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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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