Computational modeling of the effect of B and W in the phase transformation of M23C6 carbides in 9 to 12 pct Cr martensitic/ferritic steels

Abstract

The effect of B and W in the precipitation kinetic of M23C6 carbides in martensitic/ferritic steels were studied assisted by ThermoCalc and TC-PRISMA. The simulation predicts that B has low solubility in the austenitic and ferritic matrix, thus promoting the formation of M2B borides and (Cr, Fe)23(C, B)6 carbides. Furthermore, calculation carried out in ThermoCalc shown that M2B borides are stable even at austenization temperature (1100 °C). This suggests that in martensitic/ferritic steels M2B precipitates first, hence consuming most of available B. Additionally, the precipitation kinetics of (Cr, Fe)23C6, (Cr, Fe, W)23C6 and (Cr, Fe)23(C, B)6 were simulated in TC-PRISMA, results obtained predict a higher steady nucleation rate for the latter. When B atoms incorporate to (Cr, Fe)23C6 reduces the interfacial energy from 0.26 J/m2 to 0.17 J m−2, which indicates a smaller misfit between (Cr, Fe)23(C, B)6 carbides and matrix. On the other hand, B and W reduce the coarsening rate of M23C6 carbides by decreasing the interfacial energy and reducing diffusivities, respectively. Theoretical results compared with experimental data of the coarsening rate of (Cr, Fe, W)23(C, B)6 shown that it is one order of magnitude lower than the predicted by TC-PRISMA. This suggests that B can be the rate-controlling element of the coarsening of (Cr, Fe, W)23(C, B)6 carbides instead of W.