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Knowing the Basics of Tungsten Carbide Grades, Including Their Characteristics As Cutting Tool Materials, Can Help Parts Manufac

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Knowing the Basics of Tungsten Carbide Grades, Including Their Characteristics As Cutting Tool Materials, Can Help Parts Manufac FEBRUARY 2008 / VOLUME 60 / NUMBER 2 ➤ BY JOHN L. JOHNSON, GARY R U N Y O N A N D CRAI G M O R T O N , ATI All D Y N E , A N All E G H E N Y T E C HNO L O G I E S C O M PANY Powder All images: ATI Alldyne All ATI images: POWER characteristics—especially the particle A smaller WC particle size can in- crushing. These “reclaimed” powders Scanning electron microscope images Knowing the basics of tungsten carbide grades, size—of the WC powder depend pri- crease hardness, but grain size must be often provide more predictable com- of a WC powder after carburization marily on the starting W particle size preserved during the sintering process. paction behavior because they have (left) and a graded WC powder after including their characteristics as cutting tool spray drying. materials, can help parts manufacturers choose and the carburization temperature and During sintering, the WC particles less surface area than WC powders time. Chemistry control is also critical. bond and grow through a solution- made directly from the carburization of organic binder’s chemical composition the right tool for an application. The carbon content must be held con- reprecipitation process. The metal- W. Recycling scrap material is an im- can affect the density and strength of stant near its stoichiometric value of lic binder becomes liquid during the portant part of the overall economics pressed parts. A high strength binder is emented carbides, also To utilize WC’s high hardness and im- 6.13 percent by weight. Small amounts actual sintering process (known as of the WC industry. Tungsten can be desirable for handling, but results in a known as hardmetals, prove its toughness, it is coupled with a of vanadium and/or chromium can be liquid-phase sintering) to provide a chemically extracted from practically lower pressed density and can produce are the most widely used metallic binder. This provides a material added prior to carburization to control material that is fully dense. The rate all waste streams containing it and hard agglomerates that can cause de- Cclass of materials for that is considerably harder than HSS the grain size through later processing of WC grain growth can be controlled reprocessed into WC powders with the fects in the final product. high-speed machining (HSM). They while being tough enough to withstand steps. Different downstream process- by additions of other transition metal same properties and quality as if they After milling, the powders are usu- are produced by powder metallurgy the forces encountered in most cutting ing conditions and different end-use carbides, including vanadium carbide were made from ore. ally spray dried to produce free-flow- and consist of hard carbide particles, applications. It can also withstand the applications require specific combina- ing agglomerates held together by the usually tungsten carbide (WC), in a high temperatures of HSM. tions of WC particle size, carbon con- Methods of recycling organic binder. The flowability and ductile metal binder. There are hun- Today, almost all WC-Co tools and tent, vanadium content and chromium packing density of the agglomerates dreds of different WC-based combi- inserts are coated, so the role of the content. Permutations of these combi- he tungsten industry has a long (ammonium paratungstate) process or can be tailored by adjusting the organic nations. The majority of them utilize substrate may seem less important. nations result in a high number of WC Thistory of recycling scrap material the zinc-reclaim process. More informa- binder’s composition. The agglomer- cobalt (Co) as the binder, but nickel However, it is the high elastic modu- powders. For example, ATI Alldyne from all types of waste streams. Recy- tion on the tungsten recycling process ates’ size distribution can be further (Ni) and chromium (Cr) are also used. lus—a measure of stiffness—of WC- produces 23 standard WC powders and cling scrap helps control material costs, is available in a special Interactive tailored by screening out coarse or fine Other alloying elements may be added Co at cutting temperatures (about three five times that on a custom basis. preserves natural resources and elimi- Report at www.ctemag.com, or visit the fractions to insure good flowability for as well. Why are there so many grades times HSS’s room-temperature elastic Many more combinations are pos- nates the need to dispose of waste ma- ATI Alldyne Web site at www.alldyne. filling die cavities. and how does a toolmaker select the modulus) that provides a nondeform- sible when the WC powders are milled terial in landfills. ATI Alldyne recycles com or call the ATI Alldyne scrap hot- right material for a given application? ing base for the coating. The substrate with a metal binder to produce a tungsten-carbide scrap using the APT line at (256) 722-2259. Part Production To answer these questions, let’s first also provides needed toughness. These “graded” powder. Co is most com- Carbide parts are fabricated by a review what makes cemented carbides properties are fundamental to WC-Co, monly used in contents of 3 to 25 per- (VC), chromium carbide (Cr3C2), tita- Conditions for milling the WC pow- number of methods. Due to their size, desirable as cutting tool materials. but can be tailored through composi- cent by weight, but Ni and Cr are used nium carbide (TiC), tantalum carbide der with the metal binder are also key level of shape complexity and manu- tional and microstructural adjustments in applications that require enhanced (TaC) and niobium carbide (NbC). processing parameters. Two common factured quantities, most cutting in- Hardness and Toughness during processing of the powder. Thus, corrosion resistance. The metal binder They are generally added when the milling techniques are ball milling serts are pressed in a rigid die with WC-Co has a unique combination of the suitability of a tool’s properties to can be further modified through ad- WC powder is milled with the metal and attritor milling. Both processes top and bottom punches. The same hardness and toughness. WC by itself a specific application depends in large ditional alloying. For example, adding binder, although VC and Cr3C2 can homogenize the mixture and can re- amount of powder in terms of both is very hard, surpassing corundum, or part on the starting powder. ruthenium to WC-Co significantly in- also be formed during the carburiza- duce particle size. An organic binder mass and volume must flow into the aluminum oxide, in hardness and loses creases its toughness without decreas- tion of the WC powder. is usually added during milling to later die cavity for each pressing to maintain little hardness at elevated working tem- Powder Production ing hardness. Increasing the binder Graded WC powders can also be provide sufficient strength in pressed uniform part weight and size. Powder peratures. However, it lacks sufficient WC powders are produced by car- content also increases toughness, but produced from recycled scrap mate- parts to hold their shape and allow flowability is mostly controlled by the toughness to function as a cutting tool. burizing tungsten (W) powders. The at the expense of hardness. rial via a zinc-reclaim process or by handling by operators or robots. The agglomerate size distribution and the organic binder’s properties. Compac- filled with insufficient powder, resulting in undersized parts tion pressures of 10 to 80 ksi are ap- that have to be scrapped. If the powder has a high packing plied to the powder to form the pressed, density, the bag may be filled with excess powder, requiring or “green,” part. more powder to be machined away after pressing. Although Even at these pressures, the hard this powder is recycled along with that from any scrapped WC particles do not deform or frac- parts, it reduces process efficiency. ture, but the organic binder is forced Carbide parts can also be produced by extrusion or in- into the interstices between the par- jection molding. Extrusion is better suited to high-volume ticles, helping to lock them into place. axisymmetric parts, while injection molding is used for Higher compaction pressures bring producing complex geometries in high volumes. In both the WC particles closer together, in- processes, the graded powder is suspended in an organic Green (left), sintered (middle) and coated (right) inserts show the dimensional creasing the part’s pressed, or green, change from green to sintered WC. binder, which gives the mixture the consistency of tooth- density. The pressing characteristics of paste. The mixture is then either extruded through an ori- graded WC powders can vary depend- high aspect ratios, such as rods for fluid. The pressed part is often ma- fice or injected into a die cavity. The characteristics of the ing on metal binder content, particle endmills and drills, are often manufac- chined to a specific geometry before graded powder determine the optimal ratio of powder to size and shape, degree of agglomera- tured by isostatically pressing graded being sintered. The bag is oversized binder in the mixture and greatly affect the mixture’s flow tion, and composition and amount of WC powder in a flexible bag. Produc- to accommodate part shrinkage during properties through the orifice or into the die cavity. organic binder. Plots of green density tion cycles are longer than for die com- consolidation and to provide adequate After the part has been shaped by die compaction, iso- vs. compaction pressure are often con- paction, but the tooling cost is lower, stock for grinding. pressing, extrusion or injection molding, the organic binder structed by the powder producer to making isopressing more suited for Because the parts are machined af- is removed from the part prior to the final sintering stage.
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