The surface microstructure and growth behavior of an Fe-0.4mass%C alloy formed by lithium-added salt-bath nitrocarburizing were investigated. The Fe-0.4mass%C alloy was prepared by arc melting and nitrocarburized by a salt-bath containing Li+, Na+, K+, CNO-, CN-, and CO32- at 823 K for 0.1 up to 10 h. A compound layer is formed on the surface at the beginning of nitrocarburizing, and then an oxide layer is formed on the compound layer after nitrocarburizing for 1.0 h. After that, the thickness of both layers increases. Internal oxide was formed at interfaces of columnar crystals in the compound layer. On the one hand, the whole oxide consists of LixFe1-xO with an NaCl-type structure, and the growth of the oxide layer is rate-determining by diffusion of iron in the oxide layer. On the other hand, the compound layer consists mainly of an ε-Fe2(N,C)1-y phase and a slight γ’-Fe4(N,C)1+z phase near the substrate.
The remarkably high work hardening rate in high nitrogen austenitic stainless steels is generally believed to be due to the promotion of dislocation accumulation by nitrogen addition. However, analysis of dislocation accumulation behavior by the modified Williamson-Hall/Warren-Averbach method reveals that no difference exists between austenitic steels with and without nitrogen in the increment of dislocation density during deformation. Since cross slipping is markedly suppressed in high nitrogen steels, the moving dislocations are back-stressed by the planar dislocation arrays. This leads to the deformation resistance and high work hardening rate.