Characterization of Vein-Like Structures Formed in Nitrided Layers During Plasma Nitriding of 8Cr4Mo4V Steel

In this study, nitrided 8Cr4Mo4V was characterized, using optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), atom probe tomography (APT), electron probe microanalysis, and Thermo-Calc software, to study the vein-like structures formed during plasma nitriding. The structures existed in a certain zone in the nitrided layers, namely the posterior end of the N-content plateau and the anterior end of the N-content rapid-decline zones. The structures disappeared in the topmost surface layer with increasing nitriding time. For the first time, APT was used to observe these structures. Together, the APT and TEM images revealed that the main components of the structures were Fe3C and alloy nitrides. The structures and matrix were related through the (220) Fe3C//(110)α-Fe, [110] Fe3C//[001]α-Fe orientations. The appearance and disappearance of the structures were closely related to the C and N contents. In the alloy carbides, N replaced C to form alloy carbonitrides and released C and Fe to form Fe3C which appeared as the vein-like structures. In the topmost surface of the nitrided layers, the C content decreased because of decarburization. The Fe3C and alloy carbides were transformed into ferrite and alloy nitrides, and the structures disappeared.