Cemented Carbide Balls Sintering Process

The sintering process for cemented carbide balls is a key step in producing high-performance cemented carbide balls by combining refractory metal tungsten carbide (WC) with binder metal powders such as cobalt (Co) through powder metallurgy. Its core lies in precisely controlling parameters such as sintering temperature, time, atmosphere, and pressure to achieve a dense bond between powder particles, forming an alloy structure with excellent mechanical properties.

I. Process Principle

Cemented carbide ball sintering is a typical liquid phase sintering process. At the sintering temperature, the binder metal (such as cobalt) melts to form a liquid phase, while the hard phases, such as tungsten carbide, remain solid. The flow of the liquid phase and capillary forces cause the solid particles to rearrange, achieving densification. Simultaneously, through a dissolution-reprecipitation mechanism, the tungsten carbide particles gradually grow and form a uniform microstructure. After cooling, the liquid phase solidifies, firmly binding the hard phase particles together to form a cemented carbide ball with high hardness, high strength, and excellent wear resistance.

II. Key Stages

The cemented carbide ball sintering process is generally divided into four stages:

1. Forming Agent Removal and Pre-Firing Stage: During this stage, the forming agent (such as wax or glue) gradually decomposes or vaporizes and exits the sintered compact. Simultaneously, oxides on the powder surface are reduced, and the bonding metal powder begins to recover and recrystallize. Surface diffusion begins, and the compact strength increases.

2. Solid-Phase Sintering Stage: At temperatures before the liquid phase appears, solid-phase reactions and diffusion intensify, enhancing plastic flow and causing significant shrinkage in the sintered compact.

3. Liquid-Phase Sintering Stage: When the sintered compact reaches the eutectic temperature, a liquid phase appears. The flow of the liquid phase and the rearrangement of the WC particles promote rapid densification of the compact. Subsequently, the WC particles dissolve and precipitate in the liquid phase, forming a uniform microstructure. 4. Cooling Stage: During the cooling process, the alloy's microstructure and phase composition undergo certain changes depending on the cooling conditions. By controlling the cooling rate, the desired microstructure and properties can be achieved.

III. Process Parameters

The sintering process parameters for cemented carbide balls have a decisive influence on the properties of the final product. Key parameters include:

1. Sintering Temperature: This temperature is typically above the melting point of the binder metal (e.g., cobalt has a melting point of 1495°C) but below the melting point of the hard phase. The sintering temperature should be adjusted based on the alloy composition and performance requirements.

2. Holding Time: The holding time directly affects the densification and grain size of the sintered body. Excessive holding times may result in coarse grains, reducing the alloy's strength and hardness.

3. Heating Rate: Excessively rapid heating rates can increase internal stress in the sintered body, leading to defects such as cracks. Therefore, an appropriate heating rate should be selected based on the size and shape of the sintered body.

IV. Atmosphere Control

The sintering atmosphere has a significant impact on the quality of cemented carbide balls. The main atmosphere types include:

1. Hydrogen atmosphere: Hydrogen has reducing properties and can reduce oxides on the powder surface, promoting the sintering process. However, the moisture that may be present in hydrogen can cause WC decarburization, affecting the alloy's performance.

2. Vacuum sintering: Vacuum sintering can improve the purity of the furnace atmosphere and reduce the impact of harmful impurities on alloy quality. Furthermore, the vacuum environment facilitates the purification of the solid surface and improves the wettability of the bonding phase with the hard phase.

3. Other atmospheres, such as decomposed ammonia and nitrogen-based atmospheres, can also be used in the sintering process of cemented carbide balls, but their selection depends on the specific alloy composition and performance requirements.

V. Special Processes

To improve the performance of cemented carbide balls, special processes can also be used, such as:

Hot Isostatic Pressing: Applying isostatic pressure during the sintering process can further eliminate residual voids in the sintered body, improving the density and strength of the alloy. Sintering-hot isostatic pressing composite process: Combining the advantages of sintering and hot isostatic pressing, conventional sintering is performed first, and then hot isostatic pressing is performed to obtain cemented carbide balls with better performance.

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