HARD METAL MATERIALS

Abstract

A hard metal material and a method of manufacturing a component of the hard metal material are disclosed. The hard metal material comprises 5-50 volume % particles of a refractory material dispersed in a host metal. The method comprises forming a slurry of 5-50 volume % particles of the refractory material dispersed in a liquid host metal in an liquid atmosphere and pouring the slurry into a mould and forming a casting of the component.

Claims

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 1 wherein the particles of the refractory material also comprise tungsten.

3. The hard metal material defined in claim 1 comprises 5-40 volume % particles of the refractory material dispersed in the host metal.

4. The hard metal material defined in claim 1 comprises greater than 10 volume % particles of the refractory material dispersed in the host metal.

5. The hard metal material defined in claim 1 comprises less than 30 volume % particles of the refractory material dispersed in the host metal.

6. The hard metal material defined in claim 1 wherein the host metal comprises a ferrous alloy, a stainless steel, an austenitic-manganese steel, or an iron-based or a nickel-based or a cobalt-based superalloy.

7. 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.

8. The method defined in claim 7 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 into the chamber.

9. The method defined in claim 7 wherein the refractory material is less than 400 microns particle size.

10. The method defined in claim 7 comprises selecting the refractory material to have a smaller thermal contraction than the host metal.

11. The method defined in claim 7 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.

12. 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 in a microstructure of a solidified metal alloy, and allowing the melt to solidify to form the solid hard metal material.

13. The method as defined in claim 12 wherein the particles of niobium/titanium carbides have a general formula (Nb.sub.x,Ti.sub.y)C.

14. The method as defined in claim 12 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.

15. 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.

16. A casting of the metal alloy made by the method defined in claim 15.

17. The casting defined in claim 16 wherein the metal alloy is a ferrous alloy, a stainless steel or an austenitic manganese steel.

18. 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.

19. 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.

20. 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.

21. A method of forming a wear resistant hard metal material, the method comprising adding niobium, ferro-niobium, or ferro-niobium-titanium to a melt of a ferrous alloy comprising carbon and thereby forming particles of niobium carbide and/or niobium-titanium carbide; and allowing the melt to solidify to form the solid hard metal material, wherein the particles of niobium carbide and/or niobium-titanium carbide are present in a range of 10 to 40 wt. % of the total weight of the hard metal material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0087] FIG. 1 is a micrograph of a high chromium white cast iron alloy including 27 wt % chromium and 15 wt % niobium carbides.

[0088] FIG. 2 is a micrograph of martensitic stainless steel (grade 420C) including 15 wt % niobium carbides.

DETAILED DESCRIPTION

[0089] 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

[0090] 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.

[0091] 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 50 g 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.

[0092] In further embodiments 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.

[0093] 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.

[0094] The microstructure in FIG. 1 exhibits a ferrous matrix containing a fine dispersion of eutectic M.sub.7C.sub.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.

[0095] 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.

[0096] In contrast, 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.

[0097] 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.

[0098] 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. 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.

[0099] 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 microstructure that includes niobium carbides dispersed in a ferrous matrix. A similar microstructure is achieved with niobium/titanium carbide particles.

[0100] The advantages of adding 10-30 wt % NbC particles to ferrous materials are summarised below. [0101] (a) Hardness of NbC is approx 2500 HV which compares to a hardness of 1500 HV for M.sub.7C.sub.3 carbides present in high chromium white cast iron alloys. [0102] (b) Niobium is a very strong carbide former and can be added as ferro niobium or NbC powder to the ferrous melt. [0103] (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 microstructure. [0104] (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. [0105] (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. [0106] (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). [0107] (g) The presence of a high volume fraction of NbC particles in the microstructure will result in a finer ferrous matrix grain size during casting and heat treatment. This improves mechanical properties of the castings. [0108] (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. [0109] (i) 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. [0110] (j) 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. [0111] (k) The addition of 20 wt % NbC to tool steels will greatly improve tool wear life while maintaining the original material properties.

[0112] 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.

[0113] 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.

[0114] However, a laboratory test carried out by the applicant showed that segregation of NbC occurred in a high chromium white cast iron+5 wt % 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.

[0115] 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.

[0116] As 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.

[0117] 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.).

[0118] 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 (Nb.sub.x,Ti.sub.y)C. By way of example, the niobium/titanium carbides may be (Nb.sub.0.5,Ti.sub.0.5)C or (Nb.sub.0.25,Ti.sub.0.75)C or (Nb.sub.0.75,Ti.sub.0.25).sub.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.

[0119] 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.

[0120] 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 % NbC) 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.

[0121] 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.

TABLE-US-00001 Microstructure = 90 volume % white cast iron + 10 volume % NbC Furnace Charge Volume Composition (Wt %) Desc (%) Cr C Mn Si Nb Fe NbC 10 11.4 88.6 0.00 Host metal 90 27.0 2.7 2.0 0.5 67.80 Bulk 100 24.3 3.57 1.80 0.45 8.86 61.02 Chemistry

TABLE-US-00002 Microstructure = 80 volume % white cast iron + 20 volume % NbC Furnace Charge Volume Composition (Wt %) Desc (%) Cr C Mn Si Nb Fe NbC 20 11.4 88.6 0.00 Host metal 80 27.0 2.7 2.0 0.5 67.80 Bulk 100 21.6 4.44 1.60 0.40 17.72 54.24 Chemistry

TABLE-US-00003 Microstructure = 90 volume % Hadfield Steel + 10 volume % NbC Furnace Charge Volume Composition (Wt %) Desc (%) Cr C Mn Si Nb Fe NbC 10 11.4 88.6 0.00 Host metal 90 1.2 12.0 0.5 86.30 Bulk 100 2.22 10.80 0.45 8.86 77.67 Chemistry

TABLE-US-00004 Microstructure = 80 volume % Hadfield Steel + 20 volume % NbC Furnace Charge Volume Composition (Wt %) Desc (%) Cr C Mn Si Nb Fe NbC 20 11.4 88.6 0.00 Host metal 80 1.2 12.0 0.5 86.30 Bulk 100 3.24 9.60 0.40 17.72 69.04 Chemistry

[0122] In all of the work carried out by the applicant in relation to the 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] 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.