The atmosphere inside a mesh belt furnace is not just hot air — it is a precisely controlled chemical environment that determines whether the parts come out bright and clean or oxidized and decarburized. Atmosphere control separates a quality heat treater from a mediocre one.
MONTE INTELLIGENCE mesh belt furnaces operate with various atmosphere systems depending on the process requirements. This article covers the three most common controlled atmosphere types, their generation equipment, and the control parameters that determine atmosphere quality.
Endothermic gas — "endo gas" in heat treating shops — is the workhorse atmosphere for neutral hardening, carburizing, and carbonitriding of steel parts. It is produced by reacting natural gas (or propane) with air in an external generator at about 1050°C, over a nickel catalyst. The reaction is approximately CH4 + 2.38 (0.21 O2 + 0.79 N2) → CO + 2 H2 + 1.88 N2, producing a gas that is roughly 20% CO, 40% H2, and 40% N2 by volume.
The carbon potential of endo gas — its ability to add or remove carbon from the steel surface — depends on the CO/CO2 ratio and the furnace temperature. At 850°C, an endo gas with a dew point of +5°C has a carbon potential of approximately 0.35% C. Reducing the dew point to -5°C raises the carbon potential to about 0.60% C. The relationship is governed by the water-gas shift reaction: CO + H2O ↔ CO2 + H2, which means that controlling the water vapor content (dew point) controls the carbon potential.
The endothermic generator itself is a critical piece of equipment. It consists of a heated retort filled with nickel catalyst, through which the air-gas mixture passes. The reaction is endothermic — it absorbs heat — hence the name. The retort operates at 1000-1100°C and must be constructed of high-temperature alloy, typically RA330 or Incoloy 800HT, with a design life of 3-5 years of continuous operation. Retort failure is a common cause of unplanned furnace downtime, and every heat treating plant with endo gas should keep a spare retort on the shelf.
The catalyst in the generator degrades over time due to carbon deposition (coking) and sulfur poisoning from the natural gas. Sulfur is the bigger problem: nickel catalyst is permanently poisoned by sulfur at concentrations as low as a few parts per million. Natural gas specifications typically allow up to 30 ppm of sulfur, which is far above the catalyst tolerance. A sulfur removal bed — activated carbon or zinc oxide — upstream of the generator is essential, and the bed must be changed every 6-12 months depending on the gas sulfur content.
Nitrogen-methanol atmosphere is the alternative to endo gas for plants that do not want to operate an endo generator. The atmosphere is created by injecting liquid methanol (CH3OH) and nitrogen gas directly into the furnace. At furnace temperature, methanol dissociates: CH3OH → CO + 2 H2, producing the same 1:2 CO:H2 ratio as endo gas. The nitrogen dilutes the mixture to achieve the desired carbon potential.
The advantage of nitrogen-methanol is simplicity — no generator, no catalyst, no retort. The system consists of a liquid methanol storage tank, a nitrogen supply (liquid nitrogen tank or membrane nitrogen generator), flow control panels, and injection nozzles in the furnace. Start-up takes minutes instead of the hours required to heat up an endo generator.
The disadvantage is cost. Liquid methanol is more expensive per unit of atmosphere produced than natural gas. At typical methanol prices of $0.40-0.60 per liter, the atmosphere cost for a mesh belt furnace consuming 40 liters per hour of methanol is $16-24 per hour, or about $380-580 per day of continuous operation. An endo generator using natural gas produces the same atmosphere volume for about 30-40% less. The choice between the two comes down to capital cost versus operating cost, and to whether the plant has the maintenance capability to operate an endo generator reliably.
Dissociated ammonia is used for bright annealing of stainless steel, copper, and brass — processes where the atmosphere must be reducing but non-carburizing. Anhydrous ammonia (NH3) is dissociated in an external unit: 2 NH3 → N2 + 3 H2, producing a gas that is 75% hydrogen and 25% nitrogen by volume. This atmosphere is strongly reducing — the high hydrogen content reduces any metal oxides on the part surface — and contains no carbon, so there is no risk of carburization or decarburization.
The dissociator operates at about 950°C with an iron-nickel catalyst in a retort similar to an endo generator but smaller because the ammonia dissociation reaction is simpler and faster. The ammonia supply requires careful handling: anhydrous ammonia is a hazardous chemical that requires dedicated storage, leak detection, and emergency response procedures. The dissociated gas is flammable because of the high hydrogen content and must be handled with appropriate gas safety practices.
Atmosphere control instrumentation has evolved from manual dew point measurement to automated carbon potential control. The modern approach uses an oxygen probe (zirconia sensor) inserted directly into the furnace hot zone. The probe measures the oxygen partial pressure in the furnace atmosphere, from which the carbon potential is calculated based on the CO content and the furnace temperature. The probe signal controls the addition of enriching gas (natural gas or propane) to maintain the carbon potential setpoint.
Oxygen probes require periodic maintenance. The probe tip must be cleaned of soot and carbon deposits, typically every 1-3 months depending on the carbon potential being maintained. The probe should be calibrated against a reference — either shim stock carbon analysis or a portable dew point meter — at least quarterly. A probe that drifts 0.05% in its carbon potential reading can produce parts that are 1-2 HRC points off target hardness, which may be the difference between a passing and failing lot.
MONTE INTELLIGENCE supplies mesh belt furnaces with integrated atmosphere systems, including endo generators, nitrogen-methanol panels, and ammonia dissociators. We also provide atmosphere control instrumentation and commissioning support.
For atmosphere system specification for your heat treatment process, contact helenxu@cnlymonte.com.
