The background subtracted XRD patterns of the representative residues of the weld metals with different V contents are depicted in Fig. It is obvious that the carbide types within the specimens gradually changed with an increase in the V contents. M2C (M = Mo, mainly) and M3C (M = Fe, mainly) carbides were detected in the specimen without any V, while reflection peaks for the hexagonal close-packed M2C and orthorhombic M3C were easily discernible. The weak reflection peaks of the face-centred cubic MC (M = V, mainly) indicated that a small amount of MC carbides began to precipitate at the V10 specimen. However, as the V content increased to 0.18 wt%, the reflection peaks of the M2C and M3C became very weak and difficult to detect, suggesting that a large number of M3C and M2C carbides were replaced with a considerable portion of MC carbide. Moreover, it is notable that the integrated intensity of the M2C reflection peaks for the V00 weld metal was obviously stronger than that of the M3C peaks, but this intensity difference was alleviated in the V10 weld metal, indicating a decreasing M2C volume fraction owing to the Mo substituting the V in the MC carbides, instead of forming M2C. In general, the variations in the integrated intensity of the carbide reflection peaks indicated that the an increasing volume fraction of the MC was caused by the sacrifice of the M2C and M3C in the presence of V. These results revealed that the 0.18 wt% V stabilised the more thermodynamic MC carbides under the condition of the requisite C to suppress the M3C and M2C precipitation, because V was the strongest carbide forming element compared to Fe, Mo, and Cr, which resulted in a substantially greater thermodynamic advantage over the M3C and M2C precipitation with Mo contents up to 0.7 wt%.
