Atmosphere box furnaceThe atmosphere control is the core link to ensure the process effect of material sintering, annealing, brazing, etc. Different atmospheres (inert, reducing, oxidizing, etc.) need to be matched with corresponding control processes. The following are specific atmosphere control methods and operating points:
1、 Atmosphere type and applicable scenarios (choose the right atmosphere first)
Firstly, the type of atmosphere needs to be determined based on the process requirements. Common types and suitable scenarios are as follows:
Inert atmosphere (nitrogen, argon): suitable for heat treatment of easily oxidizable materials (such as copper, titanium alloys, lithium battery materials) to prevent the material from reacting with air at high temperatures.
Reductive atmosphere (hydrogen, ammonia decomposition gas): Used for metal oxide reduction (such as copper oxide reduction to pure copper), stainless steel bright annealing and other processes, the hydrogen concentration needs to be strictly controlled to avoid explosion risks.
Oxidative atmosphere (compressed air, oxygen): suitable for ceramic material sintering, metal surface oxidation treatment, etc., to enhance material surface properties.
Mixed atmosphere (such as nitrogen hydrogen mixture, argon hydrogen mixture): balancing protection and reduction effects, commonly used for heat treatment of precision alloy components.
2、 General Atmosphere Control Process (Basic Operating Steps)
Regardless of the atmosphere, the core process of "furnace chamber pretreatment → atmosphere filling → temperature linkage control → atmosphere maintenance → cooling and pressure relief" must be followed:
Pre treatment of furnace cavity (key step of clearing air)
Close the furnace door and lock it, connect the corresponding atmosphere gas source, and open the furnace chamber exhaust valve.
Introduce a small amount of atmospheric gas (with a flow rate of 1.5 times the working flow rate) and continue blowing for 10-20 minutes to replace the air in the furnace chamber (if it is a reducing atmosphere, the oxygen content should be reduced to below 50ppm).
The oxygen concentration in the furnace can be monitored in real time through an oxygen content detector, and the exhaust valve can be closed after reaching the standard.
Atmosphere filling and pressure regulation
Adjust the gas source pressure reducing valve to control the intake pressure at 0.01-0.05MPa (slightly positive pressure state to prevent air backflow), and set the gas flow meter to the required process flow rate (usually 0.5-2L/min, adjusted according to the furnace chamber volume).
For processes with high requirements for airtightness, furnace chamber pressure monitoring can be activated to maintain stable pressure within the set range and avoid atmosphere leakage caused by pressure fluctuations.
Temperature atmosphere linkage control
Start the heating program and maintain a continuous atmosphere during the heating process
Low temperature stage (<300 ℃): Increase the gas flow rate appropriately and quickly remove the trace gases released by the furnace chamber materials during the heating process;
High temperature stage (process temperature): stabilize the gas flow rate and maintain uniform composition of the atmosphere inside the furnace.
If it is a reducing atmosphere, it is necessary to achieve the oxygen content standard in the furnace chamber before heating to 200 ℃. It is forbidden to directly introduce flammable and explosive gases such as hydrogen at high temperatures.
Maintain the atmosphere during the insulation phase
Real time monitoring of atmosphere flow rate, furnace pressure, and oxygen content during the insulation process. If there is a decrease in flow rate, promptly check the remaining gas source and whether the pipeline is blocked; If the oxygen content exceeds the standard, increase the blowing flow rate and investigate the leakage point.
For a circulating atmosphere furnace, it is necessary to turn on the atmosphere circulating fan to ensure uniform atmosphere inside the furnace and avoid local differences in atmosphere composition affecting process consistency.
Cooling and pressure relief (to prevent material oxidation)
Turn off the heating system and continue to maintain the atmosphere until the furnace temperature drops below 200 ℃ (easily oxidizable materials need to be cooled to room temperature).
If it is for rapid cooling at high temperatures, the furnace cooling system can be turned on under atmosphere protection, and it is strictly prohibited to cut off the atmosphere during the cooling process.
After the furnace temperature reaches the standard, close the intake valve, slowly open the exhaust valve to release pressure to atmospheric pressure, and then open the furnace door to retrieve the parts.
3、 Key points for special control of different atmospheres
Inert atmosphere (nitrogen/argon) control
Prioritize the use of high-purity gases (purity ≥ 99.999%) to avoid material oxidation caused by trace amounts of oxygen and water in the gas.
If there is residual moisture in the furnace chamber, a molecular sieve dryer can be used in the pretreatment stage to reduce the dew point of the atmosphere to below -40 ℃.
Control of reducing atmosphere (hydrogen/ammonia decomposition gas)
It must be operated in a well ventilated environment, and the furnace body must be equipped with a hydrogen leak detector and explosion-proof device. Hot work near the furnace body is strictly prohibited.
The ammonia decomposition gas needs to be purified by a purification device to remove residual ammonia before being introduced into the furnace chamber to prevent residual ammonia from corroding the furnace body and workpieces.
When shutting down the furnace, the hydrogen gas source should be cut off first, and inert gas should be introduced to blow the furnace chamber for 30 minutes before opening the lid.
Mixed atmosphere control
Use a dedicated gas proportioner to accurately control the ratio of each gas (such as 95% N ₂+5% H ₂), with a proportioning error of ≤± 0.5%.
Before mixing, it is necessary to confirm the compatibility of the two gases, and it is strictly prohibited to directly mix combustible gases with oxidizing gases.
4、 Key equipment and monitoring methods for atmosphere control
Core control accessories
Gas flowmeter/mass flow controller: Accurately adjust the atmosphere flow rate, it is recommended to choose a model that can be digitally displayed for easy parameter recording;
Pressure sensor: monitors the micro positive pressure in the furnace chamber to prevent negative pressure from causing air backflow;
Oxygen content analyzer/dew point meter: Real time monitoring of atmosphere purity to ensure process reliability.
Common exceptions and their handling
Sudden drop in atmosphere flow: Check the pressure of the gas cylinder and whether the pipeline joints are loose, and replace the gas cylinder or tighten the joints in a timely manner;
Excessive oxygen content in the furnace: Check whether the sealing gasket of the furnace door is aging, whether the pipeline is leaking, replace the sealing parts, and re blow the furnace chamber;
Abnormal increase in pressure: Open the exhaust valve to release pressure, check if the exhaust pipeline is blocked, and clean the pipeline impurities.
5、 Safety and operational precautions
Pipelines with flammable and explosive atmospheres (hydrogen, ammonia decomposition gas) should use leak proof joints, and the furnace body should be well grounded to prevent static electricity from causing danger.
Atmosphere gas cylinders should be stored separately (inert gases and reducing gases should be placed separately), away from heat sources and open flames, and the cylinder pressure gauge should be checked regularly.
Before changing the atmosphere type, it is necessary to thoroughly blow the pipeline and furnace chamber with inert gas to avoid reactions caused by mixing different atmospheres (such as mixing hydrogen and oxygen).
Establish an atmosphere control parameter ledger to record the gas type, flow rate, pressure, and oxygen content data for each process, facilitating process traceability and optimization.