S. DERVISIC et al.: INFLUENCE OF AUSFERRITE MICROSTRUCTURE DECOMPOSITION ...
393–396
INFLUENCE OF AUSFERRITE MICROSTRUCTURE
DECOMPOSITION ON THE CORROSION PROPERTIES OF
AUSTEMPERED DUCTILE IRON
VPLIV RAZPADA AUSFERITNE MIKROSTRUKTURE NA
KOROZIJSKE LASTNOSTI AUSTEMPRANE DUKTILNE
NODULARNE SIVE LITINE
Sehzudin Dervisic
1
, Hasan Avdusinovi}
2*
, Almaida Gigovic-Gekic
2
,
Dejana Kasapovi}
2
, Sead Pa{i}
1
1
Faculty of Mechanical Engineering, Matice hrvatske b.b, Mostar 88000, Bosnia and Herzegovina
2
Faculty of Metallurgy and Technology, Travni~ka cesta 1, 72000 Zenica, Bosnia and Herzegovina
Prejem rokopisa – received: 2019-09-18; sprejem za objavo – accepted for publication: 2019-12-23
doi:10.17222/mit. 2019.224
Austempered ductile iron (ADI) belongs to the heat-treated class of ductile iron. The heat treatment consists of austenitization
and the tempering process. The microstructure of an ADI sample is ausferrite consisting of acicular ferrite, carbon-saturated
austenite and a graphite phase in the shape of nodules. The corrosion properties of ADI samples depend on the microstructure
constituents and stability of the microstructure. In this paper, the influence of ausferrite microstructure decomposition on the
corrosion properties of the ADI samples are presented. During the research, Tafel curve extrapolation and potentiodynamic
polarization were used. It was found that the ausferrite microstructure decomposition very strongly affected the general
corrosion behavior of the ADI samples.
Keywords: ausferrite, decomposition, general corrosion, austempered ductile iron
Duktilna nodularna siva litina izdelana s postopkom "austemperiranja", je posebna vrsta duktilne litine. Austempering (izraz se
je udoma~il tudi pri nas) je posebna vrsta toplotne obdelave, s katero izbolj{amo lastnosti litine. To je postopek, pri katerem so
postopki austenitizacije, hitrega ohlajanja in popu{~anje, zdru`eni v eno operacijo. Mikrostruktura austemprane litine je
sestavljena iz acikularnega (igli~astega) ferita, z ogljikom nasi~enega austenita in grafitne faze v obliki nodul (kroglic).
Korozijske lastnosti tak{ne litine so odvisne od mikrostrukturnih sestavin in stabilnosti mikrostrukture. V ~lanku avtorji
opisujejo vpliv termi~nega razpada ausferitne mikrostrukture na korozijske lastnosti litine. Med postopkom raziskovanja so
avtorji uporabili Tafelove ekstrapolacijske krivulje in tehniko potenciodinami~ne polarizacije. Avtorji ugotavljajo, da termi~ni
razpad ausferitne mikrostrukture zelo mo~no vpliva na splo{ne korozijske lastnosti duktilne nodularne sive litine.
Klju~ne besede: ausferit, razpad, splo{na korozija, austemprana duktilna litina
1 INTRODUCTION
Austempered ductile iron (ADI) belongs to the class
of ductile iron that exhibits high strength and ductility
because of its microstructure being controlled through
heat treatment. ADI offers a superior combination of
properties. It can be cast like any other type of ductile
iron, thus providing all the production advantages of a
conventional ductile-iron casting. Subsequently, it is
subjected to an austempering process to develop the
mechanical properties that are superior to those of the
conventional ductile iron, cast and forged aluminum and
many cast and forged steels. The mechanical properties
of ADI are primarily determined by the metal matrix.
The ADI matrix consists of acicular ferrite and carbon-
stabilized (saturated) austenite, called ausferrite. The
austempering process is neither new nor novel and has
been utilized since the 1930s on cast and wrought steels.
The austempering process was first commercially
applied to ductile iron in 1972 and its world-wide usage
started during 1990s.
1
Good-quality ductile-iron castings with the following
characteristics should be used as the base for an ADI
production:
• A consistent chemical composition;
• Castings free of carbides, porosity and inclusions;
• Nodularity >80 %;
Nodule count: a minimum of 100/mm
2
;
• A consistent metallic-matrix microstructure (the
pearlite/ferrite ratio).
ADI covers a group of materials whose mechanical
properties can be varied over a wide range by selecting a
suitable heat treatment. Figure 1 shows a heat-treatment
diagram for the ADI production.
A high austempering temperature (400 °C) allows the
production of the ADI with a high ductility, the yield
strength in a range of 800 MPa, good fatigue and impact
strength. A lower transformation temperature (260 °C),
results in the ADI with a very high yield strength of 1400
Materiali in tehnologije / Materials and technology 54 (2020) 3, 393–396 393
UDK 67.017:620.194:669.131.6 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(3)393(2020)
*Corresponding author's e-mail:
hasan.avdusinovic@mtf.unze.ba (Hasan Avdusinoviæ)

MPa or higher, high hardness, excellent wear resistance
and contact fatigue strength. Thus, through a relatively
simple control of the austempering conditions, ADI can
be given a range of properties unequaled by any other
material. Like other ductile-iron specifications, ASTM
A897 defines the minimum tensile properties for diffe-
rent grades of ADI.
2
The European standard that defines
the austempered-ductile-iron production and properties
is EN 1564:1997.
3
Additionally, as a consequence of the unique proper-
ties of ADI such as high strength, fatigue behavior and
ductility, ADI has been considered a very up-and-coming
engineering material and it is an economical replacement
for forged steel, or even aluminum alloys, in several
structural applications in the automotive industry (trans-
mission gears, crankshafts, connecting rods), defense
industry (aircraft landing gears, cannon shells) and
railroad industry. It is important to study the corrosion
behavior of this material while it is exposed to acid or
chlorine media as the automotive and aircraft parts made
of ADI are in continuous contact with such media. R.
Rocha-Reséndez et al.
4
stated that the ADI austempered
at a higher temperature showed a better corrosion resist-
ance than the ones austempered at a lower temperature;
the complex nature of the corrosion of ADI is influenced
by the contents of ferrite and retained austenite; it was
also reported that the percentage of retained austenite in
ADI influences its corrosion behavior – the higher the
percentage of retained austenite, the lower is the corro-
sion rate
.
Taking into consideration that the corrosion
properties depend on microstructure characteristics, the
stability of a microstructure during exploitation is very
important. The main goal of the investigation described
in this paper is to test the general corrosion properties
after the ausferrite microstructure decomposition of the
ADI samples reheated at different temperatures.
2 EXPERIMENTAL PART
2.1 ADI-sample preparation
The chemical composition of the base ductile iron
used for the ADI samples is presented in Table 1.
Table 1: Chemical composition of the base ductile iron (w/%)
CS iM nPSC rC uM g
3.56 2.09 0.482 0.011 0.003 0.111 0.340 0.040
The material was poured into U-shape castings. The
pearlite-ferrite microstructure of the base ductile iron is
presented in Figure 2.
After the cooling of the castings, seven prismatic
samples of 15×25×50 mm were cut and heat treated for
the ADI production. The heat-treating procedures were
as follows: austenitizing at 870 °C for 90 min, rapid
cooling to the temperature of isothermal transformation
(370 °C), holding the samples at the temperature of iso-
thermal transformation for 90 min in a KNO
3
salt bath
and cooling them to room temperature in air. The heat-
treatment diagram for the ADI samples is presented in
Figure 3.
S. DERVISIC et al.: INFLUENCE OF AUSFERRITE MICROSTRUCTURE DECOMPOSITION ...
394 Materiali in tehnologije / Materials and technology 54 (2020) 3, 393–396
Figure 1: Austempering-heat-treatment diagram
Figure 3: Heat-treatment diagram for the ADI-sample production
Figure 2: Microstructure of the base ductile iron, 100× mag., Nital
etched

2.2 Additional heat treatment of the ADI samples
In case of additional heat treatment of the ADI sam-
ples at an elevated temperature, the decomposition of the
ausferrite microstructure occurs. To identify the aus-
ferrite microstructure decomposition during the reheat-
ing, six ADI samples were reheated up to the transform-
ation temperature (T
t
) and held at it for 120 min; then the
samples were rapidly removed from the furnace and air
cooled to room temperature. For the experiment, six
different reheating temperatures were preset. The dia-
gram of reheating the ADI samples is presented in
Figure 4.
2.3 Test of the corrosion behavior of the ADI samples
After the additional heat treatment of the ADI sam-
ples, corrosion-behavior tests were prepared. Seven
samples (the basic ADI sample and six heat treated ADI
samples) with 10×3 mm were cut. Before the corrosion
tests, the preparation of the samples was carried out
using grinding paper of different grades. After the grind-
ing, final polishing was carried out, using 0.05 and
0.03-μm Al
2
O
3
water solutions. To test the corrosion rate
of the prepared ADI samples, electrochemical techniques
involving potentiostatic polarization were used (Tafel
curve extrapolation and potentiodynamic-polarization
technique). The survey was conducted on a Potentiostat/
Galvanostat, Princeton Applied Research Model 263A-2,
using software package PowerCORR®. The samples
were treated ina1%solution of hydrochloric acid at
room temperature (20–22 °C).
5,6
3 RESULTS
3.1 Microstructure characterization of the ADI sam-
ples
After the basic and additional heat treatment of the
ADI samples, metallographic investigations were carried
S. DERVISIC et al.: INFLUENCE OF AUSFERRITE MICROSTRUCTURE DECOMPOSITION ...
Materiali in tehnologije / Materials and technology 54 (2020) 3, 393–396 395
Figure 5: Microstructures of the ADI samples (SEM, 10,000× mag., Nital etched): a) basic ADI sample, b) ADI sample reheated at 250 °C
for 2 h, c) ADI sample reheated at 350 °C for 2 h, d) ADI sample reheated at 450 °C for 2 h, e) ADI sample reheated at 500 °C for 2 h, f) ADI
sample reheated at 600 °C for 2 h, g) ADI sample reheated at 700 °C for 2 h
Figure 4: ADI-sample reheating diagram

out. In Figure 5, SEM (scanning electron microscopy)
microstructures of the used samples are presented.
3.2 Corrosion test of the ADI samples
Table 4 and Figure 5 present the results of the tests
of the corrosion rate using the technique of Tafel curve
extrapolation.
Table 4: Current density and open-circuit potential of the tested
samples
Sample
Current density i
μA/cm
2
Open circuit potential
E(i=0) /mV
Base ADI 7.725·10
1
–467,18
S1
4.491·10
1
–473.08
S2 5.132·10
1
–470.37
S3 7.038·10
1
–471.19
S4
3.034·10
2
–478.04
S5
2.958·10
2
–483.66
S6 1.145·10
2
–488.88
4 DISCUSSIONS
After the heat treatment of ductile iron ADI, samples
with ausferrite microstructures were produced. The
resulting ausferrite microstructure was found to have a
uniform distribution of a needle-shaped ferrite phase
within the carbon-saturated austenite phase. The addi-
tional reheating of the ADI samples at different tempera-
tures led to an ausferrite decomposition that is visible on
Figure 5. With the increasing reheating temperature, the
transformation rate of carbon-saturated austenite in-
creases. Carbon-saturated austenite ( CS
) transforms into
ferrite and iron carbide according to Equation (1):
 CS
=  +Fe
3
C (1)
Table 4 and Figure 6 show different corrosion pro-
perties of the treated ADI samples. The highest current
density is found for ADI samples S
4
and S
5
. An increase
in the current density indicates a decrease in the corro-
sion resistance of the treated samples. There was no big
difference in the open-circuit potential values for the
observed samples. The samples with the increased level
of the current density (samples S
4
,S
5
and S
6
) had lower
values of the open-circuit potential.
Generally, several effects could be identified as the
reasons for this behavior, but the decomposition of the
ausferrite microstructure in the temperature region above
450 °C and the presence of the carbide phase in the
microstructure were the most pronounced. In the tempe-
rature region near the eutectoid temperature, graphiti-
zation (decomposition of the iron carbides) occurred and
it could be the reason for the decrease in the current
density during the corrosion test of sample S
6
.
5 CONCLUSIONS
The carried out investigation and analysis of the
results of the experiments on the reheated ADI samples
allow the following conclusions:
• The paths of the decomposition of austempered duc-
tile iron, reheated up to 700 °C and its influence on
the general corrosion behavior were recognized and
summarized in Figure 5 and Table 4.
• The most intensive decomposition of the ausferrite
microstructure was observed in the temperature
region above 450 °C.
• The decomposition of the ausferrite microstructure
had a negative influence on the general corrosion
behavior of the tested samples.
• The lowest corrosion resistance (the highest current
density) was recorded for samples S
4
and S
5
, pre-
sented in Table 4 and Figure 6.
6 REFERENCES
1
J. R. Keough, Austempered Ductile Iron, Ductile Iron Society,
Strongsville, Ohio, USA, 1998
2
https://www.ductile.org/didata/Section4/4intro.htm, 29.08.2019
3
European standard: EN 1564: 1997
4
R. Rocha-Reséndez, J. A. Cabral-Miramontes, C. Gaona-Tiburcio, P.
Zambrano-Robledo, F. Estupiñan-López, F. Almeraya Calderon,
Proceedings of the Symposium of Aeronautical and Aerospace Pro-
cesses, Materials and Industrial Applications, Springer International
Publishing AG 2018, doi:10.1007/978-3-319-65611-3_7
5
M. Janji}, H. Avdu{inovi}, Z. Jurkovi}, F. Biki}, S. Savi}evi}, In-
fluence of austempering heat treatment on mechanical and corrosion
properties of ductile iron samples, Metallurgy, 55 (2016) 3, 325–328
6
C. H. Hsu, J. K. Lu, K. L. Lai, M. L. Chen, Erosion and Corrosion
Behaviors of ADI Deposited TiN/TiAlN Coatings by Cathodic Arc
Evaporation, Materials Transaction, 46 (2005) 6, 1417–1424
S. DERVISIC et al.: INFLUENCE OF AUSFERRITE MICROSTRUCTURE DECOMPOSITION ...
396 Materiali in tehnologije / Materials and technology 54 (2020) 3, 393–396
Figure 6: Current density of the tested samples