Metallography of Fe–P–C and Fe–P–C–Si–N Alloys

Abstract

The microstructures of Fe–P–C and Fe–P–C–Si–N alloys have been studied to understand the phase distributions and phase transformations during heat treatment. Iron–phosphorus alloys show ghosting features when etched with Nital, since it attacks iron with lower phosphorus content preferentially. The ghostlines mark the boundaries of phosphorus content as it existed at higher temperatures in the austenite–ferrite duplex phase region. Oberhoffer’s reagent reveals the dual-phase microstructure by depositing copper on the low-phosphorus region. The ferrite-to-austenite and reverse transformations were studied and found to occur by nucleation and growth along with diffusion of phosphorus. The morphologies of the phases conform to the Dube system. The phase distribution in dual-phase microstructures was studied using quantitative metallography and compared with the phase fractions obtained from phase diagram calculated using a thermodynamic software. Silicon and nitrogen were added to improve the toughness of the base alloy. Providing homogenization treatment in the upper ferrite region will take less time.