Cast iron is an iron alloy consisting about 95% by weight iron, 2.1 to 4% by
weight carbon and 1 to 3% by weight silicon. Ferrous alloys are the most important
industrial alloy and among them 35% is cast iron (Rashid
and Edmonds, 2002). The cast iron has multi directional properties depending
on its graphite size, shape and the type of matrix. The gray cast iron contains
flaky graphite in ferrite or pearlite or a mixture of these matrixes. These
types of graphite act as crack initiator which reduces its the tensile
strength. So, the researchers try to modify the gray cast iron to change its
graphite shape as well as its properties; because, if the graphite becomes spherical
in shape, it will act as crack arrester which improves the properties of the
cast iron (Qitfer, 1983). However, the later process flourishes
very quickly for its convenience (Heine et al., 2002).
Aluminium and silicon have similar graphitization effect on Fe-C system. Ghoreshy
and Kondic (1983) proposed Fe-C-Al cast iron with the eutectic solidification
of cast iron. They proposed that solidification of Spheroidal Graphite (SG-Al)
cast iron is more complex than SG-Si cast iron. In Fe-C-Al alloy system, there
are two ranges of stabilizing effect corresponding to the formation of Fe3AlC
and Al4C3 compounds. Up to 4% of Al, its graphitizing
effect is very strong (Fargues, 1985), whereas in the
case of the silicon-bearing system, there is only one such range corresponding
to SiC (Zhukov, 1978). Aluminium promotes graphite formation
during eutectic solidification but it also stabilizes pearlite during eutectoid
reaction. Aluminium reduces the solubility of carbon in liquid eutectic iron
and increases the solubility of this element in austenite. Carlberg
and Fredriksson (1977) showed that the liquidus temperature increases by
10°C for each per cent of Al whereas for each per cent of Si decreases by
30°C. They also found that the eutectic temperature increases by 20°C
for same percent of Al whereas the temperature decreases by 7.5°C same percent
of Si content.
The magnesium treatment has a great impact on the production of SG-Al cast
iron. The use of magnesium alloy and its recovery is the crucial factor for
the production of SG-Al cast iron. It is found that the consumption of magnesium
containing alloy in the production of Fe-C-Al cast iron increases due to low
recovery of magnesium which intern increase the production cost of SG-Al cast
iron (Haque and Young, 1995; Rashid
and Edmonds, 2002; Haque, 2007). In the present
study, pure magnesium foil was used in specially designed ladle for liquid melt
treatment and recovery of magnesium was very satisfactory.
Moulding materials: For ferrous casting hard sand mould is necessary.
In this study, cold set resin bonded sand mould was prepared using synthetic
silica sand (AFS grain fineness 60), cold set resin (Grade CS 1080) and hardener
(Grade FC 200). The 18 kg silica sand was mixed with 2% cold set resin in the
Muller (Hobert Muller, Model No A 200).
|| Detail of charge materials (wt. %)
|| Chemical composition of cast iron and sorel metal (wt.%)
After 5 minutes mixing, 1% hardener was added in the sand mixture and continued
the mixing for another 1 minute. Finally, the sand mixture was used to prepare
the mould using wooden pattern.
Charge materials: The raw materials were melted in a medium frequency induction furnace. For Fe-C-Al alloy system, pig iron (Sorel Metal), mild steel and commercially pure aluminium were melted together.
After melting, the fluxing material was added, stirred and the slag was removed. The details of charging raw materials and its chemical compositions are shown in the Table 1 and 2, respectively.
Magnesium treatment: A special type of crucible was design and prepared for Mg treatment in this investigation which is shown in Fig. 1. After melting all raw materials, the liquid iron was kept in the furnace and heated up to about 1550°C temperature. Then, the liquid metal was shifted into the special type of ladle for Mg treatment where the pure magnesium foil was covered with mild steel box. To protect the liquid metal splashing during reaction, the crucible was covered with mild steel lid with refractory lining. When the reaction completed the melt was poured into the mould at about 1450°C.
After casting, the chemical analysis was performed and calculated the magnesium
recovery in the cast iron using the Eq. 1. The Metallographic
sample was prepared by polishing and etched with 5% natal solution. Hardness
was measured using Rockwell hardness tester, Model: 660RLD/T in B scale and
converted to Brinell Hardness Number (BHN) using conversion table.
||Magnesium treatment crucible for preparing spheroidal graphite
Tensile test was performed using instron (Model -3360) universal testing machine
and the fracture surface was observed under FESEM (JEOL JSM-6700F):
where, w1 is the initial weight of the magnesium which was added in the liquid metal and w2 is final weight of magnesium which was remain in the solid cast iron.
RESULTS AND DISCUSSION
Microstructure and phases: Figure 2 shows the microstructure
of ductile cast iron produced in the present study, it is noticed that graphite
nodules are surrounded by ferrite which is known as bulls eye structure
and beside the ferrite, pearlite also present as matrix. The nodularity of this
cast iron was around 90% and is due to the diffusion of carbon from austenite.
When the melt solidify from liquid to solid, it contains supersaturated austenite.
However, at room temperature, the solubility of carbon in austenite is very
low. Therefore, the carbon precipitated out in a free form which is known as
graphitization and spherodized due to the presence of aluminium and magnesium,
respectively. Since, the carbon diffuse out from the austenite, the near reason
transformed to ferrite. The similar structure was also reported by Rashid
and Edmonds (2002).
Figure 3 shows the X-ray diffraction of the ductile cast
iron. The diffraction pattern showed graphite, ferrite, cementite and iron-aluminium-silicon
complex phases. The XRD pattern indicated that the produced spheroidal graphitic
cast iron contains higher ferrite content than pearlite. It was reported that
aluminium has gamma loop closing property in iron-iron carbide equilibrium diagram
and this property improves ferrite content in the matrix (Lancaster,
||Microstructure of spheroidal graphite cast iron (etched with
|| Mechanical properties of Fe-C-Al SG cast iron
Tensile properties and fracture surface: The hardness and tensile properties
of the cast iron is given in Table 3, it is clear that the
Ultimate Tensile Strength (UTS) is 478 MPa and the percentage of elongation
is 3.50%. It shows that it has nearly the same properties with other spheroidal
graphite cast iron which is produced in different Mg treatment process (Haque
and Young, 1995).
Figure 4 shows the fracture surface of the cast alloy. It
was viaiable that Transgranular type fracture occurred in the developed cast
iron fracture surface. It was reported by Batra (2005)
that graphite nodules act as the discontinuities for crack propagation where
the crack is arrested by graphite spheroids. Due to application of load, the
matrix around the graphite deforms plastically, resulting in decohesion at the
graphite/matrix and graphite/wrapped graphite interface. Thus, microvoids first
form at the graphite nodules/matrix interface. These microvoids concentrate
stresses and may later promote cracks in the matrix adjacent to the graphite
||Fracture surface of as-cast Fe-C-Al SG cast iron under tensile
The crack, which normally initiates near the graphite nodule, propagates through
the matrix to reach the adjoining nodules.
The following conclusions can be drawn from the experiment of the present study:
||Fe-C-Al spheroidal graphite cast iron has been successfully
developed with less difficulty with designing special type crucible for
||Magnesium recovery in this process is higher compared to other
||The cast iron shows satisfactory level of strength and hardness
||Transgranular type fracture occurred in the developed cast
iron fracture surface
Authors are thankful to the Faculty of Engineering and Research Management Centre of International Islamic University Malaysia for providing laboratory facilities and financial supports from FRGS 01-06-01 research grant.