J. Mater. Sci. Technol., VoLl5 No.3, 1999 245

Study of Solidification Shrinkage of Ductile Ironin Dry Sand Molds

Jiarong LItBeijing Institute of Aeronautical Materials, Beijing 100095, ChinaBaicheng\LIUDepartment-of Mechanical Engineering, Tsinghua University, Beijing 100084, China

[ Manus~ript received June 15, 1998, in revised form August 16, 1998]

The effects of metallurgical and processing parameters on the formation of solidification shrinkagein ductile iron have been studied, considering the parameters of carbon equivalent, inoculation,casting modulus and pouring temperature within specific ranges of these variables. Based onthe orthogonal experiments, the metallurgical and processing parameters of the minimum castingshrinkage and the maximum casting shrinkage were obtained, and the effects of metallurgical andprocessing parameters on the formation of shrinkage cavities and porosities in ductile iron castingswere discussed. A regression equation relating these variables to the formation of shrinkage wasderived based upon the orthogonal experiments conducted.

1. Introduction

Ductile irons have received much attention in thepast because they have an attractive combination ofmechanical propertiesl! ,2]. The shrinkage behavior isa principal concern in the production' of ductile ironcastings. This phenomenon has been the subject ofconsiderable attention to those studying ductile ironssince soon after its introduction to the cast metalsindustry.

In the late 1950s, Reynolds et al.[3] investigatedthe effects of carbon and silicon content on shrink-age behavior, noting that sound castings could be ob-tained in rigid molds if the following parameters weremaintained:

%C + 1/7%Si > 3.9Timmons et al.[4] studied the expansions of molds

of ductile iron in dry sand molds. Bates andPattersonlf investigated the volumetric changes oc-curring during the solidification of hypereutectic duc-tile irons. Gariepy et al.[6] reported the effects of moldmaterials and feeding on the volume changes of duc-tile iron castings.

Since the mid 1980s, the studies of shrinkage be-havior have concentrated on the formation of theshrinkage cavity and porosity in ductile iron castings.Wallace et al.[7] studied metallurgical and processingfactors affecting the formation of the shrinkage cavityin ductile iron, noting that the shrinkage cavities areclosely associated with mold conditions and castingmodulus.

Su et al.[8] reported the volumetric changes oc-curring during the solidification of cylindrical ductile

t To whom correspondence should be addressedE-mail: jiarong.li@biam.ac.cn

iron castings. Kitsudou[9] studied the solidificationand shrinkage tendency of high silicon hypereutecticductile irons in metallic molds. Takamori[lO] reportedthe studied results of the solidification and expansionof ductile iron castings. Wei[ll] discussed the solidifi-cation shrinkage of ductile irons based on the experi-mental study.

In general, the previous studies have concludedthat shrinkage behavior and the formation of theshrinkage cavities and porosities during the freezingof ductile irons can be affected by a number of metal-lurgical and processing variables. However, few inves-tigators have comprehensively studied this subject [7] ,and complete clarity as to the significant control fac-tors has not been well determined.

In this work the effects of metallurgical and pro-cessing parameters on the formation of solidificationshrinkage of ductile iron in dry sand molds have beenstudied and a regression equation relating these pa-rameters to the formation of shrinkage cavities andporosities was derived based upon the orthogonal ex-periments conducted. The objective of this work 'isto provide further information of the formation of theshrinkage cavities and porosities in ductile iron cast-ings so that this. information can be applied to theproduction of ductile iron castings and included incomputer simulation of solidification.

2. Experimental

By the previous studies it is well recognized thatthe following metallurgical and processing factors areassociated with the formation of shrinkage cavity andporosity in ductile iron castings: 1) Composition: car-bon equivalent value. 2) Inoculation, or post inocula-tion. 3) Casting geometry: section modulus and

246 J. Mater. Sci. Techno!., Vol.15 No.3, 1999

Factor level

Table 1 Experimental factors and factor levels

Carbon equivalent/wt pct

Inoculation/wt pet

Setion width, T/mm

123

4.944.653.92

Casting modulus/cm

1.41.10.8

2.7051.9401.218

94.3956.9731.67

Table 2 Chemical composition, wt pet

Heats C Si C+Si/3 C+Si/7 Mn P S RE Mg2 3.16 2.28 3.92 3.49 0.27 0.040 0.027 0.011 0.0453 3.66 2.97 4.65 4.08 0.16 0.037 0.029 0.013 0.0505 4.31 1.88 4.94 4.58 0.34 0.046 0.009 0.007 0.040

Dissection position

Cope

Drag

Fig.1 Construction and dimensions of sample .castings

feeding paths.These factors as mentioned above are not all inclu-

sive, and it is recognized that other parameters alsoinfluence the shrinkage behavior and the formation ofthe shrinkage cavities and porosities of ductile ironcastings. For example, molding medium, or moldhardness and stability, has important effects on theformation of shrinkage cavities. The effect of pouringtemperature, or the temperature of the metal withinthe mold cavities, is known to have a substantial im-pact on shrinkage cavity formation. And, the effect oftrace elements is generally thought to be that theseelements influence the feed metal transfer behavior ofductile irons and result in microporosities. However,in this. work, these three factors examined at threelevels as shown in Table 1, were selected in order tostudy their effects on the shrinkage behavior and theformation of the shrinkage cavities and porosities ofductile iron in dry sand molds.

The castings selected for this study were. inverseT-shaped castings with modulus values as shown inFig.l. Carbon equivalent (CE = C + 1/3Si) val-ues for the castings are presented in Table 1, andother composition values within the following ranges:0.15",,0.35 Mn, 0.005",,0.015 RE, 0.040",0.050 Mg,< 0.05 P, < 0.03 S (wt pct).

Dry sand molds were prepared with the mold hard-nessof green sand molds within the range 85",92.

Heats were prepared using induction furnace in

a commercial foundry where liquid irons were heldat 1520C for 5",8 min prior to treatment with 1.5%MgFeSi alloy containing 5% Mg and a little rare earth.This was followed by inoculation with a total of 0.8%FeSi alloy added during tapping, the post inoculationswere performed separately with 0, 0.3% and 0.6%FeSi alloy added during transfer to the pouring ladles.Pouring temperatures were measured and thermalanalysis of selected sample castings were determined.The chemical compositions of all heats were analyzed,the microstructures of all the castings were exam-ined. By using buoyancy principle, the volumes of theshrinkage cavities and the masses of all the castingswere recorded with high precision electronic-weighingsystem of PK60MC which was made in Switzerland,and the densities of the castings were calculated. Theprecision of the electronic-weighing system was be-tween 0.01 g and 0.1 g.

3. Experimental Results

Table 2 shows the chemical composition and twokinds of carbon equivalent values of the three heats.One of carbon equivalent is expressed as: CE = C +1/3Si, this .carbon equivalent has been widely used,so this carbon equivalent was' considered in experi-mental design; the other is expressed as: CE = C +1/7Si, this carbon equivalent has been mainly used toevaluate shrinkage behaviorl ", so this carbon equiva-lent was used in the regression equation. The solidi-fication times, i.e., the time to 1080C, of hot centerof the castings are presented in Table 3. The cast-ing number (see Table 3) consists of three digits, thefirst of which refers to the heat and the last refers tothe casting modulus, e.g., 21, 22 and 23, indicatingmoduli (in em) of 2.705, 1.940 and 1.218 respectively.Density values and values characteristic of shrinkagecavities are presented in Table 4 and illustrated inFig.2, where Vd is the total volume of all depressionsor surface sinks observed in the casting, V is the to-tal volume of the casting assuming no shrinkage orporosity, VI is the volume of the first shrinkage cavityand V2 is the volume of the second, or other shrinkagecavities. Vp indicates the total shrinkage volume con-sisting of the sum of Vd, Vl and V2 in the casting. Allshrinkage volumes were determined at the midwall of

J. Mater. Sci. Technol., Vol.15 No.3, 1999 247

Table 3 Solidification time of sample castings

Casting 223 222~ 221 323 322 321 523 522 521number

Time/min 10.0 21.2 35.3 10.5 22.7 36.8 12.7 24.1 44.4

Table 4 Density and porosity ratio of sample castings

Casting Density (Vd/V) (Vl/V) (V2/V) (Y = Vp/V)number /(g/cm3) /% /% /% /% '

223 7.1823 0.187 0 0.119 0.306222 7.1714 0.418 0.144 0.616 1.178221 7.1315 0.028 1.558 0 1.586323 7.0298 0 0 0.074 0.074322 7.0130 0 0.137 0 0.137321 7.0374 0 0 0 0523 7.0250 0.088 0 0 0.088522 6.9860 0 0.404 0 0.404521 6.9775 0 0.399 0 0.399

Table 5 Range analysis of the porosity ratio of sample castings

Ki Carbon equivalent Inoculation ModulusKl 0.297 0.281 0.662K2 0.070 0.688 0.573K3 1.023 0.422 0.156

Range R 0.953 0.407 0.506

First shrinkagecavity

Depression

248

Table 6 Range analysis of the density of sample castings

J. Mater. Sci. Technol.~ VoLl5 No.3, 1999

Carbon equivalent Inoculation Modulus6.996 7.058 7.0497.027 7.049 7.0577.162 7.078 7.0790.166 0.029 0.030

1.0

0.8-e0.........g 0.6~

:~ 0.4(/')0(50.. 0.2

0.0 .__._..&...-.._.......___...L-_--L-_---L..._--J3.8 4.0 4.2 4.4 4.6 4.8 5.0

Carbon equ iva lent / %

Fig.3 Effect of carbon equivalent on the porosity ratioof sample castings

7.20,..-------------

7.15

249J. Mater. Sci. Technol., Vo1.15 No.3, 1999

1.0

0.80

0--.... -:0~ 0.6~'in e0Q_

0.2

0.0 ..___----L. __ --L- __ ....___----" __ ........0.6 0.8, 1.0 1.41.2 1.6

Inoculation/ %

Fig.7 Effect of inoculation on the porosity ratio of sam-ple castings

aspects of the ductile iron can cause various results.In general using a rigid mold the shrinkage porosity ofcasting is decreased when inoculation increases; whileusing green sand mold of low hardness with the incre-ment of inoculation the nodule counts increase, whichresults in the increment of shrinkage porosity; whenusing the sand mold of mid-hardness with the incre-ment of inoculation the formation of the shrinkagecavities and porosities is mysterious. Certainly, this isrelated to the solidification mode and to the stabilityof the mold cavity as solidification occurs. For exam-ple, the study in this paper shows that with the incre-ment of the inoculation from 0.8% to 1.1% the shrink-age porosity increases, this is, because the graphiti-zation expansion force caused by inoculation resultsin mold movement, which leads to the increment ofshrinkage porosity; but with the increment of the in-oculation from 1.1% to 1.4% the shrinkage porositydecreases, this is because the higher graphitizationexpansion force caused by higher inoculation has twoeffects, one is to cause mold movement which resultsin the increment of shrinkage porosity, the other is toincrease the pressure force in the casting to decreasethe shrinkage porosity while the mold movement isbecoming smaller and smaller.

4.4 Effect of the casting geometry (modulus)The influence of casting modulus on the shrinkage

porosity ratio of the castings is illustrated in Fig.8.With an increase in casting modulus there is an in-crease in the shrinkage porosity, which confirms theresults of Wallace et al. again[7]. In agreement withthe Chvorinov principle, the solidification time of thecastings is proportional to the square of the castingmodulus and the pouring temperature of the castingsas shown in Fig.9. This is also apparent in the re-gression equations presented in Table 7, which are ingeneral agreement with the Chvorinov relationship.In Table 7, t refers to solidification time (min), M to

1.0 ~

0.8 ~o

0--....o:.;: 0.6 ~~i;-t/) 0.4 t-o

&0.2 -

0.0 ...___ _....l _..I_..____.II..-.--L-_~ I--L--I1.0 1.4 1.8 2.2 2.6

Modulus/cm

Fig.8 Effect of casting modulus on the porosity ratio ofsample castings

50~----------~--------~~ Measured

40 0 Calculated

c 30'E-....Q)

J~ 20t-

10

o~~~~~~~~~~~~~1 2 3 4 85 6 7

Modulus square/em/

Fig.9 Effect of casting modulus on the solidificationtime of sample castings

modulus (cm) and To to pouring temperature (OC).

4.5 Effects of metallurgical and processing parametersWhen considering the variables of carbon equiva-

lent, inoculation, casting modulus and pouring tem-perature, the following regression equation was ob-tained:

Y(%) = 0.00037To - 16.068CE + 1.904(CE)2+7.7691 - 3.619/2 + 1.891M-

0.386M2 + 27.405 (2)where: CE = C% + 1/7Si%

In Eq.(2), Y refers to porosity ratio, To to pouringtemperature, CE to carbon equivalent, 1 to inocula-tion and M to casting modulus.

Using this regression equation, the effect of eachexperimental parameter on the formation of shrink-age porosity is summarized in Table 8. The decreas-ing order of the effects of metallurgical and processingfactors is: carbon equivalent, casting modulus, inocu-lation and pouring temperature, with the effect of car-bon equivalent being the most significant and pouringtemperature having least significance (over the range

250 J. Mater. Sci. Techno!., Vo1.15 No.3, 1999

Regressionequation

Determination

Table 7' Regression equations and tests

Conclusion

t = 4.847413M~+3.608828

0.961273 173.75

coefficient F computationR2

29.25 F computation >F distribution

t = 0.014371M2x{To - 900)+4.916933

0.981481 370.9~

F distribution0=0.001

29.25 time is closelyrelated to modulus

Table 8 Influences of factors on the porosity ratio of sample castings

Carbon PouringFactor equivalent Inoculation Modulus temperature

Iwt pet Iwt pet /cm rCVarying range 3.49"'4.58 0.8",1.4 1.218",2.705 1280", 1320Factor values ofmaximum and 3.49,4.08 1.1, 1.4 1.218, 2.705 1280, 1320minimum YaYI% 0.969 ,0.384 0.569 0.015

of variables considered). The range analysis of the ef-fects of all the factors on the shrinkage pososity ratiois presented in Table 5, because the experiments wasdesigned on the basis of the mathematical orthogonalmethod and range analysis was used in the discussion,the results are reliable and the accuracy is high. Whileusing this regression equation the influences of all the..factors on the shrinkage porosity ratio are listed inTable 8. By comparing Table 5.with Table 8, R valueis approximately equalto ~Y. This demonstrates thereliability of the experiment and study.

5. Conclusions

(1) The effects of metallurgical and processing pa-rameters on the formation of solidification shrinkagein ductile irons have been studied, considering theparameters' of carbon equivalent, inoculation, cast-ing modulus and pouring temperature within specificranges of these variables.

(2) Based on these factors, the factor level of theminimum casting shrinkage was observed at: carbonequivalent, 4.65%; modulus, 1.218 cm; and inocula-tion, 1.4%. The factor level of the maximum castingshrinkage was observed at: carbon equivalent, 3.92%;modulus, 2.705 cm; and inoculation, 1.1%. These val-ues .must be considered, in light of the ranges of vari-ables studied.

(3) Carbon equivalent is the most important factorin the formation of shrinkage cavities and porosities.It is evident that porosity ratio decreases and thenincreases with increasing carbon equivalent, with aminimum value at 4.65% carbon equivalent. Increas-ing carbon equivalent results in a decrease in the cast-ing density. It is obvious' that porosity ratio increasesand then decreases with the increase of inoculation,

with a maximum value of porosity ratio at 1.1% inoc-ulation. With an increase in casting modulus there isan increase in the shrinkage porosity.

(4) A regression equation relating these variablesto the formation of shrinkage cavities and porositieswas derived on the .basis of the orthogonal experi-ments conducted.

AcknowledgementThe paper was partly supported by the National Nat-

ural Science Foundation of China under the Key ProjectNo. 59235102. The authors would like to thank senior en-gineer H.B.Xiang, H.J.Tong and Y.T.xie for providing ex-perimental help, and Shougang General Machinery Plant(China) for providing the laboratory facilities.

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