In the heat treatment process of vacuum air quenching furnaces, the cooling stage is a crucial step that determines the performance of the workpiece. Its technical level directly affects the quenching hardness, uniformity of the structure and the amount of deformation. Taicang Huarui Vacuum Furnace Industry Co., Ltd. has been deeply engaged in vacuum quenching technology for many years. Through in-depth research on the characteristics of cooling media, the laws of air flow circulation and control strategies, it has formed a set of efficient and stable cooling technology system, ensuring that workpieces of different materials and shapes can all achieve ideal quenching effects.
The basic principle of vacuum air quenching
The cooling process of vacuum quenching uses inert gas as the heat transfer medium. Under high pressure, the heat on the surface of the workpiece is rapidly removed through forced convection, achieving rapid cooling from the austenitizing temperature to the martensitic transformation temperature. Compared with traditional oil quenching, its core advantages lie in the controllability of cooling rate and the non-contamination of the workpiece surface. The heat transfer efficiency of the cooling process depends on the thermophysical properties and flow state of the gas. The thermal conductivity, density and specific heat capacity of gases directly affect their heat transfer capacity. For instance, the thermal conductivity of helium is six times that of nitrogen, and under the same conditions, its cooling rate is significantly faster. Argon has a higher density and can form a denser gas flow layer under high pressure, increasing the contact area with the surface of the workpiece. When gas scour the workpiece at high speed (typically 20-50m/s), it breaks the static gas film on the workpiece surface, creating intense convective heat transfer and increasing the heat transfer efficiency by 3 to 5 times. The vacuum environment plays a dual role in this process: on the one hand, it eliminates oxygen and moisture from the air, preventing the workpiece from oxidizing during the high-temperature cooling stage; On the other hand, in a vacuum state, the free path of gas molecules increases. After high-pressure inflation, the collision frequency of gas molecules increases, further enhancing heat exchange. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. precisely controls the matching of vacuum degree and charging pressure, forming a flow state of gas in the furnace chamber, laying the foundation for efficient cooling.
The core components of the cooling system
The cooling system of the vacuum air quenching furnace is an organic whole, consisting of the coordinated operation of the gas source supply, pressurization device, gas flow circulation, flow diversion structure and exhaust gas recovery, etc. The design of each link directly affects the uniformity and stability of the cooling effect. The gas supply system is responsible for providing high-purity inert gas, which is a prerequisite for ensuring the quality of cooling. Common gases include nitrogen, argon and helium. Huarui vacuum quenching furnaces can select single gas or mixed gas according to the material characteristics of the workpiece. For instance, when handling high-speed steel, a mixture of nitrogen and helium is selected, which can not only ensure the cooling rate but also control the cost. When processing titanium alloys, argon gas is selected to avoid embrittlement caused by nitrogen. The gas source system is equipped with multi-stage filtration devices to ensure that the gas purity reaches over 99.99%, and the oil and moisture content is controlled below 5ppm to prevent contamination of the workpiece surface. The pressurization and circulation device is the "power core" of the cooling system. The Huarui vacuum quenching furnace adopts a high-flow centrifugal fan. Its impeller is optimized by fluid mechanics design and can provide a stable gas flow within the pressure range of 0.1-0.6MPa. The motor power of the fan is matched according to the volume of the furnace. For example, a 37kW motor is equipped for a 1.5m³ furnace to ensure that the gas flow rate reaches above 30m/s. Meanwhile, the system adopts variable frequency speed regulation technology, which can adjust the fan speed according to the different requirements of the cooling stage, achieving stepless regulation of the cooling intensity. The flow-directing structure determines the uniformity of gas distribution in the furnace chamber. The inner wall of the furnace chamber of Huarui vacuum air quenching furnace is equipped with multiple sets of deflector plates. These deflector plates adopt a porous honeycomb structure, which can disperse the airflow into countless fine jets and evenly cover all parts of the workpiece. For complex-shaped workpieces, such as molds with deep holes and grooves, dedicated guiding nozzles can also be equipped to enable the gas to directionally scour the hard-to-cool areas. When a certain automotive mold enterprise used the Huarui vacuum quenching furnace to treat molds with complex cavities, by optimizing the Angle of the deflector plate, the cooling rate difference between the inside and outside of the mold cavity was controlled within 5℃/s, thus avoiding cracking caused by uneven cooling. The exhaust and recovery system reflects the energy-saving nature of the equipment. The exhaust process of Huarui vacuum quenching furnace adopts a staged pressure reduction method to avoid noise and energy waste caused by the rapid discharge of high-pressure gas. For precious helium gas, the system is equipped with a dedicated recovery device. Through processes such as cooling, drying and compression, the gas recovery rate can reach over 85%, significantly reducing operating costs.
The key factors affecting the cooling effect
The quality of the cooling effect is determined by multiple factors. The Huarui vacuum quenching furnace precisely regulates these parameters to achieve accurate control of the cooling rate, meeting the quenching requirements of different materials. Gas pressure is the primary factor affecting the cooling rate. Within a certain range, the higher the gas pressure, the greater the molecular density and the higher the heat exchange efficiency. The pressure regulation range of Huarui true air quenching furnace is 0.1-0.6MPa. Through experimental data, the corresponding relationship between pressure and cooling rate was established: for example, when processing 45# steel, the cooling rate of 0.2MPa nitrogen is 20℃/s, and it can be increased to 45℃/s at 0.5MPa, which can meet the hardenability requirements of workpieces of different thicknesses. The pressure control system adopts a closed-loop feedback design, with pressure fluctuations controlled within ±0.02MPa to ensure the consistency of cooling conditions. The ratio of gas flow rate to the surface area of the workpiece is equally crucial. Too low a flow rate will cause a static air film to form on the surface of the workpiece, hindering heat transfer. If the flow rate is too high, the workpiece may deform due to the impact of the airflow. The Huarui vacuum quenching furnace automatically matches the flow rate by calculating the specific surface area (the ratio of surface area to volume) of the workpiece. For instance, when dealing with thin spring plates, a high-speed airflow of 25m/s is adopted; When dealing with heavy rolls, the flow rate should be controlled at 15m/s to ensure the cooling speed while reducing the risk of deformation. The way the workpiece is loaded into the furnace has a significant impact on the uniformity of cooling. For Huarui vacuum quenching furnaces, it is recommended to adopt the "suspended loading" or "layered interval" method to avoid gas flow blockage caused by workpiece stacking. For long shaft workpieces, a vertical bracket is used to suspend them vertically, allowing the gas to flow uniformly along the axial direction. For flat workpieces, use shelves with gaps to ensure that both the upper and lower surfaces can be fully washed by the airflow. A certain bearing enterprise has improved the cooling uniformity of bearing rollers by 40% and reduced the hardness deviation from ±2HRC to ±0.5HRC by optimizing the furnace loading method. Gas temperature is also a factor that cannot be ignored. The intake system of Huarui vacuum air quenching furnace is equipped with a pre-cooling device, which can control the gas temperature within the range of 20-40℃. When the ambient temperature is high in summer, the gas temperature is reduced by the chiller to avoid the decline in cooling capacity caused by excessively high intake gas temperature. In winter, the intake air temperature should be appropriately increased to reduce the temperature difference stress inside and outside the workpiece. This refined control ensures that the cooling effect remains stable in different seasons.
The regulation strategy of cooling rate
The requirements for cooling rate vary significantly among different materials. The Huarui vacuum quenching furnace achieves precise matching of cooling rate through flexible control strategies, ensuring that the workpiece acquires the desired structure while reducing internal stress and deformation. Segmented pressure regulation is a commonly used strategy. According to the continuous cooling transformation curve (CCT curve) of the material, high-pressure rapid cooling is adopted in the pearlite transformation zone, and the pressure is reduced and slow cooling is carried out in the martensite transformation zone. For instance, when handling 40CrNiMo steel, maintain the pressure at 0.6MPa in the 650-500℃ range (the sensitive zone for pearlite transformation) and quickly pass through the dangerous zone. Reduce the pressure to 0.2MPa in the range of 300-200℃ (martensitic transformation zone) to decrease the microstructure stress. The control system of Huarui vacuum air quenching furnace is equipped with a CCT curve database of over 100 kinds of materials, which can automatically generate segmented pressure regulation schemes. Operators only need to confirm before execution. The combined use of gas types provides more possibilities for regulating the cooling rate. The Huarui vacuum air quenching furnace supports online switching and mixing of multiple gases, and the cooling capacity can be changed by adjusting the gas ratio. For instance, when nitrogen and helium are mixed in a ratio of 7:3, the cooling rate is 50% higher than that of pure nitrogen, approaching the cooling effect of argon, but the cost is reduced by 30%. A certain tool factory adopted this mixed gas to treat high-speed steel cutting tools, which not only achieved the required cooling rate of 60℃/s but also saved a significant amount of gas costs compared to using pure helium. The dynamic adjustment of the airflow direction can further optimize the cooling effect. Traditional vacuum air quenching furnaces mostly adopt unidirectional airflow, while Huarui vacuum air quenching furnaces innovatively design a bidirectional circulation system, which can switch the airflow direction during the cooling process. For instance, when handling shaft workpieces with flanges, air should be introduced from the left end for one minute first, and then from the right end for another minute. This ensures that the cooling rates on both sides of the flange are consistent, preventing bending deformation caused by overly rapid local cooling. This dynamic adjustment technology has increased the qualification rate of complex workpieces to over 98%.
Intelligent cooling control technology
The intelligent control technology of Huarui vacuum quenching furnace transforms the cooling process from experience-driven to data-driven. Through real-time monitoring and adaptive adjustment, it ensures the stability and repeatability of the cooling effect. The temperature monitoring system is the foundation of intelligent control. Multiple sets of thermocouples are arranged in the furnace chamber to monitor the surface, center and gas temperatures of the workpieces respectively, with a sampling frequency of 10 times per second. These data are transmitted in real time to the control system, forming a three-dimensional temperature field distribution map, allowing operators to intuitively grasp the temperature changes during the cooling process. When the temperature in a certain area drops too slowly, the system automatically issues a warning and prompts to adjust the Angle of the deflector plate in the corresponding area. Adaptive control algorithms are at the core of intelligent cooling. The system automatically adjusts parameters such as gas pressure and flow rate according to the real-time temperature changes of the workpiece. For instance, when it is detected that the temperature drop at the center of the workpiece lags behind that at the surface, the gas pressure is automatically increased to enhance convective heat transfer. When the temperature approaches the martensitic transformation point, gradually reduce the pressure to achieve a "soft landing". After a certain aviation parts enterprise adopted this function, the quenching deformation of TC4 titanium alloy parts was reduced from 0.3mm/m to 0.1mm/m, meeting the requirements of high-precision assembly. The self-learning function of process parameters continuously optimizes the cooling effect. The Huarui vacuum quenching furnace will record the cooling curve and workpiece performance data of each furnace, analyze the correlation between parameters and results through machine learning algorithms, and automatically correct subsequent processes. For instance, the system found that when the nitrogen pressure was set at 0.5MPa, the standard deviation of the hardness of a certain type of gear was relatively large. It would automatically fine-tune the pressure to 0.52MPa and record the adjustment effect. After 3 to 5 rounds of iterative optimization, the stability of the product performance was significantly improved. The remote monitoring and diagnosis functions provide a guarantee for the stable operation of the equipment. The Huarui vacuum air quenching furnace can upload the cooling process data to the cloud through the industrial Internet. Technicians can view the real-time curves remotely and guide on-site adjustments in a timely manner when any abnormalities are found. A vacuum quenching furnace at a factory in another location experienced fluctuations in cooling rate. Remote technicians analyzed the pressure curve and determined that the filter was clogged. After guiding the replacement, the problem was immediately resolved, thus avoiding a batch of quality issues.
A typical case of cooling process for materials
The hardenability and cooling sensitivity of different materials vary greatly. Huarui vacuum quenching furnaces have developed mature cooling process solutions for typical materials, which have been verified through practice to achieve ideal workpiece performance. The cooling of high-speed steel requires rapid passage through the "nose tip" temperature zone to prevent the precipitation of pearlite. When the Huarui vacuum quenching furnace is used to process W6Mo5Cr4V2 high-speed steel, a mixture of 0.6MPa nitrogen and helium (30% helium) is adopted, with the cooling rate controlled at 60-70℃/s. The time for cooling from 1200℃ to 200℃ is controlled within 80 seconds. The hardness of the treated cutting tool reaches HRC63-65, the impact toughness of the cutting edge is ≥30J/cm², and the service life is 50% longer than that of traditional oil-quenched cutting tools. The cooling of die steel needs to control deformation while ensuring hardness. Take Cr12MoV as an example. The Huarui vacuum quenching furnace adopts segmented pressure control: the pressure rises to 0.5MPa within 0-30 seconds and is rapidly cooled to 600℃. Maintain at 0.3MPa for 30 to 120 seconds and cool slowly to 300℃. After 120 seconds, it drops to 0.1MPa to complete the subsequent cooling. This process enables the hardness of the die to reach HRC58-60, and the deformation is controlled within 0.05mm/m. After a certain die factory adopted this process, the grinding allowance of large stamping dies was reduced by 30%, and the processing cycle was shortened by 15 days. The cooling of stainless steel needs to take into account both corrosion resistance and mechanical properties. 316L stainless steel is cooled in the Huarui vacuum quenching furnace with 0.4MPa argon gas, and the cooling rate is controlled at 15-20℃/s to avoid intergranular corrosion caused by excessive cooling. The tensile strength of the treated stainless steel plate is ≥650MPa, the elongation is ≥40%, and it shows no rust after 72 hours of salt spray testing, meeting the strict requirements of medical devices. The cooling of titanium alloys should prevent the embrittlement of the α phase. TC4 titanium alloy was cooled in the Huarui vacuum quenching furnace with 0.3MPa argon gas, and the cooling rate was controlled at 8-12℃/s. The time for cooling from 950℃ to 300℃ was approximately 150 seconds. The tensile strength of the treated titanium alloy bar is ≥950MPa, and the impact toughness is ≥60J/cm², fully meeting the aerospace standards. The cooling technology of vacuum air quenching furnaces is a comprehensive application of materials science, fluid mechanics and intelligent control. Its development direction has always revolved around "faster cooling rate, more uniform cooling effect and more precise process control".
Through continuous technological innovation, Taicang Huarui Vacuum Furnace Industry Co., Ltd. has deeply integrated the cooling system with the heating system and vacuum system, forming an integrated heat treatment solution. Whether it is tool steel that pursues hardness or precision parts that are sensitive to deformation, Huarui vacuum quenching furnaces can provide reliable guarantees for the improvement of workpiece performance through optimized cooling technology, promoting the development of the heat treatment industry towards high efficiency, accuracy and greenness.
