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Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.4, pp 307-316, 2008
jmmce.org Printed in the USA. All rights reserved
To Study the Effect of Austempering Temperature on Fracture Behaviour of
Ni-Mo Austempered Ductile Iron (ADI)
*, Uma Batra
, D. Puri
, Amita Chawla
Mechanical Engineering Department, L.L.R.I.E.T, Moga, Punjab, India
Metallurgy Department, P.E.C, Chandigarh, India
Metallurgical & Materials Engineering, I.I.T. Roorkee, India
Chemistry Department, Govt. Brijindra College. Faridkot, Punjab, India
*Corresponding author: E-mail: firstname.lastname@example.org,
Phone: +91-9719749154, Fax: +91-1332-285243
Austempered Ductile Iron (ADI) can be as twice as strong as standard spheroidal iron at the
same level of toughness. It responds to work-hardening surface treatments and exhibits excellent
fatigue and wear property. There is extensive work done on the fracture of steel with ferrite
or/and austenite structure, but little on fracture behaviour of ADI whose microstructure also
comprises austenite and ferrite but with graphite nodules in the matrix. The present work is
aimed in this direction. The fracture behavior of Ni-Mo ADI is studied. It is found that the crack
always originates from graphite nodules and the matrix affects the propagation path.
Keywords: Austempered ductile iron (ADI), Fractography, Dimpled structure, Fracture,
Austempered Ductile Iron (ADI) has ‘come of age’ during its brief history. ADI production is
expected to grow at an annual rate of at least 5% . The microstructure of ADI also comprises
austenite and ferrite (in the form of bainitic ferrite), but with graphite nodules in the matrix. The
market of ADI is extremely large. Their attractive properties make them desirable not only for
the manufacture of existing components with improved performance but also for competing with
other materials in new applications. Advantages of ADI include high strength, ductility, wear
resistance, toughness, better machinability, high damping capacity and reduced weight in
comparison with forge steel. ADI has been widely used for engineering components such as
308 Vikas Chawla, Uma Batra, D. Puri, Amita Chawla Vol.7, No.4
gears, crankshaft, vehicle components, sprockets, and cutting tools. The matrix of ADI can
withstand a certain amount of deformation before fracture during tensile and impact testing.
However, the graphite nodules in the matrix cannot deform and hence are barriers to matrix
deformation, which give rise to crack initiation. The crack propagation and the fracture mode of
ADI are influenced by the orientation of bainitic ferrite needles with respect to the load direction
and also by the presence of carbide particles inside the needles or at interfaces.
2. EXPERIMENTAL PROCEDURE
The tensile and impact test specimens of standard dimensions (as per ASTM) were machined out
of Ni-Mo ductile iron casting (in the shape of cylindrical bar). The chemical composition (wt %)
is shown in Table 1.
Table 1. Nominal chemical composition (wt %) of Ni-Mo Ductile Iron casting.
C Si Ni Mo Mn Mg P S Fe
3.43 3.02 1.16 0.43 0.21 0.12 0.016 0.007 balance
Subsequently these specimens were annealed at 720°C for 4 hrs to homogenize the structure and
to achieve uniform distribution of alloying elements. Figure 1 shows the microstructure of Ni-
Mo ductile iron after annealing.
Figure 1. Ni-Mo Ductile Iron annealed at 720°C for 4 hrs
After annealing, all the specimens were austenitized at 900°C for 1hr and then austempered at
different temperatures in a salt bath. Table 2 indicates the designation of the specimens as per the
austempering temperature. The composition (wt %) of salt bath is shown in Table 3.
Table 2. Designation of specimen under study.
Austempered at 270°C for 1 hr. A-1
Austempered at 330°C for 1 hr. A-2
Austempered at 380°C for 1 hr. A-3
Vol.7, No.4 Study the Effect of Austempering Temperature 309
Table 3. Composition (wt %) of salt bath.
40 55 05
All the specimens austempered at different temperatures were then fractured under tensile and
impact loading. The fractured pieces were stored in plastic bags before fractography, in order to
prevent any chemical or mechanical damage to them.
The fractured surfaces were analyzed by Scanning Electron Microscope (SEM) to obtain
fractographs at various locations. For fractomicrography, the specimens were sectioned in the
direction perpendicular to the fractured surface, and then the specimens were polished.
Subsequently the samples were etched in Nital (97ml CH
OH, 3ml nitric acid). After polishing
and etching the specimens were observed under SEM and fractomicrographs were taken in order
to view the crack propagation path.
Visual, with Optical Microscope, SEM and TEM observations of as fractured surfaces, is termed
as Fractography. The effect of microstructure has been observed on the fractured surface
appearance i.e. fractography. Figure 2 and 3 show the fractures surface appearance for the
specimen A-1 broken in impact and tensile test respectively. The dimpled surface is observed.
These dimples indicate that the fracture may have occurred by the phenomenon called void
coalescence i.e. separation of the material internally, forming voids which then join to develop
the fracture surface. The shallow dimples can be seen at various locations, which indicate high
strength & low ductility of the material [2, 3].
The fractographs of specimen A-2, broken in impact test and tensile test are shown in Figures 4
and 5 respectively. These fractographs shows the dimples at various locations which indicate the
fracture may have occurred by void coalescence. The dimples are deep as compared to A-1
specimen, which indicate low strength and more ductility [2, 3].
The fractographs of specimen A-3, broken in impact test and tensile test are shown in Figures 6
and 7 respectively. These fractographs shows signs of cleavage as well as of void coalescence.
The fractographs show river like pattern as well as dimples at various locations. This indicate
that the fracture my have occurred by the mixed phenomenon i.e. quasi cleavage fracture
mechanism . According to which, the fracture may have occurred by cleavage at some foreign
particle subsequently separated from the matrix by void coalescence.
310 Vikas Chawla, Uma Batra, D. Puri, Amita Chawla Vol.7, No.4
Fractography gives information about the nature of fracture whereas the fractomicrography is the
study of the surface, which is being cut perpendicular to fractured surface. After sectioning and
mounting, the fractured specimens were analyzed by SEM in order to determine the crack
initiation and propagation, graphite nodule shape, size, distribution and matrix structure.
Fractomicrographs of specimens A-1, A-2 and A-3 are shown in Figures 10, 11 and 12
3.3 Matrix Structure
The matrix of ADI is a complex mixture of bainitic ferrite and austenite, where austenite is the
basic phase. On austempering at 250 - 330°C, the matrix of ADI comprises the banitic-ferrite
needles with carbide particles inside them and the rest is austenitic. Whereas austempering at 330
- 450°C, the matrix of ADI observed to consist bainitic-ferrite needles without carbide particles
and carbon rich / stabilized austenite, as carbon diffuses to austenite the basic phase .
Several bainitic ferrite needles or plate- lets have the same orientation, forming a cluster of
bainitic ferrite needles or platelets. Each cluster has a particular orientation, as shown in Figure
Under uniaxial external load the orientation relationship between bainitic ferrite needles and the
applied load direction can be classified into three types: -
(i) The longitudinal direction of cluster of bainitic ferrite needles is parallel to the loading
direction 'P', as shown in Figure 9(a).
(ii) The longitudinal direction of a cluster of bainitic ferrite needles is perpendicular to the
loading direction 'P', as shown in Figure 9(b).
(iii) The longitudinal direction of a cluster of bainitic ferrite makes angle 'θ' with the loading
direction 'P', as shown in Figure 9(c).
Most of the clusters of bainitic ferrite needles belong to category as shown in Figure 9(c), but
with different angles.
Figure 2: Fractographs of specimen austempered Figure 3: Fractographs of specimen austempered
at 270°C for 1 hr. and broken in Impact test. at 270°C for 1 hr. and broken in Tensile test.
Vol.7, No.4 Study the Effect of Austempering Temperature 311
Figure 4: Fractographs of specimen austempered Figure 5: Fractographs of specimen austempered
at 330°C for 1 hr. and broken in Impact test. at 330°C for 1 hr. and broken in Tensile test.
Figure 6: Fractographs of specimen austempered Figure 7: Fractographs of specimen austempered
at 380°C for 1 hr. and broken in Impact test. at 380°C for 1 hr. and broken in Tensile test.
Figure 8: The orientation of clusters of bainite Figure 9: Systematic simplified diagram showing the
ferrite needles with applied load direction “P.” possible orientation relationship between bainitic
ferrite needles with applied load direction “P.”
312 Vikas Chawla, Uma Batra, D. Puri, Amita Chawla Vol.7, No.4
Crack propagation from a graphite
Crack propagation along the
Crack propagation by cutting
through bainitic ferrite needles
Crack propagation along the
interfaces and cutting through
banitic ferrite needles
Figure 10: Fractomicrograph showing
crack propagation path in the specimen
having lower bainite microstructure (i.e.
austempered at 270
C for 1hr). The
direction of “P” indicate the load direction
Vol.7, No.4 Study the Effect of Austempering Temperature 313
Figure 11:Fractomicrograph showing crack Figure 12: Fractomicrograph showing crack
Initiation & propagation in the specimen initiation at second phase paricle and
austempered at 330°C for 1 hr. propagation in the specimen austempered at
380°C for 1 hr.
Graphite nodules being discontinuities in the ADI matrix, give rise to much higher stresses
around the graphite nodules during elastic deformation , and hence causes crack initiation at
graphite / matrix interface.
For a structure similar to Figure 9 (b), the bainitic ferrite, austenite and the interfaces of
ferrite/austenite will undergo similar external tensile stress. Atomic mismatch at ferrite/austenite
interface decreases the tensile stress bearing capacity as compared to bainitic ferrite and austenite
individually. As a result, the cracks, which originate from the graphite nodule, usually propagate
along the interfaces of ferrite/austenite as indicated in Figure l0. For a structure similar to Figure
9(a) the bainitic ferrite and austenite will deform with load. As austenite has better ductility than
banitic ferrite needles, thus can sustain more deformation . So banitic ferrite needles will
break first. The austenite deforms even after the fracture of the bainitic ferrite needles. This crack
will proceed by cutting the bainitic ferrite needles.
For the most common bainitic ferrite needles structure, the crack take the easiest way to
propagate, as is observed in Figure l0. The crack propagates along the interfaces of bainitic
ferrite/austenite when angle between the applied load direction & longitudinal direction of
cluster is greater than 45° and sometimes cut through the needles when angle is less then 45°.
For microstructure corresponding to the specimen A-1, which is lower bainite, carbide particles
appear in the bainitic ferrite needles, which act as barrier to ferrite slip. The carbide is hardest
and brittle as compared to ferrite and austenite in the matrix of ADI. There is high stress
concentration around carbide particles, during deformation under load. So the bainitic ferrite
314 V. Chawla, U.Batra, D. Puri and A. Chawla Vol.7, No.4
needles having carbide particles inside them facilitate the crack to pass through bainitic ferrite
needles or platelets , as observed in Figure 10. This creates opportunities for cracks to select
an easy way to propagate. Also the carbide particles deflect the path of crack propagation. This
results in the shallow dimpled fractured surface (as observed in figures: 2 & 3), which is due to
high strength of the material .
For microstructure corresponding to the specimen A-2, crack initiation and propagation is shown
in Figure 11. The microstructure consists of lower bainite (which consist of bainitic ferrite
needles with carbide particles) and retained austenite. The dimples appeared deeper then A-1,
which indicate this material is more ductile. The crack is initiated at graphite/matrix interface
and propagates along bainitic ferrite needles/austenite interfaces or cut through ferrite needles,
depending upon the orientation relationship of bainitic ferrite needles with applied load direction.
For the microstructure corresponding to specimen A-3, the ADI consist of bainitic ferrite needles
without carbide and high carbon stabilized austenite i.e. upper bainite is the microstructure.
Upper bainite has lower strength then lower bainite microstructure, as ferrite is the softest
structure. Figures 6 and 7 (showing the signs of cleavage i.e. river like pattern as well of void
coalescences with dimpled structure at some locations) give indication of Quasi-cleavage
fracture in the specimen A-3, which is supported by the Figure 12. This shows that the crack is
initiated at second phase particle, which is hard & brittle as compared to the matrix of ADI
(which is soft & comparatively ductile). So brittle & cleavage fracture has occurred on the
second phase particle, then separation of connecting material by void coalescence. Figure 12
show the path of propagation of crack.
According to observations the cracks are originating from graphite nodules in ADI. The easiest
propagation paths for cracks are the interfaces between ferrite and austenite, because of atomic
mismatch at the ferrite/austenite interfaces. The orientation of a cluster of bainitic ferrite needles
and the presence of precipitated carbide in the matrix can influence the crack path. However, the
orientation of the longitudinal direction of banitic ferrite needles is random and does not
influence the fracture mode of ADI. Precipitated carbides in the matrix of ADI do not
significantly influence the fracture characteristic of ADI. In order to analyze the crack
propagation path, Fan et al.  have explained the crack propagation with the help of a model as
shown in Fig.13. The same model is verified in our study, in which two clusters of banitic ferrite
needles between two graphite nodules, one nearly parallel to applied load direction and the other
nearly perpendicular to the applied load direction can be assumed. The effect of precipitated
carbide on the crack propagation path and the fracture mode of ADI can be explained with a
model as shown in Figure 13 .
Vol.7, No.4 To Study the Effect of Austempering Temperature 315
Figure 13. Two models of crack propagation in ADI .
Micro voids at graphite -matrix interfaces: -1
Bainitic ferrite needles: -2
Carbide particles in bainitic ferrite needles: -3
Possible crack path: -4
Model (I) in Figure 13 shows no carbide appear in the ADI matrix crack often pass along the
ferrite/austenite interfaces for which the needles have greater angle than 45° with the applied
load direction. However, if the longitudinal direction of the cluster of bainitic ferrite needles
tends to be parallel to the loading direction, cracks may cut through the ferrite needles (as ferrite
is softest microstructure) & austenite. In this case the fracture mode should be ductile, but due to
presence of second phase particles the fracture mode is Quasicleavage as explained earlier.
Model (II) in Figure 13 show carbide in the bainitic ferrite needles or platelets, which act as
barriers to ferrite slip, results in higher stress around carbide particles . Large number of stress
concentration locations in the needles creates particles, which further creates more opportunities
for crack to pass through needles or platelets. As the carbide particles are harder so undergoes
fracture without deformation and also deflect the crack path. This creates opportunity for crack
to select an easy way to propagate. This result has further been supported as the fractography
shows fracture in ductile mode.
Figure 13 gives just one of the main possible propagation paths of the crack. Although we cannot
predict the particular propagation path, the observed results can help us understand and develop
the appropriate microstructure of ADI.
The following conclusions can be drawn from the above study
1. Crack always originates from graphite nodules in ADI.
2. The easiest path of propagation of a crack is along the austenite/ ferrite interfaces due to
316 V. Chawla, U.Batra, D. Puri and A. Chawla Vol.7, No.4
3. The longitudinal direction of bainitic ferrite needles can be parallel, perpendicular, or inclined
at angle 'θ' with the load direction.
4. The propagation path of a crack in ADI depends upon the orientation relationship of bainitic
ferrite needles with the applied load direction.
5. The fracture mode in lower bainite structure is ductile mode and the fractographs shows
shallow dimpled structure, which indicates the high strength of lower bainite microstructure of
6. The fracture mode in upper bainite microstructure is Quassi cleavage or mixed mechanism i.e.
the fracture firstly occurs by cleavage then separation of material by void coalescence. The
fracture of second phase particle occurs by cleavage due to lack of ductility and of the matrix by
void coalescence (plastic deformation).
7. Carbide particles in bainitic ferrite needles promote the passage of cracks through the ferrite
needle, but do not significantly influence the fracture mode of ADI.
 R.J Warrick “Application of Ductile Iron Castings”. ASM Technical reports system no. 76-
 Calangels and Heiser: Metallurgical Failure: A Wiley-Interscience publications.
 J.C Morrison, “What’s’ in a name-nickel and ductile iron”. Technical paper in Indian
Foundry Journal, 1998, Vol. 44(12).
 Ashok Chaudhary and Charlie R. Brooks “Metallurgical Failure Analysis,” McGraw Hill
 Ray Elliot: Cast Iron Technology, Jaico publishing house.
 Z.Fan and R.E Smallman (1994) “Some observations of Austempered Ductile Iron”. Scripta
Metallurgica et Materials journal, Vol.31 (2), 1994