As a welding engineer, having a deep understanding of metals is essential. This is especially true for ferrous alloys since they form the foundation of modern engineering structures. Without these materials, our world today – filled with bridges, skyscrapers, power plants, and industrial machinery – would not exist.

Iron is a chemical element (Fe) with an atomic mass of 55.85. In nature, it exists in four crystalline forms depending on temperature and pressure:

1. Alpha iron (α-Fe) with a body-centered cubic (BCC) structure.
2. Gamma iron (γ-Fe) with a face-centered cubic (FCC) structure.
3. Delta iron (δ-Fe) with a BCC structure.
4. Epsilon iron (ε-Fe) with a hexagonal close-packed (HCP) structure.

Pig iron is the intermediate product obtained from extracting iron from its ores  – haematite, Fe₂O₃  and magnetite, Fe₃O₄. It is the direct liquid product of a blast furnace (see picture A). Picture B shows the typical chemical compositions of a Pig iron. From the engineering point of view, Pig iron is useless. It has poor plastic properties due to its high carbon content. It cannot be drawn, forged, bent, extruded, rolled, etc.

Pig iron is the intermediate product obtained from extracting iron from its ores – haematite, Fe₂O₃ and magnetite, Fe₃O₄. It is the direct liquid product of a blast furnace (see picture A). Picture B shows the typical chemical compositions of a Pig iron. From an engineering point of view, Pig iron is useless. This is because it has poor plastic properties due to its high carbon content. It cannot be drawn, forged, bent, extruded, rolled, etc.

To improve the pig iron, the high elemental contents are reduced. It is smelted in a converter where the C, Si and Mn contents are reduced by oxidation process. Oxygen or air is blown into the pig iron. This occurs in a Bessemer converter, Oxygen converter, or Open-hearth process (Picture C). The resultant is a granular pasty iron which is called wrought iron. Picture D shows the weight percent elements of a Wrought iron.

When the carbon content in pig iron is reduced in a range of 0.02 – 2.06 wt %, steel is produced. Steel can then be alloyed with various elements like Cr, Ni, Mo, Ti, Nb, W, Cu, Al, Co etc. to create different grades, each with unique properties tailored for specific applications.

Contributor: Dr VCI