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EffectofMoldHardnessonMicrostructureandContractionPorosityinDuctileCastIron

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JafarKhalil-Allafi

SahandUniversityofTechnology

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BehnamAmin-Ahmadi

UniversityofAntwerp

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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2011, 18(4): 44-47, 67

Effect of Mold Hardness on Microstructure and Contraction Porosity in Ductile Cast Iron

Jafar Khalil-Allafi’ , Behnam Amin-Ahmadi’ (1. Faculty of Materials Engineering, Sahand University of Technology, Tabriz 513351996, Iran; Materials Science and Engineering, Sharif University of Technology, Tehran 1136511 155, Iran)

2. Department of

Abstract: The effect of mold hardness on the microstructure of ductile iron and the contraction porosity was investi- gated. Molds with different hardnesses (0. 41, 0. 48, 0. 55, 0. 6 2 MPa) and a sand mold prepared by Coz method were used. The influence of silicon content on the induced expansion pressure owing to the formation of graphite was also investigated. The contraction during solidification can be compensated by an induced expansion owing to the graphite relief when the hardness of mold increases; therefore, the possibility of achieving a sound product without using any riser increases. Key words: ductile iron; graphite; preheating temperature; carbide; shrinkage porosity

Production of a sound ductile iron without any porosities and shrinkage defects in the green sand mold is difficult. Special consideration should be given to the control of contraction during casting of ductile iron. It is known that the contraction occurs in metals during cooling from the liquid state. This process does not happen in the ductile iron because of the graphite formation during the eutectic solidifi- cation. It is obvious that the specific volume of the graphite is higher than that of the iron phase; thus, the induced volume changes owing to the graphite formation in ductile irons can compensate the con- traction of the solidification process. Therefore, there will be no need for using riser in the casting process of these

During casting and cooling process of ductile iron, mold cavity enlarges because of the applied thermal expansion and the induced expansion pressure during the solidification process of these alloys. Conse- quently, the increase in the eutectic graphite leads to an increase in contraction defects owing to the en- largement of the mold cavity. It can be supposed that the use of high strength molds can decrease the contraction defects. Besides, there are some other typical methods to decrease the contraction problems in ductile iron including the use of additional large risers, risers with larger neck and the use of the

exothermal material in the riser. It is convenient to use a chill as well, But it leads to the decrease in the casting yield and the increase in the cost of final product^^^-^].

It is well known that the low pouring tempera- ture of ductile iron leads to the decline in the needed liquid that should be compensated by the molten metal in the feeding system. Further cooling results in the nucleation of graphite particles and the volume expansion occur. This induces large stresses on the mold cavity. Increase in the cast product module leads to an increase in the induced pressure. This behavior can be used as a self riser feeding in the ductile iron casting if the deformation of mold cavity owing to the expansion pressure is prohibitedC5’. With further cooling of the remained molten metal between grains and dendrites, small contraction de- fects occurc6’. The porosity exists in the warmest places which are called hot spots. The contraction in the solid state owing to the decrease in the tempera- ture can be compensated by applying additional di- mensions on the model design because the riser has no effect on the content of contraction at this stage of the solidification p roce~s [~ -~’ .

If the total contraction of ductile iron is com- pensated by the induced expansion owing to the graphite formation, the sound product can be ob-

Biography:Jafar Khalil-Allafi(l965-), Male, Associate Professor; E-mail: allafi@sut. ac. ir; Received Date: February 23, 2010

Issue 4 Effect of Mold Hardness on Microstructure and Contraction Porosity in Ductile Cast Iron 45 *

tained without using any riser in the casting design. But if the induced expansion cannot compensate both the needed liquid owing to the enlargement of the mold cavity and the contraction of the solidification process, porosity defects will appear in the final product. It is known that casting of the sound duc- tile iron product without using any riser can be af- fected by the chemical composition of the alloy, the mold hardness and the design of the specimen. For example, owing to the non-homogenous solidifica- tion, the contraction of one part of the specimen cannot be compensated by other part of the speci- menCR'.

I t is indicated that the carbon content has a more important effect compared with the silicon con- tent on the induced expansion pressure in the ductile cast iron at a constant carbon equivalent. However, silicon has a dominant role in prohibiting the forma- tion of primary carbides[*'.

Important rules that must be considered for the sound product casting of ductile iron without using any riser are as follows:

1) High expansion pressure should be induced by formation of graphite particles in the ductile cast iron. As mentioned above, the alloy composition plays an important role in inducing large expansion pressures. T h e high carbon equivalent guarantees no carbides formation in the microstructure and increase in the fluidity of molten metalLs1.

2) Homogenous solidification must occur during cooling; therefore, a specimen with a homogenous cross section is needed. If the specimen has a warm zone owing to the complex specimen design, a direc- tional solidification must be toward the warm zone and total additional expansions occur in the warm

3 ) Mold must be hard enough to resist against the induced expansion pressure.

In the present research, the effects of silicon content and mold hardness on the contraction behav- ior during the ductile iron casting were investigated. A balance between the induced expansion and the contraction during the solidification process has been achieved by changing the silicon content and the mold hardness"O-lll.

zone18-111

1 Materials and Methods

In the present work, the ductile iron was made by charging scraps, cast iron ingots, returned speci- mens into an induction furnace. T h e nodulizing process was performed by the sandwich method with

FeSiMg containing magnesium of 5%. The FeSi contai- ning silicon of 75% was used as nucleation agents. Two kinds of patterns with and without the riser were used in this research. T h e wood pattern was built in dimen- sions of 20 m m X 140 m m X 190 mm. T h e gating sys- tem has been shown in Fig. 1 ( a ) . T h e molds with different hardnesses were made by change of mold ramming to reach 0.41, 0. 48, 0. 55, 0. 62 MPa. T h e effect of silicon content on the graphite precipi- tation and the induced expansion pressure for molds with deferent hardnesses was examined. Different samples were cut from different positions of a cast- ing specimen. T h e density of samples was measured by the Archimedes method and the mean density of all samples was considered as the casting specimen density. T h e specimen with dimensions of 40 m m X 150 m m X 150 mm with the riser was used [Fig. 1 (b)] to examine the effect of mold hardness on the volume of contraction porosities and the influence of silicon content on the graphite precipitation. In this part of experiments, the molds with different hard- nesses of 0.413, 0.482, 0.551, 0.621 MPa and a mold

5 cm - I Fig. 1 Board pattern with a gating system (a) and

used pattern with the riser ( b )

46 Journal of Iron and Steel Research, International VOl. 18

prepared by Coz method were used and the silicon content was 2 % , 2 .5% and 3%. Specimens after casting were measured accurately to examine the movement of mold wall owing to the induced expan- sion of graphite formation. The microstructure of all samples was studied using the optical microscope (Olympus PME3 1.

2 Results and Discussion

Instability of the mold cavity owing to the in- duced expansion pressure is one of the important as- pects of the grey iron casting. This phenomenon is more severe in the ductile cast iron because of the mushy solidification of these alloys. It is well known that the shell solidification in the grey iron leads to

the decrease in the applied stresses owing to the in- duced expansion pressure on the mold cavity by transferring the pressure into the molten metalc4’. This solidified shell does not exist in the ductile iron because of its mushy solidification. If the mold wall resists dimensional changes owing to the induced ex- pansion pressure, the contraction of the austenite phase would be compensatedi5’.

The microstructure of the ductile iron with dif- ferent silicon contents is shown in Fig. 2. The in- crease in the silicon content leads to an increase in the graphite particles and the ferrite phase. The sound specimen can be produced owing to the high graphite precipitation when the chemical composi- tion of the alloy consists of high silicon content.

(a ) wsi=2%; (b) wsl=2. 5 % ; (c) wa=3%.

Fig. 2 Microstructure of ductile cast iron in sand mold with different amounts of silicon

No contraction was observed visually in the specimens cast in the sand mold without using any riser owing to the homogenous solidification of spec- imensC8’. Low thermal conductivity in sand molds and the mushy solidification of ductile iron lead to dimensional changes of final products. It was con- firmed by measuring the density of cast products. The measured density of specimens was close to the theoretical density. The density changes of specimens with different silicon contents in the ductile iron cast in a mold with different hardnesses are shown in Fig. 3 (a ) . It is obvious that the increase in mold hardness leads to the increase in specimen density. It means that the induced expansion pressure owing

to the graphite precipitation has compensated the lack of molten metal owing to the contraction of the austenite phase. At constant mold hardness, the in- crease of the silicon content in the range of 2 % - 3 % leads to a decrease in the density except for the mold with hardness of 0. 62 MPa. It is known that the graphite precipitation increases when the silicon con- tent increases and thereby the induced expansion pressure will increase. It can be concluded that in molds with low hardness, dimensional changes will be high and the specimen will be scrap. In mold with hardness of 0. 62 MPa, the increase in the silicon content leads to an increase in the induced expansion pressure. However, this induced expansion pressure

Mold ha.rdness/MPa

Variation of density ( a ) , height (b) and length (c) of ductile iron specimens with Fig. 3 different silicon contents cast in mold with different hardnesses

Issue 1 Effect of Mold Hardness on Microstructure and Contraction Porosity in Ductile Cast Iron 47

has no negative effect on the final product because the induced expansion pressure owing to the graph- ite precipitation compensates the lack of the molten metal owing to the solidification contraction of ma- trix phase instead of the enlargement of the mold cavity. T h u s , in the mold with the hardness of 0. 62 MPa, the increase of the silicon content slightly decreases the density of the specimen. It can be pre- dicted that in the mold with high hardness, the den- sity increases proportionally when the silicon content increases.

Length and height changes of specimens as a function of the mold hardness and the silicon content

are shown in Fig. 3 ( b ) and (c) , respectively. These changes confirm previous discussion. I t is clear that with the increase in the silicon content, dimensional changes of final product increases but in the mold with hardness of 0. 62 MPa, the gradient of dimen- sional changes versus the silicon content declines.

Other experiments were performed using the riser. In these experiments, a directional solidifica- tion was applied to the system. Contraction porosi- ties in the riser for different silicon contents ( 2 % , 2. 5% and 3%) and mold hardnesses ( f rom 0. 41 MPa up to 0. 62 MPa and the mold prepared by the Co, method) in ductile cast iron are shown in Fig. 4.

0.41 0.48 0.55 O.(Z 6& 0.41 0.48 0.56 0.f.Z 602 0.41 0.48 0.55 0.62 Cq

(a) ws,=2%; ( b ) ws ,=2 .5%; (c) wsl=3%. Fig. 4 Contraction porosities in riser for ductile cast iron with different silicon contents and mold hardnesses

It can be concluded that for high silicon content and low mold hardness, dimensional changes of the mold increases and more contraction exists in the riser. In the mold prepared by the Coz method, low contraction was obtained in the riser owing to high hardness of the mold.

3 Conclusions

1) The hardness of sand mold has an important effect on the casting of ductile iron without using any riser. T h e increase in mold hardness leads to an increase of induced expansion pressures owing to the graphite precipitation which decreases the contrac- tion of the ductile iron.

2 ) T h e increase in silicon content and the graphite formation in the matrix lead to the increase in induced expansion pressure and dimensional chan- ges of the mold cavity. However, in the mold with high hardness, the precipitation of more graphite has a positive effect that can compensate the lack of molten metal owing to the contraction of the liquid phase.

3 ) The increase in silicon content in the casting of the ductile iron by using the riser leads to an in- crease of dimensional changes of molds with low hardness and finally the contraction in the riser in- creases. However, in molds with high hardness, the

contraction volume in the riser will be low when the silicon content increases, which is due to the stability of the mold cavity against dimensional changes.

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I s sue 1 Effect of Mic ros t ruc tu re o n Cor ros ion Fa t igue Behavior of Dual-Phase H i g h S t r e n g t h S tee l * 67

t h r Q - , ~ of the FM steel, which was oil quenched and then tempered at 280 or 370 'C for 2 h after aus- tenitization at 900 'C for 2 min, is greater than 100 MPa at R=-1.

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