Effect of alloying on interfacial energy of precipitation/matrix in high-chromium martensitic steels

The effect of cobalt, tungsten, and boron on interfacial energy of precipitate/ferritic matrix in the 9% Cr martensitic steels on the base of creep tests at 650 °C under different applied stresses ranging from 80 to 220 MPa was investigated. An interfacial energy of M₂₃C₆ carbides, the Laves phase particles, and MX carbonitrides was estimated by comparison of theoretical curves obtained by Prisma software for the model steels for the exposure time of 2 × 10⁴ h with experimental data measured by TEM in the gage sections of crept specimens. Addition of 3 wt% Co to Co-free 9Cr2W steel led to about 1.7 times increase in the interfacial energy of M₂₃C₆ carbides and MX carbonitrides, whereas Co did not effect on the interfacial energy of the Laves phase. Increasing W from 1.5 to 3 wt% in the Co-containing steels led to increase in the interfacial energy of the Laves phase up to 0.78 J m⁻² under long-term exposure, whereas it did not effect on the interfacial energy of M₂₃C₆ carbides and MX carbonitrides. In the steel with increased B up to 0.012 wt% and decreased N to 0.007 wt%, a strong decrease in the interfacial energy of M₂₃C₆ carbides to 0.12 J m⁻² occurred. Change in the interfacial energy of the precipitates was analyzed in comparison with coarsening rate constant.