Carbide Evolution in High Molybdenum Nb-microalloyed H13 Steel during Annealing Process

Based on the Thermo-Calc thermodynamic software, the type of equilibrium precipitated carbides and their contents in high Mo Nb-microalloyed H13 steel (NMH13 steel) were calculated. The composition, morphology, and distribution of carbides after spheroidal annealing of two forged experimental steels were comparatively examined by means of optical microscopy (OM), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). VC, M23C6 and M6C are identified in H13 steel after spheroidizing annealing, while (V, Nb)C, M23C6, M2C and M6C are observed in NMH13 steel. Moreover, it is found that the addition of Nb significantly enhances the stability of MC phase and the high Mo content accelerates the precipitation of small rod-shape M2C phase in NMH13 steel. The amount of the fine carbides in NMH13 steel obviously increased with M2C and M6C precipitated from the ferrite phase, which is in accordance with the results of thermodynamic calculations.