Effect of nitriding on the transformation of alloy carbides (VC and Mo₂C) in 8Cr4Mo4V steel

The formation of vein-like structures (VLSs) in alloy steel during nitriding is a commonly known drawback that undermines its mechanical properties. However, the origin of these VLSs remains under investigation. In this study, we used transmission electron microscopy and X-ray diffraction techniques to investigate the detailed microstructure and formation process of VLSs in 8Cr4Mo4V steel nitrided at 500 °C. We also employed first-principles calculations to discuss the afforded results. In 8Cr4Mo4V steel, the alloying carbides (Mo₂C and VC) at the grain boundaries absorbed diffused nitrogen and released carbon, which then reacted with the Fe matrix to form VLSs (Fe₃C). Notably, Mo₂C displayed a stronger ability to absorb N atoms than did VC, which agreed with our theoretical results. The complex transformation mechanism for Mo₂C during the nitriding process was also elucidated as follows: hexa-Mo₂C → hexa-Mo₂C_xN_1-x → stacking-fault-type-Mo₂C_xN_1-x → γ-Mo₂N. This study provides a strong theoretical and experimental understanding for further optimization of the nitriding process and selection of alloying element types for nitriding steel.