The effect of nitriding temperature on hardness and microstructure of die steel pre-treated by ultrasonic cold forging technology

To promote the formation and growth of nitride layer, a befitting surface nano-crystallization process was introduced as a pre-treatment of nitriding, and an optimized nitriding temperature was investigated. A modification layer with a depth of 350 μm was formed by ultrasonic cold forging technology (UCFT) on AISI D2 surface. A series of plasma nitriding experiments for both treated and untreated samples were conducted at various temperatures ranging from 350 °C to 550 °C for 4 h. The influences of nitriding temperature on the hardness, microstructure, morphology and composition of the sample surface were investigated by micro-hardness tester, optical microscope (OM), 3D profile-meter and scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The diffusion of nitrogen and the formation of nitrides were markedly improved by UCFT pre-treatment. A thicker and harder nitrided layer was formed at a high nitriding temperature. However, as the nitriding temperature increased to 550 °C, a stronger sputter occurred on the sample surface. The results presents that it is an optimized process to be pre-treated by UCFT and nitriding at 520 °C for 4 h.