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International Journal of Advanced Engineering Research and Studies
E-ISSN2249–8974
Research Article
MECHANICAL PROPERTIES OF METALLIC AND NONMETALLIC CHILLED AUSTEMPERED DUCTILE IRON
(ACDI)
Yogesha K.B*, Joel Hemanth a
*
Address for Correspondence
Assistant Professor, Department of Mechanical Engineering, School of Engineering and Technology,
Jain University, Bangalore-560112, Karnataka, India.
a
Dean, Professor and Head, Department of Mechanical Engineering, Rajiv Gandhi Institute of Technology,
Bangalore-560032, Karnataka, India
ABSTRACT
This paper presents the results obtained and deductions made from a series of ultimate tensile strength (UTS) and hardness
tests involving austempered chilled ductile iron containing 0.1%Mo and Cu contents varying from 0.5 to 4.2%. By using
metallic (copper) and non-metallic (graphite) chills, the effect on ultimate tensile strength (UTS) and hardness of varying the
chill rate was also examined. All the tests were carried out in conformance with AFS (American Foundryman’s Society)
standards. It was found that austempered chilled ductile iron is highly dependent on the location on the casting from where
the test samples are taken and also on the Cu and Mo content combination of the material, as well as the rate of chilling. It
was found that the hardness and tensile strength is highly dependent on the rate of chilling. Finally comparative studies of
using different chills and without using any chills have been made to see that effect of chilling is influencing the properties
of composite under investigation.
KEYWORDS metallic and non-metallic chill; Ultimate tensile strength; hardness
1. INTRODUCTION
1.1. Chilled ductile cast iron
Chilled cast iron belongs to a group of metals
possessing high strength, high hardness, high
toughness and high wear resistance [1, 2]. They are
widely used in the manufacture of wear shoes, wear
liners, rollers, brake shoes, crushing jaws, grinding
mill liners, cam surfaces, wearing plates and other
machine components and equipments requiring such
material characteristics. For the design engineer,
there are sufficient data available on the mechanical
properties of ductile iron and austempered ductile
iron. However, there is a dearth of information on the
tensile strength and hardness of austempered chilled
ductile iron [3]. This has promoted a series of
experiments to determine the tensile strength and
hardness of austempered chilled ductile iron
containing 3.37%C, 2.5%Si, 0.1% Mo, Cu contents
varying from 0.5 to 4.2% and other alloying elements
as indicated in Table 1. The reason behind the
selection of this series of cast iron for the present
investigation is that a wide range of mechanical
properties can be obtained with different
microstructures. Strengthening of ductile iron
includes mould cooling to get a bainitic structure in
highly alloyed irons. The presence of copper as an
alloying element improves the machinability of the
cast iron. In addition, chilling, in case of cast iron
promotes directional solidification and eliminates
shrinkage, porosity, cracks, hot spots and other
casting defects and thus, resulting in production of
sound castings. Also, controlled chilling eliminates
ferrite (too soft) structure but promotes bainitic
structure. Therefore, a simple copper, graphite and
without using any chills are used in the present
investigation.
1.2 Current position of ductile iron:
Ductile iron was developed approximately 60 years
ago and has recently replaced millions of tones of
steel castings and grey iron castings. The major users
of ductile iron components include automotive and
machine tool industry. The reason for its popularity is
due to its advantages over steel castings and grey iron
IJAERS/Vol. I/ Issue II/January-March, 2012/240-243
castings. For the past few years, attempts were made
all over the world to further improve the properties of
ductile irons [4].The specific applications of ductile
iron are: (1) rolling mill rolls; (2) gears; and (3) crank
shafts, etc. The strength, hardness and wear
resistance of ductile iron can be increased through
proper heat treatment of ductile irons. The presence
of copper as an alloying element improves the
machinability of the cast iron. Moreover, a low-grade
cast iron can be converted into one of superior
qualities by chilling ductile iron and austempering
with little addition of alloying elements.
1.3 Austempered ductile iron
It is known that steel components, either hardened or
forged can be heat-treated and alloyed to get a
beneficial change in matrix structure and hence a
range of useful properties. The common steels in use
are the ones with ferritic, pearlitic, martensitic and
austenitic matrices. The demand for a tougher, harder
and stronger matrix led to the development of bainitic
matrix steels. Such a matrix was developed through a
spheroidised
heat
treatment
known
as
‘austempering’, in which the matrix gets transformed
from austenite to bainite directly without entering the
martensitic stage at any time [5]. The beneficial use
of this heat treatment had encouraged many
researchers to apply this to the ductile iron so that it
could become a tougher, more wear resistant and
stronger material [6]. Ductile iron subjected to an
austempering treatment is often referred to as
Austempered
Ductile
Iron
[ADI].in
most
international forums. The austempering heat
treatment involves the following stages:
1. Heating ductile iron castings to a temperature
above the upper critical temperature and
soaking for times varying from 1 to 5 h
depending on the section thickness and the
amount of eutectic carbides.
2. Quenching the above in a fluidised salt bath
maintained at a temperature in the range
between martensite and fine pearlite formation
temperature at rates fast enough to avoid the
transformation of austenite to pearlite.
International Journal of Advanced Engineering Research and Studies
3. Holding in the salt bath long enough to form the
desired mixture of various micro constituents of
bainite.
4. Cooling in air to room temperature.
1.4 Mechanical properties of cast iron
The mechanical properties of a metal depend very
much on its microstructure. This is especially true for
cast iron. The structure depends on the interaction
between the effects of the elements present and the
cooling rate during and after solidification in the
mould [7]. The properties of cast iron are affected by
the structure of the austenitic dendrites, graphite
flakes and eutectic ceils, as well as the formation of
different alloying additions to the base metal. The
alloying elements added should therefore be selected
on the basis of their effect in controlling both
nucleation and growth of graphite and primary
austenite [8].
1.5 Tensile properties of cast iron
It is generally acknowledged that most of the
mechanical qualities of cast iron can best be
represented by its tensile strength. Since cast iron is a
very brittle material, its yield strength can in practice
be taken to be equal to its ultimate tensile strength
(UTS). Optimum tensile strengths can be obtained in
cast iron by increasing the amount of graphite-free
area (in most cases, primary austenitic dendrites),
refining eutectic cell size and establishing the
pearlitic matrix structure [9]. Treating the base metal
with commercial inoculants and neutralizing certain
elements which inhibit graphite nucleation can result
in the formation of more eutectic cells. By increasing
the eutectic cell count, the effective span and stress
concentrating effect of the graphite can be reduced,
thus improving the tensile strength [10].
1.6 Hardness of cast iron
The chemical composition of cast iron is one of the
most important factors that influence the hardness of
the casting. The composition of iron determines the
quality and character of the graphite and the
metallurgical characteristics of the metallic matrix for
some specified set of cooling conditions. The cooling
rate is also an important factor which affects
hardness, especially when the metallic and non
metallic chills are placed at end the mould cavity [3].
It is well known to the foundryman that fully sound
castings under ordinary circumstances cannot be
produced by simple techniques since cast irons are
subjected to micro porosity and shrinkage porosity
during solidification [11].
This defect can be
minimized by making solidification directional by
setting up steep temperature gradients directed
towards the riser, and this can be achieved by
employing chill [12]. Typical benefits of employing
chills for the solidification of cast iron are the
densification of the section and the promotion of
directional solidification, resulting in sound castings
[13].
1.7 Relevance of this research
Ductile iron has the potential to replace costlier
material in many significant engineering applications.
The requirements concerning safety and reliability
are always on the increase and, therefore, the
mechanical properties are ever more crucial. Since
there is presently no published data on tensile
strength and hardness of metallic and non-metallic
IJAERS/Vol. I/ Issue II/January-March, 2012/240-243
E-ISSN2249–8974
chilled austempered ductile iron, the present research
is intended to fill the void.
2. Experimental procedure
2.1. Fabrication of the material
Ductile iron alloys of five different compositions
were produced by casting at 14500C into the form of
ingots, denoted as alloys A, B, C, D and E with
chemical compositions as indicated in Table 1.Apart
from the usual alloying elements such as Si, Mn, S
and P, copper was also added in compositions
ranging from 0.5% to 4.2% to enhance machinability
as well as to act as a grain refiner.
Table 1: Chemical composition details of the different
alloys tested alloy designation composition (wt. %)
2.2. Casting procedure
To make the mould for casting, a teak wood pattern
of size 225 mm x 150mm x 25 mm (AFS standard)
was employed with standard pattern allowances. The
moulds were prepared using silica sand with 5%
bentonite as binder and 5% moisture. The melts were
carried out using a medium frequency coreless
induction furnace. A base metal analysis, generally of
3.37% C and 2.5% Si were produced and superheated
to 15000C prior to tapping into a pre-heated 20-kg
capacity handshake, containing modularizing alloy
using sandwich techniques. Prior to casting an
inoculating addition (ferro-Si to promote uniform
structure) and other alloying elements were stirred
into the molten metal using a graphite spoon. The
treated metal was poured directly into the mould at a
pouring temperature of 14500C, which was cooled
from one end by a copper chill and graphite chill set
in the mould (as shown in Figure 1). Ingots were cast
employing different chills and without using any
chills in order to study the effect of volumetric heat
capacity on the chill. The length and breadth of the
chills were kept constant at 175 and 35 mm,
respectively. This type of casting production routine
was found to result in chilled ductile iron of
consistent chemical analysis and good nodularity.
2.3. Heat treatment
Austempering heat treatment comprises of two
stages, namely heating the plate castings to the
austenising temperature of 9500C, followed by
quenching and holding in the induction furnace
maintained at a temperature of 200-4000C for 6 to7
hrs.
Figure1. The mould for producing chilled ductile iron.
International Journal of Advanced Engineering Research and Studies
2.4 Tensile tests
The tensile test specimens were prepared according
to AFS (American Foundryman’s Society) standards
[7]. The specimens were taken from various locations
in the casting, namely 25 mm, 75 mm, 1.50 mm and
225 mm from the chill end, the last being right at the
riser end. The longitudinal axes of these specimens
were parallel to the longitudinal axis of the copper
chill during casting. The ultimate tensile strength
(UTS) of these tensometer specimens were measured
using an Instron Tension Testing Machine.
2.5 Hardness tests
Hardness tests were conducted on each of the
specimens prepared for microstructural examination.
As specified in the AFS standards [7], a Brinell
Hardness Tester with 5 mm ball indenter and 300 kg
load was used on the polished specimens. Each
hardness result was obtained from an average of at
least three repetitions on the same sample.
3.0 Results and discussion
3.1 Ultimate tensile strength (UTS):
The experimental results of the ultimate tensile
strength tests on castings using various chilled like
copper, graphite, and without chills are tabulated in
Table 2.Comparing the results in this table it can be
seen that changing the chill materials and without
using any chill does have a significant effect on the
UTS of the casting. If other factors are kept constant,
using the copper chills generally causes a marked
increase in the UTS. This implies that increasing the
rate of chilling results in an increase in the UTS of
the material. However, the chilling effect is more
significant on copper chilled casting compared to
other chilled castings and also without using any
chilled castings. When a graphite chill is substituted
with a copper chill, probably because these two chills
have different thermal conductivity a small difference
in their volumetric heat capacities (VI-K) relative to
their actual VI-KS. It can also be clearly seen that if
all other factors are kept constant, graphite chilled
casting and also without using any chills invariably
has the lowest UTS. This shows that increasing the
copper content results in an increase in the UTS of
the material. Moreover, the farther away from the
chill the specimen is taken from, the lower is the
UTS, with the exception of the specimens taken right
at the riser end. This could be because the farther
away from the chill the specimen is, the lower is the
rate of chilling. The graph showing the comparison
results of Ultimate Tensile Strength in MPa with
copper chill, graphite chill and without using any
chills (as shown in graph 2).
Graph 2: Graph showing the comparison results of
Ultimate Tensile Strength in MPa
IJAERS/Vol. I/ Issue II/January-March, 2012/240-243
E-ISSN2249–8974
Table 2: Comparison results of Ultimate Tensile
Strength (σu) in MPa
3.2 Hardness Test:
The experimental results of the hardness tests done
on castings chilled using different metallic and nonmetallic chills and without using any chills are
tabulated in Table 3. It is a well-known fact that for
metallic materials, hardness increase as tensile
strength increases. The results obtained for hardness
are therefore not surprising. Comparing the results in
Table 3, it can be seen that, as in the case of UTS,
changing the different chill does have a significant
effect on the hardness of the casting. The graph
showing the comparison results of hardness tests with
copper chill, graphite chill and without using any
chills (as shown in graph 3).
Table 3: Comparison results of hardness (BHN) test:
Graph 3: Graph showing the comparison results of
hardness (BHN) test
4.0 CONCLUSIONS
Tensile strength and hardness of austempered chilled
ductile iron is highly dependent on the chilling rate as
well as Cu and Mo combinations. An increase in the
rate of copper chilling compare to graphite chill and
without using any chill and increase in the Cu content
of the material both result in an increase in tensile
strength and hardness due to an increase in the nodule
count especially at the chill end. The mechanical
properties of austempered chilled ductile iron are also
significantly affected by bainite content of the
material. Therefore the vast areas of applications of
austempered chilled ductile iron are in the
manufacture of components where the periphery of
the part needs very good surface properties. The
following are examples:
1. gears and pinions;
2. crank shafts;
3. drive shaft yoke and related components to replace
forged and case hardened steels.
4. in gearboxes low noise may be important; and
International Journal of Advanced Engineering Research and Studies
5. the high damping-capacity of cast irons leads to
less vibration and transmission noise than occurs with
steels.
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IJAERS/Vol. I/ Issue II/January-March, 2012/240-243
E-ISSN2249–8974