The surface microstructure and growth behavior of an Fe–0.4 mass%C alloy formed by lithium-added salt-bath nitrocarburizing were investigated. The Fe–0.4 mass%C alloy was prepared by arc melting and nitrocarburized by a salt-bath containing Li⁺, Na⁺, K⁺, CNO⁻, and CN⁻ at 823 K from 0.1 h up to 10 h. A compound layer forms on the surface at the beginning of nitrocarburizing, and then an oxide layer forms on the compound layer after nitrocarburizing for 1.0 h. Afterwards, the thickness of both layers increase. A grain boundary oxide forms at the interfaces of columnar crystals in the compound layer. The oxide consists of Li_xFe_1−xO with an NaCl-type structure, and the growth of the oxide layer is controlled by the outward diffusion of iron in the oxide layer. Meanwhile, 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.

This Paper was Originally Published in Japanese in Jpn. Soc. Heat Treatment 59 (2019) 215–221.