3. The impact of WC carbon content variations on the composition, grain size, and properties of cemented carbide phases

In cemented carbide production, controlling carbon content is considered of unparalleled importance. Even minor fluctuations in carbon levels—down to as little as 0.01%—throughout the alloy system can lead to significant variations in phase composition, microstructure, and ultimately, material performance. This heightened sensitivity stems from carbon's critical role in the ternary W-C-Co system during high-temperature sintering.

3.1 The effect of WC carbon content variations in the alloy on the composition of the cemented carbide phase

To understand the influence of carbon content, one must first refer to the W-C-Co ternary phase diagram, which serves as the theoretical foundation for analyzing and predicting the phase equilibrium relationships of cemented carbides under varying temperatures and compositions.

3.1.1 W-C-Co Phase Diagram and Alloy Phase Composition

The W-C-Co ternary phase diagram is quite complex, but for typical cemented carbide compositions—where Co content usually ranges from 3% to 30%—we primarily focus on its isothermal cross-section diagrams, especially the phase regions near the sintering temperature (~1400°C). (1200℃, 1425℃).

On a typical W-C-Co isothermal section, there are three key phase regions:

(The WC+γ) Two-phase region: This is the target phase zone for producing high-quality cemented carbides. Within this region, the alloy consists of the hard phase WC and either a liquid phase (at sintering temperature) or the solidified γ-Co solid-solution binder phase. This area corresponds to a specific carbon content range—commonly referred to as the "carbon window" or "safe carbon zone." Alloys within this range exhibit the best overall mechanical properties.

(The WC + γ + C) Three-phase region: Located on the carbon-rich side of the two-phase region. When the total carbon content of the alloy exceeds the solubility limit of W and Co at the sintering temperature, excess carbon will precipitate out in the form of free graphite. The presence of this graphite phase can severely degrade the alloy's performance.

(WC + γ + η) Three-phase region: Located on the carbon-deficient side of the two-phase region. When the alloy's total carbon content is insufficient, the system will undergo a reaction to achieve a new chemical equilibrium, resulting in part of the WC and Co transforming into a ternary composite carbide of W-Co-C—known as the η phase. The formation of the η phase, however, is also something that must be strictly avoided during production.

Therefore, the primary role of carbon content is to determine the final phase composition of the alloy. Precisely controlling the carbon content of the alloy within the narrow (WC + γ) two-phase region is a prerequisite for achieving high-performance cemented carbides.

3.1.2 The allowable fluctuation range of carbon content in WC-Co alloys—the width of the "carbon window," or the permitted range of carbon content variations—is not fixed; rather, it is influenced by the alloy composition and the sintering process.

The influence of Co content: The higher the Co content, the wider the carbon window typically becomes. This is because Co acts as a solvent, dissolving a certain amount of W and C, thereby providing some buffering against fluctuations in carbon content. High-Co alloys exhibit relatively lower sensitivity to variations in carbon levels.

The influence of additives: Adding other carbides (such as TiC, TaC, and NbC) can make the phase diagram more complex and typically narrow the carbon window. These carbides interact intricately with W, Co, and C, making it even more challenging to control carbon equilibrium.

The influence of temperature: The higher the sintering temperature, the greater the solubility of each element in the liquid phase, causing the boundaries of the phase regions to shift—and typically resulting in a slight variation in the carbon window.

In actual production, this allowable fluctuation range can be extremely narrow—sometimes as little as ±0.05 wt%. This means that every step—from the tungsten carbide raw materials and cobalt powder to recycled materials, as well as subsequent processes like ball milling, pressing, and sintering atmospheres—requires precise calculation and control of carbon input and loss.

3.1.3 Types of the η Phase

The η phase is not a single compound, but rather a general term encompassing a class of complex W-Co-C ternary carbides with diverse compositions and structures. Depending on the atomic ratios of W, Co, and C, as well as their crystal structures, the η phase is primarily divided into two major categories:

M₆C type (fcc structure): In this type of η-phase, the ratio of metal atoms (where M represents W and Co) to carbon atoms is 6:1, and it adopts a face-centered cubic (fcc) structure. Its chemical formula can be written as (W, Co)₆C, with common examples including Co₃W₃C or Co₂W₄C. This η-phase typically forms under conditions where carbon depletion is relatively mild.

Type M₁₂C (hexagonal structure): In this type of η phase, the ratio of metal atoms to carbon atoms is 12:1, resulting in a more complex crystal structure. Its chemical formula can be written as (W, Co)₁₂C, such as Co₆W₆C. This type of η phase typically appears under conditions where carbon depletion is more severe.

Additionally, literature has also mentioned complex carbides containing other components, such as Co₃W₉C₄. The formation of the η phase not only consumes WC and Co but, more importantly, it appears in the alloy as a hard, brittle phase—typically in blocky or dendritic forms—that disrupts the continuity of the bonding phase. This, in turn, becomes a major source of internal defects, directly contributing to the dramatic reduction in alloy toughness. Therefore, optimizing carbon content to prevent the formation of the brittle η phase is crucial for maintaining the integrity of the microstructure.

3.2 The Influence of WC Carbon Content Changes on the Cobalt Phase Composition and Crystal Structure of Cemented Carbides

The binding phase (commonly referred to as the γ phase) is not pure cobalt but rather a solid solution of W and C dissolved in Co. Variations in the alloy's carbon content directly influence the solubility of W and C in Co, thereby altering the composition, structure, and properties of the binding phase.

3.2.1 Co Phase (γ Phase) Composition

At the sintering temperature, liquid cobalt dissolves a significant amount of W and WC. The amounts of W and C dissolved in the cobalt phase are strictly limited by the total carbon content of the alloy, following the equilibrium relationships depicted on the phase diagram.

The tungsten (W) content in the bonding phase is critical to its performance. Dissolved W atoms induce lattice distortion in the cobalt matrix, leading to solid-solution strengthening and thereby enhancing the strength and hardness of the bonding phase itself. However, excessively high W levels can make the bonding phase brittle, reducing its toughness. Therefore, controlling the carbon content to regulate the amount of W dissolved in the bonding phase is an essential approach for optimizing the alloy's overall performance.

Via: "My Cemented Carbide Journey" – Xiao Zhou