Atmosphere Control in Mesh Belt Furnace: Nitrogen, Endothermic Gas, and Hydrogen Mixes
Atmosphere control is what separates a heat treatment furnace from a heat-and-oxidation furnace. The same furnace with a poor atmosphere produces parts with heavy scale that fail quality inspection. With the right atmosphere, the parts come out bright, clean, and ready for the next operation. This is not a small difference. The atmosphere cost is typically 15 to 25 percent of the operating cost of a mesh belt furnace, and the atmosphere control system represents 20 to 30 percent of the furnace capital cost. Getting it right is worth the investment.
Why Atmosphere Matters
At temperatures above 500 degrees Celsius, steel reacts with oxygen, water vapor, and carbon dioxide in the air. The reactions form iron oxide scale, decarburize the surface, and ruin the part for many applications. The purpose of a controlled atmosphere is to displace the air and provide a chemistry that does not react with the steel.
The atmosphere also plays an active role in some processes. Endothermic gas adds carbon to the steel surface (carburizing). Ammonia-bearing atmospheres add nitrogen (nitriding). Hydrogen-containing atmospheres reduce any existing surface oxide (bright annealing). The right atmosphere is the one that delivers the required surface chemistry and prevents the unwanted reactions.
Nitrogen-Based Atmospheres
Nitrogen is the most widely used protective atmosphere in mesh belt furnaces. It is cheap, readily available, and effective for many processes. Pure nitrogen is suitable for: bright annealing of copper, stress relieving of low-alloy steels, and low-temperature processes below 700 degrees Celsius.
Pure nitrogen is not suitable for hardening of carbon steel at 850 to 880 degrees Celsius. The reason: nitrogen does not prevent decarburization in this temperature range, and the carbon content of the steel surface drops. The decarburized layer is soft and can cause the part to fail in service.
For hardening of carbon steel, the standard atmosphere is endothermic gas, sometimes with a small addition of natural gas to enrich the carbon potential. The endothermic gas is produced by reacting natural gas with air in a heated retort at 950 to 1000 degrees Celsius, with a nickel catalyst. The reaction produces a gas with about 40 percent hydrogen, 20 percent carbon monoxide, 40 percent nitrogen, and trace amounts of methane and water vapor.
Endothermic Gas (Endo Gas)
Endothermic gas is the workhorse atmosphere for mesh belt furnace hardening of carbon steel. The carbon potential of the gas is controlled at the target value for the steel grade (typically 0.4 to 0.8 percent C), and the gas adds or removes carbon from the steel surface to maintain the target.
The carbon potential is controlled by a dew point analyzer or an infrared CO2 analyzer. The controller modulates the air-to-gas ratio in the endothermic generator to maintain the setpoint. The endo generator operates at 950 to 1000 degrees Celsius and consumes 0.10 to 0.15 cubic meters of natural gas per cubic meter of endo gas produced.
The disadvantage of endo gas is the generator complexity. The retort, the catalyst, the air-gas mixing system, and the carbon potential control loop all require maintenance. Modern endo generators are reliable, but the operator must still monitor the carbon potential continuously and replace the catalyst every 2 to 3 years.
Hydrogen-Nitrogen Mixtures for Bright Annealing
Stainless steel and tool steel parts that need a bright, oxide-free surface are annealed in hydrogen-nitrogen mixtures, with hydrogen content typically 25 to 75 percent. The hydrogen acts as a reducing agent to convert any surface oxide back to metal, and the nitrogen acts as a carrier gas and safety diluent.
Pure hydrogen atmospheres are also used for the brightest finish, but they require explosion-proof furnace design and extensive safety systems. The 25 to 75 percent hydrogen mixtures are safer and deliver most of the brightness benefit at lower capital cost.
The hydrogen-nitrogen mixture is supplied either as a pre-mixed gas from a supplier or as separate gases mixed at the furnace inlet. The mixing ratio is controlled by mass flow controllers, and the dew point of the mixture is monitored continuously. A high dew point (more than -40 degrees Celsius) indicates a leak or a contaminated gas supply, and the furnace should be shut down until the issue is resolved.
Hydrogen-bearing atmospheres are explosive at concentrations above 4 percent in air. The mesh belt furnace must be designed with proper purging sequences, leak testing, and emergency ventilation. MONTE INTELLIGENCE designs mesh belt furnaces for hydrogen service with redundant safety systems and a pre-purge cycle that clears the air from the furnace before each heat.
Ammonia-Based Atmospheres for Nitriding
For nitriding of specialty parts, ammonia or ammonia-nitrogen mixtures are used. The ammonia decomposes at the nitriding temperature (500 to 600 degrees Celsius) to release atomic nitrogen, which diffuses into the steel surface. The result is a hard, wear-resistant surface layer without the need for quenching.
Ammonia atmospheres are used in mesh belt furnaces for high-volume nitriding of small parts. The ammonia consumption is typically 0.5 to 1.5 cubic meters per hour on a small furnace. The waste gas from the furnace outlet must be treated to destroy the unreacted ammonia before discharge.
Atmosphere Monitoring and Control
The atmosphere in a mesh belt furnace must be monitored continuously. The key parameters are: oxygen probe (for oxygen content), dew point analyzer (for water vapor), infrared analyzer (for CO and CO2), and flow meters (for the gas supply rates). The data is logged continuously and used for process control and quality documentation.
Modern mesh belt furnace control systems integrate the atmosphere monitoring with the temperature control. If the atmosphere chemistry drifts out of spec, the controller can adjust the gas flow, modify the carbon potential setpoint, or trigger an alarm. The integration reduces the operator workload and improves the process consistency.
Selection Criteria
For buyers specifying the atmosphere system, the key questions are: what surface finish is required, what is the steel grade, what is the process temperature, and what is the available gas supply. The atmosphere system is then matched to these parameters, with the gas type, the flow rate, the control system, and the safety systems all specified.
Talk to MONTE INTELLIGENCE About Atmosphere Systems
For buyers evaluating atmosphere systems for a mesh belt furnace, MONTE INTELLIGENCE engineering can recommend a configuration that matches the process and the surface finish requirement. Visit www.cnlymonte.com/products-mesh-belt-furnace.html for case studies. For a project discussion, email helenxu@cnlymonte.com with subject line mesh belt atmosphere and details on your process recipe and surface finish.
