamping insufficient to stand the pressure duringsolidification. Eutectic pressures up 50 kg/cm2 has been claimed!

5. Higher liquidus temperature (TL) than normal. In hypoeutectic irons thismeans too much primary austenite and more difficult feeding. Inhypereutectic it means more primary graphite which reduces the amount ofeutectic graphite.

6. Higher amount of primary austenite (S1) than normal. Too low ACEL.

7. Too high recalescence and recalescence rate causes expansion fromgraphite to occur too early. (In ductile - too much late inoculation with lowACEL).

8. Too low eutectic temperature might cause some primary carbides toform.

B4 - Which variables can be monitored by ATAS?

ATAS can monitor variables 5,6 and 8. The length of the liquidus plateaucan also be used to indicate anomalies in the initial dendrite growth andprecipitation of a pro-eutectic phase.

B5 - How can macro shrinkages be avoided?

The remedies are basically the same as for outer sunks. However to avoidmacro shrinkages the feed metal must be available longer than for Outersunks because macro defects occur at a later stage during solidification. Asolidification simulation e.g. using NovaSolid is highly recommended toensure that feed metal can reach critical areas and that feed metal isavailable during the contraction period.

C. Micro shrinkages

C1 - Definition and location

Micro shrinkages are smaller cavities with irregularsurfaces often with signs of dendrites. Sizes areoften less than 3 mm. The defects are usuallylocated close to heat centres in the casting. Micro shrinkages are oftenreferred to as porosity or "leakage" as castings with this type of defectoften leak during a pressure test. Micro shrinkages are usually not revealedunless the casting is machined or deliberately broken.

C2 - The basic mechanisms behind micro shrinkages

The defects occur at the latest stages of solidification. It is therefore moredifficult to solve micro shrinkage problems by changing the gating orfeeding system. Micro shrinkages are more of a metallurgical problem.There are three basic mechanisms behind micro shrinkages.

The first and most common mechanism behind micro shrinkages is that thecontraction of the austenite (primary and eutectic) at the end of freezingcan not be compensated for by the precipitation of carbon into graphite,which is associated with an increase in volume. (The density of the

austenite is approximately 6.9 g/cm3 at temperatures around 1100 and thedensity of graphite about 2.3 g/cm3).

Another reason especially in gray iron is too high levels of nitrogen. Inductile iron this is less likely as the high CEL-level reduces solubility ofnitrogen. High nitrogen levels cause an increased solubility of hydrogen.During the end of solidification a dispersed type of shrinkage usuallyoccurs in almost all of the casting. The cavities look like a "comma" andare often referred to as "comma" cavities or sometimes comma gas blows.Adding about 0.02% titanium usually helps. Another alternative is to usezirconium especially with ductile iron.

The other mechanism is that due to the composition of the iron there areone or more phases that solidify at a lower temperature than the austenite-graphite eutectic. The most well-known case is the formation of ironphosphide (Fe3P) in gray iron if the phosphorus level is higher than 0.15 %.The Fe3P phase segregates to cell boundaries where it continues to contracteven after most of the alloy has solidified. The solidus temperature forFe3P is about 850 C.

C3 - Variables that influence the creation of micro shrinkages

1. Mould stability. Too soft moulds, which might cause mould wallmovement.

2. Too little sea coal addition to green sand.3. Bentonite (Calcium) with low wet compression strength in green

sand. Sodium bentonites are optimal for reducing shrinkages.4. Insufficient weighting or clamping of moulds.5. Too little eutectic graphite especially at the end of freezing; its

precipitation can not compensate for the shrinkage of the austenite. Atrue eutectic composition with relatively high silicon level ispreferred as the amount of eutectic graphite is maximized. (Note thatthe eutectic point is a function of the thermal modulus!).

6. Too much inoculant in gray iron that reduces cell size.7. Too high hypereutectic composition in ductile iron. If too high then

some of the dissolved carbon will be precipitated as primary graphiteand the amount of eutectic graphite might be insufficient. Evidencedby too many large size nodules which have been growing early in theliquid.

8. Too high phosphorous in gray iron - max 0.12 % and max 0.03% ifmolybdenum or high chromium levels are used in the alloy.

9. Too much magnesium in ductile iron. Levels of Mg and RE must beconsistent with the thermal modulus of the casting.

10. Too high amount of phases which exhibit solidus below 1100 C. e.g.Fe3P.

11. Too high recalescence and recalescence rate causes expansion tooearly.

12. Too low eutectic temperature might cause some primary carbides toform.

13. Too small Graphite Factor 1 (GRF1) indicating too small amount ofeutectic graphite during the second part of the eutectic.

14. Too high Graphite Factor 2 (GRF2) indicating too little eutecticgraphite precipitation at the end of freezing.

15. Too low solidus (TS) which may induce carbides at the last portionsto freeze. Too high levels of Mg, Nb, V or similar elements tend tosegregate to the grain boundaries and form carbides that contractduring their solidification. For normal ductile iron TS should beabove 1100 C.

16. Too high silicon will increase segregation of carbide formingelements, which increases the tendency for micro shrinkage.

C4 - Which variables can be monitored by ATAS?

The mechanism behind micro shrinkages is dependent on the metallurgicalstatus of the melt, which can not be measured by chemical analysis. TheATAS thermal analysis method is therefore a powerful tool for measuringand verifying that a melt is within acceptable metallurgical thresholdvalues. ATAS can also be used to study the result of changes in meltingand treatment variables and alloying materials, whereby the most optimalalloying materials as well as melting and treatment processes can beestablished. ATAS factors 10, 11, 12,13 and14 as mentioned in theprevious chapter are good indicators for micro shrinkages.

C5 - How can micro shrinkages be avoided?

The first condition is to have a hard mould - ideally chemically bondedsand moulds. A green sand mould is compressible, however hard at roomtemperature, because of the formation of a condensation zone. In thecondensation zone, which travels from the surface of the cavity andinwards, the moisture content can be up to 3 times higher than the initialvalue. This means that the bentonite layer becomes semi-fluid and caneasily be compressed. However green sand moulds can be made morestable by means of the following recommendations:

A sodium bentonite is most favorable, as its wet compressionstrength is high.Use sea coal as carbon additive. It will "coke" and increase thefriction between sand grains. The carbon level in the sand should beat least 5 %.Use as low moisture as possible. The compactability should be lessthan 40%.The moulding machine should be able to compact the sand properly.On Disamatics check that the mould conveyor is working properly sothat the moulds are kept together tightly (sometimes a gap can beformed).For larger flasks use flasks with bars - make sure the moulds areclamped properly or that enough weights are put on the cope.The forces during solidification of cast iron can be very high -therefore normal weighting to avoid run-out at the parting line is notenough.

Metallurgically, the most important factor is to ensure that a sufficientamount of carbon is precipitated as graphite during solidification. It isimportant that the initial growth rate is not too high. This can be indicatedby studying the recalescence and recalescence rate on ATAS. A too highrate means too much graphite too early, which is associated with anexpansion that can cause mould wall movement and thus increase thelikelihood for shrinkages. Base iron nucleation status is important. Superheating above approximately 1520 C too long reduces the nucleation level.(ATAS users can easily make some tests with some melt to find the exactlimits valid with their iron composition (equilibrium temperature), type offurnace (surface area, stirring action), etc).

Graphite factor 1 indicates the precipitation of graphite during the secondphase. Higher values are more favorable. However this factor requires thatthe Quik-Cup always be filled to the brim, otherwise the weight of thesamples is different, which will influence this factor.

It is essential that enough graphite be precipitated at the end of freezing.Studying graphite factor 2, the first derivative at solidus and the solidustemperature can indicate this. A low graphite factor 2 is favorable. The firstderivative (dT/dt_TS) should be as low (deep) as possible. Solidus shouldbe high to ensure that the last metal solidifies gray and not as partly white

(which does not expand).

These metallurgical factors can be influenced by selection of chargematerials, charging sequence, the melting cycle (temp/time steps) as well astype and amount of alloying materials, inoculants and FeSiMg. With ATASit is possible to find the optimal melting and treatment practice to reducethe likelihood for shrinkages. Such an optimization can NOT be made usingtraditional chemical analysis alone. The behavior of a melt depends notonly on the percentage of each element but also on formed compounds suchas oxides, silicates, sulphides, nitrates and their interaction duringsolidification. The total behavior can however be revealed by means ofATAS thermal analysis. A quantitative analysis of the solidification canalso be made which allows the user to study the precipitation of graphiteand the associated volume changes in details. ATAS shows the weight ofprecipitated graphite during the different phases of solidification as well asthe need for feed metal.

Metallurgically it is also important to avoid phases that have a solidustemperature below the main solidus temperature. In gray iron thephosphorous level should normally be less than 0.12 %. If molybdenum orchromium are present then the phosphorus level must be lower - in somecases less than 0.4% to avoid porosity. In ductile iron phosphorous shouldbe less than 0.03%.

D. Porosities

C1 - Definition and location

Porosities are small, dispersed cavities with irregularor rather smooth surfaces, often less than 1 mm insize. The defects are usually located close to heatcenters and in grain boundaries. They are more dispersed than microshrinkages. Typically, the defect is not discovered until the casting issubjected to a leakage test with water or air.

C2 - The basic mechanism behind porosities

The defects occur at the very latest stages of solidification. Therefore it isnot possible to solve porosity problems with changes in the gating orfeeding system (unless a very steep temperature gradient is maintained).Porosities are a metallurgical problem mainly depending on the chemicalcomposition.

The main mechanism is that due to the composition of the iron, one ormore phases solidify at a lower temperature than the austenite-graphiteeutectic. The most well-known case is the formation of iron phosphide(Fe3P) in gray iron if the phosphorus level is higher than 0.15 %. The Fe3Pphase segregates to cell boundaries where it continues to contract evenafter most of the alloy has solidified. The solidus temperature for Fe3P isabout 850 C. The problem is aggravated by Mo and Cr. Another cause canbe that the iron has too high levels of elements like Nb and V that usuallyoriginate from steel and segregate to the grain boundaries and formcarbides.

In ductile iron too high magnesium levels can cause similar problems asmagnesium segregates to the rest melt and can induce formation ofcarbides which contracts and creates porosity.

C3 - Variables that influence the creation of porosities

1. Too high phosphorous in gray iron - max 0.12 % and max 0.03% ifmolybdenum or high chromium levels are used in the alloy.

2. Too much magnesium in ductile iron.

3. Too high amount of phases which exhibits solidus below 1100 C.E.g. Fe3P.

4. Too high levels of Nb, V etc.

C4 - Which variables can be monitored by ATAS?

As the mechanism behind porosities is more dependent on solidification ofminute phases with a temperature below the normal solidus it is normallynot detectable by thermal analysis. Some guidance might be available bystudying TS and the last part of the first derivative curve.

C5 - How can porosities be avoided?

The phosphorous content should be less than 0.12 % in unalloyed gray iron.(But minimum 0.08% in order to avoid expansion penetration!). The gascontent can also influence this type of defect. Metallurgically it isimportant to avoid all phases, which have a solidus temperature below themain solidus temperature. In gray iron the phosphorous level shouldnormally be less than 0.12 %. If molybdenum or chromium is present thenthe phosphorous level must be lower - in some cases less than 0.04% toavoid porosity (leakage). The amount of magnesium in ductile should bekept to a minimum.

E. "Riserless" methodology

Many castings are successfully produced without a riser (i.e. a feeder).Basically there are four possibilities:

1. The customer accepts some shrinkages as their influence on thefunctionality of the casting is negligible, e.g. a counterweight can beallowed to have inner shrinkages. Many micro shrinkage defectsinfluence neither the strength of the part nor the appearance -however the foundry often takes for granted that the customer expectsa completely sound casting. Usable castings are often scrappedwithout any rational reason! A dialogue between the foundry and thecustomer based on solidification simulations before start ofproduction can often result in more realistic quality demands, whichwill benefit both parties (higher yield, less problems!).

2. The modulus of the casting is fairly small (less than 0.3 cm) and thepouring time is relatively short. In this case the feed metal is takenfrom the pouring cup as long as feeding is possible through theingates. Note that the modulus, i.e. the solidification time of thegating system, must be related to the modulus of the casting in orderto allow feeding. A simulation e.g. with NovaSolid is ideal to findthe optimal design.

3. The modulus of the casting is fairly small and a main part of thecasting geometry is like a thin plate. In such cases the initiallysolidified thin skin is depressed inwards by the atmospheric pressure.The depression might only be fractions of a millimeter but on alarger surface this will add up to a large volume, which willcompensate for the shrinkage.

4. The alloy (ductile) should be close to true eutectic composition whichis related to the cooling rate. That means that the liquidus temperatureis very low (around 1145-1150) and the solidification starts with theprecipitation of eutectic (graphite and austenite). Thus the contractionof pro-eutectic austenite is eliminated. A hyper-eutectic solidificationin the casting should be avoided as it will reduce the amount ofcarbon during the eutectic reaction. (Note that the chemicalcomposition can be hyper-eutectic but the solidification hypo-eutectic if the cooling rate is high). The pouring temperature must bevery low in order to reduce the contraction of the melt in liquid state.Pouring temperatures for "riserless" castings are preferably around

1200 C. Due to the low pouring temperature, the pouring time mustbe short, to avoid defects. Two other conditions must also befulfilled. First the modulus of the casting must be at least 1.5 cm.This will ensure that the solidification time is slow enough to allowthe eutectic reaction to occur slowly in order to precipitate as mucheutectic graphite as possible. In order to avoid mould wall movementdue to the forces during solidification the moulds must havesufficient strength. Chemically bonded moulds using binders such asfuran or cement must be used. The moulds should be firmly clamped(instead of using weights).

Illustrations from ATAS Screens:

Pic. 1. The cooling curve shows a eutectic ductile iron with a highrecalescence.

Pic. 2. The same cooling curve evaluated by ATAS. Several parameters areoutside the allowed thermal limits. The expert system gives a warning thatthis iron is prone to micro shrinkage defects. Pressing the Risk&Explainicon will give further information.

Pic. 3. This screen shows how the expert system has interpreted the thermaldata.

Pic. 4. The cooling curve has been analyzed quantitatively by ATAS. Intotal 7.3 grams of graphite has been precipitated. Some of it has beenprecipitated just before the eutectic temperature (at the walls of the Quik-Cup). The problem is that too much is precipitated too early (Eutectic1=32.7%). The solution is to improve the treatment and inoculation process.As the effect of various modifications now can be visualized and quantifiedit is easy to optimize the various processes.

Version 02-07-22.

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