Vacuum Furnace Quenching Is Not Just About Putting It in The Furnace And Heating It And Then Cooling It Down.

It's not the expensive equipment that's the problem, but the poor workmanship-deformation, cracking, uneven hardness. A single batch of materials can cost hundreds of thousands of yuan, but a single misjudged detail can ruin everything. Especially when using vacuum furnaces for quenching, many owners assume that a clean environment and precise temperature are all that's needed. But once the parts emerge from the furnace, the surface appears shiny, but upon inspection, they reveal insufficient hardness, uneven microstructure, and even localized cracks... What's the problem? In fact, vacuum quenching may seem like a simple process: heating → holding → quenching, but each step has its pitfalls. A little carelessness can ruin all your efforts. 1. Loading method directly impacts quenching uniformity. Many people, taking the easy way out, simply pile up a basket of parts and shove them in. But have you ever considered: how is the airflow distributed? Uneven cooling means the temperature in the shadowed area drops slowly, and the core microstructure undergoes incomplete transformation, leading to a lack of hardness. Correct approach: Leave ample spacing between parts to avoid clumping; Position tall parts in the center and short parts outboard to ensure airflow around every corner; Perforate the tray appropriately to avoid blocking the cooling duct. This isn't a matter of preference; it's a process requirement. 2. Don't overly pursue a fast heating rate. In a vacuum, there's no convection; heat transfer occurs entirely through radiation. If the temperature rises too quickly, especially with thick-walled parts or high-alloy steel, the surface temperature reaches warm while the interior remains cool-this is known as "thermal stress," which can easily lead to cracking during quenching. Recommendation: For mold steels like Cr12MoV and H13, or for large workpieces, provide intermediate holding steps (e.g., 800°C for 30 minutes) to allow the internal and external temperatures to equalize. Preheat thoroughly and avoid skipping steps. Going slower is actually safer. 3. Quenching and cooling: the most critical and most prone to failure. Most vacuum furnaces use high-purity nitrogen or argon for pressurized quenching, with pressures generally ranging from 0.6–2.0 MPa. However, many factories only focus on the setpoint, not the actual performance. Common Problems: Gas purity is insufficient, oxygen or moisture content affects cooling speed and surface quality; Nozzle blockage or reduced fan efficiency leads to uneven cooling; Lack of step-by-step pressure control, resulting in high pressure from the start, can cause part deformation. Tips: Regularly check the gas dew point to ensure dryness; Check the condition of the fan and nozzle before quenching; For complex workpieces, a "staged pressurization" mode can be used: pre-cooling at low pressure before increasing the pressure to complete the phase transition. 4. Don't wait too long to remove the workpiece from the furnace. Once the furnace is stopped, many people don't rush to remove the parts, thinking that "a little longer will do the trick." However, vacuum furnaces are well insulated, and parts slowly cool down inside, potentially exceeding the optimal pre-tempering temperature, reducing the effectiveness of subsequent tempering. What is more serious is that some martensitic structures stay in the low temperature zone for too long and are prone to cracks.