1.1.2 Requirements for Particle Size Distribution
Besides the average particle size, the width of the particle-size distribution is equally crucial. Hard alloy production typically requires the WC powder to have a particle-size distribution that is as narrow and uniform as possible.
 

• Uniformity: Uniform WC particles help achieve an even distribution within the mixture, resulting in a compact with consistent density after pressing. This ensures uniform shrinkage throughout the sintering process, preventing defects such as warping and cracking.
• Narrow particle size distribution: A narrow distribution means the powder contains very few excessively coarse or fine particles. Excessively large WC particles can become "abnormally grown grains" within the alloy microstructure after sintering—these grains act as structural defects inside the material, serving as crack initiation sites and significantly reducing the alloy's flexural strength and fatigue life. Meanwhile, although ultrafine powders exhibit high reactivity and are easy to sinter, their large specific surface area makes them prone to oxygen adsorption, complicating subsequent process control. Additionally, these ultrafine particles dissolve and precipitate rapidly in the sintering liquid phase, further contributing to abnormal grain growth. Therefore, controlling the WC powder to have a tightly concentrated particle size distribution is essential for achieving a uniformly structured hard alloy with consistently stable performance.

1.1.3 Control and Review
The requirement for WC particle size essentially reflects the effort to manage the core trade-off between "hardness and toughness" in cemented carbides. In actual production, there isn’t a single “optimal” particle size; instead, the most suitable WC grade must be selected based on the specific application scenario and performance priorities of the product. Furthermore, meticulous process control is essential to ensure uniformity and stability in the particle distribution. As for measurement methods, in addition to the traditional Fischer method, modern analytical techniques such as laser diffraction particle sizing, scanning electron microscopy (SEM) image analysis (following standards like ASTM E112’s linear intercept method), and electron backscatter diffraction (EBSD) technology now provide powerful tools for precisely characterizing and controlling WC particle size and its distribution.

1.2 Requirements for Total Carbon, Free Carbon, and Combined Carbon in WC for Cemented Carbides
Carbon content is the most sensitive and critical chemical indicator, not only for WC powder but also for the entire cemented carbide system. It directly determines the phase composition of the alloy after sintering, thereby exerting a decisive influence on its performance. When discussing the carbon content in WC, it is essential to distinguish among three concepts: total carbon, free carbon, and combined carbon.
 

• Combined carbon refers to the carbon that forms a chemical bond with tungsten, constituting the WC crystal lattice. Theoretically, the combined carbon content in stoichiometric WC is 6.13% (by mass).
• Free carbon refers to carbon existing in graphite form that has not combined with tungsten.
• Total carbon is the sum of combined carbon and free carbon.

1.2.1 Requirements for Combined Carbon
The production of cemented carbides places extremely strict demands on the combined carbon content of WC powder, typically requiring it to be as close as possible to the theoretical chemical stoichiometric ratio of 6.13%. This is because the performance of WC-Co alloys reaches its optimum only within a very narrow "carbon window" or "two-phase region."
 

• Carbon Deficiency: If the combined carbon content in WC falls below 6.13%, it indicates the presence of carbon vacancies. During sintering with cobalt, such WC materials, due to the overall low carbon potential of the system, may form one or more brittle ternary W-Co-C compounds—commonly known as the η phase. The chemical formula of the η phase typically takes the form of M₆C or M₁₂C, such as Co₃W₃C, Co₆W₆C, and other similar compounds. While the η phase exhibits exceptionally high hardness, it suffers from extremely poor toughness. Its presence can severely disrupt the continuity of the cohesive phase, much like ceramic inclusions, creating ideal sites for crack initiation. This, in turn, leads to a sharp decline in the alloy's flexural strength and fracture toughness. Therefore, in the production of cemented carbides, it is crucial to rigorously prevent the formation of the η phase.
• Carbon-Rich Conditions: If the combined carbon content in WC exceeds slightly above 6.13%, it suggests that interstitial carbon atoms may be present within the crystal lattice, or that minor amounts of more advanced carbides like W₂C could have formed. (Note that while W₂C is typically more stable under carbon-deficient conditions, it may still emerge during specific non-equilibrium processes.) More commonly, when the carbon potential of the entire alloy system is excessively high, excess carbon tends to precipitate out as graphite during the sintering and cooling stages. Graphite phases are notoriously soft and exhibit extremely low strength; moreover, their flaky or flocculent structures can mimic porosity within the alloy, leading to severe stress concentrations. This, in turn, significantly undermines the alloy’s strength, hardness, and wear resistance.

Therefore, the ideal WC raw material should have a precisely controlled carbon content to ensure that the final alloy microstructure consists of a simple two-phase structure composed of the WC hard phase and the γ-Co bonding phase.

1.2.2 Requirements for Free Carbon
Free carbon refers to graphite that is either unreacted during the WC powder production process or generated due to disproportionation reactions. Like carbon-rich conditions leading to graphite precipitation, free carbon poses similar risks to cemented carbides. Therefore, the content of free carbon in WC raw powder is strictly controlled—typically required to be below 0.05%, or even lower. Excessively high levels of free carbon not only directly affect alloy performance but also introduce significant uncertainties into the carbon balance control of the production process.

1.2.3 Control and Review of Total Carbon
In actual production, what is controlled is the total carbon content. Since WC powder inevitably undergoes slight oxidation on its surface during storage and transportation, forming oxides like WO₃, these oxides will consume carbon during the carbon reduction stage of sintering. Additionally, the binder cobalt powder itself may also contain trace amounts of oxides. To compensate for this carbon loss and ensure that the sintered alloy achieves an ideal carbon balance, the total carbon content of the WC powder used typically needs to be slightly higher than the theoretical value of 6.13%. This "carbon-increase amount," however, must be precisely calculated and adjusted based on subsequent processes—such as the addition of sintering aids and the carbon potential of the sintering atmosphere.

In summary, the requirement for WC carbon content is at the core of cemented carbide production technology. It demands precise chemical analysis of both total carbon and free carbon in the WC raw materials, coupled with a deep understanding of all factors influencing carbon balance throughout the entire production process—allowing for the establishment of an optimal carbon-content range. The accuracy of this control directly determines the quality and performance stability of the resulting cemented carbide products.

1.3 Requirements for WC Purity and Microstructural Defects in Cemented Carbides
In addition to particle size and carbon content, the chemical purity and physical integrity of WC powder are also crucial indicators that cannot be overlooked.

1.3.1 Requirements for Chemical Purity
Cemented carbides demand extremely high purity of WC raw materials, requiring strict control over the content of various harmful impurity elements.

• Alkali metals (Na, K): These elements form low-melting-point compounds at sintering temperatures, which can lead to the formation of localized abnormal liquid phases—making them a major factor promoting the abnormal growth of WC grains.
• Elements such as iron (Fe), silicon (Si), aluminum (Al), and calcium (Ca): These elements may exist in the form of oxides or silicates. During the sintering process, they may not be reduced, ultimately remaining in the alloy as hard, brittle inclusions—acting as weak points in the material and compromising its strength and toughness. In particular, research has shown that an increase in iron content can promote the formation of the η phase.
• Oxygen (O): As previously mentioned, oxygen content is directly linked to carbon balance during the sintering process. Excessively high oxygen levels mean that more carbon needs to be compensated for, increasing the difficulty and uncertainty of process control.
• Other harmful elements (such as sulfur S and phosphorus P): These elements can severely embrittle the bonding phase, so they must be strictly controlled at extremely low levels.

On the other hand, the trace addition of certain elements can be beneficial. For instance, the VC, Cr₃C₂, TaC, and NbC mentioned earlier—acting as grain growth inhibitors and solid-solution strengthening elements—are deliberately added as needed. A purity analysis of tungsten cast ingots revealed that they may contain elements such as titanium, tantalum, and niobium, which, when properly controlled during the subsequent preparation of WC, can be transformed into advantageous carbides.

1.3.2 Requirements for Organizational Defects
The ideal WC powder particles should be single crystals or dense polycrystalline aggregates with intact crystallinity and no internal defects.

• Pores: The pores within WC powder particles reduce their apparent density and can become sources of defects during alloy compaction and sintering.
• Microcracks: Microcracks that may be introduced during the preparation or mechanical processing of WC powder—such as ball milling—can reduce the inherent strength of the particles, thereby affecting the performance of the final alloy.
• Pseudoparticles: Loose agglomerates formed by numerous tiny particles held together only by weak forces are referred to as "pseudoparticles." These are prone to breaking apart during mixing and compaction, leading to uneven local composition and structure—precisely the kind of issue that must be rigorously avoided when producing high-quality cemented carbides.