Tantalum and Niobium

Cemented carbides and cermets are metal-bonded transition metal carbides (or carbonitrides) used for their properties of high strength, hardness, and thermal shock resistance. Applications include metalcutting inserts; solid round tooling (drills, endmills, routers, reamers, taps, etc.); boring bars; mining tools; road-construction tools; down-hole drilling tools; tooling for ultra-high-pressure manufacture of diamond and cubic boron nitride (CBN); woodworking tools; plungers for polymer manufacturing; wear parts; dies; metalforming tools; ordnance; and seal rings.

Tantalum carbide (TaC) and niobium carbide (NbC) are important components of cemented carbides and cermets used for metalcutting applications, primarily indexable inserts. For most grades, the prime constituent is tungsten carbide (WC), while the binder metal is usually cobalt (Co), except in some specialized corrosion applications where nickel (Ni) is used instead.

However, TaC and NbC (as well as titanium carbide, or TiC) also play an important role. They are often included in grade compositions for a number of reasons, including increased high-temperature deformation resistance; increased chemical stability and reduced reactivity with Fe-group metals; and grain growth control.

These advantages result from the greater chemical stability of the Group IVA and VA carbides (titanium, zirconium, hafnium, vanadium, niobium, and tantalum) compared with WC, plus their tendency to form a 3-D carbide-grain network. Group IVA and VA metal carbides are generally stable in an ordered face-centered cubic (NaCl) structure and can incorporate some WC into this structure — hence the common terms "cubic carbides" and "solid solution carbides." WC solubility is greatest with TiC and least with TaC (NbC is intermediate in behavior); in addition, the low atomic weight of Ti, and to a lesser extent Nb, compared with W means that surprisingly small weight-percentage additions can have a major effect on structure and properties.

However, insert users pay a price for these benefits. Although TaC and NbC improve insert performance in some applications, they (especially TaC) are among the more expensive components of cemented carbides. Other disadvantages include reduced fracture toughness and strength; reduced thermal-shock resistance due to a higher thermal-expansion coefficient and lower thermal conductivity; and reduced abrasive wear resistance.

Clearly, compromises are being made and, in practice, the only applications where TaC or NbC are added in significant levels are where high temperatures and high local loads are generated in operation, where abrasive wear resistance is of secondary importance, where thermal and mechanical shock are not extremely severe, and where high-temperature reaction with iron-based materials is of concern. In practice, this mostly means the machining of steels. The same grades are commonly used for both steel and cast-iron machining. So in practice, TaC and NbC also are used in grades for cast iron machining, even though the materials-science arguments to do so are less convincing than for steels.

Product Development

A number of routes are used for making TaC and NbC, including direct carbothermic reduction of the oxides, carburization of metal powders, and crystallization from a metallic melt.

Some general trends in product characteristics and quality include:

 Reduced metallic impurity levels.

 Reduced oxygen levels at a given particle size.

 Except where a nitride or carbonitride is specifically required, reduced nitrogen levels.

 Supply of material premilled to a customer’s desired particle size range to allow ready incorporation into wet milling of carbide or cermet grades.

 Presolid solution with one another or other transition-metal carbides. Thus, NbC is generally incorporated as a solid solution with TaC. In addition, pre-solution with WC (which has long been the norm for TiC) is being increasingly used for NbC and TaC, and carbide powders containing three or even four metallic components are becoming more common. The claimed advantages of this are to ease the creation of a homogenous microstructure and improve wetting by Co during sintering; the disadvantages are in flexibility in reclaim use and materials cost.

Market Outlook

The annual quantity of cemented carbide produced in the world (excluding the former Soviet Union) is estimated at about 18,000 tons. This estimate is calculated by taking published data from trade associations where available and then adding an estimate for areas where data are less readily available. It is estimated that about 40% of this 18,000 tons, or about 7000 tons, is consumed for metalcutting inserts. The next question is: What are the Ta and Nb quantities required? In order to address this issue, chemical analyses of more than 200 metalcutting-insert grades, drawn approximately equally from North American, Western European, and Japanese sources, were summarized.

Table 1: Ta and Nb levels in metalcutting grades by geographic area.

Inspection of the data in Table 1 permits one to make the following tentative conclusions:

 Tantalum usage for cemented carbides is estimated at about 240 tons per annum.

 Niobium usage for cemented carbides is estimated at about 60 tons per annum. (Note that the above figures include reclaim, estimated at approximately 20%, and are for entire worldwide estimates, excluding the former Soviet Union.)

 The figures support the common perception that Western European manufacturers generally have been more aggressive in replacing Ta with Nb than North American or Japanese ones. However, wide differences exist between manufacturers within a given geographic segment—some use little or no Nb while some approach a 2:1 Ta:Nb weight ratio.

 It is noteworthy that the parameter Ta + 2Nb (weight %) is virtually identical for the three geographic markets. The theoretical basis for this is that the atomic weight of Ta (181) is essentially twice that of Nb (93). Thus, this parameter normalizes out the atomic-weight difference; the similarity of the number for the three geographic markets is either a remarkable coincidence or indicates that the materials analyzed are a representative sample, and that the markets are similar other than the mentioned difference in Nb for Ta substitution.

Trends Affecting Consumption

Table 2: Effects of metalworking trends on Ta and Nb demand.

The future use of Ta and Nb will be affected directly or indirectly by any trends in the metalworking industry. While it is difficult to be quantitative about these trends, Table 2 lists some major factors and their expected effects on TaC and NbC demand, and the cumulative impact of ignoring the relative significance of the effects.

Economic growth. In particular, the rapid growth being realized in the Asia-Pacific area will tend to increase demand.

Machine tool modernization. Insofar as this enables tool steels to be replaced by cemented carbides and cermets, this should be generate demand, but there are compensating factors. Very stiff, high-speed machine tools are able to use ceramics for cast-iron and superalloy machining, and may be able to use downsized inserts because they have less chatter and require less strength in the tool. Another important point is that where HSS round tools (e.g., drills, endmills, and taps) are replaced by cemented carbides, the latter are usually (but not always) "straight" grades containing little or no Ta and Nb.

Component near-net shaping. As the total amount of machining required goes down for a given component, fewer and/or smaller tools are expected to be required.

Dry machining. Many users are reducing or, in certain operations, eliminating their use of coolants. The effect of this trend, if any, on Ta and Nb demand is uncertain.

Workpiece substitutions. Two of the most important are the shift from gray cast iron to ductile cast iron, and the replacement of gray cast iron by aluminum-silicon alloys. The former has no clear effect on Ta or Nb demand, while the latter will be mildly negative—grades used for Al machining contain little or no Ta or Nb, or else diamond tooling of very long life is used.

Insert downsizing. Near-net shaping, stiffer machine tools, and improvements in toolholding and workholding permit the use of smaller inserts. This can have a major effect on material demand. For example, a square 3/8"-inscribed circle (IC), 1/8"-thick insert contains only about 37% of the material content of a 1/2"-IC, 3/16"-thick insert.

Hard coatings. One of the most important, and still most active, areas of development is that of hard coatings deposited by chemical or physical vapor deposition. Common compounds include TiC, titanium carbonitride, titanium nitride, and aluminum oxide (Al₂O₃). The coatings, although only typically in the range of about 3µm to 15µm in thickness, have a major effect in most steel and cast-iron machining applications, because they reduce wear due to both chemical and abrasive mechanisms, and often reduce cutting forces and temperatures in the cutting edge. In effect, they take over much of the role of adding solid-solution carbides to the substrate, and, in general, coated grades contain relatively low levels thereof. As coatings become more effective (thicker and/or more adherent and more uniform), the trend is for substrates to contain even lower levels of Ta and Nb.

Carbide substrate developments. Two key developments are important. The first is Co enrichment of a surface layer in the substrate below the coating, which allows a higher toughness surface with deformation-resistant bulk substrate. In practice, similar Ta and Nb levels are used for both nonenriched and enriched substrates, so the effect of this trend is uncertain. The second movement is toward finer grain size materials in "straight" grades, which are growing in application and also generally use VC or Cr₃C₂ as the grain-growth inhibitor rather than TaC (generally used in older, relatively fine-grain grades). This trend is negative toward the use of TaC.

Cermets. Worldwide, cermets comprise between 5% and 10% of insert usage, with market share being much greater in Japan than in North America or Western Europe, and their share continues to grow due to the development of stronger grades and implementation of newer machine tools. Generally, they replace cemented carbides; however, the effect on Ta and Nb demand is complicated by the fact that modern cermets themselves generally contain Ta and/or Nb. The results of a survey of the Ta and Nb levels seen in modern commercial metalcutting cermets are given in Table 3.

Table 3: Ta and Nb levels in metalcutting cermet grades by geographic area.

This survey was conducted with a much smaller population of grades than for cemented carbides, but the following tentative conclusions can be made:

After normalizing for the approximately 2:1 ratio in density between cemented carbides and cermets, the levels of Ta and Nb required for metalcutting cermets are of the same order as those used in metalcutting carbides.

As with carbides, but to an even greater degree, individual manufacturers differ in the extent to which Ta and Nb are used.

Overall, the trend toward greater use of cermets over cemented carbides is probably neutral with respect to Ta and Nb taken together; a more speculative comment is that it may be slightly negative for Ta usage and slightly positive for Nb usage.

Ceramics. Ceramics continue to grow in use, although worldwide they still only comprise no more than about 5% of metalcutting inserts. The prime growth areas are silicon nitride (Si₃N₄)-base materials for machining gray cast iron, Al₂O₃ and Si₃N₄/sialon ceramics for nickel-base superalloy machining. In general, the carbide grades replaced by ceramics are not "high cubic" grades, but commonly they will contain some TaC and perhaps NbC, so this trend is a negative one for both materials.

Diamond coating/polycrystalline diamond (PCD). In recent years, high-performance diamond-coated metalcutting inserts have become commercially available from more and more manufacturers. In addition, conventional PCD tools continue to become more important. The prime applications are aluminum and other nonferrous alloys, wood products, and nonmetallics such as fiber-reinforced polymers. While these developments clearly reduce the number of metalcutting inserts consumed, they generally compete with grades containing little or no TaC or NbC. Thus, the development is only mildly negative toward Ta and Nb usage.

CBN. Tools made of metal- or ceramic-bond CBN also are growing in use, with the main application areas being hard turning (sometimes replacing grinding, and sometimes conventional turning plus heat treatment) and cast-iron machining. As with ceramics, the substituted material is cemented carbide, which will often contain TaC and maybe NbC. Thus, this trend is a negative one for Ta and Nb usage.

Use of cemented-carbide reclaim. This means the incorporation of powder into cemented-carbide compositions, reclaimed by a process that bypasses full chemical reprocessing, and retaining at least the carbide components (and in some cases the metallic binder) in the same ratio as in the input scrap. Companies differ in the extent to which they use reclaim, and the levels may differ between different grades.

In general, those grades that contain TaC and NbC are amenable to at least partial replacement of raw materials by reclaim. Thus, the trend reduces demand for virgin TaC, but to allow more flexibility in reclaim batch use while retaining a constant final composition, it may actually encourage the incorporation of NbC. The net effect is to reduce TaC use and perhaps increase NbC use.

Conservatism. This final factor is not a trend, but rather something which needs to be kept in mind in looking at the above factors. The cemented-carbide industry is generally slow to change compositions of grades, in order to avoid requalifying materials and applications. In addition, the machine tool population is only slowly being replaced.

It takes time for components to be redrawn; patterns, molds, and dies to be redesigned and remade; machine tools to be reprogrammed; and applications to be switched from one grade to another. In short, whatever trends are occurring, there are numerous mechanisms in place to slow them down.

In brief, the outlook for Nb looks relatively better than for Ta, while demand for either is not expected to match any growth in cemented-carbide use as a whole.

About the Author
Bernard North is director of materials and process development for Kennametal Inc., Latrobe, PA.