Iron–phosphorus–carbon system: Part 3 – Metallography of low carbon iron–phosphorus alloys

Metallographic examination of archeological phosphoric iron and specially prepared iron–phosphorus alloys some times reveals unusual ‘ghosting’ features within ferrite grains when nital is used as an etch. The ghostlines mark the boundaries in phosphorus content arising from the dual phase ferrite–austenite microstructure, which existed at elevated temperatures. Nital preferentially attacks iron with lower phosphorus content, so giving a surface relief that characterises the phosphorus. Both ferrite–austenite and austenite–ferrite transformations occur by nucleation and growth, and also are accompanied by the diffusion of phosphorus. The morphologies of phases formed conform to the Dubé classification system. The microstructures observed in archeological irons are consistent with those produced by the ferrite–austenite transformation. Hence, understanding of the microstructures produced in this current work allows fuller interpretation of the microstructures found in archeological artefacts.