US20130039800A1 - Hard metal materials - Google Patents

Hard metal materials Download PDF

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US20130039800A1
US20130039800A1 US13/577,371 US201113577371A US2013039800A1 US 20130039800 A1 US20130039800 A1 US 20130039800A1 US 201113577371 A US201113577371 A US 201113577371A US 2013039800 A1 US2013039800 A1 US 2013039800A1
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particles
niobium
hard metal
host
metal material
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Kevin Dolman
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Weir Minerals Australia Ltd
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Weir Minerals Australia Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
    • B22D21/002—Castings of light metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/10—Alloys containing non-metals
    • C22C1/1036—Alloys containing non-metals starting from a melt
    • C22C1/1068—Making hard metals based on borides, carbides, nitrides, oxides or silicides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
    • C22C29/02—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
    • C22C29/14—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on borides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
    • C22C29/16—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on nitrides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/06—Cast-iron alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/10—Cast-iron alloys containing aluminium or silicon

Definitions

  • 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 100 kgs and typically more than 1 tonne.
  • the present invention also provides a method of manufacturing the components from the hard metal materials.
  • the present invention relates to hard metal materials that are useful for applications requiring wear resistance.
  • 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.
  • 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 100 kgs 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.
  • wear resistant metal alloys such as high chromium white cast irons
  • hard-facing alloys are formed on the trays of dump trucks that transport mined ore from a mine site to a minerals processing plant.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the present invention provides a hard metal material comprising 5-50 volume % particles of a refractory material dispersed in a host metal.
  • 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.
  • 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 NbC.
  • 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.
  • the particles of the refractory material may be of the type described as (M 1 , M 2 ) C, where “M” is a transition metal.
  • 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 austenitic-manganese steel such as a Hadfield steel, or a iron-based or nickel-based or cobalt-based superalloy.
  • a ferrous alloy such as a steel or a cast iron
  • a stainless steel such as a stainless steel
  • an austenitic-manganese steel such as a Hadfield steel
  • iron-based or nickel-based or cobalt-based superalloy such as a steel or a cast iron
  • the present invention also provides a method of forming a hard metal material comprising:
  • the present invention also provides a method of manufacturing a component of a hard metal material comprising:
  • 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.
  • an inert gas such as argon
  • 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).
  • 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 microstructure 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.
  • 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 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.
  • the microstructure On solidification, the microstructure 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.
  • 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.
  • Fe—Nb 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.
  • 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 CO 2 which can result in severe porosity in the castings.
  • 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 CO 2 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.
  • the density of the refractory material of the particles 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.
  • 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.
  • Niobium carbide 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.
  • 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.
  • niobium carbide and titanium carbide have similar crystal structures and are isomorphous.
  • 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 microstructure.
  • 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 microstructures 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.
  • 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.
  • a hard metal material with a microstructure 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.
  • 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.
  • 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.
  • the present invention makes it possible to achieve high wear resistance of a hard metal material without a loss of other desirable material properties.
  • a method of forming a wear resistant hard metal material 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.
  • niobium carbide and titanium carbide and “niobium/titanium carbides” are hereinafter understood to be synonyms.
  • 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 microstructure containing 10 to 40 wt % particles of niobium carbide and/or niobium/titanium carbides have excellent wear resistance properties.
  • HV Vickers hardness
  • 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.
  • 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.
  • 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.
  • the method may comprise adding the niobium to the melt in the form of ferro-niobium, for example particles of ferro-niobium.
  • 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.
  • 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 (Nb x ,Ti y )C.
  • the niobium/titanium carbides may be (Nb 0.5 ,Ti 0.5 )C or (Nb 0.25 Ti 0.75 ) C or (Nb 0.75 , Ti 0.25 ) 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 microstructure 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).
  • ferrous alloys such as ferrous alloys described as high chromium white cast irons, stainless steels, and austenitic manganese steels (such as Hadfield steels).
  • the quantity of particles of niobium carbide and/or niobium/titanium carbides in the final microstructure 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 ⁇ m 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.
  • inert conditions such as an argon blanket
  • 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.
  • inert conditions such as an argon blanket
  • 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 microstructure to achieve desired alloy properties.
  • 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.
  • the casting may comprise a uniform dispersion of the niobium/titanium carbide particles in the matrix.
  • the casting may be a pump impeller.
  • the casting may comprise a non-uniform dispersion of the niobium/titanium carbide particles in the matrix.
  • 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).
  • a ferrous alloy such as a high chromium white cast iron, a stainless steel, or an austenite manganese steel (such as a Hadfield steel).
  • FIG. 1 is a micrograph of a high chromium white cast iron alloy including 27 wt % chromium and 15 wt % niobium carbides.
  • FIG. 2 is a micrograph of martensitic stainless steel (grade 420C) including 15 wt % niobium carbides.
  • 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, austenitic-manganese steels (including Hadfield steels), superalloys, stainless steels (including duplex, ferritic, austenitic and martensitic) and hard-facing weld deposits
  • ferrous alloys including high chromium white irons, austenitic-manganese steels (including Hadfield steels), superalloys, stainless steels (including duplex, ferritic, austenitic and martensitic) and hard-facing weld deposits
  • FIG. 1 An example of a microstructure of a high chromium white cast iron alloy including 15 wt % NbC is shown in FIG. 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 ⁇ m in diameter.
  • the applicant has examined the use of various other particle size ranges of NbC, including ⁇ 45 ⁇ m in diameter, 45 to 75 ⁇ m in diameter, 75 to 150 ⁇ m in diameter and ⁇ 100 ⁇ m 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.
  • high chromium white cast iron alloys conventionally rely on some chromium remaining in the ferrous matrix and provides alloys with corrosion resistance.
  • the microstructure in FIG. 1 exhibits a ferrous matrix containing a fine dispersion of eutectic M 7 C 3 carbides (approximating 30 volume %) and a dispersion of 15 wt % NbC particles which appear as a phase of white coloured spheroids in the Figure.
  • the microstructure shown in FIG. 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 FIG. 1 .
  • NbC particles (white coloured in FIG. 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 microstructures of the test castings in FIG. 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.
  • 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.
  • niobium carbides can form as solid particles in a melt, rather than added to the melt, by adding ferro-niobium to the melt.
  • the melt contains carbon, and the weight % carbon is greater than one eighth of the weight % of niobium.
  • 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.
  • the melt is cast as a “slurry” consisting of solid niobium carbide particles suspended in the liquid melt.
  • the casting will have a microstructure that includes niobium carbides dispersed in a ferrous matrix. A similar microstructure is achieved with niobium/titanium carbide particles.
  • Niobium carbide can be added to ferrous alloys, such as high chromium white cast irons in two distinct ways, as follows.
  • 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.
  • high chromium white cast iron increases with temperature.
  • the coefficient of thermal expansion of high chromium white cast iron is double that of NbC.
  • high chromium white cast iron undergoes a step increase in volume at the solid to liquid phase change at approximately 1260° C.
  • 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.).
  • 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 (Nb x ,Ti y )C.
  • the niobium/titanium carbides may be (Nb 0.5 ,Ti 0.5 )C or (Nb 0.25 , Ti 0.5 )C or (Nb 0.75 ,Ti 0.25 )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.
  • 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.
  • 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-1.2C-2 Mn-0.5Si. Again, the bulk chemistries of these two hard metal materials are widely different and are not descriptive of the microstructures.
  • 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.
  • 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.
  • 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.

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Publication number Priority date Publication date Assignee Title
US11085102B2 (en) 2011-12-30 2021-08-10 Oerlikon Metco (Us) Inc. Coating compositions
US20140263194A1 (en) * 2013-03-15 2014-09-18 Lincoln Global, Inc. Cored non-arc consumable for joining or overlaying and systems and methods for using cored non-arc consumables
US20170080531A1 (en) * 2014-06-09 2017-03-23 Scoperta, Inc. Crack resistant hardfacing alloys
US10173290B2 (en) * 2014-06-09 2019-01-08 Scoperta, Inc. Crack resistant hardfacing alloys
US11111912B2 (en) 2014-06-09 2021-09-07 Oerlikon Metco (Us) Inc. Crack resistant hardfacing alloys
US11253957B2 (en) 2015-09-04 2022-02-22 Oerlikon Metco (Us) Inc. Chromium free and low-chromium wear resistant alloys
CN106626600A (zh) * 2016-11-08 2017-05-10 西安科技大学 一种硬金属材料
US11939646B2 (en) 2018-10-26 2024-03-26 Oerlikon Metco (Us) Inc. Corrosion and wear resistant nickel based alloys

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