Effects of microstructure and nanohardness of 8Cr4Mo4V steel under high-dose-rate N-PIII

Nitrogen ion implantation with different dose rates was conducted on 8Cr4Mo4V-bearing steel in the present study. The effects of high-dose-rate nitrogen plasma immersion ion implantation (N-PIII) on the microstructure and nanohardness of the 8Cr4Mo4V-bearing steel were studied. The surface morphology, phase microstructure, lattice parameters and nanohardness were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanoindentation tests. The results showed that finer crystallite size, higher micro-strain and dislocation density can be achieved under high-dose-rate ion implantation and the dislocation density reaches the highest level of 1.71 × 10¹⁶ m⁻² at a dose rate of 5.23 × 10¹⁷ ions cm⁻² h⁻¹. In addition, the high-dose-rate ion implantation results in changes in phase composition in the surface layer, and the Fe(M) peaks slightly shift to a lower angle. Moreover, the nanohardness of implanted samples improved notably, especially at a dose rate of 8.64 × 10¹⁷ ions cm⁻² h⁻¹, reaching the high value of 15.8 GPa, which is 42.1% higher than that of a non-implanted sample (11.1 GPa), implying that the properties of the samples were strongly affected by the implantation dose rate of nitrogen.