Abstract: TEM has been used to investigate the microstructures of the surface layer formed on 40Cr steel by nitrogen ion implantation. Different microstructures at various depths have been observed. In the outermost layer and subjacent layer, the ferrite grains have been refined, the nitrides α″-Fe16N20 γ′-Fe4N, ε-Fe3,3 (C. N) have precipitated and carbides Fe3C disappeared, while in the transition layer adjacent to matrix, the microstructure consists of a dispersion of tiny Fe10N2 precipices in the perfect ferrite matrix. The process of ferrite grains subdivided into ultra-fine grains and disappearance of retained carbides, formation of disperse nitrides were discussed qualitatively.

Key words: 铬钢, 离子注入, 显微组织