NITRIDES AND CARBIDES PRECIPITATION AND MICROSTRUCTURE EVOLUTION DURING NITROCARBURIZING AND CARBURIZING OF Fe- 4 wt. % V ALLOY

Abstract

Improving the surface properties of engineering components is essential for enhancing their operational longevity, reducing economic losses associated with frequent replacements or repairs, and mitigating the environmental impacts of energy-intensive manufacturing and recycling processes. Surface-initiated failures, often caused by wear and corrosion, highlight the critical importance of surface treatments in materials engineering. Thermochemical surface treatments, including nitriding, carburizing, and nitrocarburizing, involve the diffusion of interstitial elements such as nitrogen (N) and carbon (C) into steel surfaces, resulting in significant improvements in wear and corrosion resistances. Although these treatments are widely applied in industrial practices, a comprehensive understanding of the underlying thermodynamic and kinetic mechanisms remains incomplete and needs more fundamental investigation. Most of the fundamental understanding in this field has been derived from nitriding studies on model binary and ternary iron-based alloys, which have significantly advanced knowledge regarding the interaction of various alloying elements in steel with inwardly diffusing N. In nitrocarburizing, both N and C are simultaneously introduced to the surface, resulting in the formation of a compound layer composed of iron carbonitrides. While the nitrocarburizing behaviour of pure iron and Fe–C alloys has been extensively studied, there remains a relative paucity of detailed fundamental investigations on the nitrocarburizing response of model iron-based alloys in the existing literature. This work investigates the microstructure evolution during nitrocarburizing and carburizing of binary Fe-V alloys. Choice of vanadium in this study is due to its strong affinity for both N and C and its prevalent use in steel alloys. Unlike the typical layer-type growth of iron-nitrides observed in nitrided Fe-V alloys, nitrocarburizing resulted in an unusual plate-type morphology of γ' and ε carbonitrides within ferrite grains, alongside ε formation at grain boundaries and at the surface. The differential solubility of carbon and nitrogen in ferrite and iron-carbonitrides played a key role, with carbon enrichment at grain boundaries facilitating ε formation, while γ' developed as plates within grains due to its low carbon solubility. The ferrite matrix exhibited a six-fold increase in hardness due to VN precipitation as platelets, with these platelets influencing the nucleation and growth of γ'. Additionally, during shorter time nitrocarburizing experiments, as N gets consumed for VN precipitation, enhancing carbon solubility in the ferrite matrix which promoted cementite formation at the surface which later transforms to ε-iron nitrides in case of Fe-4 at. % V—a phenomenon which is absent in nitrocarburizing of pure iron.