CO and CO2 Control of Endothermic Gas in Heat Treatment Furnaces

It is necessary to carry out heat treatments and endothermic processes in order to guarantee high-quality alloy and metal items.

The heat treatment process requires a ‘neutral’ protective gas or carrier gas such as endothermic gas (CO/H₂/N₂), which is a common choice. In order to make sure that metal items keep their desired properties, you must have control and monitor the furnace gas composition.

In this document, the reasons behind the importance of controlling accurately the CO and CO2 levels, as well as the optimal method of measuring the levels of these gas concentrations in endothermic heat treatment furnaces will be discussed.

The physical or chemical properties of materials, like metals and alloys as well as steel, can be changed with the use of heat treatments. It is the microstructures of the metals that are altered when undergoing heat treatments, this causes an increase in their resistance to temperature, their hardness, strength and their ability to be stretched under tensile stress.

A heat treatment is a complicated process that involves carefully controlled parameters such as temperature, time and atmosphere, in order to obtain the wanted metal properties and ensure a high standard of product is produced.

Furnace Atmospheres Provide Protection and Controlled Material Modification

The use of gases in furnaces during heat treatments is typically in order to alter the metal composition or surface, by either creating a protective environment, controlled oxidation or acting as a carrier.

Endothermic gas compositions are one of the most popular atmospheres used for heat treatments and are often used as a ‘neutral’ hardening gas, or carrier gas for carbonitriding or carburizing.

The production of endothermic gas mixtures is often produced by endogas generators that increase the temperature of natural gas and air with a catalyst present. This process involves an endothermic reaction, which means it takes in heat.

Endothermic Process of Gas Composition must be Carefully Controlled to Ensure Product Quality

The function of a protective furnace atmosphere is to stop unwanted hydrogen embrittlement, surface bluing, soot formation, oxidation and decarburization from occurring. In order for the required metal treatment to be a success, you must control and monitor the gas composition with extreme care. The concentrations of gases, CO₂, H₂O, CH₄, N₂, H₂ and CO, that make up the endothermic gas atmosphere should be measured in order to aid the prevention of unwanted reactions and ensure that the endogas generator and the furnace are operating normally.

Oxidation reactions occur when there is an excess of CO₂ or any O₂ in the endothermic gas. An example of such a reaction is iron oxidation, which can be seen below:

Fe + CO2 ⇌ FeO + CO

Scale is a visible layer of iron oxide which forms on the surface of the metal and causes a dulling effect. In order to keep the brightness of steel, this oxidation reaction should be prevented by removing oxygen from the furnace and minimizing the presence of CO₂.

The properties of steels are determined by the amount of carbon in their composition, therefore it is necessary to make sure unwanted decarburization during heat treatment does not occur. Decarburization decreased the overall strength and fatigue resistance of the steel and results in the diffusion of carbon towards the surface of the steel, in the following reaction:

Fe3C + CO2 ⇌ 3Fe + 2CO

A way of preventing unwanted decarbonization is to maintain the presence of reducing gases such as an excess of CO and H₂, this works by shifting the equilibrium in the direction of the carburized product. In contrast, oxidizing gases such as O₂ and CO₂ facilitate decarbonization.

Measuring the Endothermic Process of Gas Composition in Furnaces

Dew point analyzers or oxygen sensors are used to monitor endothermic furnaces. This method, however, has the assumption that the endothermic gas mixture maintains a steady composition, which is sometimes not the case.

In an ideal situation, measurement solutions, such as CO/CO₂ analyzers, O₂ probes, and dew point analyzers, would be equipped to furnaces to allow all the necessary parameters to be controlled and monitored.

Measurement of the furnace gas composition and automatic delivery of methane and air to the endoas generator is a feature of the most advanced endothermic generators, which allows the provision of steady endothermic gas delivery to the furnace.

The furnace atmosphere can be monitored by analyzing the CO and CO₂ gas concentrations, and this gives an accurate reading of the composition of endothermic gas being delivered to the furnace. This can be done by infrared sensors, because they are easy to operate and provide quick readings, as well as producing easily understandable results.

In addition, these sensors can be seamlessly integrated into automated systems designed to change the production and delivery of endothermic gas automatically.

Infrared Gas Sensors for Endothermic Processing

The number one supplier of high-quality gas sensing solutions is Edinburgh Sensors. This includes an extensive range of infrared sensors that allow the user to reliably measure CO and CO₂.

Nondispersive infrared sensors (NDIR) sensors is provided by Edinburgh Sensors, which can be used as complete gas monitors (Guardian NG Series) and OEM infrared sensors specifically created for the purpose of integration into a huge variety of systems.

A common issue with IR sensors is the effect of their function in their exposure to temperature or pressure, however those from Edinburgh Sensors provide detailed temperature and pressure correction to make sure results are precise in a huge range of environments.

References

  1. ‘Practical Heat Treating: Second Edition’ – Dossett JL, Boyer HE, ASM International, 2006.
  2. ‘Heat Treatment: Master Control Manual’ – Bryson WE, Hanser Fachbuchverlag, 2015.
  3. ‘Heat Treatment of Steels – The Processes’ – https://www.azom.com/article.aspx?ArticleID=543
  4. ‘Considerations in Heat Treatment. Part One: Furnace Atmospheres’ – Herring DH, industrialheating.com, 2009.
  5. ‘Principles and Use of Endothermic Gas Generators’ – http://heat-treat-doctor.com/documents/Endothermic%20Gas%20Generators.pdf
  6. ‘An Overview of Endothermic Generators’ – http://www.mcgoff-bethune.com/furnace/endo.pdf
  7. ‘Thermal Process Information Book’ – ASM International, Heat Treating Progress, 2008.
  8. ‘Decarburization – Metallurgical Processes’ – https://www.azom.com/article.aspx?ArticleID=9624
  9. ‘Non-Dispersive Infrared Gas Measurement’ – Wong JY, Anderson RL, IFSA Publishing, 2012.
  10. ‘Handbook of Gas Sensor Materials: Properties, Advantages and Shortcomings for Applications Volume 1: Conventional Approaches’ – Korotcenkov G, Springer, 2013.
  11. ‘Edinburgh Sensors’ – https://edinburghsensors.com/about/about-us/
  12. ‘OEM Gas Sensors’ – https://edinburghsensors.com/products/oem/#products
  13. ‘Gas Monitors’ – https://edinburghsensors.com/products/gas-monitors/

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Sensors.

For more information on this source, please visit Edinburgh Sensors.

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