Structural analysis of AISI-SAE 4140 steel nitrided by post-discharge microwave

AISI-SAE 4140 steel pieces were nitrided in an experimental microwave post-discharge prototype reactor during 15 min at 820 K. The microstructure of the nitrided layer was analyzed by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Microhardness measurements were made to determine the microhardness–depth correlations. Molecular simulation techniques were also applied to optimize the microstructural analysis considering models based on its crystalline structure. The results obtained show a compact layer with an average thickness of 12 μm composed of ɛ-Fe2–3N and γ′-Fe4N nitrides. Both phases were identified by X-ray diffraction and HRTEM. The following crystallographic relations were determined: (0002)ɛ-Fe2–3N∣∣(111)γ′-Fe4N and [1 1 2¯ 0]ɛ-Fe2–3N∣∣[1 1¯ 0]γ′-Fe4N. The EDS analysis shows a N concentration profile from the top surface to the core material that can be correlated with the microhardness results.