Microstructure and Mechanical Properties of Synthetic Nodular Cast Iron

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meltages 6 to 10) was used. For modification FeSiMg7 modifierwas used and for inoculation FeSi75 inoculant was used. [3].

Specimens for chemical composition, experimental bars andexperimental bodies with a graded wall thickness were cast fromall the meltages.

3. Experimental results3.1. Metallographic analysis

The metallographic analysis of specimens was made by thelight metallographic microscope Neophot 32. The microstructure

was evaluated by STN EN ISO 945 (STN 42 0461) and bycoherent test grids [4-7].

From microstructural point of view all the meltages areferrite-pearlitic nodular cast irons with different content of ferriteand pearlite in a matrix, different size of graphite and count of

graphitic nodules per mm2

(Fig. 1). Graphite occurs only in aperfectly-nodular and imperfectly-nodular shape in all thespecimens. The results of evaluation of microstructure of chosenspecimens from the experimental bars by STN 42 0461 and bycoherent test grids are presented in Tab.2. The results ofquantitative evaluation of microstructure (content of ferrite andcount of graphitic nodules per mm2) are shown in Fig. 2.

Table 1. Charge composition of experimental meltages

Charge composition (kg)Numberof meltage pig iron steel crap carburizer SiC90 FeSi75 Modifier FeSiMg7 Inoculant FeSi75

1 24 6 0.27 0.09 0.5 0.2

2 18 12 0.48 0.23 0.5 0.2

3 12 18 0.69 0.37 0.5 0.2

4 6 24 0.90 0.70 0.5 0.2

5 0 30 1.00 0.90 0.5 0.2

6 24 6 0.27 0.08 0.5 0.2

7 18 12 0.40 0.69 0.5 0.2

8 12 18 0.65 0.74 0.5 0.2

9 6 24 0.90 0.80 0.5 0.2

10 0 30 1.25 0.92 0.5 0.2

a) meltage 1 b) meltage 3 c) meltage 5

d) meltage 6 e) meltage 8 f) meltage 10Fig. 1. Microstructure of chosen meltages from experimental bars, magn. 100 x, etched 1% Nital

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Table 2. Microstructure and mechanical properties of chosen meltages

Microstructure Mechanical properties

Number

of meltageSTN EN ISO 945

(STN 42 0461)

content of

ferrite (%)

count of

graphitic

nodules (mm-2)

tensile strength

Rm (MPa)

absorbed energy

K0 (J)

Brinell

hardness HBW

1 60%VI5/6+40%V6 Fe80 61.6 104.3 500.8 6.4 210.0

3 80%VI6 + 20%V6 Fe94 74.0 199.8 539.0 30.6 192.3

5 70%VI5/6+30%V6 Fe94 78.0 179.8 515.7 17.2 182.3

6 70%VI5/6+30%V6 Fe55 50.8 126.5 367.1 8.4 190.0

8 70%VI5/6+30%V6 Fe80 65.2 151.0 462.6 24.0 181.3

10 70%VI5/6+30%V6 Fe80 56.0 157.6 462.6 19.2 183.0

0102030405060708090

100

0 20 40 60 80 100

steel scrap in the charge (%)

contentofferrite(%)

I

II

0

20

40

60

80100

120

140

160

180

200

0 20 40 60 80 100

steel scrap in the charge (%)

countofgraphiticnodules

(mm-2)

I

II

a) content of ferrite in the matrix b) count of graphitic nodules

Fig. 2. Results of quantitative evaluation of microstructure (I meltages with SiC additive, II meltages with FeSi additive)

The changes of microstructure in dependence on a wallthickness of cast (i.e. cooling rate) are presented in Fig. 3. Forcomparison, it shows the microstructure of specimens from theexperimental bodies with a graded wall thickness from themeltage 3 (with SiC additive) and the meltage 8 (with FeSiadditive) with the same ratio of pig iron and steel scrap in the

charge.The content of ferrite in the matrix is being decreased with adecreasing wall thickness. The content of ferrite in the meltageswith SiC additive (i.e. meltages 1 to 5) is generally higher thanin the meltages with FeSi additive (i.e. meltages 6 to 10). Insome of the specimens with a thinner wall thickness alsocementite, which is an undesirable structural part in the matrixof graphitic cast iron, occurs. In the meltages with SiC additivecementite occurs rarely only in the specimens with a thinner

wall thickness whereas in the meltages with FeSi additivecementite occurs also in the specimens with a thicker wallthickness. The content of cementite in the meltages with SiCadditive is generally lower than in the meltages with FeSiadditive.

Graphite occurs only in a perfectly-nodular shape and

imperfectly-nodular shape in all the specimens. The proportionof perfectly-nodular graphite is being increased with adecreasing wall thickness. The size of graphite is beingdecreased with a decreasing wall thickness, on the other hand,the count of graphitic nodules per mm2is being increased with adecreasing wall thickness. The count of graphitic nodules perunit area in the meltages with SiC additive is generally higherthan in the meltages with FeSi additive.

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a) 26 mm b) 3 mm c) 26 mm d) 3 mmmeltage 3 (with SiC additive) meltage 8 (with FeSi additive)

Fig. 3. Microstructure of experimental bodies with a graded wall thickness, magn. 100 x, etched 1% Nital

3.2. Mechanical properties

The mechanical tests (i.e. static tensile test, impact bendingtest and hardness test) were realized on specimens made fromcast experimental bars.

The results of mechanical tests, that are tensile strength Rm,impact toughness KC0 and hardness HB, are given in Tab. 2.Their course depends especially on the character of matrix

(content of ferrite and pearlite) and also on the size and count ofgraphitic nodules.The mechanical properties in the meltages with SiC additive

(i.e. meltages 1 to 5) are generally better than in the meltageswith FeSi additive (i.e. meltages 6 to 10), especially in themeltages with higher amount of steel scrap in the charge.

3.3. Microfractographic analysis

The microfractographic analysis was made by the scanningelectron microscope Tesla BS 343 on fracture surfaces ofspecimens from experimental bars fractured by the static tensiletest (Fig. 4) and the impact bending test (Fig. 5). Fracturesurfaces of analysed specimens from both series of meltages are

characteristic of mixed mode of fracture.In the specimens from the meltages with low amount ofsteel scrap in the charge, transcrystalline cleavage of ferrite withan inclination to intercrystalline cleavage (Fig. 4a, 5a) wasobserved. Fracture surfaces of the specimens from the meltageswith SiC additive are characteristic of higher ratio oftranscrystalline cleavage of ferrite than fracture surfaces of thespecimens from the meltages with FeSi additive.

In the specimens from the meltages with high amount ofsteel scrap in the charge, transcrystalline cleavage with riverdrawing on facets (Fig. 4b, 5b) and transcrystalline ductilefailure of ferrite with dimple morphology (Fig. 4c, 5c) was

observed. Fracture surfaces of the specimens from the meltageswith SiC additive are characteristic of higher ratio oftranscrystalline ductile failure than fracture surfaces of thespecimens from the meltages with FeSi additive.

Transcrystalline continuous cleavage of pearlite wasobserved in all the specimens. In the specimens from themeltages with SiC additive with higher amount of steel scrapalso transcrystalline ductile failure of pearlite was observed.

The results of microfractographic analysis correspond to theresults of metallographic analysis andmechanical tests.

4. Conclusions

The substitution of a part of pig iron for steel scrap in thecharge of nodular cast iron has a considerable economiccontribution. For the regulation of chemical composition of meltit is advantageous to use metallurgical SiC additive which has

been used in this work as an alternative additive instead of FeSiin meltages with a different ratio of pig iron and steel scrap inthe charge.

Increasing amount of steel scrap in the charge decreasestotal costs of charging raw materials and in consequence of the

increasing of SiC or FeSi additive the microstructure is changed(the content of ferrite in the matrix is increased, the count ofgraphitic nodules per unit area is increased and the occurrenceof undesirable cementite is eliminated) and consequently themechanical properties are improved. This positive influence ismore significantly shown in the meltages with SiC additive.

Mode of failure of structural components depends especiallyon the content of ferrite in matrix and its purity, which has aconnection with the charge composition. Failure of ferrite wasmade from intercrystalline cleavage over transcrystallinecleavage with river drawing on facets to transcrystalline ductilefailure with dimple morphology where higher ratio of this



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micromechanism of failure (transcrystalline ductile failure) hasa connection with higher content of ferrite in the matrix. Failure

of pearlite was made especially by transcrystalline continuouscleavage with river drawing on facets.

a) intercrystalline cleavage b) transcrystalline cleavage c) transcrystalline ductile failure

Fig. 4. Micromechanisms of failure of ferrite at static stress, SEM

a) intercrystalline cleavage b) transcrystalline cleavage c) transcrystalline ductile failure

Fig. 5. Micromechanisms of failure of ferrite at impact stress, SEM

AcknowledgementsThis work has been supported by the Culture and Educational

Grant Agency of the Ministry of Education of Slovak Republic,grant No. 3/6078/08 Creation of a laboratory and textbooks forteaching of subject Properties and using of materials and bythe Scientific Grant Agency of the Ministry of Education ofSlovak Republic, grant No. 1/0208/08 Optimalization of

properties hardened Al-alloys for automotive industry castings

made of secondary aluminium.

References[1] W. D. Schubert, A. T. Ta, G. Kahr, T. Benecke, B. Lux,

Influence of SiC additions on the microstructure of gray castiron, Physical metallurgy of cast iron, Stockholm, 1984. p.119-128.

[2] S. Venkateswaran, J. Wilfing, W. D. Schubert, B. Lux, T.Benecke, Influence of SiC and FeSi additions on themicrostructure, cooling curve and shrinkage porosity ofductile iron, Physical metallurgy of cast iron, 1990. p. 171-178.

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[3] A. Vako, Influence of charge composition on the structureand properties of nodular cast iron. [Dissertation]. SjF Uilina, 2002. (in Slovak)

[4] P. Skoovsk, T. Podrbsk, Graphitic cast irons, EDIS,ilina, 2005. (in Slovak).

[5] P. Skoovsk, A. Vako, Quantitative evaluation of structureof cast irons. EDIS, ilina, 2007. (in Slovak).

[6] J. Belan, Structural analyses of advanced materials foraerospace industry, Materials science, Lithuania, Vol. 14, No.4 (2008), p. 315-318.

[7] A. Vako, The factors influencing microstructure andmechanical properties of ADI, Archives of foundry

engineering, Vol. 9, No. 1 (2009), p. 133-136


Microstructure and Mechanical Properties of Synthetic Nodular Cast Iron

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