01 Type of heating medium

When metals are heated at high temperatures, reactions with the heating medium occur on their surfaces, so the suitability of their selection is an important issue. Atmospheric heating is the least expensive heating method and is often used for annealing and tempering materials. However, oxidation and decarburization are problems with atmospheric heating.

Normally, steel superheated at high temperatures reacts with atmospheric oxygen to form a thick oxide layer on the surface, and a decarburization layer exists immediately below the oxide layer. Naturally, the higher the superheat temperature, the thicker the oxide layer on the surface, but the amount of moisture also plays a major role in the formation of the decarburized layer.

In heat treatment, to prevent this oxidation and decarburization, the heating atmosphere control, and even the salt bath and even a vacuum.

02 Atmospheric Gas Characteristics

Various atmosphere-controlled heating methods using protective gases are used to prevent or reduce oxidation and decarburization when steel is heated. As shown in the figure below, there are various gases used for atmosphere control, including inert gases, oxidizing gases, and reducing gases, and these gases are used alone or in combination in industry.

Since it is expensive to purchase and use individual gases in cylinders, etc., metamorphic gases and pyrolysis gases are used when used in large quantities. Incidentally, there are exothermic and endothermic types of metamorphic gases, and there are ammonia decomposition gases and alcohol decomposition gases as pyrolysis gases.