Iron-phosphorus-carbon system Part 3 - Metallography of low carbon iron-phosphorus alloys

Metallographic examination of archeological phosphoric iron and specially p repared iron-phosphorus alloys sometimes reveals unusual 'ghosting' feature s within ferrite grains when nital is used as an etch. The ghostlines mark the boundaries in phosphorus content arising from the dual phase ferrite-au stenite microstructure, which existed at elevated temperatures. Nital prefe rentially attacks iron with lower phosphorus content, so giving a surface r elief that characterises the phosphorus. Both ferrite-austenite and austeni te-ferrite transformations occur by nucleation and growth, and also are acc ompanied by the diffusion of phosphorus. The morphologies of phases formed conform to the Dube classification system. The microstructures observed in archeological irons are consistent with those produced by the ferrite-auste nite transformation. Hence, understanding of the microstructures produced i n this current work allows fuller interpretation of the microstructures fou nd in archeological artefacts. MST/4404 (C) 2000 IoM Communications Ltd.