A new model for growth mechanism of nitride layers in plasma nitriding of AISI H11 hot work tool steel

A new mechanism of layer growth in plasma nitriding of H11 hot work tool steel was developed. The nitriding was performed at 450, 500 and 550 °C temperature, nitriding times of 10, 45, 90, 150, 300, 450 and 600 min, in a 25% H2:75% N2 atmosphere and DC-pulsed plasma nitriding unit. The surface was studied by optical and scanning electron microscopy. According to the results, values of Q and D0 for nitrogen diffusion in steel substrate were as 55186 (j/mol) and 3.02 × 10−9 (m2/s), respectively. The dimensions of surface nitrides were determined by Clemex Vision image analyzer software and atomic force microscopy. Based on these measurements a new mechanism for white layer growth was proposed. Accordingly, the white layer could be divided into two sections: surface nitride particles and diffusion section. The values of Q and D0 for nitrogen diffusion section of white layer were calculated as, 55787(j/mol) and 4.047 × 10−12(m2/s), respectively.