As an important type of vacuum heat treatment equipment, the vacuum air quenching furnace plays a significant role in the strengthening treatment of metal materials due to its precise temperature control in a vacuum environment and the efficient cooling characteristics of gas quenching. In the research and development and manufacturing of vacuum quenching furnaces, Taicang Huarui Vacuum Furnace Industry Co., Ltd. combines process optimization with technological innovation, forming a stable and efficient working system to ensure that each process can be precisely connected, providing uniform and stable heat treatment effects for workpieces.
Workpiece pretreatment and furnace loading preparation
The working process of the vacuum air quenching furnace begins with rigorous workpiece pretreatment and loading preparation, which directly affects the quality stability of subsequent heat treatment. The core of workpiece pretreatment is to remove surface impurities and prevent the generation of volatile substances or chemical reactions in a high-temperature vacuum environment. First, the workpiece needs to undergo degreasing treatment. This can be achieved through ultrasonic cleaning or solvent wiping to remove oil stains, cutting fluids and other residues on the surface. These substances may decompose into gases at high temperatures, disrupting the vacuum environment inside the furnace and even causing carbon pollution on the workpiece surface. For workpieces with oxide scale or rust, grinding or pickling treatment is also required to expose the metal substrate. This is because the oxide scale is difficult to decompose during vacuum heating, which may lead to uneven local heating and affect the quenching effect. For instance, if there is residual oxide scale on the surface of die steel workpieces, there may be a problem of insufficient local hardness after vacuum heating, which directly affects the service life of the die. After the pretreatment is completed, it enters the furnace loading stage. When loading the furnace, appropriate tooling fixtures should be selected based on the size, shape and material of the workpiece. The material of the fixture is usually high-temperature resistant graphite or alloy material to avoid adhesion or chemical reaction with the workpiece. The placement of the workpiece on the fixture should maintain a uniform gap to ensure that the heat can be fully radiated to each part during heating, while reserving sufficient gas flow channels for the subsequent gas cooling. For instance, when dealing with slender shaft parts, the vertical suspended furnace loading method is adopted, which can reduce the deformation of the workpiece caused by its own weight and ensure that the cooling gas can flow uniformly along the axial direction. Taicang Huarui Vacuum Furnace Industry Co., Ltd. pays close attention to details in the design of furnace loading. The vacuum quenching furnaces it produces are equipped with various adjustable fixtures of different specifications, which can adapt to different batches and types of workpieces. After the furnace loading is completed, the furnace door should be closed and the sealing performance checked to ensure that the sealing surface of the furnace body and the furnace door is closely attached - this is the basis for maintaining the subsequent vacuum environment. If the seal is not tight, the infiltration of outside air will cause the workpiece to oxidize and affect the treatment effect.
Vacuum system startup and furnace vacuuming
After the sealing inspection of the furnace door is completed, the vacuum system starts up. This is the core difference link between the vacuum quenching furnace and the traditional quenching equipment. The vacuum system is composed of multi-stage vacuum pump groups such as mechanical pumps, Roots pumps, and diffusion pumps. When starting up, it operates step by step according to the preset program to gradually reduce the air pressure inside the furnace. In the initial stage, a mechanical pump operates to reduce the furnace pressure from normal to around 10Pa. This process mainly removes most of the air inside the furnace. Subsequently, the Roots pump intervened, further reducing the air pressure to below 1Pa. For more demanding working conditions, the diffusion pump will continue to operate, resulting in the furnace vacuum reaching the level of 10⁻³Pa. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. is equipped with high-precision vacuum gauges, which can display the pressure changes in the furnace chamber in real time. Operators can monitor the vacuuming progress through the control system to ensure that the pressure parameters at each stage meet the process requirements. During the vacuuming process, the composition of the gas in the furnace changes as the gas pressure decreases. When the air pressure drops below 10Pa, the oxygen content in the air is extremely low, which can effectively prevent the workpiece from oxidizing during the subsequent heating process. When the gas pressure drops below 10⁻²Pa, the gas molecules adsorbed on the surface of the workpiece start to detorb, reducing the impact of gas escape during heating on the vacuum degree. For alloy workpieces containing volatile elements, low-temperature baking is also required during the vacuuming stage - by raising the furnace temperature to 200-300℃, it promotes the early evaporation of moisture and light gases inside the workpiece, avoiding surface defects caused by intense evaporation during the high-temperature stage.
Stepwise heating and holding stage
When the vacuum degree in the furnace reaches the preset value, the vacuum quenching furnace enters the heating stage. This process adopts a stepped temperature rise mode and achieves uniform heating of the workpiece through segmented temperature control. The core of stepped heating is to prevent the workpiece from generating thermal stress due to excessive temperature differences between the inside and outside, especially for workpieces with complex shapes or uneven wall thicknesses. A slow heating rate can effectively reduce the risk of deformation. The initial heating stage rate is usually controlled at 5-10℃/min. When the temperature rises to around 600℃, it enters the holding stage, and the holding time is set according to the thickness of the workpiece. The purpose of this stage is to make the internal temperature of the workpiece tend to be consistent, laying the foundation for subsequent high-temperature heating. For instance, when processing tool and die steel with a thickness of 50mm, the holding time at 600℃ should reach 60 to 90 minutes to ensure that the temperature difference between the core and the surface is controlled within 50℃. The rate of the high-temperature heating stage can be appropriately increased to 10-15℃/min until the austenitizing temperature set by the process is reached (usually between 850-1200℃). At this point, the main holding stage begins. The holding time needs to be calculated based on the material of the workpiece and its effective thickness to ensure that the workpiece is fully austenitized. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. adopts zoned temperature control technology in the heating system. The temperature deviation of each area in the furnace chamber can be controlled within ±3℃, avoiding the problem of coarse grains caused by local overheating. During the heating process, the vacuum degree inside the furnace will fluctuate slightly as the temperature rises - the trace amount of gas released by the workpiece may cause a temporary increase in pressure, but the vacuum system will automatically activate the supplementary pumping function to stabilize the pressure within the process range. This dynamic balance ensures the stability of the heating environment and provides ideal conditions for the austenitizing process.
Pressure regulation before gas quenching
After the holding stage is completed, the vacuum quenching furnace enters the gas quenching preparation stage, the core of which is to fill the furnace chamber with inert gas and adjust it to the preset pressure. This process requires precise control of the inflation rate and pressure parameters to prepare for the subsequent rapid cooling. First, close the connection valve between the vacuum system and the furnace chamber, and open the gas inlet passage for inert gas (usually nitrogen or argon). The gas enters the furnace through a specially designed flow guide device. The porous structure of the flow guide device enables the gas to be evenly distributed, avoiding uneven cooling of the workpiece caused by local gas flow impact. The charging rate should be set according to the furnace volume and target pressure. Charging too fast may cause gas flow erosion marks on the workpiece surface, while charging too slow will prolong the transition time and affect the quenching rhythm. When the pressure inside the furnace reaches 0.1-0.6MPa (the specific value is determined based on the material of the workpiece), stop charging and keep the pressure stable. For instance, when quenching high-speed steel cutting tools, a nitrogen pressure of 0.5MPa is typically adopted to achieve a sufficient cooling rate. For some alloy steels, the pressure is controlled at around 0.2MPa to reduce quenching stress. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. is equipped with high-precision pressure sensors and automatic regulating valves, which can control pressure fluctuations within ±0.02MPa and ensure the consistency of cooling conditions. Gas purity is also a key parameter. Generally, the purity of inert gas is required to reach over 99.99%, and the oxygen content is below 50ppm. Oxygen in the impurity gas may react with the surface of the workpiece at high temperatures, affecting the surface quality. Moisture may cause fluctuations in the cooling rate, so the gas needs to undergo drying and filtration treatment before entering the furnace.
High-pressure gas circulation cooling stage
After the pressure stabilizes, the vacuum quenching furnace starts the circulating cooling system and enters the core stage of gas quenching. The cooling system is composed of a high-speed fan, deflector plates and heat exchangers. Its working principle is that the fan drives high-pressure inert gas to circulate at high speed in the furnace chamber, absorbs the heat from the surface of the workpiece, then flows through the heat exchanger, releases the heat and re-enters the furnace chamber, forming a continuous cooling cycle. The rotational speed of the fan is adjusted according to the cooling requirements, usually enabling the gas flow rate inside the furnace to reach 20-50m/s. This high-speed gas flow can break through the gas film resistance on the surface of the workpiece, significantly enhancing the heat exchange efficiency. The layout of the deflector plate has been optimized through fluid mechanics simulation to ensure that the gas can evenly cover all parts of the workpiece. For complex workpieces with grooves and blind holes, the deflector plate will guide part of the gas flow to directionally scour these areas, avoiding the formation of cooling dead zones. For instance, the intertooth area of gear-type workpieces is prone to insufficient cooling rate due to poor air flow. By setting guide holes at the corresponding positions, the cooling rate between the teeth can be kept consistent with that at the tooth top. During the cooling process, the temperature of the workpiece will rapidly drop below the martensitic transformation point (usually around 200℃). At this point, the fan needs to be turned off to end the strong cooling stage. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. can accurately determine whether the martensitic transformation is complete through infrared temperature measurement or thermocouple feedback, avoiding the pearlite transformation caused by premature stopping of cooling or the increase of internal stress in the workpiece due to excessive cooling. For materials with poor hardenability, a segmented cooling mode can also be adopted. First, cool at high speed to below the nose tip temperature, and then reduce the wind speed for slow cooling. This ensures hardening while reducing stress.
Pressure relief and workpiece removal process
After the cooling stage is completed, the vacuum quenching furnace enters the pressure relief and workpiece removal process. At this stage, it is necessary to avoid the impact of sudden temperature changes and pressure shocks on the workpieces. First, open the exhaust valve to slowly release the high-pressure gas in the furnace to normal pressure. The pressure relief rate should be controlled at 0.05-0.1MPa/min. Excessive pressure relief may cause gas flow impact on the surface of the workpiece or cause internal stress fluctuations in the workpiece due to rapid pressure changes. During the gas emission process, some vacuum quenching furnaces recover and treat inert gases - after filtration and drying, they are stored in gas storage tanks for reuse in the next quenching. This design not only reduces gas consumption costs but also decreases exhaust emissions. For gases containing volatile substances, they need to be treated by combustion or adsorption before being discharged to avoid environmental pollution. After the pressure relief is completed, when opening the furnace door, pay attention to the temperature difference between the inside and outside of the furnace chamber. If the temperature of the workpiece is still above 200℃, an insulating baffle should be set up at the furnace door to prevent the rapid influx of cold air, which may cause a temperature gradient on the surface of the workpiece and form secondary stress. When removing the workpiece, a special high-temperature resistant fixture should be used. Handle it gently to prevent deformation or collision due to high temperature and low strength. After the workpiece is removed, a preliminary quality inspection is required to observe whether there are signs of oxidation color, cracks or deformation on the surface. Only after passing the inspection can it be transferred to the subsequent tempering process. Taicang Huarui Vacuum Furnace Industry Co., Ltd. will provide customers with complete quenching process records, including temperature, pressure and time parameter curves at each stage, which is convenient for traceability and analysis, and provides data support for process optimization.
Furnace interior cleaning and equipment maintenance
After a single quenching process is completed, the vacuum quenching furnace needs to be cleaned inside the furnace and maintained to ensure the stability of the next operation. First, clean up the scattered debris in the furnace chamber, such as the oxide scale and debris fallen off from the workpieces. These impurities may volatilize or melt at high temperatures, contaminating the subsequent workpieces. Then check the condition of the heating element and the deflector plate. If any deformation of the heating wire or damage to the deflector plate is found, it should be repaired or replaced in a timely manner. The maintenance of the vacuum system is equally important. It is necessary to regularly drain the accumulated liquid in the vacuum pump and check the wear of the seals. For diffusion pumps, the oil of the diffusion pump needs to be replaced regularly to ensure its stable pumping performance. If the graphite felt or ceramic fiber insulation layer on the inner wall of the furnace is contaminated or damaged, it also needs to be cleaned or partially replaced to avoid affecting the insulation effect and vacuum degree. The vacuum quenching furnace of Taicang Huarui Vacuum Furnace Industry Co., Ltd. is equipped with an intelligent maintenance reminder function. According to the operation time and process frequency, it automatically prompts the items and cycles that need maintenance, helping operators carry out maintenance in a timely manner and extending the service life of the equipment. This full-process maintenance management, from the side, ensures the stability and reliability of the vacuum air quenching furnace's working process. The working process of the vacuum air quenching furnace is a closely interlinked systematic project. The parameter control of each link needs to take into account the material properties, workpiece shapes and performance requirements. Taicang Huarui Vacuum Furnace Industry Co., Ltd. has, through meticulous process design and precise equipment control, enabled its vacuum quenching furnace to meet diverse processing requirements ranging from precision parts to large components. While ensuring quenching quality, it has also enhanced production efficiency and process stability. With the development of material technology, the working process of vacuum air quenching furnaces is constantly being optimized. However, its core always revolves around the principle of "precise temperature control, uniform cooling, and stable reliability", providing continuous support for the performance enhancement of metal materials.
