Effect of substrate structure on properties of nitrided layers formed on pure iron

Purpose - This paper aims to investigate the structure and scratch resistance properties of gas nitrided pure iron samples. Design/methodology/approach - The effects of material strain hardening and amount of grain boundaries exposed on nitriding surface were evaluated by cold rolling the starting samples to different reduction levels before gas nitriding. Findings - The study finds that nitriding without any prior cold rolling produced a comparatively wide compound layer with a large fraction of porous zone featuring low scratch hardness values but no evidence of damage. On the contrary, cold rolling before nitriding led to a more irregular and thinner compound layer with reduced amount of porous zone and much finer nitrides in the diffusion zone. Scratch hardness was increased but failure mechanism changed by generation of conformal cracks within the track groove and the appearance of discontinuous spallation at high loads. Originality/value - One of the issues of great industrial importance concerning nitriding of steels is the need to predict the extent of the nitrided layer in products showing small variations in microstructure or in extent of cold working due to complex manufacturing cycles. Despite the practical importance, relatively little information is available in literature about these issues. The present paper is therefore aimed at investigating the structure and mechanical properties of pure iron samples, gas nitrided with different amounts of cold working and microstructural conditions.