What is an endothermic atmosphere and when is it used?

Endothermic (endo) atmosphere is produced by partially combusting a hydrocarbon gas (typically natural gas or propane) with air over a catalyst at 950–1050°C, in an endothermic reaction that absorbs heat. The resulting gas is approximately 40% N₂, 40% H₂, 20% CO, with small amounts of CO₂ and H₂O. Endo atmosphere is the standard carrier gas for carburising, carbonitriding, and neutral hardening of steel. Carbon potential is controlled by adjusting CO₂ or dew point (H₂O content). It is flammable and requires purging procedures.

What is the difference between endothermic and nitrogen-methanol atmosphere?

Nitrogen-methanol (N₂/CH₃OH) atmosphere is generated by injecting liquid methanol into the furnace hot zone, where it cracks to CO + H₂, mixed with a N₂ carrier. The resulting composition is similar to endothermic gas (N₂/H₂/CO) but is produced on-demand without a separate generator. Advantages: no generator maintenance, faster start-up, cleaner composition with less CO₂. Disadvantage: higher methanol cost vs piped gas, and methanol handling safety. Both are widely used for neutral hardening and carburising.

How is carbon potential controlled in a furnace atmosphere?

Carbon potential (Cp) is controlled through the furnace atmosphere composition. In endothermic or N₂/methanol atmospheres, Cp is measured by dew point sensor (measures H₂O content, which is in equilibrium with C and CO at temperature) or by CO₂ IR analyser. Higher Cp causes carburisation (carbon gain); lower Cp causes decarburisation (carbon loss). Oxygen probes (zirconia sensors) provide the most direct and fast-responding measurement and are standard on modern carburising controllers. Target Cp for neutral hardening is typically matched to the steel's carbon content (e.g. Cp = 0.40% for EN8).

When is vacuum preferred over controlled atmosphere for heat treatment?

Vacuum heat treatment is preferred when: the workpiece alloy contains elements that form stable oxides even in endo atmosphere (titanium, aluminium, chromium-rich alloys); bright surface finish is required without post-treatment (aerospace, medical); the process requires very low partial pressures (vacuum brazing, diffusion bonding); or when the furnace load is small and a batch vacuum furnace is more economical than maintaining a continuous atmosphere. Vacuum is more expensive per load than endo atmosphere but eliminates scale, decarburisation, and gas handling hazards.

Industrial Furnace Atmosphere Guide — Composition, Control & Selection

Furnace Atmosphere Types — Composition & Applications

Atmosphere Type	Approx Composition	C Potential	Flammable	Typical Process
Endothermic	40% N₂, 40% H₂, 20% CO	Adjustable 0.2–1.2%	Yes	Carburising, neutral harden
N₂/Methanol	~45% N₂, 35% H₂, 20% CO	Adjustable 0.2–1.2%	Yes	Carburising, neutral harden
Exothermic (lean)	~88% N₂, 1% H₂, 11% CO₂	Decarburising	No	Annealing (non-ferrous)
Pure Nitrogen	100% N₂	Slightly decarb	No	Annealing, cooling
Nitrogen-Hydrogen	95% N₂, 5% H₂	Neutral/decarb	Yes (H₂)	Bright annealing SS
Pure Hydrogen	100% H₂	Strongly decarb	Yes	Bright annealing, sintering
Vacuum	10⁻³ – 10⁻⁵ mbar	N/A	No	Tool steel, Ti, brazing
Ammonia (dissoc.)	25% N₂, 75% H₂	N/A	Yes	Nitriding, bright anneal
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Carbon Potential Control

Dew point to carbon potential conversion table
CO₂ percentage to carbon potential (Cp) chart
Oxygen probe EMF to carbon potential calculation
Harris equation for endo atmosphere equilibrium
Cp trim enrichment gas (natural gas / propane) calculation
Carbon mass balance for carburising loads

Atmosphere Tool Features

Atmosphere selection by process type and steel grade
Purge time calculator for furnace atmosphere changeover
Hydrogen safety — LEL and purge sequence guidance
Dew point target by process temperature and Cp
N₂/methanol injection rate calculator
Atmosphere safety — NFPA 86 compliance checklist

Why Controlled Atmosphere Selection Matters

Furnace atmosphere directly determines surface quality during heat treatment. An incorrect atmosphere causes decarburisation (carbon loss, reduced surface hardness), carburisation (excess carbon, brittleness), oxidation (scale, dimensional loss), or nitriding (unintended surface hardening). For aerospace and precision engineering, AMS 2759 specifies atmosphere requirements by process type and alloy — and Nadcap audits verify compliance.

Bloor Engineering operates vacuum furnaces and controlled atmosphere furnaces, and the atmosphere guide in our platform covers both the theory and the practical measurement techniques needed to maintain specification compliance in a production heat treatment environment.

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Carbon potential calculators, atmosphere selection guide, dew point tables, and purge sequence tools. Free with a registered account.

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Industrial Furnace Atmosphere Guide — Composition & Control

Carbon Potential Control

Atmosphere Tool Features

Why Controlled Atmosphere Selection Matters

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