Near-surface composition and microhardness profile of plasma nitrided H-12 tool steel

Samples of AISI H-12 tool steel were plasma nitrided at 500°C in a mixture of H2–20% N2 under a total pressure of 6 mbar, by using DC and pulsed glow discharges. The treatment time varied from 1 to 6 h. X-ray diffraction (glancing angle and θ–2θ geometry), conversion electron Mössbauer spectroscopy, conversion X-ray Mössbauer spectroscopy, electron probe microanalysis, optical micrograph and Vickers microhardness were used as analytical techniques. The obtained results suggest that, under the present experimental conditions: (i) The near-surface compound layer consists of a mixture of γ′-Fe4N and ε-Fex(N, C); (ii) the near-diffusion zone compound layer consists of a mixture of γ′-Fe4N, ε-Fex(N, C), α″-Fe16N2 and γ-austenite; (iii) the dependence of compound layer thickness on nitriding time violates the parabolic behavior and emphasizes effects from cathode sputtering and radiation-enhanced diffusion.