Phase transformations in the nitrocarburizing surface of carbon steels revisited by microstructure and property characterizations

Ferritic nitrocarburizing is a widely employed industry process by which a strengthened nitrocarburized surface can be formed on mechanical tools or parts made of steels. Mainly composed of the ε-Fe2-3(C, N) and γ′-Fe4(C, N) carbonitride phases as well as the α-Fe phase, the nitrocarburized surface has featured microstructures and properties that are directly related to the phase transformations occurring in the surface layers. Thus far, the following phase transformation sequence for the surface has generally been accepted: α-Fe + N/C → ε → γ′. In the present work, these phase transformations were systematically revisited by microstructure and property characterizations in association with a controlled nitrocarburizing process by which the microstructures and properties of the surface are adjustable. Our study demonstrates that, to fully understand the microstructure and the property of a nitrocarburized surface, it is necessary to adopt the phase transformation sequence α-Fe + N/C → γ-N/C + N/C → γ′ + N/C → ε, in which the existence of a transitional austenite phase containing N/C atoms (γ-N/C) must be assumed and the γ′ phase actually forms prior to the ε phase.