The adjustment of process parameters in vacuum air quenching furnaces is a core link that determines the quality of heat treatment, directly affecting key indicators such as the hardness, toughness and deformation of workpieces. The requirements for process parameters vary significantly among workpieces of different materials and shapes. It is necessary to achieve a balance between performance and efficiency by precisely regulating parameters such as temperature, vacuum degree, and gas pressure. Through years of practice, Taicang Huarui Vacuum Furnace Industry Co., Ltd. has summarized a set of parameter adjustment methods based on material properties and process goals. By flexibly optimizing parameters at each stage, it provides reliable solutions for diverse heat treatment demands.
Principles and techniques for adjusting temperature parameters
Temperature is a core parameter in the vacuum quenching process. Its adjustment needs to take into account both the phase transformation characteristics of the material and the uniformity of heating of the workpiece. Both excessively high and low temperatures can lead to performance defects. The determination of the heating temperature is based on the phase transformation point of the material. For materials with a clear austenitizing temperature (such as structural steel and die steel), the heating temperature should be 30-50℃ higher than the Ac3 point (the complete austenitizing temperature) to ensure that pearlite is completely transformed into austenite. For instance, the Ac3 point of 40Cr steel is approximately 780℃, and the heating temperature should be set at 810-830℃. The Ac3 point of Cr12MoV die steel is approximately 820℃. To promote the dissolution of carbides, the heating temperature needs to be raised to 980-1020℃. The process database of Huarui Vacuum Furnace Industry stores the recommended heating temperatures for over 50 commonly used materials, which can serve as an initial reference for parameter adjustment. For materials with strong thermal sensitivity (such as high-speed steel and titanium alloys), temperature adjustment needs to be more precise. If the heating temperature of high-speed steel W6Mo5Cr4V2 exceeds 1250℃, it will lead to coarse grains and a decrease in impact toughness. Therefore, it should be controlled at 1200-1220℃. When the temperature exceeds 950℃, titanium alloy TC4 is prone to coarsening of β phase grains, and the heating temperature must be strictly controlled between 850 and 920℃. The zoned temperature control system of Huarui vacuum air quenching furnace can keep the temperature deviation of each area in the furnace chamber within ±3℃, ensuring the heating consistency of each part of complex workpieces. The adjustment of the heating rate should be determined based on the thickness of the workpiece and the thermal conductivity of the material. For thick and large workpieces (such as molds with a thickness of ≥50mm), low-speed heating (5-10℃/min) should be adopted to avoid thermal stress caused by excessive temperature difference between the inside and outside. For thin and small parts (such as cutting tools with a thickness of no more than 10mm), high-speed heating (15-20℃/min) can be adopted to shorten the heating cycle. For instance, when handling a 42CrMo steel shaft with a diameter of 200mm, the temperature should be raised from room temperature to 850℃ in two stages: from room temperature to 600℃, increase the temperature at a rate of 8℃/min; from 600℃ to 850℃, increase the temperature at a rate of 5℃/min, thereby reducing the temperature difference between the shaft center and the surface. The programmable temperature control system of Huarui vacuum quenching furnace supports 10-stage heating curves. Operators can flexibly set the rate change nodes according to the characteristics of the workpiece. The adjustment of the holding time follows the principle of "thickness × coefficient". Generally speaking, for every 10mm of effective thickness, it is necessary to maintain the temperature for 20 to 30 minutes to ensure that the workpiece is thoroughly heated and the microstructure transformation is completed. For instance, for a 20mm thick 20CrMnTi gear, the holding time is set at 40 to 60 minutes. For a 50mm thick Cr12MoV mold, the holding time needs to be extended to 100-150 minutes. For complex workpieces with blind holes and grooves, the holding time should be increased by 20% to ensure that the local area is fully heated. The intelligent timing system of Huarui vacuum air quenching furnace will automatically correct the holding time according to the actual heating curve to avoid insufficient holding caused by delayed heating.
The hierarchical control method of vacuum degree parameters
The adjustment of vacuum degree needs to be coordinated with the material properties and heating temperature. Its core objective is to prevent the oxidation of the workpiece, promote gas escape, and at the same time avoid the volatilization of alloy elements. The low vacuum section (1-10Pa) is suitable for heating volatile materials. When handling materials containing low-boiling-point alloy elements such as zinc and lead, an excessively high vacuum degree can cause these elements to volatilize, affecting the stability of the composition. For instance, in the vacuum annealing of zinc alloy die-castings, the vacuum degree should be controlled at 5 to 10Pa, which not only prevents oxidation but also reduces the volatilization loss of zinc. The vacuum degree adjustment system of Huarui vacuum quenching furnace can be stably controlled within the range of 1-10Pa. Combined with low-temperature heating at 300-400℃, it effectively prevents the loss of low-melting-point elements. The medium vacuum section (10⁻¹-1Pa) is an ideal choice for most structural steels. At this vacuum degree, it can not only remove the adsorbed gases on the surface of the workpiece (such as water vapor and decomposition products of oil stains), but also prevent the significant volatilization of alloy elements. For instance, in the vacuum quenching of 45 steel, a vacuum degree of 0.5 to 1Pa is often adopted. During the heating process, there is no oxidation on the surface and the gas in the core escapes fully. After treatment, the density of the workpiece increases by more than 5%. The medium vacuum control of Huarui vacuum quenching furnace adopts a "vacuum extraction - pressure holding" cycle mode to avoid excessive fluctuations in vacuum degree affecting heating stability. The high vacuum section (≤10⁻²Pa) is suitable for materials with high purity requirements. Titanium alloys, superalloys and other easily absorbent materials need to be heated in a high vacuum environment to reduce the infiltration of oxygen and nitrogen. For instance, the vacuum degree of TC4 titanium alloy during vacuum quenching should be controlled below 5×10⁻³Pa. Even when heated to 900℃, the surface should remain clean to prevent the formation of a brittle layer. The high vacuum system of Huarui vacuum quenching furnace adopts a combination of diffusion pump and mechanical pump, which can evacuate the furnace chamber to 10⁻³Pa within 30 minutes, meeting the processing requirements of high-purity materials. The dynamic adjustment technique of vacuum degree is equally important. For complex-shaped workpieces, a "segmented vacuum" strategy can be adopted: at the initial stage of heating (from room temperature to 600℃), low vacuum (1-5Pa) is used to quickly remove volatile substances from the surface of the workpiece. During the high-temperature stage (from 600℃ to the end of the holding period), switch to high vacuum (≤10⁻¹Pa) to prevent oxidation. When a certain aviation enterprise was processing titanium alloy parts with deep holes, through this dynamic adjustment, the thickness of the oxide layer on the inner wall of the deep holes of the parts was controlled within 5μm, which was far better than the processing effect with a fixed vacuum degree.
Optimization strategies for gas pressure and flow rate
Gas pressure and flow rate are key parameters in the vacuum air quenching stage. Their adjustment needs to match the hardenability of the material and the cooling requirements of the workpiece to ensure an ideal martensitic transformation rate. The adjustment of gas pressure is based on the critical cooling rate of the material. For materials with high critical cooling rates (such as high-speed steel and high-carbon tool steel), high-pressure gas (0.5-0.6MPa) should be used to achieve sufficient cooling rates. For instance, the critical cooling rate of high-speed steel W6Mo5Cr4V2 is approximately 50℃/s. It needs to be cooled under a nitrogen pressure of 0.5MPa to prevent the formation of pearlite and ensure that the hardness reaches above HRC63. For 40CrNiMo steel with better hardenability, 0.3MPa nitrogen can meet the cooling requirements, and the hardness can reach HRC50-55. The pressure adjustment range of Huarui vacuum quenching furnace is 0.1-0.6MPa, supporting fine adjustment at the 0.01MPa level, which can precisely match the critical values of different materials. For workpieces with complex shapes, deformation needs to be reduced through pressure gradient adjustment. For parts with thin walls and sharp corners (such as gears and blades), stress concentration is prone to occur due to overly rapid local cooling during cooling. In such cases, a "low-pressure start-up + gradual pressure increase" strategy can be adopted: during the initial cooling stage (800-600℃), the pressure should be controlled at 0.2MPa to reduce air flow impact. When the temperature drops below 600℃ (the martensitic transformation begins), slowly increase it to the target pressure (0.4-0.5MPa) to ensure a complete transformation. After a certain automotive gear factory adopted this method, the root cracking rate of 20CrMnTi gears was reduced from 5% to 0.5%, and the deformation was controlled within 0.03mm. The adjustment of gas flow rate needs to be coordinated with pressure. Too small a flow rate will result in insufficient cooling speed, while too large one will increase energy consumption and may exacerbate deformation. The gas flow rate and pressure matching of Huarui vacuum quenching furnace follow the principle of "flow rate = pressure × coefficient" (the coefficient is determined according to the furnace volume). For example, at a pressure of 0.5MPa, the nitrogen flow rate in a 1m³ furnace should be controlled at 50-60m³/h. Meanwhile, the flow rate is regulated in segments: in the high-temperature section (800-500℃), the flow rate remains at the upper limit and quickly passes through the pearlite transformation zone. In the low-temperature section (500-200℃), the flow rate is reduced to 70%, reducing organizational stress. The selection of gas types also affects parameter adjustment. Nitrogen is low in cost but has limited cooling capacity, making it suitable for general structural steel. The cooling capacity of argon gas is 15% to 20% higher than that of nitrogen, making it suitable for die steels with slightly higher requirements for cooling speed. Helium has a cooling capacity six times that of nitrogen, but it is more expensive and is mostly used for special materials such as high-speed steel. The Huarui vacuum gas quenching furnace supports the individual or mixed use of multiple gases. By adjusting the mixing ratio (such as nitrogen + 20% helium), a balance can be struck between cost and cooling effect.
The coordinated adjustment of the insulation and cooling stages
The parameter adjustments during the heat preservation and cooling stages need to form a synergistic effect to ensure that the material's microstructure transformation is thorough and uniform, while reducing internal stress and deformation. The matching of the holding time with the starting temperature of cooling is crucial. The stability of austenite varies among different materials. After the holding period ends, cooling should be initiated before reaching a specific temperature. For instance, for low-carbon steel with poor austenite stability, cooling should be initiated immediately after the holding period ends (with a delay of no more than 5 seconds) to prevent the premature precipitation of pearlite. The austenite stability of high alloy steels (such as Cr12MoV) is better, and the cooling can be delayed by 10 to 15 seconds to reduce thermal shock. The "holding - cooling" connection time of Huarui vacuum air quenching furnace can be controlled within 3 seconds to ensure the stability of austenite state. The segmented adjustment of cooling rate can balance hardness and toughness. For materials that require both hardness and toughness (such as 40CrNiMo), a "two-stage cooling" approach can be adopted: rapid cooling (≥40℃/s) in the high-temperature section (800-500℃) to prevent the formation of pearlite. Slow cooling (10-20℃/s) in the low-temperature section (500-200℃) reduces the stress caused by martensitic transformation. The hardness of the treated workpiece reaches HRC50-52, and the impact toughness is ≥80J/cm², meeting the usage requirements of heavy-duty transmission shafts. The cooling system of Huarui vacuum quenching furnace supports multi-speed setting and can automatically generate cooling schemes based on the CCT curve of the material. The matching of holding temperature and cooling pressure should take into account the hardenability of the material. For materials with poor hardenability (such as carbon tool steel T10), it is necessary to increase the holding temperature (such as 850℃) in combination with high-pressure cooling (0.6MPa) to compensate for their lower critical cooling rate. For alloy tool steels with good hardenability (such as 9SiCr), the holding temperature (820℃) and cooling pressure (0.4MPa) can be appropriately reduced to ensure hardness while minimizing deformation. Through this matching adjustment, the quenching deformation of the 9SiCr tap in a certain tool factory was reduced from 0.1mm/m to 0.05mm/m, and the subsequent grinding allowance was decreased by 50%.
Cases of parameter adjustment for different materials
The composition and microstructure properties of different materials vary significantly, and the adjustment of process parameters needs to be optimized specifically. The following typical cases provide references for the treatment of similar materials. Structural steel (40CrNiMoA) : This material is often used to manufacture heavy-duty components such as drive shafts, requiring a hardness of HRC38-42 and an impact toughness of ≥70J/cm². Key points for parameter adjustment: Heating temperature 850℃ (Ac3+40℃), holding time increases by 1 hour for every 25mm thickness (for example, holding for 2 hours for 50mm thickness), vacuum degree 0.3Pa; During the cooling stage, the nitrogen pressure is 0.4MPa. A gradient pressure of "starting at 0.2MPa → rising to 0.4MPa within 30 seconds" is adopted. After cooling to 200℃, it is taken out of the furnace. The tempering temperature is 550℃. After being treated in the Huarui vacuum quenching furnace, the hardness difference between the core and the surface of the workpiece is ≤1HRC, and the impact toughness reaches 85J/cm², meeting the requirements of heavy loads. Die steel (Cr12MoV) : Used for cold working dies, it requires a surface hardness of HRC58-60 and excellent wear resistance. Parameter adjustment: Heating temperature 1000℃ (to promote carbide dissolution), holding time 1.5 hours (to ensure thick and large areas are thoroughly heated), vacuum degree 0.5Pa; During the cooling stage, the nitrogen pressure is 0.5MPa and the flow rate is 30m/s. For complex parts such as mold cavities, the Huarui vacuum quenching furnace uses deflector plates for directional air supply, ensuring that the cooling rate inside the cavity differs from that outside by no more than 5℃/s. The service life of the treated mold is 40% longer than that of the traditional process, and the surface roughness after polishing reaches Ra0.8μm. High-speed steel (W6Mo5Cr4V2) : It is used for manufacturing cutting tools, requiring a hardness of HRC63-65 and good red hardness. Parameter adjustment: Heating temperature 1220℃ (to avoid coarse grains), holding time 40 minutes (for thin small tools), vacuum degree 1×10⁻²Pa; The cooling process uses a mixed gas of nitrogen and 20% helium with a pressure of 0.5MPa. The high thermal conductivity of helium is utilized to increase the cooling rate to over 60℃/s. Tests conducted by a certain tool factory show that when the milling cutter treated with this parameter cuts 45# steel, its service life is 30% longer than that of oil-quenched tools, and the rate of edge chipping is significantly reduced. Titanium alloy (TC4) : Aerospace structural components, with tensile strength of ≥900MPa and elongation of ≥10%. Parameter adjustment: Heating temperature 920℃ (α+β phase region), holding time 60 minutes, vacuum degree 5×10⁻³Pa (to prevent suction); Cooling is carried out using argon gas (to avoid nitrogen embrittlement), with a pressure of 0.3MPa, and it is slowly cooled to 300℃ before being removed from the furnace (to reduce stress). The low-pressure control accuracy of Huarui vacuum quenching furnace ensures that the argon pressure fluctuation is ≤0.02MPa. After treatment, the fatigue strength of titanium alloy is increased by 20%, meeting the requirements for aircraft use.
Common problems and Solutions in parameter adjustment
During the process of adjusting process parameters, various quality problems often arise due to improper parameter matching. It is necessary to eliminate defects through targeted adjustments. The following are the solutions to common problems. Insufficient hardness: It is mostly due to low heating temperature, insufficient holding time or insufficient cooling rate. Solution: Increase the heating temperature by 20-30℃ (not exceeding the critical value for grain coarsening), and extend the holding time by 10%-20%. If the cooling rate is insufficient, the gas pressure can be increased by 0.1-0.2MPa or 10%-20% helium can be mixed in. When the hardness of the 20CrMnTi gears in a certain gear factory was insufficient, the nitrogen pressure was increased from 0.3MPa to 0.4MPa, and the hardness was raised from HRC55 to HRC58, meeting the design requirements. Workpiece cracking: mainly caused by overly rapid cooling or uneven heating. Adjustment method: Adopt segmented cooling (rapid cooling at high temperature and slow cooling at low temperature), and reduce the pressure in the low-temperature section (300-200℃) by 0.1-0.2MPa. For thick and large workpieces, extend the heating time and add a 600℃ holding stage (holding for 1 hour) to relieve stress. After the Cr12MoV mold in a certain mold factory cracked, the cracking rate was reduced from 8% to 1% by increasing the temperature at 600℃ and lowering the cooling pressure. Excessive deformation: It is related to uneven distribution of temperature gradient and cooling rate. Solution: Optimize the furnace loading method
