Grain refinement of Fe4N compound layer in nitrided steel

This study elucidated the microstructure of the Fe₄N (γ’) compound layer in specimens subjected to the nitriding to the specimens with various finished surfaces. Gyrofinishing produces a specimen surface with an ultra-fine-grained (UFGed) structure and a rough surface, whereas the buff-finished specimen exhibited a coarse grain but smooth surface. The γ’ grains formed on the gyrofinished specimens after nitriding were considerably finer and more equiaxial compared with those on the buff-finished specimen. The γ’ nucleated from the grain boundary in the matrix, indicating that the grain boundaries act as a nucleation site. Therefore, the UFGed structure promotes the nucleation of γ’ during the nitriding, resulting in fine grains. Numerous pores were formed in the γ’ layer of the gyrofinished specimen owing to the rough surface. The UFGed structure and surface roughness before nitriding are key factors for controlling the microstructure in the γ’ compound layer.