influence of gating system, sand grain size, and mould coating on microstructure and mechanical...

8/13/2019 Influence of Gating System, Sand Grain Size, And Mould Coating on Microstructure and Mechanical Properties of T

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Available online at www.sciencedirect.com--.; cience irectJOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(12): 38-45

Influence Gating System Sand Grain Size and Mould Coating onMicrostructure and Mechanical Properties Thin Wall Ductile Iron

Hassan Iafari : , Mohd Hasbullah Idris , Ali Ourdjini ,Majid Karirnian , Gholamhassan Payganeh

O. Department of Materials Engineering, Universiti Teknologi Malaysia, Skudai 81310, Johor , Malaysia;2. Department of Materials Engineering, Shahid Rajaee Teacher Training University, Tehran 16785-136, Iran)

Abstract: Two gating systems namely stepped and tapered runners were used to cast strip samples with differentthicknesses by COz/silicate process using sand grain sizes of AFS 151 and 171. To assess the effect of mould coatingon the properties of thin-wall ducti le iron, half of the moulds were coated whilst the rest were not coated . Moltenmetal with the carbon equivalent of 4. 29 was prepared and pou red at 1450 C. Microstructure of the specimenswas analyzed by optical and scanning electron microscopes. Count, area fraction, roundness and diameter of the graphitenodules of the samples were measured by image analyzer. Brinell hardness and tensile tests of all the samples were alsoconducted. The results show that by using s tepped runner gating system with uncoated and coarse sand mould,roundness and count of the graphite nodules decrease whereas diameter and area fraction increase. Although fine sandand coated mould cause longer d is tance of mol ten metal travel, hardness and strength of the samples decrease.Key words: thin-wall ducti le iron; gating system; sand grain size; mould coating; mechanical property; microstructure

Owing to awareness of energy conservation andenvironmental sustainability, demand in the use oflight materi als and new technologies in automotiveindustry has been created[I-2]. In recent years, thin-wall ductile iron CTWDI) has been cons idered as asubstitute for steels and light alloys owing to its prop-erties such as high strength and good ductility


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Issue 12 Influence of Gating System, Sand Gra in S ize, and Mou ld Coa ti ng on Thin-Wa ll Ductile Iron 39

and mould sand grain size on metallurgical and me-chanical properties of TWDI castings. Experimental

Two wooden patterns with 8 strips having 500 mmin length, 25 mm in width and 1, 2, 3, 4, 5, 6, 7 and8 mm in thickness attached directly to the runnerFig. 1) were designed, fabricated and painted to in-crease durability and surface smoothness. Theheight of the sprue was 250 mm and gates were de-signed in the drag. Dimensions of the pattern werecalculated in such a way that molten metal can enterthe strip cavities at the same velocity. Two differentrunners namely s tepped and tapered runners wereused to study the solidification behaviour and themicrostructure of each strip, independently. Each

a L

strip was separated at a distance of 30 mm in orderto eliminate the effect of heat from the neighbouringstrips. Eight moulds were made by a CO 2 / silicatesand molding process according to Table 1 for theinvestigation.

Graphite-based zircon-containing material MouldCoat 633, Farsiran Co, Iran) was used as coating.Low impuri ty pig i ron ingot and g ray iron and s teelscraps were charged and melted in a medium fre-quency induction furnace with 60 kg in capacity.The chemical composition of the molten metal preparedwas of carbon equivalent CE) of 4. 29 Table 2) in-dicating a eutectic composition.

A tundish sandwich method was used forspherodizing the molten metal by adding ferro-siliconalloy containing magnesium of 5 in the ladle. Theb)

) Tapered runner; b) Stepped runner.Fig Two types of patterns us in casting of TWDI specimens

Table Mould conditions used for producing the stripsGating system Sand grain size CoatingMould AFS standard)Stepped Tapered 151 171 Yes No

1 2 3 4 5 6 7 8

Table 2 Chemical composition of molten metal ass percent , )

C Si S P Mn Mg CE Fe3.492 2.399 0.010 0.029 0.495 0.003 4.291 Balance

mol ten metal was poured at temperature of 1450 C.Fig. 2 shows a typical picture of some TWDI castingsproduced in this research. To evaluate the fluidity,the length of molten metal travel in each cavity of thetapered runner mould solidified strips) was measured.For metallographic analysis, a specimen of 30 mm inl ength was cu t from each cast str ip [Fig. 3 a) ] andthen ground, pol ished and etched with 2 of Nital

Fig 2 Typical TWDI castings produced for the research


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a

Journal of I ron and Steel Research, International

b Unit:mm

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Fig.3 Schematic drawing of sampling position a and tensile test sample b

Fig.4 Variation of mould filling ability with sample thicknessreduce solidification rate of the TWDI cas tings,thereby allowing longer distance of mol ten metalt rave l. Based on t he resul ts shown in Fig. 4, it canbe stated that section thickness is directly propor-tional to mould filling ability, i e. decreasing crosssection and consequently reducing casting moduluscause increase in the solidification rate of th e castsamples thus less distance travel of the molten metal.

solution. The microstructure of the samples was ex-amined by optical and scanning electron microscopesmodel Leo 440, LEO Electron Microscopy Ltd,Cambridge England . Clemex Image Analyzer CIAwas used to qualitatively measure metallographic char-acteristics such as graphite nodule count, area frac-tion of graphite nodules , and roundness. Tensiletest was conducted according to ASTM A370-03standard using universal MTS testing machine withcapacity of 15 t and a cross speed of 2 mm/min.Fig. 3 b shows the tensile test samples used in thisresearch. The fractured surface of the selected ten-sile test samples was examined using SEM. Brinellhardness test was performed on all samples, accord-ing to ASTM EI0 standard with the size of 30 mrnX25 mm and different thicknesses, using Instron Wol-per tes ting machine. A load of 5 kN for the sampleswith 2. 3 and 3. 3 mm in thickness and the load of29. 6 kN for higher thicknesses, respectively, wereapplied in order to make t he tes t safely.

3 4 5 6Thicknesslmm 7 8 9

2 Results and Discussion2. 1 Fluidity

Fig. 4 shows the effect of sand gra in size andmould coating on the ability of molten metal to fillthe cavities with different thicknesses in taperedrunner mould. can be observed that by using finersand grain size AFS 171 and applying mould coat-ing, the ability of filling the mould by molten metalincreases for all samples. The highest mould fillingability is obtained using fine sand g ra in size withmould coating mould 5 whilst the lowest mouldfilling ability is observed on coarse sand grain anduncoated mould mould 1 Using fine sand gra insize and graphite-based zircon-containing coating,which develops a thermal barr ier between th e solidi-fied metal and the mould sand, retards the t ransferof heat to the sand mould and the surrounding at -mosphere. This phenomenon may prohibit chilling and

2. 2 Graphite nodule countThe graphite nodule count is an important fac-

to r in the characterization of the microstructure andtherefore significantly affects the mechanical proper-ties of ductile iron[12]. Fig. 5 shows that the graphitenodule count decreases with increasing sample thick-ness in all cast samples. The graphite nodule countdepends on the solidification rate[13]. Owing to thedecrease in thickness of sample, heterogeneous nu-cleation rate increases, thus leading to shorter solid-ification t imes and increase in t he graphit e nodulecount. Fig. 5 also reveals that by using mould coatand fine sand grain size, lower heat transfer fromth e mol ten metal to the surrounding air is experi-enced. This results in the decrease of cooling rateand the increase of solidification time and as a re-sult, the graphite nodule count decreases in t he TWDIcas tings. Mould 2 is observed to register the highest


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Issue 12 Influence of Gating System, Sand Gra in Size, and Mould Coating on Thin-Wall Ductile Iron 41

2.3 Graphite nodules area percentageFig. 7 i llus trates the variation of nodule a rea

percentages in s ampl es w it h d ifferent t hic knes sescast in various mould conditions. The r esul ts showthat th e percentage of nodul es a rea inc reases withincrease in the thickness of the samples for all mouldsTherefore, owing to longer solidification time, thicker

it can also be observed that mould 5 shows th e low-es t nodule count than t he o th er mou lds, inferringthat th e effect of mould coat on nodule count is moret ha n th e effect of sand gra in size

Owing to th e decreasing number of graphitenodule count or coarsening of the nodules, l es s uni-form distribution of th e graphite nodules is observedin the matrix, which produces casting of inferior me-chanical prope rt ie s. Example s of unetched micro-graphs fo r samples with 3.3, 5.4, 7.5 and 8.5 mmin thicknesses are shown in Fig. 6 It can be seent ha t t he nodule count decreases with increasing sam-ple thickness, which is consistent with th e findingsof other researchers { : . This may be due to the lon-ge r solidification time experienced by th e castings.

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Mould Mould 3Mould 5-Mould 7

5 6 7Thicknesslmm3450 _ _ _ _ _ _ _2

650

1050 . . . . ;

850

Z

Fig.5 Variation of nodule count with sample thicknessquantity of nodules compared to t he o ther moulds.In genera l, when casting in the same conditions,s amples produced f rom s tepped runne r mould showhigher nodule count than those produced from th etapered runner mould. has been reported thatnodule count between 500 and 700 mm producesTWDI castings with good mechanical propert ies- : ,Indeed, the resul ts of t he p re sent work showed thatth e majority of th e cas t samples tes ted possessessimilar nodule count within this range. From Fig. 5,

a 3. 3 mm; (b ) 5.4 mm s c 7.5 mm; (d ) 8.5 rnm,Fig 6 Microstructures of unetched samples produced in mould 1


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samples produce higher nodule percentage. Fig. 7 alsoindicates that by using fine grain size sand andmould coat in s tepped runner m ould a nd owing tolonger solidification time moulds 5 and 2 producedthe highest and lowest nodule area respectively. isknown that the amount of graphite nodule area in thematrix is independent of the size of graphite nodules[15] .2. 4 Nodularity

The effect of TWDI castings thickness an dmould condition on th e average graphite nodularityis demonstrated in Fig. 8. can be clearly observedthat th e average nodularity of the graphite nodules isinfluenced by the thickness of th e samples. As stated earlier by decreasing the thickness of the sampies the cooling rate during solidification rai ses andconsequently nodule count increases; therefore thenodularity of th e TWD I castings is enhanced[15-16].

As the mechanical properties of TWDI castings especially strength an d ductility depend di-r ect ly o n g rap hi te n od ula rit y more nodularity ofgraphite nodules leads to a higher mechanical properties in TWDI castings U. Moreover the averagenodularity of th e samples produced f rom uncoa ted

Fig 7 Variation of nodule area percentages withsample thickness

mould and from moulds made of coarse sand gra insize shows higher values. From the curves it can bestated that the same result emerges f rom samplescast in stepped runner moulds. Furthermore thecurves reveal that the average nodularity of thegraphite nodules is higher than 80 and it is morethan 90 for samples produced from mould 2.2. 5 Graphite nodule diameters

The diameter of graphite nodule is shown inFig. 9 as a function of th ic kn ess and mould conditions. I t can be observed that there is an increasingtrend in graphite nodule diameter with increasingsample thickness. Finer sand grain size used in thepreparation of the mould coating mould cav itystepped runner gating system and increase in th esample thickness l ead to the increase in solidificationtime. As a result th e diameter of graphite nodulesincreases in all samples. Samples cas t in moulds 5an d 2 show larger and smaller graphite nodule diameters respectively when compared to other samples.I t is also observed t ha t t he diameter of graphite nodules in samples ranges from 9 to 17 lm The graphi te nodule diameter ha s strong influence on mechanical This has been proven thatsmaller graphite nodule diameter in TWDI castingspromotes higher strength and ductility.2. 6 Mechanical propert ies

Fig. 10 shows the results of the hardness testfor the samples produced from different moulds as afunction of sample thickness. is clear that th ehardness in all samples decreases with increasing th ethickness from 2. 3 mm to 8. 5 mm. This may be dueto the formation of iron carbides in the microst ructure because of th e high cooling rate associated withthin sections. The precipitation of iron carbides inth e microstructure has a negative effect on th e me

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c Mould 2oMould 4e Mould foMould 8

4 5 6 7Thicknesslmm3

11109

oS 8a 70 6Z

54 2

Mould 1 nMould 2 Mould 3 oMould 4 Mould 5 t Mould 6

t lh ...... _

.... _ -- lIlo Mould 1 0 Mould 2 Mould 3 o Mould 4 Mould 5 t Mould 6 Mould 7 0 Mould 8

10094

880Z82

762 3 6 7

Thicknesslmm8 9

17

15 138 o 11Z

3 4 5 6 7Thicknesslmm

8 9

Fig.8 Variation of graphite nodularity with sample thickness Fig.9 Variation of nodule diameter with sample thickness


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Issue 12 Influence of Gating System. Sand Grain Size. a nd Mou ld Coa ting on Thin-Wa ll Ductile Iron 43

The results of the tensile s tr engt h t es t ar eshown in Fig. 12. Owing to the small size of cast-ings, tensile test was not per fo rmed fo r the sampleswith wall thickness less than 4 mm. Fig. 12 showsan increasing trend in ultimate tensile strength andyield strength of TWDI castings with a decrease inthickness. This is probably due to the formation ofiron carbide in the microstructure. also showsthat by using fine sand grain size and mould coating,a significant reduction in both ultimate tensilestrength and yield strength values is observed. Incasting of TWDI, bet ter s trength may be achievedby promoting high cooling rate, which in turn pro-duces high graphite nodule count and roundness.Fig. 13 exhibits fractured morphology of a TWDIcasting sample with thickness in 4. 4 mm after un-dergoing a tensile test. A cleavage fracture can beclearly observed which reveals a considerable amountof iron carbide precipitated in the microstructure.

The effect of sand mould size and mould coatingon the elongation of the samples taken from TWDIc ast ings w it h dif ferent t hicknesses is shown inFig. 14. can be seen that compared to the strength.

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Mould 1 Mould 2 Mould 3 0 Mould 4..Mould 5 Mould 6 Mould 7 0 Mould 8

4 5 6Thickness/mm323 ---- --- --....1..----=---=::.:==0.........:2

53

6301 11,....-----------------,

Fig.l0 Variation of hardness value with sample thickness

gj 43J33

chanical properties especially on ductilitf4.1s-19]. Freecarbide TWDI casting could guarantee sound casting,which possesses opt imum mechanical properties.Based on Fig. 9, the use of mould coating, fine sandgrain size and tapered runne r increase th e solidifica-tion time, which decreases th e formation of iron car-bide. Fig. 11 sh ows the ir on carbide formed in th esamples cast in moulds and 7. As can be seen,castings f rom mould conta in more carb ides thanthat of mould 7 and hence it s hardness value is higher.

Fig.ll Microstructures of samples cast in mould 1 (a ) and mould 7 (b )

9

550 b)

350tl0

8

_..Mould 8 \ ............... Mould 4Mould 6

550 (a)

Fig.12 Variation of ultimate tensile strength (a ) and yield strength (b ) with sample thickness


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44 Journal of I ron and Steel Research, International

Fig. 13 SEM fracture morphology of a TWDI cast ing sample with thickness of 4. 4 mm after tensile test

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Fig. 14 Variation of elongation with sample thicknessthe elongation shows an opposite trend and increaseswith increasing the thickness of the samples. More-over, by using finer sand gra in size and mould coat-ing in production of the castings, the same behav-iour is observed. The increase in ductility is attribu-ted to an increase in ferri te content Fig . 11 owingto th e slower cooling rate of th e castings. In otherwords, in thinner castings, the formation of carbidein the matrix causes a decrease in elongation. isworth noting that according to Fig. 14, the effect ofmould coating is more obvious t han the sand grain sizeowing to higher abili ty in decreasing the cooling rate.3 Conclusions

1 Using fine sand grai n size and app ly inggraphite-based zircon-containing coating on the mouldcavity can improve the fluidity of TWDI castings.

2 Using Fine sand grain size associated withgraphite-based zircon coating in mould making re -duces average nodularity, nodule count, and areapercentage. However , th e diameter of graphite in

the microstructure of TWDI casting increases.3 Thin section castings have increased the av-

erage nodularity, nodule count, and area percentageand decreased diameter of graphite in the micro-structure of TWDI casting.

4 Stepped runner gat ing sys tem Improves thegraphite nodule characteristics resulting in a positiveinfluence on hardness and strength of TWDI casting.

5 By using fine sand grain size and mould coat-ing, the hardness and strength of TWDI castings de-crease, bu t elongation increases.

6 Mould coating has a more significant effecton the mechanical and metal lurgical properties ofTWDI castings than sand gra in size.

The authors wish thank the Research Man-agement Center RMC Univers it i TeknologiMalaysia UTM for the financial support andMechanical Engineering Faculty Shahid RajaeeTeacher Training University SRTTU for provi-ding the necessary facilities.References:[ IJ Choi J O. Kim J Choi C O. et al. Effect of Rare Earth Ele-

ment on Microstructure Formation and MechanicalProperties ofThin Wall Ductile Iron Castings [J]. Materials Science and En-gineering, 2004. 383A 2 : 323.

[2 J Sosa A D Echeverria M D Moncada J et al. Surface Reac-tivity of Thin Wall Ferritic Ductile Iron. Th e Effec t of NoduleCount and Grinding Variables [J]. Materials Letters, 2008, 621 :100.

[3 J Druschitz A P. Fitzgerald D C. Lightweight Iron and SteelCastings for Automotive Applications [C J //Society of Auto-motive Engineers. SAE 2000 World Congress. Detroit: Socie-ty of Automotive Engineers, 2000: 11.


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