CLASSES OF MATERIALS
1.04 Cemented Tungsten Carbide HardmetaldAn Introduction
1.04.2 Processing of Cemented WC
Cemented WC is a composite of WC and cobalt metal (Co). In most commercially available grades of WC–Co, cobalt content ranges from 6.0 to 16.0%. There are applications using as low as 3% cobalt or as high as 25%
cobalt, but the use of the more extreme compositions is not common. WC grain sizes range from submicron (0.2mm) to 10mm in the fully sintered solid state. The manufacturing processes of cemented WC from ore to powder and from powder to products are illustrated inFigures 1and2.
AsFigure 1shows, WC powders are made through the carburizing of metal tungsten powder, which is, itself, produced from either Wolframite [(FeMn)WO4] or Sheelite (CaWO4) tungsten ores via multiple steps of chemical extractive metallurgical processes. Key intermediate products are ammonium paratungstate (APT) and either tungsten blue oxide (WO2) or yellow oxide (WO3). Tungsten oxides are produced from APT by calci- nation. These tungsten oxides are subsequently reduced in hydrogen in order to produce tungsten metal powder (TMP), which is then carburized in mixtures with carbon at high temperatures under hydrogen atmosphere to form WC. The calcination of APT and the reduction of tungsten oxides are critical steps of the overall process because the size of tungsten oxide particles has a strong effect on the size of resulting metal tungsten powders, and, therefore, the size of subsequent WC powders. Particle sizes of WC produced via the processes described above can range from 0.1 to tens of microns.
The processes transforming WC powder to tools and other products are typical for powder metallurgy processes. They involve milling of blended WC and metal binder powders, such as Co, shaping through various powder compaction techniques, and liquid phase sintering in vacuum with or without a final step of low pressure hot isostatic pressing (sinter-HIP). The processes from powder to products are illustrated inFigure 2.
Milling serves several purposes, including the mixing of WC and binder powders with a desired composition, reducing WC agglomeration and WC particle size, and to get a uniform distribution of binder powders among WC particles. Typical milling equipment includes rolling ball mills and attritor mills, and each method has particular milling characteristics. Rolling ball mills are effective in reducing the size of large particles (>2mm) to fine particles, and can produce narrow particle size distributions, which have a lower tendency toward grain growth during sintering, thus leading to a uniform sintered microstructure. When the particle size is<2mm, ball mills are less effective at further reducing the particle size.
Attritor mills are more aggressive than rolling ball mills, and are approximately 11 times more effective than ball mills with respect to particle size reduction. Fine particles (<2mm) can also be milled; however, attritor mills have a tendency to produce very fine particles and sometimes wide and/or bimodal particle size
Figure 1 General manufacturing process for tungsten carbide powder.
distributions. Such particle distributions are prone to grain growth during sintering and result in a less uniform sintered microstructure. Both methods are currently used in the industry, depending on the required micro- structure of thefinal sintered product.
After milling, the powders are dried and granulated for compaction. The consolidation of cemented carbide powders into different shapes can be achieved by die pressing, cold isostatic pressing, extrusion, or powder injection molding. Die pressing is still the most widely used and economical shaping method in the cemented carbide industry. Extrusion of cemented carbide powders mixed with a plasticizing binder is typical for the production of rods, strips, etc., especially for round rotary tools such as drilling and milling tools produced in large quantities. Cold isostatic pressing is mainly used to produce large blanks in smaller numbers. The large blanks usually need to be further machined in either the green state or presintered state in order to obtain parts close to theirfinal sintered size and shape. Powder injection molding is used to fabricate very small parts with complex geometries that cannot be readily machined.
All cemented carbides are generally sintered in the liquid phase state under vacuum conditions. Sintering is the critical step for the determination of thefinal properties of cemented carbide. One of the key innovations of the industry in the past two decades is the sinter-HIP process. Sinter-HIP is now a mature process that has been replacing the standard vacuum sintering method gradually since the late 1980s to the early 1990s. The pressure of the sinter-HIPing step ranges from 3 to 10 MPa, and the profound benefit of sinter-HIP over standard vacuum sintering is the elimination of residual porosity. This has resulted in a significant increase in the transverse rapture strength of the material, as well as improving the consistency of the strength of the material.
Figure 3shows a typical sinter-HIP furnace. The typical capacity of production furnaces ranges from 300 to 500 kg per charge. However, furnaces up to 1000 kg per charge are also available. One of the key issues for Figure 2 Typical powder metallurgy processes of producing cemented carbides.
Figure 3 A typical tungsten carbide sintering HIP furnace.
Cemented Tungsten Carbide HardmetaldAn Introduction 125
sintering cemented WC is carbon content control. The carbon content has a strong effect on mechanical properties.Figure 4shows a phase diagram of the WC–Co system with 10 wt% Co, but varying carbon content.
When the carbon content is too low, the material will form the eta phase,h, which can be complex carbides of W–Co–C. Typical formulas of the eta phase include Co3.2W2.8C and Co2W4C. The eta phase is characterized as brittle, and consequently, detrimental to mechanical properties. It is generally avoided in the final micro- structure of WC–Co. When the carbon content is too high, uncombined carbon forms a free graphite phase in the microstructure. Particles of uncombined graphite phase are also considered undesirable from the standpoint of mechanical strength, although the graphite phase is not perceived as being as detrimental as the eta phase in WC–Co microstructures.
Even when the carbon content is within the two-phasefield of WC and Co, it still has a significant effect on mechanical properties. When the carbon content is lower than the stoichiometric content of 5.52 wt% C for WC–10 wt% Co, and higher than the level that would induce the eta phase, the density and hardness of cemented carbides increase when the carbon level decreases. In this region of the phase diagram, a lower carbon level induces more W solution in the Co phase, and not only increases the density of cobalt phase but also slows down the solution–reprecipitation process during sintering and, thus, results in a smaller grain size and higher hardness in the sintered parts.
Many processing variables affect the carbon content in the final sintered material. Powder milling and sintering are the two critical processes that determine thefinal carbon content in the sintered parts. During powder milling, the carbon content is primarily dependent on raw WC materials, and can be adjusted by either graphite addition or TMP addition to increase or decrease the carbon balance in the sintered parts. During sintering, the dewax step and tungsten/cobalt oxide reduction process can also affect the carbon content in the sintered parts. For example, improper dewax may leave free carbon in the sintered parts, and excessive oxide reductions can cause an issue with carbon deficiency. Other factors, such as furnace load volume, plate tray coating, and vacuum leaks can also have distinct influences on the carbon level in the sintered parts.
Conventional sintering of cemented carbide is a long thermal cycle process (18–24 h including heating and cooling), which favors undesirable WC grain growth. In recent years, in order to control grain growth during sintering and shorten the sintering cycle, several new sintering techniques have emerged. One of these techniques is microwave sintering. Microwave heating requires little time to obtain nearly full densification, and, therefore, grain growth is relatively suppressed and a fine microstructure is generally obtained. Some reports have shown that cemented carbide parts produced by microwave sintering can exhibit an improvement in abrasion resistance, erosion resistance, and corrosion resistance without any noticeable loss in hardness or fracture toughness. These improvements in the properties are believed to be due to the fine microstructure, uniform cobalt phase distribution, and pure cobalt phase in microwave-sintered samples. Microwave sintering of cemented carbide is now under commercial exploitation for specialty carbide products.
The quality of sintered products is generally characterized and monitored by measuring density, porosity, magnetic saturation, coercive force (Hc), and hardness of the sintered products. Other inspection and charac- terization techniques, such as cobalt pools, grain size distribution, and maximum grain size may also be required to evaluate some sintered products. The measurements of these properties should conform to American Society for Testing and Materials (ASTM) standards. Different specifications are defined with respect Figure 4 Phase diagram of the WC–10 wt% Co system with varying carbon content. FromMahale (1994).
to the target and/or tolerance of the above-mentioned properties for the different sintered products, and the quality of the sintered products is basically controlled per these specifications.