Tungsten carbide (WC) is a synthetic compound recognized as one of the hardest and most durable materials manufactured for industrial applications. This composite, known as cemented carbide, combines the hardness of ceramic-like tungsten carbide with the toughness of a metallic binder, typically cobalt. Its exceptional resistance to wear and heat makes it the material of choice for high-performance products, including industrial cutting tools and mining drill bits. The production involves a precise, multi-stage process rooted in powder metallurgy, beginning with the chemical reduction of raw tungsten ore into a pure metal powder.
Preparing Tungsten Metal Powder
The foundation of tungsten carbide production is the creation of highly pure, fine-grained tungsten metal powder. The process begins with raw tungsten ore, which is chemically refined into an intermediate compound like ammonium paratungstate (APT). APT is then subjected to high temperatures to produce tungsten oxide, the direct precursor for the metal powder.
The critical step is the hydrogen reduction of the tungsten oxide powder, which removes oxygen atoms to yield pure metal. This is achieved by heating the oxide in a furnace under a controlled stream of hydrogen gas, typically ranging from 550°C to 850°C. The final particle size, usually under 5 micrometers, is carefully controlled at this stage because it directly influences the properties of the finished carbide product.
The Carburization Process
Once the pure tungsten metal powder is prepared, it is mixed with a carbon source, usually high-purity carbon black, to initiate the transformation into tungsten carbide (WC). The ratio of tungsten to carbon is precisely controlled, often including a slight excess of carbon to ensure complete conversion. This blend is thoroughly mixed, typically using ball mills, to ensure a homogeneous distribution of the two components.
The powder mixture is then subjected to high-temperature processing in a graphite furnace under a protective atmosphere, such as hydrogen or vacuum, to prevent oxidation. The carburization reaction occurs at extremely high temperatures, typically between 1,400°C and 1,700°C. During this heating cycle, carbon atoms diffuse into the tungsten crystal lattice, first forming ditungsten carbide (\(W_2C\)) and then fully converting to tungsten monocarbide (WC). This process results in a gray, hexagonal crystal powder that serves as the hard phase component of the final cemented carbide product.
Compaction and Final Sintering
The tungsten carbide powder is combined with a metallic binder, most commonly cobalt, which acts as the “cement” for the final structure. The cobalt content, usually ranging from 3% to 25% by weight, is varied based on the desired balance between hardness and toughness. Trace amounts of other carbides, such as titanium carbide (TiC) or tantalum carbide (TaC), may also be added to modify properties like heat resistance.
This mixture of WC and binder is then wet-milled, often using a liquid medium like ethanol, to achieve a uniform slurry and refine particle sizes. After milling, the slurry is dried, resulting in a ready-to-press powder containing intimately mixed WC and cobalt particles. This powder is subsequently pressed into a preliminary shape, known as a “green compact,” using high-pressure methods like cold isostatic pressing (CIP).
The final step is sintering, which transforms the fragile green compact into a dense, solid part with its ultimate mechanical properties. This is typically achieved through vacuum liquid-phase sintering, where the compact is heated to temperatures between 1,400°C and 1,600°C. At this temperature, the cobalt binder melts, forming a liquid phase that wets the solid tungsten carbide grains. The molten cobalt flows into the microscopic pores, filling the voids and bonding the structure. This action causes the compact to shrink significantly, achieving near-full density and cementing the hard carbide grains into a tough, metallic matrix.