FIELD OF THE INVENTION
Present invention, discloses a method of fabrication of grinding rolls, where
surface of wear resistant segment of grinding rolls is reinforced by hard
granules up to thickness of 55 -60 mm with improved wear resistance and
toughness.
BACKGROUND OF THE INVENTION
The phenomenon of wearing of components, is one of the most commonly
encountered industrial problem in coal fired power stations, where coal is
ground to powder state, in impact mills using grinding rolls. The major concern
of the grinding rolls (i.e. Ni-hard rolls) during the course of coal pulverization is
its “harsh abrasive interaction with the coal” due to higher ash and silica
(Quartz) content present in the coal. Silica has a Vickers hardness of > 1100 Hv,
due to this, surface of the grinding rolls (which has hardness of around 600-700
Hv) gets worn out faster and it needs to be replaced frequently.
It is known that the life of grinding rolls can be enhanced by reinforcement of
hard particles at the wear surface. Once the hardness of particles is higher than
the quartz, the wear of surface gets reduced and life of the grinding roll can be
enhanced compared to Ni-hard rolls. Ceramic composites have high hardness
(i.e. Wear resistance) and toughness (i.e., fracture resistance) and can be
reinforced at the surface of the grinding roll to improve the wear rate.

To fabricate the grinding rolls with enhanced life, the reinforced grains should
exhibit, a high abrasion resistance (hardness greater than silica) and some
ductility, to be able to withstand the mechanical stresses such as impacts. The
reinforced grains should be properly embedded in metal matrix, the poor
filtration of metal may lead to easy cleavage of reinforced grains during
grinding process. In addition to this, wear resistant segment at working surface
should be equally spaced to have homogenous wear pattern over the surface and
it should withstands against the high mechanical stresses developed during
grinding process.
As discussed above, reinforced grains should exhibit, high abrasion resistance
and some ductility. Traditional ceramic materials generally meet one or the
other of these types of requirement but are very rarely resistant to both impact
and abrasion. In this regard, document (U.S.Pat. No RE39, 998 E issued
Jan.2008) discloses a method where a combination of Al2O3 and ZrO2 is
proposed by a judicious choice to achieve the desired hardness and toughness of
ceramic composite.
Document (US. Pat. No. 3,181,939 issued May 1965) also discloses a method
for manufacturing of fused Al2O3 and ZrO2 abrasives, which combine good
wear resistance characteristics of Al2O3 and the toughness of ZrO2 and can
improve the life of wear resistant segment.
The document U.S. Pat. No 2013/0126649 A1 issued in May 2013 also
discloses a method where working face of the segment is reinforced by granules
(size 0.5 to 5 mm) of a ceramic composite of 57 Wt % Al2O3 and 43 Wt %
ZrO2. To fabricate the wear surface, compacted insert of granules is placed in

the mould before the segment is cast at about 1500°C. During the casting
operation, the ceramic insert is impregnated by the liquid cast metal and the
granules are completely embedded in the metal matrix.
Document U.S. Pat. No 8147980B2 (2012) also discloses a method for
fabrication of metal matrix ceramic composite (MMCC) wear part. In MMCC
wear part, the wear portion is made of ceramic cake comprising of Al2O3, ZrO2,
fine ceramic powder of Al2O3, any one of the carbide materials such as SiC and
WC and appropriate binder. These powders and binders are mixed in a flexible
holder and the mixture is hardened to form a ceramic cake. In order to provide
the adequate strength, the cake is heated to a temperature between 80- 2200C.
The ceramic cake is reinforced with the SG iron and finally shaped into the
grinding roll of bowl mill.
To avoid the poor infiltration of metal around the reinforced grains, document
(U.S.Pat. No RE39, 998 E issued Jan.2008) discloses a method where ceramic
pad is formed with a spongy structure (honeycomb shape), which has a three
dimensional network of open pores all of which communicate with one another.
The spongy structure promotes the arrival of the liquid metal and solves the
problem of the poor infiltration of the liquid metal around the granules.
Moreover, when the thickness of the wear surface is larger than 25 mm, two or
more pads are superimposed, separated by a minimum gap of the order of 10
mm for the infiltration of the liquid metal around the granules. In this way an
appreciable filtration is achieved around the reinforced grains.

In this document, filtration is achieved by superimposing the pads with
minimum gap of around 10 mm. However, the gap of 10 mm can reduce the life
of roll due to fast wear, as there are no hard granules.
Indian Patent Application no. 201731041870 a method is proposed, which
avoids the superimposition of pads and improves the filtration of liquid metal
around the hard granules with a working surface thickness of more than 45-50
mm. In order to enhance wettability between abrasive grains and high
chromium cast iron, the grains were coated with cobalt tungsten carbide (12 %
Co-WC) alloy. The tungsten carbide (WC) particles were selected for
reinforcement, not only because of their high hardness, but also because of their
ability to be entirely wetted by molten iron. The presence of Co further
improves the wettability for better filtration.
Even though aforesaid prior art for fabrication of ceramic composites and
ceramic pads with proper infiltration of metal around the granules are available.
Still there is a need of a process /design, which can avoid the use of costly Co-
WC alloy and superimposing of two pads to achieve the improved filtration
around the granules, when thickness of the wear surface is larger than 25 mm.
In addition to this, design of wear resistant segment should be such that,
working surface should have homogenous wear pattern over the surface and it
should withstand against the high mechanical stresses developed during
grinding process. On the basis of above issues following are the objectives of
present invention.

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a fabrication
process for high strength wear resistant grinding rolls with enhanced
performance and higher operational life.
A further object of the present invention to propose method of fabrication of
high strength grinding rolls with improved wear resistant segment having metal
filtration around the hard granules.
Another object of the invention is to propose a fabrication process for high
strength wear resistant grinding rolls with enhanced performance and higher
operational life, which can avoid the coating of Co-WC alloy around the
granules to achieve the metal filtration up to 50-60 mm.
A still another object of the invention is to propose a fabrication process for
high strength wear resistant grinding rolls with enhanced performance and
higher operational life, which avoids the superimposing of two pads to achieve
the improved filtration around the granules, when thickness of the wear surface
is around 50-60 mm.
A further object of the invention is to propose a fabrication process for high
strength wear resistant grinding rolls with enhanced performance and higher
operational life which has homogeneous wear pattern over the surface and can
withstand against the high mechanical stresses developed during grinding
process.

SUMMARY OF THE INVENTION
Present invention, disclose, propose a fabrication process for high strength wear
resistant grinding rolls with enhanced performance and higher operational life
where surface of wear resistant segment of grinding rolls is reinforced by hard
granules in plough shape structure, having excellent wear resistance and
toughness. Present invention also, proposed a design, which avoids the coating
of granules by Co-WC alloy and superimposing of two pads to achieve the
improved filtration of metal around the granules up to thickness of 50 -60 mm.
In addition to this, a design of wear resistant segment is proposed, which leads
to homogenous wear pattern over the surface and can withstand against the
mechanical stresses developed during grinding process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1. Schematic for the fabrication of grinding rolls using high wear
resistant metal-ceramic inserts.
Figure 2. Schematic of reinforced hard granules inside the metal matrix.
Figure 3. Side view of plough shape structure of hard granules over the
working surface of wear resistant segment.
Figure 4. Showing the arrangement to get the equal spacing of wear resistant
segment over the grinding surface. The wear resistant segment having male
extrusion X, is shown in Fig. 4 (a). The female slots Y, on other side of wear
resistant segment is shown in Fig. 4 (b). The male extrusion and female slots
lead to equal spacing between the consecutive segments, as shown in Fig. 4 (c).

Figure 5. Showing the top view of saw tooth structure on the top of wear
resistant segment for better griping of segment. This structure helps the wear
resistant segment to withstand against the mechanical stresses developed during
grinding process.
DETAIL DESCRIPTION OF THE INVENTION
As noted above, grinding rolls are subjected to considerable wear due to higher
ash and silica content present in the coal and need to be replaced periodically.
Embodiments of the present invention describe a process /design, which avoids
the coating of Co-WC alloy and superimposing of two pads to achieve the
improved infiltration around the granules, when thickness of the wear surface is
larger than 25 mm. In addition to this, present invention proposed a design of
wear resistant segment, which leads to homogenous wear pattern over the
working surface and can withstand against the high mechanical stresses
developed during grinding process. Embodiment described below elaborates the
process adopted for fabrication of grinding rolls.
To fabricate the grinding rolls, first ceramic inserts are fabricated using highly
abrasive grains. Then pre-shaped ceramic pads are kept in the mold and poured
with high chrome iron to have the metal-ceramic insert. These fabricated metal-
ceramic inserts called wear resistant component are arranged on the outer
periphery of the casting die and poured with SG iron /Grey iron during the
centrifugal casting of the grinding roll. The complete procedure for developing
the grinding roll is shown in Fig.1.

To fabricate the ceramic inserts, grains made of Al2O3 (60-70 wt. %), ZrO2 (30-
40 wt. %), and TiB2 (0.5-5wt. %) having size 1.4 to 2.5 mm and hardness >
1600 Hv were used (process is defined in recently filed patent, Application No.
201731041870). For fabrication of wear resistant segment, ceramic pads are
placed in the sand mold and liquid metal is poured into the mould cavity at
about 1200°C-1400°C to produce the component. In the produced component,
the thickness of wear surface embedded with hard granules ranges up to 55-60
mm. The schematic of reinforced layer with hard granules is shown in Fig.2.
In the present invention, coating of hard granules with Co-WC alloy (which
improve metal filtration) and superimposing of two pads is avoided to achieve
the complete metal filtration around the hard granules, when thickness of the
wear surface is around 50-60 mm. To achieve the improved filtration, hard
granules are embedded over the surface in the form of plough shape structure,
which follow exactly the wear pattern over the surface during grinding process.
The side view of plough shape structure is shown in Fig. 3.
Plough shape structure have the variation in wear pattern from top to bottom,
during actual grinding process. The point A in Fig. 3, which is around 20-25
mm below from the point 1, have the maximum wear of 50-55 mm during the
grinding process. The wear at point B, which is 100-105 mm below from the
point A, have maximum of around 55-60 mm, which is the maximum wear,
which occurs at grinding surface. The point C, which is above 20-25 mm from
point 2, have the lesser wear compared to point A and B. The wear at point C,
range from 40-45 mm.

The plough shape structure of embedded grains have the gap of around 5-8 mm
from the working surface, which promotes the infiltration of metal. In addition
to this, the sufficient gap of granules is maintained from point 1, and 2 for
arrival of the metal. The plough shape structure, proposed in the present
invention exhibits better infiltration, because one; its follows the actual wear
pattern and second; variation in the thickness of embedded grains from top to
bottom, which helps in maintaining the volume percentage of metal and hard
granules inside the component, and leads to improved infiltration without
coating of Co-WC and superimposing the pads. The better infiltration is
achieved when, metal volume percentage in wear resistant segment is between
60-80%, preferably from 60-75%, and more preferably from 65% to 70% of
total volume.
The final dimensions of wear resistant segment, is taken in such a way that, the
metal volume percentage is around 65% to 70% of total volume and percentage
of hard granules range from 25-30%. During the casting process, if metal
volume percentage decreases below 60%, extra force provided by molten metal
decreases which leads to poor infiltration around the granules. In the present
invention, by maintaining the volume percentage ratio between hard granules
and metals, the penetration is achieved without coating of granules by Co-WC
and superimposing the pads.
For the better grinding ability and longer life, wear resistance segment at
working surface should have homogenous wear pattern over the surface and it
should withstand against the high mechanical stresses developed during
grinding process. To have the homogenous wear pattern over the surface, an

arrangement is made in wear resistant segment, which provides equal spacing
between the consecutive segments.
The arrangement to get the equal spacing among the segments is shown in Fig.
4. The wear resistant segments have male extrusion and female slots on the
surface, perpendicular to working surface. As shown in Fig. 4 (a), the male
extrusions X, have a thickness of around 8-10 mm and height of around 15 mm.
The gap between male extrusions ranges from 350-380 mm, more preferably
350-355 mm. The female slots Y, on other side of wear resistant segment [Fig.
4 (b)], have a depth of 5 mm and can accommodate the male extrusion X, of
previous segment, which leads to equal spacing between the consecutive
segments, as shown in Fig. 4 (c). The other advantage of male / female slots is
that the segments do not move from their positions during centrifugal casting.
As discussed above, the wear resistant segment should withstand against the
high mechanical stresses developed during grinding process. To provide better
grip of wear resistant segment inside the SG iron matrix, a gripping
arrangement is made on top and bottom portion of wear resistant segment. The
gripping arrangement on the top of wear resistant segment has the saw tooth
structure, top view is shown in Fig. 5. The gripping segments have thickness of
around 50-60 mm and length of around 80-85 mm. The height of tooth ranges
from 5-8 mm from the baseline B, and two consecutive teeth have an angle of
60° for better infiltration and grip. During the centrifugal casting, SG iron
infiltrates in the space available between the saw tooth structures and provides
the grip after solidification.

Although the present invention has been described with reference to the
preferred embodiments, thereof, it is intended that the specification and
examples be considered as exemplary only, the true scope and spirit of the
invention being indicated by the following claims:


We Claim:
1. A fabrication process for high strength wear resistant grinding rolls with
enhanced performance and higher operational life, characterized in that a
surface of wear resistant segment of the grinding rolls is reinforced by
hard granules having improved wear resistance and toughness, in that the
hard granuels having a size 1.4 to 2.5 mm and hardness > 1600 Hv, and
in that the hard granules is a mixture of Al2O3, ZrO2, and TiB2.
2. The process as claimed in claim 1, wherein the Al2O3 content in the hard
granules is between 60-70 wt % of the total weight, and ZrO2 content in
the hard granules is between 30-40 wt % of the total weight, and TiB2
content in the abrasive grains is between about 0.5 to about 2 % in
weight.
3. The process as claimed in claim 1, wherein the granules are embedded
over the surface of the grinding roll in the form of plough shape structure
and wherein the plough shape structure of hard granules, exactly matches
with the actual wear pattern over the surface to maintain the volume
percentage of metal and hard granules and leads to improved infiltration.
4. The process as claimed in claim 3, wherein the variation in wear pattern
from top to bottom has the plough shape structure of hard granules, has
maximum wear of around 55-60 mm during grinding process, has gap of
around 5-8 mm from the working surface to promote the infiltration of
metal, and wherein the plough shape structure of hard granules of claim

1, also has sufficient gap from top and bottom of wear resistant
component for arrival of the metal.
5. The process as claimed in claim 1 to 4, wherein the metal volume
percentage ranges from 65% to 70% of total volume, and wherein
percentage of hard granules range from 25-30%.
6. The process as claimed in claim 1, wherein variation in the thickness of
embedded granules maintains a volume percentage of metal and hard
granules which leads to improved infiltration without coating of Co-WC
and also without using superimposing the pads.
7. The process as claimed in any of the preceding claims, wherein the wear
resistant segments have male extrusion and female slots on the surface,
which provide equal spacing between the consecutive segments in order
to produce a homogeneous wear pattern over the surface of the grinding
roll.
8. The process as claimed in claim 1, wherein the wear resistant segment
has saw tooth structure inside the SG iron matrix, top and bottom portion.
9. The process as claimed in claim 8, wherein the saw tooth structure has an
angle of 60° between two consecutive teeth for better metal infiltration
and grip.