PCD material vacuum brazing solution

Polycrystalline diamond compact (PDC) is made by sintering fine diamond powder, a cemented carbide substrate, and a small amount of additives under high temperature and high pressure. It combines the ultra-high hardness, wear resistance, and thermal conductivity of diamond with the strength, impact toughness, and excellent weldability of cemented carbide.

Vacuum Brazing PDC Bearings
PDC bearings offer the simplicity of a slurry-lubricated bearing system with the durability and strength of diamond. For drill tool manufacturers unsatisfied with the limited service life and load-bearing capacity of traditional bearings, PDC thrust bearings provide a high-performance, long-lasting solution.

Vacuum brazing of PDC is essentially brazing between the cemented carbide and a substrate, such as 4340 steel plate. Unlike conventional carbide brazing, the brazing temperature for PDC is limited by the thermal stability of the polycrystalline layer. At the same time, the brazing strength must be extremely high to withstand high cutting forces, impact loads, and fatigue strength.

Difficulties and Solutions for Vacuum Brazing

The key challenge in vacuum brazing is preventing the steel substrate from losing hardness due to the high brazing temperatures. The following are key points and detailed process recommendations during vacuum brazing:

1. Strictly Control the Brazing Temperature Range: The brazing temperature for PDC materials should be kept between 630–690°C. This is because temperatures above 700°C can cause failure of the PDC’s diamond layer (e.g., graphitization). This temperature is below the critical austenitization temperature of 4340 steel (815–870°C) and will not induce phase transformation. However, temperatures above its tempering temperature range (232–593°C) can induce a tempering effect, resulting in a decrease in hardness.
Solution: Use a vacuum furnace with high temperature control accuracy to avoid temperature fluctuations.

2. Shorten the Holding Time: The holding time at high temperatures should be minimized, typically optimized to 5–10 minutes. This is to minimize the impact of the tempering effect while ensuring adequate wetting and flow of the brazing filler metal. Solution: Determine the minimum holding time through experimentation while ensuring brazed joint quality.

3. Use a Rapid Cooling Process. Cooling Rate: Perform rapid cooling immediately after brazing (e.g., using a nitrogen circulation cooling system) to reduce high-temperature dwell time and limit the lingering effects of the tempering process.
Benefits: Rapid cooling helps preserve the martensitic structure of 4340 steel and mitigates softening. If the equipment cannot achieve rapid cooling, avoid excessively slow furnace cooling.

4. Choose Low-Temperature Vacuum-Grade Brazing Filler Material. Brazing Filler Type: Prefer low-melting-point brazing filler materials, such as silver-based alloys (melting point approximately 600–700°C), to lower the brazing temperature and minimize thermal impact on the steel substrate.
Compatibility: The brazing filler material must exhibit good wettability and bond strength with both PDC and steel. Brazing fillers containing active elements such as titanium and zirconium can improve wettability with non-metallic materials (such as PDC).
Purity Requirements: Use high-purity vacuum-grade brazing filler material to avoid contamination of the furnace atmosphere with volatile elements (e.g., Zn and Cd).

5. Optimize the Heating and Cooling Curves. Heating Rate: Control the heating rate to avoid localized overheating. The document recommends setting up a holding platform at 300–500°C to fully dissipate adsorbed gases from the workpiece and the furnace.
Cooling Control: Ensure the furnace temperature cools below 200°C before removing the workpiece from the furnace to prevent oxidation.

6. Design Preheating and Post-Heat Treatment: Preheat 4340 steel (e.g., 200–300°C) before vacuum brazing to reduce temperature gradients and thermal stresses, and avoid local overheating that can lead to hardness loss. The preheat temperature should be lower than the tempering temperature.
The key to vacuum brazing PDC steel lies in balancing heat input and material properties: Low-temperature brazing, short holding times, rapid cooling, and low-melting-point brazing filler metals are used to minimize hardness loss in 4340 steel. Since the brazing temperature inevitably exceeds the steel’s tempering temperature, completely preventing hardness loss is difficult.