Microstructural and Phase Evolution Behavior of Compound Layers of Controlled Gaseous Nitrided AISI 1015 Steel

In this study, the gaseous nitriding was carried out for enhancing surface properties of AISI 1015 steel, and the formation of compound layer was investigated accordingly. For AISI 1015 steel, the nitriding potential (K_N) was measured in the hydrogen and nitrogen atmospheres according to the decomposition of ammonia (NH₃) in the nitriding atmosphere. The formation behavior of the compound layer was observed for various process conditions. In the nitriding atmosphere, the steel surface was oxidized to Fe₃O₄ and then reduced by NH₃ to form a compound. During the initial nitriding, the γ′ (Fe₄N) phase was formed inside owing to the concentration gradient after the formation of ε phase at a high K_N. Both ε (Fe_2–3N) and γ′ phases grow over time, and finally, a compound with three layers of γ′/ε/γ′ is formed. The formation mechanisms of these nitrided compounds were thermodynamically compared and analyzed using the CALPHAD (CALculation of PHAse Diagrams) method. Based on the modified Lehrer diagram, the formation mechanisms of the ε and γ′ phases during the nitriding process could be identified.