FERRITIC STAINLESS STEEL

Ferritic stainless steels are characterized by having the same crystalline structure as pure iron, specifically a body-centered cubic (BCC) lattice, also known as the α-phase. This crystalline configuration imparts particular mechanical and magnetic properties, such as their ferromagnetism and dimensional stability, making them suitable for various industrial and domestic applications. 

Their development dates back to the period between 1904 and 1911, when various metallurgists and chemists began experimenting with iron-chromium alloys. Although there were previous attempts, it was the Frenchman Léon Guillet who managed to synthesize a ferritic stainless steel with sufficient toughness for large-scale use. Early alloys were excessively hard and brittle due to the interaction of iron and chromium with carbon, which prevented their practical application. For this reason, Guillet is rightly considered by many as the true father of stainless steel, especially in its ferritic variant. 

This family of steels represents the simplest and most accessible form within the universe of stainless steels. Their production is relatively economical, and their uses are predominantly functional and everyday. They are primarily used for their corrosion resistance, although this is limited compared to other families like austenitic or duplex steels. The chromium content typically ranges around 12%, though it can oscillate between a technical minimum of 10.5% and a maximum of 25%, provided that the carbon content is kept low and martensite formation is not induced by heat treatment.

The ferritic structure remains stable due to the partial substitution of iron atoms by chromium atoms, which allows for the formation of a passive film of chromium trioxide (Cr₂O₃) on the metal's surface. This layer is responsible for oxidation resistance, although its effectiveness directly depends on the proportion of chromium and the chemical environment. The hardness of ferritic steel is conditioned by the carbon content: a higher proportion leads to greater rigidity and hardness, but also greater brittleness. In steels like AISI 420, the hardness is between that of mild carbon steel and low-carbon martensitic steel. Generally, ferritics are not subjected to heat treatments, as their primary virtue lies in their chemical resistance rather than their mechanical properties.

These steels are malleable, though with limited ductility. They possess acceptable toughness and moderate hardness. Their corrosion resistance is the minimum required to be considered stainless, and it largely depends on the chromium content. A typical ferritic steel with up to 12% chromium can easily resist contact with alcohols, organic substances like foods and beverages, sweat, blood, fresh water, humid air, and moderately aggressive oxidizing acids. They are compatible with nitric acid (HNO₃), but suffer considerably in the presence of salt water, reducing acids, and halogens, as these agents destroy the Cr₂O₃ layer, exposing the base metal to pitting corrosion. 

Their applications include cutlery, ornamental elements, low-stress bearings, table knives, components for the food industry, and parts used in paper pulp processing. Representative examples of this family are AISI 410, AISI 420—the most popular—and AISI 430 steels. Due to their simple composition and low production cost, ferritic stainless steels are the most economical on the market.