Surface Microstructure and Its Growth Behavior by Nitrocarburizing with Lithium Added Molten Salt in Fe-0.4mass%C Alloy

Abstract

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 CO₃^2- 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 Li_xFe_1-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.