Iron–phosphorus–carbon system: Part 2 – Metallographic behaviour of Oberhoffer's reagent

Abstract

An investigation of the microstructures found in phosphoric irons has been carried out as part of a larger archeomet allurgical investigation. Oberhoffer's reagent has been used traditionally to display the macroscopic distribution of phosphorus in steels but in order to determine the microscopic response of the reagent with certainty, a systematic study of its action has been undertaken. The reagent is in current use in the metallographic analysis of archeological iron artefacts and its action needed to be clarified before further use in the current work. A series of specimens with controlled microstructures and solute profiles has been examined when etched with nital and also with Oberhoffer's reagent. The solute distribution in some of the specimens has been confirmed using wavelength dispersive X-ray microanalysis. It has been shown that Oberhoffer's reagent responds to microstructural as well as macrostructural segregations of phosphorus and also to nickel and arsenic. The etch does not deposit copper at a significant rate on carbide or phosphide particles so that they remain bright after etching. In ferritic microstructures, there may be a slight response due to variation in grain orientation.