Effect of time and temperature of plasma nitriding on microstructure, depth of nitrogen diffusion and hardness of plastic injection mould steel P20

Plasma nitriding could improve the lifecycle of plastic mould steels by modifying the surface layers. In this study, the effects of time and temperature of plasma nitriding on plasma nitriding behavior of plastic injection moulding steel P20 were studied. In this regard, pulsed plasma nitriding treatment was performed on P20 samples at different processing temperatures such as 450, 500 and 550 ºC for 2.5, 5, 7.5 and 10 h at gas mixture of 75% N2 - 25% H2. Diffusion zone and compound layer were studied by optical and scanning electron microscopy and the crystallographic phases on surface were determined by X-ray diffraction analysis. Also, micro hardness changes from surface to core of the samples were determined. Results have shown that by increasing treatment temperature and time, nitrided layer thickness and hardness were increased respectively. The SEM results showed that the interface of the compound layer and the diffusion layer formed on the sample treated at higher temperatures were more uniform than those treated at lower temperatures. Regarding the results of EDS, during the ion nitriding of P20 upward diffusion of the carbon and the downward diffusion of the nitrogen have been occurred. XRD results have shown that in all the process parameters ε-nitride was the most forming phase.