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How is ammonia used to harden the surfaces of alloy sheets?

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Difference in molecular structure between ammonia/amines and quaternary ammonium ions — ammonia Ammonia and amines have a slightly flattened trigonal pyramidal shape with a lone pair of electrons above the nitrogen. In quaternary ammonium ions, this area is occupied by a fourth substituent.

Ammonia is used to harden the surfaces of steel and alloy sheets through a process known as nitriding.

In this process, steel objects are heated in an atmosphere of gaseous ammonia (NH₃), typically at temperatures between 500 and 550 °C (950 and 1,050 °F). As the steel is heated, nitrogen from the ammonia diffuses into the surface of the metal. This nitrogen then combines with iron and other alloying elements present in the steel to form hard metallic nitrides, which significantly increase the hardness of the surface layer.12

Hardening occurs through two primary mechanisms:
* Solid-solution hardening: Occurs in all steels as nitrogen integrates into the metal structure.2
* Precipitation hardening: Occurs when nitrogen combines with specific alloying elements, such as aluminum, to form fine, hardening particles.2

Because this process occurs at lower temperatures, it results in less distortion of the metal compared to other hardening methods. Additionally, ammonia is sometimes used in carbonitriding, where it is added to a carbon-bearing furnace atmosphere to facilitate surface hardening at lower temperatures.32