Engineering, 2010, 2, 360-366
doi:10.4236/eng.2010.25047 Published Online May 2010 (http://www.SciRP.org/journal/eng)
Microstructures and Properties of Recycled Composites –
Particle Reinforced Iron Matrix Functionally Graded
Materials Fabricated by Centrifugal Casting
Yanpei Song1, Shuangxu Bi2, Xiuqing Li3
Materials Science & Engineering College, Henan University of Science and Technology, Luoyang, China
Received December 1, 2009; revised February 2, 2010; accepted February 8, 2010
Abstract
Microstructures and properties of recycled composites ring parts containing cast tungsten carbide particles
(CTCp) in a bainitic matrix fabricated from dead or waste composites roll rings by centrifugal casting after
remelting treatment, have been tested using SEM, EDS and XRD analyses as well as mechanical property
testers. The test results show that the CTCp surface were partially dissolved into the liquid iron at 1650 ºC
during remelting. The undissolved CTCp in the Fe melt moved towards the outer region under the action of
the centrifugal force during casting, which caused the formation of outer reinforced region and inner
unreinforced region along the radial direction of the parts. SEM observation displays that the undissolved
CTCp distribution in the outer region is even, and the volume fraction of them is increased with increasing
rotational speed of the mold. Besides, mechanical tests of the parts show that the outer region exhibits
superior hardness, and the inner region has sufficient impact toughness; the volume fraction of CTCP
influences the mechanical properties. The dissolution-reprecipition of CTCP during centrifugal casting was
also discussed.
Keywords: Recycled Composites; Centrifugal Casting; Mechanical Properties; Microstructures
1. Introduction
Work rolls for hot rolling mills have undergone numer-
ous developments over the past few decades. With the
development of high-speed hot rolling technology, work
rolls are demanded not only to meet high resistance to
wear and heat, but also to meet high strength-toug hness
and stable operation [1]. While cemented carbide com-
posite roll rings produced through powder metallurgy
method possess excellent wear resistance and strength
[2-4], an obvious disadvantage of them is the fact that
such high-cost rolls are unable to be used as over-size or
intermediate work rolls and are prone to fracture in
service because of the low toughness, which causes sub-
stantial waste of raw materials and energy. As a con-
sequence, the synthetic mechanical properties as well as
technology cost of work rolls for hot rolling mills has
always been of great concern to the roll designer or
operator.
Metal matrix composites (MMCs), especially particle-
reinforced metal matrix composites (PRMMCs), have
been paid increased attention and have been used exten-
sively in industry over several years, due to its excellent
mechanical and thermal properties. In recent years, by
means of centrifugal casting technique we have devel-
oped a new type of composite roll (ring) made of
iron-based functionally graded materials (FGM) rein-
forced with CTCP, i.e. FGM-PRMMC, which has sur-
face reinforced region with high hardness and excellent
wear resistance, and has inner un-reinforced region with
high strength and toughness. Previous investigations of
the newly-developed roll (ring) show that the working
surface hardness and wear resistance are as almost high
as those of the cemented carbide composite roll, and the
impact toughness is nearly 4 to 8 times higher than that
of the cemented carbide roll [5,6]. It is evident that ap-
plication of the FGM-PRMMC roll (ring) will make it
possible that the production cost of the work roll for
high-speed wire mill can be reduced by half at least,
compared to that of the cemented carbide roll (ring).
Despite of these advantages, this technique still has sev-
eral limitations such as the accumulated scrap or failure
rolls in the industrial dump sites, which, if discarded or

Y. P. SONG ET AL.361
not reused, will cause substantial waste of energy sources
and roll materials. From the point of view of economic
significance, it’s necessary to evaluate the technological
economics of the common preparation process of com-
posite roll, and thus the recycling or reuse of the scrap
rolls attracts more manufacturers’ attention. Accordingly,
the current work specifically looks at the microstructures
and mechanical performance of the recycled composites
ring, i.e. the recycled FGM- PRMMC rings, which were
fabricated by centrifugal casting technique. The aim of
the present work is to estimate the possibility of reusing
the accumulated scrap or failure roll rings made of FGM-
PRMMC mentioned above.
2. Experimental Procedures
2.1. Fabrication of the Recycled Composites
The dead or waste composites roll rings for remelting
had been Fe-alloy matrix selectively reinforced with
CTCp towards the outer surface region, and the volume
fraction of them is about 75%. The chemical composi-
tions of the Fe-alloy matrix are given in Table 1.
A 50 kg medium frequency induction furnace was
utilized to re-melt the waste roll rings after primary
crushing. When the re-melting temperature was raised to
about 1650°C, the molten material containing liquid Fe-
alloy and solid CTCp was obtained, resulting in the uni-
form dispersion of particles in the molten matrix. Then
the molten mixture was directly cast into a rotating
horizontal centrifuge, as seen in Figure 1, and the cen-
trifugal casting process was carried out with an rotational
velocity of 780-920 rpm while the pouring temperature
of the material was set at 1500°C. The ring castings
obtained were 234 mm in the outer diameter, roughly 55
mm in wall thickness, and 78 mm in length. One of the
recycled composites t ring fabricated under the experi-
mental conditions is shown in Figure 2. Figure 2(b)
presents a difference in appearance between the outer
surface region and the inner region of the centrifugally
cast ring. The thickness of the outer reinforced region
along the radial direction reaches 10 to15 mm for the
annular castings obtained in this study.
Table 1. Chemical composition of the matrix material of the roll rings for the remelting (wt, %).
C Si Mg Ni Mo Re S, P Fe
3.4 2.2 0.05 3.2 0.3 0.07 <0.03 Balance
Castin
g
Rotatin
g
mol
d
Molten mixture
ω
Pourin
g
Shaft
Reinforcin
g
p
articles
Reinforcing particles
Rotating mold
Casting
Pouring
Figure 1. Schematic illustration of the centrifugal casting system used in this research.
Centrifugal force
Outer region
Inner region
Rotational axis
(a) (b)
Figure 2. Presentation of the centrifugally cast ring: (a) photograph of the cast, and (b) schematic of the cast.

Y. P. SONG ET AL. 362
2.2. Mechanical Properties Tests
Mechanical behavior tests for the recycled composites
castings contain impact toughness and Rockwell hard-
ness testing, respectively. The relevant test specimens
were electron discharge machined from the rough cast-
ings. Impact toughness tests were conducted on an im-
pact tester (Model JB294/147A). The impact specimens
for the tests were blocks of 10 × 10 × 55 mm. Impact
toughness tests results were determined from three
specimens. Rockwell hardness used was HR150D which
was tested by using the HR150D hardness tester. The
applied load was 150 N. For each specimen, an average
value of hardness was taken from at least five measure-
ments.
2.3. Microstructure Characterization
All metallographic specimens were prepared for microst-
ructure observation by standard metallographic polishing
techniques and were etched with a solution of 4% nital to
reveal the matrix microstructure. The microstructure and
compositions were analyzed by using a scanning electron
microscopy (SEM) with energy dispersive spectroscopy
(EDS). The XRD analysis was performed on a Philips
X-ray diffractometer using Cu radiation at a voltage of
40 kV and a current of 25 mA with a step size of 0.1°.
3. Results and discussion
3.1. Hardness and Impact Toughness
The mechanical properties of the recycled composites
ring are shown in Figure 3. Along the radial direction of
the annular sample, the mechanical properties are diff-
erent. The outer surface region possesses a high hardness,
which is beneficial to increasing its surface wear resis-
tance; the inner region possesses an excellent impact
toughness, improving the impact resistance of the ring.
Therefore, such recycled composites ring can be used in
heavy load and high speed conditions such as roller rings
used in high-speed wire rolling mill. Besides, It can be
observed that with an increase in rotational speed (from
780 to 920 rpm), the hardness values of the castings
increase, while the impact toughness values somehow
decrease.
3.2. The undissolved CTCP Distribution in the
Surface Reinforced Region
The micrographs shown in Figure 4 present the CTCP
distributions in the outer surface region of the castings
fabricated at 780 and 920 rpm rotational speed of the
mold, respectively. The distribution of CTC-particles in
(
a
)
Inner regionOuter region
0
10
20
30
40
50
60
70
80
90
100
HRC
Positions along radial direction
Fabricated at 780 rpm
Fabricated at 920 rpm
(a)
Inner regionOuter region
0
1
2
3
4
5
6
7
8
9
10
Impact toughness, J/cm2
Positions along radial direction
Fabricated at 780 rpm
Fabricated at 920 rpm
(b)
(b)
Figure 3. Mechanical properties comparison of the centri-
fugally cast rings: (a) hardness and (b) impact toughness.
the outer region of the casting, as shown in Figure 4, is
even, and the volume fraction of them reaches about 54
vol. % at 780 rpm (Figure 4(a)) and 70 vol. % at 920
rpm (Figure 4(b)). That is to say, an intense segregation
of the CTCP occurs towards the outer region of the cast-
ing when a higher centrifugal rotational speed is ob-
tained.
Meanwhile, the average size and volume fraction of
the CTCP in the recycled composites become smaller
compared to those of the CTCP in the waste composites
roll ring before remelting treatment (Figure 5). This may
be duo to the fact that the CTCP were partially dissolved
into the liquid iron during remelting treatment, and the
extent of the partial dissolution varies in different
remelting treatment, as shown in Figure 4. In one word, the
segregation of the undissolved CTCP had occurred during
the centrifugal casting which contributes to improving the
hardness in the outer region as well as impact toughness in
the inner region for the recycled composites ring.

Y. P. SONG ET AL.363
(a)
(b)
(a) (b)
Figure 4. CTCp distributions in the outer region of the recycled composites rings fabricated at (a) 780 rpm, and (b) 920 rpm.
(a) (b)
Figure 5. Comparison of CTC-particle distributions in the outer region of (a) the waste roll ring and (b) the recycled composites ring.
The particle segregation mentioned above occurred
during the centrifugal casting because of the difference
in density between the molten Fe-C alloy and the CTCP.
The motion of the un-dissolved CTCP in viscous Fe-C
alloy melt under the action of centrifugal force can be
determined by Stokes’ law. Thus, the radial convection
velocity of the undissolved CTCP towards the outer
region during the centrifugal casting, υcent, can be
estimated as:
2
cen t2pm
=(-) /(18)vdρρwr η (1)
where d is diameter of un-dissolved CTCP; (ρp-ρm) is the
density difference between the undissolved CTCP and the
liquid Fe-C alloy; ω is the angle velocity based on 780 to
920rpm rotational velocity (calculated value: 41 to 48
rad/s) constant for a definite rotation but changeable; r is
radius of the rotating arm of the particle; η is the
coefficient of dynamic viscosity of Fe-C alloy melt. As a
result, the radial convection velocity of the undissolved
CTCP present in the melt, υcent, increases squarely as a
function of the rotation angle velocity ω. According to
this analysis, we can conclude that increasing the value
of ω will raise the value of υcent, which will lead to an
intense segregation of the particles towards the outer
region of the casting. That’s why the volume fraction of
the un-dissolved particles in the particle-segregated
region of the casting increases from 54 to 70 vol. %
when rotation angle velocity ω increase from 41 to 48
rad/s (based on 780 to 920 rpm rotational speed).
Therefore, it is clear that if a higher volume fraction of
undissolved CTCP in the outer region of the casting is
expected, the rotational speed of the mold must be
increased.
3.3. Analysis on the Partial Dissolution and
In-Situ Crystallization of CTCP
The SEM micrograph and EDS analysis of the recycled

Y. P. SONG ET AL. 364
composite ring, towards the outer region, are shown in
Figure 6. It is observed that small amounts of short
rod-like crystallites were dispersed in the iron matrix, in
addition to the undissolved CTCP in the iron matrix. EDS
analysis shows that the short rod-like crystallites are
composites carbides containing Fe, W, and Ni.
Figure 7 shows the microstructures of the recycled
composite ring, towards the inner region. A lot of short
rod-like and bone-like crystallites and nodular graphite
phases can be observed from the iron matrix. The EDS
result of the crystallites shows that they contain Fe, W,
Ni and C.
(a) (b)
Figure 6. Microstructure of outer region of the recycled composites ring: (a) SEM micrograph, and (b) EDS analysis.
Graphit e
(a) (b)
(c) (d)
Figure 7. Microstructure of inner region of the recycled composites ring: (a-b) SEM micrographs, and (c-d) EDS analysis.

Y. P. SONG ET AL.365
It was discovered that there were a lot of coarser in-
situ crystallites with snowflake-like or branched shapes
dispersed in the intermediate region between the outer
and inner region (in Figure 8(a)). The EDS result of
them shows that they are the composites carbides con-
taining Fe, W, and Ni (in Figure 8(b)).
All above all, some in situ crystallites with various
sizes and shapes were precipitated in the iron matrix
from the outer region to the inner region. Concerned ref-
erence [7] shows that tungsten carbide (WC) is an inter-
phase of transition-metal carbide, with simple hexagonal
lattice, considerably high microhardness value of up to
HM 1870 kg/mm2 (18.326GPa), melting temperature of
2700ºC and density of 15.7 g/cm3, and can almost be
wetted completely by Fe-C melt i.e. the wetting angle of
Fe-C melt on tungsten carbide particle, u ≈ 0o. Besides,
the solubility of WC in the Fe-melt approaches about 7%
(a)
(b)
Figure 8. Microstructure of intermediate transitional region
of the recycled composites ring: (a) SEM micrograph, and
(b) EDS analysis.
20 30 40 50 60 70 80 90
0
200
400
600
800
3
3
3
77
7
77
77
7
66
44
6
6
6
6
6
6
1
5
5
5
4
443
3
2
2
2
1
1
1
1
1
1
Intensity
2-Theta
1 WC
2 a-Fe
3 Graphite
4 W2C
5 W3C
6 Fe6W6C
7 Fe3W3C
Figure 9. XRD patterns of Fe-C alloy matrix in the inner
region.
at 1250ºC, which indicates that WC-powder is dissolved
and then separated from the matrix during the sintering
of high-speed steel powders and WC powders. Espe-
cially, the melting temperature (2525ºC) of CTCP, i.e.
eutectic composed of WC and W2C, is lower than that of
WC particle so that the dissolution of the CTCP in Fe-
melt is relatively easy. Thus, the dissolution of the CTCP
in Fe-melt occured during the remelting process, which
caused the supersaturated Fe-C-W alloy melt during the
cooling process, but this dissolution is incomplete,
namely the surface of CTCP had been partially dissolved.
Finally, the in-situ crystallites were separated from the
iron alloy during the casting process. Further XRD
analysis shows that these in-situ crystallites contain WC,
W2C, W3C, Fe6W6C and Fe3W3C (Figure 9), which are
extremely wear-resistant.
3.4. High Alloying Fe-C Alloy Matrix
As can be seen from Figure 10, the iron matrix of the
recycled composite ring shows bainitic microstructure.
The EDS-analysis result of the iron matrix shows that
they contains tungsten (W) elements, which suggests that
the Fe-C alloy matrix had been high alloyed by the pre-
viously dissolved CTCP because of the imcomplete
separation of the dissolved CTCP.
Reference [7] indicates that tungsten (W) is an impor-
tant alloying element for high-speed steel bonded carbide
products due to the fact that it can stabilize α-phase zone
and minify γ-phase zone in the Fe-C alloy phase diagram.
As for high-speed steel, it possesses a good red hardness
since the tungsten element present in the steel not only
forms wear-resistant complex interphases and raises the
decomposition temperature of tempered martensite after
quench operation, but also retards the decomposition,
precipitation and agglomeration of the martensite. Be-
sides, tungsten element present in the high speed steel

Y. P. SONG ET AL. 366
bainite structure
Figure 10. Microstructure analysis of Fe-C alloy matrix in the inner region: (a) scanning electron microscope (SEM) image,
and (b) X-ray energy spectrum (EDS).
5) EDS-analysis shows that the Fe-C alloy matrix had
been high alloyed by the partially dissolved CTCP, which
could contribute to improving the overall physical and
mechanical properties of the new products so as to meet
the requirements of the parts worked at high temperature.
can cause the tempering secondary hardening of the steel
under the 560°C tempering condition. Therefore, the
matrix alloying of the recycled composite ring can con-
tribute to the right selection of the heat treatment for the
parts, so as to improve the overall physical and me-
chanical properties of the products so as to meet the re-
quirements of the parts worked at high temperature.
4. Conclusions
In the present work, we have examined the microstruc-
tures and properties of the recycled composites rings
fabricated by centrifugal casting process. The results of
the study are stated as follows.
1) The undissolved CTCp (with higher bulk density
than liquid Fe-alloy) segregate towards the outer surface
region with a thickness of 10-15 mm. The undissolved
CTCP distribution in the outer region is even.
2) The outer surface region of the castings possesses a
high hardness, and the inner region possesses an excel-
lent impact toughness. So the rings obtained can be used
in heavy load and high speed conditions such as roller
rings for high-speed wire rolling mill.
3) With the centrifugal rotational speed increasing
from 780 to 920 rpm, the volume fraction of the undis-
solved CTCP in the outer region increased from 54 to 70
vol. %, which had led to an increase in the hardness and
a decrease in the impact toughness.
4) The partial dissolution of the CTCP surface in mol-
ten Fe-C alloy had occurred during the remelting process
and then some in situ crystallites with various sizes and
shapes were precipitated in the iron matrixfrom the outer
region to the inner region. These crystallites are compos-
ites carbides containing Fe, W, and Ni.
5. References
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