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Answering the question of material hardness after vacuum brazing

When vacuum brazing 4340 steel with PDC (polycrystalline diamond composite), it is a technical challenge to ensure that the hardness of 4340 material does not decrease. 4340 steel is a high-strength alloy steel, and its hardness depends on vacuum quenching and vacuum tempering heat treatment. The high temperature during the brazing process may induce a tempering effect, resulting in a decrease in hardness. The following are detailed process recommendations to help maximize the hardness of 4340 steel during vacuum brazing:

1. Strictly control the welding temperature and time Brazing temperature:

The brazing temperature of PDC materials is usually between 630-690°C to avoid failure of the diamond layer at above 700°C. This temperature is lower than the critical austenitization temperature of 4340 steel (815-870°C) and will not cause phase change, but it is higher than its tempering temperature range (232-593°C), which will cause tempering effect and reduce hardness. Insulation time: The insulation time at high temperature should be shortened as much as possible (such as 5-10 minutes) to reduce the impact of the tempering effect. It is recommended to optimize the insulation time to the shortest possible time while ensuring sufficient wetting and flow of the brazing material.

2. Optimize the heating and cooling curves and use fast cooling:

Fast cooling after vacuum brazing can reduce the high temperature residence time and limit the continuous effect of the tempering effect. Some vacuum furnaces are equipped with gas quenching systems (such as nitrogen circulation cooling), which can achieve faster cooling rates and help retain the hardness of 4340 steel. Control the cooling rate: If the equipment cannot achieve fast cooling, avoid too slow furnace cooling and reduce hardness loss by adjusting the cooling rate. Fast cooling helps to maintain the martensitic structure as much as possible and reduce softening.

3. Reasonable selection of brazing filler metal and welding process Low melting point brazing filler metal:

Select a brazing filler metal with a low melting point and compatible with 4340 steel and PDC, such as a silver-based alloy (melting point of about 600-700°C), to reduce the vacuum brazing temperature and reduce the thermal impact on the hardness of the substrate. Advantages of vacuum environment: Vacuum brazing can effectively prevent oxidation and ensure the quality of the joint, but high temperature may still cause tempering of 4340 steel. Therefore, the heating rate and heat input must be strictly controlled in the process design to avoid unnecessary temperature increases.

4. Design preheating and post-heat treatment process Preheating:

Preheating 4340 steel before vacuum brazing (e.g. 200-300°C) can reduce temperature gradient and thermal stress and avoid the influence of local overheating on hardness. The preheating temperature should be lower than the tempering temperature to avoid premature softening. Post-heat treatment: After vacuum brazing, if conditions permit, 4340 steel can be partially vacuum tempered or re-heat treated to restore hardness. However, since PDC materials are sensitive to high temperatures (failure temperature is about 700°C), the post-heat treatment temperature is limited and the hardness may not be fully restored, and it may not be applicable to assembled parts.

5. Comprehensive recommendations for process optimization:

By using low melting point brazing filler metal, shortening the holding time, and adopting rapid cooling measures, strictly control heat input and minimize the hardness loss of 4340 steel. Pre-heat treatment: Before vacuum brazing, heat treat 4340 steel to a state slightly higher than the target hardness, leaving room for hardness reduction so that it can still meet the use requirements after brazing. Trade-offs and challenges: Since the vacuum brazing temperature must exceed the vacuum tempering temperature of 4340 steel, it is challenging to completely avoid hardness reduction. The relationship between hardness and joint performance needs to be balanced according to specific application requirements.

6. Verification and optimization Experimental verification:

It is recommended to conduct small batch tests in actual production, measure the hardness change of 4340 steel after vacuum brazing, record key parameters (such as temperature, holding time, cooling rate), and optimize the process accordingly.

Vacuum Brazing Furnace