Carbide, more precisely cemented carbide, is a superhard composite manufactured through powder metallurgy technology. It features outstanding hardness, wear resistance, and mechanical strength, making it an ideal material for cutting tools and wear parts across industries. During the manufacturing process , the key step that determines its properties and performance is sintering. So, what exactly is carbide sintering? Now, let's take a closer look at the carbide sintering process.

The Mechanism Behind Carbide Sintering

At its core, carbide sintering is a densification process. More specifically, the sintering of cemented carbide is dominated by liquid phase sintering. This mechanism is what turns compacted powder into a fully dense, high-performance material.

In a typical tungsten carbide sintering process, the green compact consists of hard carbide particles, such as WC, bonded together by a metallic binder, most commonly cobalt. As the temperature rises and approaches the eutectic point of the WC–Co system, the binder phase begins to melt. For WC–Co, this eutectic reaction occurs at around 1320 °C. Once the binder becomes liquid, a small amount of tungsten carbide dissolves into it, forming an alloy melt.

Due to the excellent wettability of this alloy melt, it is soon drawn into the gaps between powder particles by capillary forces. As the liquid penetrates the pores, particles rearrange more efficiently, and the overall porosity is rapidly reduced. This stage is accompanied by noticeable volume shrinkage and a sharp increase in density.

At the same time, a dissolution–precipitation mechanism takes place. Carbide particles experience grain reshaping and structural reconstruction, leading to a more stable microstructure. After cooling, the result is a fully dense composite material, where hard carbide grains form the load-bearing skeleton, and the metallic binder creates a continuous bonding network. This structure defines the practical meaning of sintered carbide.

Sintering temperature must be carefully controlled according to composition. Low cobalt contents generate less liquid phase and therefore require higher sintering temperatures to achieve full densification. In contrast, higher cobalt contents allow lower temperatures but must be managed to prevent excessive grain growth.

The Carbide Sintering Process Step by Step

Stage 1: Dewaxing

Dewaxing is the first step of the carbide sintering process. Its purpose is to completely and safely remove the additives, including paraffin wax or PEG.

This stage is carried out at relatively low temperatures, typically between 200 °C and 600 °C. Heating must be slow and controlled. If the additives vaporize too quickly, internal pressure can build up inside the green compact, leading to cracking or blistering.

Dewaxing is usually performed under a flowing protective atmosphere, such as hydrogen or hydrogen–argon mixtures, or under low vacuum. The continuous gas flow carries away the decomposed hydrocarbon vapors before they can crack into carbon, which would otherwise contaminate the carbide or clog furnace pipelines.

Stage 2: Pre-Sintering

After dewaxing, the compact is mechanically weak. Pre-sintering is used to provide sufficient handling strength and to complete essential chemical reactions before full densification.

During this stage, the temperature is raised from the dewaxing endpoint to approximately 800 °C–1000 °C. At these temperatures, limited solid diffusion occurs between powder particles, significantly increasing strength. The part can now be safely transferred or even undergo pre-sintering machining, such as rough grinding.

More importantly, pre-sintering is where oxide reduction takes place. Surface oxides on powders, such as CoO or WO₃, react with hydrogen or carbon in the furnace atmosphere and are reduced back to their metallic form. For example, cobalt oxide reacts with hydrogen to form metallic cobalt and water vapor.

A very low dew point is essential at this stage. The generated water vapor must be continuously removed by gas flow. If not, the reaction may reverse, leaving residual oxides. Any unreduced oxide will later decompose at higher temperatures and release gas, forming closed pores that cannot be eliminated during final sintering.

Stage 3: Vacuum Sintering

Vacuum sintering is the core step where the sintering of cemented carbide achieves near-complete densification.

The temperature is raised to the peak range of about 1350 °C–1500 °C. Once the temperature exceeds the WC–Co eutectic point of around 1320 °C, the cobalt binder melts and forms a liquid phase. A small amount of tungsten carbide dissolves into this liquid, enabling rapid mass transport.

Driven by capillary forces, the liquid binder penetrates pores and promotes particle rearrangement. The compact undergoes intense shrinkage, with linear shrinkage typically reaching 17%–20%. Density rises quickly to more than 95% of the theoretical value.

In the early part of this stage, a high vacuum level, often around 10⁻² mbar, is applied. This helps remove residual gases and further reduces any remaining oxides. As the temperature approaches its peak, pure argon is introduced into the furnace at a controlled partial pressure, usually between 1 and 50 mbar.

This argon atmosphere stops the cobalt and tungsten carbide from evaporating, both of which have measurable vapor pressure at high temperatures. It also improves temperature uniformity and helps prevent surface cobalt depletion or abnormal sintering behavior. The peak temperature is held long enough to allow the dissolution–precipitation process to fully homogenize the microstructure.

Stage 4: Final Densification (Optional but Critical)

After vacuum sintering, small isolated closed pores may remain. The final densification is used to eliminate these pores and realize the full theoretical density, greatly improving mechanical properties (especially transverse rupture strength).

Post-HIP (Hot Isostatic Pressing)

In post-HIP, cooled sintered parts are placed in a HIP furnace. They are treated at approximately 1000 °C–1400 °C under argon pressures ranging from 100 to over 1000 bar.

Under these conditions, residual pores collapse through solid-state plastic deformation.

Sinter-HIP

Sinter-HIP combines sintering and HIP into a single step. At the peak sintering temperature, high-pressure argon, typically 50–100 bar, is directly introduced into the furnace.

Because a liquid phase is already present, pores close much more easily than in post-HIP. As a result, lower pressure is sufficient. This method minimizes grain growth while achieving full densification and is particularly effective for fine-grain and low-cobalt grades.

From an industrial perspective, sinter-HIP is also more efficient. It eliminates the need for cooling and reheating cycles and has become the standard process for high-end tungsten carbide sintering.

After completing these stages, the result is a fully densified carbide blank with a uniform microstructure and stable geometry. Although final grinding is still required, the properties established during sintering ultimately determine the performance of the finished cutting tools or wear parts.