A Study on Formation and Growth Mechanism of Nitride Layers During Plasma Nitriding Process of Plastic Injection Mold Steel

Abstract

Ion nitriding modifies composition of surface layer in steel used in plastic mold application and this consequently improves their lifecycle. In this study, pulsed plasma nitriding technique was used to produce a protecting hard layer on AISI P20 steel at three process temperatures of 450°C, 500°C, and 550°C for durations of 2.5, 5, 7.5, and 10 h at a constant gas mixture of 75% N2–25% H2. Surface morphology was studied by optical and scanning electron microscope and the phases formed on the surface layer were determined by X-ray diffraction (XRD). Elemental depth profile was measured by techniques including energy dispersive spectroscopy, wavelength dispersive spectrometer, and glow discharge spectroscopy and for identifying hardness profile, microhardness variations from surface to core of samples were recorded. Results showed that, thickness of compound layer of plastic mold steel AISI P20 was negligible. Moreover in ion nitriding of AISI P20, nitride were formed and grown in some preferred directions and upward diffusion of carbon and downward diffusion of nitrogen occurred during ion nitriding of AISI P20. XRD results showed that, ϵ-nitride is the dominant phase after plasma nitriding in all strategies. Furthermore, ion nitriding improved hardness of AISI P20 up to three times and as time and temperature increased, hardness and hardness depth of diffusion zone increased considerably.