Why Atmosphere Selection Matters
When steel is heated to heat treatment temperatures (typically 750–1200°C), it reacts rapidly with its surrounding atmosphere. Oxygen causes scaling and oxide formation that consumes material and creates a rough, pitted surface. Carbon dioxide and water vapour cause decarburisation — the loss of carbon from the steel surface, resulting in a soft skin that can compromise fatigue life by up to 50%, reduce wear resistance, and leave the surface hardness below specification. Conversely, carbon-bearing gases can cause unwanted carburisation, increasing the surface carbon content beyond specification and creating a brittle, crack-prone surface.
The role of a protective atmosphere is to control these surface reactions: preventing oxidation and decarburisation in neutral hardening and annealing, deliberately adding carbon in carburising, or adding nitrogen in nitriding and carbonitriding. Selecting the right atmosphere for the process, the material, and the furnace type is fundamental to producing quality heat-treated components. An inappropriate atmosphere choice does not merely produce poor results — it can create dangerous safety situations, as many furnace atmospheres are flammable, toxic, or both.
This guide covers the major atmosphere types used in industrial heat treatment, how to match them to specific processes, the cost implications, critical safety considerations, and monitoring methods for ongoing quality control.
Atmosphere Types: Comprehensive Comparison
| Atmosphere | Composition | Typical Use | Relative Cost | Flammable |
|---|---|---|---|---|
| Endothermic (endogas) | 40% N2, 40% H2, 20% CO, trace CH4/CO2 | Carburising, carbonitriding, neutral hardening | Low–medium | Yes (H2 + CO) |
| Exothermic (lean) | 86% N2, 1% H2, 12% CO2, 1% CO | Bright annealing of copper, brazing (non-ferrous) | Low | No |
| Exothermic (rich) | 71% N2, 15% H2, 10% CO, 4% CO2 | Annealing of steel (where slight decarburisation is acceptable) | Low | Yes |
| Nitrogen-methanol | 40% N2, 40% H2, 20% CO (in situ cracked) | Carburising, neutral hardening (replaces endogas) | Medium | Yes |
| Pure nitrogen (N2) | 99.5–99.999% N2 | Purging, inerting, annealing of non-ferrous metals | Low–medium | No |
| Nitrogen-hydrogen (N2/H2) | 75–95% N2, 5–25% H2 | Bright annealing of stainless steel, sintering | Medium | Yes (if >4% H2) |
| Pure hydrogen (H2) | 99.9–99.999% H2 | Bright annealing of high-alloy steels, sintering, brazing | High | Yes (explosive) |
| Dissociated ammonia (DA) | 75% H2, 25% N2 | Bright annealing of stainless steel, brazing, sintering | Medium | Yes |
| Argon (Ar) | 99.99–99.999% Ar | Titanium processing, reactive metals, vacuum backfill | High | No |
| Vacuum | <10−2 to <10−5 mbar | Hardening, brazing, sintering, degassing | High (capital) | No |
Matching Atmosphere to Process
Neutral Hardening and Tempering
The goal in neutral hardening is to heat the steel to austenitising temperature and quench it without changing the surface carbon content. The atmosphere must be carbon-neutral at the specified temperature and steel carbon content — in thermodynamic terms, the carbon potential of the atmosphere must equal the carbon content of the steel being processed.
- Endothermic gas: The standard choice for batch and continuous atmosphere furnaces processing carbon and alloy steels. Carbon potential is controlled by adjusting the enrichment gas (natural gas or propane) addition rate, monitored by an oxygen probe or dew point analyser. The carbon potential is matched to the carbon content of the steel being processed — for example, 0.40% C for EN19, 0.80% C for EN31 (52100) bearing steel, or 1.00% C for high-carbon tool steels. Endogas requires an endothermic generator, which adds capital cost but produces the lowest per-cubic-metre atmosphere cost for high-volume operations.
- Nitrogen-methanol: An increasingly popular alternative to endogas, particularly for smaller furnaces, low-volume operations, or installations where the capital cost and maintenance of an endothermic generator is not justified. Liquid methanol (CH3OH) is injected directly into the hot furnace through a water-cooled lance, where it thermally cracks at temperatures above 700°C to produce CO and H2 in the same nominal proportions as endogas. Nitrogen is supplied separately from bulk liquid storage or a PSA generator. The advantages are simpler infrastructure, faster start-up (no generator warm-up needed), and lower capital cost. The disadvantage is higher running cost per hour due to methanol consumption and the need for precise flow control to achieve stable carbon potential.
- Vacuum: The ultimate neutral atmosphere. No surface reactions occur in the absence of reactive gas molecules. Vacuum hardening is the standard process for tool steels (H13, D2, A2, M2), aerospace alloys, and any application where surface integrity is paramount. The capital cost of vacuum furnaces is 3–5 times higher than equivalent atmosphere furnaces, but operating costs can be lower due to the elimination of atmosphere gas consumption and the reduced need for post-treatment cleaning.
Carburising
Carburising requires an atmosphere with a carbon potential higher than the surface carbon content of the steel, creating a thermodynamic driving force for carbon to diffuse into the surface. The enriching gas is typically natural gas (CH4) or propane (C3H8) added to the endogas or nitrogen-methanol carrier atmosphere.
- Endogas + enrichment (gas carburising): The standard atmosphere carburising process used in the majority of commercial heat treatment facilities worldwide. Carbon potential is controlled to 0.80–1.10% C at 900–940°C using an oxygen probe for real-time feedback control. Boost-diffuse profiles — where a high carbon potential (0.90–1.10% C) is used during the boost phase to maximise carbon transfer rate, followed by a lower carbon potential (0.75–0.85% C) during the diffuse phase to create a smooth carbon gradient — are used to develop a uniform and optimised case.
- Low-pressure carburising (LPC): Performed in a vacuum furnace at pressures of 5–20 mbar using acetylene (C2H2) as the carbon source, pulsed into the chamber in timed cycles. LPC produces uniform, soot-free cases even in blind holes, narrow gaps, and complex geometries where gas carburising with endogas struggles to achieve uniform carbon penetration. The capital cost is higher, but the process is faster (due to higher temperatures of 950–1050°C), produces no greenhouse emissions, and eliminates intergranular oxidation (IGO) that is inherent to gas carburising.
Annealing
Annealing atmospheres must prevent oxidation and, for precision bright annealing, must also prevent any surface discolouration or tinting.
- Carbon steel annealing: Endogas (at neutral carbon potential), nitrogen-hydrogen blends (5–10% H2), or pure nitrogen. For coil annealing of cold-rolled strip, HNX atmospheres (typically 95% N2 / 5% H2) are the industry standard, supplied from bulk liquid nitrogen and hydrogen storage.
- Stainless steel bright annealing: Requires a highly reducing atmosphere with a very low dew point (<−40°C, and ideally <−60°C for highly polished finishes). Pure hydrogen, dissociated ammonia (75% H2 / 25% N2), or high-purity N2/H2 blends (25–75% H2) are used. The high chromium content of stainless steels means they are inherently more oxidation-resistant than carbon steels, but the chromium oxide (Cr2O3) that forms is extremely tenacious and difficult to remove by pickling — hence the need for a highly reducing atmosphere to prevent its formation in the first place.
Brazing
- Copper brazing of steel components: Endogas, dissociated ammonia, or N2/H2 blends with dew points below −30°C. The atmosphere must reduce any iron oxide on the steel surface to allow the molten copper filler metal (melting point approximately 1083°C) to wet and flow by capillary action into the joint.
- Nickel alloy brazing: Vacuum (preferred, at pressures below 10−3 mbar) or very dry hydrogen with dew point below −50°C. Nickel brazing alloys (BNi series) are used for aerospace, nuclear, and high-temperature applications where flux residues are unacceptable and joint strength at elevated temperatures is required.
- Aluminium brazing: Pure nitrogen with controlled flux application (Nocolok process at approximately 600°C) or vacuum brazing with magnesium as a getter to reduce the aluminium oxide layer. Aluminium brazing is a specialised process requiring very precise temperature control (±5°C) due to the narrow window between the filler metal melting point and the base metal solidus temperature.
Sintering
Powder metallurgy sintering requires atmospheres that reduce surface oxides on the metal powder particles to enable solid-state diffusion bonding between adjacent particles. The atmosphere also controls the final carbon level of the sintered component.
- Iron-based PM parts: Endogas, N2/H2 blends (10–25% H2), or dissociated ammonia at 1100–1150°C
- Stainless steel PM: Pure hydrogen or vacuum at 1200–1350°C to reduce chromium oxides
- Tungsten carbide (cemented): Hydrogen or vacuum at 1350–1500°C with precise carbon potential control to maintain the WC/Co stoichiometry
Cost Comparison
Atmosphere costs vary significantly and can be a major factor in process economics, particularly for continuous furnaces with high gas consumption rates. The following table provides indicative costs for the UK market:
| Atmosphere | Indicative Cost (£/m³ at STP) | Notes |
|---|---|---|
| Endothermic gas | 0.05–0.15 | Lowest cost per m³. Requires capital investment in generator (£30,000–£100,000). Best for high-volume operations. |
| Nitrogen (bulk liquid) | 0.08–0.20 | Delivered cost depends on volume and contract. On-site PSA generation reduces cost to £0.03–£0.08/m³ for large consumers (>500 m³/day). |
| Nitrogen-methanol | 0.15–0.30 | Higher running cost than endogas but lower capital cost (no generator). Methanol at approximately £350–£450/tonne is the major running expense. |
| Dissociated ammonia | 0.20–0.40 | Ammonia dissociator has moderate capital cost (£15,000–£40,000). Running cost dominated by anhydrous ammonia price. |
| Hydrogen (bulk liquid) | 0.40–0.80 | Highest gas cost. On-site electrolysis is an alternative for very large consumers. Justified only where hydrogen is metallurgically essential. |
| Argon (bulk liquid) | 0.50–1.00 | Expensive, no viable on-site generation route. Used only for reactive metals (titanium, zirconium) where nitrogen pick-up is unacceptable. |
Safety Considerations
Several furnace atmospheres are flammable, toxic, or both. Safety engineering is not optional — it is a legal requirement under DSEAR (Dangerous Substances and Explosive Atmospheres Regulations 2002), COSHH, and the Gas Safety (Installation and Use) Regulations 1998.
Flammable Atmospheres
Endogas (40% H2 + 20% CO), dissociated ammonia (75% H2), nitrogen-methanol, and hydrogen are all flammable and can form explosive mixtures with air. The lower explosive limit (LEL) for hydrogen is 4% in air; for carbon monoxide it is 12.5%. Explosive atmospheres can develop inside a furnace if the gas supply fails during operation, if the furnace is opened prematurely, or if an air leak introduces oxygen into the hot chamber.
- Purging: Furnaces must be purged with inert gas (nitrogen, minimum 5 volume changes verified by flow measurement) before introducing flammable atmosphere and again before opening to air after processing. Purge flow must be verified by a flow switch or pressure switch interlocked with the atmosphere gas supply valves.
- Flame curtains: All openings on furnaces operating with flammable atmospheres must have pilot burners or flame curtains to safely burn off escaping gas. The flame curtain must be proven (UV sensor or ionisation detector) before the atmosphere supply is opened, and loss of the flame curtain must trigger an immediate atmosphere shutdown.
- LEL monitoring: Continuous LEL monitoring is recommended in the furnace area, particularly at low points where heavier-than-air gases (CO) may accumulate and in overhead spaces where hydrogen may collect. Alarms at 20% LEL (warning) and 40% LEL (automatic atmosphere shutdown) are standard industry practice.
- ATEX zoning: Under DSEAR, the area around furnace openings, gas supply equipment, and exhaust systems must be classified into ATEX zones. Electrical equipment within these zones must be rated for the appropriate zone classification.
Toxic Atmospheres
Carbon monoxide (CO) is the primary toxic hazard in heat treatment atmospheres. Endogas contains approximately 20% CO, which is immediately dangerous to life and health (IDLH) at 1,200 ppm and causes impairment at concentrations as low as 50 ppm. UK workplace exposure limits per EH40 are:
- Time-weighted average (TWA, 8 hours): 20 ppm
- Short-term exposure limit (STEL, 15 minutes): 100 ppm
Fixed CO detectors with audible and visual alarms must be installed at working height in all areas where endogas or CO-containing atmospheres are generated, supplied, or used. Personal CO monitors should be worn by all personnel working in the furnace area as a secondary protection measure. Detector calibration must be verified at least every 6 months.
Asphyxiation
Nitrogen and argon are not toxic and not flammable but displace oxygen, creating an asphyxiation hazard in enclosed or poorly ventilated spaces. This hazard is often underestimated because these gases are odourless and invisible. Oxygen depletion monitors must be installed in any area where large volumes of inert gas are stored, piped, or used — including furnace pits, gas bottle storage rooms, and the immediate area around bulk liquid tanks. The minimum safe oxygen concentration is 19.5% (normal atmosphere is 20.9%). Below 16% oxygen, impaired judgement occurs; below 10%, unconsciousness follows within seconds.
Atmosphere Quality Monitoring
Continuous monitoring of the furnace atmosphere is essential for both process control and safety. The appropriate measurement method depends on the atmosphere type and the process requirements.
Key Measurement Methods
| Parameter | Measurement Method | Application |
|---|---|---|
| Carbon potential | Zirconia oxygen probe (in situ, continuous) | Carburising and neutral hardening in endogas or N2-methanol atmospheres |
| Dew point | Chilled mirror hygrometer or aluminium oxide sensor | Carbon potential cross-check; bright annealing quality assurance; brazing atmosphere verification |
| CO / CO2 | Non-dispersive infrared gas analyser (NDIR) | Endogas composition monitoring; carbon potential calculation; safety monitoring |
| O2 | Paramagnetic or zirconia analyser | Residual oxygen in inert atmospheres; combustion air ratio control |
| H2 | Thermal conductivity cell (katharometer) | Hydrogen content in N2/H2 blends; dissociated ammonia quality |
| NH3 dissociation | Glass burette or automated IR analyser | Gas nitriding process control and Kn calculation |
For atmosphere composition data, dew point/carbon potential relationships, and equilibrium calculations, use our Atmosphere Reference tool. For carbon potential calculations and equilibrium charts, see our Carbon Potential Calculator.
Troubleshooting Common Atmosphere Problems
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Parts oxidised (scaling) | Air leak into furnace, insufficient atmosphere flow rate, negative furnace pressure | Pressure test furnace shell and all penetrations; increase flow rate; verify positive pressure (2–5 mm WG) at all openings |
| Parts decarburised | Carbon potential too low, CO2 or H2O too high in atmosphere | Increase enrichment; check endogas generator air/gas ratio; reduce dew point; verify oxygen probe calibration |
| Parts carburised (unwanted) | Carbon potential too high, enrichment valve stuck open | Reduce enrichment; verify oxygen probe reading against independent dew point; check solenoid valve operation |
| Soot on parts and furnace interior | Carbon activity above unity, excessive enrichment, furnace below setpoint temperature | Reduce enrichment; ensure furnace is at temperature before starting enrichment; check for cold zones |
| Stainless steel discoloured after annealing | Dew point too high (>−40°C), insufficient H2 percentage, air leak in muffle or retort | Lower dew point by improving gas purity; increase H2 content; locate and repair air leaks; improve gas supply filtration |
| Copper brazing filler did not flow | Atmosphere not sufficiently reducing, oxide on parts, incorrect brazing temperature | Improve dew point to below −30°C; increase hydrogen content; ensure parts are thoroughly degreased; verify furnace temperature at load |
| Ammonia smell at furnace exterior | Excessive ammonia flow, furnace retort leak, blocked or non-functioning exhaust burn-off | Reduce ammonia flow rate; pressure test retort; inspect and relight exhaust burn-off pilot |
For gas flow rate calculations and pipe sizing for atmosphere supply systems, visit our Gas Flow Calculator. For comprehensive gas system references including safety standards summaries, see our Gas Systems Reference.