Microstructure evolution of martensite and carbides in 8Cr4Mo4V steel after nitrogen ion implantation

In our previous study, nitrogen plasma immersion ion implantation (N-PIII) was employed to modify the surface of the 8Cr4Mo4V steel, achieving significant improvements in surface hardness and wear resistance. To elucidate the mechanisms behind these improvements, the element concentration, phase composition, and microstructure of the implanted layer were analyzed in this study. The results show that a part of the nitrogen atoms dissolved into the martensite lattice, and the metal atoms at the boundaries of the martensite laths will capture the N atoms, causing them to move to a deeper position. While another part of the nitrogen atoms combined with hexa-M₂C (M is mainly Mo) and fcc-MC (M is mainly V) carbides in 8Cr steel to form fcc-M₂C_xN_1-x/MC_xN_1-x (M is mainly Mo) and fcc-MC_xN_1-x (M is mainly V) carbonitrides. Furthermore, the dislocation multiplication during the N-PIII process will occur at a depth deeper than the implanted layer. These solid solution strengthening, hard secondary phases, and dislocation defect strengthening all contributed to the increase of mechanical properties in the surface layer of the 8Cr steel. In this study, the microstructure evolution of 8Cr4Mo4V steel after N-PIII was analyzed in detail, which is an effective supplement to the understanding of the microstructural evolution of steels subjected to ion implantation.