Ferrous alloys (steels and cast irons) are produced in much larger quantities than non-ferrous alloys. This can be explained by their great range of mechanical properties and widespread use in construction (buildings, bridges, industrial plants, etc.). 

Steels are Fe-C alloys with a carbon content below 2%, although commercial steels usually contain less than 1%C. There are three categories of plain steels: low, medium and high carbon. Alloying elements, which can be metallic or non-metallic, increase the range of steels and provide enhanced properties (mechanical strength, wear resistance, corrosion resistance, hardenability, etc.).

When slowly cooled, the microstructures of carbon steels can be interpreted with the aid of the Fe-Fe₃C phase diagram. At room temperature, the stable phases are ferrite, α, Fe-α solid solution with interstitial C, and cementite, Fe₃C, a hard and brittle intermetallic compound with 6,67%C and 93,33%Fe. These two phases typically adopt a lamellar arrangement, resulting in the microconstituent known as pearlite.

In industrial processes, cooling rates are faster and equilibrium conditions are rarely attained. In these circumstances, the microstructures can be predicted from continuous cooling and isothermal diagrams. These diagrams include metastable microconstituents such as bainite (typically formed through isothermal transformation of austenite at temperatures below 500 °C) and martensite (transformation product of austenite that requires rapid cooling to supress diffusion).

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