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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(2): 31-35

Fracture Characteristics of Austempered SpheroidalGraphite Aluminum Cast Irons

Masoud Zandira' • Seyyed Mohammad Ali Boutorabi'(1. Department of Materials and Manufacturing Technology, Chalmers University of Technology, Goteborg 41296, Sweden;

2. Center of Excellence for Advanced Materials, Iran University of Science and Technology, Tehran 16844, Iran)

Abstract: The fracture characteristics of austempered spheroidal graphite aluminum cast iron had been investigated.

The chemical content of the alloy was C 3. 2, A12. 2, Ni O. 8 and Mg O. 05 (in mass percent, %). Impact test samples

were produced from keel blocks cast in CO, molding process. The oversized impact samples were austenitized at 850

and 950'C for 2h followed by austernpering at 300 and 400'C for 30,60, 120 and 180 min. The austempered sam­

ples were machined and tested at room temperature. The impact strength values for those samples austempered at

400'C varied between 90 and 110J. Lower bainitic structures showed impact strength values of 22 to 50J. The frac­

tures of the samples were examined using SEM. The results showed that the upper bainitic fracture revealed a honey

Comb-like topography, which confirmed the ductile fracture behavior. The lower bainitic fractures of those samples

austempered for short times revealed brittle fracture.

Key words: austempering , fracture characteristic; aluminum cast iron

All ferrous materials other than those having an

austenitic matrix may fail in a ductile or a brittle

manner according to the method of testing and the

geometry of the test piece. the temperature of tes­

ting. the structure and the composition of the mate­rial[l-3]. The randomly oriented crystals or grains of

metal undergo plastic deformation by a process of

slip which is due to a shear stress caused by the la­mellae of the crystal sliding over one another[4-5].

Failure by slip only occurs after a large amount of

plastic deformation and will be accompanied by a

considerable elongation of the test piece in a tensile

test. Besides. crystals sometimes fail by cleavage

due to separation on a plane known as a cleavage

plane. Such failure is of a sudden brittle nature and

occurs when the tensile stress. normal to the cleavage

plane. exceeds the cohesive strength of the material.

In a tensile test. when a crack is first initiated.

there is a considerable amount of elastic energy

stored in the specimen. as a large volume of material

is stressed to the maximum stress level. This energy

is released when failure starts and serves to propa­

gate the crack. The release of elastic energy speeds

up crack propagation. and depending on the chemi­

cal composition. a certain speed may be exceeded

causing the fracture to change from ductile state to

brittle state.

Works on the fracture characteristics of austern­

pered spheroidal graphite irons are only occasionally

mentioned in the literature and detailed investiga­

tions into the subject are scarce-'". This is particularly

true. in Fe-C-Al-Mg austempered ductile irons. The

previous works on this iron showed that. aluminum

is a strong graphitizer and. similar to silicon. canproduce highly stable retained austenite[7-B]. The

aim of this work is to examine the impact strengths

and fracture characteristics of austempered Fe-C-AI­

Mg alloy.

1 Procedure

The iron was melted in a medium frequency in­

duction furnace. The mechanism of Al addition. Mg

treatment. and inoculation were detailed in the pre­vious papers[7-9]. The keel blocks were cast. fol­

lowed by annealing and machining into impact test

pieces. The samples were austenitized at 850. 900

Biography: Masoud Zandirat 1983-), Male, Master Candidate; E-mail: zandira@student.chalmers.se; Received Date: October 8. 2008

• 32 • Journal of Iron and Steel Research. International Vol. 17

and 950'C for 2 h , and austempered at 300. 350,

400. and 450 'C for different times. They were ma­

chined to standard sizes. The Izod impact test was

conducted and the fracture surface of the broken

samples was examined by SEM in both as-cast and

austempered conditions.

2 Results and Discussion

The impact strengths of austempered Fe-C-Si­

Mg alloys show similar trend to Fe-C-Si-Mg alloys.

The range of values varies between 90 and 110J at

upper bainitic structure and at lower bainitic condi­

tion. the impact values range between 22 to 50 J.Fig. 1 shows the impact energy values correla­

ted with volume fraction of retained austenite.

Highly reacted retained austenite is a tough and duc­

tile phase which increases the impact strength in

ductile austempered aluminum cast iron.

This could be explained by the strong graphiti­

zing effect of Al in Fe-C-AI-Mg system, and con-

120 45 ~

100~'2

~a5 ~

SZen

80 ='... esOi 25 ....c: 0Oi 60 Impact energy c:"to 0ee 'J:!0. 40 Volume fraction of austenite 15 ~

oS <l::Oi

20 5 55 ao 60 120 180 240 300

.g:>

Austernpering time/min

Fig. 1 Volume fraction of retained austenite and impactenergy as a function of austempering time

firms the previous investigations carried out on Fe­

C-AI-Mg alloys. The as-cast structure and the frac­

tograph of SG Fe-C-AI-Mg cast iron are shown in

Fig. 2 to Fig. 4.

Fig. 5 shows a typical Iractograph of specimens

austenitized at 900 'C and austernpered at 300 'C. The

Fig. 2 As-cast structure of SG Al cast iron showing fully

pearlitic and dendritic microstructure

Fig. 3 Fractograph of as-cast SG Al cast iron

Fig. 4 Fractograph of as-cast SG Al cast iron showing arms of dendrites of primary austenite

fracture is transgranular in nature with small amounts of

intergranular fracture. Specimens austempered at

different temperatures show similar fracture sur­

faces, a fact which accounts for the consistently low

fracture toughness (in Fig. 6),

Fractographs of specimens austempered at up­

per bainitic range are shown in Fig. 7 and Fig. 8.

Generally. however, surfaces of specimens austern­

pered at 400 'C are the most ductile and show exten­

sive microductility and very few microvoids, The

Issue 2 Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons • 33 •

fracture of specimens austempered at 300 'C is mainly

intergranular in nature.

The high ductility of specimens austempered at

400C can plausibly be explained in terms of the

large contents of stable retained austenite, while the

martensite and large unstable austenite grains respec-

Fig. 5 Fractographs of specimens austenitized at 900 t: and austempered at 300 t: for 60 min.

showing brittle fracture for different magnifications

Fig. 6 Fractographs of specimens austenitized at 950 t: and austempered at 300 t: for 120 min.

showing brittle fracture for different magnifications

Fig. 7 Fractographs of specimens austenitized at 850·C and austempered at 400 t: for 30 min showing ductile fracture

• 34 • Journal of Iron and Steel Research. International Vol. 17--- ------ -- ---- -- ------------------------

Fig. 8 Fractographs of specimens austenized at 950°C and austempered at 400"C for 180 min showing ductile fracture

tively in those austempered 300e condition may ac­count for the more brittle Iractures'"" Jr,j.

Specimens austenitized at 900C and austern­

pered at 300 C for 60 min show little evidence of

ductile fracture (in Fig. 5). However. fracture

seems to have occurred predominantly by cleavage

mixed with tearing. The fracture surface is charac­

terized by small poorly defined cleavage facets con­

taining river patterns and separated by areas of tear­

ing. In addition. shallow small dimples situated

within cleavage facets are also visible.

This interesting fract ure mode is also referredto as quasi cleavage! 1.IIJ. The causes of this type of

fracture are intimately related to the microstructure.

It has been established that austernpering at 300 Cresults in the early post bainitic decomposition of re­

tained austenite into discrete aggregates of carbideand ferrite confined between ferrite plates l , . <1.

12 1.

These embrittling carbide precipitates facilitate crack

propagation. Hence. the available cleavage planes

within a wholly bainitic grain may be poorly defined

and true cleavage planes have been reduced by smal­

ler ill defined cleavage facets.

A similar fracture mode occurred in specimens

austenitized at 950 C and austempered at 300C for

120 min (in Fig. 6). However. because the structure

of this alloy contained considerable amounts of mar­

tensite (confined mainly in cell boundaries and form­

ing a continuous network throughout the structure) •

smooth and large recessions appear in these areas

where block separation might have taken place.

The fracture surface of specimen austenitized at

850 C and austempered at 400'C for 30 min (in

Fig.7) reveals a mixture of cleavage plus dimple

fractures where regions of small cleavage facets are

interspersed with minute dimples. Although this al­

loy has the highest ductility (9 % elongation) amongst

heat treated alloys. the fracture mechanism is not

fully ductile which serves to demonstrate the com­

plexity of the relationship between mechanical prop­

erties. microstructures and fracture characteristics.

Carbide precipitation between bainitic ferrite

and retained austenite phases is the main reason for

relatively low impact strength values of austempered

Fe-C-Al-Mg alloys. TEM study on this iron clearly

shows the formation of carbides in bainitic ferrite

and retained austenite interface in those samples

austenitized at 950 -C and austempered at 400"C for5 h (in Fig. 9)11'--IIJ.

In conclusion. it has been shown that although

Fig. 9 X-carbide observed by TEM at the ferrite-austenite boundary in samples austenitized at

950"C and austempered at 400°C for 5 h

Issue 2 Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons • 35 •

only a few specimens are considered, the main frac­

ture characteristics in austempered spheroidal graph­

ite aluminum irons correlate well with the micro­

structures and impact properties. However, further

work of a more detailed nature is required in this im­

portant field for correlating the microstructure withfracture mechanisms.

3 Conclusions

1) Austempered spheroidal graphite aluminum

cast irons show a mixed mode of fracture in which

ductile fracture coexisted with transgranular fracture.

2) Fracture cracks are mainly nucleated at

graphite nodule-matrix interface by decohesion ofthe matrix from graphite nodules.

3) In comparison to austempered Fe-C-Si-Mg

ductile irons, similar impact strength values can beobtained in Fe-C-AI-Mg alloys.

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