Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention concerns cemented carbide rolls for hot-
forming steel rod in multi-stand rolling mills, especially in
a finished rod diameter range of 7/32 inches to 1/2 inch.
Carbide rolls, operating at rod temperatures typically in the
1700F to 2200F range, have gained wide use in multi-stand
steel-rod rolling mills and, to a large extent, have replaced
chilled cast iron rolls, especially in finishing roll mill
stands.
The development of twist free rod mills allowed the use
of higher, more economical hot rolling speeds without sacrifice
of rod product dimensions or rod surface condition. The
successful introduction of cemented carbide rolls of homogeneous,
single-composition, tungsten carbide-cobalt alloys provided a
roll material capable of being designed to withstand higher
rolling speeds.
The sole hard carbide constituent in these roll alloys
preferred by those skilled in the art, and most successful in
application, has been tungsten carbide (WC), without additions
of other hard carbides, such as tantalum carbide, tantalum
carbide-niobium carbide solid solution, niobium carbide,
titanium carbide or tungsten carbide-titanium carbide solid
solution. The addition of such carbides is widely believed to
impair mechanical wear properties and mechanical toughness, both
desirable properties în hot steel rod mill rolls.
The realization of the benefits of still greater rolling
speeds of which improved mill design is now capable, however,
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requires roll materials possessing more resistance to wear
induced by thermal fatigue. Both the surface degradation of
roll groove surfaces, or other working surface configuration,
and massive roll fracture are related to several factors,
among which a major factor is thermal cracking caused by
alternate heating and cooling of the mill roll as it encounters
the hot steel rod.
Thermal crack patterns on used carbide roll groove, or
working, surfaces occur at every stage of the reduction process.
As heat cracks form and propagate vertically downward into the
subsurface carbide, the exposure of newly created carbide
surfaces to cooling water which becomes hot and steam enables
leaching of the cobalt binder from the crack areas of the
cemented carbide roll to occur.
As thermal cracks are deepened by the twin forces of
thermal crack propagation and leaching, the roll working surface
wear rate increases and the rolled rod surface condition
deteriorates requiring removal of the roll for grinding off the
surface damage. As surface condition of rolled rod or rod
dimensions approach tolerance, the roll working surfaces are
ground to completely remove thermal cracks prior to reuse.
Massive fracture of the roll caused by a splitting action
initiated by thermal cracks may also occur.
Rolls used for slower rolling speeds and larger rolling
diameters, such as pre-finishing mills and bar mills having a
finished rod diameter of 1/2 inch to three inches, are subject
to even greater thermal stress because thermal cycling is
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accelerated by longer time intervals of roll-to work contact
and cooling exposure.
Thermal cracking of working surfaces, therefore, operates
as a catalyst of wear, a cause of fracture failure, and a
cause of surface roughness of the rolled rod.
It is an object of this invention to provide a cemented
carbide roll for hot forming steel rod in multi-stand rod or
bar mills which is significantly more resistant to thermal
cracking.
It is an additional object of this invention to provide a
roll which possesses greater resistance to surface wear and
cobalt binder leaching as evidenced by longer roll service time
and greater steel tonnage rolled before removal of the roll
from the mill for grinding.
BRIEF SUMMARY OF THE ~NVENTION
According to the present invention, a cemented carbide roll
can have the thermal fatigue and wear resistant properties
increased by the addition of tantalum carbide to the roll
composition. Preferably, the composition of the roll near its
outer working surface will comprise, by volume: tungsten
carbide--65 per cent; tantalum carbide--l~ per cent; and
cobalt--23 per cent; however, the composition may be in the
range, by volume, of: tungsten carbide--45 to 72 per cent;
tantalum carbide--5 to 13 per cent; and cobalt--23 to 42 per cent.
Because of the cost of the tantalum carbide, which is
approximately two or three times that of tungsten carbide, it
is somewhat more economical to make a d~al compact roll with an
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outer layer as described above, but with an inner layer
preferably comprised of, by volume, 76 per cent tungsten
carbide and 24 per cent cobalt. The inner layer, however, may
be in the range of, by volume, 24 to 43 per cent cobalt, and
the balance tungsten carbide.
DETAILED DESCRIPTION OF THE INVENTION
The invention is achieved by constructing a dual-composition
carbide roll consisting of a longer-wearing peripheral or outer
cemented carbide zone in which the rolling grooves, or working
surfaces, are formed and possessing excep~ional thermal fatigue
and wear resistance derived from the incorporation of tantalum
carbide (TaC) in a tungsten carbide (WC)-cobalt (Co) base
composition, and a mechanically tough inner support core of
tungsten carbide-cobalt composition possessing a binder volume
and carbide grain structure identical with or similar to that
which exists in the peripheral or outer zone.
Both zones form a solid, integrated roll body with a
sinter-bonded interface. Carbide-robalt powder blends suitable
for each zone are first pressed together in a powder compacting
press, then sintered together as a single pressing. The use of
tantalum carbide, which is normally higher in cost than is
tungsten carbide, is thus restricted to the critical peripheral
working zone of the roll, thereby limiting its use and yet
gaining a substantial improvement in acceptable service life
between regrinds of the rolling groove.
The improved mill roll possesses as its principal and
critical feature a dual-composition cemented carbide structure,
of which the outer or rolling zone consists of a tungsten
carbide-tantalum carbide-cobalt cemented carbide composition
and an inner or core zone of cemented tungsten carbide-cobalt
having identical or similar volume per cent of binder metal.
It is recognized that dissimilarities between the zones
in carbide grain size range, volume per cent of bindex and
binder composition may not cause an unacceptable rate of
fracture failure of the roll because of inherent differences
in thermal coefficient of e~pansion between the zones, either
in use or during manufacture. Nevertheless, identical or
similar per cent binder volumes, carbide grain size ranges and
binder compositions in the outer and inner zones are a preferred
embodiment of this invention.
Two 8.125 inch diameter single-groove identical cemented
carbide rolls were made having an outer zone composition in
terms of per cent by volume of tungsten carbide--65 per cent;
tantalum carbide--12 per cent; and cobalt--23 per cent,
integrated by means of a sinter-bonded interface with an inner
cemented carbide core zone having a composition in per cent by
vol~me of tungsten carbide--76 per cent and cobalt--24 per cent.
Both zones were made to have a tungsten carbide grain size
range of approximately 90 per cent 3 to 12 micron and a
tantalum carbide grain size range of approximately 90 per cent
3 to 6 micron after sintering. The design density of the outer
zone was 13 79 grams per cubic centimeter and of the inner zone
13.95 grams per cubic centimeter. The outer or rolling zone
was 1.125 inches thick, just sufficient to provide for outer
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zone material both to accommodate the forming of the rolling
groove and the subsequent grinding of the groove between roll
passesl, but without unnecessary or excessive thickness, so as
to achie~e the greatest possible economy in the use of tantalum
carbide.
The best 8 and 1/8 inches diameter cemented carbide
finishing rolls containing, in terms of per cent by volume,
23 to 25 per cent cobalt and 75 to 77 per cent of tungsten
carbide with a sintered grain size range of approximately 90
per cent 3 to 12 microns typically attain about 72 tons of steel
rod production per 0.091 inches of grinding removal on the roll
diameter. By comparison, the dual composition 8 and 1/8 inches
finishing rolls operating in the same position and under
equivalent rolling conditions achieved about 100 tons per
0.001 inches grinding removal on the first pass before the rolls
were removed from the roll stand for grinding to remove thermal
damage to the rolling groove surfaces. This represents a 40
per cent increase in roll service life and a significant increase
in mill utilization efficiency by lessening the frequency of
mill shutdowns for changing rolls.
Carbide grain size ranges typically used in rolls in
accord with the current art vary according to the precepts of
manufacturers; it is recognized that at least some of the
economic benefit of this invention will occur independently of
grain size ranges employed in the peripheral and inner zones.
Similarly, it may be foreseen that substitutions for
tantalum carbide, all or in part, by binary or ternary soLid
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lil~BS(~
solution carbides involving combinations of niobium, tantalum
or hafnium, or by the monocarbides of niobium or hafnium in the
peripheral or outer zone may achieve at least part of the
economic benefit of this invention, owing to the relatively good
thermal crack resistance of these carbides. However, tantalum
carbide is a preferred embodiment of this invention because
its specific gravity is very close to that of tungsten carbide
thereby facilitatingg as between the outer and inner zones,
nearly equal shrinkage during sintering and nearly equal binder
volumes and sintered densities when equal weight percentages of
binder are used in the powder blend compositions, also because
tantalum carbide possesses excellent resistance to heat cracking
among all the hard carbides.
An incorporation of tantalum carbide, as an important hot
steel rod mill roll constituent, is, in itself, both novel and
useful regardless of the important economy realized from a
dual-composition roll. Therefore, it follows that a single
composition roll having the same composition throughout as the
peripheral zone will possess some, if not all, of the benefits
of a dual-composition roll.
It is understood that the volume per cent of binder, as
well as the binder composition, and the volume percentage of
tantalum carbide, may be altered in either or both zones
without impairing some or any of the economic benefits of this
invention, and that, further, the benefits of this invention
can be realized in hot rolling metals and compositions other
than steel.
Modifications may be made within the scope of the appended
claims.
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