Vacuum furnace manufacturers tell you a summary of vacuum furnace heating lag time

Summary of the research on the heating lag time of vacuum furnace:
I. Main factors affecting the heating lag time
a. Heating temperature
Principle: When the heating temperature increases, the thermal radiation efficiency of the component increases to the fourth power (radiation energy ∝T⁴), the heating speed of the workpiece increases, and the lag time shortens.
Experimental support: When the heating temperature of 40CrMnSiMoVA steel increases from 870℃ to 920℃, the lag time is significantly reduced, and the high radiation energy accelerates the burning process.
b. Heating method
Preheating effect: By comparing the step heating and direct heating of preheating, it is found that preheating can reduce the temperature difference between the workpiece and the furnace, and reduce the thermal stress and deformation risk of subsequent heating.
Applicable scenarios: Materials with poor thermal conductivity (such as stainless steel) and steels with a carbon content of >0.4% need to be preheated (650~700℃ or superimposed with 850~900℃ preheating); complex/sharp-cornered workpieces and workpieces with a hardness of >35HRC need to be preheated to reduce the risk of cracking.
c. Furnace loading quantity and loading method
Size influence: Under the same material, the lag time of φ50 mm diameter bar is about 40% longer than that of φ35 mm. The long heat conduction path of large-sized workpieces leads to obvious lag.
Furnace loading density: dense furnace loading will lead to heat shielding, the radiation absorption of outer ring workpieces is better, and the core temperature rise lags significantly (need to extend the heating time or change the furnace layout).
Ⅱ. Core method for determining heating lag time
a. Actual measurement method (direct tracking thermocouple method)
Operation: Fix the thermocouple on the surface or core of the workpiece for real-time temperature measurement, and directly obtain the temperature time interval.
Applicable scenarios: Single-chamber vacuum furnace (such as vacuum brazing, annealing), which can accurately match the timing of furnace discharge to avoid over/under burning.
b. Simulation method (conditional proximity method)
Steps: Select typical workpieces to establish a heating model (such as different thickness/materials), and form a database reference after measuring its heating curve.
Case: The heating curves of 40CrMnSiMoVA steel bars with a diameter of φ35~50 mm and a length of 150 mm at 920℃ are significantly different, and key parameters need to be recorded in layers.
c. Empirical method (extending the air furnace time)
Principle: The heat transfer efficiency in a vacuum furnace is lower than that in an air furnace, and the air furnace insulation time × 1.5 is used to roughly estimate the lag time. Exceptional conditions: Convective heat transfer is enhanced under high inflation pressure (≥75 kPa), and no additional time compensation is required.
III. Supplementary information on heat treatment after vacuum brazing
a. Temperature and time control: Taking 6061 aluminum alloy as an example, after 6 hours of solution treatment at 530℃, combined with multi-stage aging (T6I6), corrosion resistance can be improved (intergranular corrosion is converted to uniform corrosion).
b. Necessity of vacuum and humidity: High vacuum (≤10-³Pa) can inhibit oxidation and loss of volatile elements; humidity >50% can easily lead to hydrolysis of brazing filler metal to form Al₂O₃ inclusions.
IV. Conclusion and optimization measures
a. Improvement of process standardization
When mixed loading, the holding time is determined based on the workpiece with the maximum effective thickness to avoid overburning of thin parts or underburning of thick parts; preheating is quantified to 0.5~1 times the holding time, or adjusted through actual measurement.
b. Balance between efficiency and quality
Precision parts are recommended to be accurately controlled by actual measurement, and general parts can be improved by simulation; cost-sensitive scenarios allow empirical estimation (combined with risk warning).
c. Equipment parameter adaptation High-performance vacuum furnaces with a uniform temperature of ±3℃ are suitable for complex workpieces (such as heat dissipation strips), combined with radiation screen material optimization (such as polyacrylonitrile-based graphite felt with a thermal conductivity of 0.06 W/m·K) to reduce thermal inertia.
SIMUWU vacuum furnace manufacturer summarizes for you:
The heating lag time is determined by material properties, equipment heat transfer efficiency and process parameters. It is necessary to combine experiments and experience with dynamic management to ensure process stability and optimal economic benefits.

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