Role of Grain Boundaries in diffusional Phenomena during Gas Nitriding of Pure Iron

Abstract

It is known that grain boundaries are preferential ways to enhance diffusion during thermo-chemical treatments like nitriding. When a component is gas nitrided, its microstructure (grain boundaries’ orientation) may prevent diffusion of nitrogen in one direction, and enhance it along another one. Surface mechanical properties reflect the chemical anisotropy and components can be discarded after thermo-chemical treatments. In present work, samples of pure iron (Fe - ARMCO) and AISI 304 stainless steel have been rolled to a different thickness reduction to impart different anisotropy ratios. Rolling process has been conducted at room temperature to avoid recovery or recrystallization of the microstructure. Samples have then been gas nitrided at 520°C. Samples have been characterised with different techniques after the thermo-chemical treatment. Microstructure and crystal orientation have been investigated by optical and scanning electron microscopy (equipped with EBSD system). Surface mechanical properties have been evaluated through scratch and microhardness tests, both along rolling and normal direction to evaluate differences due to anisotropy. Experimental investigations demonstrated that grain boundaries’ anisotropy reflects on mechanical properties and diffusion involved in gas nitriding.