High-temperature processing (>800°C, or 1470°F) of materials such as cemented carbides, powder metals, composites and ceramics can be carried out routinely with a new and innovative continuous sintering process that uses microwave energy. The continuous process has been developed commercially to sinter cemented tungsten carbide (WC). The key to uniformity and property improvement is the quick movement of the parts into, through and out of the microwave energy zone to ensure rapid heating, sintering and quenching of the parts to impart the desired properties.
Processing differences over conventional sintering techniques usually are beneficial and contribute to improved mechanical properties of the final product. Since the energy is directly transferred internally to the part, the process is orders of magnitude shorter than conventional processing and much more energy efficient.
Continuous microwave sintering of cermets, ceramics and metals is barely explored to date. However, the implications of the properties obtained using microwave sintering of tungsten carbide are both far-reaching and exciting. Some products such as tungsten-carbide substrates for polycrystalline diamond compact (PDC) have been fully commercialized, while feasibility for cutting tools, machining inserts, bearings and a wide variety of wear parts has been demonstrated. The potential for a new family of tungsten carbide and diamond and tungsten carbide and cubic boron nitride (CBN) concretions is also significant both for cutting and wear applications.
Principal developmental applications at the current time include cutting inserts for use in oilfield and mining drill bits and tools, a variety of nozzle applications, wire drawing dies, seals and wear rings, and cutting tools. Initial field-testing is underway in several of these applications. In addition, efforts are underway to further develop the technology into an integrated manufacturing process, and to scale up the microwave reactors to accommodate larger parts. Continuous microwave sintering furnaces have been designed and manufactured in three sizes, and are commercially available. The furnaces are adaptable for microwave sintering a variety of commercial products, and compared with sinter-hip furnaces, they are both more cost effective and more energy efficient.
Microwave sintering provides significant improvements in the properties of tungsten carbide, specifically improvements in erosion resistance, abrasion resistance, corrosion resistance, hardness, toughness and density. Further, the technology enables the manufacture of materials such as extremely fine grain (nanophase) carbide without grain growth inhibitors and the manufacture of diamond carbide or CBN-carbide composites not feasible using any other current commercial technology. The property improvements are primarily due to the short sintering times (5 minutes or less) compared with conventional sinter-hip processes (10 hours or more).
Dennis Tool has developed commercial microwave sintering furnaces since 1996, as well as several tungsten carbide products with enhanced properties due to the microwave sintering process. This worked stemmed from an agreement with the Penn State Research Foundation to further develop and commercialize this technology.
Conventional vs microwave heatingHigh temperature processing of materials such as powder carbides/metals, composites and ceramics is common throughout industry using heating methods based on convection and heat transfer to the objects from a furnace heater/burner or hot reactor wall. The efficiency depends on the thermal conductivity/diffusivity of the material and the design of the furnace itself. Slow processing is required to maintain a reasonably uniform temperature within all the objects in the furnace. Typical sinter-hip processes in the carbide industry take 16 hours or more for one batch, in furnaces of 3 ft3 (~.01 m3) or more in size. During this long sintering time, several factors work to degrade the properties of the material being sintered. For example, degradation in sintered tungsten carbide occurs from grain growth, diffusion and subsequent embrittlement at the grain boundaries, cobalt pooling and carbon loss.
In microwave processing, the energy is internally absorbed and efficiently converted to heat throughout the material. Thus, the object is heated instantaneously from the inside to the outside by the direct transmission of electromagnetic energy to the grains within the material. The objects become the heaters within the furnace system and loss of heat to its surroundings is the controlling factor for uniform heating of the objects. The energy is directly transferred into the object's center and absorbed at the powder grain boundaries, which then sinter the object to near theoretical density.
Since this method takes advantage of the direct heating of the grains in the pressed powders, the heating is instantaneous and uniform throughout the volume. This eliminates the thermal gradients and the accompanying cracking of the green bodies. However, the time to temperature is very quick and external thermal gradients due to cooler external surfaces must be controlled. A major advantage of this type of processing is the improved properties imparted to the materials since grain growth is minimized and the chemistry of the grain boundaries is only minimally altered.
Continuous microwave processingTwo basic designs of microwave furnaces are batch and continuous furnaces. As in the conventional heating methods, the batch microwave furnace has many objects inside and, therefore, a large intrinsic thermal mass that has to be heated and cooled uniformly to have a successful process. A microwave furnace design developed by Dennis Tool uses a patented concept of continuous movement of the objects through the microwave hot zone. Figure 1 shows the basic design and a fully operational unit in a commercial setting.
In continuous microwave sintering, the objects are continuously moved through the microwave hot zone with a fixed feeding rate. Because of the overall low thermal mass and small working area, relatively low operating microwave power (~ 1-4 kW) is needed. Once the furnace is operating at a steady pace, the throughput is a few minutes per crucible (5-10 minutes typically). The microwave field is identical for all parts. Consequently, all have the same quick and uniform processing. There are currently three sizes of these units. The smallest furnace (1 in. diam. by 2 in. high crucible) can produce thousands of 0.5 in. parts/day and the larger furnace (3 in. diam. by 2 in. high) can produce tens of thousands of parts. This throughput is on the order of the large sinter HIP furnaces used in the carbide industry today.
Property improvements due to microwave heatingThe microwave sintering process occurs so rapidly that time-dependent adverse effects are nonexistent. The properties that make tungsten carbide suitable for severe applications (such as good resistance to abrasion, fracture and erosion/corrosion; and high hardness and toughness) are the very properties that can be enhanced by using microwave sintering to process the tungsten carbide.
The improved properties will enhance the performance of tungsten carbide in a wide range of applications including PDC and PCBN substrates, sleeves and bushings, bearings, oilfield bit inserts, flow control trim parts, nozzles, mining bit inserts, mining picks, saw teeth, drills and reamers, mills, end mills, routers, countersinks, cutting tool inserts, highway picks, circuit board drills, wire drawing dies, wear resistant seals, dies and anvils. Figures 2 shows the improvements in abrasion resistance and corrosion resistance achieved with the continuous microwave sintering process.
Microwave sintering also allows the development of materials that are not possible using conventional sintering technology, such as nanophase carbides with little or no grain growth, and the development of Co-free or extremely low Co tungsten carbides. Nanophase carbide is very abrasion resistant and erosion resistant, making it an ideal candidate for many of the applications listed above. The ability to sinter without grain growth with no other additives will also yield a tougher part than is currently available, replacing newer boron carbide nozzle materials used, for example, in water jet cutting.
Microwave sintered carbide commercializationPDC substrates. A polycrystalline diamond compact (PDC, or also called PCD) is a composite material used widely in both earth drilling and industrial machining applications. The part is formed by growing the diamond layer from many crystals (polycrystalline) onto a substrate of microwave sintered WC at pressures and temperatures to 106 psi and 3630°F (~7 GPa and 2000°C). Abrasion and impact properties are dependent on both the diamond layer and the carbide substrate.
A typical PDC is shown in Fig. 3. Dennis Tool has fully commercialized the use of microwave sintered carbide for PDC substrates used in oilfield drill bits. The improvement in impact strength (Fig. 4) allows using the PDC in drilling higher compressive strength rock.
Carbide radial journal bearings. Journal bearings (Fig. 5) were manufactured using microwave-sintered carbide for both three-cone drill bits and turbines for downhole drilling of oil and gas wells. Another application under development is microwave sintered radial journal bearings for downhole drilling motors.
Based on initial wear tests, microwave sintered carbide outperformed other journal wear materials by a wide margin. While this test measured primarily abrasion resistance, a dramatic improvement in erosion resistance was also indicated in silicon carbide blast tests (up to 50 ¥). Further, the new material has improved impact properties over standard carbide grades. Corrosion resistance was also dramatically improved, which is significant in the presence of certain drilling fluids. Property improvements are primarily due to a finer grain structure, a more uniform cobalt distribution, limited chemical interaction between cobalt and carbide phases, and the absence of grain growth inhibitors. Though no pressure is used, near theoretical densities are achieved in the microwave sintering process.
Carbide inserts for rolling-cone oilfield drill bits. The active components of a rolling-cone bit for oilfield drilling are the cutting structure inserts, or teeth, and the gage retention structure, referred to as surf row or heal row inserts. Both are subject to abrasive wear and impact damage under the usually extreme drilling environment. The cutting inserts may be any of a variety of chisel, conical, or spherical shapes. Surf row inserts are usually cylindrical or hemispherical shaped.
The usual range of cobalt content in these inserts is between 6 and 11%, depending on the range of impact and abrasion properties required. Higher cobalt gives higher impact, while higher abrasion is achieved by reducing the cobalt in the tungsten carbide. As shown in Fig. 6, the microwave sintered carbide yields improvements in both properties simultaneously for any given cobalt content. This presents the opportunity for improvement in either abrasion or impact properties without sacrifice of the other. Further improvements in wear and impact resistance are possible by modifying the grain structure, for example, by introducing submicron grain sizes, which are difficult to sinter by conventional techniques. The microwave carbide insert may also be used as a substrate to produce PDC diamond layer inserts of a variety of sizes and shapes.
Water jet-cutting nozzles. In high-pressure water jet cutting, an abrasive such as garnet is impinged on workpiece in an extremely high-pressure, low-volume stream. The nozzles used in such applications require high levels of abrasion and erosion resistance, two properties enhanced by microwave sintering.
Currently, nanophase carbide with low binder content is used in the nozzles, which focus and direct the stream to the workpiece. Boride-carbide materials are also used. The nanophase carbide currently in use suffers from grain growth during sintering, reducing erosion resistance. The nozzles are very brittle and subject to breakage during setup and operation. Boride carbide offers improvement in abrasive life, but is also extremely brittle. Microwave-sintered nanophase carbide should have superior erosion resistance and strength to either of these materials. Microwave sintered carbide water jet nozzles have been developed and successfully tested. Further prototype development and testing is underway.
Other nozzle applications, such as industrial paint spray and blast media, also can benefit from improved properties of microwave-sintered carbide. In one application using alumina blast media, microwave-sintered carbide outperformed standard carbide material by more than 50 times (Fig. 7).
Circuit-board drills. The rods from which printed circuit board drill bits are made are approximately 0.17 in. diam. by 2 in. long (4 by 50 mm). Circuit-board drill rods currently are made of a submicron carbide particle size, which is an ideal candidate material for microwave sintering. The application is very demanding due to the highly abrasive nature of advanced fiberglass-reinforced resin circuit boards and the high production rates required. A 30% performance improvement in abrasion resistance along with improved strength is potentially achievable with microwave sintered carbide rods. Prototype microwave sintered rods have been produced with very good microstructures, and development is ongoing.
Carbide wire drawing dies. There are about ten wire drawing-die blank sizes used in the majority of wire drawing applications including brass-coated tire wire, aluminum and copper electrical wire, and ferrous and nonferrous wires. The main limitations of a wire drawing die life are abrasion resistance, burst strength and corrosion induced by fluids used to facilitate the wire drawing operation. These are all properties that can be improved through microwave sintering.
Diamond-WC compositesThe relatively short times used for microwave-sintering allows the manufacture of diamond or CBN composites in a WC matrix. A variety of sizes and shapes are possible, wherein the diamond grains are spread either entirely throughout the part, or maintained in localized regions, such as near the cutting or wear resistant surface. Diamond or CBN particles are entrained in the WC matrix in varied concentrations depending on the application. Such superabrasive composites are possible with microwave sintering because near-theoretical density can be achieved with full sintering at lower than sinter-HIP temperatures within a few minutes. This type of part cannot be manufactured using sinter-HIP because the diamond would convert back to carbon at the temperatures and times necessary for sinter-HIP.
Unlike advanced carbide grades, these composites would compete in the lower performance segment of the superabrasive market, and would sell for 30 to 40% of the price of high-temperature/high-pressure (HTHP) processed compacts such as PDC and PCBN. There is a large performance gap between high performing carbides and PDC and PCBN. In some applications such as turning or milling, a performance of 100 ¥ is typical, but a price difference between the PDC or PCBN and WC may be as much as 50 times.
he feasibility of manufacturing diamond-WC composites as abrasive wear elements has been demonstrated. Applications for superabrasive composites include wear and cutting inserts for oilfield drilling equipment and mining, as well as machining inserts for cutting tools, milling tools, turning and dressing tools, and other abrasive wear elements. These is a growing range of applications for this type of composite due to increased production requirements (e.g., higher speeds and material removal rates and tighter tolerances) and new difficult-to-machine materials including aluminum alloys, bimetallic materials, metal matrix composites, graphite epoxy, Kevlar, nodular iron and powder metallurgy parts.