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23. - 25. 5. 2012, Brno, Czech Republic, EU

CHARACTERIZATION OF MICROSTRUCTURE AND FRACTURE BEHAVIOR OF GG20 AND

GG25 CAST IRON MATERIALS USED IN VALVES

Zeynep TAŞLIÇUKUR*, Gözde S. ALTUĞ*, Şeyda POLAT**,

Ş. Hakan ATAPEK**, Enbiya TÜREDİ**

*) Gedik University, Faculty of Engineering, Department of Metallurgical and Materials Engineering,

Istanbul-Turkey [email protected] , [email protected]

**) Kocaeli University, Faculty of Engineering, Department of Metallurgical and Materials Engineering,

Kocaeli-Turkey [email protected], [email protected], [email protected]

Abstract

In this study, the microstructural characterization of GG20 and GG25 gray cast iron materials were carried

out and their fracture behavior was examined. These materials are commonly used in high pressure safety

valves. In the first stage of the study, the matrix phases (ferrite/pearlite) were determined in addition to the

morphology and distribution of graphite, using light microscope and scanning electron microscope. Image

analysis was done to obtain the amount of graphite which plays an important role on fracture. In the second

stage, microhardness measurements, tensile tests and Charpy impact tests at room temperature were

performed to determine the mechanical properties of the matrices. In the third stage fractographic analysis

was carried out on the fracture surfaces, using scanning electron microscope to indicate the effects of matrix

phase, loading type and test temperature on the fracture behavior.

Keywords : Gray cast iron, characterization, mechanical properties, fractographic analysis.

1. INTRODUCTION

Gray cast iron is an attractive material used in industrial applications due to its some advantageous

properties such as good castability, corrosion resistance, machinability, low melting point, low cost, and high

damping capacity. It is used widely in the manufacturing of some machine components, disc brake rotors

and hydraulic valves [1-7].

Graphite flakes, which are formed during the solidification process, basically control the mechanical

properties and confer low strength and toughness to the gray cast iron. The microstructure of gray cast iron

is characterized by graphite lamellas dispersed in the ferrous matrix. The amount of graphite and size,

morphology and distribution of graphite lamellas are critical in determining the mechanical behavior [1, 4, 6,

8, 9]. The high damping capacity of gray cast iron, which is considerably greater than that of the steel or

other kinds of cast iron, may be attributed to its flake graphite structure [10].

In this study, the microstructural characterization of GG20 and GG25 cast iron materials used in valves was

carried out. A fractographic analysis was also done to determine fracture behavior of the matrix as function

of ferrite, pearlite and graphite distribution under tensile and impact loading.

2. EXPERIMENTAL STUDY

2.1 Materials

In grey cast iron, which is also known as flake graphite cast iron, most of the carbon is present as flake

graphite. The properties of grey cast iron depend on the distribution, size and amount of graphite flakes, and

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the matrix structure. Tab. 1 shows the standard chemical compositions of cast iron materials used in this

study.

Table 1. The standard chemical compositions of GG20 and GG25 cast iron materials.

Materials C Si Mn S P Fe

GG20 3.20-3.40 2.10-2.30 0.50-0.80 max. 0.12 max. 0.40 balance

GG25 3.00-3.25 1.85-2.10 0.40-0.70 max. 0.12 max. 0.25 balance

2.2 Mechanical tests

In order to determine the mechanical properties of cast iron samples, hardness measurement, tensile and

notched impact tests were carried out. Hardness values of the materials were obtained by macro Vickers

hardness tester as HV10. Tensile and notched impact tests were carried out at room temperature according

to TS 138 EN 10002-1 and TS EN 10045-1 standards, respectively. All the mechanical data obtained are

listed in Tab. 2.

Table 2. The mechanical properties of cast iron materials.

Materials Tensile strength, MPa Hardness, HV10 Impact toughness, J/mm2

GG20 162 187 7.2

GG25 123 159 7.8

2.3 Metallographic sample preparation and microscopic examinations

All samples for the microstructural characterization were prepared by grinding with 320, 600 and 1000 mesh

size SiC abrasives, respectively and then ground surfaces were polished using 3 μm diamond solution.

Etching was carried out with nital (% 3 HNO3) in order to determine the phases within the matrix. Zeiss

Axiotech 100 light microscope (LM) and Jeol JSM 6060 scanning electron microscope (SEM) were used for

metallographic examinations. SEM was used for fractographic analysis as well.

2.4 Image analysis

Leica QWin software package was used to determine the amount of graphite by means of image analysis.

The average and standard deviation values of this spacing were calculated from at least 60 measurements

on images of etched specimen surfaces.

3. RESULTS

3.1 Microstructural characterization

Figure 1 shows the microstructure of polished and etched cast iron materials. The flakes in dark contrast are

clearly seen in the polished matrices (Fig. 1a and c). The flakes can be broken during grinding due to the

contact with abrasives which are harder and polished microstructures exhibit globular/semi-globular regions

indicating the dropped ones. The matrices of GG20 and GG25 materials have typical pearlitic structure

which is a common phase for grey cast iron family (Fig. 1b and d). Its lamellar structure and amount affect

the mechanical properties of the material. As the distance between the lamellas in the pearlite decreases the

mechanical properties of material increase. An increase in the amount of this phase also causes an increase

in the mechanical properties. The amount of carbon has an important role on the formation of pearlite and

GG20 has a higher carbon concentration giving higher mechanical properties.


(a)

(b)

(c)

(d)

Figure 1. As polished (a and c) and etched (b and d) microstructures of cast iron materials; (a and b) GG20,

(c and d) GG25. Both of the materials have flake graphites in pearlitic matrix.

3.2 Graphite distribution

All gray cast irons contain flake graphite dispersed in a silicon-iron matrix. Amount of the graphite, its

distribution in the matrix and length of the flakes directly influence the properties of the iron. The basic

strength and hardness of the iron are provided by the metallic matrix. Graphite has little strength or hardness

and it decreases these properties of the metallic matrix. Image analysis shows that GG20 material has 8.676

% graphite in its matrix and GG25 material has only 7.483 %. Figure 2 shows selected ones for the

measurement. The amount of graphite in GG20 material is higher than GG25 due to the higher carbon

content in its composition.

(a)

(b)

Figure 2. The microstructures showing the amount of graphite in (a) GG20 and (b) GG25 materials.


3.3 Mechanical properties

The microstructure of grey cast iron is characterized by graphite lamellas dispersed into the ferrous matrix.

Foundry practice influences the nucleation and growth of graphite flakes, thus their size and type may

enhance the desired properties. The amount of graphite and size, morphology and distribution of graphite

lamellas are critical in determining the mechanical behavior of grey cast iron [1].

For a given pearlitic matrix, three concepts must be clarified to understand the matrix strengthening. Carbon

is the major element for interstitial strengthening mechanism in ferrous based materials. An increase in the

amount of carbon causes the development of the mechanical properties (hardness, strength, not toughness)

of the matrix. On the other hand, a fully pearlitic matrix can be assumed as a precipitation hardened matrix

having ferrite and cementite phases in lamellar morphology. As it is well known from the principles of

physical metallurgy, the strength of pearlite strongly depends on its interlamellar spacing. As the spacing

decreases the strength (hardness) of the matrix increases.

The mechanical properties of GG20 and GG25 cast iron materials are given in Tab. 2. In fact, all these

concepts simply indicate why the mechanical properties of GG20 cast iron is higher than those of GG25 cast

iron. Higher carbon results in higher hardness and strength in the matrix, however, impact toughness values

indicate the inverse correlation. This is possibly due to the effect of higher graphite amount in GG20 (Fig. 2).

3.4 Fractographic analysis

Hornbogen reported that the mechanical properties and also fracture behavior of grey cast irons have

captured attention due to two points of view; (i) in the case of sufficient toughness some cast irons will

compete successfully with steels and (ii) grey cast irons may serve as model materials due to their

microstructure consisting of metallic matrix with graphite which provides quasi-holes of different sizes, shape

and volume fraction [11]. Bradley et al. studied the fracture behavior of different types of cast irons: namely,

grey, white, ductile, malleable and compacted graphite (vermicular) cast irons and reported that the fracture

behavior and the fracture toughness of mentioned cast irons depend on the graphite morphology as well as

the matrix microstructure [12]. Gonzaga et al. investigated the dependency of mechanical properties on the

pearlite content of ductile irons and concluded that (i) a mixture of ferrite and pearlite or a full pearlitic

microstructure with good mechanical properties can be obtained, (ii) the higher hardness values appear in

the cast iron having 100% pearlite, (iii) pearlite hardens the matrix but elongation is lowered while the tensile

strength is increased as consequence of increasing pearlite [13].

The crack formation and propagation result in failure for a given material under loading. The secondary

phases having microvoids with their matrix are commonly responsible for the crack formation/propagation.

Fig. 3 shows the weak bonding between graphite and pearlitic matrix and there are several microvoids

surrounding the graphite. Bertolino et al. reported that lamellar grey cast iron presents a non-lineal load-

displacement record. This macroscopic behaviour has originated from the complex microstructure of these

materials that are formed by two phases (metallic matrix with distributed graphite lamellaes) that induce a

non-uniform distribution of stresses. The graphite lamellaes act as stress raisers producing self

microcracking, or interface debonding at very low stress levels and plastic deformation in the matrix. As a

consequence, the deformation mode of this type of material containing a crack, notch or defect has to be

characterized by the presence of a damaged region ahead the notch formed by a network of microcracks.

The propagation of a macroscopic crack will result from the coalescence of these microcracks together with

the matrix plastic deformation that surrounds the microcracks [14]. The tensile fracture surfaces of

experimental cast irons are given in Fig. 4 and they exhibit typical cleavage fracture. For a cast iron having

fully pearlitic matrix in addition to graphite, cleavage rupture is the dominant fracture under tensile loading.

The fracture of flake graphite cast iron mainly goes through the graphite matrix interface and the inside of the

graphite [15]. The effect of flake on the decohesion is simply illustrated in Fig. 5. All fracture surfaces in Fig.

5 exhibit cleavage type fracture indicating that there is no significant plastic deformation under loading.


(a)

(b)

Figure 3. The interfaces between graphite and matrix having microvoids; (a) GG20 and (b) GG25 materials.

(a)

(b)

Figure 4. Tensile fracture surfaces of (a) GG20 and (b) GG25 materials.

(a)

(b)

(c)

(d)

Figure 5. Impact fracture surfaces of (a and b) GG20 and (c and d) GG25 materials.


4. CONCLUSIONS

In this study, the microstructural characterization of GG20 and GG25 gray cast iron materials were carried

out and their fracture behavior was examined. The results are given as follows;

(i) Both GG20 and GG25 cast iron have pearlitic matrix having graphite flakes and image analysis showed

that GG20 material has higher graphite due to higher carbon content.

(ii) Carbon content, interlamellar spacing and also precipitated matrix directly affect the mechanical

properties of a given pearlitic matrix. GG20 cast iron has higher hardness and strength values than that of

GG25 cast iron. However, impact toughness of GG25 material is higher than that of GG20 due to low

amount of the graphite in its matrix. Fractographic analysis showed that cast irons exhibited similar fracture

surfaces after tensile/impact loading. Cleavage rupture is the dominant fracture type and there is no

indication of plastic deformation.

ACKNOWLEDGMENT

The authors wish to acknowledge Gedik Casting & Valve Inc. within the Gedik Group for their support in

mechanical tests.

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