HARD METAL MATERIALS
Field of the invention
The present invention relates in general terms to hard metal
materials comprising refractory material particles , as described
herein, dispersed in a host metal or metal alloy.
The ASM Materials Engineering Dictionary defines the term "hard
metal" as a collective term for a sintered material with high
hardness, strength and wear resistance.
The present invention also provides components manufactured from
the hard metal materials . The present invention relates
particularly, although by no means exclusively, to large
components weighing more than lOOkgs and typically more than 1
tonne .
The present invention also provides a method of manufacturing the
components from the hard metal materials.
In more particular terms , although by no means exclusive terms ,
the present invention relates to hard metal materials that are
useful for applications requiring wear resistance.
Background
It is known to use powder metallurgy to manufacture small
components from hard metal materials comprising refractory
particles dispersed in a host metal (which term is understood
herein to include metal alloy) .
Powder metallurgy processes involve sintering mechanically mixed
refractory powders at elevated temperatures under pressure,
usually in an inert atmosphere.
"Sintering" comprises bonding powdered materials, usually under
pressure, by solid-state reactions at temperatures lower than that
required for the formation of a liquid phase. During a sintering
process, at temperatures below the melting point of the metallic
binders , powders of metallic binder phase and refractory particles
are welded together by pressure and heat. Sintering is
traditionally used for manufacturing ceramic components and has
also found uses in such fields as powder metallurgy for the
manufacture of products containing very high melting point
materials.
Powder metallurgy is a useful process for manufacturing relatively
small, simple- shaped, wear resistant components such as tungsten
carbide tool bits. However, powder metallurgy is not a practical
process for manufacturing larger, complex- shaped, hard metal, wear
resistant components such as pump impellers and crusher wear parts
weighing more than lOOkgs and typically more than 1 tonne from
hard metal materials. This is an issue, particularly in
applications in the mining and minerals processing industries
where large high wear resistant components are often required.
It is known to use wear resistant metal alloys, such as high
chromium white cast irons, in the manufacture of components used
in applications in the mining and minerals processing industries,
such as applications involving transporting solid materials. For
example, hard- facing alloys are formed on the trays of dump trucks
that transport mined ore from a mine site to a minerals processing
plant. In another example, castings of wear resistant alloys are
used to form pumps for transporting slurries of ore particles
suspended in water through processing stages in flotation circuits
in a minerals processing plant.
The fracture toughness and corrosion resistance requirements for
the wear resistant alloy in each of the above examples are
different and, accordingly, the wear resistant alloy compositions
are different. The common factor between both, however, is a need
to provide wear resistance in addition to other properties.
Generally speaking, higher wear resistance can be achieved through
controlling the alloy composition, but there is a trade-off
against other properties.
For any given circumstance where wear resistance is an important
property, it is desirable to provide materials with desirable
properties and improved wear resistance by compromising less on
the balance of these properties .
It is noted that the specification includes references to weight
percent (wt.%) and volume percent (vol.%). In the context of the
references to bC in the specification, where bC has a density
similar to a host metal, these terms are interchangable .
Summary of the disclosure
The applicant has found in the course of extensive research and
development work that a liquid host metal, containing a
dispersion, typically a dispersion, of 5-50 volume % fine
particles of a refractory material that is insoluble in the host
metal, and is described herein as a liquid metal slurry, has very
good fluidity during pouring in a foundry and the slurry readily
flows to fill sand moulds to produce sound castings of the hard
metal material .
The term "insoluble" is understood herein to mean that for all
intents and purposes the refractory material is not soluble in the
host metal. There may be limited solubility. However, the
refractory particles are essentially distinct from the host metal
in that there is negligible partitioning of the transition metals
in the refractory material particles to the host metal.
The applicant has also found that mixing and dispersing the
insoluble refractory particles in the host metal may be carried
out in an effective way in the liquid state in an inert
atmosphere, such as in a vacuum furnace, to minimise oxidation of
the reactive elements in the refractory material particles .
The present invention is a departure from standard foundry
practice known to the applicant that involves the complete melting
of all alloying additions contained in a casting to form a single
phase liquid to ensure maximum fluidity during pouring into a
mould .
The applicant has also found that the fluidity of liquid metal
slurries, when cast within certain production parameters in
accordance with the present invention, is sufficient to produce a
family of sound hard metal material castings ranging from small to
large casings with specific wear resistance, fracture toughness
and corrosion resistance that suit a wide range of operating
conditions in service.
The production parameters may comprise any one or more of the
particle size, reactivity, thermal expansion or contraction,
density, and solubility of the refractory material, as discussed
further below.
In broad terms , the present invention provides a hard metal
material comprising 5-50 volume % particles of a refractory
material dispersed in a host metal.
In the context of the present invention, the term "hard metal
material" is understood to comprise particles of high melting
point carbides and/or nitrides and/or borides of any one or more
than one of the nine transition metals titanium, zirconium,
hafnium, vanadium, niobium, tantalum, chromium, molybdenum and
tungsten dispersed in a tough host metal, which acts as a binder
phase. Typically the host metal is a ferrous metal alloy. Each
of these particles is a particle of a refractory material and is
referred to herein as a "refractory material".
The particles of the refractory material may be carbides and/or
borides and/or nitrides of one transition metal, such as bC .
The particles of the refractory material may be carbides and/or
borides and/or nitrides of more than one transition metal where
the particles are a chemical mixture (as opposed to a physical
mixture) of the carbides and/or borides and/or nitrides of the
transition metals . In other words , in the case of carbides , the
particles of the refractory material may be of the type described
as (Mi ,M2) C , where M" is a transition metal. One example that is
discussed further herein in (Nb,Ti)C.
The hard metal material may comprise 5-40 volume % particles of
the refractory material dispersed in the host metal.
The hard metal material may comprise greater than 10 volume %
particles of the refractory material dispersed in the host metal.
The hard metal material may comprise greater than 15 volume %
particles of the refractory material dispersed in the host metal.
The hard metal material may comprise less than 30 volume %
particles of the refractory material dispersed in the host metal.
The hard metal material may comprise less than 25 volume %
particles of the refractory material dispersed in the host metal.
The host metal may be a ferrous alloy (such as a steel or a cast
iron) , a stainless steel, an austeni tic-manganese steel such as a
Hadfield steel, or a iron-based or nickel-based or cobalt-based
superalloy .
The present invention also provides a method of forming a hard
metal material comprising:
(a) forming a slurry of a hard metal material
comprising 5-50 volume % particles of a refractory
material dispersed in a liquid host metal, for
example in an inert atmosphere, and
(b) allowing the slurry to solidify to form a solid
hard metal material .
The present invention also provides a method of manufacturing a
component of a hard metal material comprising:
(a) forming a slurry of a hard metal material
comprising 5-50 volume % particles of a refractory
material dispersed in a liquid host metal in an
inert atmosphere, and
(b) pouring the slurry into a mould and forming a
casting of the component in an inert atmosphere.
The method may comprise forming the slurry and thereafter forming
the casting of the component in a chamber under vacuum conditions
which remove air from the chamber and supplying an inert gas , such
as argon, into the chamber. By way of example, the method may be
carried out in a vacuum melting furnace.
The method may comprise selecting the production parameters to
form the slurry in step (a) that has a required fluidity for
processing in step (b) . In any given situation, a skilled person
will be able to determine a required fluidity for processing step
(b) having regard to standard foundry practice considerations such
as the size and shape of the component to be formed and the
required dispersion (uniform or segregated) to provide the
required micros trueture for the component.
The production parameters may comprise any one or more of the
particle size, reactivity, density, and solubility of the
refractory materials, as discussed further below.
Refractory material particle size
The refractory material may be a fine particle size. A fine
refractory material particle size may be required to ensure a
homogeneous dispersion in the host metal. The melting points of
the majority of the transition metal refractory materials are in
excess of 1800°C and the refractory materials are generally
insoluble in host liquid metals. The applicant has found that
refractory powders with particle sizes less than 500 microns,
typically less than 150 microns, in diameter provide optimum flow
characteristics in liquid metal slurries and yield a desirable
uniform dispersion of the refractory particulates in the
microstructures of the Hard Metal castings.
The refractory material may be less than 400 microns particle
size .
The refractory material may be less than 200 microns particle
size .
The refractory material may be less than 150 microns particle
size .
The refractory material may be added to the host liquid metal as
follows .
(a) A s a fine powder with a selected particle size distribution.
For example, 15 wt.% of particles of a refractory material in the
form of niobium carbide (NbC) (minus 50 microns in diameter) added
to a liquid host metal in the form of a high chromium white cast
iron host metal. NbC exhibits a Vickers Hardness of 24 GPa, a
melting point of 3600°C, and a very low solubility in the host
liquid metal at a casting temperature of about 1500°C. The liquid
metal slurry comprises a suspension of insoluble NbC particles
(minus 50 microns in diameter) in the host liquid metal. On
solidification, the micros true ture exhibits a dispersion of 15
volume% fine NbC particles (minus 50 microns in diameter) in a
high chromium white cast iron matrix containing a negligible
amount (less than 0.3 wt.%) of niobium in solution in the matrix.
(b) The transition metals mentioned above or ferro-alloys of the
same transition metals can be added to a wide range of host metals
containing all the combinations and permutations of the elements
carbon, boron and nitrogen.
For example, as is described in more detail below, the applicant
has found that Fe- b readily dissolves in the host liquid metal at
1500°C and niobium immediately combines with carbon in the host
liquid metal to form niobium carbides in situ with particle sizes
less than 50 microns in diameter.
Reactive refractory materials
Most of the transition metal refractory materials described above
are classed as "reactive elements", i.e. the individual metal
elements and/or their carbide, nitride or boride compound forms
react readily with air at metal casting temperatures about 1500°C
to form undesirable metal oxides and/or copious quantities of
gases such as CO2 which can result in severe porosity in the
castings. The problems of oxidation and porosity in hard metal
castings, produced by a liquid metal slurry, and associated with
chemical reactions of the reactive refractory materials in air at
elevated temperatures are overcome by melting and pouring the
liquid metal slurry in an inert atmosphere.
Selection of refractory material particles having lower thermal
expansion or contraction than the host metal
Poor bonding between refractory particles and the host metal in
hard metal materials have been variously reported in the
literature. The applicant found no evidence of poor bonding
between the refractory particles and a wide range of the host
metals evaluated by the applicant. Whilst not wishing to be bound
by the following comment, the observed excellent bonding is
attributed by the applicant in large part to the use of an inert
atmosphere during casting of the hard metal materials and the
thermal contraction of the transition metal refractory particles
being much less, typically about 50% lower, than the thermal
contraction of the host metals during cooling from the solidus to
ambient temperature generating compressive forces on the
refractory material particles that firmly held the particles in
the host metals on solidification. All refractory particles in
hard metal material castings produced by the applicant in an inert
atmosphere were found to be under compressive loading ensuring
intimate contact and good bonding with the host metals.
Density of refractory materials
The density of the refractory material of the particles , compared
to the density of the host metal in the liquid state, is a
parameter to consider during the method of the present invention
to control the dispersion of refractory particles in the hot host
metal . In some situations it may be important to avoid
segregation of refractory material particles in the liquid host
metal. In other situations, segregation may be desirable. For
example, the nominal density of a host ferrous liquid metal at
1400 °C is 6.9 grams/cc. When tungsten carbide particles, with a
density of 15.7 grams/cc, are added to a host ferrous metal, the
WC particles will sink to the bottom of the mould prior to
solidification of the host metal. When titanium carbide particles,
with a density of 4.8 grams/cc, are added to the same host ferrous
metal, the TiC particles will float to the top of the ladle or
mould. Niobium carbide, with a density of 7.7 grams/cc at 1400 C ,
is fairly close to the density of the host liquid metal at 6.9
grams/cc and is less prone to segregation in the liquid host metal
than TiC or WC. However, the applicant has observed that NbC
particles will segregate to the bottom of large section white iron
castings during the process of the present invention when
solidification times are in the order of 30 minutes or more. As
described in more detail below, niobium carbide and titanium
carbide have similar crystal structures and are isomorphous .
Selecting the required Nb/Ti ratio in a (Nb,Ti)C chemical compound
yields a refractory material with any required density in the
range 4.8 - 7.7 grams/cc at the casting temperature. Matching the
density of the solid refractory particles and the liquid host
metal at the casting temperature eliminates segregation of the
particulates in the melt during the process of the present
invention .
Solubility of refractory materials
The addition of refractory material particles that are for all
intents and purposes insoluble , i.e. having minimal solid
solubility in the host liquid metal, to produce a casting in
accordance with the method of the present invention, produces a
hard metal material that displays physical and chemical properties
that are very similar to the host metal with substantially
improved wear resistance due to the presence of a controlled
dispersion of a high volume % of hard refractory material
particles in the micros trueture .
For example, the solubility of a refractory material in the form
of (Nb,Ti)C in liquid host metals in the form of (a) liquid
Hadfield steel and (b) liquid 316 stainless steel and (c) liquid
high chromium white cast iron at elevated temperatures is
negligible (<0 .3 weight%) . The addition of 15 weight% (Nb,Ti)C
with the required densities to these three metal alloys , followed
by standard heat treatment procedure for each host metal produces
micros tructures consisting of a uniform dispersion of 15 volume%
primary niobium- titanium carbides in the host metals which are
substantially free of niobium and titanium, i.e. there is
negligible partitioning of the transition metals in the refractory
material slurry particles to the liquid host metal.
Consequently, there is a negligible influence of the refractory
materials of the particles on the chemistry and response to heat
treatment of the host metal.
The three hard metal materials produced by the method of the
present invention display the known physical and chemical
properties of (a) Hadfield steel, (b) 316 stainless steel and (c)
high chromium white cast iron respectively with an increased wear
resistance due to the presence of a dispersion of 15 volume% of
primary niobium- titanium carbides in the microstructures .
In addition to the above, in particular the applicant has found
that providing a hard metal material with a micros trueture that
includes particles of niobium carbide and/or particles of a
chemical (as opposed to a physical) mixture of niobium carbide and
titanium carbide dispersed in a matrix of a host metal
considerably improves wear resistance of the hard metal material
without detrimentally affecting the contribution that other
alloying elements have on other properties of the hard metal
material .
In addition, in particular the applicant has found that it is
possible to adjust the density of particles of a chemical mixture
of niobium carbide and titanium carbide to a sufficient extent in
relation to the density of a host metal, which forms a matrix of
the hard metal material, to make it possible to selectively
control the dispersion of the particles in the matrix from a
uniform dispersion to a non-uniform dispersion of the particles .
This opportunity for density control is an important finding in
relation to castings of the hard metal material. In particular,
by virtue of this finding it is now possible to produce castings
of the hard metal material with controlled segregation of the
particles in parts of the castings. This is important for some
end-use applications for castings, such as where it is desirable
to have a concentration of high wear resistant particles near a
surface of a casting of a hard metal material. Equally, in other
end-use applications for castings it is desirable to have a
uniform dispersion of the particles in the matrix of the casting.
In addition, the applicant has found that forming a hard metal
material and castings of the material to include particles of
niobium carbide and/or particles of a chemical mixture of niobium
carbide and titanium carbide in a range of 10 to 25 wt% , or even
up to 33 wt% or higher, dispersed in a host metal, which forms a
matrix of the hard metal material, does not have a significant
negative impact on corrosion resistance and toughness of ferrous
material in the host metal. Hence, the present invention makes it
possible to achieve high wear resistance of a hard metal material
without a loss of other desirable material properties.
Accordingly, there is provided a method of forming a wear
resistant hard metal material, the method comprising adding (a)
niobium or (b) niobium and titanium to a melt containing a host
metal in a form that produces particles of niobium carbide and/or
particles of a chemical mixture of niobium carbide and titanium
carbide in a range of 10 to 40 wt% of the total weight of the hard
metal material, and allowing the melt to solidify to form the
solid hard metal material .
The terms "a chemical mixture of niobium carbide and titanium
carbide" and "niobium/ titanium carbides" are hereinafter
understood to be synonyms. In addition, the term "chemical
mixture" is understood in this context to mean that the niobium
carbides and the titanium carbides are not present as separate
particles in the mixture but are present as particles of
niobium/ titanium carbides .
Niobium carbides and titanium carbides each have a Vickers
hardness (HV) around 2500, which is about 1000 HV above the
hardness of chromium carbides. Accordingly, hard metal materials
having a micros trueture containing 10 to 40 wt% particles of
niobium carbide and/or niobium/ titanium carbides have excellent
wear resistance properties. However, a significant aspect of the
applicant's work has recognised that niobium carbides and titanium
carbides and niobium/ titanium carbides are substantially inert
chemically with respect to other constituents in the hard metal
material so those constituents provide the hard metal material
with the properties for which they were selected. For example,
chromium added to cast iron alloys still produces chromium
carbides and provides corrosion resistance.
The melt may be in the form of a weld pool in a hard- facing
process. In these circumstances , the niobium and/or the titanium
may be added to the weld pool in a wire alloy in order to meter
the addition of niobium and/or titanium.
The melt may be in the form of a melt for forming a casting.
The niobium and the titanium may be added to the melt in any
suitable form, bearing in mind the requirement of forming
particles of niobium carbides and/or niobium/ titanium carbides in
the solid hard metal material .
For example, the method may comprise adding the niobium to the
melt in the form of ferro-niobium, for example particles of ferroniobium.
In this situation, the ferro-niobium dissolves in the
melt and the resultant free niobium and carbon in the melt form
niobium carbides in the melt.
The method may also comprise adding the niobium to the melt as
elemental niobium.
The method may also comprise adding the niobium and the titanium
to the melt as ferro-niobium- titanium.
The method may also comprise adding the niobium to the melt in the
form of particles of niobium carbide. The method may also
comprise adding the niobium and the titanium to the melt in the
form of particles of niobium/ titanium carbides. In both cases, the
solidified metal alloy may be formed from a slurry of particles of
niobium carbide and/or niobium/ titanium carbides suspended in the
melt. It is anticipated that if the weight fraction of these
carbides in the melt slurry is too high, the flow properties of
the slurry may be adversely affected with the result that unsound
castings of the melt may be produced. Nevertheless, casting a
slurry contrasts with the standard operating procedure in
foundries which involves casting clear (single phase) liquid
melts, i.e. where the melt is above the liquidus temperature of
the highest melting point constituent of the melt.
The particles of niobium/ titanium carbides may be any suitable
chemical mixture of a general formula ( bx ,Tiy)C. By way of
example, the niobium/ titanium carbides may be (Nbo.5,Tio.s) C or
(Nbo.25,Ti0.75)C or ( b o.75 ,Ti0 .25) C .
The niobium and/or the titanium may be added to the melt to
produce particles of niobium carbide and/or niobium/ titanium
carbides in a range of 12 wt% to 33 wt% niobium carbides and
niobium/ titanium carbides of the total weight of the solidified
hard metal material .
The niobium and/or the titanium may be added to the melt to
produce particles of niobium carbide and/or niobium/ titanium
carbides in a range of 12 wt% to 25 wt% niobium carbides and
niobium/ titanium carbides of the total weight of the solidified
hard metal material .
The quantity of particles of niobium carbide and/or
niobium/ titanium carbides in the micros true ture of the solidified
hard metal material may depend on the system.
The applicant is concerned particularly with solid hard metal
materials that include host metals in the form of ferrous alloys,
such as ferrous alloys described as high chromium white cast
irons, stainless steels, and austenitic manganese steels (such as
Hadfield steels) . For ferrous alloys the quantity of particles of
niobium carbide and/or niobium/ titanium carbides in the final
micros true ture may be in a range of 10 to 33 wt% or in a range of
12 to 25 wt% of the total weight of the solidified hard metal
material .
The particle size of niobium carbide and/or niobium/ titanium
carbide may be in a range of 1 to 150 um in diameter.
The method may comprise stirring the melt with an inert gas or
magnetic induction or any other suitable means in order to
disperse particles of niobium carbide and/or niobium/ titanium
carbides in the melt.
The method may comprise adding particles of niobium carbide and/or
particles of niobium/ titanium carbides to the melt under inert
conditions, such as an argon blanket, to reduce the extent to
which niobium carbide and/or titanium carbide oxidize while being
added to the melt.
The method may comprise adding particles of ferro-niobium and/or
ferro- titanium and/or ferro-niobium- titanium to the melt under
inert conditions, such as an argon blanket, to reduce the extent
to which niobium and/or titanium oxidize while being added to the
melt.
In a situation where particles of niobium/ titanium carbides are
required in the solidified hard metal material, the method may
comprise pre-melting ferro-niobium and ferro- titanium and/or
ferro-niobium- titanium under inert conditions and forming a liquid
phase that is a homogeneous chemical mixture of iron, niobium and
titanium and solidifying this chemical mixture. The chemical
mixture can then be processed as required, for example by crushing
to a required particle size, and then added to the melt
(containing carbon) under inert conditions. The iron, niobium and
titanium dissolve in the melt and the niobium and titanium and
carbon in the melt form niobium/ titanium carbides in the melt.
The method may comprise forming the solidified hard metal material
by casting the melt into a cast product, such as a pump impeller
or a pump throatbush.
The cast product may be subject to subsequent thermal treatment
for adjusting the micros trueture to achieve desired alloy
properties .
There is also provided a hard metal material formed according to
the method described above.
There is also provided a method of casting the above-described
hard metal material with a dispersion of particles of a chemical
mixture of niobium carbides and titanium carbides in a host metal,
which forms a matrix of the casting that comprises selecting the
density of the niobium/ titanium particles in relation to the
density of the matrix material and therefore selectively
controlling the dispersion of the niobium/ titanium particles in
the matrix ranging from a uniform dispersion to a non-uniform
dispersion.
There is also provided a casting of the above-described hard metal
material made by the above-described method.
The casting may comprise a uniform dispersion of the
niobium/ titanium carbide particles in the matrix. For example,
the casting may be a pump impeller.
The casting may comprise a non-uniform dispersion of the
niobium/ titanium carbide particles in the matrix. For example, the
casting may be a pump throatbush.
The host metal may be a ferrous alloy, such as a high chromium
white cast iron, a stainless steel, or an austenite manganese
steel (such as a Hadfield steel) .
Brief description of the drawings
Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying drawings, in
which :
Figure 1 is a micrograph of a high chromium white cast iron alloy
including 27 wt% chromium and 15 wt% niobium carbides.
Figure 2 is a micrograph of martensitic stainless steel (grade
420C) including 15 wt% niobium carbides.
Detailed description
The applicant carried out an extensive series of laboratory
melting trials on the addition of 10 to 30 wt% NbC and Nb/TiC
particles to a wide selection of ferrous alloys including high
chromium white irons, austeni tic-manganese steels (including
Hadfield steels), superalloys , stainless steels (including duplex,
ferritic, austeni tic and martensitic) and hard-facing weld
deposits
The applicant has carried out further extensive work reviewing
data compiled by the applicant directly and in other sources in
relation to carbides , borides , and nitrides of transition metals ,
and chemical combinations of carbides , borides , and nitrides of
these metals, and has established that the findings of the
laboratory work reported herein are equally applicable to these
carbides, borides, and nitrides of transition metals and
combinations of elements in ferrous host metals.
An example of a micros trueture of a high chromium white cast iron
alloy including 15 wt% NbC is shown in Figure 1 . The alloy was
produced by casting a 50g ingot from a melt produced in an
electric arc melting furnace under a partial pressure of argon in
a water cooled copper hearth, i.e. the ingot was chill cast. The
NbC was added to the furnace melt as discrete particles which had
a particle size range of 2 to 20 um in diameter.
In further embodiments the applicant has examined the use of
various other particle size ranges of NbC, including <45 um in
diameter, 45 to 75 um in diameter, 75 to 150 um in diameter and
<100 um in diameter.
High chromium white cast iron alloys conventionally rely on the
high chromium content to produce a significant volume of hard
chromium carbides that provide castings with high wear resistance.
In addition, high chromium white cast iron alloys conventionally
rely on some chromium remaining in the ferrous matrix and provides
alloys with corrosion resistance.
The micros trueture in Figure 1 exhibits a ferrous matrix
containing a fine dispersion of eutectic M7C 3 carbides
(approximating 30 volume%) and a dispersion of 15 wt% bC
particles which appear as a phase of white coloured spheroids in
the Figure.
The micros trueture shown in Figure 2 is a form of 420C grade
martensitic stainless steel that was produced by the same process
described above for the high chromium white cast iron shown in
Figure 1 .
In contrast, NbC particles (white coloured in Figure 2 ) are not
regular spheroids as in the high chromium white cast iron, but
rather an irregular NbC carbide shape that appears to be typical
for various stainless steel grades that have been alloyed with
NbC.
The experimental work reported above and other experimental work
carried out by the applicant indicates that alloys produced with
niobium carbide particles in the range of 10-30 wt% NbC in a
ferrous host metal show very promising microstructures , welding
characteristics and foundry casting characteristics. The
indications are that the addition of high NbC contents to these
materials substantially increases wear resistance without
adversely affecting castability, weldability, response to heat
treatment and the mechanical properties of the original ferrous
materials .
The microstructures of the test castings in Figure 1 and other
test castings produced by the applicant show that all the NbC
particles added to the ferrous alloys are primary carbides in
suspension in the liquid metal . The analogy is that all
conventional castings above the liquidus temperature
(approximately 1300-1400°C) are "clear liquids", i.e. single phase
liquids. However, when niobium carbide particles were added, for
example 20 wt% , the particles remained in suspension so the liquid
metal and NbC particles approximate a "slurry" (2 phases) with
good fluidity, which is a mandatory requirement for producing
sound castings. The experimental work found a similar outcome
when niobium/ titanium carbide particles were added to a liquid
melt.
It will be appreciated, however, that niobium carbides can form as
solid particles in a melt, rather than added to the melt, by
adding ferro-niobium to the melt. In such cases, the melt contains
carbon, and the weight% carbon is greater than one eighth of the
weight% of niobium. In the case of ferro-niobium additions, the
iron and niobium separate in the melt. The niobium, which has a
high affinity for carbon, chemically combines with carbon from the
liquid melt to form solid niobium carbide particles dispersed in
the liquid melt. Upon casting, the melt is cast as a "slurry"
consisting of solid niobium carbide particles suspended in the
liquid melt. Upon solidification, the casting will have a
micros trueture that includes niobium carbides dispersed in a
ferrous matrix. A similar micros trueture is achieved with
niobium/ titanium carbide particles.
The advantages of adding 10-30 wt% NbC particles to ferrous
materials are summarised below.
Hardness of NbC is approx 2500 HV which compares to a hardnes
of 1500 HV for M7C 3 carbides present in high chromium white
cast iron alloys.
(b) Niobium is a very strong carbide former and can be added as
ferro niobium or NbC powder to the ferrous melt.
(c) The melting point of NbC is 3600°C, i.e. about 2000°C above the
temperature of the ferrous melt of steels, cast irons and
hard-facing weld deposits. Additionally, fine NbC particles
(e.g. 2 to 20 m in diameter) do not grow in size or coalesce
in the melt during the casting process. This is important in
terms of the castability of the melt and the resultant wear
resistance of the cast product. The wear resistance of the
cast product is optimised when a dispersion of fine NbC
particles is evenly distributed throughout the micros trueture .
(d) Other elements, e.g. Cr, Mn and Fe, do not dissolve in the
high melting point NbC particles. Accordingly, the chemical
composition of the NbC particles is not altered and they will
retain their physical properties during preparation of the
melt and after casting.
(e) The solubility of NbC in the ferrous matrix is negligible
(<0.3 wt%) which suggests that the addition of NbC to ferrous
materials will result in no observable effect on the response
to heat treatment or change in material properties of the
ferrous matrix.
(f) The density of NbC is 7.82 grams/cc at room temperature. This
is very close to the densities of ferrous materials which are
approximately 7.5 grams/cc. This means that NbC particles will
not segregate in the liquid melt by sinking (compared with
tungsten carbide, for example, which has a density of 15.8
grams/cc) or by floating (compared with titanium carbide, for
example, which has a density of 4.93 grams/cc).
(g) The presence of a high volume fraction of NbC particles in the
micros trueture will result in a finer ferrous matrix grain
size during casting and heat treatment. This improves
mechanical properties of the castings.
(h) It is estimated that 20 wt% addition of NbC to the existing
family of wear resistant high chromium white cast iron alloys,
will improve the wear resistance of these materials, in some
cases possibly by an order of magnitude.
By observing the resultant microstructures is it considered
that the addition of 10-25 weight% NbC to various stainless
steels, for example martensitic, austenitic, ferritic and
duplex, will substantially increase wear life with negligible
reduction in toughness, corrosion resistance and mechanical
properties for the various grades .
The addition of 20 wt% NbC to Hadfield steel (which is
normally used in liners of primary rock crushers, such as jaw
and gyratory crushers, where high impact toughness is
required) will produce a material with a much greater wear
life than the original Hadfield steel without diminishing the
exceptional toughness and work hardening capacity which is
inherent in this steel.
(k) The addition of 20 wt% NbC to tool steels will greatly improve
tool wear life while maintaining the original material
properties .
Niobium carbide can be added to ferrous alloys , such as high
chromium white cast irons in two distinct ways, as follows.
1 . As fine niobium carbide particles (2-100 microns in
diameter) to a melt, as per the above-mentioned laboratory work.
2 . As fine ferro-niobium powder (minus 1 mm diameter) in
the presence of the required stoichiometric amount of carbon
previously dissolved in the melt.
The density of NbC is 7.8 grams/cc at room temperature and this is
close to the density of high chromium white cast iron (7.5
grams/cc) . The presence of phases with similar densities assists
in achieving a uniform dispersion of NbC particles in the liquid
metal during a casting process.
However, a laboratory test carried out by the applicant showed
that segregation of NbC occurred in a high chromium white cast
iron + 5wt% NbC alloy by settling of the fine NbC particles to the
bottom of the ingot when the melt was allowed to stand for 15
minutes at about 150 °C below the liquidus temperature of the host
metal .
The density difference between high chromium white cast iron and
NbC increases with temperature. The coefficient of thermal
expansion of high chromium white cast iron is double that of NbC.
In addition, high chromium white cast iron undergoes a step
increase in volume at the solid to liquid phase change at
approximately 1260 °C.
A s a consequence, the density of high chromium white cast iron in
the liquid state at 1400 °C is 6.9 grams/cc whereas the density of
NbC at 1400°C is about 7.7 grams/cc. The applicant has found that
this density difference is sufficient to cause segregation of NbC
particles in liquid high chromium white cast iron at foundry
casting temperatures of 1300 °C or greater.
Titanium carbide is similar in many characteristics to NbC. The
crystal structures are the same, with group number 225. The
lattice parameter of NbC is 4.47 Angstroms and the lattice
parameter of TiC is 4.32 Angstroms. TiC and NbC are isomorphorous ,
i.e. Ti atoms will readily substitute for Nb atoms in NbC. The
hardness of TiC is similar to NbC. The melting point of TiC is
3160 °C, which is similar to the melting point of NbC (3600 °C) .
However, the density of TiC is 4.9 grams/cc at room temperature,
and this is much less than the density of NbC. Since TiC and NbC
are isomorphous, it is possible to achieve any density value for
the mixed carbide in a range 4.9-7.8 grams/cc by selecting the
corresponding chemical composition with the general formula
(Nbx ,Tiy)C. By way of example, the niobium/ titanium carbides may
be (Nbo.5,Ti0 .5) C or (Nb0 .25 ,Ti0 .7 5 ) C or (Nb0 .75 ,Ti0 .25) C . This density
difference is the basis of a cost effective method of reducing the
segregation of hard, solid carbides in liquid metal at usual
foundry casting temperatures. Specially, it is possible to
selectively adjust the density of the niobium/ titanium carbides
within the range of 4.9-7.8 grams/cc and control whether the
particles will form a uniform dispersion in or segregate in a
casting of a metal alloy, such as a high chromium white iron,
which includes the particles . This selection may be desirable for
some castings where uniform wear resistance through the castings
is desirable and for other castings where it is desirable to have
a concentration of wear resistant particles in one section, such
as a surface, of the castings.
The specification refers to the microstructures of hard metal
materials of the present invention by volume % rather than the
usual bulk chemical weight % . The table set out below is provided
to explain the reason for this selection of nomenclature.
In the first 2 cases in the table, the chemistry of the host metal
is identical and is essentially a high chrome white chromium cast
iron, with a chemistry = Fe-27Cr-2.7C-2Mn-0.5Si. It is intuitively
simple to visualize the microstructures of the two hard metal
materials (namely 10 and 20 volume % bC ) in the same host metal.
However, the bulk chemistries of the two hard metal materials (as
determined by the usual foundry spectrograghic analysis technique)
do not clearly convey the simple difference between these two hard
metal materials.
The third and fourth cases in the table, the exercise is repeated
for 10 and 20 volume % NbC in Hadfield steel. The chemistry of
the host metal is identical and is essentially Fe-12Mn-l .2C-2 Mn-
0 .5Si . Again, the bulk chemistries of these two hard metal
materials are widely different and are not descriptive of the
microstructures .
Microstructure = 90 volume% white cast iron + 10 volume% NbC
In all of the work carried out by the applicant in relation to
present invention the applicant has found that the final bulk
chemistry of each of the hard metal materials is a complex
function of the selected microstructure and the actual bulk
chemistry is not a useful means of describing the required
features of the hard metal materials . The required features of
the hard metal material of the present invention are (a) host
metal chemistry and (b) volume % of the selected refractory
particles .
It is noted that the bulk chemistry is even more complicated when
carbides and/or nitrides and/or borides of two or more transition
metals are included in the hard metal materials .
It is noted that the hard metal material of the present invention
may be cast as a final product shape and may be formed as a solid
material that is subsequently hot worked in a downstream
processing operation to form a final product shape. For example,
the hard metal material of the present invention may be formed as
an ingot and subsequently hot worked by rolling or forging as
required into a final product such as a bar or a plate.
Many modifications may be made to the embodiments of the present
invention as described above without departing from the spirit and
scope of the present invention.
It will be understood that the term "comprises" or its grammatical
variants as used in this specification and claims is equivalent to
the term "includes" and is not to be taken as excluding the
presence of other features or elements .

WE CLAIM:
1. A hard metal material comprising 5-50 volume % particles of a refractory material dispersed in a host metal, wherein the refractory material comprises particles of carbides and/or nitrides and/or borides of any one or more than one of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, and molybdenum.
2. The hard metal material defined in claim 2 wherein the particles of the refractory material also comprise tungsten.
3. The hard metal material defined in claim 1 or claim 2 comprises 5-40 volume % particles of the refractory material dispersed in the host metal.
4. The hard metal material defined in any one of the preceding claims comprises greater than 10 volume % particles of the refractory material dispersed in the host metal.
5. The hard metal material defined in any one of the preceding claims comprises greater than 15 volume % particles of the refractory material dispersed in the host metal.
6. The hard metal material defined in any one of the preceding claims comprises less than 3 0 volume % particles of the refractory material dispersed in the host metal.
7. The hard metal material defined in any one of the preceding claims comprises less than 25 volume % particles of the refractory material dispersed in the host metal.
8. The hard metal material defined in any one of the preceding claims wherein the host metal comprises a ferrous alloy
(such as a steel or a cast iron), a stainless steel, an
austenitic-manganese steel, or an iron-based or a nickel-based or
a cobalt-based superalloy.
9. A method of manufacturing a component of a hard metal
material comprising:
(a) forming a slurry of a hard metal material comprising 5-50 volume % particles of a refractory-material dispersed in a liquid host metal in an inert atmosphere, and
(b) pouring the slurry into a mould and forming a casting of the component, such as in an inert atmosphere.

10. The method defined in claim 9 comprises forming the slurry and thereafter forming the casting of the component in a chamber under vacuum conditions which remove air from the chamber and supplying an inert gas, such as argon, into the chamber.
11. The method defined in claim 9 or claim 10 comprises selecting the production parameters to form the slurry in step (a) with a required fluidity for processing in step (b).
12.. The method defined in claim 11 wherein the production parameters comprise any one or more of the particle size, shape, reactivity, density, and solubility of the refractory materials.
13. The method defined in any one of claims 9 to 12 wherein the refractory material is less than 4 00 microns particle size.
14. The method defined in any one of claims 9 to 13 wherein the refractory material is less than 150 microns particle size.
15. The method defined in any one of claims 9 to 14 comprises selecting the refractory material to have a smaller thermal contraction than the host metal.
16. The method defined in any one of claims 9 to 15 comprises selecting the density of the refractory material, compared to the density of the host metal in the liquid state to control the dispersion of the particles of the refractory material in the host metal.
17. The method defined in any one of claims 9 to 16 comprises selecting the refractory material to have minimal solid solubility in the liquid host metal.
18. A method of forming a wear resistant hard metal material, the method comprising adding (a) niobium or (b) niobium and titanium to a melt containing a host metal in a form that produces particles of niobium carbide and/or particles of a chemical mixture of niobium carbide and titanium carbide in a range of 10 to 4 0 wt% of the total weight of the hard metal material in a microstructure of a solidified metal alloy, and allowing the melt to solidify to form the solid hard metal material.
19. The method as defined in claim 18 comprising adding the niobium and/or the titanium to the melt to produce particles of niobium carbide and/or niobium/titanium carbides in a range of 12 wt% to 33 wt% niobium carbides and niobium/titanium carbides of the total weight of the solidified hard metal material.
20. The method as defined in claim 18 or claim 19 wherein the particles of niobium/titanium carbides have a general formula (Nbx/Tiy)C.
21. The method as defined in any one of claims 18 to 20 comprising adding niobium and/or titanium to the melt in the form of particles of niobium carbide and/or niobium/titanium carbides.
22. The method as defined in claim 21 comprising forming a slurry of particles of niobium carbide and/or niobium/titanium
carbides suspended in the melt and allowing the melt to solidify-to form the solidified hard metal material.
23. A method of casting a hard metal material having a dispersion of a chemical mixture of niobium carbides and titanium carbides in a host metal which forms a matrix of the hard metal material, the method comprising selecting the density of the niobium/titanium particles in relation to the density of the host metal and therefore selectively controlling the dispersion of the niobium/titanium particles in the matrix ranging from a uniform dispersion to a non-uniform dispersion.
24. A casting of the metal alloy made by the method defined in claim 23.
25. The casting defined in claim 24 comprising a uniform dispersion of niobium/titanium particles in the matrix.
26. The casting defined in claim 24 comprising a non-uniform dispersion of niobium/titanium particles in the matrix.
27. The casting defined in any one of claims 23 to 26 wherein the metal alloy is a ferrous alloy (such as a steel or a cast iron, such as a high chromium white cast iron), a stainless steel or an austenitic manganese steel (such as a Hadfield steel).
28. A method of forming a hard metal material comprising:

(a) forming a slurry of a hard metal material comprising 5-50 volume % particles of a refractory material dispersed in a liquid host metal, and
(b) allowing the slurry to solidify to form a solid hard metal material.
29. A method of forming a wear resistant hard metal
material, the method comprising adding any one or more of the nine
transition metals titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten to a melt of a host metal in a form that produces particles of carbides and/or nitrides and/or borides of any one or more of the nine transition metals in a range of 5 to 50 volume % of the total volume of the hard metal material, and allowing the melt to solidify to form the solid hard metal material.
30. A method of casting a hard metal material having a dispersion of refractory material particles of carbides and/or nitrides and/or borides of any one or more of the nine transition metals titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten in a host metal which forms a matrix of the hard metal material in a solid casting, the method comprising selecting the density of the refractory material particles in relation to the density of the host metal and therefore selectively controlling the dispersion of the refractory material particles in the matrix of a solid casting ranging from a uniform dispersion to a non-uniform dispersion.