Controlling Furnace Atmosphere in Heat Treatment

14 January 2020

A second heat treatment effector exists in an industrial furnace, one that’s sometimes forgotten by the average layperson. Besides the heat source, the gas-fuelled or electrically energized thermal energy flowing evenly around a subject workpiece, there’s the atmosphere inside that sealed chamber to consider. Sometimes, the atmosphere is totally taken out of the heat treatment formula, so the process takes place in a vacuum. At other times, that atmosphere becomes an essential process agitator.

Reviewing Atmospheric Effectors

In a regular heat treatment operation, the air itself functions as a heat load or thermal conductor. The currents convect the thermal energies from the walls of the furnace to the workpiece. Radiated heat sources function differently, without the need for air. Importantly, air can be pumped out of a vacuum-sealed chamber to add more control to the process. Alternatively, a regular atmosphere can be replaced by a second gaseous medium. This medium facilitates the formation of different surface protection finishes. Better than a coating, the gas actually impregnates the outer casing of the alloy and transforms this surface layer into a mechanically and chemically desired finish. Essentially, just like a controlled oxidation operation, the atmospherically pressurized gaseous compounds chemically alter a finite percentage of a workpiece’s surface casing.

The Demand for Controllable Metallurgical Outcomes

Even compared to twenty years ago, heat treatment technology has advanced at an unprecedented rate. Vacuum furnaces are one result of this evolutionary jump, then there are the gas-pumped furnaces. These machines pump in carbon to apply a carburizing transformative finish, which improves wear performance. Nitrogen is another gaseous medium of interest. Nitrogen atmospheres augment the annealing process. Inert argon gas environments also act as an annealing improvement agent. Even carbon dioxide, thanks to an additional oxygen atom in its chemical makeup, has become a popular supplementary furnace atmosphere. Instead of air, which only contains a small percentage of oxygen, CO2 packs a stronger oxygenated punch, so it has become something of an oxidization gas standard.

The above passages of text have barely scraped the surface of what’s possible. Modern heat treatment facilities are now working with a whole palette of different gaseous compounds. Hydrogen gas, which obviously isn’t inert, performs as a reducing agent. It purifies iron and copper oxides. Like carbon monoxide and carbon dioxide, hydrocarbons are also in use here, usually as carbon-rich compounds that divulge their chemical loads at contrasting alloy treatment temperatures. However, and this point is of critical importance, such refined chemical reactions can become corrupted. It’s, therefore, best to use clean gasses, free-flowing atmospheres that have been entirely divested of pollutants, especially water.

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