FIELD OF THE INVENTION
The present invention relates to a process of producing high strength wear
resistance hard faced spheroidal graphite (SG) cast grinding rolls and bull ring
segments for bowl mills in east Pulveriser with enhanced performance and higher
operational life.
BACKGROUND OF THE INVENTION
The power plant efficiency is largely dependent on the grinding roll and bull ring
segment (BRS) wear life. But in the current situation, especially for the Indian
coal the higher percentage of ash and silica content has become a major concern
as these are high abrasive materials which can impact the overall wear life of the
grinding rolls and BRS. This is the trigger for exploring newer innovations to have
grinding rolls with better strength and abrasive resistance. Hence, there is a
need to develop a high strength wear resistance grinding roll with enhanced
performance and higher operational life.
Pulveriser becomes one of the major components in most of the coal fired
thermal power plants, whose performance and operational life dictates the total
power plant efficiency. Performance of Pulveriser is determined by three
important parameters namely, the desired size reduction of the coal, uniform
flow rate and the higher classifier efficiency which is used to crush coal lumps
into the fines and then forwarded to the boiler section. The higher ash and silica

content in coal exerts harsh abrasive action on the grinding rolls and bull ring
segments during the course of coal pulverization. Due to this, surface of the
grinding rolls get worn out faster and reaches to the situation where it needs to
be replaced. This involves the plant shutdown and ultimately affects the plant
efficiency. Hence to overcome this problem, a novel idea has been outlined for
developing the high strength wear resistance grinding rolls with enhanced
performance and higher operational life. There are few researchers who have
reported the significant achievements in developing the high strength wear
resistance grinding rolls which are as follows:
Recently, Hubert Francois from Magotteaux International S.A. received a granted
patent USR 39998 E in which a composite wear component has been produced
by casting which consists of metal matrix whose working surface or faces include
very high resistance inserts made of ceramic pad. These ceramic pads are
fabricated with a homogeneous solid solution of 20 to 80% of Al2O3 and 80 to
20% ZrO2 by weight percentages. Then this pad being impregnated with a liquid
metal during the casting. The ceramic pad is produced from an aggregate of
ceramic grains having particle range of 0.7 mm to 5.5 mm. These grains are
made by electro fusion, by sintering, by flame spraying or by any other process
allowing the two constituents to fuse. The adhesive used for binding these grains
can be made of silicate or in the form of epoxy resin and is preferably between 2
to 3% by weight.
Sudhir Vaman Bhide from AIA Industries, Ahmedabad, India has been awarded
with a Patent US 8147980 B2 (2012) in which the process of manufacturing

metal matrix ceramic composite (MMCC) wear part has been disclosed. This
MMCC becomes part of wear portion which is impregnated by metal (SG Iron),
wherein the ceramic cake comprises of Al2O3 ZrO3 fine ceramic powder (AI2O3),
any one of the carbide materials such as boron carbide, silicon carbide and
tungsten carbide and the sodium silicate binder. All the above mentioned
powders and binders were mixed in a flexible holder and the mixture was
hardened to form a ceramic cake. In order to provide the adequate strength,
cake is heated to a temperature between 80-220° C. The ceramic cake was
reinforced with the SG iron and finally shaped into the grinding roll of bowl mill.
It was observed that the life of the grinding roll improved substantially
(quantitative value of the operational life has not been mentioned in this patent).
Gronvall et al developed the bimaterial elongated insert member for the grinding
roll as per their patent publication EP 2512680 Al (2012). The insert member
comprised a core of a first material having a first hardness, and the body of
second material having second hardness which enclosed the core. The first
hardness was greater than the second harness. The advantage is that the
reinforced insert member will be less likely to dislodge from the grinding roll due
to the elasticity created between the insert member and the grinding roll.
Mirchandani et al (US 0011965 Al) (2011), developed the wear resistance metal
matrix composite which comprises the inorganic particles dispersed in a matrix
material. Melting temperature of the inorganic particle was being greater than
the melting temperature of the matrix.

Poncin et al (US 7513295 B2 (2009) teaches development of cast parts with
enhanced wear, abrasion and impact resistance at a financially acceptable price
and also discussed about the method of their production. The method comprises
of placing two or more powdered raw materials in the mold and then after
adding a molten casting metal to the casting mold. The molten casting metal
generates the in-situ chemical reaction between the powders and finally provides
the particulate porous conglomerate. The powder raw material selected from the
group consisted of Ferro-alloys, oxides, nickel, nickel alloy, iron alloy, titanium
alloy etc.
Francois et al (US 6399176 (2002) developed the composite wear components
by centrifugal casting in which ceramic pad (in the form of honeycomb structure)
is reinforced in the molten metal. The wear component produced by centrifugal
casting consists of a metal matrix whose wear surface comprises inserts (made
of ceramic material, 20-80 wt. % Al2O3 and 80-20 wt % Zr02, Particle size
range: 0.7-5.5 mm) which is having good abrasion resistance properties. The
idea of using aluminium oxide and zirconium oxide having relatively different
properties was to adjust the hardness, toughness and thermal expansion
coefficient of the composite ceramic. Zirconium dioxide has the advantage of
having an expansion coefficient which is close to that of metal. In addition, it
contributes a good toughness, which reduces the risk of breakage. In addition,
ZrO2 particle preset in the AI2O3 increases the resistance of the Al2O3 to cracking
and hence increases the toughness greater than that of each component
considered individually (namely ZrO2 or Al2O3). Various geometries were

proposed in their invention in order to solve the infiltration of the liquid metal
within the ceramic phase. For the efficient infiltration of the liquid metal and to
reduce the risk of propagation of the crack, honeycomb type of pad was
fabricated. Finally, Ceramic inserts (hardness: 1600 Hv, expansion coefficient
~8 * 10-6/K) reinforced with the pig iron matrix (hardness: 750 Hv) and thus
composite wear component was fabricated.
Invention of James L, Overstreet, US 850036 B2 (2013) discloses development of
ultra hard sintered carbide particles hard facing for earth boring bit. The ultra
hard particles consist of tungsten carbide grains, Cobalt and Vanadium. The ultra
hard particles are dispersed within a matrix metal of iron, nickel or alloys thereof.
The composition may also have sintered tungsten carbide particles of a larger
size than the ultra hard particles. The ultra hard particles have a greater
hardness than the sintered tungsten carbide particles
Krizan et al US 4,923,511 (1990) teaches a hard facing powder to be deposited
"by plasma transferred arc welding on a substrate. The hard facing powder
includes a plurality of different types of tungsten carbide particularly W2C and
WC-Co, within a matrix alloy of Carbon 0.017-0.52% by weight, Silicon 3,3-4.7%
by weight, Boron 2.7-3.3% by weight, Iron 0.27-0.4% by weight and balance
Nickel. The total content of WC is minimum of 50% by weight.

Grover et al US 4,339,272 (1982) describes tungsten carbide-based hard metal
comprising of 75 ~ 97 weight % tungsten carbide and the balance is binder
having a composition 14 weight % manganese, 21/2% carbon, 5% nickel and
balance iron instead of the conventional binder Cobalt.
Apart from above, there are a few more patent literatures EP 0234026 A2
(1987), EP 0192972 Al (1986), US 4604781 (1986), US 4610401 (1986), US
4389767 (1983), US 3859057 (1975), US 3071489 (1963), which discuss about
the development of wear resistance surface layer. In order to make the
document brief and compact, the discussion on the abovementioned patents is
not included here.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process of
producing high strength wear resistance hard faced spheroidal graphite (SG) cast
grinding rolls and bull ring segments for bowl mills in Pulveriser with enhanced
performance and higher operational life.
Another object of the invention is to propose a process of producing high
strength wear resistance spheroidal graphite (SG) cast grinding rolls and bull ring
segments for bowl mills in east Pulveriser with enhanced performance and higher
operational life in which tungsten carbide materials having higher hardness,
strength and wear resistance with matching density, wettability and coefficient of
thermal expansion of the metal (SG iron) is selected.

A still another object of the invention is to propose a process of producing high
strength wear resistance spheroidal graphite (SG) cast grinding rolls and bull ring
segments for bowl mills in east Pulveriser with enhanced performance and higher
operational life wherein WC is sprayed by a feeder system onto the working
surface of Rolls and Bull ring segments for hard facing.
Yet another object of the invention is to propose a process of producing high
strength wear resistance Spheroidal graphite (SG) cast grinding rolls and bull
ring segments for bowl mills in east Pulveriser with enhanced performance and
higher operational life in which the binder enables deposition and adherence of
WC powders o the working surface.
A further object of the invention is to propose a process of producing high
strength wear resistance Spheroidal graphite (SG) cast grinding rolls and bull
ring segments for bowl mills in east Pulveriser with enhanced performance and
higher operational life in which the working surface is machined to the desired
depth and requisite hard facing coat with Tungsten Carbide powder is provided.
SUMMARY OF THE INVENTION
The object of the present invention is to fabricate the high strength of wear
resistance grinding roll and bull ring segments with enhanced performance and
operational life which will ultimately lead to higher plant efficiency. This can be
achieved by making a centrifugally cast Spheroidal Graphite Iron and Sand
Casting Bull Ring Segments with SG iron. Then the working surface of both the

SG Iron Rolls and Bull Ring Segments will be machined to desired depth and
then the hard facing WC powder will be coated on to the machined working
surface to the calculated height to suit to the design parameters of Pulveriser.
The WC powder so chosen will have the required hardness and wear resistance
along with comparable density, coefficient of thermal expansion and wettability
of SG Iron.
Moreover, to maintain cost effectiveness, a balance is struck between the
operating performance as well as efficiency and the final price. A feeding agent is
used to achieve the requisite strength and wear properties along with desired
bonding properties with the SG Iron matrix.
Accordingly, there is provided a fabrication process to produce high strength and
wear resistance grinding rolls and bull ring segments for bowl mill with enhanced
performance and operational life. The process involves several steps comprising
selection of a composition of Tungsten Carbide Powder materials having higher
hardness and wear resistance including the properties such as density, thermal
coefficient of expansion and wettability corresponding to the SG Iron; hard facing
the surface of SG iron cast roll using a feeder mechanism; fixing the SG Iron Bull
Ring Segments of desired shape and size into the bowl mill followed by Tungsten
Carbide hard facing of the working surface. Accordingly, it is expected that rolls
and bull ring segments exhibit enhanced performance and higher operational life.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1: shows a SG Ion Roll with outer dia machined and coated with WC
Powder.
FIGURE 2: shows a cross sectional view of the SG Iron rolls showing the depth of
coated layer of WC.
FIGURE 3: WC coated working surface of a Bull Ring Segment.
FIGURE 4: Split view of the bull ring segment across the length.
FIGURE: 5 The final assembly of the grinding roll with bull ring segments.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a Tungsten Carbide hard metal having a
composition of 85-95% (preferably 90-94%) Tungsten Carbide hard metal
(typical combination of WC-Co, Pure Nickel, Chromium and Vanadium) and
balance binder (6-14%), is used for different embodiments of hard facing metal.
The selected material essentially consisting of:

Tungsten Carbide Cobalt (WC-Co): 40-65 % by weight of the hard facing
component where Co ranges between 6 to 8 % by weight of the WC-Co (particle
range between 45-100 microns)
Chromium (Cr): 5-10% by weight of the hard facing component (particle range:
45-100 microns) and
Vanadium (V): 0.2-2% by weight of the hard facing component (particle range:
45-100 microns)
Whereas a binder being one of Cobalt and a matrix alloy of Carbon: 2-2.5%,
Silicon: 4-5%, Boron: 2-3%, Manganese: 12-14%, Nickel: up to 5% and balance
Fe, with a particle range between 45-180 microns, is used.
The constituents of hard facing component need to be thoroughly blended to a
final particle size range of 45-100 microns. Then the coating of hard facing
material along with the binder on to the working surface of SG Iron substrate of
Rolls and BRS is achieved by either Thermal spraying of Plasma Transferred Arc
welding process/ thermal spraying.
Initially the SG Iron Rolls and Bull Ring Segments are cast to desired dimension
in respect of shape and size corresponding to the parameters of a Pulveriser mill.
Then the working surface is machined to a depth of 3-5mm as depicted to the
Figures 1 to 4 for Rolls and BRS respectively.

SG Iron Rolls are manufactured to a centrifugal casting machine and the SG ion
bull ring segments are cast with CO2 sand Moulding.
The typical composition of the SG Iron metal is given below:
Carbon 3.0-3.4, Silicon: 2.3-2.5%, Manganese: 0.2% (max), Phosphorous:
0.03% (Max) and Sulphur: 0.01 (max) and residual Magnesium: 0.05-0.08%.
The mechanical properties of SG Iron cast component as produced is as given
below:
Tensile Strength: 414 N/mm2, min and % Elongation: 18 min (L=50 mm).
The microstructure of the SG Iron component is essentially ferrite and contain no
massive carbides and having 90% Type I and type II graphite as per ASTM A
247 plate I.
The working surface is either ground or machined to a desired depth for suitable
WC coating on to it. The finally fabricated grinding roll along with bull ring
segment assembly is shown in Figure 5.

WE CLAIM:
A process of producing high strength wear resistance spheroidal
graphite (SG) cast for grinding rolls and bull ring segments for bowl mills
in east Pulveriser with enhanced performance and higher operational life
comprising the steps of:
manufacturing spheroidal graphite (SG) grinding roll in a centrifugal
casting machine;
casting SG iron bull ring segments (BRS) in a CO2 sand Moulding, the
microstructure of the SG iron component being essentially ferritic and
having 90% of type I or type II graphite as per ASTM A 247 plate I;
providing a hard facing metal and blending to a particle size range of
45-100 microns;
providing a binder and mixing with the blended hard facing metal to
produce a coating; and
Coating the SG grinding roll ad SG iron bull ring segments by one of
the thermal spraying or plasma transferred arc welding process.

2. The process as claimed in claim 1, wherein said hard facing metal is
tungsten carbide having a composition by weight percentage of
tungsten carbide cobalt (WC-Co), pure nickel (Ni), chromium (Cr), and -
vanadium (V) respectively of 40-65, 20-30, 5-10, and 0.2 to 2.0, and
wherein particle range of said constituent elements are respectively at 45-
100 microns.
3. The process as claimed in claim 1, wherein the binder is mixed at 6-14
wt% with the hard facing metal wherein the binder is one of cobalt and
matrix alloy of carbon 2-2.5%, si- 4.5%, boron 2-3% manganese 12-
4%, nickel upto 5% and balance Fe, and wherein the binder (matrix alloy)
has a particle size of 45-180 microns.
4. The process as claimed in claim 1, wherein the composition of SG iron
metal consists of carbon (3-3 4%) silicon (2.3 to 2.5%), Mn (0.2%),
phosphorous (0.03%), sulphur (0.01%, and magnesium (0.05-0.08%).

5. The process as claimed in claim 1, wherein the minimum tensile strength,
minimum yield, and the minimum elongation of the SG Iron Cast
component is 414 N/MM2, 276 N/MM2, and 18% respectively.