The heating system of the vacuum oil quenching furnace is the core link that determines the quality of heat treatment. Its performance directly affects the uniformity of material heating, energy consumption level and process stability. During the research and development of vacuum oil quenching furnaces, Taicang Huarui Vacuum Furnace Industry Co., Ltd. has addressed the problems existing in traditional heating systems, such as poor temperature uniformity, high heat loss, and slow response speed. By integrating material innovation, structural optimization, and intelligent control technology, the company has achieved an all-round upgrade of the heating system, providing a reliable guarantee for efficient and energy-saving heat treatment processes.
Selection and performance optimization of heating elements
As the core component for heat output, the material selection and structural design of the heating element directly determine the heating efficiency and service life. The heating elements of a vacuum oil quenching furnace need to operate stably at high temperatures (typically 600-1300℃) and in a vacuum environment, while avoiding chemical reactions with workpieces or quenching oil vapor. Therefore, optimizing the material and shape is the primary task for upgrading the heating system. In terms of material selection, Huarui Vacuum Furnace Industry adopts differentiated solutions based on different temperature ranges: for the medium and low-temperature sections (600-900℃), nickel-chromium alloy heating tapes are selected, which have strong oxidation resistance and moderate cost. For the high-temperature section (900-1300℃), molybdenum wire or silicon-molybdenum rods are used. These materials have excellent high-temperature resistance and can maintain a stable resistivity even at 1300℃. For instance, when processing high-speed steel, molybdenum wire heating elements can operate continuously for 500 hours in a vacuum environment at 1200℃ without obvious aging, and their service life is more than three times that of traditional nickel-chromium elements. The structural form of the heating element has a significant impact on the thermal radiation efficiency. Huarui Vacuum Furnace Industry has optimized the traditional linear heating wire into a wavy or spiral shape, increasing the contact area with the furnace chamber space. The heat radiation Angle has been expanded from 120° to 180°, making the heat distribution more uniform. When dealing with large die steel modules, the radiant heating of helical molybdenum wire can keep the surface temperature difference of the module within ±5℃, while the temperature difference of traditional linear heating wire often reaches ±10℃. Meanwhile, the diameter of the heating element has been precisely calculated. For instance, a molybdenum wire with a diameter of φ0.8mm has a 20% faster thermal response speed than a similar product with a diameter of φ1.0mm, enabling it to reach the set temperature more quickly and shorten the heating time. The way components are fixed also affects stability. Huarui Vacuum Furnace Industry adopts a fixed structure combining ceramic insulators and graphite supports to avoid the short circuit risk caused by direct contact between heating elements and the metal furnace body. The ceramic insulator is made of high-temperature resistant alumina material, which can withstand temperatures above 1600℃. The graphite bracket has excellent thermal conductivity, which can evenly distribute the heat of the components and prevent local overheating and burning out. After this optimization, the replacement cycle of the heating elements in the vacuum oil quenching furnace of a certain bearing enterprise was extended from 3 months to 8 months, and the maintenance cost was reduced by 60%.
Optimization of furnace structure and thermal field distribution
As the carrier of the heating system, the spatial layout and insulation design of the furnace chamber determine the uniformity of the thermal field distribution and the heat utilization rate. Huarui Vacuum Furnace Industry optimizes the furnace structure, reduces heat loss and enhances the consistency of the temperature field by combining CFD (Computational Fluid Dynamics) simulation with actual testing. The geometric shape of the furnace chamber has a significant impact on the distribution of the thermal field. The traditional rectangular furnace chamber is prone to heat dead zones in the corners. Huarui Vacuum Furnace Industry designs the furnace chamber cross-section as a rounded rectangle, with a transition radius of ≥100mm at the corners, allowing the heat radiation to form a circulation within the furnace chamber and avoiding low temperatures in the corners. The measured data from a certain mold factory shows that the hardness difference of each part of the Cr12MoV mold treated with a rounded rectangular furnace chamber has been reduced from 3HRC to 1HRC, and the uniformity has been significantly improved. Meanwhile, the ratio of the furnace height to width is optimized to 1:1.2, keeping the vertical temperature gradient within 3℃/m, which is suitable for the overall heating of long shaft parts. The composite design of the insulation layer is the key to reducing heat loss. Huarui Vacuum Furnace Industry adopts a "multi-layer gradient insulation" structure: The inner layer is a 20mm thick ceramic fiber blanket (with a temperature resistance of 1400℃), the middle layer is a 50mm thick lightweight mullite brick (with a thermal conductivity of ≤0.15W/(m · K)), and the outer layer is a 10mm thick aerogel felt (with a thermal conductivity of 0.018W/(m · K) at room temperature). This composite structure keeps the surface temperature of the furnace body within 60℃ (when the ambient temperature is 25℃), and the heat loss rate is reduced by 50% compared with traditional single insulation materials. After this transformation, the heating energy consumption of each vacuum oil quenching furnace of a certain enterprise was reduced from 800kWh to 550kWh, and the annual electricity cost savings exceeded 100,000 yuan. The sealing and insulation of the furnace door and the furnace chamber are equally important. Huarui Vacuum Furnace Industry has added a ring-shaped heating tape (with a power of 5kW) on the inner side of the furnace door. During heating, the temperature rises simultaneously to a level close to that of the furnace chamber, reducing heat loss at the furnace door area. The sealing surface of the furnace door adopts double-layer silicone rubber sealing rings (with a temperature resistance of 200℃), filled with ceramic fiber cotton in the middle, which not only ensures vacuum sealing but also enhances the insulation effect. This design reduces the temperature deviation around the furnace door from ±8℃ to ±3℃, solving the problem of insufficient heating of workpieces near the furnace door.
Technology for improving temperature control accuracy
Temperature control accuracy is a core indicator for evaluating the performance of a heating system and directly affects the uniformity of material phase transformation. Huarui Vacuum Furnace Industry has elevated the accuracy of temperature control to a new level through the integration of multi-sensor fusion, intelligent algorithms and power regulation technology, meeting the demands of high-precision heat treatment. The multi-zone independent temperature control system enables precise regulation. Huarui Vacuum Furnace Industry divides the furnace chamber into 3 to 5 independent heating zones, each equipped with dedicated heating elements and thermocouples, and the power is adjusted respectively through the PLC control system. For instance, when processing disc-shaped parts with a diameter of 1 meter, the furnace chamber is divided into a central area and an edge area. The power in the edge area can be 10% higher than that in the central area, compensating for the rapid heat dissipation at the edge and keeping the radial temperature difference of the part within 2℃. After a certain gear factory adopted this system, the hardness difference between the top and root of the gear teeth was reduced from 2HRC to 0.5HRC, and the meshing accuracy was significantly improved. The layout optimization of thermocouples enhances the accuracy of temperature monitoring. Traditional single-point temperature measurement cannot reflect the overall temperature distribution of the furnace chamber. Huarui Vacuum Furnace Industry arranges 6 to 8 thermocouples at the top, bottom and both sides of the furnace chamber. The data is processed by the "arithmetic mean + weighted correction" algorithm: the weight of the thermocouple in the center area is 20%, and that in the edge area is 10%, ensuring that the calculation result is close to the actual temperature of the workpiece. Meanwhile, the thermocouple is treated with an anti-oxidation coating (which can work stably under vacuum) and undergoes regular metrological calibration (once a year) to keep the measurement error within ±1℃. The PID adaptive regulation algorithm enhances the dynamic response performance. The traditional PID parameters are fixed and difficult to adapt to different load changes. The control system of Huarui Vacuum Furnace Industry is equipped with an adaptive algorithm that can automatically adjust the proportional coefficient (P), integral time (I), and differential time (D) according to the real-time temperature deviation and change rate. For instance, at the initial stage of temperature rise (above 100℃ from the target temperature), a large P and small I parameter should be adopted to accelerate the temperature rise rate. When approaching the target temperature (temperature difference ≤50℃), switch to the small P and large I parameter to avoid overshoot. Tests conducted by a certain bearing enterprise show that after adopting the adaptive algorithm, the overshoot when the temperature rises from room temperature to 850℃ drops from 5℃ to 1℃, and the temperature fluctuation during the holding stage shrinks from ±3℃ to ±1℃.
Strategies for improving the efficiency of thermal cycling
The thermal cycle efficiency of the heating system directly affects the energy consumption of the equipment and the production rhythm. Huarui Vacuum Furnace Industry achieves efficient energy utilization and shortens the heating cycle by reducing ineffective heating and optimizing heat recovery. Intermittent heating and preheating recovery technology reduces standby energy consumption. During the interval period of batch production of vacuum oil quenching furnaces (such as during loading and unloading), traditional operations often keep the furnace chamber at a high temperature in standby mode, resulting in energy waste. The "Intelligent Standby Mode" developed by Huarui Vacuum Furnace Industry can automatically lower the furnace chamber temperature to 300℃ (for heat preservation), and quickly heat up after the workpieces are loaded into the furnace. This mode reduces standby energy consumption by 70%. Meanwhile, the system can recover the waste heat from the previous furnace (about 200℃ in the furnace chamber) to preheat the newly loaded cold-state workpieces, reducing the heating time by 15%. After adopting this technology, a certain auto parts factory can process two more batches of workpieces per day, increasing its production capacity by 12%. Dynamic matching of heating power avoids energy waste. Depending on the loading volume and material of the workpieces, the heating system can automatically adjust the total power output: it operates at 100% at full load and drops to 60%-70% at half load. For instance, when processing 500kg workpieces, the power is set at 80kW, and when processing 200kg, it automatically drops to 50kW to avoid the phenomenon of "a big horse pulling a small cart". Power regulation is achieved through a thyristor module for stepless speed change, with a response time of ≤10ms, ensuring smooth power variation without affecting temperature stability. Statistics from a certain heat treatment center show that dynamic power regulation has reduced the energy consumption per unit product by 25%, achieving remarkable energy-saving effects. Rapid temperature rise and fall technology shortens the process cycle. Huarui Vacuum Furnace Industry has increased the power density by thickening the diameter of the heating element (from φ0.8mm to φ1.2mm), raising the heating rate from 10℃/min to 20℃/min (for high-temperature alloys). Meanwhile, a rapid cooling interface is set at the top of the furnace chamber. After the heating is completed, a small amount of inert gas (0.05MPa) can be introduced to accelerate the cooling of the furnace chamber (reducing the time from 1000℃ to 500℃ to 30 minutes), saving time for the next furnace heating. After a certain high-speed steel tool factory adopted this technology, the single-furnace heat treatment cycle was shortened from 4 hours to 3 hours, and the average daily production capacity increased by 33%.
Adaptive optimization of heating for special materials
In response to the heating requirements of special materials such as high alloy steel and titanium alloys, Huarui Vacuum Furnace Industry has specially optimized the heating system to address issues such as easy oxidation and strong thermal sensitivity, ensuring the safety and process stability of the heating process. The optimization of low-temperature preheating for high alloy steel reduces thermal stress. High alloy steel (such as Cr12MoV) has a low thermal conductivity. Rapid heating can easily cause thermal stress and lead to cracking. Huarui Vacuum Furnace Industry has designed a "stepped preheating" process: when the temperature rises from room temperature to 600℃, it is heated at a low speed of 5℃/min, held for 1 hour to make the internal and external temperatures uniform, and then raised to the quenching temperature at 15℃/min. The heating system automatically executes this curve through the program without the need for manual intervention. After a certain mold factory adopted this process, the cracking rate of Cr12MoV molds dropped from 8% to 1%, significantly reducing the loss of defective products. The anti-pollution design of titanium alloy heating ensures surface quality. Titanium alloys are prone to absorbing oxygen and nitrogen at high temperatures to form a brittle layer. Huarui Vacuum Furnace Industry uses low venting rate materials in the heating system: high-density graphite plates (venting volume ≤0.1Pa · m³/s) are used on the inner walls of the furnace chamber, and ceramic materials are used for the heating element brackets to prevent the release of gases at high temperatures from contaminating the titanium alloys. Meanwhile, before heating, the furnace chamber should be evacuated to a high vacuum (≤10⁻³Pa) and maintained at a vacuum state above 500 ° C (the temperature at which titanium alloys are prone to gas absorption), and then slowly filled with gas to a slightly positive pressure (0.02MPa argon gas) to form a protective atmosphere. The TC4 titanium alloy parts processed by a certain aviation enterprise have a surface embrittlement layer thickness of no more than 5μm after this optimization, and the fatigue strength has increased by 20%. A non-magnetic heating environment for magnetic materials avoids performance interference. Soft magnetic materials (such as silicon steel sheets) will experience a decline in magnetic properties if affected by magnetic fields during the heating process. The heating system of Huarui Vacuum Furnace Industry adopts a non-magnetic design: non-magnetic molybdenum wire is selected as the heating element, and austenitic stainless steel (non-magnetic) is used for the furnace chamber structural components. A magnetic shielding layer (permalloy) is added around the furnace chamber to ensure that the magnetic field intensity inside the furnace is ≤1mT. After a certain motor factory adopted this system to heat silicon steel sheets, its iron loss value (50Hz, 1.5T) decreased by 5%, and the motor efficiency was significantly improved.
System integration and reliability assurance
The long-term stable operation of the heating system relies on a complete protection mechanism and integrated design. Huarui Vacuum Furnace Industry enhances the reliability and service life of the system through hardware redundancy, fault diagnosis and convenient maintenance design. Multiple safety protection mechanisms prevent unexpected malfunctions. The heating system is equipped with multiple devices such as overcurrent protection (automatically cutting off power when the current exceeds 1.2 times the rated value), over-temperature protection (cutting off heating when the temperature is 10℃ higher than the set temperature), and vacuum abnormal protection (stopping heating when the vacuum degree is lower than 10Pa), to ensure the safety of the equipment and workpieces. For instance, the vacuum oil quenching furnace of a certain enterprise showed a low displayed temperature due to a thermocouple failure. After the system detected that the actual temperature exceeded the limit, it cut off the heating power supply within 0.5 seconds to prevent the workpiece from overburning. The easy-to-replace design of the heating element reduces the difficulty of maintenance. The replacement of traditional heating elements requires the removal of the insulation layer, which is time-consuming and labor-intensive. Huarui Vacuum Furnace Industry adopts a "modular plug-in" design: the heating elements are connected to the terminal blocks through quick plugs, and the insulation layer has a maintenance window (with a sealed cover). It only takes 30 minutes to replace a single group of elements, without the need to remove the entire insulation structure. The maintenance records of a certain heat treatment workshop show that this design has reduced the average repair time (MTTR) of the heating system from 8 hours to 1 hour and increased the equipment utilization rate by 3%. Regular maintenance of the prompt system can extend its service life. The control system is equipped with an inbuilt maintenance database that records the operating time of each component: when the heating element has worked for a cumulative total of 1000 hours, it prompts for inspection; when it has worked for 2000 hours, it is recommended to replace it. Thermocouples are prompted for calibration every 12 months. Meanwhile, the system can display the resistance value changes of each heating zone (calculated through current and voltage). When the resistance value deviation of a certain area exceeds 10%, it indicates that there may be component aging, facilitating early replacement to prevent faults. By strictly implementing maintenance prompts, a certain enterprise has extended the mean time between failures (MTBF) of its heating system from 6 months to 18 months, reducing maintenance costs by 70%. The optimization of the heating system for vacuum oil quenching furnaces is a systematic project, involving the integration of multiple disciplines such as materials science, thermodynamics and automatic control.
Through continuous innovation, Taicang Huarui Vacuum Furnace Industry Co., Ltd. has taken heating uniformity, energy consumption level and process adaptability as the core optimization goals, achieving a qualitative leap in the performance of the heating system of vacuum oil quenching furnaces. Practical experience has shown that the optimized heating system not only enhances the stability of the heat treatment quality of workpieces but also significantly reduces energy consumption and maintenance costs, creating considerable economic benefits for enterprises. With the continuous improvement of the precision requirements for heat treatment in the high-end manufacturing field, the intelligent and personalized optimization of heating systems will become the future development direction. The technological accumulation of Huarui Vacuum Furnace Industry in this field will provide continuous impetus for the progress of the industry.
