Introduction to Endothermic Atmospheres

Endothermic gas (endogas) is the workhorse atmosphere for carburising, carbonitriding, and neutral hardening of steels. Generated by passing a hydrocarbon (typically natural gas or propane) and air through a heated catalyst at approximately 1040°C, the resulting gas is nominally 40% nitrogen, 40% hydrogen, and 20% carbon monoxide, with residual methane and carbon dioxide.

When the system is operating correctly, endogas provides a clean, controllable carbon potential. When it is not, the consequences range from decarburisation and soft parts to soot contamination and scrapped loads. This guide covers systematic troubleshooting of the most common endothermic atmosphere problems.

1. Incorrect Carbon Potential

Symptoms

  • Parts decarburised (carbon potential too low) or sooting (carbon potential too high)
  • Dew point readings inconsistent with expected carbon potential
  • Shim stock carbon analysis does not match setpoint

Diagnostic Steps

The carbon potential of an endothermic atmosphere is governed by the equilibrium between CO, CO2, H2, H2O, and CH4. The two primary control methods are:

  • Dew point control: Measures the water vapour content of the atmosphere. A lower dew point corresponds to a higher carbon potential. Typical target for 0.80% C at 925°C is approximately −5°C to +2°C dew point.
  • Oxygen probe (carbon probe) control: Measures the oxygen partial pressure using a zirconia sensor. More responsive than dew point and preferred for modern installations.

Common Causes and Fixes

ProblemLikely CauseCorrective Action
Carbon potential reads lowAir leak into furnace or pipeworkPressure test all connections, check door seals, verify positive furnace pressure
Carbon potential reads lowEndogas generator catalyst degradedCheck catalyst temperature, inspect retort for cold spots, replace catalyst if activity has dropped
Carbon potential reads highEnrichment gas (methane/propane) valve stuck open or over-dosingCheck solenoid valve, recalibrate enrichment controller
Carbon potential unstableOxygen probe contaminated or agedClean or replace probe, verify with portable dew point meter
Carbon potential unstableGas supply pressure fluctuatingCheck supply regulator and flow meters

For atmosphere composition calculations and equilibrium charts, see our Atmosphere Reference tool.

2. Soot Formation

Symptoms

  • Black deposits on parts, fixturing, or furnace walls
  • Carbon buildup on radiant tubes or heating elements
  • Elevated methane levels in exhaust analysis

Root Causes

Soot forms when the carbon activity of the atmosphere exceeds unity — that is, the atmosphere contains more carbon than the steel can absorb at the process temperature. This commonly occurs when:

  • Enrichment is excessive: The natural gas or propane addition rate is too high for the load size, temperature, or steel grade. Reduce enrichment and verify the carbon potential setpoint is appropriate for the steel grade being processed.
  • Temperature is too low for the carbon potential: At lower temperatures, the equilibrium carbon potential drops. If the atmosphere is set for a high-temperature process and the furnace has not reached temperature, sooting will occur during heat-up.
  • Endogas generator is producing wet gas: If the air-to-gas ratio is incorrect (too rich), the generator produces gas with elevated methane. This methane cracks in the furnace, depositing soot. Check the generator ratio and catalyst temperature.
  • Insufficient gas circulation: Dead zones in the furnace allow local carbon buildup. Verify fan operation and gas flow patterns.

Resolution

  1. Verify the endogas generator air/gas ratio using a gas analyser. Target CO should be approximately 20% for natural gas feed.
  2. Check catalyst temperature (should be 1020–1060°C across the retort).
  3. Reduce enrichment gas to bring carbon potential below 1.0% C equivalent.
  4. If soot is already deposited, a controlled burnout at temperature with air admission (carefully managed) or a lean atmosphere cycle can help remove deposits.

3. Endogas Generator Problems

Low CO Output

  • Catalyst exhausted: Nickel-based catalysts degrade over time. Typical life is 3–5 years depending on gas quality and operating conditions. Test by checking outlet gas composition.
  • Retort cold spots: Cracked or damaged heating elements create cold zones where the reaction does not complete. Inspect elements and check temperature uniformity across the retort.
  • Incorrect air/gas ratio: Use a ratio controller or check the mixing valve. The stoichiometric ratio for natural gas is approximately 2.5:1 (air:gas by volume) for endothermic generation.

Wet Gas (High Dew Point from Generator)

  • Generator outlet dew point should be approximately −5°C to +5°C for natural gas feed
  • High dew point indicates incomplete reaction: check catalyst, ratio, and retort temperature
  • Ensure the gas cooler is functioning and removing condensate before the gas enters the furnace

4. Oxygen Probe Diagnostics

The zirconia oxygen probe is the most critical single instrument in a controlled-atmosphere furnace. Probe problems mimic atmosphere problems, making diagnosis challenging.

Quick Health Checks

  • Millivolt output: A healthy probe at 925°C in a 0.40% C atmosphere should read approximately 1100–1160 mV. Readings below 1000 mV or above 1200 mV suggest probe degradation or reference air supply issues.
  • Reference air flow: Verify the reference air pump is running and the filter is not blocked. Many probe failures trace back to restricted reference air.
  • Impedance check: Probe impedance increases as the zirconia element ages. An impedance above 200 ohms at operating temperature indicates the probe should be replaced.
  • Cross-check with dew point: Compare probe-calculated carbon potential with an independent dew point measurement. Agreement within ±0.05% C confirms the probe is reading correctly.

5. Air Leaks and Furnace Pressure

Air ingress is the single most common cause of atmosphere problems. Even a small leak introduces oxygen and nitrogen, disrupting the carbon equilibrium and causing decarburisation or oxidation.

Detection Methods

  • Positive pressure check: The furnace should maintain 2–5 mm water gauge (0.02–0.05 mbar) positive pressure at all openings. Use an inclined manometer or digital pressure gauge.
  • Flame test: A small, steady flame should be visible at all furnace openings (doors, peep holes). An inward-drawn flame or no flame indicates negative pressure or a leak.
  • Soap bubble test: Apply leak detection fluid to suspected joints, flanges, and penetrations while the furnace is under slight positive pressure.
  • Tracer gas: For critical applications, helium leak testing provides the most sensitive detection.

6. Safety Considerations

Endothermic atmospheres are flammable and toxic. The gas is approximately 40% hydrogen and 20% carbon monoxide. Proper safety measures must be in place at all times:

  • Furnace must be purged with nitrogen before introducing endogas (5 volume changes minimum)
  • Flame curtains or pilot burners at all furnace openings
  • CO monitoring in the work area with alarms set at 30 ppm (TWA) and 200 ppm (STEL) per EH40 workplace exposure limits
  • Burn-off pilots must be lit before starting the endogas generator
Atmosphere analysis tools: Use our Atmosphere Reference to look up gas compositions, dew point/carbon potential relationships, and equilibrium data. For furnace calculations, try the Furnace Design Calculator. Register free to get started.