Coarsening behavior of M23C6 carbides after ageing or thermal fatigue in AISI H13 steel with niobium

Changes of the size and morphology for carbides during thermal fatigue from room temperature (20) to 700 °C and ageing at 700 °C were studied using TEM and EDS. The coarsening behavior of M23C6 carbide at 700 °C in H13 steel with and without micro-niobium was also simulated by DICTRA software package. The simulation was based on the assumption that thermodynamic equilibrium was simultaneously established at the moving phase interface. The results from the calculations were compared with transmission electron microscopy (TEM) observations. It was found that the size of Cr-rich carbide (in the type of M23C6), quickly expanded during fatigue test. However, simulations as well as the experiments indicated very low coarsening rates for the carbides aged at 700 °C. The agreement was satisfactory, although the coarsening rate in the case of ageing was larger than that of the simulated value both in H13 and H13Nb steels. The coarsening rate of M23C6 in thermal fatigue test was about 300 times bigger than in ageing for both H13 and H13Nb steels. The results from experiments and simulation showed that the addition of 0.07 wt% niobium into H13 steel could play a role to retard the M23C6 carbide coarsening.