Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02239440 1998-06-02
GRINDING ROD CHEMISTRY AND METHOD OF HEAT
TREATMENT TO ENHANCE WEARABILITY
FIELD OF THE INVENTION
This invention relates to a chemistry to enhance wearability of grinding
rods and heat treatment techniques to enhance wearability.
BACKGROUND OF THE INVENTION
Various technologies are available for manufacturing grinding rods for
use in grinding mills, such as in ore crushing, stone crushing and the like.
Grinding rods are usually 3 to 6 meters in length depending upon the size of
the
grinding device and have diameters which usually range from 7 to 10 cm. It
has been found that the useful life of a grinding rod may be improved if it
has a
hard outer shell usually of martensitic microstructure and relatively soft end
portions which are substantially of pearlitic microstructure. The soft end
portions minimize rod spalling and splitting thereof and reduce breakage and
wear of the rod mill liners. A discussion of grinding rods having soft end
portions may be found in US patent 4,589,934 as well as the several other US
patents discussed in the background of that US patent.
In an attempt to improve grinding rod longevity by way of heat
treatment, the chemistry of the steel in the grinding rod may be modified such
as described in US patent 4,840,686. The modification of the chemistry in the
steel of the grinding rod results in the rod core having a bainitic
microstructure
with less than 10% pearlite and a core hardness of at least about 40 Rockwell
C,
or 40 HRC. It is thought that making rods with the proper selection of
molybdenum and chromium to provide a rod core of mostly bainite enhances
the wear rate of the rod by nearly 20% over that of a conventional heat
treated
rod. The selected chemistry and heat treatment ensures that the core is of the
harder bainite where softer pearlitic material is to be avoided.
The rods, as made in accordance with either of US patents 4,589,934 and
4,840,686 are quenched after heating by passing the rod through a quench
spray. The quenching of the rod is commenced inwardly of the leading end of
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the rod and the quench spray turned off short of the trailing end of the rod.
It is
thought that by not applying quench water spray to the leading end and
trailing
end of the rod, softer end portions are developed. Also as taught, the rod may
have to pass through multiple quench zones in order to achieve the desired
extent of quenching to ensure the formation of the harder martensitic shell.
As
is described in US patent 4,589,934, minor amounts of quench water travelling
along the rod surface towards either the leading or trailing end portion may
create a wash effect, thereby expediting cooling of the end portion resulting
in
the formation of end portions which can have a hardness greater than 30 and
perhaps up to 45 or 50 HRC. To minimize this effect, the commencing of the
quench water spray and terminating of the quench water spray are activated or
deactivated a considerable distance from each end. A significant portion of
the
rod end is not treated resulting in a fairly large transition zone between the
quench portion of the rod which has the martensitic structure and the
untreated
end portion of the rod which has the pearlitic structure. In practice, the
softer
end portions of the rod may extend upwards of 30 cm or more with a very
gradual transition from the hard shell to the softer portion. This results in
a
grinding rod having a greater length of softer end portion with consequent
increased wear.
Although grinding rods having greater surface hardness and core
hardness have greater wearability, it has been found that durability, which
includes breakage of these rods is less than adequate particularly in severe
grinding environments. In accordance with an aspect of this invention a
grinding rod is provided which overcomes the above problems even in more
severe grinding environments.
SUMMARY OF THE INVENTION
In accordance with an aspect of the invention, a grinding rod chemistry
for enhancing wearability and durability of a steel rod comprises:
carbon .70 - 1.00 % by weight;
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manganese .60 - 1.00 % by weight;
silicon .10 - .40 % by weight;
chromium .25 - 1.04 % by weight;
molybdenum .01 - .25 % by weight; and
the balance being essentially iron and
with the proviso that a combination of carbon, molybdenum and
chromium within the above ranges are selected as follows to provide a non-
bainitic core:
a) at the lower 0.7% carbon with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 1.04% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.43%; and
b) at the upper 1.00% carbon, with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 0.80% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.28%.
In accordance with another aspect of the invention a grinding rod having
the above chemistry is characterized by:
i) a core of greater than 99% pearlite having a hardness less than 45
Rockwell C;
ii) an outer shell of tempered martensite having a hardness of greater
than 55 Rockwell C and a uniform annular thickness greater than about 1.25
cm; and
iii) a 10 cm to 15 cm soft end and having a hardness less than 35
Rockwell C.
In accordance in yet another aspect of the invention, a grinding rod of
the above characteristics may be produced by the process comprising:
i) reheating a formed steel bar to above its austenitising temperature
in a controlled manner to produce a reheated bar of substantially uniform
reheat
temperature;
ii) transferring with minimal cooling said reheated bar to an open
tubular quench vessel which is capable of enclosing an entire bar length,
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closing said vessel to provide a quench liquid tight seal about said bar while
securing said bar in said vessel to minimize bar warping in said vessel during
quenching;
iii) introducing quench water into an inlet end of said vessel and
passing said quench liquid along said vessel at high surface velocities
exceeding 4 meters per second relative to bar surface to minimize thereby
production of steam along the bar length and ensure uniform heat removal and
removing quench water at an outlet end of said vessel;
iv) quenching said bar in said vessel for a period of time which
provides a bar surface equalization temperature when removed from said vessel
of less than 350 C and greater than 150 C to provide a uniform annular layer
for said hard outer shell of tempered martensite and said softer core of
pearlite
where the end surface hardness is consistent with said hard tempered
martensite
shell, said developed uniform outer shell of martensite producing uniform
residual stress contributing to rod straightness;
v) reheating each end portion of said bar in a furnace to elevate, in a
controlled manner, said less than 15 cm end portion including its core to the
austenitising temperature, air cooling each said end portion to provide said
engineered end portion hardness of less than 35 Rockwell C.
In accordance with a further aspect of the invention, a grinding rod
chemistry for enhancing wearability and durability of a steel rod comprises:
carbon 0.70-1.00% by weight;
manganese 0.60-1.00% by weight;
silicon 0.10-0.40% by weight;
chromium 0.25-1.04% by weight;
molybdenum 0.01-0.25% by weight; and
balance essentially iron; and
with the proviso that a combination of carbon, molybdenum and
chromium within the above ranges are selected as follows to provide a non-
bainitic core:
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a) at the lower 0.7% carbon with a minimum of 0.01%
molybdenum, chromium is equal to or less than 1.04% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.43%; and
b) at the upper 1.00% carbon, with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 0.80% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.28%, said rod being
characterized by:
i) a core of greater than 99% pearlite having a hardness less
than 45 Rockwell C;
ii) an outer shell of tempered martensite having a hardness of
greater than 55 Rockwell C and a uniform annular thickness greater than about
1.25 cm;
iii) a 10 cm to 15 cm soft end having a hardness less than 35
Rockwell C; and
iv) wherein each soft end has an intermediate portion of a
hardness less than 25 Rockwell C to provide thereby a ring with improved
crack arresting properties.
In accordance with yet a further aspect of the invention, a process for
producing a grinding rod having a core of greater than 99% pearlite having a
hardness less than 45 Rockwell C and an outer shell of tempered martensite
having a hardness greater than 55 Rockwell C and a uniform annular thickness
greater than about 1.25 cm, said process comprising:
i) reheating a formed steel bar to above its austenitising temperature
in a controlled manner to produce a reheated bar of substantially uniform
reheat
temperature, said steel bar having the following chemistry:
carbon 0.70-1.00% by weight;
manganese 0.60-1.00% by weight;
silicon 0.10-0.40% by weight;
chromium 0.25-1.04% by weight;
molybdenum 0.01-0.25% by weight; and
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balance essentially iron; and
with the proviso that a combination of carbon, molybdenum and
chromium within the above ranges are selected as follows to provide a non-
bainitic core:
a) at the lower 0.7% carbon with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 1.04% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.43%; and
b) at the upper 1.00% carbon, with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 0.80% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.28%;
ii) transferring with minimal cooling said reheated bar to an open
tubular quench vessel which is capable of enclosing an entire bar length,
closing said vessel to provide a quench liquid tight seal about said bar while
securing said bar in said vessel to minimize bar warping in said vessel during
quenching;
iii) introducing quench water into an inlet end of said vessel and
passing said quench water along said vessel at high surface velocities
exceeding
4 meters per second relative to the bar surface to minimize thereby production
of steam along the bar length and ensure uniform heat removal and removing
quench water at an outlet end of said vessel; and
iv) quenching said bar in said vessel for a period of time which
provides a bar surface equalization temperature when removed from said vessel
of less than 350 C and greater than 150 C to provide a uniform annular layer
for said hard outer shell of tempered martensite and said softer core of
pearlite
where the end surface hardness is consistent with said hard tempered
martensite
shell, said developed uniform outer shell of martensite producing uniform
residual stress contributing to rod straightness.
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BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described with respect to the
drawings, wherein:
Figure 1 is a schematic of a heat treating line for heat treating and self-
tempering steel bar to form grinding rods with soft ends;
Figure 2 is a schematic cross-section through a representative type of bar
quenching device, such as described in US patent 4,376,528;
Figure 3 illustrates the steps in heat treating the bar; and
Figure 4 is an enlarged view of an end portion of the grinding rod.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Applicant has found that by the selection of a chemistry for the steel bar
which is heat treated to form a grinding rod not only can the wearability be
increased but as well as the stresses can be indirectly controlled to maintain
durability at acceptable levels. The durability in the grinding rod is
achieved by
having a tougher perilitic core which is capable of resisting the higher
stresses
in the harder martensitic shell. We have surprisingly found that the toughness
of the pearlitic core is sufficient to prevent breakage due to the higher
stresses
generated in the harder outer martensitic shell which are transferred to the
pearlitic core. In order to achieve the harder tempered outer martensitic
shell
the amount of carbon used in the steel alloy is increased and usually falls in
the
range of 0.7 to 1.0 % by weight to achieve an outer shell hardness greater
than
55 Rockwell C and up to 65 Rockwell C depending upon the manner of heat
treatment. Manganese is included at a level in the range of about 0.6 to 1.0 %
by weight and silicon is included at a level of about 0.1 to 0.4 % by weight.
In
order to achieve an annular uniform layer of martensite of substantial
thickness,
significant amounts of chromium are used to achieve the depth of martensitic
layer. The amount of chromium ranges from about 0.28 to 1.04 % by weight.
Molybdenum in the rod is equal to or less than 0.25 % by weight which in
combination with the above amount of chromium ensures a pearlitic core. It
has been found that, with these ranges for the chemistries, some guidance is
required to ensure a proper selection from these ranges to achieve the desired
wearability and durability characteristics in the rod. In this respect, the
chemistry selection is based on the proviso that a combination of carbon,
molybdenum and chromium within the above ranges are selected as follows to
provide a non-bainitic core:
a) at the lower 0.7% carbon with a minimum of 0.01%
molybdenum, chromium is equal to or less than 1.04% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.43%; and
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b) at the upper 1.00% carbon, with a minimum of 0.01%
molybdenum, chromium is equal to or less than 0.80% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.28%.
This chemistry for the rod also lends the rod heat treatment to providing
grinding rods having the desirable soft ends of hardness less than 35 Rockwell
C and the soft tough rod core of greater than 99% pearlitic and a hardness
less
than 45 Rockwell C while at the same time providing an outer tempered
martensitic shell having a hardness greater than 55 Rockwell C and up to 65
Rockwell C and greater. By virtue of the selected chemistry and a preferred
type of heat treatment, the martensitic shell is of a uniform annular
thickness
preferably greater than about 1.25 cm and up to 1.60 cm or more. The
preferred method of heat treating with this chemistry is capable of providing
soft end portions of a length of about 10 cm to 15 cm and having a hardness
less than 35 Rockwell C.
The engineered heat treating of the end portions can be modified to
provide intermediate portions of a hardness less than 25 Rockwell C to thereby
provide a ring with improved crack arresting properties. It has been found
that
with this invention a grinding rod is produced which is relatively straight by
virtue of the process and chemistry of this invention providing uniform
stresses
in the outer annular shell of tempered martensite.
The preferred process for heat treating the rod comprises quenching the
rod in an elongate vessel which delivers high velocity quench water along the
length of the bar to rapidly cool the bar with minimal generation of steam on
the bar surface. Such rapid controlled quench along the length of the bar
develops a uniform layer of martensite having the higher hardness in the range
of about 55 to 65 Rockwell C while developing uniform balances stresses
around and along the martensitic shell to provide the desired rod
straightness.
After the rapid controlled quench, the bar is withdrawn from the quench vessel
and tempered by allowing the bar to soak back to an equalization temperature
after quenching.
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In this regard, a representative heat treating line 10 for reheating steel
bar, quenching a steel bar and subsequently heat treating each bar end portion
is
shown in Figure 1. Individual bars 12 are advanced on a rack 14 which may
include a chain/dog advancing mechanism 16. Each individual bar 12 is
advanced off the rack 14 in the direction of line 18. The bar may be passed on
suitable rollers 20 into a reheat furnace 22 which is temperature controlled
to
ensure that the individual bars 12, as they advance in the direction of arrow
24
across the furnace, are reheated to the preferred austenitising temperature.
Each bar, at the desired reheat temperature, is transferred out of the furnace
22
in the direction of arrow 26 into a quenching vessel 28 which is described in
more detail with respect to Figure 2. The quenching vessel 28 delivers the
high
velocity quench water to develop a uniform annular layer of martensite which
is
tempered when the bar is allowed after exiting the quench vessel to attain a
soak back or equalization temperature in the range of less than 350 C and
greater than 150 C. The quenched bar is transferred to rack 30 with advancing
chain/dog system 32. The bar, as advanced in the direction of arrow 36 after
having been removed from the quench vesse128 in the direction in the arrow
34, is advanced in the direction of arrow 38 onto a bar conveyor system 40.
The leading end of the bar is inserted into a furnace 42 which may be an
annular induction furnace to reheat a specified portion of the bar end which
is
preferably less than 15 cm in length. The end portion is heated to its
austenitising temperature and then passed through the annular induction
furnace
42 in the direction of arrow 44, so that the end portion may be air cooled and
thereby provide an engineered end portion hardness of less than 35 Rockwell C.
After the bar end is removed from the furnace in the direction of arrow 44 and
transferred to conveyor 48, the other end of the bar is then positioned in the
furnace 42. The other bar end is now reheated in the furnace 42 to its
austenitising temperature and withdrawn in the direction of arrow 50 to permit
air cooling thereof. The bar is transferred to conveyor 52 in the direction of
arrow 54. The bar with both ends softened is transferred from the conveyor 52
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in the direction of arrow 60 onto the rack 30 for transport to a final cooling
station where the bars are inspected, bundled, identified and color coded as
required.
The aspects of the process, which provide the significant advantages in
the subject grinding rod, may be realized in the selected chemistry, in the
type
of quench vessel 28 and in the separate engineered end heat treatment to
provide a well defined softened end portion of a specified length less than 15
cm.
As shown in Figure 2, the quench vessel 28 may be of the type, for
example, as described in US patents 4,376,528 or 3,997,375. Although both of
these patents describe quenching systems for quenching tubular pipe where
water flows along the inside and the outside of the pipe, the same system may
be used to heat treat solid bar, where significant unexpected advantages flow
from use of the tubular pipe quench system in forming the harder grinding
rods.
With reference to Figure 2, a schematical cross-section of the quench vessel
28
includes a water inlet 62 and a water outlet 64. Water is forced through the
inlet in the direction of arrow 66 where it flows outwardly in the direction
of
arrow 68 over the end portion 70 of the bar 12. The water then flows along the
surface 72 of the bar and over the downstream end 74 where the water
converges and flows out through the outlet 64. The bar 12 may be supported on
suitable supports 76 which may be spaced apart along the bottom wall 78 of the
vessel, or may be one continuous support along the bottom wall. In any event,
the supports 76 make point contact with the bar 12 to maximize the surface
area
72 exposed to the water flowing longitudinally over the bar 12. Preferably,
the
quench vessel 28 includes hydraulic pistons 80 which have water sealed rams
82 extending through the vessel. The rams include plates 84 which contact the
surface 72 and thereby clamp the bar within the vessel to further resist bar
warping during the quenching process. As taught in US patent 4,376,528, the
velocity of the quench water is maintained at or above a minimum operating
level to ensure that steam does not develop at the bar surface and thereby
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CA 02239440 1998-06-02
optimizes the rate of heat transfer from the bar to the quenching water.
Cooling
water preferably travels at a minimum surface velocity relative to the bar of
about 4 meters per second and may flow at surface velocities much greater, for
example, up to 15 meters per second. The ideal flow velocity is usually in the
range of about 5 meters to 8 meters per second. At these velocities, a uniform
outer shell of martensite is produced where the bar is quenched in the vessel
for
a period of time which provides a bar surface equalization temperature, when
removed from the vessel 28, of less than 350 C and greater than 150 C. We
have determined that quenching the bar in a vessel of the type shown in Figure
2 ensures that any vapor produced at the bar surface is instantly flushed away
to
provide a uniform and rapid quenching of the bar surface. This type of
quenching ensures the development of a uniform outer shell of martensite. By
virtue of this quenching process as well as the clamping of the bar in the
vessel,
we have unexpectedly found that the bar, after cooling, maintains rod
straightness. Such rod straightness has been found preferably to be less than
1.25 cm deviation from a straight line along entire rod length. It is thought
that
the uniform quenching of the bar surface develops a uniform compressive force
in the martensite shell to maintain rod straightness.
Within the range of the above surface velocities, the length of time that
the bar is quenched in the vessel is for a defined period. Preferably, the
quench
water temperatures range from 10 C to 40 C at vessel inlet, although it is
appreciated that other quench water temperatures may be selected as long as
the
quenching achieves the desired rate of quench to provide the desired
martensite
layer. For quench water temperatures in the range of 30 C to 35 C, quench
times range from 110 seconds to 160 seconds for rods having diameters ranging
from about 7.5 cm to about 10 cm. With this period of quenching and novel
chemistry, it has been found that the tempered martensite shell has a radial
depth of at least about 1.25 cm and usually about 1.6 cm or greater.
As shown in Figure 3, the bar 12 is reheated to its austenitising
temperature. As is appreciated by those skilled in the art, the austenitizing
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CA 02239440 1998-06-02
temperature will depend on the chemistry of the material selected from the
following ranges,
Carbon .70 - 1.00% by weight
Manganese .60 - 1.00% by weight
Silicon .10 - .40% by weight
Chromium .25 - 1.04% by weight
Molybdenum .01 - .25% by weight
The selection from the above ranges requires a degree of guidance as offered
by
the proviso that a combination of carbon, molybdenum and chromium within
the above ranges are selected as follows to provide a non-bainitic core:
a) at the lower 0.7% carbon with a minimum of 0.01%
molybdenum, chromium is equal to or less than 1.04% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.43%; and
b) at the upper 1.00% carbon, with a minimum of 0.01 %
molybdenum, chromium is equal to or less than 0.80% and with a maximum of
0.25% molybdenum, chromium is equal to or less than 0.28%.
In accordance with this proviso in the chemistry selection, it is apparent
that, as the carbon content rises, with lower amounts of molybdenum the
amount of chromium is higher and with higher amounts of molybdenum, the
amount of chromium is relatively lower. Hence, for a .7% carbon chemistry,
the molybdenum and chromium may range from 0.01 % Mo and 1.04% Cr to
0.25% Mo and.43% Cr; and for a 1.0% carbon chemistry, the molybdenum and
chromium may range from 0.01% molybdenum and 0.80% chromium to 0.25%
molybdenum and 0.28% chromium. With these ranges, one skilled in the art
can readily interpolate the concentrations of molybdenum and chromium for
carbon contents between 0.7 and 1.00%. This range for the selected
molybdenum and chromium provides many advantages including achieving the
necessary balance between the amounts of molybdenum and chromium to avoid
formation of a bainitic core and to allow a selection which optimizes product
cost with varying alloy costs.
CA 02239440 1998-06-02
With this chemistry, the preferred austenitising temperature is in the
range of about 775 C to about 870 C. When the bar is quenched in vessel 28, a
uniform layer 86 of martensite is formed along the entire length of the
quenched bar 12A. The selected chemistry ensures the formation of the deep
layer of martensite. The core portion 88, on the other hand, during the heat
treatment develops a pearlitic structure in the range of at least about 99%
pearlite. The ends 70 and 74 of the bar have hardened portions 90 and 92
inwardly of the end, as depicted by the termination of the core portion at
transition line 94. The bar ends 70 and 74 are then reheated in a suitable
furnace which is preferably an induction coil furnace. A selected length of
each
end portion is reheated, preferably less than 15 cm where the end portions 96
and 98 are reheated to their austenitising temperature without appreciably
heating the rest of the bar. The end portions are then, as described with
respect
to Figure 1, air cooled to provide end portions which are of substantially
pearlitic microstructure and have a hardness of less than 35 Rockwell C. With
appropriate control of the end heating, the end portions may have a hardness
of
less than 30 Rockwell C.
In order to minimize the effects that hydrogen has on the rolled bar
stock, it is understood that the bar may be subjected to a degassing step.
This
step minimizes hydrogen build-up in the bar to enhance crack resistance of the
bar during heat treatment and in the rod during use.
As shown in Figure 4, the soft end portion 96 extends from beyond the
transition zone 100, which defines the end of the pearlitic core 88, and the
end
of the martensitic shell 86 as defined by dotted line 102. The softer end 96,
which as already noted, may have a hardness considerably less than 35
Rockwell C may be treated in a manner to include an intermediate annular ring
104 which may have a hardness less than 25 Rockwell C to provide thereby a
softer end with improved crack arresting properties. This small annular ring
of
softer material assists the end portion 96 in arresting any cracks which
attempt
to propagate along the rod.
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It is appreciated that various processing parameters may change
depending upon the size of the bar, the chemistry of the bar, the structure of
the
quench vessel, the supports in the quench vessel and the clamps for the bar in
the quench vessel. It is appreciated that such modifications are well within
the
purview of those skilled in the art to achieve all of the benefits and
advantages
of this invention which, in summary, are as follows. By providing an
engineered rod soft end portion, which is formed in a step subsequent to the
quenching step, this ensures that the rod end is well defined and is
considerably
shorter than what is produced by the prior art processes. Quenching the bar
with high velocity water quench stream ensures a uniform quenching of the bar
surface and hence the development of a uniform outer shell of martensite which
has uniform compressive stresses contributing to rod straightness. Selection
of
the appropriate low alloy composition in conjunction with the high velocity
quenching of the bar also ensures that the core content remains at least at
about
99% pearlite to give the bar the necessary toughness when used as a grinding
rod. The technology is capable of providing a tough rod structure without
having to resort to the inclusion of exotic alloys in the steel bar. The
advantage
of providing a crack arresting ring in the controlled end portion is an added
feature which is achievable by the post end treatment of this invention. A
further advantage of the soft end portion is to increase the overall wear
resistance of the grinding rod by virtue of the controlled engineered soft
ends.
Although preferred embodiments of the invention have been described
herein in detail, it will be understood by those skilled in the art that
variations
may be made thereto without departing from the spirit of the invention or the
scope of the appended claims.
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