sign in sign up
<<
Home , Carbide, Cemented carbide, Hardness, Tungsten, Tungsten carbide

FEBRUARY 2008 / VOLUME 60 / NUMBER 2

➤ By John L. Johnson, Gary R u n y o n a n d C r a i g M o r t o n , at i a L L d y n e , a n a L L e g h e n y T e c h n o l o g i e s C o m pa n y 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 images Knowing the basics of grades, size—of the WC powder depend pri- crease , 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 process. paction behavior because they have (left) and a graded WC powder after including their characteristics as spray drying. , can help parts manufacturers choose and the carburization and During sintering, the WC particles less surface area than WC powders time. 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 content must be held con- reprecipitation process. The - W. Recycling scrap is an im- can affect the 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 , 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 , it is coupled with a of and/or can be liquid- 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 same properties and quality as if they After , the powders are usu- are produced by powder the encountered in most cutting ing conditions and different end-use carbides, including vanadium carbide were made from . 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 (WC), in a high 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 () 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- (Co) as the binder, but 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 —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 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 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), 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 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 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 , 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 , 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 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. provide quantitative information on the lower volume production. terward, uniform powder packing is Sintering removes pores from the part, making it fully or pressing characteristics of graded WC In this process, the powder is poured not as critical as for die compaction, nearly fully dense. During sintering, the metallic binder powders. This information ensures that into the bag, the end of the bag is but it is still desired to ensure that the in the pressed part becomes liquid, but a combination of the powder will be compatible with the sealed off, and the powder-filled bag bag is filled with the same amount of capillary forces and particle contacts maintains the shape toolmaker’s compaction process. is placed in a chamber, which is pres- powder each time. If the powder has of the part. Large carbide parts or those with surized to 30 to 60 ksi with a hydraulic a low packing density, the bag may be After sintering, the part has the same geometry, but is smaller. To produce a specific size part after sintering, a shrink factor is designed into the tooling. The graded pow- der used for each set of tooling must be designed to have the correct shrinkage when compacted at the appropriate pressure. In almost all cases, the sintered parts undergo post-sinter- ing operations. The minimum operation performed on a cut- ting tool is honing the cutting edge. Many tool geometries require grinding after sintering. For some tools, the top and bottom will be ground. Others require the periphery to be ground with or without honing the cutting edge. All tung- Robb-Jack Corp. sten-based grinding residue can be recycled. 800-527-8883 l www.robbjack.com Putting on a Coat In many cases, the finished part is coated. The coating provides lubricity and increased hardness. It also provides a to keep the substrate from oxidizing when exposed to high temperatures. The WC substrate is critical to coating performance. In addition to tailoring the bulk properties of the substrate powder, chemistry selection and enhanced sintering practices can tailor substrate surface properties. The outermost 20µm to 30µm of the insert can be enriched with binder relative to the rest of the part via cobalt migration to give it the performance of a tougher grade with the ability to resist characteristic of the binder content below the surface layer. Toolmakers that use graded WC powders may have specific requirements based on their processes, such as dewaxing methods, heating rates, sintering times, tempera- tures and carburization potential. Some toolmakers may sinter in a furnace while others use a sinterHIP furnace, which pressurizes parts at the end of the cycle to eliminate any remaining porosity. Those that sinter in a vacuum may hot isostatically press (HIP) their parts to full density in a separate cycle. Some toolmakers may use a higher vacuum sintering tem- perature to improve densification of compositions with lower Co contents, but this can coarsen the microstructure. To maintain a fine grain size, a powder ple, ATI Alldyne produces more than properties are preferred for specific Sintered microstructures of a straight grade (left), a with a smaller WC particle size can be 600 different graded powders. Each applications, providing a means of micrograin grade (center) and an alloyed grade (right), which selected. Dewax conditions and carbu- of these lots is engineered to work ef- categorizing multiple grades. includes cubic carbides. rization potential may require a higher fectively for the targeted customer and Two common application-oriented three basic types: straight, micrograin and alloyed. Straight or lower carbon content in the starting application. classification systems are the C-grade grades are primarily WC in a Co binder but may contain powder to match specific production system and the ISO system. Neither small amounts of grain growth inhibitors. Micrograin facilities. Grade Types system completely reflects the ma- grades consist of WC in a Co binder with several tenths of

All of these factors critically affect The different types of WC powders, terial properties that influence car- a percent of VC and/or Cr3C2 to achieve a grain size of less the microstructure and performance the composition and amount of the bide grade selection, but they provide than 1µm. Alloyed grades consist of WC in a Co binder with of the sintered carbide tool and re- metal binder and the type and amount a starting point for discussion. several percent of TiC, TaC and NbC, which are known as quire close communication between of grain growth inhibitor are respon- Many manufacturers have their own cubic carbides because they show up as a blocky third phase the toolmaker and powder supplier to sible for the variety of cemented car- specific grades for each classification, in sintered microstructures. ensure that the graded WC powder is bide grades. These parameters will which contributes to the large variety tailored for the toolmaker’s process. It critically determine the microstruc- of available grades. Straight: These grades for generally is no surprise that there are hundreds ture of the and its Grades are also classified by compo- contain 3 to 12 percent Co by weight. The WC grain size of different possible grades. For exam- properties. Certain combinations of sition. WC grades can be divided into usually ranges from 1µm to about 8µm. As with other grades, a smaller grain increases hardness and transverse rupture strength (TRS), but decreases toughness. Hard- nesses of straight grades typically range from 89 to 93.5 HRA and TRSs range from 175 to 350 ksi. The powders for these grades may contain significant amounts of reclaimed constituents. Straight grades can be classified as C1 to C4 in the C- grade system or under the K, N, S and H designations in the ISO system. Straight grades with intermediate properties can be classified as general-purpose grades, such as C2 or K20, and used for turning, milling, planing and boring. Harder grades with finer grain sizes or lower Co contents Schmitt Industries Inc. can be classified as precision machining grades, such as C4 l or K01. Tougher grades, with coarser grain sizes or higher 503-590-0182 www.schmitt-ind.com Co contents, can be classified as roughing grades, such as C1 or K30. Tools made of straight grades are used to machine cast , 200 and 300 series stainless , aluminum and other nonferrous , high-temperature alloys and hard- ened . They are also used for nonmetalworking appli- cations, such as rock and earth drilling tools. These grades have grain sizes ranging from 1.5µm to 10µm or more and contain 6 to 16 percent Co by weight. Another nonmet- alworking application for straight grades is for dies and punches. These grades generally have a medium grain size with Co contents ranging from 16 to 30 percent by weight.

Micrograin: These grades generally contain 6 to 15

percent Co by weight. The VC and/or Cr3C2 additions control grain growth during liquid-phase sintering, produc- ing a final grain size of less than 1µm. The fine grain size gives very high hardness and TRS of they can be highly polished and can chip surfaces. TaC and NbC additions Compositions, properties and typical applications for example grades. 500 ksi or more. High strength com- hold an extremely sharp cutting edge. up to 25 percent by weight improve Co TiC Ta(Nb)C VC Cr Hardness TRS Grain Example bined with adequate toughness enable Micrograin grades can also be used strength, cratering resistance and ther- (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (HRA) (ksi) Size* C Code ISO Code(s) Application the use of more positive rake angles, for machining Ni-base superalloys mal shock resistance. These cubic car- which reduce cutting forces and pro- because of their ability to withstand bide additions also increase the hot Straight grades duce thinner chips by cutting instead temperatures up to 1,200° C. For high- hardness, which helps avoid thermal Precision turning 5.5 - 0.7 - - 92.4 225 F C4 K01 of pushing metal. temperature alloys and other exotic deformation in heavy-duty or other ap- Proper performance is achieved by materials, micrograin and straight plications where high temperatures are K20, N15, 6 0.17 0.34 - - 92.0 260 F C2 Turning cast iron rigorous characterization of starting grades containing ruthenium can be created at the cutting edge. In addition, S10 raw materials in the production of the used to simultaneously improve wear TiC provides nucleation sites during Rough milling 11 0.4 0.8 - - 89.7 360 M C1 K30 graded powder along with critical con- resistance, deformation resistance and the sintering process, improving the cast iron trol of sintering conditions to prevent toughness. Micrograin grades are also uniform distribution of cubic carbides formation of abnormally large grains desired for rotating tools that gener- within the parts. Micrograin grades in the microstructure. To keep grain ate shear stresses, such as drills. One of alloyed grades typi- M10, N05, Drilling Ni-base 6 - - 0.2 0.3 92.8 310 XF - size small and consistent, reclaimed development is composite drills with cally range from 91 to 94 HRA and S05 superalloys powders can only be used with com- the Co content varying within the tool TRSs range from 150 to 300 ksi. M25, N15, 10 - - 0.3 0.5 91.9 360 XF C8 Turning Ti alloys plete control of the source material and to optimize hardness and toughness at Alloyed grades are less resistant to S15 reclaim process, as well as extensive specific locations within a single bit. wear and have lower strengths Alloyed grades quality checks. Alloyed: These grades are primar- than straight grades but are more resis- Finishing high- Micrograin grades can be classi- ily used for cutting steel and typically tant to adhesive wear. 5.5 3 4.5 - - 91.4 220 M C7 P15 fied under the M designation in the contain 5 to 10 percent Co by weight. The alloyed grades can be classified strength steels ISO system in addition to the same C- The grain size ranges from 0.8µm to as C5 to C8 in the C-grade system or Milling high- 10 6.5 11.4 - - 91.2 250 M C6 P30 grade and ISO designations as straight 2µm. TiC additions range from 4 to under the P and M designations in the strength steels grades. They are used to produce tools 25 percent by weight and reduce the ISO system. Alloyed grades with inter- Slotting high- 11 5.5 3.5 - - 90.5 220 M C5 P45 for cutting soft workpieces because tendency of WC to diffuse into steel mediate properties can be classified as strength steels * XF=extra fine, F=fine, M=medium

general-purpose grades, such as C6 or used for machining nonferrous met- Powder is the critical ingredient in P30, and used for turning, threading, als, such as Ni-base superalloys. They meeting both the technical and finan- planing and milling. The hardest grades generally have grain sizes of less than cial needs of toolmakers. Powders en- can be classified as precision-machin- 1µm and contain 8 to 12 percent Co by gineered to toolmakers’ process equip- ing grades, such as C8 or P01, and used weight. Higher hardness grades, such ment and process parameters ensure for finish turning and boring. These as M10, are used for turning ductile final part performance and result in formulations typically have fine grain cast , while tougher grades, such hundreds of grades. The recyclability sizes and low Co content to achieve the as M40, are selected for milling and of tungsten materials and the ability required hardness and wear resistance, planing steel or for turning stainless to work directly with powder provid- but larger additions of cubic carbides steels or superalloys. ers enables toolmakers to control their can also achieve similar properties. Alloyed grades are also used for quality and material costs. q The toughest grades can be classi- nonmetalworking applications, primar- fied as roughing grades, such as C5 or ily as wear parts. Typical grain sizes About the Authors P50. These formulations typically have range from 1.2µm to 2µm with Co John L. Johnson, Ph.D., is R&D direc- medium grain sizes, high Co content content ranging from 7 to 10 percent tor for ATI Alldyne, Huntsville, Ala. and smaller amounts of cubic carbide by weight. These grades are commonly Contact him at jjohnson@alldyne. to achieve required toughness by in- manufactured with a significant portion com. Gary Runyon is technical service hibiting crack propagation. Enhanced of reclaimed constituents, providing a specialist for ATI Alldyne. Contact performance in interrupted turning significant cost benefit for wear appli- him at [email protected]. Craig can also be obtained by using enriched cations. Wear grades for applications Morton, Ph.D., is R&D manager for grades, which have a higher Co content requiring increased corrosion resis- ATI Stellram, La Vergne, Tenn., where at the surface as previously described. tance and higher hardness are manufac- he is responsible for R&D activities

Alloyed grades with low TiC con- tured with additions of Ni and Cr3C2. for cutting tool materials and pro- tents are used for cutting cesses. Contact him at cmorton@stell and ductile cast irons, but can also be The Power of Powder ram.com. U.S. Union Tool, Inc. CUTTING TOOL ENGINEERING Magazine is protected under U.S. and international copyright laws. 714-521-6242 l [email protected] Before reproducing anything from this Web site, call the Copyright Clearance Center Inc. at (978) 750-8400.