Endogas Atmospheres

4 August 2023

Protective atmospheres in industrial furnaces have different chemical compositions to prevent reactions (oxidation, carburization, etc.) or facilitate reactions (nitriding, carburization, etc.) in materials subjected to heat treatment, depending on the nature of the heat treatment. Protective atmospheres can consist of inert gases or gas mixtures that do not participate in reactions, as well as high-carbon potential gases. Table 1 provides information about the effects of gases used as protective atmospheres in the heat treatment industry on different metals (Note: The effect of the atmosphere gas on steel varies depending on the carbon percentage of the steel, as shown in Figure 1). The applications of endogas atmospheres are listed below:

  1. Bright hardening of steels with different carbon percentages without carburizing or decarburizing.
  2. Annealing and normalizing of steels with varying carbon percentages without carburizing or decarburizing.
  3. Bright copper or silver soldering of steels with different carbon percentages without decarburization.
  4. Carbon balancing of decarburized forged or bar steels.
  5. Sintering of medium and high carbon steels with a high probability of decarburization and requiring a reducing atmosphere in metallurgical processes.
  6. Carrier gas for gas carburizing and carbonitriding processes. Endogas atmospheres are obtained by allowing rich air-fuel mixtures to react at high temperatures through nickel-based catalysts inside a stainless steel reactor heated from the outside. Depending on the application, these atmospheres are produced by rapid cooling after a complete or partial reaction inside the reactor to prevent soot deposition and carbon dioxide formation. Endogas atmospheres are stable, easy to produce, and have a consistent chemical composition, making them suitable for protecting most of the steels subjected to heat treatment inside the furnace against oxidation and decarburization. However, due to its high reactivity with chromium, endogas can form explosive mixtures with air (due to the high content of CO and H2). It also has the disadvantage of depositing carbon at low temperatures. The production of endogas atmospheres involves measuring the fuel-air mixture to ensure it only contains the required amounts of carbon monoxide and hydrogen, without generating carbon dioxide and water vapor.

According to the given compositions, the endogas obtained from propane have a higher carbon monoxide content but a lower hydrogen content compared to natural gas. Consequently, the dew point temperature of endogas obtained from propane is lower (-15…10°C) than that of natural gas (-5…10°C). The gas composition produced by the endogas generator can be determined by taking samples from the generator outlet and using three gas analysis devices, enabling the assessment of whether the atmosphere meets the desired quality. Table 2 presents the values that need to be determined using the three gas analysis devices, and it is desired to have a carbon potential expressed as a %C value between 0.3 and 0.4. It is recommended to continue the measurements with the three gas analysis devices for at least one hour and ensure the stability of the produced endogas atmosphere. The quality of the endogas atmosphere supplied to the furnace can be continuously monitored and controlled using dew point sensors, infrared sensors used for determining the composition of different gases, and oxygen probes.

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