Effect of Boron on Carbide Coarsening at 873 K (600 A°C) in 9 to 12 pct Chromium Steels

The addition of small amounts of boron to 9 to 12 pct chromium steels has been found to decrease their creep rate at 823 K to 923 K (550 A degrees C to 650 A degrees C). In this article, the behavior of boron during austenitizing, tempering, and isothermal heat treatment at 873 K (600 A degrees C) is studied using high-resolution microscopy and microanalysis as well as using atomistic modeling. It was found that increasing the boron content from 9 to 40 ppm decreased the coarsening constant of M23C6 by a factor of almost 2. Most of the added boron was incorporated in M23C6. Atomistic modeling showed that boron diffusion in ferrite is dominated by an interstitial mechanism at 873 K (600 A degrees C). However, the generation of vacancies when carbide precipitates dissolve may promote a distribution with substitutional boron atoms. The absence of a fast mechanism for the transition from substitutional to interstitial occupancy will make the slow substitutional boron diffusion in the matrix rate controlling for the coarsening process.