FIELD OF INVENTION
The present invention relates to a process to fabricate the high strength wear
resistance grinding rolls to enhance its life. More particularly, the invention
relates to a process by which wear surface is reinforced by the ceramic pads
having excellent wear resistance with toughness.
BACKGROUND OF INVENTION
Various industrial processes use the wear resistant materials to prevent or
decrease wear loss and to reduce downtime of the equipment running in contact
with abrasive environment, and also to increase the performance and quality of
the processes. Wear is one of the most commonly encountered industrial
problems in coal fired power stations, where coal is ground to powder in impact
mills by grinding rolls. The major concern of the grinding rolls during the course
of coal pulverization is its "harsh abrasive action with the coal" due to higher ash
and silica (Quartz) content present in the coal. Silica has the 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 reaches the condition where it
needs to be replaced. This involves the plant shutdown and ultimately affects the
plant efficiency.

The life of grinding rolls is generally enhanced by reinforcement of hard particles
(i.e. ceramic grains) in ductile matrix. The wear rate of the harder reinforcement
is typically much smaller than that of the matrix. Once the hardness of particles
is higher than the quartz, then it will not be adversely affected by the quartz
content of the ash as compared with those of grinding elements composed of
materials softer than quartz.
In this context, Tatsuo Natori (U.S. Pat. No. 5,052,464 issued Oct. 1991)
disclosed a method to fabricate the improved wear surface. A mixture of ceramic
particles (such as SiC particles) with metallic powder such as iron-base particles
is placed in a predetermined position of an internal wall of a mold and the
molten metal (i.e. cast iron) is poured into the mold. In this manner a wear
resistant layer is formed in the required portion on a surface of the casting at the
same time when pouring is being done for manufacturing the casting. The
specimen taken from the cast block of the present invention was subjected to
hardness measurement. As a result, it was found that the SiC particle had
hardness of 2700-3000 Hv and improved wear resistance.
The document U.S. Pat. No 2013/0126649 A1 issued in May, 2013 discloses a
method for fabrication of grinding ring for centrifugal vertical roller grinding mills
with higher wear resistance. In the present invention, the working face of the
segment is reinforced depth wise with ceramic particles. The wear layer is

reinforced by granules (size 0.5 to 5 mm) of a ceramic composite of alumina-
Zirconia (57 Wt % Al2O3-43 Wt % ZrO2) from which a compacted insert is placed
in the mould before the segment is cast at about 1500°C. During the cast
operation, the ceramic insert is impregnated by the liquid cast metal and the
ceramic particles are completely embedded in this way in the metal matrix. This
impregnation leads to a substantially pore-free composite structure on the
working face of the segment having a wear layer of about 20 mm.
Claude Poncin, (U.S. Pat. No 7,935,431 B2 issued May, 2011) discloses a method
for production of cast parts with enhanced wear resistance by an improvement in
the resistance to abrasion whilst retaining acceptable resistance to impact in the
reinforced areas. In the present invention cast wear part, is reinforced by at least
one type of metallic carbide, borides, oxides etc. The raw materials acting as
reagents for said components have been put into a mould, before casting, in the
form of inserts or pre-shaped compacted powders or in the form of barbitones,
in that the reaction of said powders is triggered in-situ by heat of the molten
metal. The materials that react in-situ produce high wear resistance surface
having hard particles of carbides, borides, oxides, etc. The hardness values of
embedded particles into the reinforced surfaces range from 1300 to 3000 Hv.
Document U.S. Pat. No 8147980B2 (2012) also discloses a method for fabrication
of metal matrix ceramic composite (MMCC) wear part. In this MMCC, the wear

portion formed by a ceramic cake impregnated by metal (i.e. SG Iron), wherein
the ceramic cake comprises of Al2O3, ZrO2, fine ceramic powder of Al2O3, any one
of the carbide materials such as boron carbide, silicon carbide and tungsten
carbide and the sodium silicate 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-
220°C. The ceramic cake is reinforced with the SG iron and finally shaped into
the grinding roll of bowl mill.
The wear resistance components in plants are often subjected to high
mechanical stresses and to a high wear by abrasion at the working face. It is,
therefore, desirable that these components should exhibit a high abrasion
resistance and some ductility, to be able to withstand the mechanical stresses
such as impacts. Another problem arises from the fact that, above a thickness of
25 mm of the ceramic composite, poor infiltration of the metal is observed.
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 adjust the
hardness and the toughness of ceramic composites. Al2O3 contributes to good
hardness, however, ZrO2 present in the alumina make it possible to increase the
resistance of the latter to cracking and thus to obtain a toughness greater than
that of each of the components considered in isolation.

In the same document, inventors proposed a method to fabricate the ceramic
pads to avoid the problem of poor infiltration. According to this document a
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. This ceramic pad is formed by pouring grains of ceramic materials into
an appropriate mould with a liquid adhesive. Honeycomb shape geometries
permit the arrival of the liquid metal and solve the problem of the poor
infiltration of the liquid metal within the ceramic phase. Moreover, when the
thickness of the ceramic pad 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 into various pads. In this way an appreciable
increase in the proportion of the ceramic phase within the insert is obtained
without being confronted with the problem of the poor infiltration by the metal.
Even though aforesaid varieties for fabrication of ceramic composites and
ceramic pads with proper infiltration of metals are available, still there is a need
for further improvements in the wear resistant and toughness of ceramic
composites. In addition to this, a method is required to fabricate the ceramic
pads with proper infiltration of metal without superimposing the ceramic pads to
make the process more efficient and economical.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a process to fabricate the
high strength wear resistance grinding rolls to enhance its life, which is capable
of reinforcing the wear surface of the grinding rolls with material of excellent
wear resistance and toughness.
Another object of the invention is to propose a process to fabricate the high
strength wear resistance grinding rolls to enhance its life, which is able to avoid
the use of organic binder for synthesis of abrasive grains (i.e. wear resistance
material).
A further object of the invention is to propose a process, wherein proper
infiltration of metal can be achieved within the ceramic phase without
superimposing the ceramic pads.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - Shows schematic for the development of grinding rolls using high
wear resistance metal-ceramic inserts.
Fig.2 - Shows optical micrograph of Vickers indentation on the surface of
abrasive grains, fabricated by solid state sintering process.

Fig.3 - Shows image of the metal-ceramic inserts reinforced by ceramic
phase of thickness 50-60 mm on working surface.
Fig.4 - Shows schematic of reinforced ceramic grains inside metal matrix.
Fig.5 - Shows cross-section of the metal-ceramic insert, showing the
distribution of ceramic grains inside the metal matrix.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As noted above, grinding rolls (1) 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 disclose a fabrication process
for high strength wear resistance grinding rolls with enhanced performance and
higher operational life. In the present invention, cast wear part of grinding rolls is
reinforced by the ceramic pads having excellent wear resistance with good
toughness. Below embodiment describes the process adopted for fabrication of
grinding rolls.
To fabricate the grinding rolls (1) 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 (1) are arranged (3) on the outer periphery of the casting die (2)

and poured with SG iron/Grey iron (4) during the centrifugal casting of the
grinding roll (6). The complete procedure for developing the grinding roll (6) is
shown in Figure 1.
The abrasive grains to fabricate the ceramic pads are mixture of Aluminum Oxide
(Al2O3)(60-70 wt.%), Zirconium Oxide (ZrO2)(30-40 wt.%), and Titanium
Diboride (TiB2)(05-5wt.%). Aluminium oxide contributes to good hardness.
Zirconia particles present in the Alumina improve the resistance to cracking and
thus the toughness of composite/grains. After judicious choice of Al2O3 and ZrO2,
the addition of small amount of TiB2 in the mixture, leads to drastic improvement
in the wear resistance of composites, due to its inherent high hardness (~ 34
GPa).
Whereas the addition of large amounts of TiB2 grains do not show any further
improvement in wear resistance/hardness. To fabricate the highly abrasive
grains, the amount of TiB2 is preferably between about 0.5 to about 5% of the
total weight, more preferably between about 0.5 to about 2% in weight. The
particle size of TiB2 in the mixture ranges from 5-10 microns.
To avoid the use of organic binder and improve the toughness of the grains,
metals like cobalt were selected for sintering of Al2O3, ZrO2 and TiB2. During the
sintering, cobalt forms liquid state, which helps in binding /diffusing of Al2O3, ZrO2

and TiB2 grains together. The presence of cobalt particles along the grain
boundary makes a great contribution to the fracture toughness. The ductile
cobalt deforms plastically and dissipates energy during crack initiation and
propagation.
In the mixture of Al2O3, ZrO2 and TiB2, the amount of cobalt is preferably between
about 2 to about 10% of the total weight, more preferably between about 2 to
about 5% in weight. The particle size of cobalt in the mixture ranges from 20-50
nm. It is also within the scope of this invention that, other metals may be used
such as, for example, aluminum, stainless steel, copper, nickel, alloys of any of
these.
However, these transition metals react with TiB2 to form metallic borides of the
MB, M2B and M23B6 types, which are even more brittle than TiB2 itself. In the
present invention the formation of secondary phases are avoided by adding
controlled amounts of Ti/Al to the powder mixtures before sintering.
The densification of mixture was carried out at around 300 MPa for 5 min. After
densification, sintering was carried out between 1400°C-1800°C, preferably at
1600°C. Sintered material is crushed in order to obtain grains, the size of which
varies from 1 to 10 mm, preferably from 1 to 5mm, and more preferably from
1.4 to 2.5 mm.

The Vickers hardness (Hv) of the sintered samples was measured by Vickers
hardness tester under the force of 30 kg, using the equation Hv = 1.854 F/d2
where F is the indentation load and d is the arithmetic mean of two diagonals (d1
and d2). The length of the diagonals are calculated using the impressions shown
in Fig. 2. The average hardness of grains ranges from 1650-1800 Hv.
To fabricate the ceramic inserts, a desired patternjiaving various size cavities is
created, in plaster of paris (POP) with the help of metallic dies. To avoid the poor
infiltration of the metal into ceramic phase, initially ceramic grains are mixed with
SG iron powder (or flakes) /adhesive and placed into POP mold to form the
desired shape for the inserts. The mixture is sintered at around 100-120°C to get
the ceramic pads (1) having adequate strength. In the present invention poly-
vinyl alcohol (PVA) is used as an adhesive. The adhesive may be inorganic or
organic.
The weight percent of SG iron powder in ceramic grains is preferably between
about 20 to about 50% of the total weight, more preferably between about 45 to
about 50% in weight. The percentage of adhesive ranges from 3-5 wt. % of the
total weight. In the present invention, the ceramic pads (1) include number of
passages to facilitate flow and penetration of the molten metal during the casting
process. The size and shape of the passages may be sized to facilitate good
permeation of the base metal into the porous pads.

To fabricate the metal-ceramic inserts (1), ceramic pads are placed in the sand
mold and liquid metal at about 1500°C is poured into the mould cavity to
produce metal-ceramic inserts (1). In order to solve the problem of the poor
infiltration of the liquid metal within the ceramic phase, pressurized gate ways
are designed. During the casting process, pressurized gate ways provide extra
force to the molten metal for better infiltration inside the ceramic phase. In
addition to this, metal powder inside the ceramic inserts (1) is melted by the
heat of the molten metal and better infiltration of metal within the ceramic phase
is achieved. Wear-parts thus produced are subjected to special heat treatment so
that metallic portion develops better wear resistance. After pouring the image of
metal-ceramic insert (7) is shown in Fig. 3.
In the present invention, working face of the metal-ceramic inserts is reinforced
in depth (7) by ceramic phase. The thickness of the reinforced layer (8) is
generally higher than 20 mm, preferably higher than 30 mm and generally
situated between 50 and 60 mm. The schematic of reinforced layer (8) is shown
in Fig.4. To see the infiltration of metal into ceramic phase, the fabricated
segment is cut into many parts. The cross section of the segment is shown in
Fig.5. From the figure it is evident that, ceramic grains are completely embedded
(10) in the metal matrix (9) and pore-free composite structure on the working
face of the segment is achieved. In the present invention, proper infiltration of
metal is achieved with the help of metallic powders and pressurized gate ways.

Finally, fabricated metal-ceramic inserts (7) are arranged on the outer periphery
of casting die (2) and poured with molten metal during the centrifugal casting
(2) of the grinding roll (1). In the present invention a suitable liquid metal
composition is selected based on the application of the wear part. The metal
composition contains about 0.2% to about 1.2% C and about 2% to about 8%
Cr, and have additional alloying elements such as Mn, Mo, Ni & Cu. The hardness
of metal matrix ranges from 700-750 Hv. Grinding rolls of the claim 1, is
fabricated by arranging metal-ceramic insert on the outer periphery of casting
die and pouring with molten metal during the centrifugal casting.
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 process to fabricate the high strength wear resitance grinding rolls to
enhance its life comprising;
selecting mixture of Aluminium oxide (Al2O3), Zirconium Oxide (ZrO2) and
Titanium Diboride (TiB2) as abrasive grains to fabricate ceramic pads/inserts;
adding cobalt for sintering of Al2O3, ZnO2 and TiB2 to bind/diffuse the said
grains together;
carrying densificaiton of the mixture and then sintering;
crushing the sintered material to form grains;
creatintg a pattern having various size cavities in plaster of Paris (POP) with
metallic dies;
mixing ceramic grains with SG iron powder in POP mold to form desired
shape of the inserts;
stirring the mixture at around 100-1200C to obtain ceramic pads of adequate
strength;
wherein, ceramic pads (1) are placed in the sand mould and liquid metal at
about 15000C is poured into a mould cavity to produce metal-ceramic insert
(7) where pressurized gate ways provide extra force to the molten metal for
better inflatration inside the ceramic phase, and metal powder inside the
ceramic inserts (1) is melted by the heat of the molten metal resulting better
infiltration of metal within the ceramic phase wherein the produced wear

parts are subjected to special heat treatments to develop better wear
resistance on metallic portion wherein, working face metal-ceramic inserts (7)
is reinforced in depth by ceramic phase when fabricated metal-ceramic inserts
(7) are arranged on the outer periphery (3) of casting die (2) and poured
with molten metal for the centrifugal casting (2) of the grinding roll (6).
2. The process as claimed in claim 1, wherein the abrasive grains to fabricate
the ceramic pads are mixture of 60-70 wt% of Al2O3, 30-40 wt% of ZrO2 and
0.5-5 wt% of TiB2.
3. The process as claimed in claim 1, wherein the amount of cobalt in abrasive
grains is about 2%-10% of the total weight, preferably between about 2%-
5% in weight.
4. The process as claimed in claim 1, wherein the presence of cobalt in the
abrasive grains provide binding action and toughness in the abresive grain.
5. The process as claimed in claim 1, wherein sintering of abrasive grains is
carried out in between 14000C – 18000C preferably at 16000C.
6. The process as claimed in claim 1, wherein the size of abrasive grains varies
from 1 to 10 mm preferably from 1 to 5 mm and more preferably from 1.4 to
2.5 mm.

7. The process as claimed in claim 1, wherein the weight percent of SG iron
powder in ceramic pad (1) is preferably between 20-50% of the total weight,
more preferably bwtween about 45-50% in weight.
8. The process as claimed in claim 1, wherein the percentage of adhesive in the
ceramic pads (1) ranges from 3-5 wt% of the total weight.
9. The process as claimed in claim 1, wherein the thickness of the reinforced
layer of the working face of the metal-ceramic inserts by ceramic phase is
higher than 20 mm, preferably highrr than 30 mm and generally between 50
and 60 mm.