Journal of Minerals & Materials Characterization & Engineering, Vol. 6, No.2, pp 109-120, 2007 Printed in the USA. All rights reserved
Evaluation of the Influence of Bainitic Transformation on some
Mechanical Properties of Calcium Treated Cast iron
Umoru, L.E.,
Ali, J.A. and
Afonja, A.A.
Department of Metallurgical and Materials Engineering,
Obafemi Awolowo University, Ile-Ife, Nigeria.
Nigeria Building and Road Research Institute (NIBRRI)
Garki, Abuja
This work has investigated the effect of austempering variables on hardness,
strength and toughness of 0.48wt. % calcium treated cast iron. The iron was subjected to
varying austenitizing and isothermal transformation temperatures and times. Samples
were austenitized for 15, 30, 45 and 60 minutes at each of the temperatures 850, 875,
900, 925 and 950 degrees centigrade. Also for 950
C and 60 minutes of austenitizing
conditions, samples of the 0.48wt. % calcium treated irons were isothermally transformed
at 300 330, 360 and 420
for 2 hours each.
The results of this work show that austenitizing temperature enhances the
hardness property of the austempered iron. The Brinell hardness values of austempered
iron increased from 275 BHN to368 BHN after austenitizing for an hour between 850 and
C respectively. The austenitizing time does not show any significant influence in the
same temperature range. The tensile strength increased from 277.5MPa at the
transformation temperature 300
to a maximum of about 430MPa at 374
decreased afterwards. The percentage elongation on the other hand, reduced to a
minimum of 2.7% at 360
and increased thereafter with increase in transformation
temperature. The austempered microstructures were the accicular lower bainites that are
fine for isothermal temperatures below 374
and coarse upper bainites for higher
isothermal transformation temperatures.
Key words: Austempering, cast iron, calcium, austenitizing, isothermal, transformation,
bainites, strength, elongation
Iron foundry was the nucleus of industrial revolution of the developed nations and
is still the foundation for effective industrial development of any nation. The
technological demand for high toughness and superior quality components has forced the
developed nations to substitute wrought and powder metallurgy components for cast
components from conventional gray cast iron. In spite of their inferior mechanical
properties in certain engineering uses, the developed nations have not lost sight of the
advantages of iron castings over forged products, such as ease of casting of intricate
110 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
components to near-net shape, lower raw materials and overall production cost and
possession of certain desirable physical properties.
In order for cast iron products to compete with forged products in applications
demanding high strength, the inherent problem of low toughness and ductility associated
with cast iron products must be overcome. When graphite is present in the form of
nodules, higher toughness and ductility can be achieved in the castings. Additionally,
because of the volume of graphite nodules, a ductile iron casting will weigh
approximately 10% less than steel forging if both have exactly the same shape (Warrick
et al, 2000). Also, ductile iron castings will provide greater shape flexibility as the result
of draft angle requirements and the ability to cast in features through the use of cores.
These nodular irons are produced mainly by addition of elements such as magnesium and
cerium. Other elements such as calcium, lithium, rare earths have also been identified as
having potentials to nodualrize cast iron graphites. The work of Umoru et al (2005) has
confirmed calcium as a resourceful graphite nodularizer particularly for the Nigerian Iron
Founders. Their research determined the calcium quantity required for optimum
nodularization as 0.48% by weight. Compared with the traditional materials ( Mg and
Ce) used for graphite nodularization in iron, calcium is not only cheaper and more readily
available in Nigeria, its higher melting point and boiling point make it more suitable for
the nodularizing process. While one of the major problems with magnesium is
manipulation of the vapour generated during nodularizing, calcium is not subject to this
problem. The reason is because Mg has a boiling point of 1107
C which is far less than
the melting point of cast iron that is between 1153 and 1400
C depending on the carbon
content while Ca has its boiling point as 1372
C. In addition to exhibiting similar affinity
for oxygen and sulphur according to Ellingham diagram, calcium has affinity for other
impurities like silicates in iron that perhaps explains why it is able to slag off impurities in
iron melts.
Another microstructural parameter that is known to influence the mechanical
properties of nodular iron is the structure of the matrix surrounding the free graphite
particles. This has been another area of research concentration in recent years particularly
with respect to magnesium or cerium graphite nodularized cast irons. Research efforts in
this area have been focused on heat treatment and alloy additions to achieve the desired
structure. The research and foundry practice in the developed countries in recent years
according to Dorazil (1982) and Imasogie, et al (2000) have firmly established the use of
isothermal heat treatment to produce bainitic matrix structure (i.e. austempering) as an
effective means of obtaining the desired combination of properties.
The technological advances made thus far in the production of graphite nodules in
bainitic matrix structures have led to significant improvement in properties and
performance. While conventional ductile iron grades have long been used successfully
for automobile components, the high strength and toughness of austempered ductile iron
offers much greater potential for weight reduced designs (Warrick et al, 2000). It is now
possible to use this class of materials for critical engineering components requiring
excellent combinations of high strength and ductility (i.e. high toughness). Ductile cast
iron is now being used to make pinion and bevel gears, cam and crankshafts in
automobile engineering. It is also being used to make tractor and earth moving
equipment components, heavy duty cutting knives and railway wagon wheels.
Vol.6, No.2 Evaluation of the Influence of Bainitic Transformation
Having obtained 0.48wt.% calcium as that required for maximum nodularization
of graphite in iron (Umoru, et al 2005), this work seeks to establish an austempering heat
treatment schedule for the inducement of the best combination of mechanical properties
in the nodular cast iron with 0.48wt.% calcium as nodularizer.
The 0.48 wt. % calcium treated iron was prepared according to the procedure in
Umoru et al (2005) from gray iron engine block and a grade of ferro-silicon alloy. The
tensile test pieces were machined to geometrical specification according to B.S. tensile
specification No. 18 (1950) as contained in Monsanto Hounsfield tensometer machine
manual. Prior to austempering treatment the surface of the test-pieces were ground on a
machine to 600 grit on the same emery paper. Hardness and metallographic specimens
were cut from the annealed rods prior to austempering.
Austempering involves basically two processes: solution treatment and isothermal
transformation. Each of these processes is identified with two variables, viz. treatment
temperature and time. Rossi and Gupta (1981) had once described the austempering
process of nodular irons to require the austenitizing variables of 850 to 925
C and 2-4
hours. To determine the suitable combination of the above variables to use for the
austempering of the calcium treated iron, twenty samples of the 0.48wt.% Ca treated
irons were austenitized at temperatures of 850
C, 875
C, 900
C, 925
C and 950
C for
15 , 30, 45 and 60 minutes respectively at each temperature. Throughout the experiments,
the transformation conditions were maintained at 350
C and 3 hours because according to
literature this temperature falls in the bay of the isothermal transformation diagram of the
ductile iron and 3 hours will be sufficient to drive the bainite transformation to
completion (Rossi and Gupta, 1981). Each of the samples under investigation was cut to
the average thickness of the tensile samples, i.e. 3.50mm for the purpose of fair inference.
The results have been synthesized and used to determine the effect of the temperature and
time variables on the strength and the ductility of the austempered iron variables.
A pair each of the tensile samples used for the austempering process was
austenitized at the optimum condition determined above and austempered at 300
C, 330
C, 360
C and 420
C for an austempering time of 2 hours. The tensile properties of the
samples were subsequently measured.
The Brinell Hardness accessories of Monsanto type W-tensometer were employed
for the hardness tests with results expressed in BHN. The surfaces of austempered
samples were all prepared metallographically to 600 grit and cleaned before the hardness
test. An average of four impressions was made on each sample and their dimensions
measured with the help of the scale attached to the Brinell reading microscope. The mean
diameters of indentations were used to obtain the BHN hardness values.
Ten austempered test pieces of the 0.48wt.% Ca treated irons were tested in
tension to failure on the tensometer used. The percentages of elongation were
subsequently calculated.
The bainitic matrices of the austempered cast irons were microscopically observed
using Wild M50 Metallurgical microscope. The universal EPI-Apochromat objective of
40x and eye-piece of 20x magnifications were used for observing and taking the
112 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
micrographs of the austempered calcium treated irons A Photoautomat MKa and a motor
driven 35 magazine were attached to the HZ phototube of the M50 microscope and used
for the photomicrography.
Prior to investigating the effect of isothermal transformation temperature on the
austempered properties of the calcium treated iron produced in this work, the effect of the
austenitizing conditions were studied.
Figure 1 shows the graphical plots of the hardness versus austenitizing times for
the cast irons austenitized at different temperatures. In general, the graphs show that
hardness of the austempered irons increases with solution treatment temperature. The
increases for temperatures 850, 875, 900 and 925 degrees centigrade are similar in the
sense that they are cyclical. That is, their increases fluctuated around mean values. As
for the 950
C austenitizing temperature, the increase was gradual and tended to saturate to
a hardness value in the austempered iron. Rundman and Klurg (1982) reported an
increase in carbon content of austenite from 0.06 to 1.40 wt. %C in the temperature range
770 to 1100
The quantity of carbide precipitated after austempering has direct
relationship with the carbon content of austenite and the final hardness of austempered
iron. Accordingly, the increased carbon content of austenite with increase in the
austenitizing temperature between 850 and 950
explains the increase in the hardness
values shown in Fig.1.
Fig.1 Effect of time at various austenitizing temperature on 0.48wt.% Ca-treated cast iron
austempered at 350oC for 3hours
010 20 30 40 50 60 70
Austenitizing time, minutes
Hardness values, BHN
850oC 875oC 900oC 925oC 950oC
Figure 2 shows the influence of solution treatment time on the austempered
calcium treated iron. Plots of hardness versus solutionizing temperatures at the
austenitizing times 15, 30, 45, and 60 minutes were used to evaluate the effect
Vol.6, No.2 Evaluation of the Influence of Bainitic Transformation
austenitizing times. The curves in Fig. 2 do not establish any clear relationship between
hardness values and the austenitizing times. In other words, the solutionizing time at the
gauge thickness level of the samples investigated does not have a significant effect on the
austempered properties.
After analyzing the effect of austenitizing conditions on the hardness of calcium
treated iron, 950
C and 1 hour were chosen as the most appropriate austenitizing
conditions for the austempering of the irons produced in this work. This choice
corroborates the work of others like Rossi and Gupta (1981).
During austenitizing, the pearlitic zones generally get enough supply of carbon to
transform immediately to graphite nodules through diffusion, a process that is both time
and temperature dependent (Rajan, et al, 1988). As a result of this, an hour was chosen to
austenitize the starting ferritic matrix of the annealed iron in this work at 950
C. With
these solutionizing conditions, the effect of isothermal transformation temperature on
some austempered mechanical properties of 0.48wt.% calcium has been studied. The
results are contained in Figs. 3 and 4 .
Figure 3 shows the variation of tensile strength of the austempered irons with
transformation temperatures between 300 and 420
C. The strength increased initially
from an average value of about 277.5 MPa at 300
C transformation temperature to about
430Mpa at 374
C. With increase in austempering temperature beyond 374
C the tensile
strength of the iron decreased rapidly to lower values. The temperature of 374
therefore marks the transitional temperature of transformation from the lower bainite
regions to the upper bainites. Other researchers have also reported transformation in the
neighbourhood of 370
C ( Imasogie et al, 2000) .
Fig.2 Effect various austenitizing temperatures on 0.48wt.% Ca
treated cast iron
austempered at 350oC for 3hours
940 960
Austenitizing temperature
Hardness values, B
114 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
The influence of isothermal transformation temperature on average percentage of
elongations (ductility) shows that the ductility of the calcium treated iron decreases
slightly until about 360
C and increases afterward with increase in the transformation
temperature (Fig.4).
Fig.3 Variation of isothermal transformation temperature with tensile strength of 0.48wt.% Ca-
treated cast iron after solutionizing at 920oC for 1 hour and austempered for 2 hours
300 320 340 360 380 400 420 440
Isothermal Transformation temperature, oC
Tensile strength, MPa
Figures 5 to 9 show the austempered microstructure of the 0.48wt.% Ca treated
irons. Figure 5 represents the resultant microstructure after treatment at 950
C for an hour
and 300
C for 2 hours. In a similar manner, Figs. 6 to 9 show the structures after
transformation at 330
, 390
, and 420
C, respectively. Even though the magnification of
800x was not enough it can still be seen that the structure of the iron austempered at
C is coarser than those of 390
and 360
C. Similarly, the 300
C microstructure is
coarser than those of 330
C and 360
On the basis of the morphological study of bainitic structure in silicon steel and
ductile iron, and taking into account the transformation kinetics, the course of
transformation of austenite in the bainite region has been divided into three stages
(Dorazil, 1982). During the first stage characterized by an average transformation rate,
nucleation sets in and ferrite plates begin to grow. The untransformed austenite at this
stage gradually becomes enriched with carbon and transform into a considerable amount
of martensite on cooling. This structure markedly affects the properties of the
austempered iron, resulting in most cases in premature fracture in tension.
Vol.6, No.2 Evaluation of the Influence of Bainitic Transformation
Fig.4 Variation of isothermal transformation temperature with percent elongation of 0.48wt.%
Ca-treated cast iron after solutionizing at 920oC for 1 hour and austempered for 2 hours
300 320 340 360 380 400 420 440
Isothermal transformation temperature, oC
Percentage elongation, %
Fig.5 Micrograph of austempered 0.48wt.%Ca treated iron. Austempering condition=
C/2hrs+air cooling; acicular structure=bainites; light areas=retained
austenite and Dark spots=graphite nodules. Picral etached. x 800
116 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
Fig.6 Micrograph of austempered 0.48wt.%Ca treated iron. Austempering condition=
C/2hrs+air cooling; acicular structure=bainites; light areas=retained
austenite and Dark spots=graphite nodules. Picral etached. x 800
Fig.7 Micrograph of austempered 0.48wt.%Ca treated iron. Austempering condition=
C/2hrs+air cooling; acicular structure=bainites; light areas=retained
austenite and Dark spots=graphite nodules. Picral etached. x 800
Vol.6, No.2 Evaluation of the Influence of Bainitic Transformation
Fig.8 Micrograph of austempered 0.48wt.%Ca treated iron. Austempering condition=
C/2hrs+air cooling; acicular structure=bainites; light areas=retained
austenite and Dark spots=graphite nodules. Picral etached. x 800
Fig.9 Micrograph of austempered 0.48wt.%Ca treated iron. Austempering condition=
C/2hrs+air cooling; acicular structure=bainites; light areas=retained
austenite andDark spots=graphite nodules. Picral etached. x 800
In the second stage, reaction rate is slow and transformation continues mainly by
lateral growth of bainitic ferrite plates. The untransformed austenite becomes enriched
with carbon to a concentration which does not allow martensite to form on subsequent
118 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
The third stage is characterized by increased average transformed ferrite plates
and reduced portions of retained austenite in the structure. The mixture of ferrite and
carbide found in areas where bainitic ferrite plates and carbide grow into each other is
said to be indicative of austenite transformation in the form of the eutectoid reaction, in
which the carbide phase is the leading one. The carbide phase increases with increase in
the carbon content of austenite.
According to Rundman (1983) the carbon content of austenite varies from
0.60%C at 770
C to about 1.4%C at 1100
C. This results in the increase in carbide phase
of the ductile iron with an increase in the austenitizing temperature after transformation at
a particular temperature and time. But carbide is a hard phase, its hardness increases the
austempered hardness of nodular iron as the amount of carbide increases. Thus, it can be
rightly inferred from the effect of solutionizing temperature on the quantity of carbide
phase in austempered matrix that the hardness of the austempered iron should increase
with austenitizing temperature. This is evident in the trend shown in the result of this
work as the hardness of the austempered iron increases with the austenitizing
temperatures 850
, 875
, 925
, and 950
centigrade respectively.
The variation of tensile strength with isothermal transformation temperatures is
shown in Fig.3. The strength increased initially up to a value of about 430 MPa at 374
This simply marks 374
C as the transitional austempering temperature from lower bainite
to the upper bainites. About 370
C is reported by Harding (1986) as the transition period
in nodular irons.
From the continuous time temperature transformation diagram for unalloyed irons
it can be seen that for transformation in regions close to the Ms point, the start and finish
of bainite reaction is drastically delayed. What this implies in effect is that at the end of
the two hours of transformation allowed in this project, the austempered iron was
probably in the first stage of transformation; so that on cooling, the carbon enriched
austenite transformed into martensite, leading to premature fracture in tension (Rossi and
Gupta, 1981). This behaviour accounts for the lower values of the tensile strength (277.5
Mpa) of the specimen austempered at below 374
With increase in the transformation temperature following the slope of TTT –
diagram before the nose of the curve, the start and finish of transformation decrease
accordingly leading probably to the approach of the second stage of bainite
transformation given the same 2 hours transformation time. The second stage according
to Rossi and Gupta (1981) is characterized by maximum tensile strength in the
austempered iron. This explains the increase in strength of the iron up to a maximum at
the transformation temperature of 374
C. The subsequent fall in the tensile strength and
rise in the percentage elongation in regions above 374
C (Figures 3 and 4) is explainable
from the standpoint of upper bainite morphology.
In the region between 374
C and the nose of the CCT – diagram the bainitic
ferrites undergo increased rate of lateral growth (Pickering, 1967). This is due to the
favourable temperature condition that enhances increased rate of diffusion of carbon into
the austenite phase (Gulaev, 1980). This gives rise to increase in size of ferrite which
Vol.6, No.2 Evaluation of the Influence of Bainitic Transformation
according to Hall and Petch relation (Rajan et al, 1988) results in reduced tensile strength,
in accordance with the following equation
= s
+ kd
………………. (1)
- represents the tensile strength,
- the strength to overcome internal friction
d - the grain size, and
k - a constant
Figures 7 and 9 show the different ferrite structures at 360
and 420
, respectively.
The feature of larger ferrite grains at 420
C (Upper bainite) is in agreement with the
above definition.
When viewed from another metallurgical standpoint, the corresponding decrease
in tensile strength and increase in percentage elongation of the austempered 0.48wt.%Ca
treated iron above 374
C can be attributed to the retained austenite in the micrographs of
Figs. 8 and 9 (light background). The austenite structure is the closest packed structure
with 12 slip systems. And hence is responsible for the high toughness in the upper bainite.
The following conclusions can be drawn from the results of this study:
1. The hardness property of austempered calcium treated iron is significantly influenced
by austenitizing temperature and not by austenitizing time between 850 and 950
2. The hardness values of the austempered calcium treated iron are comparable with those
of other austempered ductile iron in the literature.
3. The hardness of the austempered cast irons increases with austenitizing temperature
from 275BHN at 850
C to 368BHN at 950
C. The tensile strength of the same
austempered iron varied with the isothermal transformation temperature (ITT) as the
strength increased from 277.5MPa at 300
C ITT to a maximum value of 430MPa at
C and thereafter decreased.
4. The ductility of the 0.48wt. % Ca iron as measured by percentage elongation decreased
with isothermal transformation temperature to a minimum value of 2.7% at about 360
and increased afterwards to higher values.
5. The microstructures of the bainites formed from the austempering process were finer at
isothermal transformation temperatures lower than 374
C and coarser with some retained
austenite content at higher temperatures.
120 Umoru, L.E., Ali, J.A. and Afonja, A.A. Vol.6, No.2
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