J Mater Sci (2009) 44:1069–1075

DOI 10.1007/s10853-008-3203-z

Effect of austempering treatment on microstructure

and mechanical properties of high-Si steel
D. Mandal Æ M. Ghosh Æ J. Pal Æ P. K. De Æ
S. Ghosh Chowdhury Æ S. K. Das Æ G. Das Æ
Sukomal Ghosh

Received: 8 August 2008 / Accepted: 17 December 2008 / Published online: 14 January 2009
Ó Springer Science+Business Media, LLC 2009

Abstract In the present investigation, the influence of conditions. In recent years, the development of cast aus-
austempering treatment on the microstructure and mechan- tempered steels gained interest in automobile industries
ical properties of silicon alloyed cast steel has been because of unique combinations of properties achieved
evaluated. The experimental results show that an ausferrite through alloying with inexpensive elements. Researches
structure consisting of bainitic ferrite and retained austenite have been directed towards the study of austempered high-
can be obtained by austempering the silicon alloyed cast steel silicon cast steel having similar austempered structure (aus-
at different austempering temperature. TEM observation and ferrite structure) [1, 3]. This kind of steel shows better
X-ray analysis confirmed the presence of retained austenite mechanical properties in respect of higher strength, hard-
in the microstructure after austempering at 400 °C. The ness and toughness in comparison to ADI. It also exhibits
austempered steel has higher strength and ductility compared higher life expectancy.
to as-cast steel. With increasing austempering temperature, It is well known that conventional bainitic (austem-
the hardness and strength decreased but the percentage of pered) structure is an aggregate of bainite and carbide.
elongation increased. A good combination of strength and However, research reports indicate that the bainitic struc-
ductility has been obtained at an austempering temperature ture in high silicon containing steels consists of bainite
of 400 °C. plates and lath or interlath thin films of carbon-enriched
retained austenite instead of carbide because silicon
strongly retards the formation of carbide [4–7]. The car-
bon-enriched retained austenite undergoes strain-induced
Introduction transformation to martensite [8, 9], greatly increasing the
strain hardening rate, uniform elongation and the ultimate
The study of austempered ductile cast irons (ADI) is a tensile strength. Therefore, the bainitic structure consisting
world-wide hot point [1–3], as ADI shows excellent com- of bainite and retained austenite is attractive in the devel-
prehensive mechanical properties explicitly higher strength opment of high-strength steel with good ductility [7, 10,
and toughness when compared with ductile irons. The most 11]. The amount of retained austenite present, after
notable limitation of ADI is the micro-segregation of some quenching from austenitising temperature, depends on the
elements such as manganese and phosphorus, which induce composition, cooling rate, austenite morphology, austem-
brittleness. The other constraint is that the graphite nodules pering temperature and time [12]. The formation of
in the ADI act as ‘crack source’ and reduce service life of retained austenite, its morphology and influence on the
components under heavy abrasive wear and impact mechanical properties have been studied by several
investigators [7, 10, 11, 13]. Bhadeshia and Edmonds [7]
reported that film like retained austenite was most desirable
D. Mandal (&)
M. Ghosh
J. Pal
P. K. De
morphology to get good combination of strength and
S. Ghosh Chowdhury
S. K. Das
G. Das
S. Ghosh
ductility. It had been reported that the austenite retention
Metal Extraction and Forming Division, National Metallurgical
Laboratory, Jamshedpur 831007, India was especially promoted by addition of Mn that stabilised
e-mail: durbadal73@yahoo.co.in; durbadal@nmlindia.org austenite and Si retarded cementite formation [14, 15].

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Silicon is a good ferrite strengthener. Manganese con- by applying 10-kg load for 15 s. Tensile tests were per-
tent below 2% has a beneficial effect on hardenability, formed according to the ASTM standard method using 50T
which contributes to solid solution strengthening. Simul- Instron machine at a constant strain rate of 0.5 mm min-1
taneous addition of small amount of micro alloying at room temperature. At least two tensile samples were
elements may further increase hardenability and precipi- tested under similar condition. Fracture surfaces of broken
tation strengthening. tensile samples were observed under scanning electron
The aim of this study is to develop high-silicon cast steel microscopy to identify the mechanism of fracture of this
with microalloying addition and investigate the correlation alloy.
between microstructure and mechanical properties of this
steel at various austempering temperature.
Results and discussion

Experimental procedure Optical microstructure analysis

In the present investigation, low-carbon high-silicon steels As cast microstructure of steel (Fig. 1) shows a predomi-
(10 kg heat) were prepared through induction melting. nant ferrite matrix with pearlite dispersed in it. Figure 2a–c
After melt down of low-carbon steels, Fe–Si, Fe–Mn and show the microstructure at various austempering tempera-
Fe–V were added in calculated amounts to achieve the tures (350, 400 and 450 °C) while the austenitising
composition of the steels. The chemical composition of the temperature (900 °C) and austempering time (10 min)
steel is shown in Table 1. remained constant. The austempered microstructure shows
The cast blocks (120 mm 9 90 mm 9 38 mm) were a matrix consisting of two phases. A dark etched bainite,
homogenised at 1,000 °C for 6 h. The samples (120 mm 9 which is of needle shape, and ferrite (white phase) are
90 mm 9 8 mm) were austenitised at 900 °C for 30 min in a observed in the microstructure. Most of the bainites shown
resistance heating furnace and subsequently quenched into a in these figures have lath morphology. It has been observed
NaNO3–KNO3 salt bath capable of maintaining different that the length of bainites remained almost equal with
austempering temperatures such as 350, 400 and 450 °C. increasing austempering temperature yet width increased.
The samples were isothermally held for 10 min followed by At lower transformation temperature, the bainite laths
air-cooling. The samples were sectioned for microstructure are finer and the orientation of lath is irregular. Increasing
analysis, hardness and tensile strength measurement. the austempering temperature from 350 to 450 °C, the
The specimens were ground, polished and etched with bainitic ferrite laths widen and the carbon-enriched
nital (2%) solution for microstructure analysis. Optical and retained austenite between the adjacent bainite thickens.
electron microscopy were used to study the microstructure The initial stage of the transformation nucleates bainite at
of the specimens under different austempering conditions. the austenite grain boundary that grows in parallel plates or
The retained austenite volume fraction was determined by
X-ray diffraction. The angular range (2h) was 40°–130°
with Cu Ka radiation at 40 kV with a scanning rate of
1° min-1. Thin foil specimens were prepared for trans-
mission electron microscopy (TEM) studies from 1-mm
thick disc slit of austempered specimens. The discs were
thinned down to below 100 lm by abrasion on 1,200 grade
emery paper and then electro polished in a twin jet electro
polisher using a mixture of 5% perchloric acid and 95%
ethanol solutions at ambient temperature at about 50 V.
Hardness was measured with the help of Vickers hard-
ness tester. A square-base diamond pyramid (included
angle between the opposite faces of the pyramid is 136°)
has been used as an indenter. The hardness was measured

Table 1 Chemical composition of steel

Elements C Mn Si V P S Fe

wt% 0.13 1.03 1.2 0.08 0.042 0.05 Bal.

Fig. 1 Optical micrograph of the steel in as cast condition

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laths (Fig. 2a) with rejection of carbon into the austenite.

The microstructure of specimens, thus, reveals bainite and
a low level of retained austenite. The structure shows
higher strength but low toughness.
Some acicular bainite also have been observed at higher
austempering temperature. The average length of lath and
that of acicular ferrite are almost equal while these have
different thickness. In other words, the morphology of
bainite at the early stage of transformation is rather acicular
and changes to lath-like morphology as a consequence of
lateral growth as reported in the studies of austempered
ductile iron [16]. The bainite formed at the beginning of
transformation could have maximum lateral growth
because those are surrounded by austenite having lower
carbon content. The increase in carbon content of austenite
is a consequence of the bainite decarbonisation that, in
turn, reduces the driving force for lateral growth of bainite
at the last stage of transformation. The results of the
microstructural study show that austempering temperature
has a minor effect on the bainite length while bainite width
shows a pronounced variation with austempering temper-
ature. It is assumed that sheaves of bainite form with an
almost equal length and then lateral growth occurs.

TEM microstructure analysis

Bright field electron image (Fig. 3) shows general mor-

phology of bainitic lath in a sample austempered at 350 °C.
The bainitic lath is advancing parallel to the prior-austenite
grain boundary and the packet of lower bainite sheaves
originates from the prior-austenite grain boundary. The lath
boundaries are slightly wavy and lack clarity due to their
low-angle misorientation. The micrograph shows that the
bainite sheaves are composed of smaller subunits of

Fig. 2 Optical micrographs of steel isothermally transformed for Fig. 3 TEM micrograph of the steel isothermally transformed at
10 min at different temperatures: a 350 °C, b 400 °C and c 450 °C 350 °C for 10 min

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Fig. 4 TEM micrograph of carbide formation inside the bainite lath

austempered at 350 °C for 10 min Fig. 6 XRD pattern of the steel austempered at 400 °C for 10 min

bainite. In the packet of lower bainite, the widths of the

subunits are very fine (25 lm) and reveal high angle lath retention time of 10 min. It is well established that silicon
boundaries. The interior decoration of a subunit has been inhibits the formation of cementite during bainite trans-
elaborated through a magnified bright field electron image formation and encourages the formation of carbon-
shown in Fig. 4, where the dislocation density in the lath is enriched retained austenite [17] after partial transformation
very high. The lower morphology of bainite subunit is due to bainite. The present high-carbon austenite did not
to carbon constraints imposed by the matrix commonly transform completely to martensite.
observed (shown in Fig. 4), such microstructure is a result The stability of this retained austenite is attributed to the
of the displacive transformation mechanism. high carbon and manganese content in the steel. Some of
In Fig. 5, the TEM photography shows a thin film of the austenite, adjacent to the grain boundaries transformed
retained when austempered at 400 °C for 10 min. The to bainite first after quenching at 400 °C in the salt bath,
presence of retained austenite is also confirmed through the formation of bainite caused the remaining austenite to
X-ray diffraction analysis. X-ray diffraction pattern is be enriched with carbon. During isothermal transformation
shown in Fig. 6. It is seen that the amount of retained at 400 °C, bainite grew and the remaining austenite was
austenite in this steel reaches a maximum of 6% at the further enriched with carbon. On cooling to room tem-
isothermal transformation temperature of 400 °C and at a perature, part of the austenite was retained, whereas the rest
transformed into martensite. The retained austenite in the
present isothermal transformed specimens was smaller in
size than what was observed by the other investigators
[13, 18]. Saleh and Priestner [13] observed that the width
of retained austenite was a few microns in size and it was
spaced uniformly in the bainite. In the present investiga-
tion, no carbides were identified in the structure because
the presence of substantial amount of silicon delayed the
cementite transformation from austenite. Silicon not only
stabilised bainite in the austempered structure but also
prohibited the formation of cementite.

Mechanical properties

Variation of mechanical properties with austempering

Temperature:

Fig. 5 TEM micrograph of the steel austempered at 400 °C: film-like The mechanical properties of high-silicon steel austem-
retained austenite is visible between the bainite lath pered at various austempering temperatures are shown in

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Table 2 Mechanical properties of the steel at different austempering combined strengthening effects of dislocation density and
temperatures the ultrafine bainite grain size are very influential in the
Sample Heat treatment Hardness UTS % case of low-carbon bainitic steel. In bainite steel, retained
Condition (VHN) (MPa) El austenite is responsible for the increase in ductility. Yield
strength (YS), however, is found to be low due to the
Cast Homogenised 183 553 21
relative softness of the austenite.
35010 Austempered at 350 °C for 238 676 20
10 min
40010 Austempered at 400 °C for 224 663 27 Fracture surface analysis
10 min
45010 Austempered at 450 °C for 216 620 28 The fracture surface morphology of the specimens aus-
10 min tempered at different temperature and tested under
monotonic tensile loading, is shown in Fig. 8a–c. The
fracture surfaces of all the samples show cup and cone
formation that indicate ductile fracture. It is observed that
the dimple size increases but the number of dimples
decreases with increasing austempering temperature. Dur-
ing tensile testing, initially some microvoids develop in the
neck region. The microvoids usually nucleate at the regions
of localised strain, discontinuity such as grain boundary,
dislocation pile ups, second-phase particles inclusions etc.
With increased strain, the microvoids grew, coalesced and
eventually produced an internal crack by normal rupture.
The final separation of the materials occurred by shear
rupture, which produced the wall of the cup and cone.
Dimple rupture is the dominant mechanism of fracture as
shown in Fig. 8. The size of the dimple is governed by the
number and distribution of microvoids. When microvoids
nucleate at the grain boundaries or at dislocation pile-ups,
intergranular dimple rupture can occur. The maximum
Fig. 7 Stress versus strain curves of the steel as cast condition and at numbers of dimples on fracture surface are observed in
different austempering temperatures samples those are austempered at 350 °C. Increase in
austempering temperature enhances the dimple size by
Table 2. Representative stress and strain curves are shown decreasing the number of dimples. The fracture exhibits
in Fig. 7. Ultimate tensile strength (UTS) of austempered numerous cup-like depressions caused by coalescence of
steels is always higher compared to ‘as-cast’ steel. With an microvoids.
increase in austempering temperature, the tensile strength The effects of austempering temperature on the strength
and hardness decreased and the percentage of elongation, and ductility are shown in Fig. 7. The specimens were aus-
an index of ductility, increased. The optimum combination tempered at different temperatures and were kept at an
of strength and ductility was obtained at an austempering identical holding time (10 min). Therefore, any change in the
temperature of 400 °C for 10 min of holding time. The mechanical properties should be due to the effect of tem-
steel showed high-tensile strength, high hardness and pering temperature only. Figure 7) shows that the highest
reduction in ductility when austempered at 350 °C. Also, UTS is obtained at 350 °C, but the percentage of elongation
with austempering at 450 °C, the steel revealed a higher is low. The highest percentage of elongation is observed at
percentage of elongation, but reduction in the UTS was 450 °C austempering temperature with a low strength. A
observed. good combination of UTS and percentage of elongation is
The hardness and tensile strength of bainitic steel observed at an austempering temperature of 400 °C and
decrease with increasing austempering temperature. It is 10 min holding time. The microstructures of the austem-
observed that the strength at any stage of interrupted pered steel mainly govern the mechanical properties. The
tempering correlated well with the microstructure. These high strength at low-austempering temperature (350 °C) is
results imply that the strength depends mainly on carbide associated with the formation of high strength lower bainite,
dispersion strengthening but unfortunately, the grain size, which features dominantly in the structure.
particle size and distribution of dislocation density are not The strength is mainly influenced by the carbon content
independent parameters. It is an established fact that the in the bainite and the thickness of the bainitic laths. The

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ductility (%El) of the steel is principally decided by the

sub-structure of the bainite lath and the quality and mor-
phology of the carbon enriched retained austenite occupied
between bainite laths [19]. In other words, the mechanical
properties of the bainitic structure mainly depend on the
diffusion of carbon during bainite transformation. At a
lower transformation temperature (350 °C), the bainite
laths become finer and close-net as shown in Fig. 2a, the
amount of retained austenite is low. TEM studies have
revealed the presence of carbides in the bainite lath at the
austempering temperature of 350 °C as shown in Fig. 4.
The microstructure results higher strength and hardness but
accounts for reduction in the percentage of elongation or
ductility.
The strength is also increased by impeding dislocation
motions. The factors, which contribute to this aspect, include
the fine structure of the bainite platelets, dispersed carbide
formation and high-dislocation density. A low level of
retained austenite was observed, originating from untrans-
formed austenite, contributed to the low ductility at 350 °C
austempering temperature. There is evidence [20] that the
lower bainite generated after low-temperature transforma-
tion contains a concentration of carbon, which is located at
defects such as dislocations within the ferrite lattice. The
majority of dislocations in bainite are believed to be mobile,
since sharp yield points are not observed during tensile tests.
With increase in austempering temperature, the mor-
phology of bainite changes from fine to coarse (carbide free
upper bainite). As a result, the strength is reduced and the
ductility is increased. As the transformation temperature
increases, the diffusion ability of the carbon atom increases
and the degree of super cooling of the bainite transforma-
tion decreases. All these factors reduce the amount of the
bainite laths, widen the laths, reduce the strength and the
hardness and increase the ductility. At a higher austem-
pered temperature (450 °C), the structure becomes coarse
as shown in Fig. 2c, on the contrary, the dimple size on
fracture surface increases (Fig. 8c), which results in
increased ductility.

Conclusions

1. The microstructure of Fe–1.2Si–1.0Mn–0.08 V steel

consists of bainitic ferrite and carbon-enriched retained
austenite after austempering at different austempering
temperatures.
2. The film-like retained austenite is observed when the
isothermal transformation temperature is low, whereas
Fig. 8 SEM fractography of the steel austempered for 10 min at blocky morphology of retained austenite appears when
different temperatures: a 350 °C, b 400 °C and c 450 °C the temperature was high.

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